The minimum temperature required for a copolymer emulsion to form a continuous film, T,r, and the glass transition temperature T, of the copolymer were studied for vinylidene chloride-acrylate copolymers and vinylidene chloride-acrylonitrile copolymers.
The T,r and T, were studied as a function of the nature of the surfactant used in the preparation of the emulsion. The T,r was found to deviate markedly from the T, for most compositions and the T,r-composition curves were found to be similar in shape to the T,-composition curves, exhibiting a maximum in the case of the acrylate copolymers.
The quantity T,r -T, changes from negative to positive with decreasing polar character of the methyl and ethyl acrylate copolymers. This is especially obvious in the ethyl acrylate copolymer. The high T, t values for copolymers containing acrylonitrile is attributed to strong interchain forces, which orientate the neighboring segments in a latex particle unfavorably for interpenetration with other particles. The nature of the surfactant used in the preparation of the emulsion was found to affect the T,r. Emulsions of nonpolar, hydrophobic copolymers containing a blend of anionic and nonionic surfactants have a higher T, r than those containing only anionic surfactant; however, with an increase in the amount of hydrophilic units in the copolymer the converse is true, except in the case of the highly polar copolymers of vinylidene chloride-methyl acrylate, in which the Tm<s for both surfactant systems are similar.