MSE Doctoral Dissertation Defense: “Polymer Infiltrated Nanoporous Gold: Kinetics and Optical Properties of Novel Polymer Nanocomposites” (Weiwei Kong)

Abstract: One of the biggest challenges in the field of polymer nanocomposites (PNCs) is to disperse high nanofiller loadings into the polymeric matrix. The high loading and uniform dispersion are limited by the unfavored polymer/nanofiller thermodynamics and the tendency for nanofiller to aggregate. In this thesis, these are circumvented by using nanoporous gold (NPG) as a scaffold for polymers to fill. The ultra high loading (>50 vol%) is achieved by infiltrating polymer melts into NPG to produce a polymer infiltrated nanoporous gold (PING) composite . This novel composite provides promise for the next generation of advanced materials for coating, optical sensors, actuators, and batteries.

This thesis contributes to our understanding of polymer kinetics under moderate confinement by varying the interfacial energy between polymer and pore wall, and investigating the temperature dependence of infiltration. The confinement ratio in this thesis is defined by Γ = R_g/R_p. For polystyrene (PS) infiltrating into the NPG (Γ = 0.47 – 0.77), a weakly attractive interaction, the infiltration time scales with molecular weight (M_w) as 𝛕 ~ M_w^1.30, weaker than bulk predictions.  Moreover, PS infiltration is much faster compared to the bulk behavior because confinement reduces the number of entanglement and the effective polymer-wall friction decreases as M_w decreases.  For poly(2-vinylpyridine) (P2VP) infiltration into the NPG (Γ = 0.18 – 0.78), a strongly attractive interaction, infiltration time also exhibits a weak dependence as 𝛕 ~ M_w^1.43.  However, compared at similar conditions, P2VP infiltration is slower than PS which is attributed to the formation of a physisorbed layer during P2VP infiltration, which is supported by the MD simulations. Lastly, the temperature dependence of P2VP:NPG infiltration at moderately confined (Γ = 0.55) conditions follows the bulk WLF behavior, while more confined P2VP (Γ = 0.97) exhibits a weaker temperature dependence  than predicted  by WLF.  This deviation is attributed to a confinement induced reduction in thermal expansion coefficient. Those fundamental studies on polymer kinetics enable the optimization of preparing  PING composites so they can be used for previously mentioned applications.

The optical response of the NPG during polymer infiltration is studied using UV-Vis spectroscopy. As the dielectric constant of the gold nanopores increases during filling, the absorbance spectra intensity increases and the plasmon peak undergoes a red shift. The extent of infiltration measured by the evolution of the absorbance spectra is in good agreement with in-situ ellipsometry measurements. In Discrete Dipole Approximation (DDA) simulations are used to model the absorbance spectra at various stages of annealing time. Importantly, a “T” shaped gold structure is found to better represent the plasmonic absorption of the NPG ligaments compared to a nanorod used in a prior study. These optical absorption studies demonstrate that UV-Vis is a facile method for studying polymer infiltration in metal scaffolds.

This thesis advances the understanding of polymer infiltration kinetics for polymers that weakly and strongly attract to the pore walls, and show that confinement reduces thermal expansion of the polymer. To complement ellipsometry, a new approach is presented to follow polymer infiltration using UV-vis spectroscopy. These advancements enable scientists to better understand polymers under confinement and advance the tool box for creating interconnected polymer/filler systems at high filler concentrations.