H. Henning Winter, University of Massachusetts (Amherst)
This study concerns clay/polymer composites in which nanometer-thick clay sheets ("exfoliated clay") get randomly distributed in a polymer matrix, with random orientation of the clay sheets. The focus of the study is on a model for the exfoliation mechanism that describes a recently discovered phenomenon where exfoliation occurs nearly instantaneously (10 minutes at suitable conditions). Surprising is that the self-exfoliation is so fast and that it occurs without the help of stirring or sonication, just by itself after having gently distributed clay particles in the polymer. Unclear is the underlying mechanism that causes the rapid self-exfoliation. Knowledge of the mechanism will allow optimal processing of layered materials, not only clay but also graphite. Such knowledge is important since a suitable arrangement of nm-thick solid sheets in a nanocomposite can result in advantageous properties such as increased heat resistance, low gas permeability, favorable mechanical properties over pure polymer components, increased electrical conductivity, and light weight compared to conventionally filled polymers. Here, we propose a novel mechanism, check its consistency with known observations on nanocomposites, and select an experiment to further test the proposed mechanism.
This experimental study is designed to focuses on nanocomposites with layered silicate (stack of nm-thick organo-clay sheets), such as montmorillonite, as the reinforcing component of a polymer (end-functionalized polybutadiene). The experiments are guided by a newly proposed exfoliation mechanism which consists of three steps. Novel is especially the first step: the proposed pulling of anchored, telechelic ("sticky") macromolecules on the outer surface of clay particles. The macromolecules form a polymer brush. The second step is the swelling of the clay particles to a new quasi-equilibrium state in which the outer brush force, due to the tethered macromolecules, is balanced by the inner cohesion forces between the sheets in the stack. The third and final step is diffusion and pressure driven flow of matrix molecules into the expanded clay galleries to the point that they fully exfoliate. The model can especially explain the nearly instantaneous exfoliation since little time is required for brush absorption and expansion of the clay stacks and the establishing of the force balance. The external brush will form quickly and its entropic force gets instantaneously balanced by swelling the stack to raise the internal cohesion force. The rapid expansion of the clay stacks is followed by a slow approach of the final state by molecular diffusion. It is a two-tier process which became visible in the time-resolved rheology data of this study.
The newly proposed exfoliation mechanism is consistent with the exfoliation dynamics as observed through rheological observations. It also predicts a maximum exfoliation rate at an intermediate temperature. A test of this prediction is at the core of first year's research: The anchored polymer chains are in constant thermal motion. At higher temperatures the entropic pulling force increases but macromolecules have less probability to remain absorbed on the clay surface as the hydrogen bonds are overcome by the increasing thermal energy. These competing phenomena can be expected to increase the entropic pulling of the brush at a moderate temperature rise but will eventually break down the entopic pulling action since fewer and fewer surface molecules will be able to anchor. Fastest exfoliation is expected at an intermediate temperature. Experiments of this study confirm the predicted temperature dependence. The exfoliation rate increases when raising the temperature by a reasonable amount. However, beyond a certain temperature, the entropic pulling force weakens due to decreased surface coverage (diminished brush).
A research paper is about to be submitted to Macromolecules, an ACS journal.
A related study focused on the relaxation dynamics of amorphous materials, which are in the approach a liquid-to-solid transition from the liquid side (LSTLS). Linear viscoelastic experiments on two representative materials focus on the distribution of relaxation modes and the increasing elasticity in the LSTLS approach. The first material is a concentrated colloidal suspension; it represents the glass transition. The second material is a crosslinking polymer far above its glass transition; it represents gelation. For both, the relaxation time spectrum broadens significantly near LSTLS and was found to share the same powerlaw format. A distinctive difference comes from the powerlaw exponent, n, which is positive for the glass transition, n>0, and negative for gelation (-1<n<0). The spectrum, H= 0 , is cut off above the longest relaxation time, which belongs to the diverging, largest cluster in the glass or gel. The front factor, H0, determines the basic stress level in a relaxation process. Stress under deformation is governed by a wide range of relaxation modes; as argued here, short modes overpower the long modes in gelation and long modes dominate over short ones in the glass transition, i.e. the relaxation patterns are inverse with respect to each other. Several examples are shown for each class of materials in order to test the proposed transition behavior for glasses (colloidal and molecular) on the one hand and chemical/physical gels on the other. Among several results, this experimental study provides a decisive criterion that distinguishes the glass transition from gelation.
The study was submitted to "Soft Matter" under the title: The Glass Transition as Rheological Inverse of Gelation. The review process is in progress.