Rheological characterization of mechanical properties of chemically crosslinked microspheres

This work presents a rheological method to characterize degradation of microspheres made of chemically crosslinked hydrogels. Conditions to measure rheological properties of microspheres, remaining as individual microspheres during measurement, were established. Relevant and reproducible measurements could be obtained with a rheometer equipped with a parallel plate geometry in which a homogenous and continuous monolayer of microspheres was inserted in a gap between the plates. The storage modulus of microspheres was determined under an imposed strain of 0.04% with an oscillatory measurement mode at various frequencies. The microspheres showed almost purely elastic behavior while their storage modulus was affected by the degree of crosslinking. In the second part of the work, the rheological method was applied to investigate the degradation of microspheres made of a hydrolyzable crosslinked hydrogel. The method was found suitable by measuring the storage modulus over time. A good correlation was identified between acidification of the incubation medium because of the release of degradation products and the decrease of the storage modulus of the microspheres indicating a reduction in the crosslinking of the hydrogel resulting from the degradation process. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013