The effects of nano-size fillers on shape memory (SM) properties of polyurethane (PU) nanocomposites were evaluated. Organoclay, carbon nanofiber (CNF), oxidized carbon nanofiber (ox-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 indicated that exfoliated organoclay significantly augmented SM performance, while CNF and SiC diminished it by interfering with crystallization of the soft segment. The shape memory properties reduced significantly beyond a certain loading of CB. Better SM performance with organoclay can be attributed to significant mechanical reinforcement and very little detrimental effect on 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. The effects of thermal expansion on shape memory performance of SMPU composites were also evaluated. It was observed that strain due to thermal expansion can significantly reduce the recoverable strain. Such reduction depends on the magnitude of temperature gradient in shape recovery (heating) step, thermal expansion coefficient of system, and the level of recoverable strain.
The effectiveness of carbonaceous, electrically conductive fillers, CNF, ox-CNF, and CB, in shape memory actuation of polyurethane composites by resistive heating was also evaluated. Specifically, the dependence of electrical resistivity on specimen temperature and imposed tensile strains encountered in shape memory test cycles was determined for CNF/SMPU, ox-CNF/SMPU, and CB/SMPU composites. A reduction in soft segment crystallinity was observed in the presence of CNF and ox-CNF; the reduction was smaller in the case of ox-CNF. Only the composites of CB showed pronounced positive temperature coefficient (PTC) effects. The observed PTC effects bore a close relationship with non-linear thermal expansion during heating. The composites of CNF and ox-CNF did not show PTC effects due to low levels of soft segment crystallinity. The resistivity of composites of CB increased by several orders of magnitude with imposed strain while composites of CNF and ox-CNF showed weak dependence.
The filler-matrix interactions were anticipated to be stronger in ox-CNF/SMPU composites compared to those in CNF/SMPU composites due to the presence of polar functional groups on ox-CNF particles. However experimental analysis of hard segment interactions with CNF and ox-CNF particles as revealed from fluorescence emission spectroscopy data could not distinguish between the relative strengths of these interactions.
The study also investigated if styrene-isoprene-styrene (SIPS) and styrene-isobutylene-styrene (SIBS) copolymers would exhibit shape memory properties, especially in light of the possibility of strain induced crystallization of these polymers. It was found that the pristine and silica filled SIBS and SIPS copolymers did not exhibit any shape memory properties due to absence of crystals at room temperature.