Date of Award

2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Materials Science & Engineering

First Advisor

Russell J. Composto

Abstract

This dissertation describes experimental studies on the dynamics of polymer nanocomposites (PNC), namely, center-of-mass (COM) polymer diffusion in PNCs, and COM nanoparticle (NP) diffusion in polymer melts. Elastic recoil detection (ERD) is used for polymer diffusion studies and Rutherford backscattering (RBS) is used for NP diffusion studies. Diffusion of the tracer polymer, deuterated poly(methyl methacrylate) (dPMMA) is slowed down in a PMMA matrix filled with hydroxyl-capped spherical silica nanoparticles. A confinement parameter, ID/2Rg, where ID is interparticle distance and 2Rg is probe size is defined to account for the NP crowding effect. For highly crowded region where ID < 2Rg, D decreases by up to 80% relative to the bulk value. Surprisingly, D is reduced by 15% relative to the bulk value even when ID is eight times larger than 2Rg in the weakly confined region. A comparison between the current PMMA and polystyrene nanocomposites indicates that attractive interactions in the PMMA system do not significantly alter the center of mass diffusion of macromolecules in polymer nanocomposites.

Diffusion of deuterated polystyrene (dPS) is probed in PS matrices containing string-like chained nanoparticles (cNP) grafted with PS. This investigation connects prior diffusion studies in model spherical and cylindrical NP systems, and provides insight for technological applications, which typically involve irregularly-shaped NPs such as carbon black. We report that the presence of chained NPs in PS matrices induces a minimum in the diffusion coefficient (D) with increasing cNP concentration when the key length scale, 2Rg/L ≤ 1.5, where Rg is the gyration radius of dPS and L is the mean length of the impenetrable core of the chained NPs. The diffusion minimum is attributed to anisotropic diffusion in the vicinity of the chained NPs and requires that the long dimension of the cNP be comparable to or longer than the tracer molecule. Two normalizations are explored to account for the brush effect on polymer diffusion. These studies show that the NPs not only act as impenetrable obstacles for polymer diffusion, but that the polymer brush grafted to the cNP provides an alternative pathway to control polymer dynamics.

The relative mobility of nanorods in PNC is shown to impact chain diffusion. Nanorod (NR) mobility was tuned by varying the molecular weight of the matrix and NR concentration. When the tracer polymer diffuses faster than the NRs, the tracer diffusion coefficient in the PNC decreases similarly to the immobile (fixed) NR case as NR concentration increases. However, when the tracer diffuses slower than the NRs, enhanced tracer diffusion is observed with respect to the fixed NR case below the overlap concentration for NRs. This enhancement is attributed to NR mobility which allows for removal of topological constraints present in PNCs with fixed NRs. At NR concentrations above the overlap concentration, where NR mobility is reduced by interactions with neighboring “overlapping” NRs, tracer diffusion becomes independent of matrix molecular weight. These experimental results establish criteria by which the mobility of NRs relative to long chains assists polymer diffusion and will motivate a broader inspection of the role of mobile nanoparticles on the properties of polymer nanocomposites.

We also study the diffusion of PMMA-grafted iron oxide nanoparticles (core diameter = 5 nm) in PMMA melts. Dry and wet brush architectures are obtained by tuning brush molecular weight (16 and 21 kg/mol), brush grafting density (0.17, 0.33 and 0.55 chains/nm2) and PMMA matrix molecular weight (4 – 50 kg/mol). The diffusion of nanoparticles is slowed down relative to the Stokes-Einstein relation prediction, suggesting that the interpenetration between the brush and matrix influences nanoparticle mobility. Self-consistent field theory is performed to predict the structure of brush and matrix in the vicinity of the particle to quantify the effect of brush-matrix interpenetration on NP diffusion. These experiments demonstrate that the structure of the brush could affect nanoparticle center of mass diffusion and the brush-nanoparticle interpenetration should be considered.

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