NEW MEASURES OF POLYDISPERSITY FROM RHEOLOGICAL DATA ON POLYMER MELTS

The rheological properties of polymer melts depend strongly on the und erlying molecular structure: molecular weight, molecular weight distri bution, and long chain branching. It is of considerable importance, bo th fundamental and practical, to relate the molecular architecture to polymer melt theology. The focus of the present work is in extracting a measure of polydispersity from theological data. Various polydispers ity measures that have been proposed in the literature are critically examined and their limitations are pointed out. New measures of polydi spersity are proposed that overcome these limitations. The evaluation of the various polydispersity measures is performed by reference to th eology fundamentals, with model calculations and examples drawn from i ndustrial practice. The issues of eliminating molecular weight and tem perature effects in characterizing polydispersity are comprehensively addressed. The presence of small levels of long chain branching in an otherwise linear polymer alters most of these measures of polydispersi ty dramatically, while no detectable change appears in the molecular w eight distribution obtained using a gel permeation chromatograph. It i s demonstrated that the polydispersity measures proposed in the presen t work, and which are extracted from frequency response data in the li near viscoelastic region, can be used reliably to characterize polydis persity in polymer melts. (C) 1995 John Wiley & Sons, Inc.