Theoretical influence of polyester molecular weight distribution variation on melt viscosity during injection molding and extrusion as influenced by ester‐ester interchange

Abstract

AbstractIn this study Flory's initial molecular weight derivation involving polyester polymerization and ester‐ester interchange was expanded to address two types of blends relative to both the number average molecular weight and the weight average molecular weight. First, an add blend was considered where the numbers of molecules of the same length were simply added together to calculate new number and weight average molecular weights. Second, the complete transesterification blend was calculated using Flory's most probable distribution with the same number average as the add blend but with a very different weight average molecular weight. The ratio of the weight averages of the add blend to the transesterification blend were found to be significantly larger if the initial blending molecular weights were significantly different from one another. This ratio was also found to go through a maximum that occurred at approximately 50% by weight if the two different molecular weights had the same monomer molecular weight. If the monomer molecular weights were different, the location of this maximum could be quite different from 50% by weight. Since melt viscosity is proportional to the weight average molecular weight to a power ranging from 1 to 3.4, a very large ratio between weight average molecular weight of the add blend and the ester‐ester interchange blend was found to have a significant influence on the melt viscosity. A polyester blend mixture of micro‐domains of both a nearly pure add blend and a nearly complete ester‐ester interchange blend was used to explain how a short shot could be followed a few shots later with a part that flashed the mold in an injection molding application.