Session F20: Invited Session: Polymer Physics Prize

Polymer Prize Lecture: A molecular perspective on the deformation of polymer glasses

The mechanical properties of polymer glasses, including yield and plastic flow, are important for many applications. In contrast to the flow of polymer melts, plastic flow is poorly understood at a fundamental level. One reason for this is that the deformation of polymer glasses typically occurs in a highly nonlinear regime, e.g., doubling the strain rate has little impact on the flow stress. Eyring proposed that stress increases the rate of molecular rearrangements in solids and this is the source of nonlinearity in many models. Our group has developed an optical technique to measure molecular mobility and shown that mobility during deformation can increase by more than a factor of 1000. In this talk, I will highlight recent progress including understanding the role of deformation temperature and a comparison between molecular and mechanical relaxation times. Results from recent computer simulations and molecular theories will be discussed. Finally, some comments will be made on the deformation of polymer glasses in comparison to colloidal and metallic glasses.   

Nanoparticles in liquid crystals, and liquid crystals in nanoparticles

Liquid crystals are remarkably sensitive to interfacial interactions. Small perturbations at a liquid crystal interface, for example, can be propagated over relatively long length scales, thereby providing the basis for a wide range of applications that rely on amplification of molecular events into macroscopic observables. Our recent research efforts have focused on the reverse phenomenon; that is, we have sought to manipulate the interfacial assembly of nanoparticles or the organization of surface active molecules by controlling the structure of a liquid crystal. This presentation will consist of a review of the basic principles that are responsible for liquid crystal-mediated interactions, followed by demonstrations of those principles in the context of two types of systems. In the first, a liquid crystal is used to direct the assembly of nanoparticles; through a combination of molecular and continuum models, it is found that minute changes in interfacial energy and particle size lead to liquid-crystal induced attractions that can span multiple orders of magnitude. Theoretical predictions are confirmed by experimental observations, which also suggest that LC-mediated assembly provides an effective means for fabrication of plasmonic devices. In the second type of system, the structure of a liquid crystal is controlled by confinement in submicron droplets. The morphology of the liquid crystal in a drop depends on a delicate balance between bulk and interfacial contributions to the free energy; that balance can be easily perturbed by adsorption of analytes or nanoparticles at the interface, thereby providing the basis for development of hierarchical assembly of responsive, anisotropic materials. Theoretical predictions also indicate that the three-dimensional order of a liquid crystal can be projected onto a two-dimensional interface, and give rise to novel nanostructures that are not found in simple isotropic fluids.   

Broadband Coherent Raman Scattering for Rapid Spectroscopic Imaging

Over the past ten years, coherent Raman imaging (CRI) has evolved from a curiosity to a practical tool for investigating some classes of biological and material questions. An important key to this evolution has been the ability to rapidly obtain information from many spectral peaks. Most vibrational spectroscopic information is found in the fingerprint region where spontaneous Raman can be used to achieve \textgreater 3:1 signal to noise ratio for weak fingerprint peaks in biological systems, but typically requires acquisition times of several seconds; too slow for imaging. Coherent Raman methods have previously been unable to acquire high quality fingerprint spectra. We have overcome this limitation by developing a highly efficient signal excitation paradigm and appropriately harnessing the nonresonant background (NRB) signal that accompanies the resonant signal of interest. With these and other innovations, we have developed a CRI approach based on broadband coherent anti-Stokes Raman scattering (BCARS) that provides an unprecedented combination of speed, sensitivity, and chemical selectivity [1]. Using this system we are able to obtain high quality Raman spectra in the fingerprint and CH stretch regions from biological specimens at 3.5 ms, enabling rapid, label-free chemical imaging of even delicate samples. I will briefly put our approach in context with the broader CRI field, describe key technical features of the present imaging system and provide application examples in materials and biology. I will also briefly discuss focus areas for future advances, and speculate on ultimate performance limits for coherent Raman imaging. \\[4pt] [1] Camp, J. J., Lee, Y. J., Heddleston, J. M., Hartshorn, C. M., et al. High-speed coherent Raman fingerprint imaging of biological tissues. Nat Photon 8, 627-634 (2014).   

What have we learned after 20 years of measuring glass transitions in thin polymer films?

For the past 20 years, there has been significant experimental, computational, and theoretical work on anomalous dynamics in thin films of glass forming polymers. Even considering only the single material, polystyrene(PS), the number of experiments, and the wide range of conclusions that have been reached from these experiments is striking. In this talk, I will discuss dilatometric measurements of the glass transition temperature in thin PS films, measures of enhanced surface mobility in glassy PS, and a simple picture of how we can use surface mobility to develop an understanding of the thin film glass transition. Finally, I will discuss the relation between the length scale of surface mobility to the long discussed length scale for dynamic correlation in general glass forming materials.   

Surface Mediated Self-Assembly of Amyloid Peptides

Amyloid fibrils have been considered as causative agents in many neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, type II diabetes and amyloidosis. Amyloid fibrils form when proteins or peptides misfold into one dimensional crystals of stacked beta-sheets. In solution, amyloid fibrils form through a nucleation and growth mechanism. The rate limiting nucleation step requires a critical concentration much larger than those measured in physiological conditions. As such the exact origins of the seeds or oligomers that result in the formation of fully mature fibrils in the body remain topic intense studies. It has been suggested that surfaces and interfaces can enhance the fibrillization rate. However, studies of the mechanism and kinetics of the surface-mediated fibrillization are technologically challenging due to the small size of the oligomer and protofibril species. Using smart sample preparation technique to dry the samples after various incubation times we are able to study the kinetics of fibril formation both in solution and in the vicinity of various surfaces using high-resolution atomic force microscopy. These studies elucidate the role of surfaces in catalyzing amyloid peptide formation through a nucleation-free process. The nucleation free self-assembly is rapid and requires much smaller concentrations of peptides or proteins. We show that this process resembles diffusion limited aggregation and is governed by the peptide adhesion rate, two -dimensional diffusion of the peptides on the surface, and preferential interactions between the peptides. These studies suggest an alternative pathway for amyloid formation may exist, which could lead to new criteria for disease prevention and alternative therapies.