Fibrillar Structure and Networks in Methylcellulose Hydrogels
2018-12
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Fibrillar Structure and Networks in Methylcellulose Hydrogels
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2018-12
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Methylcellulose is a water soluble, commercial polymer which is used in applications including consumer products, food and pharmaceutical products, construction materials, and industrial pastes. For many applications the thermogelation behavior of MC is of particular interest. Upon heating, MC forms a turbid hydrogel. The mechanism of gelation has been debated for decades; however, recent insight from cryo-TEM has revealed that MC forms a network of 15 nm diameter fibrils that entangle and crosslink resulting in the macroscopic gelation of MC. This thesis focuses on developing a deeper understanding of the fibrillar structure in MC gels by i) studying the molecular weight dependence on the fibrillar structure, ii) determining the sub-fibril structure, and iii) investigating the impact of salt on the fibril morphology. The gel modulus has been previously reported to depend on the molecular weight of the polymer. This dependence is confirmed in Chapter 3 via small amplitude oscillatory shear (SAOS) rheology. The structure of the gel is measured using cryogenic transmission electron microscopy (cryo-TEM), small angle X-ray scattering (SAXS), and small angle light scattering. Through these techniques it is revealed that the fibril length is dependent on the molecular weight of the chain, while the fibril diameter and the heterogeneity of the gel remain relatively constant. By measuring the length of the fibrils in cryo-TEM it is revealed that the length of the fibrils and the length of the polymer chains are correlated. As a result it is proposed that polymer chains within the fibril are oriented parallel to fibrils. Although the dimensions of the fibril structure have been studied, the sub-fibril structure has not been determined. Using SAXS, mid-angle X-ray scattering (MAXS), and wide-angle X-ray scattering (WAXS), the correlations between the polymer chains in the gel state is investigated in Chapter 4. From these investigations the inter- and intra-polymer correlations are found to be related to the sub-fibril structure. Using dried films of MC gels the fibrils are found to be ~ 30% crystalline. To determine the orientation of the inter- and intra-polymer chain correlations, oriented films of dried MC gels are prepared and fiber diffraction patterns are generated. From these investigations it is determined that the associated domains of MC chains form an orthorhombic unit cell. It is proposed that these small associated domains ultimately form a helical structure, which controls the overall fibril diameter. The addition of salt has been reported to impact the gel temperature in MC; however, the effect of salt on the fibril structure has not been reported. Using small amplitude oscillatory shear (SAOS) rheology the change in the gel temperature with the addition of salt was investigated. Using SAXS and cryo-TEM it was demonstrated that the fibril structure changes with the addition of salt. By varying the concentration and the identity of the anions, it was determined that the fibril radius decreases linearly with increasing ionic strength independent of the anion identity. This was attributed to the increase in the osmotic pressure on the fibril, which dehydrates the fibrils. From these investigations it is determined that the fibril structure and the properties of MC gels are strongly correlated. The addition of salt and change in molecular weight both result in changes in the fibril structure. These results and the deeper understanding of the fibril structure are useful to future studies on MC and product development to understand the key design parameters that impact fibrils and as a result, the properties of MC gels.
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University of Minnesota Ph.D. dissertation. 2018. Major: Material Science and Engineering. Advisors: Timothy Lodge, Frank Bates. 1 computer file (PDF); 146 pages.
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Schmidt, Peter. (2018). Fibrillar Structure and Networks in Methylcellulose Hydrogels. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/202142.
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