Pressure induced crystallization and in situ simultaneous SAXS/WAXS investigations on structure transitions

Abstract

Pressure effect is of great importance in injection and compression of poly(L-lactide) (PLLA). However, pressure induced crystallization (PIC) and structural change of PLLA have not been thoroughly studied. In this study, we investigated PIC and structure transitions of PLLA using in situ WAXS and SAXS. A mixture of α′ and α crystals were observed at temperatures above 170 °C at high pressures. A frustrated structure was induced at P_c ≥ 100 MPa owing to sliding of chains and crystal blocks upon volume shrinkage. A mesomorphic phase was observed at P_c ≥ 100 MPa or at 190 °C ≥ T_c ≥ 165 °C by SAXS and WAXS. Upon heating the samples prepared at high P_c, doubling lamellar thickening occurred via mutual chain rearrangement between the adjacent lamellae. Based on the results, a phase diagram was revealed for the crystal structures formed and under different crystallization conditions.