Photo-Cross-Linked Poly(trimethylene carbonate)/Poly(ε-caprolactone) Triblock Copolymers with Controlled Architectures: From Phase-Separated Structures to Shape-Memory Materials

Efficient and rapid organocatalyzed ring-opening polymerization (ROP) of trimethylene carbonate (TMC) and cdprolactone (epsilon-CL) was reported for the synthesis of PTMC-b-PCL-b-PTMC block copolymers with controlled architectures and desired block proportions. These block copolymers were processed as thin films, and the improvement of their stability and mechanical properties was achieved via an additional reticulation step of previous chain-end methacrylation. Subsequently, the morphology (phase separation and crystallinity) of these semicrystalline block copolymers was investigated by differential scanning calorimetry (DSC) and atomic force microscopy (AFM). DSC measurements of such block copolymers with chi N-AB values higher than 20 showed two distinct glass-transition temperatures, thus indicating a phase-separated system. As expected, the crystallinity-driven phase-separated morphology predominated as revealed by AFM analysis of the block copolymers. The cross-linking step at the melt state, hence creating a dense polymer network, revealed to not disturb the crystallinity phenomenon. However, melt-state cross-linked block copolymer thin films manifested to be sensitive to rapid liquid nitrogen quenching. Thereby, as evidenced by AFM, under these circumstances, thermodynamically driven phase separation clearly predominates over the originally crystalline one. The cross-linked thin films remained stable even after 4 months with access to thermodynamical phaseseparated morphologies at temperatures below the poly(epsilon-caprolactone) melting point. Precisely, this coexistence of dual cross-linked/crystalline networks in the same copolymer structure allowed us to establish, for the first time, the shape-memory properties in such block copolymers, as verified by thermomechanical analysis.