(94a) Ubiquity of Soft Glassy Dynamics in Polypropylene-Clay Nanocomposites

Polymer-clay nanocomposites (PCNs) exhibit rich rheological behavior, including significant time-dependence over exceptionally long time scales. Furthermore, the relaxation of flow-induced clay orientation has been shown to be non-Brownian and perhaps influenced by attractive interactions between organoclay domains.

Central to the discussion of organoclay disorientation kinetics and relaxation in PCNs is the analogy to the dynamics of soft colloidal glasses put forth by Ren et al. Glassy colloidal suspensions have mechanical properties similar to metastable soft solids (e.g., solid-like behavior with a finite yield stress, thixotropy under deformation, and slow recovery from a deformation), properties that may evolve continuously over time. The continuous time evolution of mechanical properties is an ?aging? phenomenon characteristic of systems that are far from equilibrium and associated with the metastable, heterogeneous structure of soft materials on microscopic to mesoscopic length scales. It has been hypothesized that structural disorder creates energy barriers (like those associated with the rearrangement of droplets in an emulsion) that prevent reorganization into states of lower free energy, and those energy barriers become greater the longer a system is aged such that the longest relaxation time continuously increases. In this environment, particle motion has been described by a cage-diffusion process. Small excursions within the ?cage? formed by neighboring particles occur on fast time scales, while rearrangement of the cages themselves, tantamount to particles escaping from their cages, occurs over increasingly long time scales. During this slow "gelation" process, particles may form clusters that eventually collect into a percolated network. The imposition of a deformation in excess of the yield stress, however, is postulated to rejuvenate the energy landscape, analogously to increasing the temperature above the glass transition. Aging effects in soft materials are seldom reported, regarded instead as artifacts obfuscating the true behavior of the material. In fact, melt blended PCNs should be inherently out-of-equilibrium, having experienced intense shearing and subsequent rapid cooling. Consequently, the time-dependent phenomena discussed previously that are characteristic of soft glassy aging dynamics is germane to fundamental PCN research and industrial processing.

Melt-blended PCNs are inherently out-of-equilibrium, having experienced intense shearing and subsequent rapid cooling. Here, we focus upon a series of melt-blended polypropylene-clay nanocomposites whose rheology bears the logarithmic time dependence characteristic of glassy systems, irrespective of clay concentration, exfoliation, dispersion, and compatibilization. Both small-amplitude oscillatory shear, steady shear, and a combination of the two are employed in order to contrast the transient rheology of unsheared (i.e., as-processed) and sheared (i.e., flow-aligned) samples and the influence of time, thermal, and deformation history.