Shape-Memory Actuators on the Basis of Binary and Ternary Blends of Polyethylenes

摘要

The unconstrained reversible shape-memory effect (URSME), which appears as anomalous elongation of pre-deformed sample during non-isothermal crystallization and the following contraction during heating in the stress-free conditions, was recently discovered in crosslinked semi-crystalline polymers. The preconditions for the appearance of the URSME are (1) deformation of molten covalent network, (2) its cooling under load below the lowest crystallization temperature, and (3) subsequent unconstrained partial melting of crystalline phase. The goal of present work was to test the binary and ternary crosslinked blends of high-density polyethylene with ethylene-1-octene copolymers with two degrees of branching for the URSME. The thermal analysis revealed the considerable enthalpic effects of crystallization and melting of the blend components in wide temperature range, indicating the fundamental possibility of choosing the temperatures, at which partial melting of crystals can be achieved. The SM investigation was carried out by means of thermo-mechanical tests, which implied preliminary melting of crystalline phase/-s with lower thermal stability, while the high-temperature crystalline phase was crystallized storing a part of programming deformation. Obtained results have shown the distinct manifestation of the URSME resulting in about 10-13% of strain increment during crystallization. It is concluded that the most pronounced URSME is observed when the high-temperature crystalline phase is enough elastic and does not prevent the oriented growth of crystals with lower crystallization temperature. At the same time, blend component with higher crystallization/melting temperature has to possess enough high crystallinity and to be well coupled with other phase/-s in order to store a deformation, which causes driving entropy-elastic forces enough to cause exactly oriented crystallization of low-temperature blend component/-s. Undoubtedly, received results enables creating of a new class of actuators on the basis of polymer blends.