Thermo-mechanical properties and hydrolytic degradation of tyrosine-derived polymers for use in biomedical applications

The macromolecular properties of polymers based on tyrosine derivatives, intended for use as degradable biomaterials, were extensively evaluated as a function of their chemical structures. Since these polymers were designed to incorporate flexible units that could be chemically varied in a systematic fashion, a variety of structural features were screened for chemical and physical properties that would make them optimal candidates for biomedical applications.; In tyrosine-derived polycarbonates, the effect of pendent chain structure on mechanical, thermal properties and degradability was investigated. Increasing pendent chain length of the polycarbonates was found to slightly lower their hydrolytic degradation rates. By using a set of model compounds and a set of differently shaped polymeric specimens, the hydrolysis rate of the carbonate bond in the polymer backbone was found to be higher than that of the esterified pendent chain. The highly hydrophobic structure of these polycarbonates as well as the low solubility of the degradation residues in aqueous medium accounted for a lower water uptake and slower resorption rate of these polymers as compared to a commercially available poly(D,L-lactic acid) used as a reference.; Mechanical strength parameters were determined through tensile testing. Increasing the pendent chain length decreased the tensile strength and stiffness of the polymer, but increased its ductility. The strength of the polycarbonates could be increased by a pre-orientation process in which the polymeric chains were re-oriented. At the same time, increasing the pendent chain length reduced the glass transition temperatures (T_g) and raised the rate of chain relaxation. On the contrary, increasing bulkiness of the pendent chain by using a branched alkyl group produced a reversed effect. A further investigation revealed that the magnitude of T_g and the kinetics of structural relaxation were clearly affected by the location of flexible units in the polymeric structure. These last findings were obtained from the thermal analysis of tyrosine-based polyarylates, in which the modifiable flexible units were not only at the pendent chain as in the polycarbonates, but also at the backbone diacid section.