Dynamics retardation in hybrid POSS-NIPAm nanocomposites. Thermoplastic and thermally-responsive hydrogel behavior

Abstract

A series of uncrosslinked and unentangled hybrid copolymers of N-isopropyl acrylamide (NIPAm) and polyhedral oligomeric silsesquioxane (POSS) exhibited dual, thermoplastic and hydrogel behavior. The products of the synthesis were characterized by nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (FTIR), and dynamic light scattering (DLS). The dried hybrids were amorphous and exhibited a reduction of glass transition temperature Tg as POSS content increased, i.e., ΔTg = Tg − Tg,bulk < 0°C, suggesting nanoconfinement-like effects. The hybrid melts exhibited linear viscoelastic behavior. Master curves were constructed using time–temperature superposition (TTS), and PNIPAm displayed Rouse dynamics. The hybrid melts however displayed terminal and transition regimes where the scalings G″ ≠ ω2 and G′ = G″ ≠ ω1/2, did not hold. The mechanical damping evidenced a molecular retardation mechanism suggesting POSS-POSS interactions in the melt. Strikingly, the melt viscosity was reduced up to one order of magnitude and exhibited yield stress at higher POSS content, a consequence of POSS associations. In the hydrated state the “sticky” POSS-POSS associations produced hydrogel behavior, with at least ∼3 wt% POSS content. The hydrogels were thermo-responsive, and POSS tuned the LCST, swelling ratio, and modulus. The physical associations are reversible as the hydrogels healed immediately after a nonlinear deformation. Hence, the thermoplastic-hydrogel behavior of PNIPAm-POSS hybrids enables thermoforming (injection molding, extrusion, and fiber spinning) followed by hydration. This bottom-up approach opens opportunities for easily procesable, environmentally friendly, and versatile smart hydrogel-based medical devices.