Controllable Preparation Of Buckled Fibers And Silver Nanoparticle Chains

Owing to being stretched, bended and twisted to a certain deformation, flexible or stretchable optoelectronics are the future trend of electronic devices, and flexible or stretchable conductors play an important role in flexible or stretchable optoelectronics. “Structure that can be stretched” is a major strategy to prepare flexible or stretchable conductors. Electrically conductive materials with wave shape or spring-like can be stretched to a certain strain and treated as stretchable conductors. Based on the mechanical buckling of jet in the electrospinning, curved fiber can be prepared, including wavy fibers or 3D helical fibers, considering that electrospinning is a simple, effective and low-cost preparing technology. However, the formation conditions and the formation mechanisms of wavy or 3D helical fibers in the electrospinning have been seldom reported. In this thesis, wavy or spring-like polystyrene fibers were controllably prepared based on mechanical buckling of jets in electrospinning. The formation conditions and the formation mechanisms of wavy or 3D helical fibers were investigated. As-prepared curved fibers are expected to be utilized for preparing stretchable conductor. Silver nanoparticles chain were prepared by combining elelctrospinning with calcination. The preparation conditions and formation mechanism of silver nanoparticles chain network were explored and silver nanoparticles chain network is promising to be applied for flexible sensors. The main work are summarized as follows.(1) We investigated the morphology change and formation mechanism of wavy microfibers via electrospinning by taking polystyrene(PS)/dimethylformamide(DMF) solution as an example. The effect of electrospinning parameters, i.e. voltage and nozzle-to-collector distance, on the amplitude of wavy fibers was explored. Several kinds of arrays consisting of wavy microfibers were prepared. Our results showed that with the increase of collector speed, PS microfibers experienced the conversion in morphology from translated coiling to “W”-shaped microfibers, then to wavy microfibers, and finally to straight fibers. Further analysis on the relationship between the amplitude and collector speed indicates that the conversion from wavy fibers to straight fibers follows the Hopf bifurcation, i.e. the nonlinear perturbation in the direction perpendicular to the movement of collector generates Hopf bifurcation with O(2) symmetry and initiate the formation of wavy fibers. The conversion between wavy fiber and “W”-shaped fiber may be induced by the change in the kinematic mode of perturbation in the longitudinal and transverse direction to the movement of collector. Wavy fiber with maximum amplitude located at the position where the fiber changed from “W”-shaped fiber into wavy fiber. The value of maximum amplitude and critical collector speed increased with the increase of voltage and nozzle-to-collector distance. We managed to produce arrays consisting of uniaxially aligned, 90° cross-stacked and 45° cross-stacked wavy fibers, which are potential candidates to prepare stretchable electronics.(2) Spring-like polystyrene fibers were controllablly prepared with liquid surface as collector via electrospinning. The formation conditions of spring-like polystyrene fibers and the influential factors that determined the wire diameter and the outer diameter of spring were investigated. Moreover, in order to verify reliability of spring-like fibers in flexible electronics, the length change and stress distribution in hollow spring-like silver fibers when subjected to tensile were simulated, and the mechanical match of hollow silver spring with polymer matrix was also explored. It was found that straight jet and large moving speed of nozzle(>40 mm/s) are essential conditions to obtain aligned coiling spring shaped polystyrene fibers. The wire diameter and the outer diameter of coiling spring increase with the arise of nozzle-to-collector distance or solution concentration. Furthermore, based on finite element analysis for the mechanical property of hollow silver spring, the maxinum elastic elongation of silver spring with the equal size of coling polystyrene fibers is 53%, which matches with elastic polymer matrix.Finally, silver nanoparticle chains located on quartz were fabricated by combining electrospinning with subsequent thermal treatment. The electrospinning parameters to prepare precursor nanofibers containing silver nitrate were investigated. The influence of calcinating temperature on the arrangement and diameter of silver nanoparticles was explored. Our results showed that smooth nanofibers without beads were prepared when the volume ratio of silver nitrate aqueous solution to PVP ethanol solution was 2:5. Then silver nanoparticles with the diameter less than 150 nm were arranged like a chain by calcinating precursor nanofibers at a low temperature of 500 oC. However, when the calcinating temperature was 850 oC, the silver nanoparticles with the diameter of 500~700 nm were in disorder. As-prepared silver nanoparticle chain possessed surface plasmon resonance at wavelength of 362 nm and 506 nm. The formation of silver nanoparticles was attributed to two phases of precursor fibers, owing to the incompatability of silver nitride and PVP in ethanol.