Polymer-layered silicate nanocomposites: Rheology and intercalation kinetics

Layered silicate based polymer nanocomposites have provided a convenient macroscopic system to study fundamental scientific issues concerning confined and tethered polymers.; In this thesis, the kinetics of intercalation of monodispersed polystyrene in organosilicates were studied using x-ray diffraction. The kinetics of the neat polymers and their functionalized derivatives and copolymers were measured as a function of molecular weight, extent of functionalization or block ratio and silicate surface organic modification, at various temperatures. The kinetics data are interpreted in terms of an effective diffusion coefficient (D _eff) of the polymer, which undergoes a dramatic decrease with stronger silicate surface-polymer affinity. The intercalation kinetics exhibits a similar temperature dependence as the self-diffusion in polymer bulk, but a very different molecular weight dependence. At low molecular weight, i.e. below M_c, D_eff ∞ N−0.3, and at higher molecular weight, D_eff ∞ N−1. Competition and substitution experiments have shown that at low temperature, the faster, kinetically controlled intercalation (with longer surfactant modified silicate) dominates; but at higher temperature, the more stable, thermodynamically controlled intercalation (with shorter surfactant modified silicate) prevails. In addition, the intercalation kinetics under shear flow were also investigated. High frequency (inside the plateau zone in the master curves of both the neat polymer and the intercalated nanocomposites) shear can align the silicate layers to some extent, but has a more significant effect on the polymer chains. The copolymer of poly(styrene-b-isoprene) can also intercalates into organosilicates by annealing. Such intercalation can occur at temperature above as well as below its T_ODT. The temperature dependence of the effective diffusion coefficients of copolymer intercalation does not display any discontinuity when T_ODT is traversed, same as the temperature behavior of its self-diffusion.; In addition, end-tethered dispersed polystyrene nanocomposites were synthesized by TEMPO-mediated free living radical polymerization and their rheological behavior studied. They exhibit unique behavior in both the terminal zone and the plateau zone. Non-terminal behavior was observed with as low as 1.7% silicate loading and a two fold increase in entanglement molecular weight was obtained with less than 5% silicate loading.