ORI GIN AL PA PER

Nanostructured amino acid containing poly(amide-imide)s from different diisocyanates: synthesisand morphology properties in molten TBAB as a greenmedia

Shadpour Mallakpour • Marziyeh Khani

Received: 2 September 2011 / Accepted: 17 February 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract In this investigation, a series of nanostructured optically active poly

(amide-imide)s (PAI)s were prepared via solution polycondensation of N-trimelli-

tylimido-L-isoleucine as a chiral diacid monomer containing naturally occurring

a-amino acids with different aliphatic and aromatic diisocyanates in the presence of

molten tetrabutylammonium bromide (TBAB) as an environmental friendly med-

ium and a volatile organic solvent and the results of them were comparable. In these

step-growth polycondensations amino acid was used as a chiral inducing agent. The

obtained polymers were characterized with respect to chemical structure and purity

using specific rotation experiments, FT-IR techniques and elemental analysis. The

surface morphology of these polymers was investigated by field emission scanning

electron microscopy. The results indicated that the aforementioned polymers have

nanostructured morphology. The obtained polymers have inherent viscosities in a

range of 0.12–0.45 dL/g. Thermal stability of the resulting PAIs was studied by

thermogravimetric analysis. A comparison with conventional solvents was exhibited

smoothly higher inherent viscosities and better yields and thermal stability. The

results showed that the molten TBAB was superior polymerization media. This

process was safe and green, since toxic and volatile organic solvent such as

N-methyl-2-pyrrolidone was eliminated. The observation of optical rotation con-

firms the optical activity of these polymers.

Keywords Poly(amide-imide)s � Nanostructured polymers � Diisocyanate route �Morphology study � Optically active polymers

S. Mallakpour (&) � M. Khani

Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University

of Technology, Isfahan 84156-83111, Islamic Republic of Iran

e-mail: mallakpour84@alumni.ufl.edu; mallak@cc.iut.ac.ir; mallak777@yahoo.com

S. Mallakpour

Nanotechnology and Advanced Materials Institute, Isfahan University of Technology,

Isfahan 84156-83111, Islamic Republic of Iran

123

Polym. Bull.

DOI 10.1007/s00289-013-0945-9

Introduction

Polyimides (PI)s are high-performance macromolecules which belong to the most

thermally stable polymers. These polymers are one of the most significant

candidates for diversity applications like: microelectronics industries, aerospace and

other advanced fields because of their excellent mechanical, thermal, dielectric

properties, chemical resistance, superior thermal stability, low color, flexibility,

radiation resistance, outstanding tensile strength and modulus [1]. In spite of the

excellent properties of PIs, their prevalent application was frequently limited due to

their poor solubility and high processing temperature which was caused by the rigid

macromolecules backbones and the strong interchain interactions [2–5]. Introduc-

tion of flexible linkages for instance –O–, –CH2–, and –COO– in a PI backbone

brings about a decreased glass transition temperature and an improved moldability

[6].

Although, it is known that polyamides (PA)s have high thermal stability and

exceptional mechanical properties and became breakthrough materials in commer-

cial applications as early as the 1960s, but they are very difficult to fabricate because

of their poor solubility in organic solvents and high glass transition temperature.

Great attempts have been made to improve solubility parameters of these

macromolecules, whereas maintaining their optimal, thermal, and chemical

resistance [7, 8]. Polyamide-imide (PAI) brings together both greater mechanical

properties typically linked with PAs and the high thermal stability and solvent

resistance characteristics of PIs. Compared to the other PIs, PAI is more rigid

structure and higher hydrophilicity owing to the enclosure of amide group, which

makes them especially attractive for the dehydration of organic solvents by

pervaporation [9]. PAIs have been commercially available for several decades.

Their synthesis, properties and applications have been extensively described along

with those of PAs and PIs. Their greater mechanical, thermal and oxidative

properties have made them suitable for a variety of applications in industrial

processes, transportation, electrical equipment finger mounted tactile sensors, cold

valves for superfluid helium, membranes for separation and purification of fluid

mixtures, alignment surfaces for liquid crystals, and as an organic host for inorganic

materials and so on [10–12].

Room temperature ionic liquids (IL)s are molten organic salts that comprise a

cationic group (e.g., dialkyl-imidazolium or alkyl-pyridinium) and an anionic group

(e.g., halides, tetrafluoroborate, or hexafluorophosphate). Owing to the asymmetric

structures of the cations, the melting point of the IL is normally below 100 �C. The

past decade has witnessed a rapid expansion in the field of ILs, chiefly due to their

distinctive physical and chemical characteristics, for example: low vapor pressure

and adjustable solvent properties that can get better chemical processes over

conventional ones and decrease adverse impact on the environment. The utilization

of ILs has been actively studied in many fronts, which include organic synthesis,

catalysis, separation and extraction, electrochemistry, polymerization [13–15],

inorganic nanomaterials, etc. [16–18]. Consisting of an organic cation and an

inorganic anion, ionic liquids have numerous advantages over organic solvents: high

chemical and thermally stability, non-flammability, negligible vapor pressure and in

Polym. Bull.

123

some cases high electrochemical stability and hydrophobicity [19–24]. But, high

price of most of the room temperature ILs and trepidation concerning the toxicity of

some of them has led to the employ of further benign salts in the molten state as

useful alternatives. Lately, molten tetrabutylammonium bromide (TBAB) was used

as an efficient catalyst in several practical synthetic transformations [25–27].

For nanotechnology, macromolecules are extremely significant class of materials

which physical and chemical properties are strongly related to their molecular and

supermolecular structure. Explorations of macromolecules structure have decisive

significance for their potential application in nanotechnology [28]. Submicron- and

nano-sized polymer particles have been the subject of active study in recent years,

and some of the resulting particles have been exhibited to be of practical utilized.

The size and morphology of the particles received meticulous attention. In recent

years, there has been growing demand for particles with characteristic features and

functions for practical application in diverse fields [29–31].

Recently, we have synthesized a variety of chiral extended polymers by

incorporation of amino acid segments [32–34] for the preparation of biologically

active polymers. The aim of this study is the synthesis, characterization, and

morphology study of functional amino acid-based PAIs by the direct polyconden-

sation reaction of a chiral diacid with different diisocyanates using molten TBAB as

an efficient and green method by different catalysts. L-isoleucine was used as an

amino acid building block to produce diacid which can act as a biodegradable

source in the polymer chain. After confirming the structure of the resulting polymer,

their thermal properties and solubility were also investigated. These results were

compared with previous work (conventional solvents) [35].

Materials and methods

Materials

Reagents were purchased from Fluka, Aldrich and Riedel–deHaen AG. Pyridine

(Py), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), N,N-dimeth-

ylformamide (DMF), and N-methyl-2-pyrrolidone (NMP) were dried over BaO and

then were distilled under reduced pressure. 4,40-methylenebis(4-phenylisocyanate)

(MDI) (Aldrich) was used as received. Trimellitic anhydride was recrystallized in

acetic acid/acetic anhydride mixture (3/1) and dried under vacuum at 60 �C for 6 h.

TBAB (mp = 100–103 �C) was purchased from Merck Co. (Darmstadt, Germany)

and was used without further purification. Diacid 1 was prepared as described

previously in glacial acetic acid [36]. The yield of the diacid 1 was 95 %,

mp = 198 �C (dec), and ½a�25D ¼ �62:73 (0.0504 g in 10 mL of DMF, 25 �C).

Equipments

Proton nuclear magnetic resonance (1H-NMR, 300 MHz) spectra were recorded in

DMSO-d6 solution using a Bruker (Germany) Avance 300 instrument at Shahid

Polym. Bull.

123

Beheshti University, Tehran, Iran. FT-IR spectra were recorded on a Nicolet impact

400D IR spectrophotometer. Spectra of solids were carried out using KBr pellets.

Vibrational transition frequencies are reported in wavenumber (cm-1). Band

intensities are assigned as weak (w), medium (m), strong (s) and broad (br). Inherent

viscosity was measured by a standard procedure with a Cannon–Fenske (Mainz,

Germany) routine viscometer. Specific rotation was measured with a Jasco Osaka,

Japan, P-1030 polarimeter at the concentration of 0.5 g/dL at 25 �C. Thermal

gravimetric analysis (TGA) data were taken on STA503 WinTA instrument

(Hullhorst, Germany) in a nitrogen atmosphere at a heating rate of 10 �C/min.

Elemental analyses were performed by the Research Institute of Polymer and

Petrochemical of Iran. Field emission scanning electron microscopy (FE-SEM) was

done using HITACHI (S-4160) (Tokyo, Japan).

Polymer synthesis

The following procedures were applied to all polymerizations: a mixture of aromatic

diacid 1 (0.10 g, 3.27 9 10-4 mol) and TBAB (0.400 g, 1.24 9 10-3 mol) was

ground until a powder was formed. After the mixture was completely ground,

triethylamine (TEA) (0.018 g, 2.78 9 10-5 mol) was added; then, it was transferred

into a 25-mL, round-bottom flask, and MDI (2a) (0.08 g, 3.27 9 10-4 mol) was

added to the mixture, which was heated until a homogeneous solution was formed.

Then, the solution was stirred for 12 h at 120 �C. The viscous solution was

precipitated in 30 mL of methanol. The white solid was filtered off and dried to give

0.10 g (65 %) of white PAI2. This polymerization was also repeated using

dibutyltin dilaurate (DBTDL), Py and without catalyst, respectively. In this way, the

reaction catalysts were optimized and the reaction with other diisocyanates

performed in the presence of DBTDL as the catalyst.

Results and discussion

Polymerization reactions

Monomer 1 was synthesized according to the reported procedure [36]. The

polymerization reactions of monomer 1 with diisocyanates were carried out via

conventional solution polymerization techniques in TBAB as a molten IL in the

presence of different catalysts and without catalyst to afford PAI1-PAI7

(Scheme 1).

The results obtained are summarized in Table 1. The polymers PAI1-PAI7 were

obtained in moderate yields and inherent viscosities.

The replacement of a volatile and toxic organic solvent in the polymerization

with a nonvolatile solvent will reduce losses through evaporation, therefore, it will

be considered as green process. Table 2 shows the PAIs were prepared in volatile

organic and more toxic media such as NMP, which is not suitable, so using

nonvolatile TBAB instead of NMP will be preferred [35].

Polym. Bull.

123

Structural characterization of PAIs

The resulting polymers were characterized by 1H-NMR, FT-IR and elemental

analyses. The FT-IR spectra of resulting PAIs showed the presence of the

characteristic peaks of amide and imide functions and the absence of the original

peaks arising from the COOH and NCO groups in the corresponding diacid 1 and

N

O

O

COOH

HHO

O

R NCO

N

O

O H

O

O

NHRHN

C

CH2CH3H3C

CH

H3C CH2CH3

CH2 or

CH3

H3C CH3

CH2

CH3

or CH2

OCN

H

R=

n

or

1

6

a b c d

PAI1-PAI7

TBAB

2a-2d

Scheme 1 Polycondensation reactions of monomer 1 with different diisocyanates

Table 1 Reaction conditions for the polymerization of monomer 1 with different diisocyanates in the

presence of TBAB as an ionic liquid and some physical properties of PAI1–PAI7a

Polymer Diisocyanate Catalyst Reaction time Yield (%) g (dL/g)ba½ �25

Dc

PAI1 MDI No catalyst A 88 0.35 -4.5

PAI2 MDI TEA A 65 0.42 -11.3

PAI3 MDI Py A 72 0.35 -9.3

PAI4 MDI DBTDL A 80 0.45 -6.5

PAI5 IPDI DBTDL A 85 0.12 -15.3

PAI6 TDI DBTDL A 68 0.28 -1.68

PAI7 HDI DBTDL A 72 0.26 -12.1

A 12 h 120 �Ca All of these polymers were precipitated in methanolb Measured at a concentration of 0.5 g/dL in DMFc Measured under the same conditions as inherent viscosity

Polym. Bull.

123

diisocyanates precursors. For example, the FT-IR spectrum of PAI1 exhibited

characteristic absorption bands of the amide group around 3,322 (N–H stretching)

and 1,667 cm-1 (C=O stretching), together with peaks at 1,786 (C=O asymmetric

stretching), 1,720 (C=O symmetric stretching), 1,416, 1,383 (CNC axial stretching),

1,316, 1,111 (CNC transverse stretching), 734 (CNC out-of-plane bending) cm-1

for the imide heterocyclic ring [35]. The 1H-NMR spectrum of PAI4 and PAI7

showed the peaks that confirm their chemical structure [35].

PAI4

1H-NMR peaks (ppm): 0.83 (distorted dd, 3H, CH3), 1.07 (distorted d, 3H, CH3),

1.50-1.90 (m, 2H, CH2), 3.35 (m, 1H, CH), 3.78 (s, 2H, CH2), 4.47 (d, 1H,

J = 6.99 Hz, CH), 7.11 (d, 4H, J = 7.36 Hz, MDI), 7.35 (d, 4H, J = 7.36 Hz,

MDI), 7.70–8.40 (m, 3H, Ar–H, TMA), 8.70 (s, br, 1H, NH), 10.50 (s, 1H, NH).

PAI7

1H-NMR peaks (ppm): 0.74–0.95 (m, 6H, CH3), 1.25–1.75 (m, 10H, CH2),

2.85–2.90 (distorted t, 4H, CH2), 3.35 (m, br, 1H, overlaped with H2O, CH), 4.40

(d, br, 1H, CH), 7.85–8.35 (m, 3H, Ar–H, TMA), 8.00–8.85 (s, br, 2H, NH) [35].

The elemental analyses results are also in good agreement with calculated

percentages of carbon, hydrogen and nitrogen contents in the polymer repeating

units (Table 3).

The thermal behaviors of resulted PAIs were examined by TGA (Fig. 1). Table 4

gives the results related to the PAI1 and PAI6. The initial weight loss of PAI1

and PAI6 occurred at 250 and 180 �C, respectively. The 5 and 10 % weight loss

(T5, T10) of the polymers and residue at 800 �C (char yield) and also limiting oxygen

index (LOI) based on Van Krevelen and Hoftyzer equation used as criterions for

their thermal stability by TGA [37] (Table 4)

Table 2 Reaction conditions for the polymerization of monomer 1 with different diisocyantes and some

physical properties of PAIs in NMP as a solvent [35]

Polymer Diisocyanate Catalyst Yield (%) g (dL/g) a½ �25D

d

PAIa MDI DBTDL 70 0.30a –

PAIb MDI Py 64 0.36b –

PAIc MDI No catalyst 77 0.37b -5.6

PAId MDI TEA 53 0.44b –

PAIe IPDI TEA 99 0.27b -1.2

PAIf TDI TEA 46 0.24b -6.8

PAIg HDI TEA 57 0.25c –

a Measured at a concentration of 0.5 g/dL in DMF containing 0.2 % W/W LiCl at 25 oCb Measured at a concentration of 0.5 g/dL in DMFc Measured at a concentration of 0.5 g/dL in DMF containing 0.4 % W/W LiCl at 25 oCd Measured under the same conditions as inherent viscosity

Polym. Bull.

123

LOI ¼ 17:5þ 0:4 CR CR ¼ char yield:

From these data, it is clear that all of the PAIs are rather thermally stable which

could be due to existence of various linkages such as imide bonds and aromatic

moieties in the polymer backbones. These polymers have LOI values calculated based

on their char yield at 800 �C which are higher than 35. On the basis of LOI values, such

polymers can be classified as self-extinguishing macromolecules. According to

Table 4, it is clear that the PAI1 has higher thermal stability than PAI6. It could be

pertained to aromatic and rigid structure of diisocyanate MDI. According to previous

Table 3 Elemental analysis of PAIs

Polymer Formula C % H % N %

PAI1 C28H25N3O4 Calcd. 71.32 5.51 9.18

457.52 g/mol Found 70.63 5.35 10.02

PAI7 C21H27N3O4 Calcd. 65.44 7.06 10.90

385.46 g/mol Found 64.78 6.92 11.12

Fig. 1 TGA thermogram of PAI1 under a nitrogen atmosphere at heating rate of 10 �C/min

Table 4 Thermal properties of PAI1 and PAI6

Polymer T5 (oC)a T10 (oC)b Char yield(%)c LOId

PAI1 325 395 44 35.1

PAI6 300 355 45 35.5

a Temperature at which 5 % weight loss was recorded by TGA at heating rate of 10 �C/min under a

nitrogen atmosphereb Temperature at which 10 % weight loss was recorded by TGA at heating rate of 10 �C/min under a

nitrogen atmospherec weight percentage of material left undecomposed after TGA analysis at a temperature of 800 �C under

a nitrogen atmosphered Limiting oxygen index (LOI) evaluating char yield at 800 �C

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data [35], it is clear that PAIs were synthesized in molten TBAB as media of reaction

have higher T5, T10 and char yield than PAIs in NMP as a solvent.

Solubility of PAIs

The solubility properties of PAIs were studied in different solvents. Polymers could

be dissolved in the amide type solvents, such as NMP, DMAc, and DMF. They are

insoluble in solvents such as water, methanol, chloroform and dichloromethane. The

PAI1–PAI7 showed the better solubility than the others which were prepared by the

classical method [35].

Morphology of polymers

The surface morphology of the polymers was observed by FE-SEM and the images are

shown in Fig. 2. The PAI4 and PAI6 exhibit relatively a spherical shape with the

Fig. 2 FE-SEM micrographs of PAI4, PAI6 and PAI7 before and after sonication; a, a’ PAI4 beforesonication; b, b’ PAI4 after sonication; c, c’ PAI6 before sonication; d, d’ PAI6 after sonication; e, e’PAI7 before sonication; f, f’ PAI7 after sonication

Polym. Bull.

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diameter about 50 nm (Fig. 2a–d’). Different morphology was observed for PAI7: a

cylindrical-like nanostructure with a diameter below 78 nm (Fig. 2e–f’). These

cylindrical-like shapes possess an in-plane random orientation. As shown in Fig. 2,

nanoscale morphology of the obtained polymers was clearly observed. It is interesting

to mention that the sonication of polymer samples provided valuable effect on polymer

nanostructure (size and shape). All of the obtained polymers were sonicated for 1 h,

and for all the macromolecules changes such as shapes and sizes were observed.

Generally, the size of polymer particles became smaller after sonication process

(Fig. 2). These phenomena will give higher surface area for the nanostructured

macromolecules for further applications. Effective fabrication of ordered nanostruc-

tured macromolecules is a precondition for the mixing of nanostructured materials into

functional building blocks and a variety of functional units/nanodevices of realistic

significance [32]. Nanostructured polymers have been developed for a wide range of

advanced uses such as medical application (drug delivery devices), conducting wires,

catalysts support and optoelectronic devices [38].

Conclusions

A series of optically active PAIs with inherent viscosities in the range of

0.12–0.45 dL/g were synthesized by the direct polycondensation of chiral

N-trimellitylimido-L-isoleucine (1) as a diacid monomer with different diisocya-

nates according to isocyanate route under conventional solution method in molten

TBAB as the reaction medium. These polymers displayed good solubility and high

thermal stability compared to the PAIs which were synthesized in conventional

solvents. Morphology study of the resulting PAIs showed the nanostructured

phenomena of these polymers by FE-SEM technique. Owing to the naturally

occurring amino acids as biological chiral resources, it is predictable that these

amino acid-derived polymers could be biodegradable, and thus, considered as eco-

friendly polymers. These compounds are good candidates for the use in the various

applications such as the chiral medium in asymmetric synthesis and chiral stationary

phases in high-performance liquid chromatography for the resolution of racemic

mixtures. Furthermore, due to their nanoscale structure, they could be applied as

fillers for the formation of nanocomposite materials.

Acknowledgments We wish to express our gratitude to the Research Affairs Division Isfahan

University of Technology (IUT), for financial support. Further, financial support from National Elite

Foundation (NEF) and Center of Excellency in Sensors and Green Chemistry Research (IUT) is gratefully

acknowledged.

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