VISCOELASTIC PROPERTIES AND FILM MORPHOLOGY OF HETEROGENEOUS STYRENE-BUTADIENE LATEXES

Heterogeneous carboxylated styrene-butadiene (S/Bu) latexes were prepa red by a two-stage emulsion polymerization process, using three PS see ds with different molecular weights. The second-stage polymer was a co polymer with a fixed S/Bu ratio of 1 : 1 and a methacrylic acid (MAA) content of either 1 or 10 wt %. Morphological studies by transmission electron microscopy (TEM) as well as studies of the viscoelastic prope rties by mechanical spectroscopy have been performed on films prepared from the latexes. The studies showed that the glass transition temper ature, T-g, of the second-stage polymer was considerably affected by c opolymerization with MAA. An increase in the MAA content in the second -stage polymer increased the T-g of this phase significantly. Addition of DVB as a crosslinking agent in the preparation of the PS seed phas e substantially increased the rubbery moduli of the films, whereas the glass transition temperature of the second-stage polymer was unaffect ed. On the other hand, the presence of a chain transfer agent reduced the glass transition of the second-stage copolymer containing 1 wt % M AA dramatically, whereas the rubbery modulus was unaffected. When the MAA content was increased to 10 wt % the influence of the MAA monomer had a dominating effect on T-g. Latexes containing 10 wt % MAA had T-g values close to each other, regardless of chain transfer agent presen t in the second-stage polymerization. It was found that the morphology of the latex particles influenced the rubbery modulus of the films. T he presence of irregularly shaped seed particles in samples prepared f rom a crosslinked PS seed bad a considerable reinforcing effect on the films, whereas spherical seed particles originating from core-shell p articles had a less reinforcing effect. (C) 1996 John Wiley & Sons, In c.