Effect of the Fixation Temperature Tlow on the Crystallization Behavior and Shape-Memory Performance of Crystallizable Copolyesterurethanes


Multiblock copolymers (PDLCL) composed of crystallizable poly(ω-pentadecalactone) (PPDL) segments forming hard domains and crystallizable poly(ε-caprolactone) (PCL) segments forming switching domains, have been recently introduced as multifunctional shape-memory material. The shape-memory properties of PDLCL are related to the crystallization and melting behavior of PCL switching domains, which enable the temporary fixation of an applied deformation via crystallization as well as the recovery of the original shape by melting of the PCL crystals upon heating. In this work, we explored the effect of different fixation temperatures (Tlow = 0, 10, 20 and 25 °C) on the crystallization behavior of PCL domains in compression-molded films prepared from PDLCL with identical weight contents of PCL and PPDL segments in the starting composition by atomic force microscopy (AFM) and differential scanning calorimetry (DSC). The results demonstrated that lower Tlows ≤ 10 °C supported the nucleation of PCL domains, while a Tlow ≥ 20 °C facilitated the growth of PCL crystals. Reducing Tlow, on one hand, increased the degree of crystallinity of PCL domains, which resultantly improved the shape fixation ratio (Rf) from 83% at 25 °C to 89% at 0 °C. Furthermore, the onset temperature of the recovery process (Ts) related to the crystalline PCL domains, was shifted from Ts = 29 °C to 14 °C when Tlow decreased from 25 to 0 °C, causing an increase in the width of the shape-memory transition. In contrast, the shape recovery ratio, with constant high values of Rr ≥ 96% and the almost identical characteristic switching temperature at Tsw ≈ 42 °C, were found to be independent from the applied Tlow. The obtained results confirmed that the shape-memory performance of multiblock copolymers with crystallizable switching domains can be tailored by altering the fixation temperature during programming.
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