Microscopic structural changes during the freeze cross-linking reaction in carboxymethyl cellulose nanofiber hydrogels

Abstract

Freeze cross-linking increases the mechanical strength of carboxymethyl cellulose nanofiber (CMCF) hydrogels. The structural changes of CMCF hydrogels during the freeze cross-linking reaction were investigated. The SEM images of CMCF hydrogels prepared with different reaction times showed that the CMCF hydrogel in the initial state of the cross-lining reaction (t ≤ 1 h) has a fibrous structure due to the original CMCF. As the reaction progressed (t ≥ 12 h), the fibrous structure was no longer present and a sheet structure with the average thickness of 1.5 ± 0.2 µm was observed. The XRD profiles indicated that the sheet consisted of a stable crystalline structure of cellulose II, in which CMCF was aligned along the (110) plane. It was also confirmed that using acidic aqueous solutions such as citric acid, DL-malic acid, lactic acid, or hydrogen chloride, as cross-linking agents yields CMCF hydrogels with the sheet structure and high compressive strength. On the other hands, when alkaline aqueous solution such as trisodium citrate, tripolyphosphate, or urea were used as cross-linking agents, brittle CMCF hydrogels with inhomogeneous fibrous structures were formed.