The non‐isothermal and isothermal crystallizations of extruded poly(l‐lactic acid) (PLLA) blends with 10, 20 and 30 wt% poly(ethylene glycol) (PEG) were investigated with differential scanning calorimetry. The formation of α‐form crystals in the blend films was verified using X‐ray diffraction and an increase in crystallinity indexes using Fourier transformation infrared spectroscopy. Crystallization and melting temperatures and crystallinity of PLLA increased with decreasing cooling rate (CR) and showed higher values for the blends. Although PLLA crystallized during both cooling and heating, after incorporation of PEG and with CR = 2 °C min−1 its crystallization was completed during cooling. Increasingly distinct with CR, a small peak appeared on the lower temperature flank of the PLLA melting curve in the blends. A three‐dimensional nucleation process with increasing contribution from nuclei growth at higher CR was verified from Avrami analysis, whereas Kissinger's method showed that the diluent effect of 10 and 20 wt% PEG in PLLA decreased the effective energy barrier. During isothermal crystallization, crystallization half‐time increased with temperature (Tic) for the blends, decreased with PEG content and was lower than that of pure PLLA. In addition, the Avrami rate constants were significantly higher than those of pure PLLA, at the lower Tic. Different crystal morphologies in the PLLA phase were formed, melting in a broader and slightly higher Tm range than pure PLLA. The crystallization activation energy of PLLA decreased by 56% after the addition of 10 wt% PEG, increasing though with PEG content. Finally, PEG/PLLA blends presented improved flexibility and hydrophilicity. © 2019 Society of Chemical Industry
In this study, hydroxyapatite (HA) was incorporated in a poly(L-lactic acid) (PLLA) matrix and the thermal properties and crystallization behavior of the derived composites were investigated. The nanocomposites, containing 0–20 wt% HA, were prepared by melt extrusion employing a twin-screw extruder. XRD experiments verified an increase in the intensity of the characteristic diffraction peak of the α-form crystalline phase of PLLA with increasing HA content. By DSC experiments it was observed that the presence of HA increased the crystallinity during cold crystallization, leading to a shift of cold-crystallization temperature to lower values and to an increase in the melting temperature of the PLLA phase. Isothermal crystallization experiments at 100, 110, 115 and 120°C, revealed a maximum in crystallization kinetic around 100°C after the addition of HA compared to 115°C for pure PLLA. The crystallization rate of PLLA matrix in the nanocomposites decreased with increasing crystallization temperature. By using the Avrami and Lauritzen-Hoffman equations the exponent n was calculated in the range 2–3 and a theoretical approach verified that the HA/PLLA systems belong to Regime II of crystallization behavior. The investigated melting behavior of PLLA was attributed to better organized crystalline structure with increasing isothermal crystallization temperatures and might be related with the longer time necessary for the completion of crystallization.
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