Study On The Evolution Of Hierachical Structures Of Poly-L-Lactic Acid (Plla)under Pressure-Induced-Flow (Pif)Processing

In recent years, polylactic acid (PLA), as a new environmentally friendly and biodegradable polymer material, has gained widespread interest in the reseachers. In this paper, we have studied the evolution of hierachical structures and performances of poly-L-lactic acid (PLLA) under pressure-induced-flow (PIF) processing using small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXD) with synchrotron radiation, X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and other test methods. The main results are as follows:(1) During the PIF processing, with the increase of the compression ratio, the lamellae firstly yielded and formed the lamellae clusters arranging along the flow direction (FD) in a certain angle. Then, the lamellae crushed furtherly. At the same time, the molecular chain was pulled out of the original lamellae and oriented along the FD. And then the new oriented lamellae clusters arranging periodically along FD were formed. When the compression ratio further increased, the initial yielded lamellae completely crushed and disappeared, and then replaced by a new structure which is packed by crushed crystals along the FD. The molecular chain acrossed many crystals and strenched along FD. Moreover, the molecular chain bound the crushed crystals, forming the microfibrillar srtucture. Between the microfibers, the microvoids along the FD formed and these microfibers would be stripped when they are fractured.(2) When the PIF processing temperature increased, the moving ability of the lamellae became stronger, and the lamellae was easier to twist and oriented along the FD. Similarly, the thicker lamellae acquired from higher temperature was more stable and easier to twist, but harder to crush.(3) The tensile strength and impact strength were simultaneously improved after PIF processing, and with the increase of the initial thickness of lamellae, the tensile strengh and impact strengh of PLLA materials after PIF processing could be3.5and1.5times of those before PIF processing. The starting melting temperature (ronset), melting point (Tm) had a trend of decrease, suggesting that some crystal crushed and the regularity weakened, but PIF processing method would not change the crystallinity of materials at small compression ratio. At the same time, the molecular motion and relaxation of amorphous areas were restricted, the glass transition temperature increased and the peak of a relaxation became larger.