Polymer Phase Behavior and Morphology Control: From Blend Compatibilization to Aerogel Formation

Abstract

This dissertation covers polymer blend compatibilization and improvements in the mechanical properties of semicrystalline polymer aerogels. Both blends and aerogel properties are governed by phase separation processes. While in polymer blends, domain size and interfacial adhesion related to the phase separation can be detrimental for the properties, in aerogels obtained through thermally induced phase separation (TIPS), the phase separation is responsible for the network formation. Therefore, in both areas, controlling the phase behavior is necessary to control the properties of the final blend and aerogels. In the first half of this dissertation, the compatibilization of blends of polysaccharides with polyesters is investigated. Polymer blends are the physical mixture of at least two polymers, which are designed to achieve improved properties compared to the pure polymers. While polysaccharides are sustainable polymers, their use as plastics is limited due to frequent low toughness, poor melt processability, high water sensitivity, and high production costs. Therefore, making blends of polysaccharides is an alternative to mitigate this shortcoming and expand the use of sustainable polymers. Due to a small entropy of mixing, polymer blends tend two phase separate. The phase-separated morphology is characterized by sharp interfaces with low adhesion, which leads to poor properties. To mitigate the consequences of phase separation, compatibilization is achieved by adding compatibilizers that favorably interact with both components in the blend. Chapter 1 discuss the fundamentals of polymer blends and polymer blends compatibilization. Chapter 3 and Chapter 4 investigate the use of a block polymer and a graft polymer, respectively, in the compatibilization of polysaccharide/polyester blends to advance the knowledge about blend compatibilization. In both chapters, phase contrast optical microscopy (PCOM) and small-angle laser-light scattering (SALLS) were used to track changes in the phase-separated morphology as the compatibilizers were added. In Chapter 3, ethyl cellulose (ECel)/poly(ethylene terephthalate) (PET) 70/30 blends were compatibilized with a block polymer of ethyl cellulose (ECel) and poly(benzyl glutamate), named ECel-block-poly(BG). Different amounts of the block polymer, 5, 10, 20, and 30 wt.%, were tested as compatibilizers. The uncompatibilized blend presented a highly phase-separated morphology composed of large and small domains, characteristic of late stages of spinodal decomposition. As the compatibilizer content increased, the size of the large domains decreased until a bi-continuous spinodal texture was obtained with 30 wt% of compatibilizer. A decrease in average domain size from 15 ± 4 μm in the uncompatibilized blend to 2 ± 1 μm when using 30 wt% of the compatibilizer was observed. These changes in domain size highlight the ability of the compatibilizer to steric stabilize these blends, preventing coarsening of the phase-separated morphology. Chapter 4 investigated the impact of amylose acetate-graft-poly(D,L-lactic acid) (AmAc g-PDLLA) graft density and graft length on the compatibilization of starch acetate/PDLLA 70/30 blends. Graft polymer contents of 5, 10, and 20 wt% with varying graft density and graft length were investigated. The results showed that in order for the compatibilizer to reduce the interdomain distance of the blend, it has to entangle with the polymers of the blend. Furthermore, the ability of the compatibilizer to entangle was related to the chain entanglement molecular weight (Me) of the polymers in the blend. The series of graft polymers with the same graft length (29.4 kg/mol) but different graft densities (between 0.5 and 19 %) showed that the graft density has to be low enough so that the segments between grafts are at least the Me of the starch acetate, allowing the graft polymer to entangle with the starch acetate and promote compatibilization. In parallel, the series of graft polymers with the same graft density (1 %) but different graft lengths (between 7.9 and 29.4 kg/mol) showed that the grafts have to have a molecular weight above the Me for PLA, allowing the grafts to entangle with PDLLA and promote compatibilization. The second half of this dissertation investigates semicrystalline polymer aerogels obtained through TIPS. On TIPS, the polymer is dissolved at a high temperature, and upon cooling, the gel network is formed through a phase separation process. Chapter 2 discusses the fundamentals of the TIPS process and polymer aerogels. For the systems investigated here, the phase separation happens through solid-liquid phase separation, where the polymer crystallizes from the solution to yield the aerogel network. On TIPS, there are many parameters that can be tuned to control the morphology of the aerogel, including the initial polymer content, the solvent, the dissolution temperature, the gelation temperature, and the presence of additives. In Chapter 5, the impact of the gelation temperature on the properties of poly(ether ether ketone) (PEEK) aerogels was studied. It was observed that by increasing the gelation temperature the aerogel network connectivity was enhanced. As the mechanical properties of aerogels depend on the network connectivity, this increase in connectivity resulted in up to a 111.5 % improvement in the compressive modulus of the aerogels, while crystallinity, density, and porosity remained unchanged. Chapter 6 explored another approach to improve the mechanical properties of aerogels while maintaining their porosity. Specifically, the incorporation of sodium montmorillonite, Cloisite 10A, and Cloisite 25A nanoclays to polyphenylene sulfide (PPS) aerogels was investigated. The addition of 1 wt% of any type nanoclay did not impact the morphology of the PPS aerogel, but adding 5 wt% of Cloisite 10A resulted in a less connected morphology and therefore worse mechanical properties. An increase of 30 % in the compressive modulus of the aerogel was observed when 1 wt% of the montmorillonite was added to 15 wt% PPS aerogels. While some intercalation was observed for montmorillonite, we believe that the compressive modulus was not further enhanced because the nanoclay was not fully exfoliated in the aerogel matrix. Lower improvements in compressive modulus were observed by the addition of Cloisite 10A and Cloisite 25A, which were related to the degradation of the organic modifier. The degradation of the organic modifier can lead to a worse distribution of the nanoclays in the polymeric matrix, which is detrimental to mechanical properties. Some dependence on the nature of the organic modifier was also observed, highlighted by the better performance of Cloisite 10A compared to Cloisite 25A. Finally, the degradation temperature of the aerogels was increased by the addition of the nanoclays.