Polyimide/montmorillonite nanocomposites based on thermally stable, rigid-rod aromatic amine modifiers
Introduction
Polymer/inorganic nanocomposites based on the intercalation of polymer chains into organically modified layered silicates (OLS) form a class of nanocomposites that have recently received considerable attention [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], because they often exhibit superior physical, mechanical and thermal properties to conventional mineral-filled composites or unfilled polymers. These performance improvements depend greatly on the distribution, arrangement of OLS and synergism between the layered silicate and the polymer.
Clay is a type of layered silicate, and the most commonly used clay in the preparation of polymer/clay nanocomposites is montmorillonite (MMT). MMT is composed of silicate sheets of about 1 nm thickness with adsorbed exchangeable alkali or alkaline earth cations such as Na+, K+, Ca2+. Before the preparation of polymer/MMT nanocomposites, modification is generally required through ion exchange reaction between organic cations and inorganic cations to render hydrophilic MMT more organophilic and to increase interlayer spacing of MMT, aiming at providing a better physical and chemical environment for the polymer.
The commonly used organo-modification agents are long carbon-chain alkyl ammonium salts. It has been widely accepted that the interlayer spacing of OLSs depends greatly on the length of the carbon chain [1]. Although these modification agents have been gaining significant success in the preparation of polymer/MMT nanocomposites, their common shortcoming is the poor thermal stability. Xie et al. [20], [21] have studied the thermal stability of MMT modified by long carbon-chain alkyl quaternary ammonium ions using TGA-MS and found that the on-set decomposition temperature of the resultant OLSs was approximately 180 °C. Unfortunately, the preparation and processing of most of the polymer/OLS nanocomposites require a temperature much higher than this value, and the thermal decomposition of the long carbon-chain alkyl quaternary ammonium salts is inevitable. Delozier et al. [22] observed that during the preparation of polyimide/clay nanocomposites, the decomposition of the organic modifier led to the 'collapse of the clay particles into larger agglomerates’. This may affect the morphological structure, properties and service life of nanocomposites. Therefore, the thermal stability of organic modifier may pose significant effect on the preparation, performance and application of nanocomposites. However, little attention has been paid to improve thermal stability of the organic modifier while making nanocomposites so far.
Polyimide is considered to be one of the most important engineering plastics, and has outstanding mechanical and electrical properties, especially at elevated temperature, being an attractive candidate for many applications in electronics and photonics [29]. Recently, the studies on polyimide have been focused on the improvement in the mechanical properties, size stability and solvent resistance to satisfy its application field. The study on polyimide/MMT nanocomposites was first reported by Yano et al. [30], [31]. Agag et al. [17] have studied polyimide/MMT nanocomposites based on BPDA/PDA and PMDA/ODA polyimide. Our group [24] has studied the morphology, solvent resistance and thermal properties of poly(etherimide)/MMT nanocomposites and found that the introduction of MMT led to an obvious decrease in the solvent uptake. These property improvements were based on the good dispersion of MMT and strong interaction between MMT and polyimide matrix. In all these studies, aliphatic amines were used as the modification agents. Very recently, works on the design of new structure organo-modifier have been reported [26], [27].
In this paper, we report our study on the organo-modification of MMT using thermally stable aromatic amines. The amines contain thermally stable phenyl structure and imide moiety and, unlike aliphatic amines, have very rigid chemical structure (Scheme 1 (a) and (b)). These organo-modified MMTs were also used to prepare polyimide/MMT nanocomposites. They may possess better compatibility with aromatic polymers such as polyimide because of their structure similarity. Their properties were also studied.
Section snippets
Materials
Sodium montmorillonite (Na-MMT) with a cation exchange capacity (CEC) of 100 meq/100 g was supplied by the Institute of Chemical Metallurgy, Chinese Academy of Sciences. The average particle size is 50 μm. N-[4-(4′- aminophenyl)]phenyl phthalimide (OM-l) and N-[4-(4′-aminophenoxy)]phenyl phthalimide (OM-m) were synthesized in our lab. 1-Hexadecylamine (OM-16C, C16H33NH2, lab reagent) was purchased from Merck. 4,4′-Diamino-3, 3′-dimethyldiphenylmethane (MMDA) was synthesized by a reaction of o
Organo-modification of MMT
As previously mentioned, the organo-modification of MMT is an important step in the preparation of polymer/MMT nanocomposites and primary aliphatic amines such as 1-hexadecylamine and its quaternary ammonium salt were commonly used organic modifiers. IR and XRD were used to verify the organic modifiers designed by us have the same efficacy as the commonly used modifiers. Fig. 1 is the IR spectra of MMT, MMT-16C, MMT-l and MMT-m. The absorption bands at 1038 and 1090 cm−1 were characteristic of
Conclusions
Two thermally stable, rigid-rod aromatic amines have been synthesized and used as organo-modifiers for MMT. The MMT treated by them exhibited higher basal spacings than that treated by 1-hexadecylamine. The MMT layers were basically exfoliated in the PI/MMT nanocomposites prepared by in-situ polymerization when the MMT content was below 3 wt%. The MMT content influenced the properties of PI/MMT nanocomposites significantly. When the MMT content was below 3 wt%, the PI/MMT nanocomposites were
Acknowledgements
The authors would like to thank the Ministry of Education of China (Kuashiji Scholars' Project) and Science and Technology Commission of Shanghai Municipal Government (Nano-Project) for their financial support.
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