(306e) Enthalpic and Entropic Competitions in Solvent-Free Polymer-Grafted Nanoparticles

Polymer grafted nanoparticles, in the past, have drawn significant attention owing to their ability of forming uniformly distributed polymer-nanoparticle mixtures. One class of such grafted nanoparticles is the ‘self-suspended hairy nanoparticles’, where polymer chains are densely grafted onto nanoparticle surface, such that they show good phase stability even in absence of a dispersing medium. Here, we report on the dynamics and structure of covalently grafted polyethylene glycol (PEG) onto silica nanoparticles, which show several interesting phenomena like temperature induced jamming, slow-down of polymer chain dynamics as well as stress overshoot during start-up of steady shear. The entropic attractions between the polymer chains owing to the particle curvature can explain many of these observed behaviors. Further, we show that the attraction between the polymer chains can be manipulated in mixtures of nanoparticles with different polymer chemistry to design materials with unusual characteristics. Specifically, we blend nanoparticles with PEG and PMMA corona chemistries, which are known to have negative Flory Huggins interaction parameter. These blends show similar soft glassy rheological behavior but with a significantly higher plateau modulus and yielding energy. SAXS measurements and DFT calculations further reveal that the grafted chains are highly stretched and interdigitated, owing to the favorable enthalpic attractions. As an extension to these ideas, we study the polymer-grafted nanoparticle blends with polymer chains (PEG and PI) having positive Flory-Huggins interaction parameter. Previous studies on block-copolymers have shown interesting micro-phase separation and self-assembly in contrast to polymer-blends, owing to their higher entropic penalty. Such nanostructures in the block-copolymers have led to their usage in various bulk applications including energy storage, water purification as well as thin-film coatings. In SiO₂-PEG/SiO₂-PI blends, the rheological experiments reveal large reduction of storage modulus and energy of dissipation compared to their single component counterparts. We explain this phenomenon in terms of competitions between enthalpic repulsions among incompatible corona chains and the entropic attractions due to the space filling requirement. More subtle effects of the enthalpic-entropic competition are well captured by quantifying the stretching and terminal relaxation dynamics of tethered polymer chains.