ma071517z_si_002.pdf (632.13 kB)
Two-Way Reversible Shape Memory in a Semicrystalline Network
journal contribution
posted on 2008-01-08, 00:00 authored by Taekwoong Chung, Angel Romo-Uribe, Patrick T. MatherCooling-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 (Tc) and contraction on heating (Tm) 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.