Dynamic Heterogeneity in Optically Perfect Bulk Polymers

Patterson本体聚合物的分子动力学可以通过具有弛豫速率分布的机械过程来表征。 无定形聚合物的可测量性质是由宏观变量与这些微观过程的耦合决定的。 本研究的目的是解释纵向声波的行为，在光学完美的块体聚合物的纵向弛豫过程中的材料。 确定纵向模量的过程的弛豫率分布的知识将用于预测这些材料的机械性能。 许多应用，例如光纤，需要仅散射由液体的可压缩性确定的最少量的光的聚合物材料。 将在研究中使用一系列达到该澄清度限度的甲基丙烯酸正烷基酯（n= 1、2、4、6）样品。 样品的光学完整性将通过瑞利-布里渊光谱测量。 瑞利-布里渊频谱取决于负责光散射的纵向声波的频率。 当存在弛豫速率低于声音频率的微观过程时，频谱显示中心峰，其形状由与这些运动相关的弛豫强度和速率确定。 在对机械过程（如韧性）重要的频率范围内（106至10-2 Hz），使用光子相关光谱法进行动态光散射测量。 引起光散射的纵向密度波动的弛豫函数将被测量为温度、压力和化学结构的函数。 样品将保持在高于玻璃化转变的平衡液体状态。 光散射数据将用于计算与纵向模量相关的弛豫率分布。 所得分布将与分子的化学结构相关。 作为压力的函数的分布的形状的研究将允许识别弱耦合到液体的体积的运动模式。 这些过程在玻璃态下保持其松弛强度，并且较少受到物理老化的影响。 典型的坚韧聚合物材料在结构上是充分不均匀的，因此它们是不透明的。 聚合物最重要的新应用之一是光纤。 为了在该应用中有用，材料必须是足够光学均匀的。 挑战在于生产一种在光学透明度和机械韧性方面都得到优化的材料。 这项研究将采用光学完美形式制备的散装聚合物样品。 光散射光谱非常适合于研究无定形聚合物的光学和机械性能。

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.