Structure evolution and orientation mechanism of long-chain-branched poly (lactic acid) in the process of solid die drawing

Abstract

Highly oriented long-chain-branched poly (lactic acid) (LCB-PLA) was prepared and the structure evolution was studied in the process of solid die drawing by compared with poly (lactic acid) (PLA). During drawing, samples underwent not only die drawing process but also free drawing process. Drawing speed presented a prominent effect on the free drawing process, while die thickness showed a more obvious influence on the die drawing process. For PLA, free drawing process mainly contributed to its final orientation degree and crystallinity, and thus the mechanical properties of PLA were greatly influenced by drawing speed. However, for LCB-PLA, die drawing process made a greater contribution to the final orientation degree and crystallinity, and its mechanical properties were mainly affected by die thickness. Under the same drawing condition, the tensile strength and modulus of LCB-PLA were always higher than those of PLA, and reached up to 228 MPa and 7.2 GPa, respectively, which basically met the requirement for born fixation materials. Samples which only underwent die drawing showed obvious “sandwich” structure, and the thickness of the oriented skin layer for LCB-PLA was thicker than that for PLA, suggesting that shear-induced orientation can be easily retained due to the enhanced entanglement between long branched chains. After drawing, LCB-PLA samples showed smaller lamellae size (Llateral) but larger long period (Lac) compared with PLA, suggesting that the low chain mobility restricted the motion of chain slipping of LCB-PLA and thus resulted in the fragmentation of neighboring crystal lamella by chain stretched-out.