Sparse long chain branching, LCB(side chains attached to the main polymer backbone), i.e., branching levels typically less than one branch per 1000 backbone carbon atoms, and arm molecular weights, Ma, significantly greater than the critical molecular weight (MW) for entanglements, Me, have been reported to have remarkable effects on the rheology and processing behavior of polyolefins. For example, what is believed to be one branch in every three chains on average can have similar effects on the rheology of a polyethylene(PE) as having multiple branches of various types(long and short), high molecular weight, and a broad molecular weight distribution. The goal of this research is to develop a quantitative theory for the rheological response and associated processing performance of a melt of known polymerization kinetics. In other words, it is desired to predict polymerization conditions and catalyst structure which will lead to the molecular architecture needed to produce the desired rheology, processing performance, and properties. Furthermore, it is desired to evaluate the use of LCB to render ultra-high MW resins melt processable opening the door for producing high performance materials with many applications(e.g. prosthetics, ballistics protection, coatings, etc.).
In order to theoretically design the molecular architecture of polymer chains for generating the desired processing performance, a highly interdisciplinary effort will be required which incorporates experts in experimental and theoretical rheology, polymer processing, polymerization kinetics and catalysts, and polymer synthesis and characterization (this expertise cannot be found in any one location). Scientists from two U.S. universities (Virginia Tech and the University of Tennessee) with expertise in extensional and non-linear rheology, flow birefringence, polymer processing and polymer synthesis and characterization will join forces with scientists from English(7), Dutch(1) and Greek(1) universities. The research effort will capitalize on the Leeds-based Microscale Polymer Processing (MuPP) consortium with contributions from Leeds (molecular rheology, reaction kinetics), Durham and Imperial College-London (chemistry), Sheffield (chemistry and crystallization), Cambridge and Bradford (small-scale processing), and Oxford and Eindhoven (solid state). The group at Imperial College, London is joining this co-operative program with expertise in polymerization catalyst development for tailored molecular structure. Scientists at Leeds are the world leaders in the development of molecular theories for branched polymers. The general approach is to use model systems to establish a rheological standard by which to identify the structures present in commercially produced PEs (standard analytical techniques cannot provide the required information) and then develop correlations between polymerization kinetics, molecular architecture and processing performance.
The basis for designing the branching topology of polyolefins and other resins for processing performance and establishing polymerization conditions for generating this range of topologies will be established which has not been done before for branched polymers. In addition, this program will feed into the developing fundamental research program on the dynamics of branched polymers. Improvements in the molecular theory for the melt rheology of branched polymers will also be an outcome of this cooperative research effort.
This research will have practical implications as it will allow industry to optimize the molecular architecture for processing performance and properties theoretically and thereby shorten time-intensive experimental programs. Graduate students will learn first hand the latest developments in molecular rheology and its use in designing molecules for processing performance. Exchanges between groups will accelerate the learning of both the theory and experimental techniques used in this program. Carefully selected undergraduates from underrepresented groups will be brought into the program to enhance their interest in polymer science and engineering.
