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

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 (Tm and Tc) and in the crystallinity content. When subjected to constant non-zero stress and to cooling-heating cycles from above Tm to below Tc, 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.