Title: Tuning Nanoparticle Organization and Mechanical Properties in Polymer Nanocomposites
Description: Polymer nanocomposites (PNCs), mixtures of nanometer-sized particles and polymeric matrices, have attracted continuing interest over the past few decades, primarily because they offer the promise of significant property improvements relative to the pure polymer. It is now commonly accepted in the community that the spatial organization of nanoparticles (NPs) in the polymer host plays a critical role in determining the macroscopic properties of the resulting PNCs. However, till date there is still dearth of cost-effective methods for controlling the dispersion of NPs in polymeric hosts. In this dissertation, we are dedicated to developing practically simple and thus commercially relevant strategies to controllably disperse NPs into synthetic polymer matrices (both amorphous and semicrystalline). We first investigate the influence of casting solvent on the NP spatial organization and the thermomechanical properties in a strongly attractive PNC consisting of bare silica NPs and poly(2-vinylpyridine) (P2VP) hosts cast from two different solvents - methylethylketone (MEK) or pyridine.

In MEK, we show that P2VP strongly adsorbs onto the silica surface, creating a stable bound polymer layer and thus helping sterically stabilize the NPs against agglomeration. On the contrary, in pyridine, P2VP does not adsorb on the silica NPs, and the phase behavior in such case is a subtle balance among electrostatic repulsion, polymer-induced depletion attraction, and the kinetic slowdown of diffusion-limited NP aggregation. Using Brillouin light scattering, we further show that in pyridine-cast films, there is a single acoustic phonon, implying a homogeneous mixture of silica and P2VP on the mesoscopic scales. However, in MEK-cast samples, two longitudinal and two transverse acoustic phonons are probed at high particle content, reminiscent of two metastable microscopic phases. These solvent-induced differences in the elastic mechanical behavior disappear upon thermal annealing, suggesting that these nanocomposite interfacial structures in the as-cast state locally approach equilibrium upon annealing.

Next, to disperse silica NPs into an energetically unfavorable polystyrene (PS) matrix in a controllable fashion, we have proposed a simple and robust strategy of adsorbing a monolayer of PS-b-P2VP block copolymer onto the silica surface, where the short P2VP block is densely coated around the silica particles and thus helps to reduce the inter-core attraction while the long PS block provides a miscible interface with the matrix chains. As a result, we have found that the silica particles can be uniformly dispersed in the PS matrices at a low grafting density of 0.01 chains/nm2. Even more interestingly, we have shown that the BCP coated NPs are remarkably better dispersed than the ones tethered with bimodal PS-P2VP brushes at comparable PS grafting characteristics. This finding can be reconciled by the fact that in the case of BCP adsorption, each NP is more uniformly coated by a P2VP monolayer driven by the strongly favorable silica-P2VP interactions. Since each P2VP block is connected to a PS chain we conjecture that these adsorbed systems are closer to the limit of spatially uniform sparse brush coverage than the chemically grafted case.

Finally, we have examined the interplay between NP organization and polymer crystallization in a melt-miscible model semicrystalline nanocomposite comprised of poly(methyl methacrylate) or poly(methyl acrylate) grafted silica NPs in poly(ethyleneoxide) matrices. Here we have achieved active NP organization at a length scale of 10-100 nm by isothermal polymer crystallization. We have shown that the melt-miscible spherical NPs are engulfed by the polymer crystals and remain spatially well-dispersed for crystallization faster than a critical growth rate (G > Gc ~ 0.1 um/s). However, anisotropic sheet-like NP ordering results for slower G - the NPs are preferentially segregated into the interlamellar zone of the multiscale, hierarchical polymer crystal structure spanning lamellae (10-50 nm), fibrils (um) and spherulites (mm). This NP ordering is found to favorably impact the elastic modulus while leaving fracture toughness unaffected. We thus conclude that polymer crystal growth kinetics coupled to the unusual morphology of semicrystalline polymers represent a novel handle for in-situ fabricating hierarchical, anisotropic NP structures in a synthetic semicrystalline polymer, which could inspire significant applications.