Concentration Dependence of Crystalline Poly(L-lactide) Prepared by Freeze-drying Solutions

Poly(L-lactide) was freeze-dried from p-xylene solutions with varying concentrations. The samples were characterized by differential scanning calorimetry. It was found that the sample produced from the dilute solution exhibited higher non-isothermal crystallization temperature and faster crystallization rate. There was a critical concentration corresponding with the critical overlap concentration proposed by De Gennes in the polymer solutions. In the solution below the critical concentration, the PLLA chains were isolated from each other, resulting in an acceleration of melt crystallization for the recovered samples. It appears that the chain entanglement is a barrier to the melt crystallization of polymer.