Distinction between two molecular mechanisms for deformation of glassy amorphous polymers

Abstract

During deformation of glassy amorphous polymers the rotation of rigid chain segments around skeletal bonds is restricted simultaneously by configurational (intramolecular) and chain-chain (intermolecular) energy barriers. These barriers are modeled in a self-consistent manner for six polymers by use of an approximate analytical treatment. The comparative contribution of these barriers to the small-strain modulus of the bulk solid is used as a basis for distinguishing between two mechanisms of stiffening. With polycarbonate, polyphenylene oxide, and an aromatic polyimide, in which the rigid chain segment is relatively long, the modulus derives its value primarily from the resistance to displacements across chains due to intermolecular barriers. With vinyl polymers such as polystyrene, poly(vinyl chloride) and poly(methyl methacrylate), however, in which the rigid chain segment is short, resistance to displacements along the main chain due to intramolecular barriers contributes equally significantly to the modulus. Our calculations also show that the length of the rigid chain segment, acting as a mechanical moment arm, affects the resistance to intramolecular displacements much more than does the height of the rotational energy barrier.