Poly(lactide)s (PLAs) are well known hydrolytically degradable polymer
s, that are widely used for biomedical applications. However, the intr
insic 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 amor
phous polymers. In order to investigate the effect of tacticity, high
molecular weight poly(lactide) stereo-copolymers with D-lactide conten
ts in the range 0-50 mol-% were synthesised. These polymers were used
in light scattering and differential scanning calorimetry (DSC) experi
ments 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 incre
ases with the tacticity of the polymer from 9.5 for racemic PDLLA to a
n 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 th
e characteristic ratio from the scattering data is not as obvious as i
t may seem and remains a point of discussion. The tacticity of poly(la
ctide)s also influences the glass transition temperature, which varies
from 54 degrees C for racemic to 63 degrees C for isotactic poly(lact
ide).