Structure Of Individual Poly(Vinylidene Fluoride) Electrospun Nanofibers

Electrospinning is a convenient and versatile technique for the fabrication of continuous polymer nanofibers with tunable diameters ranging from tens of nanometers to several micrometers.Electrospun nanofibers often exhibit unusual properties as compared to bulk materials,one of which is an exponential increase in modulus when reducing the fiber diameter below an onset value.This is due to their complex internal structure and molecular orientation.However,due to the limitations of previous research methods,analysis of individual electrospun nanofibers with small diameters is challenging.In this thesis,molecular orientation and polymorphic distribution in individual electrospun poly(vinylidene fluoride)(PVDF)fibers were investigated by atomic force microscopy-infrared(AFM-IR).The main finidings and conclusions are summarized as follows:1.Molecular orientation in individual electrospun nanofibersThe molecular orientation in individual nanofibers was analyzed by the polarized AFM-IR technique.We first introduced correction factors to eliminate the effect of laser energy difference between different polarizations of the IR radiation,and then dichroic ratio of the absorption intensity of parallel spectrum to that of vertical spectrum was derived to reflect the degree of molecular orientation,establishing a method for the quantitation of molecular orientation by polarized AFM-IR.By using this method,we analyzed individual nanofibers with different diameters spun under identical conditions,and found that the molecular orientation along the fiber of a uniform thickness remained constant,whereas for the fibers of different sizes,the molecular orientation increased with the fiber diameter decreasing,showing a similar trend as literature results for thicker fibers.The method reported here provides a powerful tool for the determination of molecular orientation in nanofibers,which is critical for understanding of the structure-property relationship for electrospun fibers.2.Melting temperature of individual electrospun PVDF nanofibersAnalyses of four PVDF fiber mats electrospun under different conditions showed that the PVDF crystallized into a and ? phases,exhibiting typical crystallinity and Tm values with no significant difference among the four mat samples.However,analyses at single fiber level revealed broad distribution in diameter and Tm for the fibers produced under each identical electrospinning condition.The Tm of individual nanofibers remained constant at large diameters and increased quickly when reducing the fiber diameter toward the nanoscale,and Tm values of 220?230? were observed for the thinnest nanofibers,much higher than the typical values reported for bulk PVDF.The Tm and molecular orientation at different positions along a beaded fiber were analyzed,showing a similar distribution pattern with a minimum at the bead center and higher values when moving toward both directions.The results indicate that Tm dependence on the diameter of nanofibers is a superficial phenomenon,and molecular orientation is the underlying driving mechanism for the observed correlation between the Tm and the diameter of the nanofibers.3.Polymorphic distribution in individual electrospinning PVDF nanofibersIn electrospun PVDF nanofiber,the PVDF crystallizes into a and ? phases.The spatial distribution of the ? and ?-form crystals in single PVDF nanofibers along the fiber axis was studied by AFM-IR imaging,which revealed that the two crystalline polymorphs were randomly distributed.Based on the experimental observations,a possible mechanism was proposed.