Synthesis and Rheological Characterization of Poly(vinyl\nacetate‑<i>b</i>‑vinyl alcohol‑<i>b</i>‑vinyl acetate) Triblock Copolymer Hydrogels

Abstract

We report the synthesis and shear\nrheology of a series of hydrogels\nderived from poly­(vinyl acetate-<i>b</i>-vinyl alcohol-<i>b</i>-vinyl acetate) (PVAc-<i>b</i>-PVOH-<i>b</i>-PVAc) triblock copolymers. Bidirectional, reversible-addition–fragmentation\nchain transfer free radical polymerizations of vinyl acetate (VAc)\nand vinyl chloroacetate (VClAc) are optimized to produce a series\nof relatively narrow dispersity PVAc-<i>b</i>-P­(VAc-<i>ran</i>-VClAc)-<i>b</i>-PVAc triblock copolymers with\ncontrolled molecular weights and compositions. Rapid and selective\nhydrolysis of P­(VAc-<i>ran</i>-VClAc) blocks with K₂CO₃/CH₃OH furnishes access to a series\nof PVAc-<i>b</i>-PVOH-<i>b</i>-PVAc amphiphiles.\nHydration of solvent-cast films of these copolymers, comprising PVAc\nblocks of comparable degrees of polymerization with variable PVOH\nsegment lengths, yields soft hydrogels (<i>G</i>′(ω)\n∼ 1–10 kPa). Synchrotron small-angle and wide-angle\nX-ray scattering reveals that these hydrogels microphase separate\ninto spherical hydrophobic PVAc domains that are interconnected by\nsolvated PVOH network strands. The strain-dependent rheology of these\nhydrogels depends sensitively on the length of the center PVOH segment.\nThis gel rheology is shown to stem from the hydrogen bond donor–acceptor\ncapabilities of PVOH, which leads to the formation of weak, dynamic\nnoncovalent crosslinks in the aqueous domains of these gels. Thus,\nthe observed gel rheology may be rationalized in terms of the shear-induced\nreorganization of these H-bonding crosslinks as a function of the\napplied strain.