Molecular Composites of Rodlike/Flexible Polyimides
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MOLECULAR COMPOSITES OF RODLIKE/FLEXIBLE POLYIMIDES S. R. ROJSTACZER, D.Y. YOON, W. VOLKSEN AND B.A. SMITH IBM Research Division, Almaden Research Center, San Jose, CA 95120 ABSTRACT Mixtures of a rodlike and a flexible polyimide were prepared by solution-blending of the respective poly(amic alkyl ester) and poly(amic acid), followed by solvent evaporation and thermal imidization. The size scale of the phase separation, as measured by light scattering, is primarily set during the demixing of the precursor polymers, with no significant coarsening observed due to the imidization performed at 400'C. The observed variation of the domain size with parameters such as composition, molecular weight and film thickness is discussed in terms of the miscibility of the precursor polymers as well as the thermal history to which these were exposed. INTRODUCTION Mixtures of rodlike and flexible polyimides have the potential to achieve the desired properties for applications in microelectronic packaging by combining the required thermal and mechanical properties characteristic to the rodlike component along with enhanced adhesion strength provided by the flexible polyimide. The use of such mixtures as thin film coatings, however, requires that any heterogeneity on composition has to be kept on a scale well below the micron level, raising the question of the feasibility of polyimide based molecular composites. Of particular interest in such systems is the effect of the imidization carried out at high temperatures on the morphology of the precursor blends. Yokota et al. [1] studied mixtures of polyimides prepared by imidization of mixtures of different poly(amic acid) (PAA) precursors. Feger [2] and Ree et al. [3] showed, however, that mixtures of poly(amic acid)s can lead to the formation of copolymers rather than physical blends of homopolymers. The exchange reaction leading to the copolymer formation occurs due to the fact that the poly(amic acid) coexists with a small amount of anhydride and amine form through chain scission and recombination. The exchange reaction takes place either slowly at room temperature or during a slow curing procedure. Recently, Ree et al. [4] showed that polyimide/polyimide blends can be formed from stable precursors mixtures if at least one of the poly(amic acid)s is replaced by its alkyl ester derivative. The poly(amic alkyl ester) (PAE) form provides a more stable polyimide precursor system than its PAA counterpart by eliminating the possibility for reequilibration reactions. In the present paper, the polyimide blend system introduced by Ree et al. [4] is used to study the relationship between the miscibility of the precursor mixtures, the drying and curing conditions and the final morphology of the imidized mixtures.
Mat. Res. Soc. Symp. Proc. Vol. 171. ©1990 Materials Research Society
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EXPERIMENTAL Fig.l shows the chemical structure of 6F-BDAF and PMDA-PDA polymers synthesized for the present study. 6F-BDAF polyamic acids were prepared via classical solution polycondensation of sublimed hexa
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