Polymer Brush-Modified Silica Nanoparticles: Characterization of the Glass Transition Temperature, Fragility, and Physical Aging

Polymer Brush-Modified Silica Nanoparticles: Characterization of the Glass Transition Temperature, Fragility, and Physical Aging

Shadid Askar, Tian Lan, Hannah Seo, and John M. Torkelson

It is widely known that polymer matrices loaded with inorganic nanofillers exhibit properties that can be enhanced compared to the neat polymer. Polymer nanocomposites have a wide range of applicability in areas of biomedical implants, structural materials, and electronics. A significant challenge pertinent to polymer nanocomposites involves preventing particle agglomeration within a polymer matrix. For instance, post polymerization processing of polymer nanocomposites such as melt processing may lead to dramatic particle agglomeration thereby mitigating any potential thermal or mechanical property benefits of the nanocomposite. One strategy to prevent particle agglomeration involves polymerizing brushes onto the surfaces of nanoparticles using controlled polymerization techniques. While a considerable amount of research has been done to characterize surface-grafted nanoparticles dispersed in a polymer matrix, significantly less research has been done to characterize the surface-grafted nanoparticles themselves. In this study, polystyrene (PS) brushes polymerized from silica nanoparticle surfaces using ARGET ATRP are characterized. Nanoparticles grafted with PS of varying molecular weight are synthesized. Using differential scanning calorimetry, glass transition temperature (T_g), fragility, and physical aging of PS-grafted silica nanoparticles are measured. Information regarding the thermal properties of the modified nanoparticles is useful for designing polymer nanocomposites with optimal properties. It has been found that T_g and fragility are elevated relative to neat PS. These results are attributed to the fact that polymer chains are tethered to the silica nanoparticle. In addition to thermal characterization, dispersion of PS-grafted nanoparticles within a PS matrix is investigated using a novel fluorescence technique. In agreement with reports in literature, the fluorescence technique indicates that PS-grafted nanoparticles exhibit improved dispersion compared to the unmodified counterparts.