2025 Theses Doctoral

Fundamental Insights into Directed Filler Dispersion & Nanoplastic Formation in Semicrystalline Polymers

Mendez, Nicholas

From the start of the synthetic plastic era in the early 20th century to today, where over 400 million tonnes of plastic waste are produced each year, plastics have truly become one of the most dominant materials in our society in a relatively short amount of time. For materials that are still in their effective infancy, humans have made incredible progress in engineering polymers for everything from disposable cutlery to biocompatible medical implants. Yet despite this progress, there is still much more ground to cover. This thesis explores two main avenues of interest in polymer nanoscience, focusing specifically on semicrystalline polymers which comprise over two-thirds of polymer production: i) understanding and utilizing a method of directed filler dispersion and its impact on polymer composite properties and ii) degradation of semicrystalline polymers into micro- and nanoplastic.

Semicrystalline polymer composites and blends have been the subject of interest for the last several decades to tailor polymer properties and morphology towards specific applications. In chapters 2-4 of this thesis, we develop a fundamental understanding of crystallization-induced ordering of semicrystalline polymers. Crystallization-induced ordering results from the interplay between the semicrystalline polymer crystal growth rate and the diffusion of the nanofiller. This ordering process impacts the dispersion of the nanofiller resulting in assembly at different potential length scales (interlamellar, interspherulitic, etc.).

In chapter 2 we use poly(ethylene oxide)/silica nanocomposites to explore the impact of manipulating matrix molecular weight (which impacts matrix viscosity and therefore nanoparticle diffusion) on the crystallization-induced ordering of the nanoparticles. Through the use of small angle x-ray scattering and the Hermans orientation function we find that ordering in these systems can be divided into 3 regimes: i) when the crystal growth rate is much faster than nanoparticle diffusion, no change to dispersion occurs ii) when crystal growth rate and nanoparticle diffusion are balanced the strongest interlamellar ordering behavior is observed iii) when nanoparticle diffusion is much faster than the crystal growth rate then nanoparticles accumulate at the crystal growth front (interspherulitic).

Chapter 3 further develops our understanding by quantifying the diffusion of nanoparticles in the respective poly(ethylene oxide) matrices by using x-ray photon correlation spectroscopy (XPCS). Quantification of nanoparticle diffusion allowed for calculation of a dimensionless Peclet number between crystal growth rate and nanoparticle diffusion which showed that the highest interlamellar ordering was achieved when the Peclet number was of order unity. In addition, the dynamic XPCS measurements showed that the nanoparticle diffusion follows Stokes-Einstein predictions if a bound polymer layer (that scales with the polymer R_g) is considered in the size of nanoparticle.

Using the understanding gained from chapters 2 and 3, in chapter 4, we combine crystallization-induced ordering with a direction crystallization method called zone annealing to investigate the mechanical properties of miscible polymer blends with at least one crystallizing component. We use a model system of poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) due to their favorable miscibility in the melt state. Utilizing a slow crystallization rate allows for ordering of the amorphous PMMA in the interlamellar region of the semicrystalline morphology. Furthermore, we use zone annealing to orient the morphology unidirectionally resulting in anisotropic composite materials with an alternating nanostructure. We find that zone annealing had the most impact on the neat PEO mechanical properties, while modest property improvements were observed in the blends of PEO/PMMA. We found that having a glassy polymer (PMMA) in the interlamellar regions leads to a loss of an anisotropic property response, potentially due to an erasure of the property gradient between the crystalline lamella and amorphous interlamellar regions. Zone annealed blends of PEO/PMMA did show improved toughness compared to unorganized samples suggesting that properties of blends can be changed and improved using processing techniques to tune the composite nanostructure, which may serve as a potential way to upcycle polymer blends with one semicrystalline component.

Following the development of our fundamental understanding of crystallization-induced ordering, we investigate a new research area in nanoscience focused on the environmental degradation of plastic into micro- and nanoplastic. This research stems from the large amount of plastic waste in our environments that is subject to both mechanical and chemical degradation. Growing concerns over the fate of environmental plastic waste and the worrying potential health effects of nanoplastic accumulation have driven us to better understand these materials. While much of the research in this field has been focused on downstream effects of nanoplastic pollution (potential health effects, environmental identification, etc.) we focus primarily on the upstream processes that lead to the formation of nanoplastic. Gaining insights into the formation mechanisms of nanoplastic can allow us to develop better model systems for studies and potentially reveal ways to engineer plastics that reduce nanoplastic production. In chapter 5 we examine the formation and properties of poly(ethylene terephthalate) (PET) nanoplastic under quiescent accelerated environmental weathering conditions (hydrolysis) and correlate the degradation-induced embrittlement to the release of nanoplastic. In chapter 6 we begin to investigate the formation of PET nanoplastic through mechanical wear and compare these results to those from chemical degradation (hydrolysis).

In summary, we have used a suite of experimental tools to investigate the structure and properties that arise from crystallization-induced ordering in addition to understanding the formation of nanoplastic through different degradation mechanisms. As polymer production continues to increase, we will have higher performance polymeric materials, but we will also have additional plastic waste. Both topics covered in this thesis will continue to be relevant for the future development of polymeric materials. In the final future work section, we discuss further areas of research for all the sections covered in this thesis and propose a potential way for the two threads in this thesis to work together cooperatively. The fundamental polymer physics insights gained from these works lay the foundation to pursue and understand new routes to improve polymer performance while also looking to address the long-standing issues with environmental polymer waste that have come with the dominance of polymeric materials.