Changes of Crystal Structure and Morphology during Two-Way Shape-Memory Cycles in Cross-Linked Linear and Short-Chain Branched Polyethylenes

Abstract

The present work comprehensively describes the formation of crystal structure and morphology of cross-linked linear and short-chain branched polyethylenes during their nonisothermal crystallization under constant mechanical load. The crystalline phase of linear as well as medium branched polyethylenes with about 30 CH3/1000C—formed as a result of the crystallization process under load and accompanied by an anomalous elongation—is represented by lamellae oriented perpendicular to the stretch direction with tilted folded chains. In contrast, highly branched polyethylene with about 60 CH3/1000C processed under similar conditions contains only small crystallites, whose c-axis is oriented parallel to the applied force. Experimentally determined crystallinity, type of crystalline structure as well as size and orientation of the crystallites were compared with theoretical predictions got by modeling the two-way shape-memory (SM) behavior. Qualitative and quantitative characteristics of the two-way SM effect and the experimental curves of temperature dependent strain as well as the features of generated crystalline structures are in good agreement with the theory.