Title: Crystallization Kinetics of Semicrystalline Polymer Nanocomposites
Description: Semicrystalline polymers constitute the majority of the commercially manufactured polymers, mostly known as commodities with low modulus and inferior properties. A robust approach used in tailoring such commodity’s properties for more advanced applications is through the incorporation of inorganic nanoparticles (NPs). Over the past half century, polymer nanocomposites (PNCs) have attracted extensive interest in fundamental research and technological applications. However, NPs have been found to result in complicated alterations in the semicrystalline polymer crystallization kinetics, and their crystalline morphology, which could either synergistically or adversely affect the final composite properties. A comprehensive understanding of this topic is still lacking, which with one could tune the final polymer properties for various cutting-edge applications. In this dissertation, we focus on the crystallization kinetics of semicrystalline PNCs and the connection between the morphology and the mechanical (and rheological) properties of such hybrid systems.

First, we control the NP dispersion and self-assembly in a semicrystalline poly(ethylene oxide) (PEO) matrix using both bare and polymer-grafted NPs. We show that bare NPs (with different sizes) and unimodal poly(methyl methacrylate) (PMMA)-g-SiO2 NPs uniformly disperse in a PEO matrix because of the favorable interaction between the matrix and the NP surface (or the PMMA brush). Grafting the latter NPs with a short dense polystyrene brush that is immiscible with PEO while varying the PMMA grafting parameters induces self-assembly and leads to various NP structures: well-dispersed, connected sheets, strings, and large aggregates. Next, we systematically investigate the role of bare and self-assembled grafted NPs on the spherulitic growth kinetics of semicrystalline polymers. In all cases, the incorporation of spherical NPs suppresses the polymer growth kinetics. Using rheological measurements, we show that the reduction in growth is mainly attributed to the NPs increasing the melt viscosity; whereas, they minimally alter the secondary nucleation process.

Surprisingly, the PNC growth kinetics is suppressed in two apparently universal manners when plotted as a function of confinement: NP dominated and brush-controlled regimes. Bare NPs and large aggregates of polymer-grafted NPs appear to nearly follow the same dependence for the role of additives on polymer viscosity, weakly suppressing the growth kinetics. On the other hand, all the other self-assembled NP structures showed much stronger growth reductions because of the larger increase in the melt viscosity by the chemically bonded brush. Given our prior knowledge of the PNC growth kinetics, we then draw generalized trends for the role of bare and grafted NPs in nucleating semicrystalline polymers. This is achieved by comparing the polymer crystallization kinetics in the presence of large, asymmetric, immobile fillers (selected from the well-established literature) to those smaller, spherical, mobile NPs (examined throughout this thesis). Generally, NPs serve as heterogenous nucleation sites when incorporated at smaller amounts, leading to accelerated crystallization kinetics.

At larger filler contents, NPs confine the polymer chains into smaller domains and become more susceptible to aggregation, which results in antinucleating effects and suppressing the crystallization rate. Such competing effects result in a maximum nucleation efficiency at moderate filler contents. It is also worth noting that the degree of nucleation enhancement and subsequent suppression depends on the system and is controlled by NP dispersion, geometry, and surface chemistry. For example, one- and two-dimensional NPs usually result in a higher nucleation power compared to spherical NPs. Another major difference between mobile and immobile fillers is that when slowly crystallizing from the melt, the smaller diffusive NPs can be segregated and ordered into hierarchal structures (interlamellar sheets and interfibrillar and interspherulitic aggregates). This provides a much richer class of materials with a kinetics route in controlling NP assemblies. Finally, we create robustly toughened semicrystalline polymers by confining the PEO crystallization using a densely grafted PMMA brush (i.e., PMMA-g-SiO₂) with different molecular weights.

For comparison, we prepare linear PMMA/PEO blends with equivalent PMMA molecular weights and volume fractions to those of the nanocomposites. We show that PMMA-g¬-SiO₂ NPs surpass linear PMMA homopolymers in terms of toughening the PEO matrix, with the grafted system experiencing relatively higher connectivity and lower crystallinity. At moderate confinement, the nanocomposite sustains a maximum modulus increase of 42%, with around a 200-fold increase in the PEO toughness. This provides a novel route for toughening semicrystalline polymers using noncrystallizable polymer-grafted NPs.