Title: Directional Nanoparticle Organization in Semicrystalline Polymers
Description: The commodity plastics industry is dominated by semicrystalline polymers, which generally display high toughness relative to amorphous polymers but typically suffer from low strength and modulus. Researchers have shown that the addition of nanoparticles (NPs) to these semicrystalline matrices can result in materials with enhanced properties relative to the neat systems. The arrangement of these NPs into anisotropic sheet-like structures appears to endow these processed polymer nanocomposites (PNCs) with further improved mechanical properties relative to PNCs where the NP morphology remains well-dispersed. However, there is currently no appropriate methodology in the literature with which to quantitatively correlate the extent of NP organization to the enhancement in mechanical properties. Additionally, isothermal crystallization (the current processing technique of choice for this class of PNCs) results in numerous grain boundaries. While entanglements across grains can limit issues associated with failure, grain boundaries can also be undesirable for the modulus of the material.

In this dissertation, we methodically investigate several key topics related to the above. We first present our modifications to the correlation function approach of Strobl and Schneider, which was originally developed to characterize the structural parameters of neat semicrystalline polymers and their blends, that allow us to apply it to isothermally crystallized poly(ethylene oxide) (PEO) PNCs. We select PEO due to the relative ease with which mobile silica NPs can be dispersed within the matrix. Next, we characterize these materials using the generally used large beam size typical of laboratory-scale and synchrotron X-ray scattering instruments. In this study, we show that our adaptations to the correlation function approach allow for the quantitative evaluation of the NP ordering process as a function of isothermal crystallization temperature. The same systems are then characterized with a microfocus synchrotron X-ray scattering beam guided by an autonomous experimentation protocol, which allows for a detailed, granular mapping of the structural parameters of these materials.

The much smaller beam reveals spatial morphological heterogeneity in both the neat and PNC systems due to the grain size being on the order of the dimensions of the microbeam as opposed to those of the larger beam. Hence, the combination of the large and microfocus beam provides a comprehensive view of these systems, with varying degrees of granularity. We also find quantitative evidence that demonstrates that NPs organize parallel to the direction of polymer crystal growth, a phenomenon which has previously only been shown in the literature in a qualitative fashion. Having established the physics of the NP ordering process in isothermally crystallized PNC systems, we turn to the zone annealing (ZA) technique as inspiration to approach more uniform, unidirectionally oriented NP morphologies. ZA, which has found extensive use in the production of ultra-pure semiconductors for electronics applications, proceeds by translating a sample at a constant velocity over a well-defined temperature gradient.

This directional processing technique has been shown to result in the reduction of grain boundaries when applied to semicrystalline polymers. Since the PNC is a more complicated system than the neat matrix, we first perform studies of zone annealed neat PEO. Our experimental, analytical, and numerical investigations validate a crucial directional crystallization theory proposed by Lovinger and Gryte, who were among the first to apply ZA to semicrystalline polymers; our experimental evidence confirms the existence of a critical ZA velocity (v_crit) below which directional crystallization occurs and above which the process is closer in spirit to isothermal crystallization. Having determined the mechanism driving the ZA of neat PEO, we then turn to the ZA of PEO-based PNCs. Through our studies, we find that it is imperative to minimize or eliminate sample flow during the procedure, as otherwise the NPs order in disparate directions. Our subsequent redesign of the sample preparation protocol, such that the material is pressed between two glass coverslips separated by Teflon spacers, leads to extensive unidirectional organization of NPs that persists throughout the film at slow enough ZA velocities, as evidenced from X-ray scattering experiments.

Hence, this dissertation systematically examines questions relevant to understanding how to obtain uniform, unidirectional NP organization in semicrystalline PNCs, with relevance to applications requiring enhanced properties.