Linear dynamic mechanical properties of an SBS block copolymer

Abstract

AbstractDynamic viscosity and elastic modulus for a low molecular weight styrene‐butadiene‐styrene (SBS) block copolymer ate measured as a function of temperature (80‐170°C) and frequency using the eccentric rotating disc geometry. These linear properties are superimposed to yield master curves each of which exhibits two branches below different (critical) reduced frequencies. At lower temperatures, the non‐Newtonian behavior characteristic of SBS block copolymers is observed. In contrast, Newtonian response occurs at higher temperatures. As a consequence, plots of the viscoelastic properties vs temperature exhibit discontinuities below the critical frequencies, reflecting a narrow transition at about 142°C. Above this temperature, it is inferred, consistent with the equality of dynamic and steady state viscosities, that the polystyrene (S) blocks, existent in dispersed domains at low temperatures, exceed a critical degree of compatibility with the continuous polybutadiene phase. The activation energies indicate that the S blocks affect the temperature dependence of the dynamic properties in proportion to their presence in an interphase which is assumed to continuously grow in size as temperature is raised to the transition temperature. Below the critical reduced frequencies, it is inferred that S domain disruption may increasingly occur in conjunction with the observed property enhancement due to these domains, relative to the miscible blocks, as reduced frequency is lowered. However, above these frequencies, the presence of frequency‐temperature superposition implies that the S domains and the miscible blocks are equivalent in their effects on properties. At still higher reduced frequencies, the domains present at the low temperatures studied are assumed to remain intact, but plateau behavior similar to the response characteristic of homopolymers is observed.