Nanoparticle Stability in Semidilute and Concentrated Polymer Solutions

Uniform nanoparticle dispersion in semidilute and concentrated polymer solutions is of great importance in industrial and medical applications. Despite their widespread applications, colloid stability in these systems has received relatively little attention. The overarching goal of this dissertation was to connect experiments and theories to elucidate the factors that control nanoparticle dispersion in semidilute and concentrated polymer solutions. To this end, monodisperse silica nanoparticles were synthesized and grafted with polydimethylsiloxane (PDMS) to create model systems of PDMS- g-silica nanoparticles which were suspended in semidilute and concentrated PDMS solutions to investigate nanoparticle stability in these solutions. The first objective was to determine the mechanisms that govern depletion restabilization using model PDMS-g-silica nanoparticles in semidilute and concentrated PDMS/cyclohexane solutions. Using dynamic light scattering (DLS), the stability of the particles was examined as a function of graft and free polymer molecular weight and free polymer volume fraction. The experimental results were connected to self-consistent mean field (SCF) predictions, showing that nanoparticle stability is governed by interfacial wetting of the graft polymer by the free polymer.;The second objective was to elucidate the effect of solvent quality on nanoparticle dispersions in semidilute and concentrated polymer solutions. Model systems of PDMS-g-silica suspensions in PDMS/bromocyclohexane solutions were used, bromocyclohexane being a theta solvent for PDMS. The solvent quality was varied by changing the suspension temperature as well as using a range of marginally poor solvents. Using visual observations, DLS, SCF, we found that solvent quality had no effect on nanoparticle dispersion in semidilute and concentrated polymer solutions, thus, interfacial wetting governed nanoparticle dispersion.;The final objective was to quantify nanoparticles interactions in pure solvents and concentrated polymer solutions. Model systems of PDMS- g-silica were used in pure solvents---octamethyltrisiloxane (good solvent) and t-butanol (marginally poor solvent), and concentrated PDMS solutions. Nanoparticle interactions were quantified in terms of the second virial coefficient.;Overall this study contributes toward understanding the dispersion of polymer-grafted nanoparticles in semidilute and concentrated polymer solutions. The results presented in this dissertation should aid the improvement of product quality by controlling aggregation and phase separation of soft materials under a variety of conditions.