Doctoral Dissertations

Date of Award

8-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Mark Dadmun Dr.

Committee Members

Charles Feigerle Dr, Bin Zhao Dr., Verlee Keppens Dr.

Abstract

The dissertation presents work that improves our understanding of the impact of soft nanoparticles on the dynamics of linear polymer in all-polymer nanocomposites and the impact of graphene on the thermal and mechanical properties of PLA in fused deposition modeling. Polymer nanocomposites in which soft, polymer-based nanoparticles are dispersed in the polymer matrix have received great interest lately due to their potential use in a range of applications, including drug delivery and self healing materials. However, the impact of this new class of nanoparticles on the dynamics of a linear polymer matrix in an all-polymer nanocomposite is still largely unknown. In the first chapter, we determine the impact of 10 nm radius polystyrene soft nanoparticles on the diffusion of high molecular weight linear PS chains as a function of nanoparticle loading. Our results show that at loadings below 1% of the nanoparticle, the diffusion of the linear matrix increases by a factor of two presumably via a constraint release mechanism, while at loadings above 1% the increase in diffusion is mitigated by confinement effects of the nanoparticles. The transition appears to happen when the distance between nanoparticles is similar to the size of the polymer chain of the matrix (ID/2Rg ~ 1). The next project presents a protocol for determining tracer diffusion coefficients of soft nanoparticles and correlate its topology to observed dynamics. The results suggest that the nanoparticle softness and deformability dictate its motion. Increasing the crosslinking density of the nanoparticle increases its hardness and suppresses its motion in the linear matrix. Additionally, the nanoparticle molecular weight dependence deviates from the exponential dependence for star polymer suggesting that these nanoparticles resemble fractal microgels that benefit from the cooperative motion of the matrix to open pathways for the nanoparticle. The next project, examines the effect of graphene on thermal transport and inter-filament bonding in 3D printing of PLA. The incorporation of graphene at law loadings appears to enhance thermal conductivity and lead to more uniform thermal gradients. Additionally, at low graphene loading, high bed temperatures can be utilized to enhance thermal transfer in the z direction and improve mechanical strength. At higher loadings the improvement in heat transfer is undermined by the slow diffusion of polymer chains due to confinement. Finally, the last project evaluates the impact of graphene on irreversible thermal strains of PLA in FDM. The results demonstrate the potential to mitigating warping through graphene incorporation and control of thermal evolution throughout the printing process.

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