Grain boundaries in extended-chain polymers: Theory and experiment

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Abstract
A general geometrical classification scheme for grain boundaries in extended-chain polymers is presented. The analysis is an extension of a scheme originally presented by Bevis to describe deformation twins in polymers. Bevis recognized the importance of the orientation of the covalently bonded chain backbone within the crystallite, and referred to boundaries as ‘chain invariant’ or ‘chain rotation’ depending on whether chains in adjacent domains were parallel or not. We consider an important additional parameter: the orientation of the grain boundary plane. We classify those grain boundaries which are parallel to the two chain directions as ‘lateral’, and those which are not ‘axial’. These two parameters lead directly to four types of grain boundaries which are, in order of increasing energy: lateral chain invariant, lateral chain rotation, axial chain invariant, and axial chain rotation. Experimental evidence for these different types of boundaries are presented from High Resolution Electron Microscopy (HREM) studies of the rigid-rod polymer poly(paraphenylene benzobisoxazole) (PBZO or PBO). Possible molecular mechanisms for grain boundary motion are also discussed.