Branched polymers rheological methods

A successful theory, or simulation method, for branched polymer rheology should predict the frequency-dependent linear viscoelasticity of an arbitrary mixture of branched (and linear) spedes, whether those species are stars, H molecules, combs, or irregular branched structures. Such a theory, if it could be developed, would allow one to design polymer branching structures to produce the desired linear viscoelastic response. It could also potentially be used to infer from linear viscoelastic data information about the type of branching present in the melt [56, 57,5]. This latter use of linear viscoelastic data is referred to as analytical rheology. 

An important class of commercial polymers is that of copolymers of ethylene and alpha-olefins, which are commonly referred to as linear low density polyethylenes (LLDPE). The use of a copolymer introduces short-chain side branches onto the polyethylene backbone, and the effect of these short-chain branches on rheological properties depends very much on the method of polymerization. If a heterogeneous, Ziegler catalyst is used, the side-chains tend to be distributed in blocks rather than randomly along the backbone, and Wardhaugh and Williams [74] point out that this can lead to microphase separation in the melt, which could have an important effect on rheological behavior. 

Alditols polyols are readily renewable, inexpensive and harmless to the environment. By incorporation of polyols into aliphatic polyesters, functional linear or hyperbranched polymers can be prepared with specific biological activities and/or that respond to environmental stimuli. Polyesters with carbohydrate or polyol repeat units in chains have been prepared by chemical methods. " In some cases, the reaction conditions led to hyperbranched polymers (HBPs). The highly branched architecture of HBPs leads to unusual mechanical, rheological and compatibility properties. " These distinguishing characteristics have garnered interest for their use in numerous industrial and biomedical fields. Chemical routes to linear polyol-polyesters require elaborate protection-deprotection steps ". Furthermore, condensation routes to hyperbranched polymers generally require harsh reaction conditions such as temperatures above 150 C and highly acidic catalysts ". ... 

By use of chlorosilane chemistry, various branched structures can be prepared. For example, star-branched PBd can be prepared [27] and hydrogenated to produce analogs of star-branched polyethylene [46]. Hadjichristidis etal. [47] have reported the latest developments in the preparation of polyethylene analogs based on butadiene. Using the methods they describe, a remarkable array of structures can be produced, including stars, H-shaped molecules, super-H molecules (three-armed stars at both ends of a backbone segment), pom-poms (multiarmed stars at the ends of a backbone) and combs of various types. Rheological data have been published for the polymers they described [48]. 

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