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 series of variable extensibility polystyrene-based Boger fluids has been investigated 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 phthalate-based or good, tricresyl phosphate-based solvents. This rational manipulation of Boger fluid solvent quality (assessed based on previously reported light scattering and intrinsic viscometry measurements) and chain length yields a unique opportunity to correlate viscoelastic extensional response to finite extensibility and molecular interactions: 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 strain hardening is observed for these polymer solutions, as well as certain deviations of the experimental flow from ideal uniaxial extension. As opposed to Tirtaatmadja and Sridhar (1993), no steady-state extensional viscosities are obtained for the comparatively small strains of our experiment. The transient experimental results are roughly consistent with simple one-dimensional FENE-P calculations. However, uncertainties due to the nonideal nature of the flow at short times and the sensitivity of the measured extensional stress growth coefficient to the details of the imposed elongation make it difficult to unambiguously assign L based solely on short time filament stretching results. These results are considered in light of the substantial effects solvent quality and molecular weight have on the measured drag in flow past a sphere [Chmielewski et al. (1990); Solomon and Muller (1996b)]. (C) 1996 Society of Rheology.