Rheology and Microstructural Changes during Enzymatic Degradation of a Guar−Borax Hydrogel

Abstract

Hydrogels composed of borax cross-linked guar galactomannans are enzymatically degraded using endo-β-mannanase, an enzyme which cleaves the polymer chain backbone. Dynamic rheological measurements show the elastic (G‘) and viscous (G‘ ‘) moduli to be sensitive to gel structure and to reduce significantly during the enzymatic hydrolysis process. The reduction in rheological properties shows three distinct regimes: an initial large decrease, a slower reduction rate at intermediate times, and an accelerated reduction at longer degradation times. In contrast, the polymer chain molecular weight, obtained from gel permeation chromatography, reduces rapidly at short times and at a slower rate subsequently. We therefore find the kinetics of moduli reduction to be dictated by the relationship between gel structure and rheological properties, rather than purely the rates of chain scission. At short times, the large decrease in moduli is analogous to changes in molecular weight and can directly be attributed to chain scission. At long times, corresponding to when the product of polymer concentration and intrinsic viscosity, c[η], reaches a critical value (≤2.5), the chains are too short to overlap and the long range network breaks down rapidly, leading to accelerated moduli reduction. Additionally, a synergistic increase in the degradation rate is observed on using a combination of backbone-cleaving β-mannanase enzyme and a side-chain-cleaving α-galactosidase enzyme, as compared to using only β-mannanase. This can be attributed to an enhancement of mannanase activity due to removal of the sterically hindering galactose side chains. Finally, a comparison of gel and solution degradation reveals very similar behavior in molecular weight changes for both but contrasting trends in rheology.