On the chain stiffness of poly(lactide)s

Poly(lactide)s (PLAs) are well known hydrolytically degradable polymers, that are widely used for biomedical applications. However, the intrinsic brittle nature of poly(lactide)s is still poorly understood. The stiffness of a polymer chain, expressed as the characteristic ratio C-infinity plays an important role in the deformation mechanism of amorphous polymers. In order to investigate the effect of tacticity, high molecular weight poly(lactide) stereo-copolymers with D-lactide contents in the range 0-50 mol-% were synthesised. These polymers were used in light scattering and differential scanning calorimetry (DSC) experiments for the determination of the characteristic ratio C-infinity and glass transition temperature T-g. The results of the light scattering experiments in acetonitrile indicate that the characteristic ratio C-infinity for poly(lactide)s depends on the L/D-lactide ratio and increases with the tacticity of the polymer from 9.5 for racemic PDLLA to an estimated 11.8 for isotactic PLLA. These results are consistent with the observed brittle behaviour of poly(lactide) stereo-copolymers and agree well with predictions obtained from group contribution methods. The choice of the relevant bond length used for the calculation of the characteristic ratio from the scattering data is not as obvious as it may seem and remains a point of discussion. The tacticity of poly(lactide)s also influences the glass transition temperature, which varies from 54 degrees C for racemic to 63 degrees C for isotactic poly(lactide).