The spatial structure of gels of sodium carboxymethyl cellulose (NaCMC-gel) and carboxymethyl cellulose in the free acid form can be imaged in the transmission electron microscope (TEM). The freeze-fracturing technique is suitable for this. Experiments with test preparations (10 % aqueous glycerol solution) show that cooling rates during freeze-fixing are decisive for visualization, and that cooling rates can be improved substantially by using a cryojet. The increase of the cooling rate to more than 15 000 K/s makes it possible to obtain extremely fine network structures with a mesh width of 5-25 nm and with a filament thickness of 2-3 nm. The results obtained after jet-freezing show differences in the structure of the two gels: quasi-crystalline microaggregates in the HCMC-gel can be seen, and they cause an increased elasticity and opalescence of the aged HCMC-gels.

Aqueous gels of carboxymethylcellulose in the free acid form (HCMC) are obtained when dialyzed sodium carboxymethylcellulose (NaCMC)-gels are treated with a strongly acidic ion exchanger. These gels age rheologically. During the aging process (maximum 60 days), thixotropy and increased viscoelasticity occur. The rheological changes are measured by shear viscosity r/, coefficient ofthixotropy H, complex dynamic shear modulus G*, shear storage modulus G', shear loss modulus G", and tangent of the phase angle. The aging process is caused by the formation of quasi-crystalline microaggregates of HCMC which can be detected by electron microscopy (Part II).