Thermal properties and physical ageing behaviour of tyrosine-derived polycarbonates.

Tyrosine-derived polycarbonates are new carbonate-amide copolymers. These materials have been suggested for use in medical applications, but their thermal properties and their enthalpy relaxation kinetics (physical ageing behaviour) have so far not been evaluated in detail. Since structure-property correlations involving enthalpy relaxation are rarely investigated for biomedical polymers, a series of four tyrosine-derived polycarbonates was used as a model system to study the effect of pendant chain length on the thermal properties and the enthalpy relaxation kinetics. The chemical structure of the test polymers was identical except for the length of their respective pendant chains. This feature facilitated the identification of structure-property correlations. Quantitative differential scanning calorimetry was utilized to determine the thermal properties and to measure enthalpy relaxation kinetics. The glass transition temperature of this family of polymers decreased from 93 to 52 degrees C when the length of the pendant chain was increased from two to eight carbon atoms. Successive additions of methylene 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 independent of the number of methylene groups in the pendant chain. The enthalpy relaxation data were fitted to the Cowie-Ferguson model and the relaxation times obtained were about 90 min. Dynamic mechanical analysis was employed to study the viscoelastic properties. The available observations indicate that the polymers become more flexible with increasing length of the pendant chain. The results suggest that the length of the pendant chain can be used effectively to control important material properties in this series of polymers.