Journal of physical chemistry b vol:108 issue:47 pages:18164-18173
Wide-angle light scattering studies were performed on the atactic poly(methacrylic acid), HPMA, in 0.002 and 0.1 M HCl at 25 and 55 degreesC in a wide concentration range. The weight-average degree of polymerization of a single poly(methacrylate), PMA, chain (N) over bar (w) (=1528) was estimated from scattering experiments on poly(sodium methacrylate) in 0.1 M NaCl. The intermolecular association between PMA chains in carefully prepared acidic aqueous solutions is evidenced in both angular and concentration dependencies of scattered light. The dependence of the reciprocal reduced scattering intensity on the square of the scattering vector is heavily curved at high angles, and it exhibits a minimum as a function of concentration. In 0.002 M HCl, the associated forms deaggregate on a time scale of several days. This process is accompanied by a decrease of the second virial coefficient, A(2), and by chain expansion; the radius of gyration, R-g, increases from about 17 to about 28 nm after 0.17 and 576 h, respectively. The intermolecular association in 0.1 M HCl is stronger than in 0.002 M HCl and seems to be stable on a much longer time-scale. The time-average R-g, value (about 18 nm) of the associated HPMA form in 0.1 M HCl indicates a more compact nature of entanglements; this fact decisively contributes to the overall stability of intermolecular aggregates. For HPMA dissolved in 0.1 M HCl, an increase in temperature from 25 to 55 degreesC brings about no increase in R-g. The changes in thermodynamic functions of association point to a process driven by hydrophobic interactions.