The effect of molecular weight (MW) on the polymorphous crystallization and melting behavior of poly(l-lactide) (PLLA) were systemically studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide-angle X-ray diffraction (WAXD), and time-resolved Fourier transform infrared (FTIR) spectroscopy. It was found that the polymorphism of PLLA is not influenced much by MW, and the α‘- and α-form crystals are produced at low and high crystallization temperature (T c), respectively, regardless of the MW. However, MW significantly affects the crystallization kinetics, and the crystallization rate reduces greatly with MW increasing. Moreover, the T c- and MW-dependent melting behavior of PLLA was clarified with combining the DSC and FTIR results. It was found that the α‘- to α-crystalline phase transition occurs prior to the dominant melting in both the low- and high-MW PLLA crystallized at low T c. Unlike the high-MW PLLA, in low-MW PLLA crystallized at low T c, the α‘-form crystals only partially transform into the α-one, and some amounts of α‘-form crystals melt directly without transition during the heating process. With increasing T c, the melting of PLLA with various MWs changes from the phase transition + melting mechanism to the usual melt−recrystallization mechanism.

Effects of annealing conditions and molecular weight (MW) on the crystalline phase transition in poly(L-lactide) (PLLA) were studied by wide-angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC). The disordered crystal (R′-form) of PLLA was found to transform into the R one during annealing process at elevated temperatures. The R′-to-R transition is quite dependent on the annealing period (t a : 0-1440 min) and annealing temperature (T a : 120-160°C). With increasing T a , the polymorphic transition progresses much more rapidly. The R′-to-R transition is mainly involved by the slight rearrangement of the chain conformation (especially related to the side groups) and packing manner in the unit cell to the more energy-favorable state, corresponding to the reduction of unit cell dimension. Besides, it was proposed that the R′-to-R transformation mainly proceeds by the direct solid-solid transition mechanism. Moreover, it was found that MW affects the crystalline phase transition significantly. In the low-MW PLLA sample, the R′-to-R transition becomes much faster, and it can proceed prominently even when annealed at relatively lower temperature.

The crystalline structure of poly(L-lactide) (PLLA) have been found to quite depend on the crystallization temperatures (T c s), especially in the range of 10021208C, which is usually used as the crystallization temperature for the industrial process of PLLA. The analysis of wide-angle X-ray diffraction and Fourier transformed infrared spectroscopy revealed that 1108C is a critical temperature for PLLA crystallization. At T c < 1108C and T c 1108C, the a 0 and a crystals were mainly produced, respectively. Besides, the structural feature of the a 0 -form was illustrated, and it was found that the a 0 -form has the larger unit cell dimension than that of the a-form. Moreover, the crystallization kinetics of the a 0 and a crystals are different, resulting in the discontinuousness of the curves of spherulite radius growth rate (G) versus T c and the half time in the melt-crystallization (t 1/2 ) versus T c investigated by Polarized optical microscope and Differential scanning calorimetry, respectively.

The effects of miscible blending on the crystallization kinetics and crystalline structure of poly(Llactide) (PLLA) were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), Fourier transform infrared (FTIR) spectroscopy, and wide-angle X-ray diffraction (WAXD). It was confirmed that the blends of PLLA and poly(D,L-lactide) (PDLLA) are miscible at all compositions. Due to the dilution effect, the crystallization of PLLA was hindered with the presence of PDLLA. The PLLA/PDLLA blends show the irregular or ring-banded spherulites. The polymorphic behavior of PLLA is influenced by the miscible blending with PDLLA. FTIR and WAXD data indicates that the formation of the R-form crystals of PLLA is favored in the blends. With incorporation of 50 wt % PDLLA, the critical temperature for the formation of R′-and R-form crystals decreases from ∼110 (of neat PLLA) to ∼80°C. The factors affecting the formation of the PLLA R′-and R-form crystals were discussed from the thermodynamic and kinetic considerations. It was proposed that the formation of the metastable R′-form crystals of PLLA is kinetically preferential, while that of the thermally stable R-form crystals is thermodynamically favored. The blending effects on the crystalline structure of PLLA are ascribed to two factors, that is, the lowered equilibrium melting point and decreased crystallization rate.

Poly(L‐lactide) (PLA) was melt‐blended with four rubber components—ethylene–propylene copolymer, ethylene–acrylic rubber, acrylonitrile–butadiene rubber (NBR), and isoprene rubber (IR)—in an effort to toughen PLA. All the blend samples exhibited distinct phase separation. Amorphous PLA constituted a topologically continuous matrix in which the rubber particles were dispersed. According to Izod impact testing, toughening was achieved only when PLA was blended with NBR, which showed the smallest particle size in its blend samples. In agreement with the morphological analysis, the value of the interfacial tension between the PLA phase and the NBR phase was the lowest, and this suggested that rubber with a high polarity was more suitable for toughening PLA. Under the tensile stress conditions for NBR and IR blend samples, these rubbers displayed no crosslinking and showed a high ability to induce plastic deformation before the break as well as high elongation properties; this suggested that the intrinsic mobility of the rubber was important for the dissipation of the breaking energy. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009