Investigations On The Interfacial Polymer Crystallization Induced Homogenneously And Heterogenneously By Electrospun Polymer Nanofibers

Interfacial crystallization occurring at the interface between fillers and polymer has been a research hotspot in fibrous fillers(e.g.,carbon fibers)reinforced semicrystalline polymer composites.Generally,fillers with the diameter at the microscale can serve as one-dimensional(1D)heterogeneous nucleating agents for polymer crystallization and result in the formation of a transcrystallinity accross the interface.Whereas,a nanoscale filler e.g.,carbon nanotubes,always leads to the nanohybrid shish-kebabs.Electrospun nanofibers have attracted considerable attention in reinforcing a polymer,due to their excellent properties involving continuity,high length-diameter ration,and larger specific surface area,etc.However,the research regarding interfacial crystallization in electrospun nanofiber/polymer composites is still lacking and the relevant mechanisms have not yet been fully understood.In view of this,we systematically studied the interfacial crystallization in both PCL solution and melt,induced homogeneously and heterogenneously by electrospun nanofibers of poly(?-caprolactone)(PCL),PCL/poly(left-hand lactic acid)-co-Poly(?-caprolactone)(PLCL)composite fibers and nylon-66(PA66)fibers.This study is aimed at revealing the morphology of interfacial crystallization and its mechanisms in electrospun nanofiber filled polymer composites.To this end,a series of characterizations involving scanning electron microscopy(SEM),atomic force microscope(AFM),polarized optical microscope(POM),differential scanning calorimetry(DSC)and wide-angle X-ray diffraction(WAXD)were further employed to investigate the crystal morphology and structure,crystallization kinetics and crystallization behaviors.The main content of this work is illustrated as follows:Firstly,electrospun PCL fibers with different average diameters were prepared,and subsequently incorporated into PCL solution to study the interfacial crystallization induced by the homogeneous PCL fibers.It was found that PCL fibers acted as a onedimensional nucleating site for the epitaxial growth of PCL on the surface of PCL fibers.The resulting morphology and structure of interfacial crystallization showed a stronger dependence on the diameter of fibers and crystallization time.As for PCL fibers with diameter in hundreds of nanometers,the typical periodical nanofiber hybrid shishkebab(NFSK)structure can be observed perpendicular to the fiber axial.Whereas,when the the diameter of PCL fibers was above 2 ?m,PCL lamella arranged randomly on the surface of PCL fibers,formimg interface crystallization morphology with interlocked kebabs.The formation of the crystallization morphologies is likely attributed to the arrangement and/or regularity of PCL lamella of PCL fibers.Secondly,the interfacial crystallization was further studied in PCL solution with electrospun PCL/PLCL composite fibers.Remarkably,the content of PCL composite fibers dictated the generation of interfacial crystallization.Specifically,when PCL content was below 30 wt%,the composite fiber was unable to induce PCL crystallization on the surface of fibers.By contrast,with PCL content exceding 50 wt%,the hybrid shish-kebab was observed on the surface of composite fibers.It was revealed that the crystallization and especially the lamellar arrangement of PCL in composite nanofibers played a dominant role in inducing the epitaxial crystallization of PCL chains on the surface of fibers.Lastly,the interfacial crystallization behaviors of PCL induced by electrospun PA66 nanofibers in both PCL solution and melt were investigated.As for solution crystallization,the nanofiber hybrid shish-kebab(NFSK)was also obtained where PA66 fibers serve as the shish and PCL lamella as periodical kebab perpendicular to the PA66 fiber axial direction.More interestingly,for the first time,a hierarchical morphology of interfacial crystallization with the coexistence of NFSK and transcrystallization was found on the surface of PA66 fiber was observed for the crystallization from melt.The possible mechanism responsible for such a hierarchical interfacial crystallization was proposed: PCL chains is firstly absorbed onto the surface of PA66 electrospun fibers due to the strong intermolecular interaction by hydrogen bonding between PCL and PA66,and a layer of PCL chains is thus formed on the surface of PA66 nanofibers,which subsequently induces the epitaxial crystallization of PCL,leading to the NFSK structure.As crystallization proceeds,the outer PCL kebabs of the NFSK grow in the form of dendritic crystal and consequently results in the formation of transcrystallinity.