No abstract

The past several years have witnessed significant advances in the field of shape memory polymers (SMPs) with the elucidation of new compositions for property tuning, the discovery of new mechanisms for shape fixing and recovery, and the initiation of phenomenological modeling. We critically review research findings on new shape memory polymers along these lines, emphasizing exciting progress in the areas of composites, novel recovery triggering, and new application developments. 445Annu. Rev. Mater. Res. 2009.39:445-471. Downloaded from www.annualreviews.org by NORTH CAROLINA STATE UNIVERSITY on 09/29/12. For personal use only.

Cooling-induced crystallization of cross-linked poly(cyclooctene) films under a tensile load results in significant elongation and subsequent heating to melt the network reverses this elongation (contracting), yielding a net two-way shape memory (2W-SM) effect. The influence of cross-linking density on the thermal transitions, mechanical properties, and the related 2W-SM effect was studied by varying the concentration of cross-linking agent dicumyl peroxide (DCP) and using differential scanning calorimetry (DSC), gel fraction measurements, dynamic mechanical analysis (DMA), and customized 2W-SM analysis. The latter showed that there is crystallization-induced elongation on cooling and melting-induced shrinkage on heating (2W-SM), with lower cross-link density leading to higher elongation at the same applied stress. For a given cross-link density, however, increasing the tensile stress applied during cooling resulted in greater stress-induced crystallization. We further observed that the onset temperatures for elongation on cooling (T c ) and contraction on heating (T m ) shifted to higher temperatures with decreasing cross-link density. Similarly, the degree of molecular orientation achieved upon deformation was found to increase with decreasing cross-link density. The impact of stress on the 2W-SM effect was examined using wide-angle X-ray diffraction (WAXD), revealing a transition from bimodal to unimodal orientation. As the crystalline structure evolves from bimodal (low stress) to unimodal (high stress), the crystallization occurs along a single preferred orientation thus inducing greater elongation along the stretching direction. We anticipate that the observed 2W-SM property in a semicrystalline network will enable applications heretofore possible only with costly shape memory alloys and liquid crystalline elastomers.