Browsing by Subject "Methylcellulose"
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Item Input and data for "Simulating precursor steps for fibril formation in methylcellulose solutions"(2019-05-08) Sethuraman, Vaidyanathan; Dorfman, Kevin D; dorfman@umn.edu; Dorfman, Kevin D; Dorfman Research GroupWe use coarse-grained molecular dynamics simulations to study the precursor steps for fibril formation in methylcellulose solutions. Simulations of ring stacking between two collapsed methylcellulose chains demonstrate the existence of a capture radius that is much larger than that predicted by polymer diffusion alone. When two rings are in very close proximity, they stack together to form a fibril precursor. Simulations of stacks of such rings suggest that this structure is metastable. In contrast, chains that are within the capture radius but not in close proximity, as well as for systems containing both ringlike and relaxed chains, fibril-like structures form via a distinctly different mechanism. Irrespective of their initial arrangement, the chains undergo two specific conformational changes: (i) a part of either a ring or a randomly coiled chain splays out and (ii) the splayed chain subsequently engulfs a nearby chain if it is within a certain capture distance. The latter results are consistent with recent experimental measurements of fibril formation by short methylcellulose chains, which suggests the formation of a twisted bundle.Item Polymer Solutions in Complex Flows: Fibrils, Filaments, and Flocs(2020-08) Metaxas, AthenaThe behavior of polymers in solution under complex physicochemical and hydrodynamic flow fields is of interest to a variety of industrial processes, such as polymer processing and water treatment. In this thesis, two main areas are presented: (1) self-association of polymer chains in flow, resulting in the formation of fibrils and filaments, and (2) association of polymer chains with suspended particulate, resulting in the formation of flocs. In the first area, extensional properties of methylcellulose (MC) solutions were characterized using millimeter scale capillary thinning and micrometer scale filament stretching methods. The addition of NaCl to MC solutions results in self-assembly of a fraction of the MC chains into MC fibrils, which imparts elastic characteristics to the solution. Capillary Breakup Extensional Rheometry (CaBER) studies demonstrate the extensional relaxation time and extensional viscosity increased with increasing MC concentration in the presence of salt. Likewise, microfluidic filament stretching studies demonstrate the extensional viscosity increased with increasing NaCl concentration. By reducing the characteristic length scale of the thinning filament, the microfluidic platform enabled new measurements extensional properties for low molecular weight and low viscosity MC solutions. In the second area, the assembly of charged polymers, or polyelectrolytes, with bentonite clay into flocs was studied in complex flow fields using macroscale Taylor-Couette (TC) flows. A custom-built TC cell allowed for injection of the polyelectrolyte solution into the particle-laden flow to investigate in-situ floc nucleation and growth in varied hydrodynamic flow states. Faster floc growth rates and decreased 2-D perimeter-based fractal dimensions were observed for higher order flow states, indicating improved mass transfer of the polymer flocculant and shear rounding of the flocs, respectively. Additionally, the effects of ionic strength and polyelectrolyte molecular weight on flocculation in the TC cell were investigated. Smaller flocs were formed with increasing ionic strength, due to the role of charge screening on the initial bentonite aggregate size and polyelectrolyte chain persistence length and conformation in solution. Overall, this thesis seeks to provide additional understanding of how polymers assemble in solution under a variety of physicochemical conditions and flow, which can inform predictive processing capabilities and performance.Item Structure and Phase Behavior of Aqueous Methylcellulose(2015-09) McAllister, JohnMethylcellulose is a chemically modified polysaccharide that is partially substituted by methoxyl groups. When the average substitution per monomer is intermediate, MC is water soluble, and these materials see use in a wide variety of commercial products. Aqueous solutions of MC undergo gelation and phase separation upon heating. Using dynamic mechanical spectroscopy, frequency-independent loss tangents were used to identify the gel point (Tgel) in MC solutions well over the chain overlap concentration (c 10c). Transmittance of 633 nm laser light through the solutions revealed that MC solutions cloud upon gelling, with a relative transmittance of 86% closely associated with the gel point. The gelation temperature of MC solutions was found to decrease with increasing MC concentration and the results for all molecular weights superposed. The fibrillar structure of aqueous MC gels was probed using a combination of small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (USANS), and cryogenic transmission electron microscopy (cryo-TEM). The effect of molecular weight (Mw) and concentration on the gel structure was explored. The fibrillar morphology was consistently observed at elevated temperatures ( 70 C), independent of concentration and Mw. Moreover, the fibril dimensions extracted from SANS by fitting to a scattering function for semiflexible cylinders with disperse radii revealed that the fibril diameter of ca. 141 nm is constant for a concentration range of 0.01% to 3.79% and for all Mw investigated (150-530 kg/mol). Comparison of the measured SANS curves with predicted scattering traces revealed that at 70 C the fibrils contain an average volume fraction of 40% polymer. The linear and nonlinear viscoelastic response of MC gels can be described by a filament-based mechanical model. In particular, large-amplitude oscillatory shear experiments show that aqueous MC materials transition from shear thinning to shear thickening behavior at the gelation temperature. The critical stress at which MC gels depart from the linear viscoelastic regime and begin to stiffen is well predicted from the filament model over a concentration range of 0.18-2.0 wt%. These predictions are based on fibril densities and persistence lengths obtained experimentally from neutron scattering, combined with cross-link spacings inferred from the gel modulus via the same model. Mw, z-average radius of gyration (Rg), and second virial coefficient A2 have been determined between 15 and 52 C for dilute aqueous solutions of methylcellulose (MC) with three different molecular weights and constant degree of substitution (DS) of 1.8 using static light scattering. These measurements, conducted within 1 hour of heating the homogeneous solutions from 5 C, reveal that the theta temperature for MC in water is T = 48 2 C, with A2 < 0 for T > T, indicative of lower critical solution temperature (LCST) behavior. However, after annealing a solution for 2 days at 40 C evidence of high molecular weight aggregates appears through massive increases in the apparent Mw and Rg, a process that continues to evolve for at least 12 days. Cryogenic transmission electron microscopy images obtained from a solution aged for three weeks at 40 C reveal the presence of micron size fibrils, which is analogous to the fibrils that form upon gelation of aqueous MC solutions at higher concentrations and elevated temperatures. Growth of fibrils from a solution characterized by a positive A2 indicates that semiflexible MC dissolved in water is metastable at T < T, even though the solvent quality is apparently good. The minimum temperature required for MC solutions to aggregate is estimated to be 30 C, based on the rate independent gel-to-solution transition determined by small amplitude oscillatory shear measurements conducted while cooling 0.5 and 5.0 wt% solutions. These results cannot be explained based solely on separation into two isotropic phases upon heating using classical Flory-Huggins solution theory. It is speculated that the underlying equilibrium phase behavior of aqueous MC solutions involves a nematic order parameter.