Adiabatic compressibility of polyelectrolytes: Poly(N-dimethylaminoethyl methacrylate) and its copolymer with acrylic acid

Abstract

The adiabatic compressibility of poly(N-dimethylaminoethyl methacrylate) and of three copolymers of N-dimethylaminoethyl methacrylate and acrylic acid, ranging in composition from 33 to 58 mole-% amino groups, has been studied. The ϕV of the polymer shows a slight decrease (2.4 cc/mole), while the ϕK is found to have increased considerably (51 × 10−4 cc bar−1 mole−1) compared to that of the monomer. The latter is apparently due to the more compressible nature of the polymer than that of its monomer. The experimentally observed ϕK20 and ϕV20 values for the three copolymers containing 58%, 43%, and 33% amino groups are −2.5 × 10−4 cc bar−1 mole−1 and 164.5 cc/mole, −32 × 10−4 cc bar−1 mole−1 and 177.5 cc/mole, and −55 × 10−4 cc bar−1 mole−1 and 211.3 cc/mole, respectively, whereas the calculated values are less by 19.4 × 10−4 cc bar−1 mole−1 and 3.2 cc/mole, 49.5 × 10−4 cc bar−1 mole−1 and 19.9 cc/mole, and 73 × 10−4 cc bar−1 mole−1 and 16.4 cc/mole, respectively. This decrease is attributed to the interaction of acid and base groups in the molecules. The ϕK20 and ϕV20 values have been resolved into their ionic components ϕK and ϕV. Since the magnitude of electrostriction is higher in fully neutralized salt than in unneutralized salt, the ϕK2i0 and ϕV2i0 values are lower as expected. The difference in these values for the polybase and its salt is 23.7 × 10−4 cc bar−1 mole−1 and 7.5 cc/mole, respectively, which may be due to the electrostrictive effect. In excess NaCl (1.0M), the magnitude of electrostriction is somewhat reduced and ϕV2i0 and ϕV2i0 approach values more or less equal to those of the unneutralized polymer. The 100% neutralized hydrochloride salt of poly(N-dimethylaminoethyl methacrylate) shows greatly increased reduced viscosity over that of the feebly basic parent polymer due to the characteristic polyelectrolytic expansion in dilute aqueous solution. The copolymer containing excess amount of amino groups (58%) shows similar behavior, while the other two copolymers containing fewer amino groups (43% and 33%) show a contraction of chains, which may be ascribed in interaction of the carboxyl ions that are freshly formed on dilution with the amino groups in the copolymer chain.