Polycaprolactone−POSS Chemical/Physical Double Networks

Biodegradable shape memory polymers are exciting materials, based on a combination of controlled degradation and triggered actuation that enables the design of unique medical devices. However, their synthesis requires precise control over thermomechanical properties often not possible with common building blocks for biodegradable polymers. In the present work, we report the preparation of “double networks” that feature a superposition of a covalent network with a percolative physical network, the latter being derived from a well-defined crystalline phase of strongly hydrophobic polyhedral oligosilsesquioxane (POSS) moieties. Each covalent network chain features polycaprolactone (PCL) tethers on a single POSS moiety by virtue of its use as a difunctional initiator for PCL ring-opening polymerization. Such network chains were synthesized to feature molecular weight values that ranged from 2000 to 4000 g/mol (polydispersity index <1.35) and POSS contents ranging from 49 to 24 wt %, respectively. Thermal analysis of the uncrosslinked network chains indicated competitive crystallization between POSS and PCL domains, with the PCL melting temperature (T_m) and percent crystallinity increasing with increasing PCL content in the diol, while the POSS T_m decreased, though always remaining higher than PCL. Covalent networks were prepared by end-capping the POSS−PCL telechelics with acrylate groups and photocrosslinking with stoichiometric addition of a tetrathiol cross-linker. Only those POSS-containing networks with high POSS loading, P-CL2-net (42 wt %) and P-CL2.5-net (34 wt %), showed evidence for POSS crystallization, while samples with lower POSS loading were dominated by PCL crystals that ostensibly prevented POSS crystallization. Dynamic mechanical analysis of P-CL2-net showed two rubbery plateaus, one between the glass transition temperature of PCL and the POSS melting point, and the other above the POSS melting point and extending to high temperatures. The first plateau, at a surprisingly high value of 150 MPa, was further investigated using rheometric crystallization studies on the diol precursor, P-CL2, which revealed a prominent sol−gel transition upon POSS crystallization during cooling. We reason that, in the cross-linked form, the same POSS crystal percolation yields a “double network”, with a combination of both physical and chemical network junctions, as evidenced by two rubbery plateaus. The duel network structure enabled both one-way and two-way shape memory behavior, showing recoverable deformation (strain) and increased performance with repetitive cycling.