The Online Research Of The Cold Crystallization Behavior Of Poly (Lactic Acid) (PLA) Based On In Situ FTIR

For the environmentally friendly polymer material, poly(lactic acid)(PLA) has a wide prospect in the important filed of medicine carring, biological scaffold and bone tissue regeration, due to its well biocompatibility and biodegradability.However, PLA is such a lower crystallization rate that the product can be very easy to present an amorphous state in the process of the actual machining, which causes the sharp decline of the thermostablilty and mechanical property, and finally restricts the practical application of the material. There is a great deal of research that shows that reprocessing PLA, such as annealing and external force exertad, is a key means of altering PLA’s microstructure, which improves its property. Therefore, the topic studied in this paper is that the cold crystallization behavior of PLA amorphous sample is researched by in-situ FTIR analytical technique under the temperature filed and the strain field. The dissertation is simply discribed as follows:(1) The PLA film is prepared by the solution method originally and the PLA annealed sample is obtained through the way of melt-quenching. The structure change of the raw sample, at the same time, is characterized with the help of WAXD,DSC and FTIR. It turns out that the PLA annealed sample is in an amorphous state.(2) The in situ FTIR analytical technique is utilized to online study the structure evolution of PLA amorphous sample when annealing, analyze the formation mechanism of mesophase as well as the transition from mesophase to crystal phase and also explore the influence of the annealing temperature on the crystallization process and the degree of crystal perfection. The results show that there is a critical crystallization temperature, within which range is 63 ~ 68oC. That is, the cold crystallization of sample can be realized through partial adjustment of the molecular chain when the annealing temperature is above this critical value, conversely, none of this can happen. Besides, it is found that mesophase always first appear in the cold crystallization process, followed by a mesophase-to-crystal phasetransformation, while the transition from amorphous to mesophase takes place through the synchronous adjustment of the molecular intrachain conformation and interchain packing. The higher the annealing temperature increase, the earlier the mesophase emerge, and the resulting crystal structure would become more and more regular.(3) In situ polarized FTIR measurement technique coupling with a tensile heating stage is employed to research the structure envolution of PLA amorphous sample and the relationship between the structure envolution and mechanical behavior, at the same time, discusses the influence of the drawing temperatures on strain-induced crystallization of PLA. It is found that the onset of the ordering process of the intermolecular packing is always the precursor of the crystalline form.There is no formation of mesophase before the crystallization onset during stretching,and its crystalline phase directly grows from the oriented amorphous phase. At the same time, PLA sample products the orientation in the process of in situ stretching and the interchain packing becomes more and more regular, the crystallinity will be improved after reaching the critical crystallization strain and the crystal structure will be more and more perfect. When the drawing temperature rises higher than it, the thermodynamics contribution becomes more and more significant, which is able to accelerate the intermolecular packing ordering and reduce the critical orientation value of amorphous phase for the crystallization onset. For the samples under drawing, additionally, the strain-hardening obviously occurs beyond the critical strain for the crystallization onset, and its profile consisted well with the changes of their crystallnity, suggesting that crystallinity controlled the strain-hardening process of the drawing samples.