Elsevier

Polymer

Volume 46, Issue 10, 25 April 2005, Pages 3343-3354
Polymer

Polyamide-6,6/in situ silica hybrid nanocomposites by sol–gel technique: synthesis, characterization and properties

https://doi.org/10.1016/j.polymer.2005.02.104Get rights and content

Abstract

The organic–inorganic hybrid nanocomposites comprising of poly(iminohexamethyleneiminoadipoyl), better known as Polyamide-6,6 (abbreviated henceforth as PA66), and silica (SiO2) were synthesized through sol–gel technique at ambient temperature. The inorganic phase was generated in situ by hydrolysis–condensation of tetraethoxysilane (TEOS) in different concentrations, under acid catalysis, in presence of the organic phase, PA66, dissolved in formic acid. Infrared (IR) spectroscopy was used to monitor the microstructural evolution of the silica phase in the PA66 matrix. Wide angle X-ray scattering (WAXS) studies showed that the crystallinity in PA66 phase decreased with increasing silica content. Atomic force microscopy (AFM) of the nanocomposite films revealed the dispersion of SiO2 particle with dimensions of <100 nm in the form of network as well as linear structure. X-ray silicon mapping further confirmed the homogeneous dispersion of the silica phase in the bulk of the organic phase. The melting peak temperatures slightly decreased compared to neat PA66, while an improvement in thermal stability by about 20 °C was achieved with hybrid nanocomposite films, as indicated by thermogravimetric analysis (TGA). Dynamic mechanical analysis (DMA) exhibited significant improvement in storage modulus (E′) for the hybrid nanocomposites over the control specimen. An increase in Young's modulus and tensile strength of the hybrid films was also observed with an increase in silica content, indicating significant reinforcement of the matrix in the presence of nanoparticles. Some properties of the in situ prepared PA66-silica nanocomposites were compared with those of conventional composites prepared using precipitated silica as the filler by solution casting from formic acid.

Introduction

Inorganic–organic hybrid nanocomposites using the sol–gel process where the inorganic phase is grown in situ is being actively pursued globally [1], [2], [3]. The shift in emphasis from the traditional practice of mechanically blending the reinforcing fillers into a polymeric matrix to the sol–gel process is due mostly to the subtle control over morphology and/or surface characteristics of the growing inorganic phase in the polymer matrix by control of various reaction parameters like pH, concentration, temperature, etc. More importantly, unlike the traditional composites, which have macroscale domain size varying from micrometer to millimeter scale, the inorganic–organic hybrids have domain sizes varying typically from 1 to 100 nm [4]. Thus, the inorganic–organic hybrids are usually optically transparent, even though microphase separation may exist. Sol–gel hybrid preparation mostly centers on the growth of the inorganic phase from the hydrolysis–condensation of alkoxysilanes like TEOS in a solution containing the organic polymer. The mechanism of hydrolysis–condensation of TEOS is well known [5]. Scheme 1 depicts the formation of the three-dimensional silica network arising as an outcome of the hydrolysis and condensation reactions of TEOS. So far, many hybrids have been prepared in this way using poly(vinyl acetate) [6], [7], poly(methyl methacrylate) [8], [9], poly(vinyl pyrrolidone) [9], poly(ethylene oxide) [10], poly(dimethylsiloxane) [11], Nafion® [12], [13], poly(vinyl alcohol) [14] and several other polymers. From our laboratory, we have reported several nanocomposites including those prepared by the sol–gel technique [15], [16], [17], [18].

The novelty of this work lies in the fact that the silica nanophase has been grown in situ within the Polyamide-6,6 (PA66) matrix probably for the first time. The present paper describes a comparison of spectroscopic, morphological, thermal, mechanical, dynamic mechanical and water absorption properties of silica composites with those of PA66 and an attempt has been made to explain the properties with the structure of the nanocomposites. For comparison, optical and morphological properties of precipitated silica-PA66 composites prepared by solution blending technique have been reported. It is worth mentioning that nanocomposites based on polyamides (particularly polyamide-6) and nanoclay has commercial potential [19].

Section snippets

Preparation of PA66-silica nanocomposites

A commercial grade of PA66 (Zytel 101L, DuPont, India) was dissolved in 85% formic acid (synthesis grade, Merck Ltd., India) to prepare a 10 wt% PA66 solution. Different amounts of TEOS (Acros Organics, USA, density=0.93) were carefully added to this 10 wt% PA66 solution with vigorous stirring with a magnetic stirrer bar at room temperature (30 °C). No water was added externally, as water was already present in the formic acid to the extent of 12–15%. The stirring was carried out for 1 h under

IR studies

The IR spectrum of N66T0 (neat PA66) and N66T5 are shown in Fig. 1(a) with all the characteristic peaks labeled. Neat PA66 films show the characteristic peaks at 3320 cm−1 (N–H stretch), 2938 cm−1 (CH2 stretch), 1640 cm−1 (C6-point double bondO stretch, amide I), 1540 cm−1 (in-plane N–H deformation, amide II), 1370 cm−1 (CN stretch+in-plane NH deformation, amide III), 1200 cm−1 (amide III coupled with hydrocarbon skeleton) and 934 cm−1 (C–CO stretch, crystalline band) respectively as reported previously by other

Conclusions

PA66/silica hybrid nanocomposites were prepared by the sol–gel reaction between TEOS and PA66 matrix. IR studies revealed the formation of both linear and network silica growth structures. Dimensions of these silica structures were less than the optical wavelength and thus the hybrid films had optical clarity comparable to neat PA66 film. AFM studies further confirmed that one of the dimensions of the silica structures was within 100 nm and Si mapping revealed the homogeneity of the hybrid

Acknowledgements

We acknowledge the financial assistance provided by DAE, BRNS, Mumbai vide sanction no. 2002/35/7/BRNS/172. We are indebted to Mr Kausick Auddy and Mr Anirban Ganguly for the UV–Vis and AFM measurements. The AFM was acquired via an equipment grant from DST, New Delhi and MHRD, New Delhi, India.

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