Abstract
By using simultaneous small/wide-angle X-ray scattering (SAXS/WAXS), Fourier-transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC), we have investigated in detail the effects of physical aging on cold-crystallization behavior of enantiomeric PLLA α′ phase and racemic PLLA/PDLA βc phase. We firstly found that the α′ crystals are intrinsically metastable phase which continuously increasing packing density towards the α form. It release very limited latent heat upon emergence and then underwent continuous perfection for decreasing lattice spacing during which a majority of latent heat was released. Small amount of the better-packed α′ crystals will transform to smaller α form when the lattice parameters were closer but still finitely different from those of the α phase, implying the transformation is achieved via a first-order transition. We show from DSC traces that annealing of amorphous PLLA below the glass transition temperature (Tg) increases the crystallization rate and shortens induction period during cold-crystallization at 80 °C. Emergence of (110)/(200) reflection of α′ crystals during crystallization was earlier in physically aged PLLA than the as-quenched counterpart. Time evolution of the SAXS profile generally shows shifting/intensification of interparticle correlation peak toward higher q range with crystallization time, indicating increased nanograin population and hence dominantly sporadic nucleation. From dispersed-ellipsoids model fitting, the sporadic nucleation rate for aged PLLA deduced from the slope of np vs. tc data is 1.4 times the value of the as-quenched case in the beginning of crystallization. Results from FTIR indicate that the shortening in crystallization induction period and faster in crystallization kinetics for aged PLLA was caused by the preformed α′ precursor with interchain methylene-methylene interaction in physically aged specimen. We conclude that these precursors subsequently transform into nuclei of α′ crystals upon heating above Tg without consuming the amorphous phase in the beginning, resulting in increased sporadic nucleation rate (as compared to the as-quenched glass) and shortened induction period during cold-crystallization. By aging the polylactide mixture with weight ratio of PLLA to PDLA equal to 1:1, the α′ precursor instead of βc precursor (paired 32/31 helices) will formed during aging, as illustrated in the unchanged peak position of asymmetric C-H stretching band at ca. 2995 cm-1. We discover that the aged specimen can produce more paired 32/31 helices in the induction period as the βc precursor and increase the sporadic nucleation rate. In addition, the crystallization always begins from decreasing the FTIR amorphous band at 1267 cm-1 and increasing the interchain interaction band at 1210 cm-1, followed by the emergence of the paired 32/31 helices no matter whether the sample is aged or not. This is contrary to the case of physically aged pure PLLA sample in which formation of 107-helix precede the consumption of amorphous phase. This implies that α′ precursory may act as the nucleating agent of βc crystal and facilitate the formation of regular paired 32/31 helices from the amorphous matrix and subsequent nucleation process. We further demonstrated that the structure form during aging are hardly removed even the lower heating rate (5 °C /min) is apply when dynamic heating experiment was conducted. The result reveals that the physically aged sample nucleate at lower temperature (82.4 °C) compared with the as-quenched counterpart (85.6 °C). The earlier nucleation of aged sample is directly related to higher level of interchain interaction contributed from α′ precursory before nucleation of βc phase.