Rheological monitoring of long-term degrading polymer hydrogels

This paper reports on the rheological characterization of long-term degradi ng hydrogel slabs using a rotational rheometer. A rheological methodology w as developed and validated using hydrogel slabs made of cross-linked methac rylated dextran. The method was based on the identification of the optimal amount of compression of the hydrogel slabs between the plates of the geome try, needed for reliable G' measurements. A G' plateau (G'(p)) could be ide ntified for each hydrogel slab as a leveling off of G' upon further compres sion. This G'(p) was reproducible and of the same order of magnitude as G' of hydrogels prepared in situ between the plates of the geometry. The metho d allowed doing rheological measurements without prior knowledge of the exa ct dimensions of the hydrogels. Using this method, the enzymatic degradatio n of dextran methacrylate (dex-ma) hydrogel slabs with incorporated dextran ase was characterized theologically. It was proven that the observed change s in the rheological behavior were enzyme linked. Being able to reproducibl y measure the rheological properties of hydrogel slabs prepared outside the rheometer has as a great advantage that the influence of swelling, degrada tion, or any treatment can be theologically studied. (C) 1999 The Society o f Rheology. [S0148-6055(99)01104-9].