The two-way shape memory behaviour of crosslinked poly(ε-caprolactone) systems with largely varied network density

Abstract

The two-way shape memory behaviour of semicrystalline networks was investigated on systems based on poly(ε-caprolactone) featuring significantly different network architecture. Crosslinked poly(ε-caprolactone)s were prepared by thermal curing from methacrylic end-capped linear chains having various methacrylation degrees. By conveniently reducing the methacrylation degree, the crosslink density of cured materials was varied over a range of one order of magnitude, leading to comparable changes in the material compliance in the rubbery region, but only to moderate variations in melting and crystallization temperatures (T_m and T_c) and in the crystallinity content. When subjected to constant non-zero stress and to cooling–heating cycles from above T_m to below T_c, the materials undergo a reversible two-way elongation–contraction effect, whose extent depends on material structure and applied stress. The structural changes in the crystalline phase accompanying the cooling-induced elongation were studied through differential scanning calorimetry and X-ray diffraction analyses. The elongation process involves different contributions of entropy- and crystallization-driven processes, whose amounts were investigated as a function of the loading conditions and the molecular architecture. The role of the network density towards a controlled two-way response is evidenced, showing that a proper value of the crosslink density has to be identified to maximize the two-way elongation capabilities.