FLOW CURVE MOLECULAR-WEIGHT DISTRIBUTION - IS THE SOLUTION OF THE INVERSE PROBLEM POSSIBLE

Several different theoretical models have been developed relating the flow curve of a polymer melt to its molecular weight distribution (MWD). These models allow calculation of the flow curve if MWD is known beforehand. According to one model the non-Newtonian behavior of a polymer melt is considered as a consequence of the gradual transition of high molecular weight (MW) fractions to the rubbery (non-fluid) state. This model, which gives realistic predictions of the flow curve, can be transformed into the equation for MWD, which appears to be directly related to the flow curve. The derived equation seems to present the exact solution of this inverse problem. Nevertheless, calculation tests show the instability of such a solution. This means that any inevitable experimental error in the flow curve measurements can lead to an unexpected and arbitrary wide divergence of the calculated MWD from the true one. However, if definite preset forms of MWD are used, the MWD width can be determined from the flow curve. This has been confirmed by experiments on different polymers, such as low-density polyethylene (LDPE), polyisoprene (PI), butyl rubber (BR), and polystyrene (PS).