Abstract
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~โ~ plays an important role in the deformation mechanism of amorphous polymers. In order to investigate the effect of tacticity, high molecular weight poly(lactide) stereoโcopolymers with Dโlactide contents in the range 0โ50 molโ% were synthesised. These polymers were used in light scattering and differential scanning calorimetry (DSC) experiments for the determination of the characteristic ratio C~โ~ and glass transition temperature T~g~. The results of the light scattering experiments in acetonitrile indicate that the characteristic ratio C~โ~ for poly(lactide)s depends on the L/Dโlactide 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) stereoโcopolymers 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 54ยฐC for racemic to 63ยฐC for isotactic polyโ(lactide).