Effect Of Nano - Layered Silicate On Phase Behavior Of LCST - Type Polymethyl Methacrylate / Polyvinyl Acetate Blends

Most polymer materials used in the academic and application fields are multiphase/multicomponent polymer blends/composites, whose performance mainly depends on their phase structure. It is essential to control the formation and evolution of morphology for polymer alloys with the desired properties. It is a common method that introducing a third component, filler, especially those with large specific surface area of nano-fillers to obtain the materials with more comprehensive and more targeted performance. Typically, the incorporation of nano-filler will increase the modulus, impact strength, heat resistance, electrical conductivity and flammability for the materials, while its influence on the compatibility and evolution of phase structure for polymer blends, especially for those with lower critical solution temperature (LCST) and upper critical solution temperature (UCST) cannot be ignored. The sufficient understanding on the effect of filler on the thermodynamics and kinetics of phase behavior for binary polymer blends will help us predict and optimize the permanent stability and ultimate performance.In this dissertation, poly (methyl methacrylate)/poly (vinyl acetate)/nano-clay nanocomposite was selected as model system. Within the comparatively wider heating rates and annealing temperatures regions, the effect of nano-clay on the phase separation behaviors of polymer blend matrix was studied through time-resolved small angle laser light scattering (SALLS). The dispersion of nano-clay and the evolution of phase separation process were observed by transmission electron microscope (TEM) and phase contrast microscope (PCM). Besides, the influence of nano-clay on the phase separation of ternary system was investigated through rheological measurements.It is found that the nano-clay exists in the polymer matrix mainly in the intercalated and aggregation state. Due to its affinity to PMMA, the nano-clay migrates from the homogeneous blend matrix to the PMMA-rich domain with the evolution of phase separation. The incorporation of nano-clay leads to the decline of two thermodynamics parameters, the cloud points Tc and the delay time of phase separation tD, for polymer blend matrix, indicating that the introduction of nano-clay may destroy the phase stability and cause the concentration fluctuation of PMMA/PVAc blend at the early stage of phase separation. Furthermore, their decrease amplitude is remarkably dependent on the composition of PMMA/PVAc, which may be attributed to the redistribution of blend components in the border layer and the bulk owing to the selective absorption of clay. On the other hand, the reduction of the apparent diffusion coefficient (Dapp) at the early stage and the coarsening rate at the late stage of phase separation is caused by the mechanical barrier effect of clay on the macromolecular diffusion of blend matrix. Hence, such seemingly counterintuitive results on the thermodynamics and kinetics of phase-separation are attributed to different dominant factors.Based on the rheological results, it can be found that the incorporation of small amount of nano-clay may hardly affect the linear viscoelastic region of PMMA/PVAc system, but it causes the suppression of phase separation and the decrease of their phase separation rate. The second plateau in low frequencies of ternary system is wider than that of binary system and shifts to the higher frequency. Besides, their peak of loss tangent tanδ also shifts toward the higher frequency and the deviation in the terminal region of Han curve becomes more remarkable. Furthermore, the incorporation of nano-clay causes the decrease of the Binodal temperature Tb and Spinodal temperature Ts obtained by using the theoretical approach of Ajji and Choplin based on the mean field theory. Such above-mentioned rheological results also indicate that the nano-clay may reduce the phase stability of polymer blend at the early stage and hinder the coarsening of domain at the late stage of phase separation, which are consistent with the SALLS results.