| Polymer melt intercalation of organically modified mica-type layered silicates (OLS) is a viable route for the synthesis of polymer-ceramic nanocomposites. Hybrids are formed by annealing a mixture of polymer and OLS above the softening point of the polymer. During the anneal, polymer chains diffuse from the melt into the silicate interlayer, producing hybrids with nanoscale phase dimensions. Within the interlayer, the polymer chains adopt a collapsed, two-dimensional conformation and exhibit properties that are qualitatively different from polymer chains in the bulk, such as a marked suppression of the glass transition. Polymer intercalation from the melt depends critically on silicate functionalization and constituent interactions. Using infrared spectroscopy (FTIR), the interlayer structure of the OLS was studied as a function of temperature, packing density and length of the functionalizing chains. As the packing density or length decreases or the temperature increases, the chains adopt a more disordered, liquid-like structure; at intermediate cases, the chains exhibit liquid crystalline character. To clarify the importance of enthalpic and entropic factors, a mean-field, lattice-based description of polymer melt intercalation was developed. The entropy loss associated with polymer confinement is compensated by entropy gains associated with layer separation. In agreement with experimental observations, the thermodynamic model identified three possible phase states (exfoliated, intercalated and immiscible). Using the thermodynamic model and experimental results to determine the effect of various system parameters on polymer intercalation, general guidelines for hybrid synthesis were developed. Polymer intercalation is most favorable for OLSs with intermediate interlayer structures, and it depends on polar interactions between the OLS and polymer. The molecular weight of the polymer does not affect the final equilibrium structure. The kinetics of polystyrene melt intercalation were studied using x-ray diffraction and transmission electron microscopy. Hybrid formation is limited by mass transport into the primary particles of the silicate and not specifically by diffusion of the polymer chains within the silicate galleries. The activation energy is similar to that previously measured for polystyrene self-diffusion, implying that the diffusivity of the polymer chains within the silicate galleries is at least comparable to that in the melt. |
