ON THE STABILITY OF MOLECULAR-WEIGHT DISTRIBUTIONS AS COMPUTED FROM THE FLOW CURVES OF POLYMER MELTS

In this work we analyze the possibility of improving the reliability o f the conversion from the viscosity function to the molecular weight d istribution (MWD) for linear flexible polymers. A constrained inverse procedure has been applied to guide the inverse problem to the correct solution, i.e., an analytical MWD function utilizing the generalized exponential function (GEX) has been assigned. The self-consistency of this flow curve-MWD constrained conversion has been tested on copolyac etal samples of different molecular weights by determining the chi(2) surface in the space of the GEX parameters around the minimum. The res ult is that the constrained inverse method is capable of yielding mean ingful results in terms of MWDs and molecular weight averages provided that the viscosity data fall into the commonly accepted experimental error range and cover an adequately wide shear rate range. Discrepanci es between the MWDs calculated from rheological measurements and the c orresponding GPC data have been detected in the case of the polymers o f lower molecular weight. This event probably occurs because of the la ck of viscosity data at very high shear rates due to experimental diff iculties. In addition, failure can occur in the rheological model util ized for the lower molecular weight samples. (C) 1996 Society of Rheol ogy.