Surface, bulk, and rheological properties of polyhedral oligomeric silsesquioxane/high density polyethylene nanocomposites
In the formulation of high performance nanocomposites, control of miscibility and dispersion of filler material through a polymer matrix is of utmost importance. Due to their inorganic nature most nanofillers are insoluble in polymers, leading to costly/complicated surface modification as a primary means of increasing miscibility and interaction with organic matrices. Polyhedral oligomeric silsesquioxane (POSS) nanostructured chemicals offer an attractive alternative to conventional nanofillers. Due to their hybrid organic-inorganic nature, POSS has the potential to be tailored for miscibility in a wide range of organic matrices not by chemical surface modification but through modification of the molecular structure of the filler itself. The overall goal of this research is to investigate how changes to POSS molecular structure affect miscibility and dispersion in physically blended high density polyethylene (HDPE)/POSS blends. The primary objective of the first section is to understand the effect of POSS cage structure, physical state and R-group alkyl chain length on miscibility and blend performance through a wide range of characterization techniques. Special attention will be paid to rheological, bulk and surface performance of the blends as compared to the neat HDPE matrix. The primary objective of the second section is to determine the utility of theoretical solubility parameter calculations as a means of predicting POSS miscibility in the HDPE matrix. This section will focus on solubility parameters calculated using both group contribution and molecular dynamics simulation methods, determining their proximity to each other, and qualifying their applicability in predicting POSS miscibility and blend performance. This dissertation is comprised of six chapters. Chapter I provides an introduction to nanocomposites, as well as background information on HDPE, POSS, pertinent POSS blends and solubility parameter theory. Chapter II gives an overview of the research goals and specific objectives of this research. Chapter III probes the influence of POSS functionality, cage structure and physical state on the bulk properties (thermal, rheological, mechanical) of the melt-processed HDPE/POSS blends. Chapter IV explores HDPE surface modification as a function of POSS incorporation, as well as aggregation and migrational behavior of the POSS molecules. Chapter V surveys POSS theoretical solubility parameter calculations via both group contribution theory and molecular dynamics simulations and correlates these values with observed blend behavior due to incorporation of POSS. Finally, Chapter VI provides recommendations for future work in an attempt to further refine our understanding of the complex behaviors and trends observed in our HDPE/POSS systems.