The MSF model:: relation of nonlinear parameters to molecular structure of long-chain branched polymer melts

The elongational viscosity data of model PS combs ( Hepperle J, Einfluss der Molekularen Struktur auf Rheologische Eigenschaften von Polystyrol- und Polycarbonat-schmelzen. Doctoral Thesis, University Erlangen-Nurnberg, 2003) are reconsidered by including the interchain pressure term of Marrucci and Ianniruberto [Macromolecules 37: 3934-3942, 2004] in the Molecular Stress Function model [ Wagner et al., J Rheol 47(3): 779-793, 2003, Wagner et al., J Rheol 49:1317-1327, 2005d]. Two nonlinear model parameters are needed to describe elongational flow, beta and f(MAX)(2). The parameter beta determines the slope of the elongational viscosity after the inception of strain hardening. It is directly related to the molecular structure of the polymer and represents the ratio of the molar mass of the ( branched) polymer to the molar mass of the backbone alone. beta follows from the hypothesis of Wagner et al. [J Rheol 47( 3): 779-793, 2003] that side chains are compressed onto the backbone. We consider also the case that side chains are oriented by deformation, but not stretched, and found little difference in the model predictions. The parameter beta MAX represents the maximum strain energy stored in the polymeric system and determines the steady-state value of the viscosity in extensional flows. The relation of this energy parameter to the molecular structure is discussed. Good correlations between the energy parameter and different coil contraction ratios, as determined either experimentally or calculated theoretically by considering the topology of the macromolecule, are found. The smaller the relative size of the polymer coil, the larger is the energy parameter and the more strain energy can be stored in the polymeric system.