Solution and Solid-State Behavior of Amphiphilic ABA\nTriblock Copolymers of Poly(acrylic acid-<i>stat</i>-styrene)-<i>block</i>-poly(butyl acrylate)-<i>block</i>-poly(acrylic\nacid-<i>stat</i>-styrene)

Abstract

A combination of\nstatistical and triblock copolymer properties\nis explored to produce stable aqueous polymer dispersions suitable\nfor the film formation. In order to perform an extensive structural\ncharacterization of the products in the dissolved, dispersed, and\nsolid states, a wide range of symmetrical poly(acrylic acid-<i>stat</i>-styrene)_<i>x</i>-<i>block</i>-poly(butyl acrylate)_<i>y</i>-<i>block</i>-poly(acrylic acid-<i>stat</i>-styrene)_<i>x</i>, poly(AA-<i>st</i>-St)_<i>x</i>-<i>b</i>-PBA_<i>y</i>-<i>b</i>-poly(AA-<i>st</i>-St)_<i>x</i>, (<i>x</i> = 56, 108 and 140, <i>y</i> = 100–750;\nthe AA:St molar ratio is 42:58) triblock copolymers were synthesized\nby reversible addition–fragmentation chain transfer (RAFT)\nsolution polymerization using a bifunctional symmetrical RAFT agent.\nIt is demonstrated that the amphiphilic statistical outer blocks can\nprovide sufficient stabilization to largely hydrophobic particles\nin aqueous dispersions. Such a molecular design provides an advantage\nover copolymers composed only of homoblocks, as a simple variation\nof the statistical block component ratio provides an efficient way\nto control the hydrophilicity of the stabilizer block, which ultimately\naffects the copolymer morphology in solutions and solid films. It\nwas found by small-angle X-ray scattering (SAXS) that the copolymers\nbehaved as dissolved chains in methylethylketone (MEK) but self-assembled\nin water into stable and well-defined spherical particles that increased\nin size with the length of the hydrophobic PBA block. These particles\npossessed an additional particulate surface structure formed by the\nstatistical copolymer stabilizer block, which self-folded through\nthe hydrophobic interactions between the styrene units. SAXS and atomic\nforce microscopy showed that the copolymer films cast from the MEK\nsolutions formed structures predicted by self-consistent field theory\nfor symmetrical triblock copolymers, while the aqueous dispersions\nformed structural morphologies similar to a close-packed spheres,\nas would be expected for copolymer particles trapped kinetically due\nto the restricted movement of the blocks in the initial aqueous dispersion.\nA strong correlation between the structural morphology and mechanical\nproperties of the films was observed. It was found that the properties\nof the solvent cast films were highly dependent on the ratios of the\nhard [poly(AA-<i>st</i>-St)] and soft (PBA) blocks, while\nthe aqueous cast films did not show such a dependence. The continuous\nphase of hard blocks, always formed in the case of the aqueous cast\nfilms, produced films with a higher elastic modulus and a lower extension-to-break\nin a comparison with the solvent-cast films.