Date of Award

2018

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Brian Benicewicz

Abstract

This dissertation presents the design, synthesis, and characterization of polydiene grafted nanoparticles as a way to tailor nanocomposite interfaces and properties via interface design. The polymerization of dienes was done via reversible addition fragmentation chain transfer (RAFT) polymerization. The grafting of polymer chains on the surface of silica nanoparticles can be controlled through the molecular design of the RAFT agents attached to the nanoparticles surface. The properties of the nanocomposites largely depended on the interface between the particles and the polymer matrix. In the first part of this work, the polymerization of diene monomers was done on 15 nm diameter silica nanoparticles. SI-RAFT polymerization of isoprene and chloroprene on silica NPs was studied in detail and revealed living character for all these polymerizations. Composites of matrix-free grafted NPs were prepared and analyzed to find the effects of chain length on the dispersibility and organization of particles throughout the matrix. A wide range of grafted polydiene brush molecular weights and graft densities were polymerized on SiO2 NPs to investigate mechanical properties of composites. Multiple vii characterizations such as DSC, WAXS, and SAXS were applied to study the interaction of the polydiene brushes on the inorganic fillers. The surface modified particles with diene polymer brushes were capable of creating a welldispersed state that resulted in improved mechanical properties of matrix-free composites. High loadings of inorganic particles were attained while avoiding particle aggregation and the improvement in mechanical properties correlated with the loading of the core silica loading level. In the second part, both free and SI-RAFT polymerization of 2,3-dimethyl butadiene (DMB) was studied. The kinetic study of DMB monomer was studied with free and SI-RAFT polymerization and compared to other diene monomers. The SI-RAFT polymerization was done with two different graft densities to represent both low and high-density graft regimes. The dispersion of particles was investigated and showed that for both low and high graft density an acceptable level of dispersion was observed throughout the final composite which was confirmed with TEM and SAXS studies. The resulting polydimethyl butadiene (PDMB) grafted silica nanoparticles were directly crosslinked to obtain matrix-free nanocomposites that showed good nanoparticle dispersion and much improved mechanical properties compared with the unfilled crosslinked matrix. viii The third part of this study examined the reversible additionfragmentation chain transfer polymerization of chloroprene on the surface of 15 nm diameter silica nanoparticles to obtain polychloroprene-grafted-silica nanoparticles which were dispersed in an industrial matrix of polychloroprene to obtain PCP nanocomposites with different silica core loadings. Two graft densities and a wide range of molecular weights were studied to examine the effects of these key parameters on the cured composite properties. The dispersion of the grafted nanoparticles in a commercial PCP matrix were excellent for both high and low graft densities. The mechanical properties were enhanced for all composites compared to unfilled cured matrix and proportionally improved with increasing silica loading and grafted polymer chain length. Stress-strain properties were most improved in composites using nanoparticles with low graft density and high molecular weight grafted chains. Finally, polyisoprene (PIP) grafted nanoparticles were prepared and studied for use in rubbery nanocomposites. Scale up approaches were successful and detailed mechanical property studies were conducted to evaluate the advantages of these new polymer grafted nanoparticle based rubbery composites. These trans-PIP grafted particles were dispersed in commercial cisPIP and in-house prepared trans-PIP matrices to obtain PIP nanocomposites with different silica loadings and a single graft density. Miscibility and dispersion of ix particles in both matrices were also studied to examine the compatibility of the different isomers. The trans-PIP-g-NPs were relatively well-dispersed in the cisPIP matrix where the molecular weights of the grafted and matrix polymers were nearly the same (35 kDa-grafted and 40 kDa matrix). However, the mechanical properties of the trans-PIP-g-NPs in the trans-PIP matrix showed better mechanical properties, likely due to the polymer compatibility even though the molecular weights of the grafted and matrix chains (35 kDa-grafted and 52 kDa matrix) were mis-matched and the particles were not dispersed as well in the matrix.

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