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John M. Torkelson
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Polymers:
An environmentally benign, continuous process called solid-state shear pulverization is being studied in order to achieve compatibilization of immiscible polymer blends lacking functional groups and without chemical modification or use of melt processing. Additionally, the fundamental, molecular-scale mechanisms of compatiblization are being investigated. These studies have revealed that absolute compatibilization of a variety of immiscible blends can be achieved via pulverization and that the origin of this compatibilization is in situ block copolymer formation. The block copolymers result from small to moderate levels of chain scission in the process, leading to polymeric radicals that can combine at interfacial regions that are continually being reformed in the pulverization process. We have also shown that pulverization is useful in producing well-exfoliated polymer-clay nanocomposites in systems for which conventional melt processing yields relatively little exfoliation, such as nonpolar polymer (unmodified polypropylene) hybrids with clay. Work is also underway with regard to issues of sustainability; this work involves blends and nanocomposites made with polymers that are from renewable resources and/or that are biodegradable Novel optical sensors have been developed to characterize the glass transition temperature (Tg), gelation, and physical aging in polymers, resulting in the observation and characterization of great differences in behavior of nanoconfined, ultrathin films relative to bulk. In the most recent work (see Nature Materials 2, 695 (2003)), we have completed the first study that quantitatively probes the distribution of glass transition temperatures within polymer films. This study employed a novel, multilayer film method in which optical labels that reveal the value of Tg are covalently attached at tracer levels in only one layer (12- to 14-nm thick) in the multilayer film. This study found a major increase in polymer dynamics associated with Tg near the free surface of the films, including buried layers several tens or nanometers into the film, yielding significant reductions in Tg. In cases in which there is an attractive interaction between polymer and the substrate on which the polymer film sits, e.g. a poly(2-vinyl pyridine) film on glass with hydroxyl groups at the glass surface, there is also a dramatic reduction in the polymer dynamics (major increase in Tg) some tens of nanometers into the film from the substrate. These effects are much larger than had been hypothesized in simple two- and three-layer models that purportedly explained Tg-nanoconfinement effects, and clearly show that a perturbation to dynamics at one location in the glass former can have long-range influence elsewhere in the glass former due to the highly cooperative dynamics associated with Tg. We have also used single-molecule spectroscopy and simulation to follow probe diffusion in polymers, providing an explanation of how diffusion of small molecules whose motion is coupled to cooperative segmental mobility is affected by nanoscale dynamic heterogeneity. We are also developing optical sensors as pressure-sensitive paints and systems for cure monitoring. An understanding of the role that oligomer and polymer radical diffusion plays in autoacceleration (the “gel effect”) of free radical polymerization is being achieved. This work, combining experiment with simulation, has also revealed the role of chain transfer reactions in mitigating reaction runaway. Controlled radical polymerization methods are also used to prepare and study a new class of polymers called gradient copolymers that have molecular structures intermediate between random copolymers and block copolymers. Additionally, some of the limitations associated with the production of high molecular weight polymers via controlled radical polymerization methods have been explored. Finally, a novel strategy for producing block copolymers from mixtures of homopolymers made by controlled free radical polymerization processed in the melt state has been demonstrated. The application of this method for compatibilization of immiscible blends is currently under investigation. Recent Publications P. Rittigstein and J.M. Torkelson, “Polymer-Nanoparticle Interfacial Interactions in Polymer Nanocomposites: Confinement Effects on Glass Transition Temperature and Suppression of Physical Aging,” J. Polym. Sci. Part B Polym. Phys. 44, 2935-2943 (2006). Y. Tao, J. Kim, and J.M. Torkelson, “Achievement of Quasi-Nanostructured Polymer Blends by Solid-State Shear Pulverization and Compatibilization by Gradient Copolymer Addition,” Polymer 47, 6773-6781 (2006). J. Kim, M. Mok, R. Sandoval, D.J. Woo, and J.M. Torkelson, “Uniquely Broad Glass Transition Temperatures of Gradient Copolymers Relative to Random and Block Copolymers Containing Repulsive Comonomers,” Macromolecules 39, 6152-6160 (2006). J. Kim, H. Zhou, S.T. Nguyen, and J. M. Torkelson, “Synthesis and Application of Styrene/4-Hydroxystyrene Gradient Copolymers Made by Controlled Radical Polymerization: Compatibilization of Immiscible Polymer Blends via Hydrogen Bonding Effects,” Polymer 47, 5799-5809 (2006). R.D. Priestley, C.J. Ellison, L.J. Broadbelt, and J.M. Torkelson, “Structural Relaxation of Polymer Glasses at Surfaces, Interfaces and in between,” Science 309, 456 (2005). C.J. Ellison, M.K. Mundra, and J.M. Torkelson, “Impacts of Polystyrene Molecular Weight and Modification to the Repeat Unit Structure on the Glass Transition-Nanoconfinement Effect and the Cooperativity Length Scale,” Macromolecules 38, 1767-1778 (2005). J. Kim, M.K. Gray, H.Y. Zhou, S.T. Nguyen, and J.M. Torkelson, “Polymer Blend Compatibilization by Gradient Copolymer Addition during Melt Processing: Stabilization of Dispersed Phase to Static Coarsening,” Macromolecules 38, 1037-1040 (2005). Awards and Honors
Prof. John M. Torkelson
tel: 847/491-7449, 491-3553
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Administrative & Emeritus Faculty Master of Biotechnology Program Faculty and Staff
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