Water uptake and relaxation processes in mixed unlimited swelling hydrogels

The rheological oscillatory test parameters have been observed for highly c oncentrated hydroxypropylmethyl cellulose (HPMC), carboxymethylcellulose-so dium (NaCMC) and mixed HPMC/NaCMC hydrogels obtained by swelling of matrix tablets in 0.1 mol cm(-3) HCl and pH 6.8 phosphate buffer. The mechanical s pectra of the gels have been analysed using theoretical models, i.e. a gene ralised Maxwell model and an adapted Maxwell model, both based on Ferry and Williams approximations. The relaxation time spectra as well as the parame ters characteristic of linear viscoelastic behaviour have been calculated: zero shear viscosity (eta (0)), plateau moduli (G(N)(0), G(0)' and G(0)"), zero-relaxation time (tau (0)) and mean relaxation time (theta). The mechan ical spectra of mixed HPMC/NaCMC hydrogels differ considerably from those o f the pure ones, the type of the spectrum depending on the two polymers' ra tios. In both media, the rheological models applied define the HPMC gels as homogeneous entangled networks, and those of NaCMC and mixed HPMC/NaCMC as heterogeneous physical gels. The relationship between the kinetic constant s of water penetration and the mean relaxation times suggests that the mole cular relaxation controls the water uptake velocity. With all the systems t ested irrespective of pH of the aqueous phase, an inversely proportional de pendence between the viscosity and the water penetration velocity has been noted. Since the degree of hydration is one of the factors determining the degree and velocity of drug release from the hydrogel matrices, the relatio n between the kinetic parameters of water penetration and the Viscosity is a characteristic indicator for the gel structure, the degree of swelling an d the drug release rate. (C) 2000 Elsevier Science B.V. All rights reserved .