Rheological study of polymer/clay nanocomposites and optical linear rheometer instrumentation
Nanocomposite gels were formed by mixing organically modified clay into a linear polymer, dicarboxyl-terminated polybutadiene (CTPB). Two similar but differently sized counterions were chosen for preparing the organoclay. The polymer-clay interaction causes the clay aggregates to break up into particles, which swell, exfoliate, and eventually assume a stable dispersion in the polymer matrix. The bulkiness of the different-sized counterions affects the dispersion process and the final, stable clay-polymer network structure. The use of two different organoclays allows comparison of the observed phenomena. Parameters of the study are clay concentration &phis; and distance from the gel point. As a main result, the nanocomposite with the larger counterion exfoliates faster but requires more clay to form a percolating structure. It also forms the weaker gel. These seaming contradictions (fast exfoliation but weak gel) are attributed to steric effects of the large counterion. The low frequency linear viscoelastic behavior was analyzed using a percolation model (near the percolation threshold &phis;c) and the power law (far above the percolation threshold). The degree of agreement between the experimental data and models was used to theorize that the particle-polymer interactions are the controlling factor in the increasing solid-like behavior with increasing clay content.
Rheology is also employed to study the dynamics of structure development for a model nanocomposite of clay in a low Mw polymer. The clay, organically modified with macro-counterions, gradually exfoliates in an endfunctionalized linear polymer. Polymer and clay connect into a sample spanning network with increasing modulus and decreasing relaxation time as the exfoliation proceeds and more and more clay surface becomes accessible. The product of modulus and relaxation time stays constant as observed earlier for another physical gel. The rheological measurements suggest that the classical diffusion model of exfoliation is incomplete. Trends in the ripening time, the distance to the final stable clay dispersion and in changing concentrations support a layer-by-layer peeling mechanism over the traditional exfoliation theory. The layer-by-layer peeling mechanism proposes that the polymer chains follow Langmuir isothermal adsorption and pull on the clay stacks, first breaking the clay pack into smaller stacks, and then possibly peeling the clay sheets off one-by-one if the force required to break the stack becomes too great.
Intermittent flow forward (IFF) and intermittent flow reverse (IFR) protocol are used to study the structure breakdown and reformation in CTPB/organoclay nanocomposites, which is found to be caused by the clay layers orientation. The orientation relaxation is accelerated due to the strong interaction between the CTPB and clay surface via hydrogen bonding.
A new optical linear rheometer was developed that allows simultaneously conducting rheological experiments, light scattering, and microscopy. Unlike rotational motor in commercial rheometers, a linear motor is used, which ensure uniform shear through samples.