Dipole moment and conformational properties of poly(methacrylic acid) in solvents with hydrogen bonding

Dielectric polarization of solutions of un-ionized linear poly(methacrylic acid) in polar associated liquids is studied in the temperature range 20–50°C. The solutions are in methanol, with the molar fraction of polymer units x2 = (3 × 10−3)−(1.5 × 10−2), and in water, with x2 = (4 × 10−5)−(4 × 10−3). The permittivity ε12 of the polyacid solutions in methanol is shown to be lower than the permittivity of the pure solvent ε1; the permittivity of the polyacid solutions in water exceeds ε1 of water in the concentration range x2 = (4 × 10−5)−(2.13 × 10−4) and becomes lower than ε1 as the polymer concentration in the solution increases further. A procedure for estimating the dipole moment μ2 per monomer unit of the polymer macromolecule in solution is proposed. The estimation is based on Buckingham’s statistical polarization theory for a two-component mixture of polar molecules under the conditions of infinite dilution. The μ2 values amount to 2.76–2.14 D (x2 < 1.5 × 10−2) in methanol at 20–50°C and to 11.4−3.8 D (x2 < 2.13 × 10−4) in water at 20–40°C. The difference in the dipole moments of the polyacid and in the patterns of their temperature dependences in methanol and in water is due to the effects of the polyacid-solvent hydrogen bonding, to intramacromolecular hydrogen bonds, and to specificity of the local structure of the solvent. It is shown that the μ2 value corresponds to the dipole moment of the solvates and decreases with temperature owing to changes in the stoichiometry of the solvates, to the formation of cyclic associates in the macromolecule, and to conformational changes in the chain.