PEG-POSS Multiblock Polyurethanes: Synthesis, Characterization, and Hydrogel Formation

We report the synthesis and characterization of a series of multiblock thermoplastic polyurethane hydrogels synthesized with PEG as the soft segment and incorporating an isobutyl-functionalized POSS diol (TMP POSS diol) in the hard segment. The molecular weight of PEG was systematically varied to include 10, 20, and 35 kg/mol, while the molar ratio of POSS diol (as chain extender) to PEG was varied from 3:1 to 8:1 reported for a polymer series with PEG held constant at 10 kg/mol. The diisocyanate employed for TPU polymerization was 4,4'-methylenebis(phenyl isocyanate) (MDI). Wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) experiments revealed that both hydrophilic soft segments (PEG) and hydrophobic hard segments (POSS) can form crystalline structures driven by microphase separation due to the thermodynamic incompatibility. Consequently, the POSS nanocrystals serve as physical cross-linking points consisting, themselves, of an inorganic-organic hybrid networks. Interestingly, the polymers swell in water to an extent well-governed by composition, with equilibrium swelling ratio increasing with PEG loading from similar to 70% to similar to 600%. The water diffusion in the hydrogel was found to be non-Fickian and to depend strongly on PEG crystallinity. The shear modulus of the hybrid hydrogels spanned the range 0.3 MPa < G < 4.0 MPa, a range higher than most hydrogels studied previously. The high stiffness of such thermoplastic hydrogels was found to be controlled by the POSS:PEG mole ratio and was uncorrelated with molecular weight of PEG segment. Given their thermoplastic nature in the dry state, we envision applications involving melt processing of complex shapes and coatings, followed by hydration to hydrogel form.