||Chain packing and its anomalous effect on mechanical toughness for poly(lactic acid)
Huang, Tong ,
Miura, Motohiro ,
Nobukawa, ShogoYamaguchi, Masayuki
1666 , 2015-04-15 , American Chemical Society
In this study, the effect of chain packing on tensile properties is studied employing amorphous poly(lactic acid) PLA. It is found that the samples cooled in the temperature range from 60 to 80 oC, i.e., slightly higher than the glass transition temperature T_g, show ductile behavior with a low brittle-ductile transition temperature. Furthermore, the sample obtained by prolonged cooling at 56 oC also shows ductile behavior, whereas a shorter cooling time at the same temperature leads to brittle fracture. Even for the samples quenched at 40 oC, the exposure to post-processing annealing operation at 60 oC enhances the toughness greatly, which is an anomalous phenomenon for a glassy polymer. The dynamic mechanical analysis and thermal characterization reveal that the ductile samples show slightly higher T_g than the brittle samples, presumably due to the high packing density of polymer chains. Moreover, it is found from infrared spectroscopy that the ductile samples show strong absorbance at 1267 cm-1, ascribed to high-energy gg conformers. Following the classic Robertson’s descriptions of plastic flow, it is concluded that the increase in the gg conformers, which show the conformation change under a low stress level, reduces the critical onset stress for shear yielding. The results demonstrate that the mechanical toughness of PLA can be controlled by the cooling conditions during processing and the post-processing annealing operation.