(268c) From Molecular Topology to Viscoelasticity: Predicting the Change of Flow Properties for Entangled Polymers Under Sol-Gel Transition

When polymer molecules are crosslinked, the presence of intramolecular loops and hyperbranched structure make the prediction of viscoelastic properties rather difficult. Here, on the basis of the effective relaxation potential defined at the junction of each polymer strand, we propose a novel strategy to determine the timely movement of relaxation “front” between the fully-relaxed, outermost layer and the unrelaxed, innermost core of crosslinked polymer structure. In this scheme, kinetic simulation is used to predict topological information from a kinetic Monte Carlo method for simulating crosslinking reactions. Based on the predicted distribution of polymer molar masses and the category of a specific strand, i.e., linear polymer, (hyper)-branched, part of network or loop, different relaxation dynamics can then be applied allowing, for the first time, to quantitatively predict the linear viscoelasticity of an arbitrary molecular architecture during the sol-gel transition of entangled polymers.

To evaluate this approach to the calculation of viscoelastic properties, predictions of our model are compared to two separate experimental systems. First, for monodisperse polymers under well-defined synthesis and crosslinking mechanisms, i.e., the curing of NHS-ester modified polynorbornenes by adding ethylenediamine, the predictions of the above model on the evolution of rheological moduli with respect to different conversions are found to be consistent with those of experimental measurements. Second, the model is also compared to a series of industrial grade polyolefin samples being modified by reactive extrusion. With an ensemble of molecular architecture inferred from high temperature GPC of unmodified polyolefin, the model predicts quite well on the change of power law behaviors of rheological moduli under a various combination of process conditions.