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PROPERTIES AND STRUCTURES OF SULFONATED SYNDIOTACTIC
POLYSTYRENE AEROGEL AND SYNDIOTACTIC POLYSTYRENE/SILICA
HYBRID AEROGEL
A Thesis
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirement for the Degree
Master of Science
Huan Zhang
August, 2014
ii
PROPERTIES AND STRUCTURES OF SULFONATED SYNDIOTACTIC
POLYSTYRENE AEROGEL AND SYNDIOTACTIC POLYSTYRENE/SILICA
HYBRID AEROGEL
Huan Zhang
Thesis
Approved: Accepted: ____________________________ _____________________________ Advisor Department Chair Dr. Sadhan C. Jana Dr. Robert A. Weiss ____________________________ _____________________________ Faculty Reader Dean of the College Dr. Bryan Vogt Dr. Stephen Z.D. Cheng ____________________________ _____________________________ Faculty Reader Dean of the Graduate School Dr. Nicole Zacharia Dr. George R. Newkome ____________________________ Date
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ABSTRACT
This study focuses on hybrid aerogels of syndiotactic polystyrene (sPS) and silica
where silica concentration is varied and silica condensation conditions are varied to
produce various mesoporous materials. In addition, the study investigates hybrid
materials obtained by coating sulfonated syndiotactic polystyrene on a cellulose filter
paper.
In first part of this study, syndiotactic polystyrene is modified by sulfonation to enable
further chemical modifications. Specifically, sulfonated syndiotactic polystyrene (ssPS) is
coated on macroporous cellulose filter paper using a dip coating process, ssPS is turned
into gel by thermoreversible gelation, and finally aniline is polymerized on ssPS strands
to obtain a hybrid aerogel with electrical conductivity. The aerogels are recoved by
removing the solvents under supercritical conditions. The ssPS aerogels are coated on
cellulose filter materials to derive two benefits: first, to capitalize on the large surface
area of ssPS aerogels and second, to exploit the mechanical strength of the cellulose
filter.
In second part of this study, silane precursor is absorbed inside the macropores of sPS
gels and silica gels are grown under both a two-step reaction and a one-step reaction.
Type A silica aerogel is synthesized by acid-base catalyzed sol-gel method, while type B
silica aerogel is prepared only by acid catalyzed sol-gel process. Type A silica gels
exhibit pearl-necklace structure, while type B silica gels are composed of strand-like
iv
combination of organic and inorganic materials and their associated surface area and
surface energy offer a number of attractive properties.
The sPS/silica hybrid aerogels are fabricated from different concentrations of sPS in
solution and from different weight ratio of sPS to silica. The sPS/silica hybrid aerogels
exhibit predominant pores located in the mesopore range (2~50 nm), and at higher silica
content, silica particles undergo more aggregation as evident from larger fractions
mesopore size (~15 nm). In contrast to native sPS aerogel, the compressive modulus of
hybrid aerogel is increased up to 100%. In addition, the hybrid aerogels present specific
surface area as high as 693 m2/g, high porosity, fast absorption and high absorption
capacity of crude oil. Type A hybrid aerogels are composed of both fiber-like strands of
sPS and pearl-necklace particles of silica. At low weight ratio of sPS to silica, the silica
forms aggregates on sPS backbones as conformal coating. At higher weight ratio of sPS
to silica, the silica particles not only coated on the polymer backbones, but also filled the
macropores of sPS. Type B hybrid aerogels present fiber-like interpenetrating networks
formed by sPS strands of 50-100 nm and silica strand of 200 nm.
silica particles. In the process, the mesoporous hydrophilic silica particles are combined
with macro- and microporous hydrophobic sPS in single articles. This unique
v
ACKNOWLEDGEMENTS
I would like to express my sincere appreciation to my advisor, Prof. Sadhan C. Jana,
for his selfless guidance, and for his perseverance and constant encouragement to
complete this thesis.
I would like to extend my thanks to all my friends in my groups for their support to my
work. I would like to express my deep gratitude to Xiao Wang for her invaluable help
and encouragement to my research.
I would like to express special thanks to Dr. Rong Bai and Dr. Boji Wang for training
me for the instrument
vi
TABLE OF CONTENTS Page
LIST OF TABLES ............................................................................................................. ix
LIST OF FIGURES ............................................................................................................ x
CHAPTER
I INTRODUCTION ......................................................................................................... 1
II REVIEW OF LITERATURE ....................................................................................... 5
2.1 What is an aerogel? ................................................................................................ 5
2.2 Supercritical drying ................................................................................................ 6
2.3 Silica aerogel .......................................................................................................... 8
2.3.1 Silica sol-gel chemistry ................................................................................ 8
2.3.2 Structure of silica aerogels .......................................................................... 11
2.4 Syndiotactic polystyrene aerogels........................................................................ 13
2.4.1 Syndiotactic polystyrene/solvent system ................................................... 13
2.4.2 Thermo-reversible gelation of sPS ............................................................. 16
2.4.3 Types of sPS aerogels ................................................................................ 17
III SULFONATED SYNDIOTACTIC POLYSTYRENE AEROGEL-COATED MESOPOROUS SURFACES ................................................................................. 19
3.1 Introduction .......................................................................................................... 19
3.2 Experimrntal ........................................................................................................ 20
3.2.1 Materials .................................................................................................... 21
3.2.2 Preparation of sulfonating agent ................................................................ 21
vii
3.2.3 Process of sulfonating sPS ......................................................................... 22
3.2.4 Measurement of the degree of sulfonation ................................................. 23
3.2.5 Process of making ssPS-coated-filter gels ................................................. 24
3.2.6 Polymerization of aniline on the ssPS-coated-filter aerogel ...................... 25
3.2.7 Characterization methods........................................................................... 26
3.3 Results and discussion ......................................................................................... 28
3.3.1 Density ....................................................................................................... 28
3.3.2 Morphology................................................................................................ 29
3.3.3 Surface properties ...................................................................................... 33
3.3.4 Thermal property ....................................................................................... 35
3.3.5 Electrical resistivity ................................................................................... 37
IV TAILORING MORPHOLOGY AND STRUCTURE PROPERTIES OF SYNDIOTACTIC POLYSTYRENE-SILICA HYBRID AEROGEL .................... 40
4.1 Introduction .......................................................................................................... 40
4.2 Experimental ........................................................................................................ 42
4.2.1 Materials .................................................................................................... 42
4.2.2 Preparation of sPS/Silica Hybrid Aerogels ................................................ 42
4.2.3 Characterization ......................................................................................... 44
4.3 Results and discussion ......................................................................................... 46
4.3.1 Type A hybrid aerogels .............................................................................. 46
4.3.2 Thermal stability ........................................................................................ 48
4.3.3 Morphology property ................................................................................. 50
4.3.4 Pore size distribution and surface area....................................................... 51
viii
4.3.5 Contact angle ............................................................................................. 53
4.3.6 Compressive properties .............................................................................. 55
4.3.7 Oil and water absorption ............................................................................ 57
4.3.8 Type B hybrid aerogel ............................................................................... 62
V SUMMARY ............................................................................................................... 68
REFERENCES ................................................................................................................. 69
ix
LIST OF TABLES
Table Page
1. Critical constant for some solvents [66]. ................................................................. 7
2. Physical properties of cellulose filter and ssPS-coated-filter aerogel. The specimens of radius 24.52 mm were used in measurement. ...................... 28
3. Surface area and average pore size of different materials. .................................... 35
4. Composition and properties of sPS/silica hybrid aerogels .................................... 46
5. Surface areas of native sPS aerogel and hybrid aerogel ........................................ 52
6. Contact angles of water for sPS-0.02-TEOS aerogels ........................................... 54
7. Contact angles of hydrocarbon oil for sPS-0.02-TEOS aerogels……...………...53
8. Crude oil absorption data for all specimens. .......................................................... 57
9. Water absorption data for sPS-0.02-TEOS aerogel. .............................................. 58
10. Surface area of native sPS aerogel, acid-derived silica aerogel and Type B hybrid aerogel. ……………………………………………………….…..63
11. Oil absorption and water absorption data for sPS-0.02-TEOS-3 acid-derived aerogel. ......................................................................................................... 65
x
LIST OF FIGURES
Figure Page
1. Pressure-temperature phase diagrams. Supercritical drying (arrow 1) goes beyond the critical point of the working fluid. The arrow 2 shows ordinary drying, and the arrow 2 shows two phase changes in freeze-drying. ....................... 7
2. Hydrolysis mechanism of alkoxysilanes under acid/base catalyzed conditions .... 10
3. Condensation mechanisms of alkoxysilanes under acid/base catalyzed conditions . ............................................................................................... 11
4. Gel network structure for acid and base catalyzed reactions . ............................... 12
5. Schematic representations of temperature-concentration phase diagrams for sPS and (a) good solvent, (b) bad solvent ....................................................... 14
6. Schematic of polyaniline formation on sulfonated polystyrene ............................ 20
7. Chemical formula of sPS ....................................................................................... 21
8. Schematic view of apparatus used in synthesis of sulfonated sPS ........................ 23
9. Schematic view of dip coater ................................................................................. 24
10. The PANI-coated-filter is shown in graph (a). The cellulose filter shown in graph (b). .............................................................................................................. 25
11. SEM images of the surfaces of cellulose filter (a), sPS-coated-filter aerogel (b), ssPS-coated-filter aerogel (c), and polyaniline-coated-filter aerogel (d) 29
xi
12. SEM images of the cross-section of cellulose filter (a), ssPS-coated-filter aerogel (b). The internal space of the ssPS-coated-filter aerogel is shown in (c) . 31
13. SEM images of the structure of sPS aerogel (a), ssPS aerogel (b), and polyaniline-coated aerogel (c). ............................................................... 32
14. Nitrogen adsorption-desorption isotherms at 77 K of cellulose filter, ssPS-coated aerogel and polyaniline-coated-filter aerogel. ........................................ 34
15. Pore size distribution of pure filter, ssPS-filter aerogel and polyaniline-ssPS-filter aerogel..................................................................................................... 34
16. TGA traces of sPS and ssPS ................................................................................. 36
17. TGA traces of cellulose filter, ssPS-coated-filter aerogel, and polyaniline-coated-filter aerogel .............................................................. 37
18. The relationship between the voltage and current. ssPS-coated aerogel is shown in graph (a), polyaniline-coated aerogel is shown in graph (b). ............... 39
19. Chemical formula of TEOS. ................................................................................ 42
20.(a) TGA curves of sPS-0.02-TEOS aerogels, (b) TGA curves of sPS-0.04-TEOS aerogels. ................................................................................................ 49
21. The hybrid aerogels before (a) and after (b) heated to 800 0C in the air. From left to right, the concentration of TEOS decrease from zero to 0.7 mol/l. ..... 49
22. SEM images of sPS aerogel (a), hybrid aerogels (b)~(e), and native silica aerogel (f) ................................................................................................................ 51
23. Pore size distribution of two series of hybrid aerogels. ....................................... 52
24. Images of aerogels exposed to water. (a) 0.7 mol/l silica aerogel, sank to the bottom of water and collapsed under a tender touch. (b) sPS-0.04-TEOS-0.7 aerogel, floated on the surface of water, kept a strong structure after immersed in the water using a external force. .................. 54
25. (a) Compressive moduli of hybrid aerogels. (b) compressive stress-strain curves of selected hybrid aerogel. ........................................................................ 56
26. Images of sPS-0.04-TEOS-0.7 aerogel and sPS-0.04-TEOS-0.1 aerogel at different time during oil absorption test, sPS-0.04-TEOS-0.7 aerogel : (a): 1s, (b) 15s, (c) 30s; sPS-0.04-TEOS-0.1 aerogel : (d): 1s, (e) 15s, (f) 30s. .................................................................................................................. 59
xii
27. Image (a) and (b) show the hybrid aerogel and native sPS aerogel before and after oil absorption test. Image (c) and (d) show the hybrid aerogel before and after water absorption test ................................................................ 59
28. (a) TGA curves of Type B hybrid aerogels, (b) The Type B hybrid aerogels before and after heated to 800 0C in the air. ........................................................ 62
29. SEM images of acid-derived silica aerogel (a) and Type B hybrid aerogel (b). .. 63
30. Pore size distribution of native sPS aerogel, acid-derived silica aerogel and type B hybrid aerogel……… …………...63
31. Image (a) and (b) show Type B hybrid aerogel before and after oil absorption test. Image (c) and (d) show Type B hybrid aerogel before and after water absorption test ......................................................................................... 66
1
CHAPTER I
INTRODUCTION
Aerogels are highly porous, low density, solid state materials with three dimensional
network structures. The network structures are produced first in the starting gels and
aerogels are derived by removing the liquid component from gels under supercritical
conditions. Aerogels attract significant academic and industrial interest owing to its
extremely low density, high porosity, and high surface area. However, their applications
are limited due to some of their inherent defects. For example, silica aerogels are fragile
and friable materials. The structure of native silica aerogel can be easily crushed by a
stress of 31 kPa [1]. Accordingly, many strategies emerged in last decades to tailor the
network structures and to obtain reinforcement.
Aerogels have been made from polymers [ 2 3 4 5 ], transition metals [ 6 7 8 9 ], and
montmorillonite clay/polymer composites [10,11]. Since first reported in the 1930s [12],
silica aerogels have became most widely used and extensively studied due to versatile
chemistry and remarkable properties, e.g., density ~3-350 mg/cm3 [13,14], high porosity
(>95%), large surface area (ca. 1000 m2/g), low thermal conductivity (0.004-0.03
W/mˑK), low dielectric constant (1.1-2.2) [15,16], and low index of refraction (~1.05) [17]
2
Due to these interesting properties, silica aerogels found many potential applications in
thermal insulation, energy media, molecular separation, high energy physics, to name a
few [14],[1819202122]. Two unfavorable properties limit the applications of silica aerogels –
first, the hygroscopic nature and second, the inherent fragility of the silica networks. The
network structures of silica aerogels are formed by the condensation reaction between
"Si-OH" groups. However, not all Si-OH groups are fully condensed. Thus, silica
aerogels can be easily destroyed by placing a small drop of water on its surface [23,24].
The hygroscopic silica particles absorb water into its pores. The capillary stress causes
compressive stress and crushes the networks [25]. A native silica aerogel derived from
tetramethoxysilane (TMOS) with a density of 0.12 g/mL can be completely shattered into
dust under a small load of 31 kPa [1]. This is due to weak network structures formed by
many "pearl necklaces" that are tied together by a limited number of "Si-O-Si" bonds at
the necks of the secondary silica particles.
Many effective methods of reinforcement of silica aerogel have been developed. The
most straightforward way is to increase the total number of connecting points between
the secondary particles by increasing the density [26]. In addition, strengthening the neck
region using multifunctional particles, and reinforcing the network with nanofibers
showed large increases in mechanical strength [27282930313233]. It is reported [34,35] that silica
aerogels can be effectively strengthened by a conformal coating of polymer on silica
backbone. In this context, one can capitalize on the unique properties of silica aerogels if
silica aerogels are efficiently packaged in another porous material and the carrier porous
material provides mechanical integrity.
3
Since first reported by Daniel et al. in 2005 [36], syndiotactic polystyrene (sPS)
aerogel has also attracted significant interest. sPS aerogel is a highly porous, high surface
area and low density material obtained from physical sPS gel by replacing the liquid
component with a gas by supercritical drying or freeze drying methods. The network
structures of sPS aerogel are composed of three-dimensional polymeric strands, and the
connectivity of network is obtained from the intermolecular physical bonding at the
crystalline junctions [37]. sPS aerogels offer unique three-dimensional network structures,
consisting of micro- (<2 nm) and macropores (>50 nm), formed by the intermolecular
physical bonds at the crystalline junctions [36]. The micropores can trap volatile organic
compounds, e.g., 1,2-dichloroethane in the crystalline nanocavities of sPS strands [37]. It
is reported that -form sPS aerogels are effective in adsorption of volatile organic
compounds (VOC) [37, 38 ]. Hydrogen adsorption experiments indicated that the
adsorption occurred in the ordered cavities of the nanoporous polymeric crystalline states
[39,40] and the VOC diffusivity in -form sPS aerogels with porosity of 98.5% is about
7 orders of magnitude faster than in -form sPS films [37]. However, hydrophobicity of
sPS chains, inferior compressive mechanical properties although much better than silica,
and relative chemical inertness also limit the applications of sPS aerogels.
This thesis is divided as follows. Chapter II presents the review of associated literature.
Chapter III discusses the properties of sulfonated syndiotactic polystyrene (ssPS) aerogels.
Syndiotactic polystyrene was modified by introducing the sulfonic acid groups on the
polymer main chain such that further chemical modification is later possible. The ssPS
gel was then coated on cellulose filter to take advantage of its mechanical strength. A
further modification was studied by polymerizing aniline on the ssPS-filter. It is reported
4
[ 41 ] that highly conductive polyaniline/sulfonated polystyrene composites can be
synthesized by in-situ polymerization of aniline within the sulfonated syndiotactic
polystyrene matrices. The randomly distributed sulfonic acid groups were localized
within the polyaniline backbones and formed the ionic domains which act as templates
for protonation of aniline to form the aniline salt. Polymerization of aniline within these
fixed ionic domains was initiated by allowing diffusion of an oxidant. In this study, the
ssPS-filter was used as template for in-situ polymerization of aniline. The polyaniline
domains formed on ssPS polymer strands. As a result, a conductive filter film was
obtained, which can find application as a capacitor.
Chapter IV extends a novel type of hybrid aerogel reported by Wang and Jana. [25].
The sPS/silica aerogel was obtained by a two-step sol-gel process. sPS gel was first
prepared and the silica precursor was absorbed into the sPS gel. Subsequently, the silanes
were turned into gels. The silanes were converted into gels following two methods – first,
a two-step acid-base catalyzed sol-gel process and second, a one-step acid catalyzed
sol-gel process. The ratio of sPS to silica was varied to obtain several morphological
forms, surface area, and compressive properties. Wang and Jana [25] used a fixed
concentration of silane in this work. Also they followed a two-step sol-gel process to
yield silica particles with pearl-necklace morphology. The sPS strands were prevented
from buckling by the rigid silica pearl necklace particles grown inside the macropores.
On the other hand, the sPS networks acted as cage for silica particles and prevented their
brittle rupture.
5
CHAPTER II
REVIEW OF LITERATURE
2.1 What is an aerogel?
Aerogel is a highly porous, ultralight material derived from a gel where the liquid is
replaced with a gas. The first aerogel was reported by Steven S. Kistler in 1931 [12].
Kistler dispersed colloidal silica particles in distilled water. The silica particles then
connected with each other by chemical bonds during condensation of the silanol groups
and formed a gel. The aerogel was obtained by using a supercritical drying process
[12],[42] in which the liquid is removed above its critical point such that the distinct
liquid and gas phases do not exist. A xerogel in comparison is obtained from a gel which
is dried under ambient conditions. The absence of surface tension in supercritical drying
preserves the perfect network structure of aerogel. Due to the presence of a large volume
of gas, the aerogel has many outstanding properties, such as extremely low-density
(3-150 mg/cm3) [14],[43], high porosity (>95%), large surface area (500-1200 m2/g), low
dielectric constant (1.1-2.2) [14-16],[44,45], low thermal conductivity (ranging from
0.004 to 0.023 W/ Km ), and low index of refraction (~1.05) [17], In addition, they
present different morphologies (monoliths or powders) and offer optical properties
[46,47]. These remarkable properties promote a wide range of applications. Aerogels
6
found usage in high-energy particle physics [21],[48] to generate Cherenkov radiation by
passing charged particles through the aerogel at speeds faster than the speed of light [49].
Recently, more academic and industrial attention is focused on the aerogels, and new
applications have emerged in thermal and acoustic insulation [18],[20,21], catalysis [50],
as storage media [51], and filtration media [37]. In addition, silica aerogels find
applications in life sciences such as in dealing with biocatalysis [525354], with a lipase
enzyme, or in a process to detect a viral particle by immobilized bacteria [555657585960].
2.2 Supercritical drying
An aerogel is obtained when the wet gel is dried under supercritical condition. The
drying process is extremely important. During evaporation of the liquid within the pores
of gel, the capillary stresses inevitably occur whenever gas-liquid menisci appear at the
pore boundaries. Consequently, the aerogels tend to crack. Strategies to avoid cracking
have been studied. There are three main techniques of drying of silica gels [61],[62]
(shown in Figure 1.1):
1. Freeze-drying – this necessitates bypassing the triple point during drying.
2. Evaporation – this implies the crossing of the liquid-gas equilibrium curve during
drying.
3. Supercritical drying – this necessitates bypassing the critical point.
In general, freeze-drying leads to cracked pieces or even powder-like products [63].
Evaporation without specific surface treatments and/or aging treatments [64] results in
“dense” (e.g., >0.25 g/cm3) [ 65 ] and even cracked materials called xerogel.
Supercritical drying is considered a much better way of making aerogels, because it
7
eliminates capillary stresses. This also allows fabrication of aerogels with large
dimensions. In supercritical drying, the solvent in the gel is removed above its critical
point (shown in Figure 1), thus the distinction between the liquid and gas no longer exists.
In principle, supercritical drying can be performed in different organic solvents in their
supercritical state. The critical constants of the common drying fluids are listed in Table 1,
where Tc and Pc are the critical temperature and pressure, respectively. Carbon dioxide is
the most popular liquid used for supercritical drying, because of its very low critical
temperature and pressure.
Figure 1. Pressure-temperature phase diagrams. Supercritical drying (arrow 1) goes
beyond the critical point of the working fluid. The arrow 2 shows ordinary drying, and
the arrow 3 shows two phase changes in freeze-drying.
Table 1. Critical constant for some solvents [66].
liquid
solid
gas
critical point
supercritical
state
Temperature T(K)
Pre
ssu
re
P(b
ar)
1
2
3
Solvent Tc (oC) Pc (MPa)
8
Table 1. Critical constant for some solvents (continued)
2.3 Silica aerogel
Silica aerogel are most widely used and extensively studied. The structure of silica
aerogel strongly depends on the process of gelation. Two-step acid-base catalyzed sol-gel
process is the most widely used synthesis method for silica aerogel.
2.3.1 Silica sol-gel chemistry
The sol-gel process is primarily used to produce a variety of inorganic networks from
silica or metal alkoxide monomer precursors. This process usually involves hydrolysis
and condensation of metal alkoxides, in which a colloidal suspension (sol) gradually
evolves to form a network in a continuous liquid phase (gel) [67],[68]. The most widely
used precursors for silica aerogel are alkoxysilanes, such as tetraethxoysilane (TEOS) and
tetramethoxysilane (TMOS).
Hydrolysis occurs when a metal alkoxide (M-OR) reacts with water, forming a metal
hydroxide (M-OH). It can proceed in either acidic or basic conditions. For acid-catalyzed
reaction, the transition state is positively charged and the hydrolysis is strongly
Methanol 240 7.9
Ethanol 243 6.3
Acetone 235 4.7
H2O 374 22.1
CO2 31 7.3
9
accelerated [69] since the silicon atom is more susceptible to attack by water due to the
protonated alkoxide, which reduces the electron density of connected silicon atom. The
silicon is attacked by a water molecule from the opposite side of the protonated alkoxy
group and the water molecule gains a positive charge because the alkoxy groups are more
electron-donating than the hydroxyl groups. The mechanisms of hydrolysis reaction in
acidic and basic conditions are shown in Figure 2.
Acid-catalyzed hydrolysis:
Base-catalyzed hydrolysis:
10
Figure 2. Hydrolysis mechanism of alkoxysilanes under acid/base catalyzed conditions
[67]
Two partially hydrolyzed molecules combine together in a condensation reaction to give
a molecule of metal oxide species (M-O-M). The condensation reaction yields one water
molecule. These reactions can proceed under either acidic or basic conditions. The
condensation reaction occurs relatively slowly when catalyzed by acid. The rate of
nucleophilic attack on a protonated silanol is slow because the nucleophiles tend to be
inhibited in the presence of the acid. The base-catalyzed condensation reaction is widely
used and its mechanism may involve penta- or hexa-coordinated silicon intermediates or
transition states [70]. The schemes of condensation reaction under both acidic and basic
conditions are shown in Figure 3.
Acid-catalyzed condensation:
Base-catalyzed condensation:
11
Figure 3. Condensation mechanisms of alkoxysilanes under acid/base catalyzed conditions.
[67],[71].
2.3.2 Structure of silica aerogels
According to Iler, the process of basic sol-gel polymerization can be divided in three
stages [69]. In the first stage, the monomers react to form particles. The second stage
produces the growth of particles. In the third stage, the final gel is obtained, and the
particles link together into chains to form the networks. In these three stages, many factors
affect the structures of silica aerogel, such as pH, monomer concentration,
water/monomer molar ratio, reaction time, etc. [67],[72]. For acid-catalyzed sol-gel
process, the hydrolysis reaction is the fastest, and the condensation is slow. A silanol with
more alkoxy groups produce more stable positively charged transition states since the
alkoxy groups are more electron-donating than hydroxyl groups. Therefore, an open
network structure is formed initially, which undergoes further hydrolysis and
condensation reactions. Hence, the silica gels are formed by linear or randomly branched
long chains. They ultimately present a bushy network as shown in Figure 4 (a). On the
other hand, for base-catalyzed sol-gel process, the hydrolysis steps proceed rapidly and
12
the hydrolyzed specie undergo the faster condensation reactions. Thus, highly
cross-linked sol particles are formed initially and link together to form a gel with large
pores between the interconnected particles as shown in Figure 4 (b).
(a)
(b)
Figure 4. Gel network structure for acid and base catalyzed reactions [70] (Adapted from
Ref 70).
13
2.4 Syndiotactic polystyrene aerogels
Since syndiotactic polystyrene (sPS) was synthesized by researchers at Idemitsu Kosan
Central Research Laboratory in 1986 [73], it attracted strong interests in academia and
industries because of its complex polymorphic behaviors under different conditions.
2.4.1 Syndiotactic polystyrene/solvent system
sPS/solvent system exhibit complicated behavior due to the complex polymorphic
crystal shape of sPS. A lot of sPS/solvent systems have been well studied, such as the
solution of sPS with toluene [74-75], benzene [76], chloroform [74],[77] chlorobenzene
[78], 1,2-dichorobenzene, 1,2-dichloroethane [79], tetrahydrofuran [80], trans-decalin
[81], cis-decalin [81], etc. The complex polymorphic behavior can be described in terms
of the formation of different polymer-rich phases. The stable conformations in sPS
crystalline forms are classified into two types. One is characterized by all-trans (T4)
planar zigzag chain conformation [82], and the other is characterized by the formation of
polymer-solvent compound which adopt a trans-trans-gauche-gauche (T2G2) helix
conformation [83]. The sPS can crystallize in four crystalline modifications. The
crystalline in which the polymer chains adopt the T4 conformation is called α-, β-
modifications, and the γ-, and δ- modifications are characterized by the T2G2 helical
conformation. As shown in Figure 5, the polymer-rich phase structure is determined by
the result of the combined effect of thermodynamics and kinetics. In the case of good
solvent, i.e., o-xylene, only the δ-phase (helical conformation) is formed at low polymer
concentrations. At high polymer concentrations, the β-phase (planar zigzag conformation)
is thermodynamically stable phase and the metastable δ-phase is observed when the
14
Low
High
concentrated solutions are quenched first (Figure 5 (a)). In the case of poor solvent, the
β-phase is the most stable phase over the whole concentration range, and quenching to
room temperature results in the formation of δ-phase (Figure 5 (b)).
Figure 5. Schematic representations of temperature-concentration phase diagrams [84] for
15
sPS and (a) good solvent, (b) bad solvent (Adapted from Ref 84).
According to Roels et al. [78], the phase behavior of sPS in different solvent is strongly
related to the solvent qualities. The different solvent can be listed in the following order
with increasing solvent quality: decalin < 1, 2, 4-trichlorobenzene < o-xylene < 1,
2-dichlorobenzene < chlorobenzene < chloroform. The stability of the zigzag
conformation phase which is considered as a “classical” crystalline phase increases with
decreasing the quality of the solvent. This phase represents a stable phase in poor solvents,
such as decalin. The helical phase shows an opposite solvent quality dependence,
presenting stable phases in good solvent, such as chloroform. It is also reported [85] that
the stability of the TTGG ordered conformation in both gel and crystalline phases are
strongly dependent on the solvent-polymer interactions, including the kind of solvent and
the gelation temperature.
Daniel et al. [ 86 ] studied the rheological behavior of sPS gels formed with
1-chlorotetradecane (CTD), a non-volatile bulky solvent. The gel presents a zigzag planar
conformation. The rheological behavior was compared for s-PS/CTD gels with different
crystalline phase. The rheological measurements have shown that the gels with the s(2/1)
helical crystalline phase are generally more stable than trans-planar sPS gels. In fact, for
equal polymer concentrations, the dynamic storage modulus G′ of the gel with helical
crystalline phases is much higher than that of gels with trans-planar crystalline phases.
This rheological behavior can be attributed to different morphologies. For the crystalline
phase with planar zigzag conformation, the crystalline structure of the gel is observed as
lamellar. For the crystalline phase with helical conformation, sPS gel shows fiber-like
network structures.
16
2.4.2 Thermo-reversible gelation of sPS
Gel is a unique class of materials possessing network structures. According to the
connection type, gels can be subdivided into two categories-chemical gels and physical
gels. Like isotactic polystyrene (iPS) and atactic polystyrene (aPS), the sPS
thermo-reversible gels can be obtained easily by heating sPS/solvent mixture till
formation of a homogeneous solution, and cooling the solution down to room temperature.
The solution then undergoes the sol-gel process, where the sPS polymer chains act as the
framework and the liquid component is confined within the framework. The sol-gel
transition can be reversed by cycling the temperature. According to Daniel et al. [87], the
thermo-reversible gel can be defined as follows: (1) The gel is primarily a network which
is considered as a large system of lines, tubes, etc. which are connected with each other.
(2) The phase transition responsible for the creation of the gel junction must be first order.
(3) The gels do not disaggregate when immersed in an excess of preparation solvent.
The process of thermo-reversible gelation of crystalline phases with helical
conformation is investigated by small angle neutron scattering. It is found that the chains
adopt a worm-like conformation in the gel state [88], indicating that the chains do not
fold in the gel state as with the spherulitic structures. Since the structure of sPS differs
significantly from that of isotactic polystyrene (iPS), a new model is offered for
understanding the gelation phenomenon. This model considers that the helix stabilization
and the unfolded polymer chains are attributed to the solvent molecules which are housed
within the cavities formed by the adjacent phenyl groups of the helix. The only remaining
possibility for the chains to organize is to form a fibrillar morphology as is expected for a
gel.
17
2.4.3 Types of sPS aerogels
In the crystalline state, sPS polymer chains adopt two different conformations. One is
all-trans planar zigzag (T4) conformation that can be separated into α- and β- crystalline
forms. These can be obtained from melt or by annealing [899091]. Another one is s(2/1)2
helical conformation (T2G2) which is observed in the γ-, δ- and ε- crystalline forms of
polymer[92]. The crystal structures of δ- and ε- forms with empty cavities are able to
absorb low molecular weight guest molecules even from extremely diluted solutions.
The crystalline phase and the structures of sPS aerogels depend on the structure, e.g.,
cocrystalline or form of the crystalline junctions formed at the start of gelation of sPS
solutions [36]. The polymer-rich phase of sPS gels (type I gels) obtained with
low-molecular-mass guest molecules [81] present a cocrystal phase. The sPS aerogel
obtained from type I gels exhibit the nanoporous crystalline -form. For example, the
sPS aerogel prepared from chloroform is δ-aerogel, where the polymer-rich crystalline
phase is the δ-crystalline structure [36]. The crystalline structures of the aerogel derived
from sPS/chloroform are inferred from X-ray diffraction pattern. The diffraction pattern
displays strong reflections located at 2 (CuK ) =8.3, 13.5, 16.8, 20.7, and 23.5°, and a
weak diffraction peak at 2θ≈10.6°, which represent the microporous δ-phase [36]. The
crystalline phase of δ-aerogel possesses two identical cavities and eight styrene monomeric
units per unit cell. The sPS form aerogels showing a lamellar crystals structure are
derived from form gel, which is obtained with bulky solvent molecules [36] and
shows pastelike solid state with low elasticity. The corresponding X-ray diffraction
pattern displays strong reflections at 2 =6.1, 12.3, 13.6, 18.5 and 20.2° [36].
18
The -aerogel is discovered recently by immersion of -aerogel (film or powder) in
chloroform for 12 h followed by supercritical drying at a pressure of 20 MPa,
temperature of 40°C for 2 h. [ 93 ]. The -aerogel is obtained from a sPS/1,
2-dichloroethane gel by supercritical carbon dioxide extraction of the solvent at 130°C.
The X-ray diffraction pattern of -aerogel displays strong reflections at 2 =6.9, 8.1,
13.7, 16.2 20.5 23.5 and 28.5° [2]. Unlike the δ-aerogel with isolated cavities, the
-aerogel presents channel-shaped cavities crossing the unit cells along the c-axis [2]. The
SEM analysis shows that the obtained -aerogel exhibits open pore structures with
macropore diameters in the range of 0.5-2 m. The sPS aerogels are effective in
adsorption of volatile organic compounds (VOC) from aqueous diluted solutions, only
when a nanoporous crystalline phase (δ or ε) is present and the guest uptake depends
strongly on the guest molar volume. [93].
19
CHAPTER III
SULFONATED SYNDIOTACTIC POLYSTYRENE AEROGEL-COATED
MESOPOROUS SURFACES
3.1 Introduction
As presented in Chapter II, syndiotactic polystyrene (sPS) can easily form
thermo-reversible gels formed with fiber-like strands. The network structure of sPS is
composed of three-dimensional polymeric strands, where the connectivity of network is
formed by the intermolecular physical bonding at crystalline junctions [37]. sPS aerogel is
a highly porous, high surface area and low density material obtained from physical sPS
gel by replacing the liquid component with a gas by supercritical drying or freeze drying
methods. sPS aerogel has attracted significant academic and industrial interest not only
for its outstanding properties, but also for its unique physically bonded network structures.
However, the nature of high chemical inertness and lack of flexibility in planting
functional groups limit applications of sPS aerogels.
Syndiotactic polystyrene can be functionalized by introducing the sulfonic acid groups
in the polymer backbones. Hsu and coworkers [94] developed a sulfonation procedure of
sPS. In their research, purified chloroform was used as the solvent and acetyl sulfate was
used as the sulfonating reagent. The sulfonated syndiotactic polystyrene (ssPS) consists
of non-polar polystyrene backbone and polar sulfonated acid groups which form a unique
20
microphase separated ionic-rich domains [95]. It was also reported [45] that highly
conductive polyaniline/sulfonated polystyrene composites could be synthesized by in situ
polymerization of aniline within the sulfonated syndiotactic polystyrene matrices. The
randomly distributed sulfonic acid groups are localized within the polyaniline backbones
and form the ionic domains which act as templates for protonation of aniline to form the
aniline salt. The aniline within these fixed ionic domains is polymerized by allowing
diffusion of an oxidant. The process is shown in schematically Figure 6.
Figure 6. Schematic of polyaniline formation on sulfonated polystyrene.
3.2 Experimental
In this part, the process of sulfonating sPS and process of making ssPS-coated filter gels
were described, and several characterizations were used to analyze the properties of
materials.
SO3H SO3H
SO3H
SO3H
SO3H
SO3H
SO3H
SO3Hsulfonation
polymer strands
aniline
SO3H SO3H
SO3H
SO3H
SO3H
SO3H
SO3H
SO3H
aniline
aniline
aniline
aniline
SO3H SO3H
SO3H
SO3H
SO3H
SO3H
SO3H
SO3H
polymerization
of aniline
polyaniline
21
3.2.1 Materials
The syndiotactic polystyrene (sPS) used in these studies (molecular weightwM
=300,000 g/mol, density 1.05 g/cm3) was purchased from Scientific Polymer Products,
Inc. (Ontario, NY). The sPS in the form of pellets was grinded into powder for easy
dissolution in solvents. Chloroform (>99% purity), sulfuric acid ( 96.2% purity), acetic
anhydride (94%) were purchased from Fisher Scientific. Chloroform (400 mL) used as
the solvent was washed with 300 mL of distilled water at least 4 times to remove the
ethanol preservative. Figure 7 illustrates the chemical structure of sPS.
Figure 7. Chemical structure of sPS
Raw materials for synthesize polyaniline. Aniline (>99%) and ferric chloride were
obtained from Fisher Scientific. Whatman, Grade 1 filters were used as the macroporous
surfaces to support coating layers of ssPS.
3.2.2 Preparation of sulfonating agent
Chloroform (35 mL) was taken into a vial (50 mL) and acetic anhydride (0.108 mol)
was added to it. The vial was put into ice bath. Sulfuric acid (0.036 mol) was slowly
added into the solution under vigorous stirring for about 5 minutes. The solution was then
diluted with chloroform in a 50 mL volumetric flask at room temperature. In this study,
22
the molar ratio of acetic anhydride and sulfuric acid was kept at 3:1. It was assumed that
sulfuric acid reacted completely and produced acetyl sulfate as the active component of
the sulfonating agent.
3.2.3 Process of sulfonating sPS
The sPS was sulfonated in a 1000 mL three-neck flask. The flask was immersed in the
oil bath and equipped with a dropping funnel and condenser shown in Figure 8.
Chloroform (400 mL) and sPS (6 g) were added into the flask. The mixture was heated at
180 °C under vigorous stirring by using a stir bar. When all the sPS was dissolved in
chloroform, the solution was cooled to 70 °C. The sulfonating agent was added into
solution slowly by using the dropping funnel. After the solution was stirred for 3 hours,
the reaction was terminated by adding 10 mL methanol. The sulfonated sPS (ssPS) then
was precipitated in deionized water and the precipitate was washed by deionized water
several times until the pH of the mixture did not change any more. The ssPS was dried in
the vacuum oven 2 days at 80 °C.
23
Figure 8. Schematic view of apparatus used in synthesis of sulfonated sPS.
3.2.4 Measurement of the degree of sulfonation
In this study, the degree of sulfonation was analyzed by using the methods reported by Li
and coworkers [96]. The degree of sulfonation was determined by titration process and is
defined as the mole percentage of styrene units that was sulfonated, and was calculated
using following equation:
Degree of sulfonation % = 10081
104
nVm
nV (1)
In equation (1), n and V are the concentration and volume of sodium hydroxide used in
titration, and m is the weight of ssPS sample. The numbers 104 and 81 are the molar mass
of styrene unit and sulfonated acid group, respectively. ssPS with 10 mol% sulfonation
was used in the rest of the work.
24
3.2.5 Process of making ssPS-coated-filter gels
Sulfonated syndiotactic polystyrene was added to THF in a hermetically sealed vial.
The mixture was heated to 100 °C in the oil bath under vigorous stirring. When all the
ssPS was dissolved in THF, a yellow, homogenous solution was obtained. Then the
boiling solution was cooled to 70 °C. By using a dip coater, the filter was immersed into
the solution for one minute and taken out. In this manner, a coating layer of ssPS formed
on the surface (shown in Figure 9). The thickness of ssPS coated on the surface of the
filter was controlled by the speed of pulling of the filter and the viscosity of the solution.
The filter was then transferred to ethanol in a sealed bottle for 24 hours where the
gelation process occurred.
Figure 9. Schematic view of dip coater.
25
3.2.6 Polymerization of aniline on the ssPS-coated-filter aerogel
After the ssPS gelled completely on the surfaces of the filter, the solvent ethanol was
exchanged with distilled water. Then, the ssPS-coated-filter was immersed in 0.35 mol/L
aniline aqueous solution for 12 hours at room temperature so that the ssPS-coated-filter
had adsorbed enough aniline monomer. The ssPS-coated-filter was washed by distilled
water to remove excess aniline and immersed in 1.0 mol/L FeCl3 aqueous solution. After
12 hours, the material was taken out and the dark-green PANI-coated-filter was obtained
as shown in Figure 10. The filter was washed with 1.0 mol/L HCl aqueous solution and
subjected to solvent exchange with ethanol.
Figure 10. The PANI-coated-filter is shown in graph (a). The cellulose filter shown in
graph (b).
The PANI-coated-ssPS aerogels were obtained after supercritical drying. First, the
solvent in the gels was exchanged with liquid CO2 and the liquid CO2 was removed after
taking the system above its critical point. For this purpose, the gels were soaked in liquid
CO2 for 30 minutes, followed by drainage of the solvent, and refilling the specimen with
fresh liquid CO2 [97]. This process was repeated 4 times and the gels were heated to
(a)
(b)
26
50 °C and kept at 11.5 MPa pressure for 1 hour above the critical point of CO2 (31 °C,7.4
MPa) [98]. The aerogels were obtained after all CO2 in the chamber were vented under
the supercritical condition.
3.2.7 Characterization methods
3.2.7.1 Bulk density, skeletal density, and porosity
The bulk density of aerogels was obtained by measuring the mass and volume of the
circular disc specimens. The radius and thickness of circular disc specimens were
measured by a micrometer with a tolerance of 0.01 mm. The skeletal density was
obtained by using an Accupyc 1340 Helium Pycnometer (Micrometrics Instrument Corp).
The porosity was calculated by using the following equations:
%100)1( s
bP
(2)
tD
mb 2
4
(3)
In equations (2) and (3), P is porosity, b and s are bulk density and skeletal density,
respectively, m is the weight of the aerogel specimen, and R and t are the radius and
thickness, respectively.
3.2.7.2 Morphology
The morphology of aerogels was characterized by scanning electron microscopy
(SEM), JEOL JSM5310 with operating voltage 1 kV. The morphologies of surface and
cross-section of the specimens were observed. The PANI-coated aerogels were fractured
after dipping in liquid nitrogen for 10 minutes and the specimens were then mounted on
an aluminum stub by using adhesive carbon tape. The fractured surface and cross-section
27
of the specimens were then sputter-coated with silver by using Sputter Coater, Model ISI
5400.
3.2.7.3 Surface properties
The surface area, average pore size, and pore size distribution were obtained from the
analysis nitrogen adsorption-desorption isotherms using Micromeritics Tristar II 3020
Analyzer. The surface area of the specimen was calculated by using
Brunauer-Emmet-Teller (BET) method and the pore size distribution was calculated by
using Brunauer-Joyner-Halenda (BJH) method.
3.2.7.4 Thermal properties
The thermal stability of specimens was investigated using a thermogravimetric analysis
(TGA) 2050 device (TA Instrument, New Castle, DE) under nitrogen atmosphere. The
specimens were placed in a platinum pan and heated from room temperature to 800 °C at
a heating rate of 20 °C/min. In a mass versus temperature plot, the temperature at the
onset of thermal degradation or 5% mass loss was estimated.
3.2.7.5 Electrical resistivity
The resistivity of PANI-coated aerogels was measured by using a Keithley
picoammeter (Keithley 487) and Model 8009 Resistivity Test Fixture. The volume
resistivity ( v ) was calculated from the following equation:
vv Rt
86.32 ; Ohm-centimeter (4)
In equation (4), v is the volume resistivity, t is the thickness of the specimen in
centimeters, and vR is the resistance measured by Keithley picoammeter.
The surface resistivity is given as:
28
ss R13.35 , in Ohm (5)
where s is the surface resistivity of the sample, sR is the resistance measured by
Keithley picoammeter.
In equations (4) and (5), the numerical constants 22.9 and 53.4 are based on the
physical dimensions of the electrodes of the Model 8009.
3.3 Results and discussion
3.3.1 Density
The physical dimensions, density, and porosity of cellulose filter and filter coated with
ssPS aerogel are presented in Table 2. The thickness of the filter increased by 33.3
percent due to coating layer of ssPS aerogel, while the mass increased by 27.8 percent.
This is supported by a reduction of bulk density – the bulk density decreased from 0.478
g/cm3 for the filter to 0.414 g/cm3 for ssPS-coated filter. In addition, the skeletal density
decreased by 4.2 percent. In addition, the skeletal density of ssPS (1.138 g/cm3) is lower
than that of the cellulose filter (1.58 g/cm3). It is apparent that the coating of ssPS aerogel
caused a 4.3% increase in porosity of the ssPS-coated filter.
Table 2. Physical properties of cellulose filter and ssPS-coated-filter aerogel. The
specimens of radius 24.52 mm were used in measurement.
Cellulose filter ssPS-coated-filter aerogel
mass 0.1627 g 0.1875 g ( 15.2%)
radius 24.52 mm 24.52 mm
29
Thickness 0.18 mm 0.24 mm ( 33.3%)
Table 2. Physical properties of cellulose filter and ssPS-coated-filter aerogel. The
specimens of radius 24.52 mm were used in measurement (continued).
Bulk density 0.478 g/cm3 0.414 g/cm3 ( 13.4%)
Skeletal density 1.5807 0.00189 g/cm3 1.5144 0.0058 g/cm3(
4.2%)
porosity 69.7% 72.7% ( 4.3%)
3.3.2 Morphology
The morphology of the cellulose filter, sPS-aerogel filter, sulfonated sPS-coated-filter
aerogel, polyaniline-coated-filter aerogel was characterized by SEM. Representative
images are shown in Figure 11, Figure 12, and Figure 13.
30
Figure 11. SEM images of the surfaces of cellulose filter (a), sPS-coated-filter aerogel (b),
ssPS-coated-filter aerogel (c), and polyaniline-coated-filter aerogel (d).
(d)
31
Figure 12. SEM images of the cross-section of cellulose filter (a), ssPS-coated-filter
aerogel (b). The internal space of the ssPS-coated-filter aerogel is shown in (c).
(b) (c)
(a)
32
Figure 13. SEM images of the structure of sPS aerogel (a), ssPS aerogel (b), and
polyaniline-coated aerogel (c).
It is observed in Figure 11 (a) that the surface of the filter is covered by
interconnected cellulose fibers of typical diameter ~ 20μm. Compared with
sPS-aerogel as shown in Figure 13 (a), the surface of ssPS-aerogel appears much
rougher (Figure 13 (b)). The polyaniline appears as small particles on the surfaces of
ssPS aerogel as shown in Figure 11 (d). From Figure 12 (b) and (c), it is seen that the
ssPS aerogel coated the surface of the filter, and some strands of ssPS also diffused
inside the cellulose fiber networks. Figure 13 (a) and (b) show that the sPS aerogel
and ssPS aerogel have similar fibrillar structure. However, a denser fibrillar
structure is observed in the case of sPS. The less dense packing in the case of ssPS is
attributed to the loss of crystallinity of sPS due to sulfonation. In Figure 13 (c), it is
seen that polyaniline formed aggregates on the polymer strands and appeared as
small particles.
(b) (c)
33
3.3.3 Surface properties
The surface area and pore size distribution of the cellulose filter, ssPS-coated-filter
aerogel, and polyaniline-coated-filter aerogel were characterized by the nitrogen
adsorption–desorption isotherms as shown in Figure 14. According to IUPAC
classification, the porosity is identified in a wide range of diameter including micropores
(< 2 nm), mesopores (2-50 nm), and macropores (> 50 nm) [99]. For P/P0 < 0.06, the rise
in quantity adsorbed is due to adsorbing molecules which are interacting with the
micropores. Note that the micropores are the most energetic regions of the solid surfaces.
The contributions of micropores to the adsorption process ceases after the knee in the
curves (point K in Figure 14). The multilayer adsorption on the surfaces of mesopores
and macropores show a low slope region in the middle of the isotherm. The hysteresis
loop at the end of the isotherm is due to capillary condensation of nitrogen in mesopores.
The pore size distribution determined using the BJH method is shown in Figure 15. The
BET surface area of cellulose filter, ssPS-coated-filter aerogel, and
polyaniline-coated-filter aerogel are listed in Table 3. It is seen that the surface area of
cellulose filter is extremely small (0.95 m2/g) compared to the aerogel specimens.
However, the surface area increases by about 60 times when the mass of the filter only
increases by 15.2% after ssPS aerogel is coated onto it. The surface area coated with
polyaniline reduced to 12 m2/g which is still a factor 12 higher than the cellulose filter.
34
0.0 0.2 0.4 0.6 0.8 1.0
0
20
40
60
80
100
120
140
160 polyaniline-ssPS-filter aerogel
cellulose filter
ssPS-filter aerogel
Qu
an
tity
Ad
so
rbe
d (
cm
3/g
ST
P)
Relative Pressure (P/P0)
0 2 4 6 8 10
0
2
4
6
8
10
Mic
rop
ore
fill
ing
K
Multilayer adsorption
Hystersis
Figure 14. Nitrogen adsorption-desorption isotherms at 77 K of cellulose filter,
ssPS-coated aerogel and polyaniline-coated-filter aerogel.
0 10 20 30 40 50 60 70
0.00
0.05
0.10
0.15
0.20
0.25
polyaniline-ssPS-filter aerogel
cellulose filter
ssPS-filter aerogel
dV
/dD
(cm
3/g
.nm
)
pore diameter (nm)
Figure 15. Pore size distribution of cellulose filter, ssPS-filter aerogel and
polyaniline-ssPS-filter aerogel.
35
Table 3. Surface area and average pore size of different materials.
Cellulose
filter
ssPS-coated filter
aerogel
Polyaniline-coated
filter aerogel
surface area:m2/g 0.95 60 12
Average pore size:
nm
12 11
3.3.4 Thermal property
The thermal stability of sPS, ssPS with 10 mol% sulfonation, cellulose filter,
ssPS-coated-filter aerogel, and polyaniline-coated-filter aerogel was investigated as
reflected from the TGA traces in Figure 16 and Figure 17. It is observed in Figure 16 that
in comparison to sPS, the onset temperature of the mass-loss process reduced slightly for
ssPS. Figure 17 shows that the decomposition temperature of cellulose filter is higher
than that of ssPS-coated filter aerogel and polyaniline-coated-filter aerogel due to lower
decomposition temperature of ssPS and polyaniline.
36
0 100 200 300 400 500 600 700
0
20
40
60
80
100
We
igh
t (%
)
Temperature oC
ssPS
sPS
Figure 16. TGA traces of sPS and ssPS.
37
0 100 200 300 400 500 600 700 800-10
0
10
20
30
40
50
60
70
80
90
100
110
We
igh
t (%
)
Temperature oC
cellulose filter
ssPS-coated-filter aerogel
polyaniline-coated-filter aerogel
Figure 17. TGA traces of cellulose filter, ssPS-coated-filter aerogel, and
polyaniline-coated-filter aerogel.
3.3.5 Electrical resistivity
The aerogels specimens were trimmed to a rectangular shape for facilitating electrical
conductivity measurement. Figure 18 shows the relationship between the voltage and
current. It is apparent that the polyaniline-coated aerogel is a conductive material.
The surface and volume resistivity of the polyaniline-coated-filter aerogel was
obtained from the following relationships:
kRss 10167535.1313.35 (6)
38
cmkkcm
centimeterohmRt
vv 22923.0033.0
86.32)(86.32 (7)
where, s and v are the surface resistivity and volume resistivity, respectively, and t
is the thickness of the aerogel , sR and vR are the resistance value measured by
Keithley picoammeter.
-2 -1 0 1 2
-8.00E-008
-6.00E-008
-4.00E-008
-2.00E-008
0.00E+000
2.00E-008
4.00E-008
6.00E-008
8.00E-008
Current
Linear Fit of Current
Curr
ent (A
)
Voltage (V)
Equation y = a + b*
Adj. R-Squar -0.01262
Value Standard Erro
Current Intercept 4.34536E-1 9.41637E-10
Current Slope 4.34109E-1 8.05469E-10
(a)
39
Figure 18. The relationship between the voltage and current. ssPS-coated aerogel is
shown in graph (a), polyaniline-coated aerogel is shown in graph (b).
-2 -1 0 1 2
-0.0000015
-0.0000010
-0.0000005
0.0000000
0.0000005
0.0000010
0.0000015
Current
Linear Fit of Current
Curr
ent (A
)
Voltage (V)
Equation y = a + b*x
Adj. R-Square 0.99977
Value Standard Error
Current Intercept 2.31664E-10 1.11046E-9
Current Slope 5.63637E-7 9.49876E-10
(b)
40
CHAPTER IV
TAILORING MORPHOLOGY AND STRUCTURE PROPERTIES OF
SYNDIOTACTIC POLYSTYRENE-SILICA HYBRID AEROGELS
4.1 Introduction
Aerogels are characterized by high porosity, low density, large pore surface area,
fractal pores, and low thermal conductivity. However, the synthesis process and
conversion of parent gel into an aerogel structure are often cumbersome. The pore
networks and the pore volumes of the gel are sometimes compromised in the drying
process if supercritical conditions are not used.
Silica aerogel have became most widely used and extensively studied due to versatile
chemistry and remarkable properties. However, the hygroscopic nature and the inherent
fragility of the silica networks strongly restrict the application of silica aerogel. The silica
aerogel can be easily destroyed by placing a small drop of water on its surface due to the
residual “Si-OH” bonds in the silica particles [23],[24]. The fragile property of native
silica aerogel is due to weak network structures formed by many “pearl necklaces” that
are tied together by a limited number of “Si-O-Si” bonds at the necks of the secondary
silica particles. In past decades, many studies have been devoted to polymer
reinforcement and surface modification of silica aerogels. In 1990s, the silica aerogel
derived from tetraethoxysilane (TEOS) was first successfully reinforced by reacting with
41
hydroxyl-terminated polydimethylsiloxane (PDMS) [12]. The surface area of this
aerogel was up to 1200 m2/g and the strength of reinforced aerogel was better than that of
corresponding silica aerogel. In 2000s, Leventis and coworkers [100],[101] reported that
the silica aerogels could be reinforced by reacting the residual –OH groups on the
surfaces with the isocyanates. By this method, the weak neck zone between the particles
was coated by crosslinked polyurethane. In this research, two different methods were
used to synthesize silica aerogel: one-step base catalyzed sol-gel process and two-step
acid-base catalyzed sol-gel process. The reinforced silica aerogel required more than ~30
times breaking force than that of corresponding native aerogel with increasing density up
to ~3 times. The silica aerogel reinforced by incorporating methyltrimethoxysilane
(MTMS) and bis(trimethoxysilylpropyl)amine (BTMSPA) was studied by Nguyen et al.
[29]. The dipropylamine spacer from BTMSPA worked as both a flexible linking group
and a reactive site via its secondary amine to reacting with a tri-isocyanate. The
compressive strength increased owing to the tri-isocyanate which provides an extra
degree of branching. According to Wang and Jana [25], the compressive modulus of silica
aerogel increased when the sPS gel grew in the silica gel.
In this chapter, the pores of sPS gel were used to grow silica aerogel. The
organic-inorganic hybrid aerogels were synthesized by “growing gel in a gel”. Two types
of silica aerogel networks were prepared. The work specifically focused on the effects of
silica content on pore surface area, porosity, hydrocarbon liquid absorption capacity, and
compressive properties of the hybrid aerogel.
42
4.2 Experimental
4.2.1 Materials
All solvents and reagents were used as received. Syndiotactic polystyrene pellets
(molecular weight =300,000 g/mol, density 1.05 g/cm3) were purchased from
Scientific Polymer Products, Inc. (Ontario, NY). The sPS pellets were ground into
powder for easy dissolution in solvents. Tetraethylorthosilicate (TEOS, reagent grade,
98%), ammonium hydroxide solution (28-30%, NH3 basis), and nitric acid (purity
64-66%) were purchased from Sigma Aldrich. Reagent grade tetrahydrofuran (THF) and
ethanol were obtained from Fisher Scientific. Figure 19 shows the chemical formula of
TEOS.
Figure 19. Chemical formula of TEOS.
4.2.2 Preparation of sPS/Silica Hybrid Aerogels
As the gelation mechanisms of sPS and silica are quite different, it is hard to obtain a
sPS/silica hybrid aerogel by simply mixing these two gel precursor materials together. In
this work, first, sPS gel was prepared by thermoreversible gelation. Second, the silica gel
precursor, TEOS was absorbed into the macropores of sPS gel and allowed to undergo
hydrolysis and condensation reactions to produce silica particle networks. In this work,
two types of silica aerogel were grown in sPS gel. In Type A, silica aerogel was
wM
43
synthesized by acid-base catalyzed sol-gel process. In Type B, silica aerogel was
prepared by acid catalyzed hydrolysis and condensation process. The hybrid aerogel
specimens are denoted as “sPS-g/ml-TEOS-mol/L” in the rest of the work. For example,
sPS-0.02-TEOS-0.25 aerogel represents the hybrid aerogel derived from 0.02 g/mL
solution of sPS and 0.25 mol/L solution of TEOS. Eight different hybrid aerogel samples
were prepared in this study including two different concentrations (0.02, 0.04 g/mL) of
sPS and four different concentrations (0.1, 0.25, 0.5, 0.7 mol/L) of TEOS.
For synthesis of Type A hybrid aerogel, here we describe the process of a
representative aerogel specimen, sPS-0.02-TEOS-0.25. sPS power (0.2 g) was dissolved
in THF (10 mL) in a sealed vial and the mixture was heated to 160 °C in oil bath under
vigorous stirring until the sPS power dissolved completely and a transparent,
homogeneous solution was obtained. The solution was transferred to a cylindrical mold
and allowed to stand for 24 hours at room temperature to obtain sPS gel. The gel was
immersed in ethanol to remove THF from the gel.
Solution A containing TEOS (8.3332 g), ethanol (100 mL), and deionized water (2.16
mL) and solution B containing ethanol (46.04 mL) and deionized water (2.88 mL) were
prepared separately. The sPS gel was immersed in solution A for 12 hours so that TEOS
and deionized water could diffuse inside the gel. Subsequently, the pH of solution A was
adjusted to a value of 2.0 by adding nitric acid. The gel was kept in this solution for 1.5 h
to conduct hydrolysis of TEOS within the gel network. The sPS gel with hydrolyzed
TEOS was transferred to solution B and the pH was adjusted to 9.0 using ammonium
hydroxide so that silica sol could transform into silica gel by silane condensation. The gel
was allowed to stand in solution B for 24 h for complete gelation of the silane. The
44
residual water and TEOS were removed by washing hybrid gel with ethanol five times.
The hybrid gel was dried using supercritical carbon dioxide at 50 °C and 11.5 MPa
pressure. The hybrid aerogel samples were kept in a vacuum oven overnight to remove
residual solvent.
For Type B hybrid aerogel, the preparation process of specimen of sPS-0.02-TEOS-3 is
presented. The sPS gel was first synthesized by thermoreversible gelation scheme
described above. The solution of TEOS was prepared by dissolving TEOS (62.499 g) and
deionized water (27 mL) in ethanol. The pH of solution of TEOS was then adjusted to a
value of 1 by adding nitric acid. sPS/silica hybrid gel was formed after keeping the sPS
gel in a solution of TEOS for 2 weeks. The hybrid aerogel was obtained by solvent
exchange and supercritical drying.
4.2.3 Characterization
4.2.3.1 Bulk density, skeletal density, and porosity
The dimensional and structural changes induced by hybrid aerogel preparation
methods are shown in Table 1. The bulk density ρb of hybrid aerogels was obtained by
measuring the mass and volume of the each cylindrical specimen. The skeletal density ρs
was obtained using an Accupyc 1340 Helium Pycnometer (Micrometrics Instrument
Corp). The diameter shrinkage δ is defined as the change of diameter from hybrid gels to
the diameter of corresponding aerogels samples. The porosity P of hybrid aerogel and
bulk density ρb were calculated using the equation (2), (3) and the diameter shrinkage δ
was calculated using the following equations:
oD
D1 (8)
45
In equations (8), D is diameter of the hybrid aerogel specimen measured by a
micrometer with a tolerance of 0.01 mm, and D0 is the diameter of the hybrid gel.
4.2.3.2 Thermogravimetric analysis
The thermal stability of hybrid aerogels was studied by subjecting the specimens to
thermal scans in thermogravimetric analyzer (TGA), model TGA 2050 device (TA
Instrument, New Castle, DE) under nitrogen atmosphere. The temperature range was
from room temperature to 800 oC at a heating rate of 20 oC/min.
4.2.3.3 Nitrogen adsorption-desorption measurement
The surface area and pore size distribution were obtained by analyzing nitrogen
adsorption-desorption isotherms [10] using Micromeritics Tristar II 3020 Analyzer. The
surface area was calculated using Brunauer-Emmet-Teller (BET) method and the pore
size distribution was calculated using Brunauer-Joyner-Halenda (BJH) method.
4.2.3.4 Morphology
The morphology of hybrid aerogels was examined by scanning electron microscope
(SEM), JEOL JSM5310 with operating voltage 5 kV. For this purpose, the hybrid aerogel
specimens were fractured at room temperature and mounted on an aluminum stub using
adhesive carbon tape. The fractured surface was then sputter-coated with silver under
argon atmosphere by using Sputter Coater, Model ISI 5400.
4.2.3.5 Compressive test
The compressive modulus was obtained from stress-strain data using Instron 5567
(Canton, MA) tensile tester following ASTM D595 method. The crosshead speed for the
measurement was chosen as 1.27 mm/min.
46
4.2.3.6 Contact angle measurement
The contact angles of hybrid aerogel were measured using a Rame Hart contact angle
goniometer (model 500) under ambient conditions. The cylindrical aerogel specimens
were compressed into discs at 3500 psi to eliminate the effect of pore structures. In static
sessile drop method, a 6 drop of water and a 4 drop of hydrocarbon oil were placed
on the surface of aerogel specimens. The values of contact angles of water and
hydrocarbon oil were measured by using the ImageJ software. Five measurements were
taken for each aerogel specimen.
4.3 Results and discussion
In this part, the properties of type A hybrid aerogels and type B hybrid aerogels were
discussed, separately.
4.3.1 Type A hybrid aerogels
The physical properties, thermal stability, morphology property, pore size distribution,
surface area, compressive properties, contact angles, oil and water absorption were
discussed as following.
4.3.1.1 Density, shrinkage, and porosity
Table 4. Composition and properties of sPS/silica hybrid aerogels
Sample name Diameter
shrinkage
(%)
Bulk density
(g/mL)
Skeletal
density (g/mL)
Porosity
(%)
sPS-0.02 12.3 0.030 1.0678
0.0040
97.2
47
Table 4. Composition and properties of sPS/silica hybrid aerogels (continued)
As shown in Table 4, the bulk density of the aerogels increased with an increase of
concentration of TEOS. This is due to an increase in the amount of silica particles
residing in the fixed volume of marcopores of sPS. The skeletal density of native silica
sPS-0.02-TEOS-0.1 9.2 0.040 1.2170
0.0098
96.6
sPS-0.02-TEOS-0.25 7.5 0.050 1.4215
0.0039
96.4
sPS-0.02-TEOS-0.5 4.2 0.068 1.6301
0.0043
95.9
sPS-0.02-TEOS-0.7 3.4 0.099 1.6845
0.0032
94.1
sPS-0.04 7.2 0.054 1.0571
0.0069
94.9
sPS-0.04-TEOS-0.1 6.32 0.062 1.1964
0.0116
94.8
sPS-0.04-TEOS-0.25 5.76 0.076 1.2940
0.0048
93.0
sPS-0.04-TEOS-0.5 5.44 0.098 1.4003
0.0038
92.6
sPS-0.04-TEOS-0.7 4.5 0.134 1.4893
0.0040
90.0
48
aerogel is larger than that of sPS aerogel. Thus, the value of skeletal density of hybrid
aerogel samples shows the same trend as bulk density with an increase in the
concentration of TEOS. All aerogel specimens exhibit high values of porosity as listed in
the Table 4. However, a slight reduction of porosity is observed in the case of hybrid
aerogels, corresponding to an increase of bulk density. The shrinkage of hybrid aerogel is
much smaller compared to native sPS aerogel. For example, the value of shrinkage of
sPS-0.02 aerogel is 12.3% which is about four times the shrinkage observed for
sPS-0.02-TEOS-0.7 aerogel. This is attributed to rigid silica particles filling the pores of
hybrid aerogel. This rigid networks prevent the weak networks of sPS from shrinkage.
4.3.2 Thermal stability
0 100 200 300 400 500 600 700 800 900
0
20
40
60
80
100
we
igh
t (%
)
Temperature (0C)
sPS-0.02TEOS-0.1
sPS-0.02TEOS-0.5
sPS-0.02TEOS-0.7
sPS-0.02TEOS-0.25
sPS-0.02
61.5%
53.1%
40.0%
9.2%
0.0%
(a)
49
0 100 200 300 400 500 600 700 800 900
0
20
40
60
80
100
we
igh
t(1
00
%)
Temperature
sPS-0.04-TEOS-0.1
sPS-0.04-TEOS-0.25
sPS-0.04-TEOS-0.5
sPS-0.04-TEOS-0.7
sPS-0.04 47.4%
32.8%
24.1%
7.1%
0.0%
(b)
Figure 20.TGA curves of Type A aerogels (a) sPS-0.02-TEOS aerogels, (b)
sPS-0.04-TEOS aerogels.
(a) (b)
Figure 21. The Type A hybrid aerogels before (a) and after (b) heating to 800 0C in the air.
From left to right, the concentrations of TEOS decrease from 0.7 mol/L to zero.
The thermal stability of sPS aerogel and hybrid aerogels are inferred from the TGA
traces as shown in Figure 20. It is observed that sPS aerogel began to decompose at
around 400 oC, due to degradation of polystyrene backbone; complete decomposition
occurred at around 450 oC. Silica aerogels also show small weight loss at around 100 oC
50
due to residual moisture and greater weight loss at around 300 oC due to decomposition
of unreacted TEOS and methyl groups [102]. The residues from TGA analysis – all silica
– yield information on the amount of silica present in hybrid aerogels. Two trends are
observed from TGA traces in Figure 20 (a) and (b). First, the weight percent of silica in
hybrid aerogel increased with the increase of concentration of TEOS in solution for a
given concentration of sPS in THF solution. Second, the amount of silica in hybrid
aerogel decreased with an increase of concentration of sPS in THF solution. For example,
the weight percent of silica in sPS-0.02-TEOS-0.25 aerogel specimen is much higher than
that of sPS-0.04-TEOS-0.25 aerogel specimen, or of sPS-0.04-TEOS-0.5 aerogel
specimen. One can attribute such observation to a reduction of macropore volume
defined by sPS concentration – the higher density of sPS strands at 0.04 g/mL polymer
concentration left much less pores for absorption of TEOS [103].
Figure 21 shows the images of aerogels before and after heated to 800 oC in air. It is
seen that the native sPS aerogel decomposed completely after the heat treatment, while
the hybrid aerogels yielded continuous silica aerogel structure as residue. This proves that
the hybrid aerogels possess a co-continuous network structure.
4.3.3 Morphology property
(a) sPS-0.04 (b) sPS-0.04-TEOS-0.1 (c) sPS-0.04-TEOS-0.25
51
(d) sPS-0.04-TEOS-0.5 (e) sPS-0.04-TEOS-0.7 (f) TEOS-0.7
Figure 22. SEM images of sPS aerogel (a), hybrid aerogels (b)-(e), and native silica aerogel
(f)
The morphology of aerogels of sPS, hybrid material, and native silica are compared in
Figure 22. Figure 22(a) shows that sPS aerogel had fiber-like network structure formed
by sPS strands of 50-100 nm in diameter. The macropores (>50 nm) of sPS aerogel are
readily apparent in Figure 22(a). The native silica aerogel in Figure 22(f) shows
aggregated particles of typical diameter 85-100 nm. It is obvious that the hybrid aerogels
were composed of both fiber-like strands of sPS and pearl-necklace particles of silica. At
low concentration of TEOS, the silica particles could not form gel networks. Instead, they
formed aggregates on sPS backbones as conformal coating (Figure 22b-c). At higher
concentration of TEOS, however, the silica particles not only coated on the polymer
backbones, but also filled the macropores of sPS, as evident from Figure 22 (d-e).
4.3.4 Pore size distribution and surface area
The data on surface area and pore size distribution of native aerogel and hybrid aerogel
obtained from nitrogen adsorption-desorption isotherms are presented in Table 5 and
Figure 23 respectively.
52
1 10 100
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
dV
/dD
(cm
3/g
*nm
)
pore diameter (nm)
sPS-0.02
sPS-0.02-TEOS-0.1
sPS-0.02-TEOS-0.25
sPS-0.02-TEOS-0.5
sPS-0.02-TEOS-0.7
(a)
1 10 100
0.00
0.02
0.04
0.06
0.08
0.10
0.12
sPS-0.04
sPS-0.04-TEOS-0.1
sPS-0.04-TEOS-0.25
sPS-0.04-TEOS-0.5
sPS-0.04-TEOS-0.7
dV
/dD
(cm
3/g
*nm
)
pore diameter (nm)
(b)
Figure 23. Pore size distribution of two series of hybrid aerogels.
Table 5. Surface areas of native sPS aerogel and hybrid aerogel.
TEOS-0 TEOS-0.1 TEOS-0.25 TEOS-0.5 TEOS-0.7
53
Table 5. Surface areas of native sPS aerogel and hybrid aerogel (continued).
sPS -0.02 398 m2/g 432 m2/g 610 m2/g 632 m2/g 693 m2/g
sPS -0.04 368 m2/g 417 m2/g 538 m2/g 550 m2/g 618 m2/g
The higher surface area of hybrid aerogels compared to sPS is due to the contributions
of mesopores of silica. The surface area increases with silica concentration. Figure 23
shows that native sPS aerogel has no predominant pores located in the mesopores range
(2~50 nm). The pore size distributions of aerogel specimens sPS-0.02-TEOS-0.1 and
sPS-0.02-TEOS-0.25 show a small peak in the mesopores zone. These mesopores are
contributed by the silica aerogel containing a predominant pore size of around 9 nm [1].
At higher silica content, silica particles underwent more aggregation as evident from
larger mesopores size (~15 nm), e.g., for the aerogel specimen sPS-0.02-TEOS-5.
Mesopores are much more prominent in the case of aerogel specimen sPS-0.02-TEOS-7.
The hybrid aerogels at higher sPS concentration show similar trends. However, in
contrast to native silica aerogel, all hybrid aerogels show larger predominant mesopores.
We attribute this to restricted growth of silica networks with the macropores of sPS, e.g.,
30 nm predominant pore of sPS-0.04-TEOS-0.7 aerogel compared to ~20 nm for
sPS-0.02-TEOS-0.7 aerogel.
4.3.5 Contact angle
The contact angles of water on compressed disc of aerogel specimens are shown in
Table 6, and the contact angles of hydrocarbon oil are shown in Table 7.
54
(a) (b)
Figure 24. Images of aerogels exposed to water. (a) 0.7 mol/L silica aerogel, sank to the
bottom of water and collapsed under a tender touch. (b) sPS-0.04-TEOS-0.7 aerogel,
floated on the surface of water, kept a strong structure after immersed in the water using a
external force.
Table 6. Contact angles of water for sPS-0.02-TEOS aerogels.
sample name sPS-0.0
2
sPS-0.02-TEO
S-0.1
sPS-0.02-TEO
S-0.25
sPS-0.02-TEO
S-0.5
sPS-0.02-TE
OS-0.7
contact angle ° 102.3 100.5 95.7 82.5 75.2
Table 7. Contact angles of hydrocarbon oil for sPS-0.02-TEOS aerogels
sample name sPS-0.02 sPS-0.02-TEO
S-0.1
sPS-0.02-T
EOS-0.25
sPS-0.02-TEOS-
0.5
sPS-0.02-
TEOS-0.7
contact angle ° 2.4 7.7 10.4 13.4 19.0
As shown in Figure 24 (a), when exposed to water, the native silica absorbed water and
the whole body of silica aerogel collapsed under a tender touch because the “pearl
necklace” structure was destroyed by the capillary force. The hybrid aerogels, on the
other hand, show a different behavior. It is seen that sPS-0.04-TEOS-0.7 aerogel floated
on water and could keep its structure intact after being immersed in the water using an
55
external force. The contact angles of water on compressed disc of aerogel specimens are
shown in Table 6. It is seen that sPS aerogel is hydrophobic and the value of contact
angle of hybrid aerogels decreased with an increase of the concentration of TEOS. The
silica particles are the hydrophilic in nature. The contact angles of hydrocarbon oil are
shown in Table 7. For native sPS aerogel, it shows strong lipophilicity due to the presence
of hydrocarbon groups in sPS. Also higher values of contact angles of hydrocarbon oil
are observed in the case of hybrid aerogels, especially of higher concentration of TEOS.
This is due to the hydrophilic nature of silica aerogel.
4.3.6 Compressive properties
(a)
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
2
4
6
8
10
12
co
mp
ressiv
e m
od
ulu
s (
MP
a)
concentration of TEOS (mol/l)
sPS-0.02
sPS-0.04
(mol/L)
56
(b)
Figure 25. (a) Compressive moduli of hybrid aerogels. (b) Compressive stress-strain
curves of selected hybrid aerogel.
Typical compressive stress-strain curves of selected aerogel specimens are shown in
Figure 25(a). At small compressive strain, both hybrid aerogel and native aerogel exhibit
linear elastic response. The silica aerogels cracked under a very low compressive load at
a strain of ~20%, while sPS aerogel can bear higher deformation without shattering.
However, the sPS aerogel underwent buckling at low compressive stress and turned into
dense solids at large load. The hybrid aerogel, however, show much better compressive
stress-strain behavior – they did not shatter under large load. These results can be
interpreted as follows. (1) Silica particles shattered at low strain due to a limited number
of Si-O-Si bonds at the neck of the secondary silica particles and weak “pearl necklace”
structures [104]. (2) The sPS aerogels experience buckling at low compressive load due
to weak network structure and a large number of macropores. (3) In hybrid aerogels, the
57
sPS strands are prevented from buckling by rigid silica necklace particles grown inside
the macropores. On the other hand, the sPS networks act as cage for silica particles and
prevent their brittle rupture. The compressive moduli of hybrid aerogels in Figure 25 (b)
show an increase with an increase of concentration of TEOS and sPS.
4.3.7 Oil and water absorption
Table 8. Crude oil absorption data for all specimens.
Sample name
Weight increase percentage
(%) at Equilibration
time
Theoretical
weight
increase
percentage
(%)
10 s 15 s 30 s 1 min
sPS-0.02 720 731 746 755 2 min 2581
sPS-0.02-TEOS-0.1 617 665 679 697 1min
1992
sPS-0.02-TEOS-0.25 719 750 761 760 30s 1371
sPS-0.02-TEOS-0.5 739 796 798 797 30s 1128
sPS-0.02-TEOS-0.7 620 623 628 625 15s 760
Sample name 0.5
min 1 min 5 min 15min
Equilibration
time
Theoretical
weight
increase
percentage
(%)
58
Table 8. Crude oil absorption data for all specimens (continued).
sPS-0.04 125 180 230 248 20 min 1397
sPS-0.04-TEOS-0.1 90 128 211 223 20 min 1224
sPS-0.04-TEOS-0.25 243 296 300 302 5 min 968
sPS-0.04-TEOS-0.5 186 250 308 310 5 min 760
sPS-0.04-TEOS-0.7 300 332 334 335 1 min 537
Table 9. Water absorption data for sPS-0.02-TEOS aerogel.
Sample name Weight increase percentage (%) at
Equilibration time
(min)
0.5 min 1 min 5 min 15min
sPS-0.02 3 3 3 4 5 min
sPS-0.02-TEOS-0.1 8 8 15 16 5 min
sPS-0.02-TEOS-0.25 20 25 25 28 1 min
sPS-0.02-TEOS-0.5 24 39 61 66 5 min
sPS-0.02-TEOS-0.7 48 89 98 158 15min
59
(a) (b) (c)
(d) (e) (f)
Figure 26. Images of sPS-0.04-TEOS-0.7 aerogel and sPS-0.04-TEOS-0.1 aerogel at
different time during oil absorption test, sPS-0.04-TEOS-0.7 aerogel : (a): 1s, (b) 15s, (c)
30s; sPS-0.04-TEOS-0.1 aerogel : (d): 1s, (e) 15s, (f) 30s.
Figure 27. Image (a) and (b) show the hybrid aerogel and native sPS aerogel before and
after oil absorption test. Image (c) and (d) show the hybrid aerogel before and after water
absorption test
In order to learn more synergistic effects of the hybrid aerogel, the oil and water
absorption tests were performed by putting the specimens in hydrocarbon oil and
weighing these aerogels at set intervals. For the series of sPS-0.02-TEOS-mol/L aerogels,
as soon as the samples were placed on the crude oil, plenty of tiny air bubbles were
evicted from the aerogels and showed a fast absorption rate due to the high surface area,
60
large capillary force, and the inherent fast absorption kinetics of sPS aerogel. The same
phenomenon is seen for the sPS-0.04-TEOS-0.7 aerogel as shown in the Figure 26. It is
observed from Table 8 that the equilibration time of absorption of crude oil decreased
with the increase of concentration of TEOS. This situation is more obvious in the case of
sPS-0.04-TEOS aerogel. For example, as shown in Figure 26, the sPS-0.04-TEOS-0.1
aerogel exhibits much slower absorption rate compared with sPS-0.04-TEOS-0.7 aerogel.
The theoretical weight increase percentage listed in Table 8 is calculated using
following equation.
Theoretical weight increase percentage (%) =
%100801.0
%100%100 initial
aerogel
initial
oiloil
initial
oil
m
PV
m
V
m
m (11)
In the equation (11), moil is the mass of crude oil absorbed by hybrid aerogel specimen
when all pores are filled with crude oil, minitial is the mass of specimen before exposure to
the oil, oil is the density of crude oil which is equal to 0.801 g/mL. oilV and aerogelV
are the volume of absorbed oil and specimen, respectively. P is the porosity of aerogel. It
is seen that all aerogels show high absorption capacity attributed to high porosity and low
bulk density. The theoretical weight increase percent of specimens decreased with the
increase of concentration of TEOS due to a larger bulk density and smaller porosity. For
sPS-0.02-TEOS-0.1 aerogel and sPS-0.04-TEOS-0.1 aerogel, the weight gain of
specimen after absorbing oil (697% and 223%, respectively) are smaller than that of
native sPS (755% and 248%, respectively). This corresponds to the trend of theoretical
value of weight gain. However, an opposite trend is observed for aerogels at a higher
concentration of TEOS, e.g., 697% increase for sPS-0.02-TEOS-0.1 aerogel, 760%
increase for sPS-0.02-TEOS-0.25 aerogel, 797% increase for sPS-0.02-TEOS-0.5 aerogel.
61
It can be attributed to the increasing amount of pores in the mesopore size. There is a
sudden drop in weight increase for sPS-0.02-TEOS-0.7 aerogel due to the large increase
of bulk density and reduction of porosity. It is also seen that in contrast to native sPS
aerogel, the hybrid aerogel with high concentration of TEOS almost reaches the
theoretical absorption level. The results of water absorption test are presented in Table 9.
It is seen that the native sPS aerogels almost have no capacity of absorbing water due to
its hydrophobic nature. The hybrid aerogels with a higher concentration of TEOS, on the
other hand, exhibit a larger capacity of absorbing water attributed to the residual "Si-OH"
groups on the silica aerogel. Figure 26 (a) and (b) show the aerogel specimens before and
after absorbing oil, respectively. It is seen that large shrinkage of native sPS aerogel
occurred after the test, because the weak sPS networks were destroyed by the capillary
force when absorbing the crude oil. However, the shrinkage of hybrid aerogel was much
smaller due to the support of silica rigid particles, and there is almost no shrinkage for
sPS-0.02-TEOS-0.7 aerogel. On the other hand, the hybrid aerogel with high
concentration of TEOS shows large shrinkage after water absorption due to the presence
of hygroscopic silica aerogel.
62
4.3.8 Type B hybrid aerogel
0 100 200 300 400 500 600 700 800 9000
20
40
60
80
100
We
igh
t (%
)
Temperature (oC)
sPS-0.02-TEOS-3
55%
(a)
(b)
Figure 28. (a) TGA curves of Type B hybrid aerogels, (b) The Type B hybrid aerogels
before and after heated to 800 °C in air.
Thermal stability of Type B hybrid aerogels is analyzed from TGA traces as shown in
Figure 28 (a). It is seen that the Type B hybrid aerogels present similar thermal
decomposition trend. The hybrid aerogels show small weight loss at low temperature due
63
to residual moisture and due to decomposition of unreacted TEOS and methyl groups.
They show much higher weight loss at the temperature between 300-450 °C due to
degradation of polystyrene backbone. According to TGA analysis, there are about 55 %
residues which are due to silica present in hybrid aerogels. It is also seen that the Type B
hybrid aerogels leave continuous, integral silica aerogel networks (Figure 28 (b)) after
heated to 800 °C in air. Thus, one can infer that the Type B hybrid aerogels possess an
interpenetrating polymer network structure within the sPS aerogel.
(a) (b)
Figure 29. SEM images of silica aerogel from acid hydrolysis and condensation (a) and
Type B hybrid aerogel (b).
The morphology of acid-derived silica aerogel and Type B hybrid aerogel are shown in
Figure 29 (a) and (b), respectively. The sol-gel process of TEOS is much different under
acidic condition. The hydrolysis reaction is greatly accelerated [69], while the
condensation reaction proceeds relatively slowly. In this manner, the silica gels are
formed by linear or randomly branched long chains and ultimately present a bushy
network of short strands of silica, as shown in Figure 29 (a). Typical diameter of short
strands is about 200 nm. The morphology of Type B hybrid aerogel (Figure 29 (b)) is
64
similar to that of native sPS aerogel. The network structure is a combination of silica
aerogel in Figure 29(a) and sPS aerogel, forming interpenetrating networks. The bulk
density of hybrid aerogel (0.0925 g/cm3) is higher than that of native sPS aerogel (0.0303
g/cm3) due to the presence of about 55 wt% of silica. The skeletal density of hybrid
aerogel (1.5659 g/cm3) is also higher than that of native sPS aerogel (1.0678 g/cm3)
owing to higher skeletal density of silica aerogel (1.7973 g/cm3). The presence of solid
silica particles in the macropores of sPS aerogel result a slightly lower porosity (94.1%)
in hybrid aerogel compared to native sPS aerogel (97.2%).
Table 10. Surface area of native sPS aerogel, acid-derived silica aerogel and Type B
hybrid aerogel.
sPS-0.02 sPS-0.02-TEOS-3 acid-derived silica aerogel
Surface area (m2/g) 399 363 325
1 10 100
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
dV
/dD
cm
3/g
*nm
pore diameter (nm)
sPS-0.02
sPS-0.02-TEOS-3
acid-derived silica aerogel
Figure 30. Pore size distribution of native sPS aerogel, acid-derived silica aerogel and
type B hybrid aerogel.
65
The surface area and pore size distribution of native sPS aerogel, acid-derived silica
aerogel, and type B hybrid aerogel are shown in Table 10 and Figure 30, respectively.
Compared to native sPS aerogel and acid-base derived silica aerogel, the acid-derived
silica aerogel exhibit smaller surface area, and the surface area of hybrid aerogel is
smaller than that of native sPS aerogel due to the presence of acid-derived silica aerogel
in them. As discussed eariler, native sPS aerogel has no predominant pores located in the
mesopores range (2-50 nm). In contrast, the pore size distributions of type B hybrid
aerogel show a small peak in the mesopores zone due to the presence of the acid-derived
silica aerogel which exhibits a predominant pore size of around 4 nm as shown in the
Figure 30.
Table 11 Oil absorption and water absorption data for sPS-0.02-TEOS-3 acid-derived
aerogel.
Weight increase percentage (%) at
Equilibration
time (min)
Theoretical
weight
increase
percentage
(%)
10 s 15 s 30 s 60 s
Oil
absorption 601 698 718 720 30 s 851
0.5 min 1 min 5 min 15min
Water
absorption 401 421 530 533 5 min 1062
66
Figure 31. Image (a) and (b) show Type B hybrid aerogel before and after oil absorption.
Image (c) and (d) show Type B hybrid aerogel before and after water absorption.
Type B hybrid aerogel specimens were used in oil and water absorption tests, as
described in section 4.3.7. It is seen from the data in Table 11 that the Type B hybrid
aerogels exhibit higher capacity of absorption of crude oil. The experimental absorption
levels show weight increase almost close to the theoretical weight increase. In addition,
the Type B hybrid aerogel present a high absorption rate. In this case, high porosity, large
capillary force, and the inherent fast absorption kinetics of sPS aerogel are responsible.
Compared with native sPS aerogels and Type A hybrid aerogels made form low
concentration of TEOS which exhibit large shrinkage after absorbing oil, the Type B
hybrid aerogel showed no shrinkage after oil absorption test. It is interesting to observe
that the Type B hybrid aerogel also show high capacity of absorbing water as seen from
the data presented in Table 11. This can be explained as follows: Under acidic condition,
67
the rate of hydrolysis process of TEOS is much higher than that of condensation process.
This results in many residual hydroxyls on the silica particles. Thus, silica aerogel
produced from acid hydrolysis and condensation reaction should exhibit higher
hygroscopic nature. The Type B hybrid aerogel also showed almost no shrinkage after
absorbing water. In comparison, Type A hybrid aerogels showed much larger shrinkage,
as shown in Figure 27 (c), (d). The strand-like silica networks in acid derived silica
aerogel are apparently much stronger than the “pearl necklace” structures in acid-base
derived silica aerogel.
68
CHAPTER V
SUMMARY
Chapter III established that macroporous cellulose filters can be easily coated with
ssPS aerogel to obtain a porous material showing both mesoporous and macroporous
surfaces. The ssPS aerogel was coated on the top surface of the cellulose filter. In
addition, some ssPS chains diffused into the macropores of the cellulose filter to offer
good bonding between the materials. The sulfonic acid groups facilitated polymerization
of aniline monomer. The polyaniline formed aggregates on the polystyrene main chain
and produced electrically conductive networks. The surface area of ssPS-coated filter was
about 60 times higher than that of the cellulose filters. This is attributed to the mesopores
of ssPS aerogels. The surface area reduced upon polymerization of aniline on ssPS,
indicating filling of a large fraction of mesopores. The value of electrical conductivity
shows that this material has the potential to be capacitosr. A future study should
investigate the capacitance, mechanical properties of the polyaniline-coated-filter
aerogels study and evaluate the effect of degree of sulfonation of sPS on capacitance of
aerogel.
Chapter IV established that novel sPS/silica hybrid aerogel can be fabricated by a
simple two-step process, in which the sPS gel is prepared first and its macropores act as
cage to contain the silica gel. The Type A hybrid aerogels exhibit much higher surface
area than sPS aerogel due to the contributions of silica aerogel, and present mesopores
distribution. A synergy in mechanical properties is found. The Type A hybrid aerogel also
69
present a fast absorption and high absorption capacity of crude oil. Compared with Type
A hybrid aerogel, the Type B hybrid aerogel exhibit an IPN structure, and high absorption
capacity of water in addition to high absorption capacity of crude oil.
70
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