Thermal and cosolvent-based modulation of polymer solution microstructure and processability

Abstract

Many industrial operations such as spraying, fiber spinning, and roll-coating are used to process polymer-containing formulations for myriad applications including separations and water remediation, energy storage and generation, and regenerative medicine. In these processes, polymer-containing formulations are subjected to strong extensional deformations downstream from rollers or as the fluid is ejected from a nozzle, forming an unstable liquid bridge or fluid sheet that undergoes capillarity-driven thinning and breaks up into droplets over time. As extensional flows deform polymer chains much more dramatically than shear flows, even a small amount of polymer additive can result in filament formation and delayed break up. Filament formation and delayed break up is advantageous for certain processes such as fiber spinning but detrimental for some processes such as spraying and roll-coating, leading to undesired effects such as droplet coarsening and misting. Moreover, polymer solutions containing high molecular weight polymers are desirable as increasing molecular weight imparts advantageous properties such as enhanced mechanical strength and stability to applications; however, increasing polymer molecular weight exacerbates the nonlinear extensional flow behavior and thus decreases processability in spraying and coating operations. To address these challenges, this thesis explores strategies for modulating polymer solution processability via temperature- and cosolvent-based modulation of polymer solution microstructure. First, a temperature control chamber is developed and integrated into dripping-onto-substrate extensional rheometry (DoS) to enable measurements of temperature dependent extensional flow behavior in low viscosity, low elasticity polymer solutions. The temperature-controlled DoS (TC-DoS) instrument is then validated by measuring the extensional rheology of aqueous poloxamer, a thermoresponsive model triblock polymer with well-characterized temperature dependent solution microstructure. TC-DoS measurements reveal that the poloxamer solution exhibits inertiocapillary through elastocapillary thinning behavior as a result of temperature-induced micelle lengthening, demonstrating the breadth of extensional flow behavior that can be characterized by TC-DoS. Measurements of poloxamer WLM solutions reveal that WLMs composed of amphiphilic polymers exhibit comparable behavior to those composed of surfactants. Finally, comparisons of poloxamer WLM extensional rheology measured using TC-DoS to surfactant WLM extensional rheology measured using incumbent techniques such as capillary breakup extensional rheometry (CaBER) reveal that CaBER measurements underpredict the extensional relaxation time of WLM solutions, and thus TC-DoS is more suitable for measuring microstructured fluids. Using TC-DoS, a proof of concept for using temperature to modulate polymer microstructure, solution extensional flow behavior, and thus processability is then demonstrated. To demonstrate the viability of this strategy, a model PNIPAM-based triblock containing poly(N-isopropylacrylamide) (PNIPAM) is chosen for its temperature-dependent microstructure in solution. Using turbidimetry, dynamic light scattering, and small angle x-ray scattering, the existence of unimers at ambient conditions and self assembled spherical micelles above the transition temperature is confirmed. The temperature-dependent extensional rheology of the PNIPAM-based polymer solution is then measured using TC-DoS. Below the transition temperature, unimer solutions exhibit weakly elastic extensional flow behavior. Above the transition temperature, purely aqueous PNIPAM-containing triblock polymer solutions form a viscoelastic film at the air-water interface in DoS measurements. To circumvent this issue, N,N-dimethylformamide (DMF), a better solvent for PNIPAM, is added to solution. Above the transition temperature, solutions of spherical micelles exhibit Newtonian flow behavior without interfacial film formation. Thus, this work demonstrates that temperature can be used to modulate between two distinct polymer microstructures, giving rise to distinct extensional flow behaviors. The second goal of this work is to explore the effect of cosolvent addition on polymer microstructure, solution extensional flow behavior, and processability. DoS is used to measure the extensional rheology of PNIPAM in DMF/water mixtures as a function of DMF content. Interestingly, the solution extensional relaxation time increases by up to twenty-fold as DMF content is increased in the water-rich, one-phase regime while solution elasticity is greatly diminished in the DMF-rich, one-phase regime. Meanwhile, steady shear flow behavior is largely invariant with DMF content. To explain this unexpected behavior, a mechanism is proposed to link extensional flow behavior to solution microstructure, which is shown to vary with DMF content in light scattering measurements. Preferential interactions between PNIPAM chains and DMF molecules are proposed to vary with DMF content, giving rise to changes in solution microstructure and extensional rheology. This study demonstrates the importance of polymer-cosolvent interactions in ternary systems and that the extensional flow behavior and processability of PNIPAM-containing formulations can be modulated by simple cosolvent addition. Finally, the effects of different formulation parameters on DMF-mediated PNIPAM solution extensional rheology are examined. DoS measurements conducted on PNIPAM/DMF/water solutions with varying polymer concentration reveal that extensional relaxation time scales unexpectedly with PNIPAM concentration in the presence of DMF. A mechanism is proposed based on the DMF-mediated PNIPAM microstructure. DoS measurements of low and high molecular weight PNIPAM in DMF/water mixtures are also compared, revealing an expected decrease in solution elasticity with molecular weight. Further, the decreasing polymer dispersity is shown to decrease the extent to which DMF addition increases solution elasticity in the water-rich regime. Finally, DoS measurements are conducted on solutions of poly(N,N-dimethylacrylamide) (PDMA), a polymer with similar structure to PNIPAM but lacks the amide hydrogen bonding donor site. The extensional flow behavior of PDMA is invariant with DMF content, highlighting the importance of the PNIPAM amide hydrogen bonding donor site in DMF-mediated PNIPAM solution extensional flow behavior. Overall, this thesis uses extensional rheology to probe the effects of temperature- and cosolvent-mediated polymer microstructure on solution processability. The measurement of polymer solution extensional rheology is advanced with the development of temperature control capabilities for a droplet-based technique suited for microstructured fluids. Fundamental insights into structure-extensional flow relationships in polymer solutions are also uncovered. The findings in this work ultimately inform the design of polymers and formulations that achieve optimal processability while retaining desirable polymer functionality.