(241b) Thermally-Driven Nanostructure Evolution of Phase-Change Elastomer Gels

Phase-change elastomer gels (PCEGs), which combine an elastomeric network with a phase-change (hydrocarbon, HC) additive, are promising candidates for shape-memory and self-supported passive temperature control materials. The use of a thermoplastic elastomer (TPE) further enhances the potential of these materials due to their ability to be repeatedly processed and easily recycled. However, the competing structural formation processes of TPE (i.e., block copolymer) self-assembly and HC crystallization on ultimate morphology are not well understood and may translate to a better understanding of macroscopic properties. In this presentation, PCEGs composed of poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS), a commonly utilized TPE, and crystalline HCs are considered. The shape-memory and thermal properties of SEBS / HC blends will be provided to motivate the current work, and then the focus will shift to characterization of the blends’ nanostructural details discerned from a combination of small- and wide-angle X-ray scattering (SAXS and WAXS, respectively). Within the composition range considered, the molten-state blends (T_m,HC < T < T_g,S) exhibit a body-centered cubic (BCC) morphology in which S spheres reside in an EB / HC mixed matrix. Surprisingly, HC crystallization (T < T_m,HC) does not impact the shape of the S spheres, but completely distorts their lattice positions. This change, along with scattering from the HC crystals, can be tracked as a function of temperature to provide structural transitions in the blends. Further analysis of PCEGs containing two different HCs reveal that crystallization can be split into two separate, yet co-existing, crystal populations. These systems also indicate that the distortion of the BCC lattice is dependent on the overall crystallinity.