Effect of regiochemistry on the viscoelastic properties of cellulose acetate gels

Abstract

Cellulose acetate (CA) solutions have been shown to undergo phase-separation-induced gelation upon the addition of polar nonsolvents involving both hydrogen bonding and hydrophobic interactions. Using cellulose acetates with specific regiochemistry, only free hydroxyl groups at the C6 position of the anhydroglucopyranose unit, the viscoelastic behaviour and gel properties were evaluated. Steady-state and dynamic rheology indicate that both degree of acetylation (DA) and regiochemistry impact gel formation. Increasing nonsolvent content from 15 to 25 wt% substantially increased the elastic modulus (G′) of all ternary systems. At 15 wt% water G′ is significantly larger for the regioselective CA as compared to that of the random CA; consistent with the fact that the random CA is still in the solution phase while the regioselective CA are gels. At 20 wt% nonsolvent, the random CA and the regioselective CA of the same DA (~2.4) have approximately the same G′, while that of the regioselective CA with DA ~2.8 is significantly higher. Moreover, at the same DA, the random CA ternary system is less prone to rupture as it possess a longer linear viscoelastic region, i.e. drop in modulus occurs at higher stress than the corresponding regioselective CA ternary systems. However, at the same nonsolvent content the critical stress of the regioselective CA with DA ~2.8 is comparable to that of the random CA; suggesting the DA and not specifically regiochemistry affects gel modulus, but regiochemistry does influence stress behaviour.