Processing-structure-property relationships for polymer nanodielectrics

Abstract

The prepared nanocomposites were also characterized by dielectric spectroscopy and dielectric breakdown strength measurements. As the prepared nanocomposites showed a variation in dispersion state, at the same nanoparticle loading of 2 wt%, the effect of dispersion state and surface modification on the dielectric properties of the nanocomposite was successfully studied. It was found that for the polar amino-silane modification and chloro-silane modification in the PMMA matrix, the dielectric permittivity of the nanocomposite reduced with improved dispersion of the nanoparticles. In contrast, the non-polar octyl-silane modification in PMMA led to an increase in permittivity with improved dispersion. This difference in trend was attributed to the interaction between the surface silane and the polymer matrix, leading to a difference in properties of the interfacial polymer. For the PS systems, the permittivity was seen to increase with improved dispersion. PS being a non-polar polymer did not interact strongly with the different silane end groups and hence the same trend was observed for all surface treatments. All nanocomposites showed increase in the dielectric breakdown strength with improved dispersion, smaller nanoparticle clusters and smaller distances between the clusters. The study showed that properties of nanocomposites can be controlled by changing the dispersion state of the system.

This thesis reports work done to understand the effect of different extrusion processing parameters on the dispersion state of the nanocomposites and additionally, the effect of the dispersion state on the dielectric permittivity and breakdown strength of the nanocomposites. SiO¬2 nanoparticles were surface modified with different monofunctional silanes to change their wetting characteristics with respect to PMMMA and PS matrices. Compatibility between the modified nanoparticle and polymer matrix was determined by the ratio of work of adhesions of polymer-filler and filler-filler (WPF/WFF). Nanocomposites were prepared in a twin screw extruder and the microstructure was characterized using TEM imaging and image analysis tools. It was found that the final dispersion state was dependent on the WPF/WFF of the nanocomposite system. For WPF/WFF <1, the systems were heavily agglomerated and the final dispersion state indicated an erosion dominated mechanism of deagglomeration. For WPF/WFF ≥ 1, the systems exhibited good dispersion which was indicative of a rupture dominated mechanism. Further analysis showed that the infiltration of the polymer matrix into the nanoparticle agglomerate greatly reduced the cohesive strength between the nanoparticles and facilitated the transition of deagglomeration mechanisms.

The addition of a small amount of nanofillers can lead to a significant change in the dielectric properties in polymer nanocomposites. The cause for the change has been identified as the presence of an interfacial layer of polymer surrounding the nanoparticle, which has properties different from the bulk matrix polymer. Due to the large surface area of nanometer sized particles, the volume of interfacial polymer is significant enough to influence the properties of the nanocomposite as a whole. Controlling the properties of the nanocomposite requires control over the dispersion state of nanoparticles and an understanding of the interaction between the nanoparticle and the matrix polymer.