Microstructure and Phase Behavior of POSS/PCL Shape Memory Nanocomposites

This contribution concerns the microstructure of POSS-based networks previously reported for their shape memory properties [Lee et al. Macromolecules 2008, 41, 4730-4738]. Simultaneous wide-angle and small-angle X-ray scattering (WAXS/SAXS) analysis has revealed highly ordered nanoscale structures in polyhedral silsesquioxane poly(epsilon-caprolactone) (POSS-PCL) semicrystalline cross-linked nanocomposites. Architecturally, the oligomers used for network formation resemble a highly asymmetrical triblock copolymer featuring a single POSS moiety centered between two PCL chains, yielding two short tethers for a total molecular weight of 2600 g/mol. WAXS patterns of the diol-terminated POSS-PCL sample showed the existence of both crystalline reflections of the PCL orthorhombic phase and crystalline reflections of the POSS rhombohedral phase, indicating microphase separation that allows independent crystallization. Accordingly, two endothermic thermal transitions associated with the melting of each crystal phase were exhibited by the material. Strikingly, SAXS revealed two long period spacings: one associated with the POSS nanobuilding blocks (long period of 66 A) and the other associated with PCL lamellar nanophase (long period of 151 A). Surprisingly, end-capping of the PCL alcohol groups with acrylate groups (needed for cross-linking) greatly reduced the crystalline Order of POSS nanocages, whereas the orthorhombic phase of PCL remained unchanged: The acrylate groups also produced a significant reduction of melting transition temperature of POSS crystals. Moreover, SAXS showed only one long period (117 A) associated with PCL crystalline nanostructure. Macromolecular end-linked networks exhibiting shape memory behavior were obtained by photocuring the acrylate-terminated nanocomposites utilizing a tetrathiol cross-linker. Despite the architectural constraints of cross-links, POSS-PCL networks also featured the POSS rhombohedral crystalline phase. However, PCL crystallization was suppressed, resulting in an amorphous PCL phase. DSC analysis of the POSS-PCL networks showed only one endothermic transition. SAXS showed the existence of long-range order in the cross-linked material with the intensity maxima indexed to a cubic lattice with parameter a = 13 nm. That is, a superstructure is present in the cross-linked networks, and the cubic superstructure is attributed to crystalline clusters formed by POSS molecules. To our knowledge, this is the first evidence for such a nanoscale superstructure in a polymer network.