Influence of annealing-induced phase separation on the shape memory effect of graphene-based thermoplastic polyurethane nanocomposites

Thermoplastic polyurethane (TPU) is a multiblock copolymer that exhibits an attractive shape memory effect (SME). Its morphology consists of a soft segment (SS), which corresponds to the polyol or a long-chain diol, while the hard segment involves the intercalation of a diisocyanate and a chain extender. Due to the distinct thermodynamic parameters of each monomer, these segments are not miscible with each other, resulting in a phase-separated structure in their morphology. This structure is characterized by the formation of soft and hard domains (SD and HD), respectively. When incorporating 0.1 wt% of graphene nanoplatelets (GNP) or 0.1 wt% of multilayer graphene oxide (mGO) into the TPU matrix using solution casting process, a contribution to the phase separation of these domains is observed. This phenomenon becomes even more pronounced when graphene-based nanocomposites are subjected to annealing at 110 degrees C for 24 hours, indicating a good interaction between the GO and GNP with the HD and SS, respectively. After annealing, the nanocomposites (TPU + GNP and TPU + mGO) exhibit improved performance in SME, as evidenced by an approximately 9% increase in the shape recovery ratio compared to the nonannealed TPU. Additionally, all nanocomposites maintained a high strain during SME programming, surpassing that of pure TPU, both before and after annealing. This suggests a direct influence of the graphene-based nanoparticles on the shape memory effect. Solution casting processing was used to incorporate 0.1 wt% of graphene nanoplatelets (GNP) or 0.1 wt% of multilayers graphene oxide (mGO) into TPU matrix. The results have shown that GNP with less functional groups than mGO, is mainly disperse in the soft segment, while mGO acting as nucleating agent mainly interacts with the hard segments. According to the shape-memory thermomechanical cycle, analyzed in terms of stress and strain, higher strains are observed for all nanocomposites when compared to neat TPU. Among the graphene nanoparticles, GNP presented higher strains, mostly due the lubricating effect of graphene, as mGO, presents greater stiffness which can be explained by the higher polarity inherent to the functional groups that present better affinity with the TPU, which anchors the polymeric chains.image