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The thermal expansion behavior for various aromatic polymides was investigated. Usally polymers, including polyimides, have high thermal expansion coefficients (3–6 × 10−5 K−1), compared with metals and ceramics. However, there are some polyimides which have very low thermal expansion coefficients below 1 × 10−5 K−1. This property was observed for the polymides obtained from pyromellitic dianhydride or 3,3′ 4,4′‐biphenyltetracarboxylic dianhydride and aromatic diamines which were constituted of only benzene rings fused at para positions. It was proposed that their low thermal expansion coefficient related to the linearity in their polymer molecular skeltons.

SYNOPSISTo clarify the relationship between crosslinking density and physical properties of phenolformaldehyde novolac cured epoxy resin and factors governing their physical properties, we studied various properties of cured resins having different crosslinking densities. The resins were prepared with various curing accelerators and raw epoxy resins having different molecular weights. We found that as the crosslinking density of a cured resin increases, glass transition temperature ( Tg) rises and the relaxation time becomes longer. Furthermore, in the rubbery region, the coefficient of linear thermal expansion drops and the elastic modulus become larger, while, in the glassy region, the coefficient of linear thermal expansion, specific volume, water absorption, diffusion coefficient, and permeability all increase but the elastic modulus becomes smaller. The WLF analysis on the relaxation behaviors of typical cured resin showed that cured resin with a higher crosslinking density decreases in the fractional free volume. This behavior is completely opposite from the relationship predicted from the temperature dependency of specific volume. While the coefficient of thermal expansion of free volume decreases as the crosslinking density increases for the cured resin, it coincides well with the tendency predicted from the difference in coefficient of cubic thermal expansion in the rubbery and glassy regions of each cured resin. That the free volume obtained from WLF analysis shows a relationship opposite to the predicted free volume as based on the temperature dependency of specific volume is explained as follows: Namely, the free volume obtained from the WLF analysis is a hole free volume V , which contributes to fluidity and v h decreases with the crosslinking density. On the other hand, the free volume predicted from the specific volume is a sum of the interstitial free volume Vi and v h . V, increases with the crosslinking density and this Vi increase exceeds the decrease of v h . Therefore, the free volume predicted from the specific volume increases with the crosslinking density. Consequently, the influence of free volume on the relationship between the crosslinking density and physical properties of cured resin can be interpreted as follows. As the crosslinking density increases on cured resins, Tg rises, the relaxation time is lengthened, and the coefficient of linear thermal expansion becomes smaller in the rubbery region because, as the crosslinking density increases, v h decreases. Since crosslinking density increases on cured resins, the coefficient of linear thermal expansion, water absorption, diffusion coefficient, and permeability become larger, and the elastic modulus becomes smaller in the glassy region because, as the crosslinking density increases, Vi increases and, accordingly, molecular chain packing becomes looser; i.e., the specific volume increases. 0 1993 John Wiley & Sons, Inc.

The relationships, between chemical structures of various aromatic polyimides and their thermal expansion coefficients, were investigated and the properties of low thermal expansion polyimides were elucidated. Such low values were observed for polyimides obtained from pyromellitic dianhydride or 3,3'.4,4'-biphenyltetracarboxylic dianhydride and aromatic diamines. which included only benzene or pyridine rings fused at para-positions without a flexible linkage. It was proposed that these low thermal expansion coefficients were related to the lineerity of their polymer molecular skeletons. In particular, PIQ-Ll 00 (Hitachi Chemical Co. Ltd) i s one such low thermal expansion polyimide and, it has excellent mechanical properties, thermal stability, and low absorbed moisture content.