An Investigation of Mixed Cellulose Esters and Acyclic Polyacetates: Effects of Side-Chain Lengths and Degrees of Ring-Opening

Sammanfattning: Bio-based polymers produced from natural sources are gaining an increased interest as potential replacement for today’s conventional fossil-based plastic polymers. Their use is already wide in many large-scale industrial areas such as healthcare, personal care, and food. To widen the potential of biopolymers in new applications such as plastics, their properties need to be tuned by modification to handle factors like relative humidity, which is especially important for gas barriers in food packaging. This thesis explores the effect of two structural variations of cellulose esters, one where the average side-chain length is increased, going from cellulose acetate to cellulose acetate propionate and then cellulose acetate butyrate, and another where the polymer backbone of cellulose acetate is ring-opened. These two modifications affect the glass transition temperature, an important structural factor. The effect of the average side-chain length is explored to a greater extent where they are studied for impact on mechanical properties, water content, water sorption at different RH, the kinetics of water sorption at different RH, mechanical properties at different RH and oxygen permeation at different RH. The focus is on how water interacts with the different esters with regard to the average side-chain length and how water affects their properties. An increase of average sidechain length and the ring-opening were shown to decrease the glass transition temperature. Together with the water sorption and Hansen solubility parameter, it was concluded that longer average side-chain length screens out hydrogen bonding between the polymers. The studies on the average side-chain length and water sorption indicated that water entering the cellulose acetate creates clusters. These formed water clusters create cavities in the polymer which makes the polymer hold more water than before introducing of the water clusters. Oxygen permeation studies on prewetted films prove that these cavities created by water clustering are still present after drying the material at 0% RH and thus resulted in a higher oxygen permeation compared to films that had not been exposed to higher than 50% RH.

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