Free Volume and Water Sorption by Cellulose Esters

Abstract

The results ofthe sorption properties of cellulose acetate (CA) with different degrees of substitution (SD) are summarised. It has been shown that the sorption capacity of CA in water vapour decreases naturally with increasing content of acetate groups in monomeric units of cellulose ethers. The experimental isotherms are analysed according to the double sorption model. Hydrate numbers of hydroxyl and acetate groups were determined. The paired parameters of the Flory–Huggins interaction were calculated. It is shown that the decrease of the Langmuir component is due to the replacement of hydroxyl groups by ester groups, whose local sorption capacity by water vapour is lower than the sorption capacity of OH groups. In the area of high humidity, there is an increase in vacancy sizes due to plasticisation of the sorbents.

Study of water vapor sorption by cellulose esters with different degrees of substitution

“…Various mechanisms of WV sorption by cellulose esters have been proposed including multimolecular surface adsorption, clustering of water molecules, the combination of monomolecular with multimolecular surface adsorption, or monomolecular surface adsorption with the dissolution of water molecules in amorphous polymers, and some others [2,[7][8][9][10]. However, it has long been known that common cellulose esters are usually low-or non-porous materials with a small specific surface area (<1 m 2 /g) [14].…”

“…4). Therefore, it is doubtful that polar water molecules can dissolve in these hydrophobic esters and cause a strong rise in sorption value according to the Flory-Huggins equation at enhanced relative vapor pressures, as postulated in reference [10].…”

“…An important method for characterizing the hydrophobicity of cellulose derivatives is to study the sorption of water vapor (WV). The main attention was paid to the sorption of WV by cellulose acetates (CA) [7][8][9][10]. It was shown that with an increase in the degree of substitution of CA, the sorption ability of WV decreases due to an increase in hydrophobicity of this ester [10].…”

“…The main attention was paid to the sorption of WV by cellulose acetates (CA) [7][8][9][10]. It was shown that with an increase in the degree of substitution of CA, the sorption ability of WV decreases due to an increase in hydrophobicity of this ester [10]. As for other cellulose esters, when moving from acetates to mixed esters, aceto-propionates and acetobutyrates, the hydrophobicity rises, and as a result, the sorption of WV declines, while the isotherms look less concave [2].…”

“…Various models of WV sorption for cellulose esters have been proposed, such as multimolecular surface adsorption, clustering of water molecules, as well as mixed models combining monomolecular and multimolecular surface adsorption, or monomolecular surface adsorption and dissolution of water molecules in amorphous polymers, and some others [2,[7][8][9][10]. Thus, the WV sorption mechanism by various esters remains still a subject of discussion.…”