Relating rheology and molecular structure of model branched polystyrene melts by molecular stress function theory

A quantitative analysis of the experimentally observed strain-hardening characteristics of comb-shaped polystyrene melts with different numbers and lengths of grafted sidechains is made using the molecular stress function (MSF) model. This is based on a specific strain energy function, which assumes that the backbone of the grafted molecule is stretched by deformation, while the side chains are compressed. It is demonstrated that the experimentally observed slope of the elongational viscosity after inception of strain-hardening depends on the ratio 6 of total molar mass to backbone molar mass as predicted by the model. The steady-state (plateau) value of the elongational viscosity depends on the maximum relative stretch, f(2)MAX which can be supported by chain segments. It is found that f(2)MAX increases with the molar mass fraction of sidechains, and at the same molar mass fraction. more short sidechains lead to higher values of f(2)MAX than a few long sidechains. Surprisingly, even sidechains which are shorter than the entanglement length are found to contribute to strain hardening. Using the same nonlinear parameters as in elongational flow plus a parameter taking into account the additional constraint release in rotational flows, the shear damping function data of the grafted polystyrene melts are modeled quantitatively by the MSF theory. (C) 2004 The Society of Rheology.