Isothermally crystallization behavior of poly (L‐lactide) from melt under high pressure

Abstract

The poly (L‐lactide) (PLLA) samples from melt were isothermally crystallized at 175°C and 190°C under pressures (PC) ranging from 0.1 to 400 MPa. The crystalline structures and thermal properties were investigated by using wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry (DSC). On the basis of the DSC and WAXD results, it is confirmed that an α crystal was formed in the low‐pressure region (PC ≤ 50 MPa), and the disordered α crystal (α′) was formed in the medium‐pressure region (100 MPa ≤ PC ≤ 200 MPa) for the PLLA samples isothermally crystallized at 175°C. However, the only α crystal was formed in the pressure range from 0.1 to 200 MPa, and no α′ crystal could be found in the PLLA samples isothermally crystallized at 190°C. Interestingly, the full amorphous PLLA was obtained at 175°C and 190°C, when the PC was above 250 MPa. There seems to exist a cut‐off pressure for the crystallization of PLLA, which divides the pressure into high and low regions. The peculiarities of crystalline structure could be explained by the enhanced melt supercooling and the confined crystallization behaviors of PLLA under high pressure. The effect of the compression rate on the formation of amorphous PLLA was also researched with different compression rates. The results indicate that the molten PLLA could be easy to solidify as full amorphous phase under the higher pressure at a lower critical compression rate.