Dynamic micromechanical properties of styrene-butadiene copolymer latex films

Abstract

Polymer films of various gel fractions, obtained by coalescence of styrene-butadiene (SB) copolymer latices, are investigated for their viscoelastic behaviour by dynamic micromechanical spectroscopy. Isochronal temperature dependences of the storage modulus ( E′), loss modulus ( E″) and loss tangent (tan δ) are measured in the temperature range −50 to +130°C including the glass transition of the polymer films. Then, it is shown that the criteria of applicability for the superposition principle are fulfilled. It follows that master curves for the isothermal frequency dependence of the moduli ( E′, E″) can be obtained over a wide range of frequency (10 −7 to 500 rad s −1), with shift factors obeying a law of the Williams-Landel-Ferry type. The key parameters describing the effects of degree of crosslinking and molecular interactions on the viscoelastic behaviour of the films are worked out. They are investigated at different scales (macroscopic, molecular, local) and their dependence upon chain-transfer-agent concentration, which controls the crosslinking process in the system, are determined. Moreover, it is shown that a local parameter, such as the monomeric friction coefficient, can be connected with the glass transition temperature of the SB copolymer film. Finally, the influence of the structure of the films arising from incompletely achieved coalescence of latex particles is discussed.