Miscibility studies of polyimide/poly(ether imide) molecular composites T. Fukai, J. C. Yang and T. Kyu* Institute of Polymer Engineering, The University of Akron, Akron, Ohio 44325, USA and S. Z. D. Cheng, S. K. Lee, S. L. C. Hsu and F. W. Harris Institute of Polymer Science, The University of Akron, Akron, Ohio 44325, USA (Received 31 May 1991; revised 26 September 1991; accepted 28 September 1991 ) A new group of miscible mixtures (molecular composites) has been studied by means of differential scanning calorimetry, dielectric relaxation and dynamic mechanical techniques. Three kinds of rigid aromatic polyimides ( PI ) and a commercial-grade poly (ether imide ) ( PEI ) were mixed by dissolving them in m-cresol and subsequently coagulating in methanol. The mixtures show a single glass transition temperature (Ts) in all compositions in which the Tg increases with increasing PI content. These T u values are reproducible in repeated heating cycles, suggesting the true miscibility of the blends. This miscibility may be attributed to the similarity of imide structure between the rigid PI and PEI matrix. The bulk PI/PEI indeed shows improved modulus and tensile strength as compared to the pure PEI matrix. (Keywords: miscibility; molecular composites; polyimide; poly(ether imide); differential scanning calorimetry; dielectric relaxation; dynamic mechanical analysis) INTRODUCTION Molecular composites 1-3 have received considerable attention owing to their potential for development of new polymeric materials that may be employed in the area traditionally dominated by metals and ceramics. Conceptually, a molecular composite is a mixture of rigid-rod and flexible-coil molecules, in which rigid-rod molecules reinforce a flexible matrix in a manner similar to a chopped-fibre-filled composite. The optimum reinforcement would occur in composites if rigid rods and flexible coils were mixed at a molecular level. According to Flory 4, such a molecular-level dispersion is very difficult to achieve because of the small entropy of mixing and high orientability of rigid-rod molecules. Most studies on molecular composites also indicate the immiscible character of the mixtures 3'5'6. In this study, rigid aromatic polyimides synthesized in our laboratory are used as the reinforcing molecules 7. These polyimides, having excellent chemical and thermal stabilities, could be dissolved in a limited type of solvents such as m-cresol. Considerable attention has been directed to the synthesis of polyimides 7's, but there are limited studies addressing polyimide-based molecular composites 9. Three kinds of aromatic polyimides (PI) of different chemical moieties were mixed with a commercial- grade poly(ether imide) (PEI). PEI was chosen as a matrix material primarily because of its similarity in chemical structure to rigid-rod polyimides and also because of its availability on a commercial basis. The miscibility behaviour of PI/PEI has been explored based on differential scanning calorimetry (d.s.c.), *To whom correspondenceshould be addressed dynamic mechanical and dielectric realxation methods. Subsequently, tensile properties of these molecular composites are evaluated. EXPERIMENTAL The chemical structures of the three polyimide moieties and their corresponding inherent viscosities are shown in Table 1. The synthesis procedure of the PIs was reported elsewhere 7. These polyimides contain a common diphenylpyromellitic dianhydride (DPPMDA) unit, the other units being different in chemical structure, namely 4,4'-diaminodiphenyl ether (oxydianiline) (ODA), 6,6'- perfluoromethylbenzidine (PFMB) and 3,3'-dimethoxy- benzidine (DMB). The polyimide DPPMDA-ODA has an ether bond in the main chain, and therefore it probably has greater chain flexibility relative to the other two. There is no identifiable T~ in all three PIs owing to their chain rigidity. Degradation is suspected to occur before Tg could be detected via thermal analysis. The flexible matrix used in this study is poly(ether imide ) (PEI), namely poly (2,2'-bis ( 3,4-dicarboxyphenoxy ) phenylpropane-2-phenylenebisimide) (Ultem 1000, General Electric), which is an amorphous thermoplastic with a Tg around 215°C. The number- and weight-average molecular weights were 12 000 and 30 000, respectively1 o. Various blends were prepared by dissolving PI/PEI in m-cresol at 130°C for 5 h. The concentration of polymer solutions was 2 wt%. The solutions of various PI/PEI compositions were coagulated in methanol. The precipitates were washed copiously with methanol to remove residual solvent and then dried at 150°C in a conventional vacuum oven for over a week. Additionally, PI/PEI films were made by casting the ternary solutions 0032-3861/92/173621-06 © 1992 Butterworth-Heinemann Ltd. POLYMER, 1992, Volume 33, Number 17 3621