Thermal behaviour of poly(dimethylsiloxane) hybrid silicas prepared by radiation grafting

1 23

Journal of Thermal Analysis andCalorimetryAn International Forum for ThermalStudies ISSN 1388-6150Volume 112Number 2 J Therm Anal Calorim (2013)112:703-711DOI 10.1007/s10973-012-2610-1

Thermal behaviour ofpoly(dimethylsiloxane) hybrid silicasprepared by radiation grafting

Ornella Ursini, Giancarlo Angelini,Edo Lilla, Donatella Capitani, FrancoCataldo & Claudio Villani

1 23

Your article is protected by copyright and

all rights are held exclusively by Akadémiai

Kiadó, Budapest, Hungary. This e-offprint is

for personal use only and shall not be self-

archived in electronic repositories. If you wish

to self-archive your article, please use the

accepted manuscript version for posting on

your own website. You may further deposit

the accepted manuscript version in any

repository, provided it is only made publicly

available 12 months after official publication

or later and provided acknowledgement is

given to the original source of publication

and a link is inserted to the published article

on Springer's website. The link must be

accompanied by the following text: "The final

publication is available at link.springer.com”.

Thermal behaviour of poly(dimethylsiloxane) hybrid silicasprepared by radiation grafting

Ornella Ursini • Giancarlo Angelini •

Edo Lilla • Donatella Capitani • Franco Cataldo •

Claudio Villani

Received: 13 November 2011 / Accepted: 13 July 2012 / Published online: 26 August 2012

� Akademiai Kiado, Budapest, Hungary 2012

Abstract This paper reports our investigation regarding

the thermal properties of new polymer-silica hybrid materials

obtained by radiation grafting. The polymer poly(dimethyl-

siloxane),bis(3-aminopropyl)terminated is c-grafted on a

silica gel surface. The thermal behaviour of c-grafted hybrid

materials reveals remarkable differences compared to the

thermal behaviour of physically adsorbed polymers. These

differences allow us to assess the ability of c-rays to produce a

polymer chemically bonded on a silica surface. The chemical

bonds formed by irradiation give to the polymer a high

conformational stability confirmed by DTA analysis.

Keywords Hybrid silica material �Thermogravimetric analysis � Radiation grafting �Poly-dimethylsiloxane

Introduction

Hybrid materials are an important new family of amor-

phous solids in which organic and inorganic constituents

are enclosed in a unique composite material.

The birth of soft inorganic chemistry processes has

opened the way for preparing tailor-made materials in

terms of chemical and physical properties. Numerous silica

and siloxane-based hybrid inorganic–organic materials

have been developed over the past few years [1–5].

The most common procedures, described in the litera-

ture, for the preparation of ‘hybrid inorganic–organic’

materials are grafting reactions [6, 7] and the sol–gel (SG)

method [8, 9].

Radiation grafting is often used to improve polymer

stability [10, 11] against the degrading effect of heat,

oxygen and light and to modify the physical–chemical

properties of a polymer such as wettability, adhesion,

adsorption and surface reactions [12, 13].

Chemical reactions induced by radiation can be carried out

at any temperature, and in every phase of the matter, whether

solid, liquid or gas without the use of a catalyst. High-energy

radiation such as gamma rays can penetrate thick polymeric

materials whether they are or not transparent.

So, one advantage offered by the radiation process is the

possibility of carrying out the reactions at room tempera-

ture and in the solid state.

Rarely, c-irradiation is tested to build some stable stationary

phases that can be useful in chromatographic employments

[14, 15] and in preparing some hybrid materials [16, 17].

The aim of our study is to investigate the thermal

behaviour of the hybrid material obtained through the

c-ray grafting process, using as an initial polymer the

poly(dimethylsiloxane),bis(3-aminopropyl)terminated and

as an inorganic surface the amorphous silica gel.

The specific properties of the chosen polymer are the

presence of two amino-functionalized symmetrical arms.

These amino groups could become reactive sites acting

like hooks to which a specific organic molecule can be

anchored by means of a subsequent reaction.

O. Ursini (&) � G. Angelini � E. Lilla � D. Capitani

Institute of Chemical Methodologies, CNR, Area della Ricerca

di Roma 1, 00015 Monterotondo, Rome, Italy

e-mail: [email protected]

F. Cataldo

Lupi Chemical Research Institute, Via Casilina 1626/A,

00133 Rome, Italy

C. Villani

Dipartimento di Chimica e Tecnologie del Farmaco,

Universita ‘‘La Sapienza’’, Rome, Italy

123

J Therm Anal Calorim (2013) 112:703–711

DOI 10.1007/s10973-012-2610-1

Author's personal copy

We wanted to investigate the thermal behaviour of the

polymer-silica hybrid composites, the influence of the sil-

ica surface on the grafting process, and to assess the

influence of irradiation parameters on the chemical prop-

erties of the newly prepared hybrid material. This study

was carried out using the thermogravimetric analyses and a

Fourier transform-infrared spectroscopy (FT-IR).

Experimental

Materials and c-rays radiation source

The silica and the poly(dimethylsiloxane),bis(3-aminopro-

pyl)terminated were purchased from Sigma-Aldrich and

were used without any further purification. The surface

area of the silica (Merck grade) was 300 m2 g-1, the par-

ticle size was 63–200 lm and the pore size was 100 A.

The polymer had an average molecular mass of about

2,500 Da and a mean amine group number of 0.06–

0.08 meq g-1. The average number of the repetitive

intermediate units [(CH3)2–Si–O–]n in the single polymeric

chain was about 30.

The solvents used were dichloromethane and ethyl

acetate purchased from Carlo Erba.

The samples were irradiated using a Nordion 220

Cobalt-60 c-source with a dose rate of 2 kGy h-1. The

irradiation time was modulated to obtain the different total

doses of 100, 200, 300, 400 and 500 kGy.

Chemical and physical modifications of the silica

The silica active sites useful in finding new interactions

between the silica and a general ‘foreign’ compound are:

– the pores useful for physical adsorption,

– the silanol groups useful in building new covalent

bonds between the silica surface and the organic

molecules, i.e. sites useful for the net chemical grafting

process.

In order to modify the active sites so as to improve the

grafting ability, the pure silica underwent specific chemical

and physical pre-treatment.

The various kinds of pre-treatment of the silica are:

– Heating at 120 �C, 760 mmHg for 24 h.

– Heating at 400 �C, 760 mmHg for 72 h.

– Heating at 120 �C, 0.01 mmHg for 24 h.

– c-irradiation at the total dose of 43.5 kGy.

– Treatment with HNO3 at 45 �C.

A thermogravimetric analysis was carried out on each of

these different silica samples to investigate the modifica-

tions that took place on the silica active sites. A weighted

amount (0.5 g) of each different silica sample was used to

examine the adsorption and to determine the radiation

grafting abilities.

Preparation of the samples for the adsorption

and radiation grafting studies

The polymer (0.51 mL) was dissolved in 7 mL of dichlo-

romethane at room temperature. Pretreated pure silica

(0.5 g) was added to the solution, and the resulting dis-

persion was stirred for 3 h in a sealed vial to prevent the

solvent from evaporating. After the stirring time, the vials

were opened to allow the solvent to evaporate very slowly

and to guarantee a uniform distribution of the polymer on

the silica. In order to detach the polymer which had not

been adsorbed on the silica, the obtained samples under-

went an extraction procedure in a Soxhlet apparatus with

dichloromethane for 5 h and successively with ethyl ace-

tate for 5 h. Only the samples characterized by a physical

polymer adsorption, at the end of the extraction process,

will be called ‘adsorbed samples’.

The procedure followed to prepare the samples for

c-grafting is analogous to that used for the adsorption

process. Once the polymer was well distributed on the

silica surface, the samples underwent the c-irradiation

process in closed vials at the total doses of 100, 200, 300,

400 and 500 kGy. At the end of the c-ray treatment, the

irradiated samples underwent an extraction procedure in a

Soxhlet apparatus following the above-mentioned proce-

dure. Only the samples characterized by a chemical poly-

mer grafting, at the end of c-ray treatment and extraction

process, will be called ‘grafted samples’.

Characterization

Simultaneous thermogravimetric analyses (TG) and dif-

ferential thermal analyses (DTA) were performed on a

Linseis apparatus model L81 ? DTA at a heating rate of

10 �C min-1 under air flow.

FT-IR spectra were obtained in transmittance mode on

an IR300 spectrometer from Thermo-Fisher Corp. The

spectra of the silica and the solid grafted samples were

recorded using the KBr plates. The spectrum of the crude

polymer was recorded in the pure liquid state.

Results and discussion

In the work here presented, the polymer was used in a

liquid phase, and the evaluation of the thermal stability of

the polymer-silica hybrid material was followed by TG

investigations, taking into account the different behaviour

of the materials obtained by the simple adsorption of the

704 O. Ursini et al.

123


polymer on the silica and of the hybrid material obtained

after radiation grafting.

In order to induce some changes on the active silica

surface, the pure silica underwent specific chemical and

physical pre-treatment, and then these preliminary pro-

cesses were investigated to see if they were able to modify

the grafting and the adsorption abilities. The FT-IR spec-

troscopy confirmed the difference in behaviour between

adsorption and chemical grafting.

The silica gel: thermogravimetric analyses of pristine

and pretreated silica

The thermogravimetric analysis of the silica shows that the

untreated silica undergoes a first weight loss at a temper-

ature less than 100 �C followed by a second loss at a higher

temperature (Fig. 1a).

Regarding the pretreated silica, the most significant

indications are obtained when the silica are heated at a

temperature of 400 �C for 72 h and when the silica are

treated with HNO3 (Fig. 1a).

It is worth noting that the weight loss at the high tem-

perature is a little less than in the case of the pristine silica.

In fact in the latter case, the weight loss is 13 %, whereas in

the case of both pretreated silica, the loss weight is about

8–9 %. It is supposed that the modifications of the active

sites due to the different silica pretreatment, induce two

correlated effects: first, a minor weight loss and second, a

reduction in adsorption ability.

The differential thermal analyses (DTA) of the pre-

treated silica (Fig. 1b), reveal that they undergo a first

weight loss at a temperature lower than 90 �C, followed by

a second, slow, gradual loss at a higher temperature. The

first loss can be attributed to a dehydration process that

occurs easily at a low temperature (hydration water) fol-

lowed by a framework dehydroxylation process that occurs

at a high temperature.

Besides, the HNO3-treated silica shows a DTA curve

with a more noticeable minimum with respect to the silica

preheated at 400 �C, and this behaviour is closely related to

an increase in the number of silanol groups present in the

framework silica, which increased as a result of the acid

treatment.

X-ray analysis and FT-IR spectroscopy

The XRD curve of the pristine silica gel used in our

experiments is absolutely analogous to the XRD curves

described in the literature [18, 19] showing the typical

broad profile typical of a amorphous silica where the pri-

mary building units are randomly connected to each other,

without exhibiting a regular pattern characteristic of the

crystalline silica.

The IR spectrum of the silica is characterized by some

peaks in the range between ca 1,100–400 cm-1 which

correspond to the vibrations of the Si–O–Si bonds of the

SiO4 tetrahedron. In particular, the dominant broad peak at

1,099 cm-1 is due to the asymmetric stretching vibration

of the Si–O–Si atoms mas Si–O–Si), while the moderate

band at 797 cm-1 corresponds to the symmetric stretching

vibration ms Si–O–Si). The bending of the Si–O–Si bond

corresponds to the band at 465 cm-1. The presence of the

surface silanol groups Si–OH is evident from the broad

band at 3,444 cm-1.

The polymer: the thermogravimetric analysis

and the FT-IR spectroscopy of the crude polymer

The TG curve of the crude polymer shows a substantial

weight loss of about 89 % that starts at about 400 �C and

gradually increases with a rise in temperature. The first

derivatives of the thermogravimetric curve, the DTG curve,

presents an indented, quite broad curve indicative of a

decomposition that occurs as the temperature rises (Fig. 2).

98

96

94

92

90

88

86

100

2

1

0

–1

–2

–4

–3

–5

–6

–7

–8

–9

–10

0 100 200 300 400 500 600 700 800

Temperature/°C

0 100 200 300 400 500 600 700 800

Temperature/°C

DTA

sig

nal/

VM

ass/

%

83.0 °C

517.7 °C

518.3 °C

DTA SiO2 HNO3

DTA SiO2 400 °C 72h

530.4 °C

677.3 °C

677.3 °C

–13.1 %

–9.8 %

–8.3 %

TG SiO2 pristineTG SiO2 HNO3

TG SiO2 400 °C

(a)

(b)

µ

Fig. 1 a Thermogravimetric analysis of the untreated silica and

pretreated silica: heating at 400 �C at 760 mmHg for 72 h and pre-

treatment with HNO3, b DTA curves of the pretreated silica heated at

400 �C at 760 mmHg for 72 h and for the silica treated with HNO3 at

45 �C

Thermal behaviour of poly(dimethylsiloxane) hybrid silicas 705

123


The IR spectrum of the crude polymer shows the bands

at 2,964–2,903 cm-1 which corresponds to the aliphatic

C–H stretching vibrations of the CH3 and CH2 polymer

groups. The polymer backbone is built from two basic

different units: the Si–(CH3)2 groups and the Si–O unit.

The Si–(CH3)2 groups correspond to the sharp peaks at

1,262 and 801 cm-1, due respectively, to the symmetric

bending and stretching modes. The stretching vibrations of

the Si–O units take place between the 1,091–1,021 cm-1

range.

Polymer adsorption

The porous silica particles are able to adsorb the polymer,

by a physical process. The adsorption behaviour depends

on two factors: the affinity that exists between the polymer

and the silica particles, and the presence of pores. The

chemical–physical pre-treatment of the silica, modifies the

active silica surface and changes the adsorption behaviour.

The equation useful to value the adsorption process, for

each pretreated silica, is:

%Ads:Pol: ¼ ½ðgInit:Pol: � gExtr:Pol:Þ=gInit:Pol� � 100

where gInit.Pol. is the amount in g of the initial polymer and

gExtr.Pol. is the amount in g of the recovered polymer [20].

The polymer adsorption value for each silica treatment

is shown in Table 1 and represents, according to our

experimental conditions, the percentage of adsorbed poly-

mer calculated with respect to the initial amount of poly-

mer. The results are the mean value of five independent

adsorption experiments.

First of all, it should be noted that the adsorption of the

polymer in the cases of silica pretreated by heating is

always less than the adsorption value on the pristine

untreated silica. It should be noted that the more the tem-

perature and the time of heating increases the more the

adsorption of the polymer decreases.

In fact, in the case of silica heated at 400 �C, the amount

of the adsorbed polymer in the silica porous surface is

remarkably reduced. The influence of the pressure variation

is not so remarkable. At the same heating temperature of

120 �C, the pressure variation (0.01 mmHg compared to

760 mmHg) does not change the polymer adsorption

ability. This behaviour, can be attributed to the fact that the

silica gel surface consists of two primary active sites: sil-

anols (Q2 and Q3 sites) and siloxanes (Q4 sites).

Polymer

HO HOOH OHOO O

O

H H

O

Si Si Si Si Si Si

Q2 Q3 Q4 Q3 Q3* Vicinal and Bridged Si-OH

Polymer

The silanol groups are dominant in the adsorption pro-

cess but the thermal pre-treatments are able to decrease

their surface concentration [21, 22]. Consequently, if the

silica is preheated, the smaller number of silanol groups

produce a decrease in the adsorbed polymer on the silica.

As far as the preliminary c-irradiated silica is concerned,

the ability of the polymer adsorption does not change, as it

is exactly similar to the un-treated silica (Table 1).

According to the literature in the case of the precipitated

amorphous silica, the Q4/Q3 units ratio greatly changes

after the irradiation process, it is important to observe that

this process starts at a c-ray dose of 300 kGy [23, 24].

Consequently, in the case of a dose of 43.5 kGy, as used in

this work, it can be presumed that the silica structure does

not change at all.

Regarding the percentage value of the adsorbed polymer

in the mineral acid pretreated silica, it is a little less than

that in the untreated silica. In the literature, it is reported

100

90

80

70

60

50

40

30

20

10

0

Mas

s/%

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800

Temperature/°C

0

–0.05

–0.1

–0.15

–0.2

–0.25

–0.3

–0.35

–0.4

–0.45

–0.5

–0.55

–0.6

Der

iv. m

ass/

mg

°C–1

TG PDMS

DTG PDMS678.7 °C

514.0 °C

496.6 °C

421.8 °C

–89.2 %

Fig. 2 Thermogravimetric

analysis of the crude polymer

TG, DTG curves


123


that the mineral acid treatment increases the number of the

silanol groups located on the silica surface ([25] and ref-

erences therein). In this way, it is possible to promote two

types of silanol groups, Q2 and Q3 units. If Q3 units are

very close, they form Q3* vicinal units. The Q3* vicinal-

bridged groups prevent polymer adsorption, so that the

amount of the adsorbed polymer is less than that in the

untreated silica.

Polymer radiation grafting

The assessment of the sorption process allows the grafting

yield of the polymer on the silica surface to be calculated

accurately. In fact, by evaluating the amount of adsorbed

polymer on the silica, the value of the grafted polymer can

be estimated, assuming that the adsorbed polymer does not

give any grafting processes.

The amount of the grafted polymer was evaluated by a

simple mass balance as follows:

gGrafted Polymer ¼ gInit:Pol: � ½1�%Adsorbed Polymer=100�� gExtracted Polymer

where the gGrafted Polymer is the amount in g of the grafted

polymer and the %Adsorbed Polymer is the percentage of the

adsorbed polymer previously calculated.

These values are shown in Table 2.

First of all, we can note that silica, regardless of the

preliminary chemical–physical treatments carried out,

show a clear growth of the grafted polymer after increasing

the adsorbed dose. Besides, all the investigated pretreated

silica show a better grafting behaviour compared to the

untreated silica. The higher value of grafting (0.337 g) is

obtained at a dose of 500 kGy in the case of the 400 �C

preheated silica. This value is directly correlated to the

minor adsorption value (Table 1).

However, both the cases of thermal-treated silica show a

greater amount of grafted polymer with respect to the

untreated silica. This behaviour can be attributed to the fact

that the thermal process [21, 22] induces a decrease in the

silanol groups present on the silica surface. Consequently,

this means an increase of Q4 units on the silica. It can be

supposed that these Q4 units, act in two distinct ways.

On the one hand, the Q4 units, located on the surface,

reduce the extent of physical adsorption, so that by

increasing the temperature in the silica pre-treatment, there

is a net decrease of the physically adsorbed polymer. On

the other hand, the Q4 units located along the pores of the

silica, allow the polymer to slide more easily in depth in

the pores during the irradiation. Then, the c-rays allow the

formation of the covalent bonds between the polymer

which has slid deeply into the silica pores, and the active

sites of the silica. A decrease in the silanol groups on the

silica is well confirmed by IR spectra. In fact, the intensity

of the broad band, characteristic of silanol group Si–OH,

decreases with the silica pre-treatment temperature.

Adsorbed and grafted samples: thermogravimetric

analysis and FT-IR spectroscopy

Taking into account, the various types of treatment

imposed on the silica before grafting irradiation, only those

samples, polymer adsorptions (Table 1) of which shows a

significantly different behaviour with respect to the pristine

untreated silica, were chosen to undergo the thermogravi-

metric and IR investigations.

So, the IR and thermogravimetric behaviour of the silica

preheated at 120 �C for 24 h at reduced pressure, the silica

preheated at 400 �C and the silica HNO3 treated were all

investigated.

The bare pretreated silica loses about 2 % of water as a

consequence of the dehydroxylation process at around

400–500 �C (Fig. 1a). So the values of the gross weight

loss shown in the TG curves in the case of all adsorbed and

Table 1 Percentage of the adsorbed polymer calculated with respect

to the initial amount of polymer (starting from about 0.5 mL of

polymer and about 0.5 g of silica)

Pristine untreated silica 52.3 %

Heated silica 120 �C for 24 h (760 mmHg) 35.7 %

Heated silica 120 �C for 24 h (0.01 mmHg) 35.0 %

Heated silica 400 �C for 72 h (760 mmHg) 25.6 %

c-Irradiation of the silica at the dose 43.5 kGy 53.4 %

HNO3 treatment 42.6 %

Table 2 Grafted polymer and extracted polymer on several pre-

treated silica in function of the total dose of the c-rays

Silica Dose

kGy

Initial

polymer/g

Extracted

polymer/g

Grafted

polymer/g

Untreated 100 0.484 0.165 0.066

200 0.501 0.090 0.149

300 0.486 0.058 0.174

400 0.481 0.050 0.180

500 0.492 0.038 0.197

Heated 120 �C

10-2 torr

100 0.463 0.142 0.156

200 0.491 0.105 0.211

300 0.450 0.065 0.225

400 0.483 0.073 0.238

500 0.505 0.071 0.254

Heated 400 �C 100 0.5230 0.111 0.278

200 0.5000 0.055 0.317

300 0.4930 0.044 0.323

400 0.4770 0.033 0.322

500 0.4920 0.029 0.337


123


grafted samples, must take into account the dehydroxyla-

tion water loss. The comprehensive information resulting

from thermogravimetric investigations can be validated by

the FT-IR spectroscopy, the spectra of the silica preheated

at 120 �C for 24 h at reduced pressure (Fig. 3) is reported

here as a model where it is evident that the band at

2,963 cm-1 corresponds to the C–H stretching mode and

the peak at 1,255 cm-1 corresponds to the Si–(CH3)2

groups bending mode. These peaks, which are totally

absents in the IR silica spectrum, confirm the presence of

the polymer on the silica surface. Similar spectra were

obtained of the other pretreated silicas.

Thermogravimetric analysis of pretreated silica

The TG profiles of three pretreated silica reveal that ther-

mal weight decrements greatly change as a consequence

of the grafting process. The results are summarized in

Table 3.

The small weight loss registered by the TG behaviour in

the case of adsorbed sample is directly related to the lower

percentage of adsorbed polymer. Whereas, the high weight

loss observed in the case of the grafted samples, reflects a

greater amount of polymer present on the surface after the

irradiation process.

The thermogravimetric behaviour of the silica heated at

120 �C at 0.01 torr, and of the silica preheated at 400 �C,

indicates that:

– The DTG curves of the grafted samples, (Fig. 4a, b)

show well-defined profiles, corresponding to clear and

sharp decomposition temperatures. This behaviour

suggests that the grafted polymer achieves a unique

and neat interaction at the silica-polymer interface after

the grafting process. It should be noted that the simple

adsorbed sample presents a larger DTG curve which

can be attributed to a very slow softening by heat.

Whereas, the DTG of the grafted polymers are aston-

ishingly sharp and correspond to a faster and more

stable decomposition.

– The differential thermal analyses (DTA) of the grafted

samples (Fig. 5a, b), for both preheated silica, reveal a

well defined and sharp profile with positive curves,

characteristic of exothermic processes.

Two distinct peaks are present. These sharp peaks fall

in a zone where the adsorbed sample is ‘silent’. There

is reason to assume that these peaks can be attributed to

that part of the polymer that has actually been grafted.

The new chemical bonds formed on the polymer/silica

hybrid material by irradiation, give to the polymer a

high conformational stability that in the DTA analysis

appears as a definite temperature. In fact, it is rea-

sonable to suppose that to break the new covalent

bonds only a specific amount of energy is required that

results as a definite peak in the DTA profile. It is worth

noting that in the case of silica preheated at 400 �C

for 72 h, the heights of the peaks are quite similar

1.0

0.5

0.01.0

0.5

0.01.0

0.5

0.0A

bsor

banc

eWavenumbers/cm–1

Grafted polymer : 500 KGy

Grafted polymer : 100 KGy

Absorbed polymer

2963

2963 12

5512

60

1065

1073

1013

859 78

6

1062

1015

856

802

791

2963

Fig. 3 FT-IR spectrum

of adsorbed and grafted polymer

on the silica preheated at 120 �C

0.01 torr

Table 3 Thermal weight decrements of pretreated silica registered by thermogravimetric analyses

Silica treatments Adsorbed sample

(mass loss/%)

100 kGy grafted sample

(mass loss/%)

500 kGy grafted sample

(mass loss/%)

Preheated at 120 �C at 0,01 torr 12.8 21.7 20.4

Preheated at 400 �C (72 h) 6.4 20.6 22.7

Treatment with HNO3 10.7 20.0 24.8


123


(Fig. 5b). This means that the silica treatment changes

the surface properties so that the grafted polymer

adopts different conformational shapes. It is well

documented that by heating the silica the Q4 siloxane

units increases [21, 22]. This means that the silica

becomes gradually less hydrophilic and the interface

interactions between the hydrophobic polymer and the

silica surface take place easily. These favourable

interactions determine the sharp profile in the thermo-

gravimetric analysis.

– The DTA curve in Fig. 5a of the adsorbed sample

presents a first peak at low temperature of 287.9 �C

whereas the DTA curves in Fig. 5b shows a peak at

slightly higher temperature 292.0 �C. These peaks can

be explained as a polymer conformational framework

reorganization that occurs as a result of heating, just

when the polymer is present on the silica.

It should be noted that the height of this peak decreases

in the case of the grafted samples. Due to irradiation,

the covalent bonds take place and the polymer confor-

mational framework reorganization becomes gradually

more difficult. The polymer conformational framework

reorganization decreases with an increase in the level of

the covalent bonds. So, in the case of the 100 kGy

grafted sample, these peaks become very insignificant

and completely disappear in the case of the 500 kGy

sample.

These results show as the changes of silica surfaces

induced by heating, improve the radiation grafting

process and consequently the thermal properties of the

hybrid polymer-silica material.

Thermogravimetric analysis of silica treated

with HNO3

– The DTG curve of adsorbed polymer on the silica

(Fig. 6a) presents peaks at different stages of a gradu-

ally increasing temperature. The shape of the DTG

curve could be rationalized by the presence of different

‘families’ of adsorbed polymers. These families are due

to various conformational shapes of the adsorbed

polymer with different thermal stabilities. By increas-

ing the temperature, the more stable shape of the

polymer becomes more evident. The various types of

mineral acid treatment ([25] and references therein)

give rise to an increase of the silanol groups present on

the silica surface, thus increasing the hydrophilic

character of the surface. The Q3* vicinal units prevent

0.02

–0.02–0.04–0.06–0.08

–0.1–0.12–0.14–0.16–0.18

–0.2–0.22–0.24–0.26–0.28

0

0.013

–0.007

–0.027

–0.047

–0.067

–0.087

–0.107

–0.127

Der

iv. m

ass/

mg

°C–1

Der

iv. m

ass/

mg

°C–1

150 200 250 300 350 400 450 500 550 600 650

Temperature/°C

Temperature/°C

120 170 220 270 320 370 420 470 520 570 620

DTG Grafted polymer 500 kGy

Grafted polymer 100 kGy

Absorbed polymer


DTG Grafted polymer 100 kGy

439.3 °C

426.3 °C

414.2°C

465.5 °C

444.0 °C441.7 °C

(b)

(a)

Fig. 4 a First derivatives of the TG curves of the adsorbed and

grafted polymers on the preheated silica at 120 �C, 0.01 torr, b first

derivatives of the TG curves of the grafted polymers on the silica

preheated at 400 �C

140

120

100

80

60

40

20

0

140

120

100

80

60

40

20

0

190 240 290 340 390 440 490 540

Temperature/°C

Temperature/°C200 250 300 350 400 450 500 550 600

DTA

sig

nal/

VD

TA s

igna

l/µV


Absorbed polymer



Absorbed


292.0 °C

481.7 °C

452.6 °C

442.9 °C

429.9°C

418.5 °C

287.9 °C

450.1 °C

450.3 °C

475.5 °C487.1 °C

(a)

(b)

µ

Fig. 5 a Differential thermal analysis of the adsorbed and grafted

polymers on the preheated silica at 120 �C, 0.01 torr, b differential

thermal analysis of the adsorbed and grafted polymers on the silica

preheated at 400 �C


123


adsorption of the polymer. In fact, we can observe a

decrease of the adsorption value compared with the

pristine silica (Table 1), but the adsorbed polymer is

present with different conformational shapes.

– The DTA curve of adsorbed polymer (Fig. 6b) presents

three sharp peaks, with a sharp profile.

– The first peak at 285.4 �C can be attributed to a

polymer conformational framework reorganization that

occurs by heating and that becomes progressively

less marked as the covalent bonds are formed by

c-irradiation. The other two peaks (432.8 and 461.4 �C)

could be related to the greater number of Q2 and

isolated Q3 silica sites created by the acid treatment.

These sites could be responsible for two different kinds

of conformational adsorption that in the DTA curve

appear as two different peaks.

– The DTA curve of the 500 kGy grafted sample shows a

noteworthy behaviour. Compared to the previously

discussed pretreated silica, in the 400 and 500 �C range,

there is closely knitted group of sharp curves. The

presence of these four sharp peaks can be explained to a

combination of two consecutive effects. First, the acid

treatment increases the Q2 and the isolated Q3 silica

sites. Successively, the adsorbed dose required for the

grafting process changes the nature of these sites. Some

of the Q2 and isolated Q3 silica sites become, respec-

tively, Q3 and Q4 sites. The polymer feels this evolutive

change during irradiation and it adopts different

conformational housing.

Conclusions

In general, the immobilization of a polymer on a silica

surface is obtained by heating the polymer/silica system in

the presence of a radical initiator.

In our process, the grafting process is obtained using

irradiation by c-ray.

– The most important advantages offered by the

c-grafting process are:

– The reaction proceeds without the use of radical

initiators.

– The reaction can be carried out at any temperature.

– There are no solvent limitations.

– The obtained materials are very clean.

The thermal stabilities of our polymer-silica hybrid

materials obtained after irradiation are investigated by

thermogravimetric analysis and confirmed by FT-IR

investigations.

The polymer-silica grafted hybrid materials are remark-

ably different from the corresponding polymer-silica adsor-

bed materials.

Both the processes, physical adsorption and radiation

grafting are strongly influenced by the structural changes of

the silica surface, obtained by submitting the bare silica to

different physical–chemical pre-treatment. The different

ratios between the silica active sites such as silanol groups,

(Q2 and Q3 units) and siloxane Q4 sites deeply influence the

adsorption and the grafting processes. The results show as

the changes of silica surfaces induced by silica, pre-treat-

ments improve the radiation grafting process and conse-

quently, the thermal properties of the hybrid polymer-silica

material.

Heating the silica in the treatment before the irradiation

process encourages an increase in the number of siloxane

Q4 units. The resulting large number of Q4 units act in two

distinct ways. The Q4 units, located on the surface, reduce

the extent of the physical adsorbed polymer. The Q4 units

located along the pores of the silica allow the polymer to

slide more easily and more deeply inside the pores where it

binds during the irradiation process. Then, the c-rays allow

the formation of the covalent bonds between the polymer,

slid deeply into the silica pores, and the active sites of the

silica.

The thermogravimetric behaviours of the hybrid poly-

mer-silica materials obtained with preheated silica (heated

Temperature/°C

Temperature/°C

200 250 300 350 400 450 500 550 600

220 270 320 370 420 470 520 570

120

100

80

60

40

20

0

0

–0.05

–0.1

–0.15

–0.2

–0.25

Der

iv. m

ass/

mg

°C–1

DTA

sig

nal/

V


Absorbed polymer



Absorbed polymer


452.3 °C426.8 °C

398.1 °C

407.6 °C

412.9 °C

402.2 °C

285.4 °C

292.3 °C

432.8 °C

454.7 °C

479.2 °C

495.0 °C

(a)

(b)

µ

Fig. 6 a First derivatives of the TG curves of the adsorbed and

grafted polymer on the HNO3 pretreated silica, b differential thermal

analysis of the adsorbed polymer on the HNO3 pretreated silica


123


at 120 and 400 �C) suggest that the polymer interacts more

neatly after the grafting process. The new chemical bonds

formed by the c-rays in the hybrid system give a high

conformational stability to the polymer that in the DTA

analysis appears as peaks at specific temperatures.

On the other hand, the mineral acid pre-treatment

modifies the hydrophilic character of the silica and con-

sequently, the adsorbed polymer and the radiation-grafted

hybrid material show peculiar behaviours evidenced by the

TG analyses. In general, the acid treatment increases the Q2

and the Q3 silica sites. The TG behaviour of the physical

adsorbed material can be explained by the presence of a

double effect. The isolated-Q3 sites and the Q2 sites are

responsible for the different conformational adsorption

ways shown in the DTA curve, while the Q3* vicinal units

reduce the total amount of adsorbed polymer.

In the case of the radiation-grafted hybrid material, the

DTA curve shows numerous and dense families of positive

sharp curves. During irradiation, some of the Q2 and iso-

lated-Q3 silica created by the acid treatment change their

nature and the silica becomes gradually less hydrophilic.

During irradiation, the polymer feels these silica evolutive

changes that appear in the DTA analysis as a closely

knitted group.

Acknowledgements This work was supported by the National

Research Council of Italy.

References

1. Xia B, Xiao SJ, Wang J, Guo DJ. Stability improvement of

porous silicon surface structures by grafting polydimethylsilox-

ane polymer monolayers. Thin Solid Film. 2005;474:306–9.

2. Collins KE, Sa ALA, Bottoli CBG, Collins CH. Thermal

immobilization of poly(methyloctylsiloxane) in the pores of

chromatographic silica. Chromatographia. 2001;53:661–4.

3. Sanchez C, de Soler-Illia GJAA, Ribot F, Lalot T, Mayer CR,

Cabuil V. Designed hybrid organic–inorganic nanocomposites

from functional nanobuilding blocks. Chem Mater. 2001;13:

3061–83.

4. Sanchez C, Lebeau B, Chaput F, Boilot JP. Optical properties of

functional hybrid organic–inorganic nanocomposites. Adv Mater.

2003;15:1969–94.

5. Bauer F, Ernst H, Decker U, Findeisen M, Glasel HJ, Langguth

H, Hartmann E, Mehnert R, Peuker C. Preparation of scratch and

abrasion resistant polymeric nanocomposites by monomer graft-

ing onto nanoparticles, 1 FTIR and multi-nuclear NMR spec-

troscopy to the characterization of methacryl grafting. Macromol

Chem Phys. 2000;201:2654–9.

6. Cataldo F, Ursini O, Lilla E, Angelini G. Radiation-induced

polymerization and grafting of b(-)pinene on silica surface.

Radiat Phys Chem. 2008;77:561–70.

7. Cataldo F, Ursini O, Lilla E, Angelini G. Radiation-induced

synthesis of fullerene-silica hybrid nanomaterials. Fuller Nanotub

Carbon Nanostruct. 2007;15:445–63.

8. Brinker CJ, Scherer GW. Sol–gel science. The physics and

chemistry of sol–gel processing. San Diego: Academic Press;

1990.

9. Lebeau B, Sanchez C. Sol–gel derived hybrid inorganic–organic

nanocomposites for optics. Curr Opin Solid State Mater Sci.

1999;4:11–23.

10. Ranogaiec F, Mlinac-Misak M. Improvement of the polymer

stability by radiation grafting. Radiat Phys Chem. 2004;71:

227–31.

11. Cleland MR, Parks LA, Cheng S. Applications for radiation

processing of materials. Nucl Instrum Methods Phys Res B.

2003;208:66–73.

12. Zhirong L, Huiliang W. Radiation-induced grafting of glycidyl

methacrylate onto high-density polyethylene (HDPE) and radia-

tion lamination of HDPE. J Appl Polym Sci. 2005;96:772–9.

13. Khalil MMI, El-Sawy NM, El-Shobaky GA. c-Irradiation effects

on the thermal and structural characteristics of modified, grafted

polypropylene. J Appl Polym Sci. 2006;102:506–15.

14. Jardim ICSF, Collins KE, Anazawa TA. Radiation immobiliza-

tion of poly(methyloctylsiloxane) on silica for use in HPLC: a

uniform layer model. J Chromatogr A. 1999;849:299–307.

15. Bachmann S, Melo LFC, Silva RB, Anazawa TA, Jardim ICSF,

Collins KE, Collins CH, Albert K. Synthesis and solid-state NMR

investigations of radiation-immobilized polysiloxanes on bare,

titanium-grafted, and zirconium-grafted silicas. Chem Mater.

2001;13:1874–9.

16. Gomes SR, Margaca FMA, Miranda Salvado IM, Leal JP,

Marques C, Alves E, Ferreira LM, Falcao AN. Elemental and

RBS analysis of hybrid materials prepared by gamma-irradiation.

Nucl Instrum Methods Phys Res B. 2008;266:288–94.

17. Gomes SR, Margaca FMA, Ferreira LM, Miranda Salvado IM,

Falcao AN. Thermal analysis of hybrid materials prepared by

c-irradiation. J Therm Anal Calorim. 2009;95:99–103.

18. Xu Y-M, Qi J, He D-M, Wang D-M, Chen H-Y, Guan Jun,

Zhang Q-M. Preparation of amorphous silica from oil shale res-

idue and surface modification by silane coupling agent. Oil Shale.

2010;27:37–46.

19. Gun’ko VM, Dyachenko AG, Borysenko MV. CVD-titania on

mesoporous silica gels. Adsorption. 2002;8:59–70.

20. Anazawa TA, Jardim ICSF. Polymethyloctilsiloxane adsorbed on

porous silica as a packing material for reversed phase high per-

formance liquid chromatography. J Liq Chromatogr. 1994;17:

1265–79.

21. David Sunseri J, Cooper WT, Dorsey JG. Reducing residual sil-

anol interactions in reversed-phase liquid chromatography.

Thermal treatment of silica before derivatization. JChromatogr A.

2003;1011:23–9.

22. Leboda R, Mendyk E, Gierak A, Tertykh VA. Hydrothermal

modification of silica gels (xerogels). Effect of treatment tem-

perature on their porous structure. Colloids Surf A. 1995;105:

181–9.

23. Cataldo F, Capitani D, Proietti N, Ragni P. c Radiolyzed amor-

phous silica: a study with 29Si CP-MAS NMR spectroscopy.

Radiat Phys Chem. 2008;77:267–72.

24. Patel M, Morrell PR, Murphy JJ, Skinner A, Maxwell RS.

Gamma radiation induced effects on silica and on silica–polymer

interfacial interactions in filled polysiloxane rubber. Polym

Degrad Stab. 2006;91:406–13.

25. Barrett DA, Brown VA, Watson RC, Davies MC, Shaw PN,

Ritchie HJ, Ross P. Effects of acid treatment on the trace metal

content of chromatographic silica: bulk analysis, surface analysis

and chromatographic performance of bonded phases. J Chroma-

togr A. 2001;905:69–83.


123


Documents

Thermal behaviour of poly(dimethylsiloxane) hybrid silicas prepared by radiation grafting