Dynamic mechanical properties of polyimide/poly(silsesquioxane)-like hybrid films

Abstract

Polyimide/poly(silsesquioxane)-like (PI/PSSQ-like) films have three-dimensional structure with linear PI blocks and a crosslinked PSSQ-like structure. They have higher thermal stability and char yields than pure PI from 4,4'-diaminodiphenyl ether and 3, 3'-oxydiphthalic anhydride (ODPA). In a series of X-PIS [PI modified with p-aminophenyltrimethoxysilane (APTS), where X is the molecular weight of each PI block] hybrid films, decreasing the PI block chain length enhances the storage modulus, tensile modulus, and glass-transition temperature (T-g) but reduces the alpha -relaxation damping peak intensity, density, and elongation. The change in the former three properties may be caused by an increase in the crosslinking density and rigidity of the network structure. The changes in the next two properties are caused by an increase in the free volume or the PI interblock separation and decreases in the interblock PI chain interaction. The change in the last property (i.e., a decrease in elongation) is related to the increase in the rigidity of the network structure. The activation energy of the a transition depends on the chain length of the PI block. A maximum value is reached at the chain length of 10,000 because of two different factors: crosslinking density and free volume. In a series of X-PIS-y-PTS [X-PIS modified with phenyltrimethoxysilane (PTS), where y is the weight-ratio percentage of PTS to APTS-polyamic acid] films with a constant PI block length, the storage modulus, tensile modulus, T-g, density, and alpha -relaxation damping peak intensity decrease with the PTS content. This could be because of the increase of the PSSQ-like domain size with the PTS content, which leads to the introduction of more free volume or interblock separation and to a decrease in the interblock chain interaction force. (C) 2001 John Wiley & Sons, Inc.