Fundamental understanding of activated glassy dynamics and kinetic arrest is relevant to key properties of functional polymeric materials such as nanocomposites, single ion conductors, separation membranes, and thermoplastics. I will present a new microscopic, force-based statistical mechanical theory for structural relaxation and diffusion in dense mixtures of glass-forming polymers and particles. The approach treats in unified manner the entire range of particle sizes from the small penetrant or ion limit to the large nanoparticle regime. The focus is on miscible systems stabilized by an attraction between polymers and particles. Dynamical constraints are quantified using the equilibrium PRISM theory of structural pair correlation functions which captures packing motifs associated with entropic depletion, steric stabilization, and polymer or particle mediated tight bridging. Central issues include whether the addition of particles plasticize or anti-plasticize the polymer liquid, the degree of dynamical coupling between segments and particles, penetrant diffusivity, and the emergence of kinetically arrested nonequilibrium states. For small enough particles (penetrants, ions), large decoupling can occur corresponding to a two-step relaxation process which depends strongly on segment to particle diameter ratio, particle volume fraction, and attraction strength. When the size ratio is of order unity, structural relaxation is cooperative corresponding to strongly coupled or slaved segmental and particle activated motion which results in maximizing anti-plasticization effects. As the particle becomes increasing larger than the polymer segment (nanocomposites), strong decoupling again occurs with segments relaxing in an effectively static environment of immobile nanoparticles. Moreover, in this regime the addition of nanoparticles can either speed up or slow down segmental relaxation relative to the neat polymer melt due to a competition between free volume creation which weakens caging and the formation of long-lived physical bonds.

Activated relaxation, diffusion and kinetic arrest in dense polymer-particle mixtures: From nanoparticles to penetrants

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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