Microscopic theory for dynamics in entangled polymer nanocomposites

Nanocomposites;Polymer Dynamics;Entanglement;Nanoparticles

Yamamoto, Umi

关键词: Nanocomposites;    Polymer Dynamics;    Entanglement;    Nanoparticles;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/49732/Umi_Yamamoto.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

New microscopic theories for describing dynamics in polymer nanocomposites are developed andapplied. The problem is addressed from two distinct perspectives and using two different theoreticalapproaches.The first half of this dissertation studies the long-time and intermediate-time dynamics ofnanoparticles in entangled and unentangled polymer melts for dilute particle concentrations. Using a combination of mode-coupling, Brownian motion, and polymer physics ideas, the nanoparticle long-time diffusion coefficients is formulated in terms of multiple length-scales, packing microstructures, and spatially-resolved polymer density fluctuation dynamics. The key motional mechanism is described via the parallel relaxation of the force exerted on the particle controlled by collective polymer constraint-release and the particle self-motion. A sharp but smooth crossover from the hydrodynamic to the non-hydrodynamic regime is predicted based on the Stokes-Einstein violation ratio as a function of all the system variables. Quantitative predictions are made for the recovery of the Stokes-Einstein law, and the diffusivity in the crossover regime agrees surprisingly well with large-scale molecular dynamics simulations for all particle sizes and chain lengths studied. The approach is also extended to address intermediate-time anomalous transport of a single nanoparticle and two-particle relative diffusion.The second half of this dissertation focuses on developing a novel dynamical theory for aliquid of infinitely-thin rods in the presence of hard spherical obstacles, aiming at a technical and conceptual extension of the existing paradigm for entangled polymer dynamics. As a fundamentaltheoretical development, the two-component generalization of a first-principles dynamic meanfieldapproach is presented. The theory enforces inter-needle topological uncrossability and needlesphereimpenetrability in a unified manner, leading to a generalized theory of entanglements thatincludes the sphere excluded volume effect. Coupled self-consistent equations for the generalized diffusion tensors are constructed, and the expressions for the transverse localization lengths and the long-time diffusion coefficients are derived. In the static sphere limit, we find the effectivetube diameter is generally reduced as a function of a single confinement parameter that quantifies the number of particles penetrating into the pure-polymer tube. A preliminary extension to treat flexible chain melts has also been achieved, and shown to agree reasonably well with simulations. The anisotropic needle diffusion constants are rich functions of the length-scale ratios, needle concentration and particle volume fraction. We show that the steric blocking of the longitudinal motion causes a literal and simultaneous localization of the two diffusion channels, and entangled needles can diffuse via a modified reptation dynamics over a window of polymer concentration but the compression of the tube and the blocking of the reptation motion must be accounted for. Generalization to treat mobile spheres is also possible and fully formulated.