Two-Way Reversible Shape Memory Polymers Made of Cross-Linked Cocrystallizable Random Copolymers with Tunable Actuation Temperatures

Two-way reversible shape memory polymers (2W-SMPs) are highly desirable for many applications. We report for the first time the use of random copolymers with cocrystallizable monomeric units for the preparation of such polymer networks. Homopolymers and random copolymers of ε-caprolactone and ω-pentadecalactone were designed and made by ring-opening polymerization with Candida antarctica lipase B as catalyst. The melting temperatures of these prepolymers may be adjusted by the use of various molar ratios of the comonomers. Upon thiol–ene cross-linking, the polymer network exhibited two-way reversible shape memory effects under both stress-free and stress conditions. The actuation temperature (T_A) of the 2W-SMP under stress-free condition can be tuned in a broad range using a selected mixture of prepolymers followed by photo-cross-linking with a multifunctional cross-linker. Increasing the initial stretching stress amplitude led to an increased absolute strain change under both stress-free and stress condition; it led to a reduced relative strain change under stress-free condition, but almost no change under stress condition. The evolution of the microstructure of 2W-SMPs under stress-free condition cycle was studied by the use of X-ray diffraction (2D-WAXD and SAXS). This is the first report on the tuning of T_A of 2W-SMPs under stress-free condition by the use of one or two prepolymers to form a chemically cross-linked network. We have also eliminated the use of toxic metal catalysts in the synthesis of polymeric biomaterials. The materials are shown to be capable of performing reversible bending–unbending and coiling–uncoiling motions.