Structural Effect of Linear and Star-Shaped Poly(L-lactic acid) on Physical Properties

The linear and star-shaped poly(L-lactic acid) (PLLA) with similar molecular weight were prepared and their physical properties such as thermal properties, rheological properties, and crystallization behavior in quiescent and dynamic states were compared. The differential scanning calorimetry showed that the linear PLLA gave higher glass transition, melting, and crystallization temperatures than the star-shaped one. In dynamic crystallization, the linear PLLA gave longer induction time and longer overall crystallization time than the star-shaped one, although the former gave higher rate of crystallization in quiescent crystallization. However, wide-angle X-ray diffractometer(WAXD) analysis revealed that the linear and star-shaped PLLA developed the same crystal structure and application of shear had little effect on crystal structure. As predicted, the linear PLLA gave higher crystallinity than the star-shaped PLLA. In the dilute solutions, the linear PLLA exhibited higher intrinsic viscosity than the star-shaped one. In the concentrated solutions, the star-shaped PLLA gave higher values of dynamic viscosity, storage, and loss moduli than the linear one. Further, the former exhibited more noticeable shear thinning behavior and greater dependence of rheological properties on temperature than the latter. For both PLLA melts, the modified Cole-Cole plot gave slope less than 2. Of two PLLA polymers the star-shaped PLLA gave smaller slope than the linear one. In addition, the former showed greater change of the slope with temperature than the latter.