Thermoreversible gelation in polymer systems. Part 2. Gel/sol transition in vinylidene chloride/methyl acrylate copolymer gels

Abstract

Those vinylidene chloride/methyl acrylate copolymers that show crystallinity in the solid state form reversible gels when dissolved in benzene, chlorobenzene, m-dichlorobenzene or o-dichlorobenzene. The temperature T_m of the gel/sol transition increases with the vinylidene chloride content of the copolymer and with decreasing dipole moment of the solvent. Polymer concentration and molecular weight have only a small effect on T_m and so the Ferry and Eldridge relationship, ∂ ln C/∂T^–1_m=ΔH_x/R, gives very large values (80–240 kJ mol^–1) for the junction point energy ΔH_x. This increases with the vinylidene chloride content of the copolymer and with the dipole moment of the solvent.

Equating the gel/sol transition with the crystalline melting point enables Flory's relationships between copolymer composition, solvent power and the melting point to be applied to T_m. This gives χ values for poly(vinylidene chloride) increasing from 0.67 to 0.80 as the dipole moment of the solvent decreases and a heat of fusion of 2.64 kJ mol^–1 per vinylidene chloride unit. It is suggested that the network junction points of the copolymer are between 30 and 100 units long and this accounts for the absence of gelation in copolymers having less than 80 mol % of vinylidene chloride units.