Study On Crystallization Kinetics And Structure Of PLA Under External Field

Polylactic acid(PLA),as an biodegradable and environmentally friendly material that can replace petroleum-based polymers,has a wide range of applications in biomedicine,packaging,textile and automobile interiors.However,the glass transition temperature and crystallization rate of PLA is relatively low which make the crystallinity change continuously during processing and use,and affect the performance of PLA products.Therefore,it is helpful to improve the properties of PLA products by studying the effect of external field on the crystalline structure and properties of PLA.At present,such methods to improve the crystallinity of PLA include pressure field,shear field,tensile field and temperature field.By controlling the nucleation and growth process,the crystallization kinetics of polylactic acid is improved and the crystallinity is increased.In this thesis,the isothermal and non-isothermal crystallization kinetics of PLA,the crystallization formation and structure changes of PLA under different temperature fields,and the orientation and structure development of PLA induced by tensile field were studied respectively.The evolution of crystalline and structure under different conditions was analyzed,and the effect of structure on thermal and mechanical properties was analyzed,and the relationship between structure and properties was established.It is hoped that this research can provide reference for processing.Differential scanning calorimetry(DSC),polarizing microscope(POM),dynamic thermomechanical analysis(DMA)and synchrotron radiation small angle and wide angle X-ray scattering(WAXS and SAXS)were used to study the.crystalline structure and properties of PLA.In the first part,the isothermal and non-isothermal crystallization kinetic processes of PLA,structure and thermal properties of PLA from melt isothermal crystallization and cold crystallization were analyzed.The Avrami equation was used to analyze the isothermal crystallization kinetics of PLA.The Avrami exponent n is between 2 and 3,which indicates that the growth mode of PLA is mainly two-dimensional lamellar crystal and three-dimensional spherulite.Isothermal crystallization studies at different temperatures showed that PLA has the fastest total crystallization rate at 110 ?,which indicates that the crystallization kinetics is accelerated.A relatively low Avrami exponent n was obtained at crystallization temperature 110 C which indicated that more two-dimensional growth mode of crystals may exist resulting from a relatively fast dynamic process.Several methods were used to analyze the non-isothermal crystallization process of PLA Ozawa model has limitations because of the great difference in the crystallization temperature range of PLA at different cooling rates.Mo method combining Avrami with Ozawa gets more reasonable results in analyzing the non-isothermal crystallization kinetics of PLA.Non-isothermal crystallization kinetics studies show higher crystallinity could be obtained by accelerating cooling rate per unit crystallization time.PLA products can improve its crystallinity in two ways.Firstly,annealing treatment is carried out directly from melt,the second is the cold crystallize treatment before use.In the part of the thesis,structure and thermal properties of PLA from melt isothermal crystallization and cold crystallization were analyzed.It was found that the nucleus formation was faster at low temperature,while the segment movement was more favorable at high temperature.It was found that the fastest rate of spherulite growth was at 130?,which was different from the optimum temperature for crystallization total rate.Both treatments showed lower crystallinity at low temperature;with the increase of temperature,crystallinity increased,and the phenomenon of melt recrystallization could be observed in DSC heating curve,which indicated that defective crystallization was further improved before melting.This conclusion also supports the results of the first part of the isothermal crystallization kinetics research.Compared with the cold crystallization,the melt crystallization sample has an obviously better crystalline structure especially at low temperature.The results of impact and tensile tests of cold crystallized PLA showed that the impact strength and elastic modulus increase while the tensile strength decreases under high temperature cold crystallization conditions,indicating that cold crystallization treatment at higher temperature not only increased the crystallinity,but also improved the toughness.In order to simulate the structural changes of PLA during polymer processing,the third part mainly deals with the induction cold crystallization of PLA under different tensile strains.The tensile strain and the temperature of cold crystallization are used as parameters to analyze the crystalline structure and properties of PLA.With the increase of strain,the internal stress increases,the orientation of the polymer segments increases which helps better cold crystallization process.The results of DSC and XRD all support that when the strain reaches a certain level,the orientation structure will be destroyed and relaxed.When PLA is tensile stretched in the temperature range of Tg-Tm,the orientation structure can be maintained intact under larger strain.In a certain temperature range,the appearance of more stable a crystalline form leads to more complex rules of crystallinity.The orientation analysis by synchrotron radiation show that both the microcrystalline structure and the lamellar crystal have obvious orientation.The orientation at low temperature is obvious,and the orientation structure relaxes at high temperature.At low temperatures,especially for microcrystalline structure,there shows a cyclic process of orientation structure which is orientation-destruction-disorientation-orientation-destruction.