| It is well recognized that adding fillers into polymers can result in significant improvement of thermal, mechanical, optical and dielectric properties. The enhancement of the mechanical performance by the addition of fillers draws much attention in particular. There have been some very well-established theories developed by Einstein and Guth that explained the stiffening of polymers caused by colloids or particles by considering the hydrodynamic interactions and distortion of strain field. However, there have been some experimental and simulation results that deviate from the prediction by classic theory, and many of them have been explained by slowdown of chain relaxation, and formation of a percolated network of particle and polymer chains.;Recently, an unusual and unique property was observed in polymer nanocomposite systems by Senses et al. (Senses, E.; Isherwood, A.; Akcora, P. ACS Appl. Mater. Interfaces 2015, 7, 14682). These nanocomposite systems show stiffening behavior upon heating that is reversible and repeatable. This unique thermal stiffening behavior was attributed to the dynamic coupling of high glass transition (Tg) grafted chains and low-Tg matrix chains. To better study the stiffening mechanism, we first studied the viscoelasticity and dynamics of a model dynamically asymmetric binary polymer blend which consists of two type of chains with significantly different Tg. Our study shows that the good dispersion of relatively immobile polymer chains with high-Tg in the mobile polymer chains with low-Tg leads to a significant storage modulus increase compared to the phase separated blend. It is suggested that the obstacle effect and the percolated network of the high-Tg polymer chains are responsible for this phenomenon. We later studied the composite system containing nanoparticles grafted with high- Tg chains dispersed in a low- Tg matrix polymer. We found that these nanocomposites exhibited significantly greater storage modulus when the high-T g grafted chains assumed stretched conformations (and were well-mixed with the low-Tg matrix chains) compared to when they assumed collapsed conformations. Further detailed static and dynamic analysis showed that stretched grafted chains could significantly reduce the mobility of matrix chains, which was ultimately one of the most important factors that were responsible for the stiffening of the whole system. Next, we studied the static and dynamic properties of nanocomposite system with chemically identical brush and matrix chains. By increasing the ratio of brush length to matrix chain length, we can control the conformation of the brush chain changing from self-avoiding random walk to random walk. The relative viscosity of the nanocomposite system shows the similar trend of blend system with polymers of same composition, which suggests that the viscosity of the nanocomposites with long brush chains can be modelled by a blend of grafted chains and matrix chains.;These studies provide a new insight into how viscoelasticity and dynamics of polymer nanocomposites can be controlled by graft chain conformation at nanoparticles/polymer interfaces. In the future, one can explore more ways to drive the enthalpic and entropic conformation change, such as changing the Lenanrd-Jones interaction parameter, Tg difference, grafting density, and chain length. The goal of this thesis is to build a comprehensive understanding of the effect of the conformation and stiffness of the grafted chains on the interfacial interactions, phase behavior, static and dynamic properties of the nanocomposites and to provide a guideline for future experimental researchers in this field to select the materials for the optimum properties. |
