| Shape memory polymers are smart materials which can remember their original shapes. However, the low recovery stress and low mechanical strength limit the commercial applications of shape memory polymers. In this study, nanoclays were introduced to shape memory polyurethanes (SMPU) to augment these properties by enhance the network of SMPU. Several factors which influence the shape recovery stress were evaluated, including the nature of polymer chain by using different monomers, type of clay particles, extent of filler dispersion, clay content and deformation conditions.; It was found that only reactive clay particles were well dispersed into polyurethane matrix by the tethering between --CH2CH 2OH functional groups in clay surfactants and polyurethane chains. Two different shape memory polyurethanes (Systems I & II) prepared by bulk polymerization were compared. The shape memory effect of System I was triggered by melting of the soft segment crystals, while that of System II was by glass transition of the soft segments. It was seen that the reactive clay particles dispersed well in both polyurethane matrices and augmented the recovery stress, e.g., 20% increase with 1 wt % nanoclay in System I and 40% increase with 5 wt % nanoclay in System II were observed.; In System I, clay particles interfered with soft segment crystallization, and promoted phase mixing between the hard and soft segments, thus affecting the fixity and recovery ratio. Nevertheless, the soft segment crystallinity was still enough and in some cases increased due to stretching to exhibit excellent shape fixity and shape recovery ratio. The higher loading of clay particles accelerated the stress relaxation, resulting in reduction of recovery stress.; In System II, no significant effect of clay particles in phase separation was observed, so there was no influence of clay on shape fixity and recovery ratio. The recovery stress increased with reactive nanoclay content. It was also found that the recovery stress could be tailored by the processing conditions. The recovery stress increased with decrease of stretching rate, and increase of stretching temperature and stretch ratio. The recovery stress of polyurethane/clay nanocomposites largely depended on the degree of clay exfoliation. Higher recovery stress was found in nanocomposites with better clay dispersion. The dependence of stress relaxation on stretching conditions, clay type, and clay content was also investigated and related to shape recovery stress. It was found that stress relaxation occurred more easily in the presence of nanoclay. |
