Macromolecules, Vol.53, No.4, 1368-1379, 2020
Fully Reversible Spherulitic Morphology in Cationically Photopolymerized DGEBA/PCL Shape-Memory Blends
Films with shape-memory properties were obtained after photocuring blends of diglycidyl ether of bisphenol A (DGEBA) with previously crystallized poly(epsilon-caprolactone) (PCL). The effect of the epoxy photopolymerization on the semicrystalline PCL was studied, and transesterification reactions between epoxy and PCL were confirmed to occur in the amorphous regions of the DGEBA/PCL samples by Fourier transform infrared (FTIR) and gel content determination. Polarized light optical microscopy (PLOM) observations showed that because curing and transesterification reactions were confined to the amorphous regions of the sample, a spherulitic template was created with remarkable reversibility upon melting and recrystallization. To analyze the effect of this template on PCL crystallization, self-nucleation experiments were performed by differential scanning calorimetry (DSC). Within the spherulitic templates of photocured samples, in the amorphous regions, chains are confined near cross-linking points. The crystalline memory vanishes upon melting because the cross-link points induce an elastic entropy-driven response that changes the direction of chain orientation in the melt, as revealed by changes observed in the birefringence sign of the sample. The effect of the DGEBA/PCL composition on the shape-memory properties was analyzed, and better fixation was obtained at higher contents of epoxy resin. Instead, better recovery could be obtained for samples with higher amounts of PCL.