Huang Tong, Miura Motohiro, Nobukawa Shogo, Yamaguchi Masayuki
School of Materials Science, Japan Advanced Institute of Science and Technology 1-1 Asahidai, Nomi, Ishikawa 923-1292 Japan.
Biomacromolecules. 2015 May 11;16(5):1660-6. doi: 10.1021/acs.biomac.5b00293. Epub 2015 Apr 24.
The effect of chain packing on tensile properties was studied employing amorphous poly(lactic acid) PLA. It was found that the samples cooled in the temperature range from 60 to 80 °C, that is, slightly higher than the glass transition temperature Tg, showed ductile behavior with a low brittle-ductile transition temperature. Furthermore, the samples obtained by prolonged cooling at 56 °C also showed ductile behavior, whereas a shorter cooling time at the same temperature provided a brittle product. Even for the samples quenched at 40 °C, they showed ductile behavior after the exposure to postprocessing annealing operation at 60 °C; that is, the strain at break is larger than 3. This is an anomalous phenomenon for a glassy polymer. The dynamic mechanical analysis and thermal characterization revealed that the ductile samples show slightly higher Tg than the brittle ones, presumably due to high packing density of polymer chains. Moreover, it was found from infrared spectroscopy that the ductile samples show strong absorbance at 1267 cm(-1), ascribed to high energy gauche-gauche gg conformers. Following the classic Robertson's descriptions of plastic flow, it is concluded that the increase in the gauche-gauche gg conformers, which shows the conformation change under a low stress level, reduces the critical onset stress for shear yielding. The results demonstrated that the mechanical toughness of PLA can be controlled by the cooling conditions during processing and the postprocessing annealing operation.
采用非晶态聚乳酸（PLA）研究了链堆积对拉伸性能的影响。结果发现，在60至80°C的温度范围内冷却的样品，即略高于玻璃化转变温度Tg，表现出韧性行为，且脆韧转变温度较低。此外，在56°C下长时间冷却得到的样品也表现出韧性行为，而在相同温度下较短的冷却时间则得到脆性产物。即使对于在40°C下淬火的样品，在60°C下进行后处理退火操作后也表现出韧性行为；即断裂应变大于3。这对于玻璃态聚合物来说是一种反常现象。动态力学分析和热表征表明，韧性样品的Tg略高于脆性样品，这可能是由于聚合物链的堆积密度较高。此外，通过红外光谱发现，韧性样品在1267 cm(-1)处有强烈吸收，这归因于高能左-左（gg）构象体。根据经典的罗伯逊对塑性流动的描述，可以得出结论，左-左（gg）构象体的增加，即在低应力水平下显示出构象变化，降低了剪切屈服的临界起始应力。结果表明，PLA的机械韧性可以通过加工过程中的冷却条件和后处理退火操作来控制。
