The study of water uptake in degradable polymers by thermally stimulated depolarization currents

Poly(DTH succinate) is a new, degradable polymer with potential applications as a medical implant material. This polymer can be classified as an alternating copolymer of succinic acid and desaminotyrosyl-tyrosine hexyl ester (DTH), a diphenolic monomer derived from the natural amino acid L-tyrosine. In this study, the effects of water uptake (hydration) on the secondary relaxations of poly(DTH succinate) were investigated using the technique of thermally stimulated depolarization currents (TDSC). Four relaxation peaks were precisely characterized by means of Gaussian activation energy distributions. Drying and rehydration treatments show that only small amounts of water, at most 0.5% (w/w), are necessary to hydrate two of the four polar moieties in poly(DTH succinate): the pendent chain ester carbonyls and the amide carbonyls in the polymer backbone. Water appeared to be more tightly bound to the amide carbonyl group and more loosely bound to the ester carbonyl group in the pendent chain. Even at a high state of hydration (1.2% w/w), TDSC indicated that no water was associated with the phenyl ester bond in the polymer backbone. This finding may explain the unexpectedly high stability of this polymer toward hydrolysis under physiological conditions. Polymer packing was also affected by hydration. In an intermediate hydration state (water content: 0.5% w/w) polymer packing was less dense than in the wet state (water content: 1.2% w/w). This study represents the first application of the TDSC technique to the study of hydration in a degradable biomedical polymer. The results obtained indicate that the TDSC technique may be useful to assist in the understanding of the mechanisms of hydration and subsequent hydrolytic degradation in degradable biomaterials.