New measures of polydispersity from rheological data on polymer melts

Abstract

SYNOPSISThe rheological properties of polymer melts depend strongly on the underlying molecular structure: molecular weight, molecular weight distribution, and long chain branching. It is of considerable importance, both fundamental and practical, to relate the molecular architecture to polymer melt rheology. The focus of the present work is in extracting a measure of polydispersity from rheological data. Various polydispersity measures that have been proposed in the literature are critically examined and their limitations are pointed out. New measures of polydispersity are proposed that overcome these limitations. The evaluation of the various polydispersity measures is performed by reference to rheology fundamentals, with model calculations and examples drawn from industrial practice. The issues of eliminating molecular weight and temperature 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 polydispersity dramatically, while no detectable change appears in the molecular weight distribution obtained using a gel permeation chromatograph. It is demonstrated that the polydispersity measures proposed in the present work, and which are extracted from frequency response data in the linear viscoelastic region, can be used reliably to characterize polydispersity in polymer melts. 0 1995 John Wiley & Sons, Inc.

Effect of long chain branching on linear-viscoelastic melt properties of polyolefins

“…[7] When the LCB content is very low (less than 1 LCB /10 4 C), its effect on the radius of gyration goes unnoticed and values of g ≈ 1 are obtained [42]. The effect of such a low amount of LCB has been explored in LDPE and in modified linear polymers of ethylene/α-olefins and propylene (PP) by irradiation, electron-beam treatment, peroxide reaction, and thermal/mechanical degradation [7,10,17,[44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59]. In principle, lowpressure processes (chromium-based and Ziegler-Natta type catalysts, developed in the 1950s and 1970s, respectively) yield linear species.…”

“…Shroff and Mavridis [142] stated that DRI is a measure of rheological polydispersity and as such depends on both MWD and LCB. DRI could be used as a measure of LCB for polymers of known similar MWD, as could any other measure of rheological polydispersity [54]. …”

“…Mavridis and Shroff [54] described several indices to quantify the rheological polydispersity of polymeric materials. MWD and LCB are specific molecular features that affect these elastic indices.…”

“…Fig. 10 shows the difference between the chromium-based PE series and linear PEs of similar polydispersity index taken from the literature [54]. In principle, the enhanced values of E R in the former could be attributable to a non-symmetric relaxation time spectrum, with a tail of high relaxation times corresponding to large molecular species giving rise to extremely high values of E R .…”

Nitroxide‐mediated controlled degradation of polypropylene

“…In general, it has been concluded that CRPP resins have lower molecular weight, narrower MWD and reduced viscosity. In addition, it has become clear that one of the most significant challenges for the polymer processing engineer is the problem of inter-conversion between rheological data and the MWD (Shroff and Mavridis (1995)). …”