We formulate a theoretical mechanism for the physical origin of the massive dynamic fragility range observed in long chain glass-forming polymer melts within the context of the force-level elastically collective nonlinear Langevin equation theory of coupled local–nonlocal activated segmental relaxation. The hypothesis involves how the cage scale barrier hopping process on the three Kuhn segment length scale is quantitatively coupled to the longer range collective elastic distortion required to sterically allow large-amplitude events to occur. The key nonuniversal aspect is proposed to be an effective microscopic jump distance, a dynamical quantity associated with the activation barrier, which is influenced by nanometer-scale conformational transition physics and monomer chemistry. By introducing a single numerical factor that breaks the universality of the jump distance in our mapping of polymers to liquids of disconnected Kuhn-sized hard spheres, one can account rather well, and simultaneously, for the vitrification temperatures and dynamic fragilities of 17 polymer liquids of diverse chemistry. The very low fragilities of polyisobutylene (PIB) and polyethylene are suggested to be a consequence of suppression of the collective elastic distortion effect. The large fragility variations displayed by polymeric materials appears special to long chain melts, consistent with their absence in molecular and oligomer liquids. Connections between cooperativity and fragility are identified. The theory very accurately captures segmental relaxation time experimental data for PIB, polypropylene, and polycarbonate melts over 11–13 decades. The present work sets the stage for attempting to understand the failure of time–temperature superposition in the deeply supercooled regime.

Polymer nanocomposites (PNCs) are important materials that are widely used in many current technologies and potentially have broader applications in the future due to their excellent property tunability, light weight, and low cost. However, expanding the limits in property enhancement remains a fundamental scientific challenge. Here, we demonstrate that well-dispersed, small (diameter ∼1.8 nm) nanoparticles with attractive interactions lead to unexpectedly large and qualitatively different changes in PNC structural dynamics in comparison to conventional nanocomposites based on particles of diameters ∼10-50 nm. At the same time, the zero-shear viscosity at high temperatures remains comparable to that of the neat polymer, thereby retaining good processability and resolving a major challenge in PNC applications. Our results suggest that the nanoparticle mobility and relatively short lifetimes of nanoparticle-polymer associations open qualitatively different horizons in the tunability of macroscopic properties in nanocomposites with a high potential for the development of advanced functional materials.

We construct a new theory for transient aspects of the shear rheology of entangled chain liquids. Within an established tube model constitutive equation framework, four new physical features are introduced: a tension blob scaling derivation of the interchain grip force that generates chain stretch, a force imbalance condition for the termination of affine stretch deformation, a delayed chain retraction process that after loss of grip is accelerated for fast deformations, and a distribution of tube diameters. Nonclassical predictions are made for the stress–strain curve to just beyond the overshoot, the existence of a master curve, and fractional power law scaling of the overshoot strain and stress at high shear rates, all in good agreement with experiment and simulation. Testable new predictions are made for chain stretch dynamics.

We generalize our recent new ideas for the continuous startup shear rheology of entangled polymer liquids in the transient stress overshoot regime to formulate a theory for the full constitutive response and nonequilibrium dynamics over all time scales, deformation rates, and degrees of entanglement. The convective constraint release (CCR) idea that chain retraction locally triggers disentanglement in a shear-rate-dependent manner is significantly modified to be physically consistent with our nonclassical treatment of delayed retraction due to an entanglement grip force. A detailed numerical study of the predictions of the theory is presented for the full stress–strain response, scaling behavior of the stress overshoot and undershoot features, orientational stress, primitive path contour length dynamics, and nonequilibrium steady-state properties spanning the slow and fast nonlinear deformation regimes. For deformations slow enough there is little or no chain stretch, our results are qualitatively the same as in prior tube-based models. However, under fast deformation conditions, we make qualitatively new predictions for all rheological and dynamic properties that are not contained in any existing models. No-fit-parameter quantitative comparisons are made with experimental and simulation studies, and very good agreement is found; testable predictions are made. Strong connections between properties (stress and degree of chain stretch) at the overshoot and in the steady state are found, which suggests common physics exists at the elastic–viscous crossover (stress overshoot) and for long time flow associated with a delayed onset of primitive path retraction and emergence of CCR.

We propose a microscopic theory for the decoupling of self-diffusion and structural relaxation in glass-forming liquids within the Elastically Collective Nonlinear Langevin Equation (ECNLE) activated dynamics framework. Our central hypothesis is that the heterogeneity relevant to this problem is static fluctuations of local density on the scale of 3–4 particle diameters and how this changes local packing correlations. These fluctuations modify the degree of dynamical cage expansion that mechanistically couples intracage large amplitude hopping and longer range collective elasticity in ECNLE theory. Decoupling only emerges in the deeply supercooled regime where the strongly temperature dependent elastic barrier becomes non-negligible relative to its noncooperative local analog. The theory makes predictions for various aspects of the decoupling phenomenon, including apparent fractional power law Stokes-Einstein behavior, that appear to be consistent with experiments and simulations on hard sphere fluids and molecular liquids. Of central importance is a microscopic connection between the barrier fluctuation variance and most probable barrier height. Sensible results are also obtained for the nonexponential stretching of a generic relaxation time correlation function and its temperature evolution. Nonuniversality can arise from the relative importance of the local and collective barriers (related to fragility) and the precise magnitude of the length scale that defines the transition from local cage to elastic physics. Comparison is made with a traplike model based on a Gaussian distribution of barriers.