Preparation Of Oriented Polylactic Acid Film And Its Stretching Behavior

The melt stretching is a special flow field between the conventional extrusion weak flow field and the spinning strong flow field.The research on semi-crystalline polymer materials such as polypropylene,polyethylene and polyvinylidene fluoride has shown that by controlling the melt stretching flow field,an initial film with oriented lamellar crystal structure can be formed.Slow crystallization rate and brittleness of PLA biodegradable materials limit their application.The processing flow field directly affects the crystalline morphology and properties of crystalline polymer materials.For the weakly crystallized PLA materials,the crystal morphology and corresponding properties brought by the special flow field of melt stretching are worth discussion.In this paper,poly(lactic acid)(PLA)casting film with oriented structure was prepared by melt stretching.We study from the following four aspects: the influence of low molecular weight on the structure and properties of PLA casting film;the annealing of PLA casting film and the stress-induced crystallization by stretching at room/near-glass transition temperature.The raw material parameters and melt draw ratio have a great influence on the structure and properties of PLA casting film.PLLA1 with low molecular weight fraction content has higher melt strength,and it is easier to establish entanglement network and disentanglement in melt drawing process,and form oriented structure at lower melt drawing ratio.The "shish-kebab" like structure is formed with the increase of melt draw ratio.The PLA casting film with oriented structure shows high "toughness" in the process of drawing at room temperature,and the maximum elongation at break is 275%.The structure evolution of PLA film with oriented structure during room temperature tensile process is studied,and the source of high elongation at break and the mechanism of room temperature tensile toughening are clarified.The results of SEM,SAXS and optical microscope show that the oriented PLA casting film forms craze in the stretching process.DSC shows that the tensile induced crystallization also plays an important role in the room temperature stretching process of the oriented PLA casting film.The in-situ infrared test shows that the gg conformation is reduced in the stretching process,and there is breaking and recombination in the crystal and amorphous areas.In-situ WAXS test results show that although the stress-induced crystallization occurs in the process of room temperature stretching,the perfect crystal structure is not formed,but it is related to the formation of PLA mesophase.High elongation at break at room temperature is due to the contribution of stress-induced craze and the formation of mesophase.The annealing process at 60～90 ℃ is studied.Compared with the non-oriented structure,the oriented structure significantly improves the crystallization rate of PLA and shortens the crystallization induction period.The two melting peaks of isothermal crystallization at 90℃are related to the orientation perfection of the film along the direction of TD and MD.The formation of "cross" crystal structure is found by SEM.According to Avrami index,one-dimensional crystal growth of casting film with oriented structure is observed at 60～90℃.The tensile process of oriented casting film at 60℃ is studied.When the strain is less than 96%,the high energy conformation content of the molecular chain is more,the crystal structure can not exist stably,and the transformation between the crystal structure and the amorphous structure is repeated.But when the strain is higher than 96%,the stable crystal structure can be formed by stress induction.DSC results show that the damage and recombination of the crystal have little effect on the crystallinity of the sample after stretching,but the melting point is greatly affected by the damage and recombination of the crystal.The results of mechanical properties test show that the mechanical properties of the oriented PLA casting film are significantly improved after stretching near the glass transition temperature,the tensile strength is up to 248 Mpa,and the modulus is up to 4.2Gpa,which provides a new method for the reinforcement and toughening of PLA.The improvement of mechanical properties is related to the formation of strain-induced crystallization near the glass transition temperature.