Evolution of the molecular weight distribution and linear viscoelastic rheological properties during the reactive extrusion of polypropylene

1995 ◽  
Vol 57 (2) ◽  
pp. 151-173 ◽  
Author(s):  
D. W. Mead
Keyword(s):  
Molecular Weight ◽  
Rheological Properties ◽  
Molecular Weight Distribution ◽  
Weight Distribution ◽  
Reactive Extrusion ◽  
Linear Viscoelastic

Viscoelastic Characterization of Long Branching and Gel in Butadiene-Acrylonitrile Copolymer Elastomers

1980 ◽  
Vol 53 (1) ◽  
pp. 14-26 ◽  
Author(s):  
N. Nakajima ◽  
E. R. Harrell
Keyword(s):  
Molecular Weight ◽  
Molecular Weight Distribution ◽  
Weight Distribution ◽  
General Rule ◽  
The Other ◽  
Model Systems ◽  
Newtonian Flow ◽  
Long Chain Branching ◽  
Linear Viscoelastic ◽  
Definition Of

Abstract Difficulties in relating long-chain branching to processability may be attributable to two causes: one is the definition, pertinent to processability, of what long branches are and the other is a method of determining long branching which is free from interference by other material variables, such as molecular weight distribution, gel, and “short” branches. Measurements of the dilute solution properties are tedious, time-consuming, and require skill for precision. In addition, the requirement for filtering the solution practically obliterates the result, regardless of how precise the measurement may be, because elastomers, as a general rule, have or are suspected to have an insoluble gel fraction. Recent advances in viscoelastic studies of model polymers show that the branches must be 2–3 times longer than the “entanglement coupling” distance in order to exhibit enhancement of viscosity in the Newtonian flow. Whereas Newtonian flow provides a precise definition of the long branches, it is not accessible for most of the elastomers. In the observed time scale, the linear viscoelastic properties as well as the steady-state viscosities are affected not only by branches but also by gels and molecular weight distribution. When these material variables are changed one at a time in the properly designed model systems, their effects are separately observable. On the other hand with a sample of unknown background, the effect of long branching is usually inseparable from those of other variables.

Rheology of Flexible Homopolymers

2007 ◽  
Author(s):  
Chang Dae Han
Keyword(s):  
Molecular Weight ◽  
Shear Flow ◽  
Rheological Properties ◽  
Molecular Weight Distribution ◽  
Rheological Behavior ◽  
Weight Distribution ◽  
Polymer Processing ◽  
Random Copolymers ◽  
Profound Influence ◽  
Theoretical Predictions

Numerous flexible homopolymers and flexible random copolymers are commercially available. Thus, understandably, a number of research groups have reported on the rheological behavior of flexible homopolymers and flexible random copolymers in the bulk and solution states. There are too many studies to cite them all here. In Chapters 3 to 5 we presented the rheological behavior, in general terms, of linear flexible homopolymers in steady-state shear flow, elongational flow, and/or oscillatory shear flow. In this chapter, we present the effects of temperature, molecular weight (although in Chapter 4 we presented theoretical predictions of the effect of molecular weight), and molecular weight distribution on the rheological behavior of linear flexible homopolymers, and also flexible homopolymers with long-chain branching. The rheological behavior of much more complex polymer systems is presented in other chapters of this volume. From the point of view of polymer processing, temperature is one of the most important variables that greatly affect the rheological behavior of polymeric liquids. Therefore, it is very important to present the effect of temperature on rheological behavior, placing emphasis on the methods that enable one to obtain temperature-independent correlations for rheological properties. Such correlations, when available, will help one to estimate the rheological properties of the same polymer without conducting additional experiments. With respect to polymer synthesis and polymer processing, a better understanding of the effects of molecular weight and molecular weight distribution on the rheological behavior of a polymer is of fundamental importance. In Chapter 4 we have presented molecular theory, demonstrating that the molecular weight of a linear flexible homopolymer has a profound influence on its rheological properties. Thus, information on the relationships between molecular weight and rheology, when available, will help one to choose, with little waste of time and effort, optimum processing conditions. One of the common features of all commercial homopolymers is that they are polydisperse and, therefore, it is not difficult to surmise that the molecular weight distribution of a polymer also has a profound influence on its rheological properties.

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