Dynamical and structural behavior of hydroxyethylcellulose hydrogels obtained by chemical gelation

Rheological, dynamical, de-swelling, turbidity, and structural features during gelation with a chemical cross-linking agent of semi-dilute aqueous solutions of hydroxyethylcellulose (HEC) were investigated at different cross-linker concentrations. A higher level of cross-linker leads to faster gelation and stronger incipient hydrogel. The rheological results favor the percolation model. The cross-linking reaction proceeds over a long time in the post-gel region, and this produces gels with a solid-like response. Dynamic light-scattering experiments reveal a fast and a slow relaxation mode. The slow relaxation time increases and the sample becomes more heterogeneous in the course of gelation. In the post-gel region, the gels shrink during a long period of time and this phenomenon is accompanied by strong turbidity enhancement. Small-angle neutron scattering results from samples quenched at a certain stage in the post-gel regime disclose growth of heterogeneity in the gel with increasing level of cross-linker addition. At smaller length scales, no effect of cross-linker addition could be detected on the structural organization, and scattered intensity in this domain suggests that the HEC chains are locally stretched. (C) 2006 Society of Chemical Industry.