4. Cellulose Solubility, Gelation, and Absorbency Compared with Designed Synthetic Polymers

Swelling and solubility of polymers, and in particular cellulose, are controlled by interactions, molecular symmetry, chain flexibility, and order/disorder. Theory is used to explain and predict which liquid systems, polymer structures, and chemical modifications form gels and polymer solutions. Extension of these principles leads to super-absorbent polymers. Cellulose is not water soluble, though some water systems can dissolve cellulose, particularly alkaline or strongly hydrogen-bonding solutions. Less hydrophilic derivatives such as methyl cellulose dissolve in water; while with increasing substitution with methyl groups, cellulose becomes soluble in organic solvents such as dichloromethane. Sometimes temperature can enhance solubility or gelation; alternatively adjusting chemistry through functional group modification to reach an optimum between intermolecular versus solvation interactions will create exceptional changes in absorbency. The solvation power can be increased by adding strongly ionic, hydrogen bonding or acid–base solutes such as lithium chloride, urea, or sodium hydroxide. Synthetic polymers have been designed and commercialized with specific solubility, solution rheology, gelation, and absorbency for many applications. Synthetic water-absorptive polymers begin with the choice of monomer(s), molar mass, and chain architecture. Cellulose is separated with exact structure that can be derivatized, grafted, or modified to change its native resistance to super-absorbency, gelation, or dissolving in water. Molecular modeling and simulation are used to evaluate parameters that will describe super-absorbent character. This review explores and evaluates the chemistry and structural symmetry of celluloses and synthetic polymers, leading to solubility, and gelation leading to super-absorbency. Cellulose is emphasized and compared with synthetic polymers where chemistries are designed and created at all levels of structure.