Abstract
In previous studies of poly(l-lactic acid) (PLLA) with specific solvent, the solvent-induced ε crystals with a small amount of α crystals at low temperature (＜ 0 °C) had been identified, but details of the phase transition upon heating process, crystalline behavior in both ε and α phases, and solvent effect in PLLA are unclear. Here we select the PLLA with cyclopentanone (CPO) as a system to analyze and report the results of our attempt to clarify these questions. Phase transition during programmed heating from 30 to 90 °C at a rate of 3 °C/min was monitored via simultaneous small/wide-angle X-ray scattering (SAXS/WAXS) and thermogravimetric analysis (TGA). WAXS observations along with supporting SAXS results show that the phase transition above 46 °C involves three stages: (1) beginning of phase transition, in which displays that ε phase starts to transform into meso- and α phases at the same time, (2) nucleation and growth of α phase from amorphous matrix, accompanied with faster transformation of ε phase, (3) final melting of the residual ε phase, representing the end of phase transition. Most importantly, the dominant mechanism of phase transition is the direct transformation from ε to α phases, with additional nucleation and growth of α phase from amorphous phase. Moreover, result obtained from TGA indicates that the rate of weight loss is faster when the temperature is above 46 °C, implying the progress of phase transition accelerates the evaporation rate of solvent. As more solvent molecules trapped in crystal lattice are released after the ε phase transforms into meso- and α phases, the rate of solvent evaporation increases. Isothermal experiments of WAXS/SAXS and TGA were also conducted to examine the phase transition more closely. Results at 46 °C show that the primary mechanism of phase transition is direct transformation from ε to α phases, with a small amount of α phase formed from amorphous phase, which is consistent with the results obtained from heating process. However, there is an incubation period before phase transition due to slow solvent transport. By comparing the time-dependent WAXS 1D profiles of isothermal experiments at 46 and 48 °C, it can be observed that the critical temperature of phase transition is indeed around 46 °C, which also interprets why there is an incubation period before phase transition. By preparing samples immersed into CPO for different periods of time, quantitative analysis for WAXS measurement reveals the relation between ε to α phases. As immersion time increases, the amount of ε phase rises moderately while that of α phase remains largely unchanged, indicating that ε and α phases are formed from amorphous phase and independent from each other. By comparing the WAXS results of sample with or without solvent treatment, it is found that PLLA with specific solvent treatment tends to give α (rather than α′) phase upon heating, implying that the presence of solvent enhances chain mobility, hence favoring direct formation of the α phase of higher packing density.