Dynamic Heterogeneity in Optically Perfect Bulk Polymers

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Patterson The molecular dynamics of bulk polymers can be characterized by mechanical processes with a distribution of relaxation rates. The measurable properties of amorphous polymers are determined by the coupling of the macroscopic variable with these microscopic processes. The goal of this research is to explain the behavior of longitudinal sound waves in optically perfect bulk polymers in terms of the longitudinal relaxation processes in the material. The knowledge of the distribution of relaxation rates for the processes which determine the longitudinal modulus will then be used to predict the mechanical properties of these materials. Many applications, such as optical fibers, require polymeric materials which scatter only the minimum amount of light determined by the compressibility of the liquid. A series of n-alkyl methacrylate(n=1,2,4,6) samples which achieve this limit of clarity will be used in the research. The optical perfection of the samples will be measured by Rayleigh-Brillouin spectroscopy. The Rayleigh-Brillouin spectrum depends on the frequency of the longitudinal sound waves responsible for the light scattering. When there are microscopic processes with relaxation rates lower than the sound frequency, the spectrum displays a central peak whose shape is determined by the relaxation strengths and rates associated with these motions. In the frequency regime important for mechanical processes such as toughness, (106 to 10-2 Hz), the measurement of dynamic light scattering is carried out using photon correlation spectroscopy. The relaxation function for the longitudinal density fluctuations which give rise to light scattering will be measured as a function of termperature, pressure, and chemical structure. The samples will be maintained in the equilibrium liquid state above the glass transition. The light scattering data will be used to calculate the distribution of relaxation rates associated with the longitudinal modulus. The resulti ng distribution will be correlated with the chemical structure of the molecules. The study of the shape of the distribution as a function of pressure will allow identification of modes of motion that couple weakly to the volume of the liquid. These processes retain their relaxation strength in the glassy state and are less subject to physical aging. %%% Typical tough polymeric materials are sufficiently structurally heterogeneous that they are opaque. One of the most important new applications of polymers is optical fiber. In order to be useful in this application, the material must be sufficiently optically homogeneous. The challenge is to produce a material which is optimized both for optical clarity and for mechanical toughness. The research will employ bulk polymer samples that have been prepared in optically perfect form. Light scattering spectroscopy is well suited for the study of both the optical and mechanical properties of amorphous polymers.