THE TRANSIENT EXTENSIONAL BEHAVIOR OF POLYSTYRENE-BASED BOGER FLUIDS OF VARYING SOLVENT QUALITY AND MOLECULAR-WEIGHT

The behavior under transient uniaxial elongation of a homologous serie s of variable extensibility polystyrene-based Boger fluids has been in vestigated in an apparatus similar to that developed by Sridhar and co -workers. The test fluids are dilute solutions of either 2.0X10(7) or 2.0x10(6) g/mol monodisperse polystyrene dissolved in poor, dioctyl ph thalate-based or good, tricresyl phosphate-based solvents. This ration al manipulation of Boger fluid solvent quality (assessed based on prev iously reported light scattering and intrinsic viscometry measurements ) and chain length yields a unique opportunity to correlate viscoelast ic extensional response to finite extensibility and molecular interact ions: here we search for such effects in transient filament extension between parallel plates, an approximation of purely extensional flow. The test device, which can impart a maximum Hencky strain, epsilon, of 4.5 at rates, epsilon over dot, between 0.3 and 3.0 s(-1), is similar to that reported by Sridhar et al. (1991). In agreement with results communicated there and in Tirtaatmadja and Sridhar (1993), large strai n hardening is observed for these polymer solutions, as well as certai n deviations of the experimental flow from ideal uniaxial extension. A s opposed to Tirtaatmadja and Sridhar (1993), no steady-state extensio nal viscosities are obtained for the comparatively small strains of ou r experiment. The transient experimental results are roughly consisten t with simple one-dimensional FENE-P calculations. However, uncertaint ies due to the nonideal nature of the flow at short times and the sens itivity of the measured extensional stress growth coefficient to the d etails of the imposed elongation make it difficult to unambiguously as sign L based solely on short time filament stretching results. These r esults are considered in light of the substantial effects solvent qual ity and molecular weight have on the measured drag in flow past a sphe re [Chmielewski et al. (1990); Solomon and Muller (1996b)]. (C) 1996 S ociety of Rheology.