Rheology and Microstructure of Cellulose Acetate in Mixed Solvent Systems

Abstract

Cellulose is a natural abundant polymer used in a variety of applications. Its use however, is hampered by its poor solubility in various solvents. This is primarily due to the hydrogen bonds between the hydroxyl groups on the anhydoglucose chain. In view of that, various cellulose derivatives have been synthesized to aid dissolution and this impacts a variety of solubility characteristics. Cellulose acetates (CA) are cellulose esters that are partially substituted at the C-2, 3 and 6-positions of the anhydroglucopyranose residue. Their solubility in various solvents depends on the degree of substitution (DS) of the acetyl groups. For instance, CA is soluble in water at low DS of between 0.5-1. But it is insoluble in aqueous solutions at higher degree of substitution (DS > 1). CA is employed in various applications such as textile manufacture, tool handles, specialty papers, cigarette filters and is a polymer of choice in majority of reverse osmosis membrane preparation. These applications often exploit semi- to concentrated cellulose acetate solutions in appropriate solvents. Such systems can be induced to form aggregated structures such as gels which can be initiated by the addition of a non-solvent. Thus, depending on the solvent and non-solvent adopted, the cellulose acetate mixed solvent system can be tailored to exhibit sol-gel characteristics utilizing the inherent intra- and intermolecular interactions present in solution. However, such systems and the interactions influencing their behavior is not very well understood. In this regard, the main objectives of our study are as follows — to develop a ternary mixed solvent system comprising of cellulose acetate, N,N dimethylacetamide and water and manipulate the system to form aggregated structures leading to phase separated gel network. The tools employed in this project to investigate and characterized the macroscopic properties as well as the microstructural changes are rheology, scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM).

The first part of this study involves addition of water- N,N dimethylacetamide solutions in different ratios to bulk 20% cellulose acetate in N,N dimethylacetamide solutions with emphasis on increasing water content in the system. Using rheology as the main analytical tool, the steady state viscosity was found to increase with water content increase. Above water concentrations of 19%, there is a solution to gel transition, which also showed enhancement in dynamic viscoelastic properties with water content increment. The SEM micrographs showed similar patterns with gels having lower water content exhibiting larger voids in comparison to gels with higher water content at same cellulose acetate concentration. Using LSCM, we obtained microstructural formation with more open networks at lower water content, whiles a more compact homogenous structure was exhibited for higher water content gel samples.

In the second part of this study, addition of cellulose acetate to different ratios of N,N dimethylacetamide/water solutions are investigated. At low water content, the system showed steady state viscosity increase with water content as was observed in the first part of this thesis. Typically beyond 19% water concentration, the systems phase separates into two layers consisting of a clear solution on top of a viscous bottom layer. This is in contrast to the first part of the study where we observed a uniform rigid material. Heating the two-phase system to 100°C and cooling back to room temperature led to the formation of a one-phase physical gel matrix. With increasing water content, the elastic and viscous moduli of the gels increased at constant cellulose concentration.

Finally, we investigate the gel properties with emphasis on yield stress when mechanical stress is applied to the gels. In addition, the gel-sol transition for the gels are investigated by subjecting them to temperature variations.