H.H. Winter, University of Massachusetts Amherst
The program goal is to gain a deeper understanding of flow-induced crystallization of polymers. These advances include rheo-optical experiments under shear, rheo-optical experiments under uniaxial extension, and modeling with several of the most advanced rheological constitutive equations for predicting flow-induced molecular stretch and orientation.
A unique optical micro-rheometer and a novel filament stretching extensional rheometer, recently obtained under an NSF MRI grant, are capable of simultaneously measuring the evolution of stress and material structure as a function of time and accumulated strain in homogeneous shear and extensional flows. Simultaneous observation of light scattering, microscopy, fluorescence, and birefringence from the sample make it possible to obtain time-correlated structural information of the polymer melts. These new instruments were designed for studies on very small samples at the large shear and extension rates often encountered in polymer processing. They will allow polymer crystallization exploration by facilitating the first simultaneous measurements of stress and crystal structure growth in response to homogenous shear flows, homogeneous uniaxial extensional flows, and precisely controlled sequential homogeneous shear and uniaxial extensional flows.
Modeling the effect of strain on the polymer conformation and stretch will use newly developed cyber infrastructure (computer aided) methods that have become available by combining rheological experiments with molecular theory for non-linear viscoelasticity. Advanced molecular dynamics theories will predict the flow-induced molecular stretch and orientation in polymer melts of various molecular topology (linear, short-chain branched, stars, pom-pom) and various molecular weight distributions. Model predictions might be able to classify crystallization dynamics at large stresses and strains and connect the observations back to the molecular architecture.
Intellectual Merit: The proposed flow-induced polymer crystallization research represents a strong synergy between newly developed experimental techniques and facilities and state-of-the-art constitutive models and theories to explore the stress and structure of fluids near the gel point. A series of well-designed experiments will be performed into several key areas of quiescent crystallization and flow-induced crystallization of polymer melts. Strong homogenous shear and extensional flows will be used to explore the role of shear rate, extension rate, strain, strain energy, and branching on the rate of crystal nucleation and growth of polymer melts.
Broader Impacts: The proposed research can have significant impact on a host of commercial applications from facilitating the design of materials with application specific properties to reducing the time and cost of manufacturing polymer based parts. These advances require not only a detailed understanding of the role of shear and extensional flows on the crystallization process, but the development of processing protocols and equipment which can precisely control the dynamics of crystallization. In addition, the proposed work will enhance education, by including both graduate and undergraduate students in the research project. The college's successful Minority Engineering Program will help involve underrepresented groups in the proposed research. Cyberinfrastructure (CI) students will directly access several of the world's most advanced theories of polymer dynamics. The combination of experimental techniques and the CI constitutes a powerful teaching tool to share with faculty of Northeast Alliance for Graduate Education Partner (NEAGE) institutions. Dr. Estevez (U. of Puerto Rico) will collaborate in the development of CI modules specifically designed for implementation in his materials science curriculum. With the aid of the NEAGE, he will visit UMass Amherst to learn more about the CI platform and the proposed experiments and will then integrate them into his courses. Activities like CI build bridges to the science and technology community making it possible for students of all backgrounds to engage in science and technology at a very high level.
