Microphase Separation Mode-Dependent Mechanical Response in Poly(vinyl ester)/PEO Triblock Copolymers

The morphology and mechanical properties of a series of poly(vinyl ester)/poly(ethylene oxide) triblock copolymers are reported. Reversible addition fragmentation chain transfer (RAFT) polymerizations of vinyl ester monomers mediated by an alpha,omega-telechelic poly(ethylene oxide) bearing terminal xanthate functionalities enable the bidirectional syntheses of monodisperse POP, AOA, and BOB (P = poly(vinyl pivalate); A = poly(vinyl acetate); B = poly(vinyl benzoate); O = poly(ethylene oxide)) triblock copolymers. By controlling the extent of monomer conversion in these RAFT polymerizations using a single difunctional macro-RAFT chain transfer agent having M-n,M-O = 11.3 kg/mol, a series of polymers having O mass fractions 0.25 <= W-O <= 1.00 and M-n,M-total = 14.9-45.1 kg/mol (M-w/M-n < 1.40) were synthesized. Through a combination of differential scanning calorimetry (DSC) and variable-temperature small-angle X-ray scattering (SAXS) analyses, the three triblock series are shown to exhibit three different solid-state morphologies that arise directly from (i) O crystallization from a microphase-separated melt, (ii) O crystallization-induced segregation, and (iii) complete suppression of O crystallization. The mechanical properties of these materials are correlated with the observed mode of microphase separation.