Fabrication And Properties Of Electrospun Enantiomeric Polylactides/Fluorescent Carbon Quantum Dots Based Hybrid Micro/Nanofibers

In view of energy shortages,harvesting the environmental available renewable energy such as solar energy,mechanical energy and wind energy into electrical energy is particularly important for the realization of a sustainable modern society.Compared with the traditional piezoelectric materials,the natural piezopolymers have demonstrated more advantages in converting kinetic energy into electricity,and promise applications in medicine and self-powered electronics.Polylactic acid(PLA)is a degradable natural polymer material with abundant sources,chiral molecular conformation and unique piezoelectricity.However,the piezoelectric properties of PLA are very mild,and it needs to undergo uniaxial orientation to exhibit shear piezoelectric responses.When PLA fibers are prepared by electrospinning,their morphologies and orientation can be adjusted by the control of electrospinning parameters in order to improve piezoelectric properties and expand the application range.Furthermore,a multi-functional piezoelectric fibrous material,with multi-color fluorescence properties and high fluorescence stability,will be produced by the introduction of carbon quantum dots(CQDs)into PLA during the electrospinning,making the material promising for self-powered systems for biological implants or energy harvesting.In this paper,PLLA,PDLA,PLLA/PDLA and PLLA/PDLA/CQDs micro/nanofibers with uniform diameter and smooth surface were prepared with the utilization of electrospinning technique,and the crystallization behaviors,morphologies and hydrolytic degradation in different pH environments were investigated for those electrospun bionanofibers.Moreover,the piezoelectric and photoluminescent properties of the electrospun PLLA/PDLA/CQDs hybrid micro/nanofibers were further studied in this thesis.The electrospun nanofibers of single PDLA or PLLA only have homogeneous crystals.In contrast,the fibers obtained by mixing PDLA with PLLA by 1:1 mass ratio have stereocomplex crystals,and the amount of homogeneous crystals is almost negligible.At the same time,electrospinning at high voltage enhances the orientation and formation of the polylactic acid stereocomplex crystal,and the heat resistance of the nanofiber is significantly improved.In terms of polylactic acid hydrolysis properties,the electrospun bionanofibers hydrolyzed fastest in alkaline environment,but slower in acidic and neutral conditions.Compared with PDLA or PLLA,the hydrolytic degradation rate of PDLA/PLLA fibers was relatively lower.Under the same pH value,the hydrolysis degradation rates of PDLA/PLLA and PDLA/PLLA/CQDs fibers were almost the same.Multicolor photoluminescent PDLA/PLLA/CQDs bionanofibers,with optimal piezoelectric output,were obtained by adjusting the preparation conditions of electrospinning.The piezoelectric output of PDLA/PLLA/CQDs nanofibers was further enhanced with the increase of nanofiber network layers and the amplitude/frequency of excitation force.Especially at higher frequency,very high piezoelectric output density could be achieved at relatively low dynamic load amplitude.The maximum open-circuit voltage and short-circuit current output density reached 74.2 V/cm~3 and 4.9?A/cm~3,respectively.In addition,PDLA/PLLA/CQDs based nanogenerators demonstrated good stability and durability together with a self-perfection mechanism in electromechanical conversion,and no attenuation of electrical signal output was observed in more than 10000 continuous working cycles.In addition,electrospun hybrid nanofibers emit different fluorescent colors under laser excitation of different wavelengths.As an developed eco-friendly bio-nanofiber,it can be used as a self-powered biosensor,bio-piezoelectric nano-generator,in future transient electronics,implantable medical devices and so on.