Crystal structural evolution and cavitation in biodegradable polyglycolide: An in-situ WAXD/SAXS study during hot-stretching

The structural evolution of high-crystallinity polyglycolide (PGA) during hot-stretching from 50 degrees C to 150 degrees C was investigated using synchrotron in-situ WAXD/SAXS techniques. The structural evolution during stretching at different temperatures can be divided into three stages. The stage preceding the yield point, the lamellar crystals undergo extensive slip due to shear forces. The crystallite size decreases by 43.9 % during stretching at 50 degrees C, while it only decreases by 23.8 % at 150 degrees C. In this stage, cavities with their long axes vertical to the stretching direction form between the lamellar stacks parallel to the stretching direction. Higher stretching temperatures are less favorable for the formation of these cavities. Stage from yield point to onset of strain hardening, some of the small crystals formed in the previous stage accumulate in the oblique direction. After reaching the critical strain, they reorient towards the stretching direction. This reorientation causes fragmentation and recrystallization, promoting the further development of cavities. Both the cavities and the recrystallized crystals align along the stretching direction. In the strain-hardening stage, new crystal formation occurs under stress, and higher temperatures favor the formation of highly oriented crystals. As a result, the crystallinity and crystal orientation of PGA at the end of stretching are positively correlated with the stretching temperature.