THERMAL-PROPERTIES AND PHYSICAL AGING BEHAVIOR OF TYROSINE-DERIVED POLYCARBONATES

Tyrosine-derived polycarbonates are new carbonate-amide copolymers. Th ese materials have been suggested for use in medical applications, but their thermal properties and their enthalpy relaxation kinetics (phys ical ageing behaviour) have so far not been evaluated in detail. Since structure-property correlations involving enthalpy relaxation are rar ely investigated for biomedical polymers, a series of four tyrosine-de rived polycarbonates was used as a model system to study the effect of pendant chain length on the thermal properties and the enthalpy relax ation kinetics. The chemical structure of the test polymers was identi cal except for the length of their respective pendant chains. This fea ture facilitated the identification of structure-property correlations . Quantitative differential scanning calorimetry was utilized to deter mine the thermal properties and to measure enthalpy relaxation kinetic s. The glass transition temperature of this family of polymers decreas ed from 93 to 52 degrees C when the length of the pendant chain was in creased from two to eight carbon atoms. Successive additions of methyl ene groups to the pendant chain made a fairly constant contribution to lowering the glass transition temperature. For pendant chains of four or more methylene groups, the rate of enthalpy relaxation was indepen dent of the number of methylene groups in the pendant chain. The entha lpy relaxation data were fitted to the Cowie-Ferguson model and the re laxation times obtained were about 90 min. Dynamic mechanical analysis was employed to study the viscoelastic properties. The available obse rvations indicate that the polymers become more flexible with increasi ng length of the pendant chain. The results suggest that the length of the pendant chain can be used effectively to control important materi al properties in this series of polymers.