Evaluation of nanoparticulate fillers for development of shape memory polyurethane nanocomposites

The effects of nano-size fillers on shape memory (SM) properties of polyurethane (PU) nanocomposites were evaluated. Organoclay, carbon nanofiber (CNF), silicon carbide (SiC), and carbon black (CB) were selected as the fillers in an attempt to reinforce the PU and to obtain significantly increased shape recovery stress. The shape memory PU was synthesized from diphenylmethane diisocyanate, 1,4-butanediol, and poly(caprolactone)diol, the latter with a molecular weight of 4000 g/mol. The composites were prepared by melt mixing of extended chain PU with the fillers. The shape memory behavior was triggered by heating the specimen above the melting point of the crystalline soft segment. Our results indicate that exfoliated organoclay significantly augments SM performance, while CNF and SiC diminish it by interfering with crystallization of the soft segment. CB destroys the shape memory properties beyond a certain loading. Better SM performance with organoclay can be attributed to mechanical reinforcement without much interference with the soft segment crystallinity. The reduction of soft segment crystallinity in the presence of CNF and SiC was analyzed. It was found that the extent of crystallinity, as well as the crystallization temperature, was significantly reduced in the presence of these fillers.