Abstract:

The non-isothermal crystallization behavior of polylactide(PLA) was investigated by differential scanning calorimetry(DSC), including heating from glass state and cooling from melted state for the PLA polymer. Crystallization kinetic parameters were calculated using Ozawa equation, Mo method, Khanna law and Kissinger method. These methods are all suitable for handling non-isothermal crystallization process of PLA.The crystallization rate coefficient(CRC) proposed by Khanna can easily evaluate the relative crystallization rate of PLA. In contrast to the cooling process from melted state, the heating process of PLA from glass state stimulates crystal nucleus generation, while cooling process is conducive to the growth of crystals. When the heating rate is at 2.0 ℃/min, crystallization enthalpy(ΔH_c) reaches the maximum of 27.1 J/g. When the crystallization starts from the melted state, ΔH_c increases with the decreasing cooling rate. The cooling rate is lowered to 0.25 ℃/min, and ΔH_c is increased to 28.3 J/g. At a lower temperature, the process of crystallization from the glass state, is mainly heterogeneous nucleation of two-dimensional crystal growth pattern. At higher temperature, crystallization from the glass state and melted polymer largely attributes to the homogeneous nucleation for three-dimensional growth pattern. Compared with the heating process, the cooling process is not conducive to the formation of crystal nucleus, so that in the cooling process ΔH_c is much lower, and the diffusion activation energy is a little higher than that in the heating process.

Key words: polylactide, non-isothermal crystallization, Ozawa equation, Khanna method, polymeric melted state, glass state