PA66拉伸形变测试

Tensile Deformation of Electrospun Nylon-6,6NanofibersE.ZUSSMAN,1M.BURMAN,1A.L.YARIN,1R.KHALFIN,2Y.COHEN21Faculty of Mechanical Engineering,Technion,Israel Institute of Technology,Haifa32000,Israel2Department of Chemical Engineering,Technion,Israel Institute of Technology,Haifa32000,IsraelReceived25July2005;revised25December2005;accepted24February2006DOI:10.1002/polb.20803Published online in Wiley InterScience().ABSTRACT:Nylon-6,6nanofibers were electrospun at an elongation rate of the orderof1000sÀ1and a cross-sectional area reduction of the order of0.33Â105.The influ-ence of these process peculiarities on the intrinsic structure and mechanical proper-ties of the electrospun nanofibers is studied in the present work.Individual electro-spun nanofibers with an average diameter of550nm were collected at take-up veloc-ities of5and20m/s and subsequently tested to assess their overall stress–straincharacteristics;the testing included an evaluation of Young’s modulus and the nano-fibers’mechanical strength.The results for the as-spun nanofibers were compared tothe stress–strain characteristics of the melt-extruded microfibers,which underwentpostprocessing.For the nanofibers that were collected at5m/s the average elonga-tion-at-break was66%,the mechanical strength was110MPa,and Young’s moduluswas453MPa,for take-up velocity of20m/s—61%,150and950MPa,respectively.The nanofibers displayed a-crystalline phase(with triclinic cell structure).V C2006WileyPeriodicals,Inc.J Polym Sci Part B:Polym Phys44:1482–1489,2006Keywords:electrospinning;nanofiber;nylon-6,6;tensile stress;Young’s modulusINTRODUCTIONNanomaterials can possess different mechanical properties than those of the bulk materials from which they were derived.This is perhaps due to different processing conditions that can affect the material’s structure or to the characteristic length scale,that is,confinement arrangements. For electrospun nanofibers made from polymer solutions with a typical diameter ranging from 20to1000nm,the extreme processing condi-tions can affect the macromolecular conforma-tion,the nanofiber structure,as well as the me-chanical characteristics.Projected uses for such nanofibers includefilter media and catalyst sup-ports,drug delivery carriers,tissue scaffolds,so-lar sails,reinforcing elements in composite ma-terials,nanosensors,and airborne structures.1–6 In many of these applications,the mechanical characteristics of electrospun nanofibers are of utmost importance.Evaluating these character-istics is the goal of the present work.A characteristic feature of the electrospinning process is the extremely rapid formation of the nanofiber structure,which is on a millisecond scale and can apparently promote the nucleation and growth of the polymer crystallites.Other no-table features of electrospinning are a huge mate-rial elongation rate of the order of1000sÀ1,and a cross-sectional area reduction of the order of 105.7,8These unusual characteristics of the elec-trospinning process have been shown to affect the orientation of the structural elements within the fiber,9–12which,in turn,is believed to have a direct impact on the mechanical properties of the nanofibers.Apparently,structural orientation and crystallinity inCorrespondence to: E.Zussman(E-mail:meeyal@tx.technion.c.il)Journal of Polymer Science:Part B:Polymer Physics,Vol.44,1482–1489(2006)V C2006Wiley Periodicals,Inc.14821.29governing parameters of the electrospinning pro-cess,and the nature of the polymer solutions(their rheological behavior,surface charge density,and solvent characteristics).13It was expected that structural characteris-tics would affect tensile mechanical properties, and studies to characterize the mechanical properties of individual electrospun nanofibers have already been performed.Tan et al.14con-ducted tensile tests of individual poly(e-capro-lactone),PCL,electrospun nanofibers.When comparing the results to gravity-spun PCLfi-bers,they found that the modulus and maximal tensile stress of the electrospun nanofibers were larger.In another work,Tan et al.15found that the tensile modulus of an individual elec-trospun PEO nanofiber is of the same order of magnitude as the corresponding bulk material. Inai et al.16investigated the mechanical prop-erties of as-spun poly(L-lactic acid)PLLA nano-fibers.The tensile strength of these nanofibers was close to that of the melt-spun PLLAfibers. In contrast,the tensile modulus and the strain-at-break of the electrospun nanofibers were lower than those of the melt-spunfibers.A higher tensile modulus and strength,but lower strain-at-break were obtained by increasing the take-up velocity.For stretched PEO nanofibers collected using a rotating wheel,multiple neck-ing and(in certain cases)fibrillar patterns resembling crazing in the necked regions have been observed;this is probably due to stretch-ing by the wheel.17The crazing was attributed to the formation of polymer crystals of the shish-kebab type.11,17,18The aim of the present work is to study the intrinsic structure and mechanical properties of individual nylon-6,6(PA66)electrospun nanofib-ers.The individual nanofibers were mounted between the tip of an AFM cantilever and a stainless steel wire.The tensile stress–strain curves of the individual nanofibers were calcu-lated from the force-displacement curve of the AFM cantilever as it was bent.The structure of the as-spun nanofibers was analyzed by means of wide-angle X-ray(WAXD)and small-angle X-ray scattering(SAXS).The paper is organized as follows:in the experimental section we de-scribe the material preparation,the fabrication technique for the nanofiber specimens,and the experimental design.In the results and discus-sion section we present and discuss the essential results of the tensile tests.The conclusions are presentedfinally.EXPERIMENTALPA66pellets were obtained from Nilit,Israel. The pellets were dissolved in formic acid to yield a20wt%solution.All reagents were used with-out further purification.The polymer solution was electrospun from a5mL syringe with a hypo-dermic needle(inner diameter of0.1mm)and with aflow rate of5mL/h(a Harvard PHD 2000syringe pump was used).A copper elec-trode was placed in the polymer solution and the latter was electrospun onto the sharp edge of a grounded collector disk19(cf.the work of Theron et al.for additional details),where the aligned solidified nanofibers were collected.The strength of the electrostaticfield was1.7kV/cm with a potential of30kV.The linear speed of the edge of the disk collector was V¼5,or 20m/s.All the experiments were performed at ambient temperature(about258C)in air with a relative humidity of$40%.A forkedfiber holder made of aluminum foil was attached to the col-lector disk edge.As the electrospun nanofibers reached the disk,they were wound around the edge and aligned on thefiber holder.The nano-fibers were stored in vacuum(10À2Torr)for 24h before the tensile test experiments were conducted.The characterization of the nanofibers was done using a HRSEM(High Resolution SEM, Leo Gemini)operating at an accelerating voltage of3kV and a current of120pA.The nanofibers were placed on a silicon wafer for observation. Wide-angle X-ray(WAXS)and small-angle X-ray scattering(SAXS)were performed using a small-angle diffractometer(Bruker AXS Nanostar,tube: KFF CU2K-90)with Cu K a radiation,pinhole col-limation(that produced a100l m-diameter beam), and a10Â10cm2two-dimensional position-sensi-tive wire detector that was positioned at6.8cm (WAXS)and65.05cm(SAXS)behind the exam-ined sample.The X-ray beam was directed par-allel to the normal of thefiber bundle,which was detached from the edge of the collector disk. Therefore,the beam was perpendicular to the winding direction of the nanofibers.To perform the tensile tests,an individual electrospun nanofiber was removed from the forkedfiber holder.One end was mounted to the tip of an AFM cantilever(that served as a force-sensing element)and the other end to the etched tip of a stainless steel wire(that served as a pulling element20,21[cf.Refs.20and21for additional details on the measurement technique]).TENSILE PROPERTIES OF PA66ELECTROSPUN NANOFIBERS1483Journal of Polymer Science:Part B:Polymer PhysicsDOI10.1002/polbThe nanofiber ends were secured at these posi-tions with epoxy glue (Attex no.27,purchasedfrom Loctite).The AFM cantilevers were pur-chased from Mikromasch (series NSC38,NSC12,and NSC36).The cantilevers were considered as the Euler–Bernoulli beams of a rectangular cross-section with b and h being the width and thickness,and the length L being more than 5Áb .Since in the present case the cantilever deflection angles were always smaller than 108,the general solution for a clamped cantilever transversally loaded at the free end (tip)22reduces toP ¼K e dð1Þwhere P is the applied force acting perpendicu-lar to the cantilever at the tip,d is the resulting tip displacement and the elastic constant K e ¼3EI/L 3.In this equation,I is the cross-sectional moment of inertia and E is Young’s modulus.The first fundamental eigenfrequency of the cantilever is given by Timoshenko 23x ¼1:8752½EI =q S 1=2=L 2ð2Þwhere q is the cantilever density (q ¼2.3g/cm 3in the present case)and S ¼b Áh the cross-sec-tional ing,eq 2the expression for K e becomesK e ¼0:2427q bhL x 2ð3Þwhere 3/1.8754¼0.2427.This expression was used by Sader et al.in his work 24and in the present work.Each cantilever’s first eigenfrequency,x ,was found using a vibrometer (Polytec OFV 3001)in vacuum,and then K e was calculated as per eq 3.The results are presented in Table 1.Note that the value of x could also have been calculated using the expression for the moment of inertia I ¼bh 3/12and a value of E .However,as E is not known with a high degree of accuracy,better results can be achieved using direct measure-ment of x as was done in the present work.For the tensile test,the cantilever end was loaded normally to its neutral axis at the tip via the attached nanofiber pulled by a wire at a lin-ear velocity of 4l m/s.The other end of the can-tilever was clamped.The deflection,d ,of the AFM cantilever was observed using an Olympus BX51microscope with 500Âand 1000Âmagnifi-cations (long distance focal length objective),T a b l e 1.E x p e r i m e n t a l R e s u l t s f o r t h e E l a s t i c C o n s t a n t (K e )O b t a i n e d f o r t h e A F M C a n t i l e v e r sK e (N /m )M o d e l :N S C 38/A I /15K e (N /m )M o d e l :N S C 12/A I /15K e (N /m )M o d e l :N S C 36/A I /15b Âh ÂL (l m )35Â1Â300b Âh ÂL (l m )35Â1Â350b Âh ÂL (l m )35Â1Â250b Âh ÂL (l m )35Â2Â300b Âh ÂL (l m )35Â2Â350b Âh ÂL (l m )35Â2Â250b Âh ÂL (l m )35Â1Â110b Âh ÂL (l m )35Â1Â90b Âh ÂL (l m )35Â1Â1300.0720.0510.1250.4510.2840.7960.6521.1770.4140.0550.040.1710.3690.3270.6210.7101.2320.4680.0990.0670.1040.5210.2260.9170.5931.0900.5131484ZUSSMAN ET AL.Journal of Polymer Science:Part B:Polymer PhysicsDOI 10.1002/polbequipped with a CCD camera (Olympus DP12).A series of images taken during a tensile test is shown in Figure 1.The gauge length of the fiber,L f ,was defined as its length between the apex of the AFM tip and the tip of the stainless steel wire.The length change,d L f ,as a function of the applied load was measured,and the strain was obtained from dL f =L f 0,where L f 0is the initial length of the nanofiber.This resultedin a nominal strain rate of _e %8Â10À2s À1.RESULTS AND DISCUSSIONFigure 2shows a SEM micrograph of the elec-trospun PA66nanofibers.The individual fibersare observed to have uniform cross sections with an average diameter of 550nm (with the stand-ard deviation,SD ¼120nm)for the take-upwheel velocity of 20m/s,and a larger diameter of 570nm (SD ¼40nm)for the take-up velocity of 5m/s.The X-ray diffraction patterns (WAXS)of bundles of aligned electrospun nanofibers pro-duced at 5and 20m/s are shown in Figure 3(a,b)respectively.The nanofibers exhibited four reflections corresponding to,001,002,and 100spacing and a [010]/[110]doublet for the a -crys-talline form of PA66(triclinic structure,a ¼4.9A˚,b ¼5.4A ˚,and c ¼17.2A ˚,a ¼48.58,b ¼778,and c ¼63.58).25,26The diffracted intensity of the nanofibers collected at 20m/s [Fig.3(b)]manifests itself in the form of equatorial arcs indicating a preferred axial orientation of nylon crystallites inside the fiber,wherein the normals to the ac and bc planes are perpendicular to the fiber axis.In contrast,crystallites in the nano-fibers electrospun at the lower take-up velocity of 5m/s manifest themselves by rather uniform diffraction rings [Fig.3(a)].Consider once more Figure 3(b),which corresponds to the higher take-up velocity of 20m/s.The equatorial dif-fraction arcs are not fully separated,but are instead merged into a single arc that resembles that of PA66,which has been rapidly quenched from melt.26It is useful to compare these find-ings to those of Hsiao et al.27in which the dif-fraction patterns of highly oriented PA66were found to be much sharper when examining the melt-drawn fibers produced at room tempera-ture with draw ratios of 1.01–1.37at a drawing speed of 0.05m/s.The azimuthal scans of the individual diffraction rings in Figure 3(a,b)for the [010]/[110]doublet are shown in Figure 3(c,d).A quantitative orientation factor,f ¼90Àu 090(u 0is Figure 1.A series of images taken during a tensile test up to break of an electro-spun PA66nanofiber.The fiber was attached to the tip of an AFM cantilever and pulled transversally by a stainless steel wire.The deflection of the cantilever,d ,is proportional to the applied force and was measured throughout the tensiletest.Figure 2.SEM micrograph of the electrospun PA66nanofibers.TENSILE PROPERTIES OF PA66ELECTROSPUN NANOFIBERS 1485Journal of Polymer Science:Part B:Polymer Physics DOI 10.1002/polbthe half-width of a reflected peak at half its height),of polymer crystallites can be calculated from the azimuthal scans.This yields the orienta-tion factor of f ¼0.8for V ¼20m/s and f ¼0.58for V ¼5m/s.A two-dimensional SAXS image of the electro-spun nanofibers is shown in Figure 4.The image does not reveal any lamellar superstruc-ture (a structure on a scale larger than the unit cell with a repeating long period).The elliptical shape of the diffuse small-angle scattering indi-cates elongated nanostructures,possibly nanofi-brils or voids between them.Melt-drawn PA66fibers exhibit a long period of 100A˚.26There-fore,it seems that the extremely rapid struc-tural formation and the high draw ratio have no significant effect on crystallite structure forma-tion within the nanofibers during the electro-spinning process.Figure 5shows a typical stress–strain curve (until failure)of the individual electrospun nanofibers collected at 20m/s,and at 5m/s,along with a commercial microfiber (Nilit,P55;diameter 38l m)that was prepared by melt ex-trusion followed by cold drawing.The average values of Young’s modulus,maximal stress,andFigure 3.Two-dimensional WAXS patterns of the electrospun PA66nanofibers.(a)The nanofibers collected at the take-up wheel velocity of 5m/s,(b)at 20m/s;(c)the azimuthal scan of the [010]/[110]diffraction ring from part (a),and (d)the azimuthal scan of the [010]/[110]diffraction ring from part (b).1486ZUSSMAN ET AL.Journal of Polymer Science:Part B:Polymer PhysicsDOI 10.1002/polbelongation-at-break of the electrospun nanofib-ers are presented in Table 2.The results repre-sent 10experiments at a take-up wheel velocity of 5m/s,and 22experiments at 20m/s.At the maximal tensile stress,the nanofibers were bro-ken far from the clamped ends;hence,the stress concentration at the nanofiber ends was ex-cluded.The measured Young’s moduli and maxi-mal stresses of the electrospun nanofibers have broad distributions even though the fibers were processed under similar conditions.The distri-butions may probably be partially attributed to the experimental inaccuracies;however,the main contribution is probably due to an increase in the nanostructures’heterogeneities.The na-nofibers collected at 5m/s revealed an initial plateau at $20MPa (cf.Fig.5),which is consist-ent with alignment of the macromolecules in the amorphous regions during the initial stage of stretching.Above 20MPa,the load is also sup-ported by the crystalline phase and the stress increases significantly.Similarly shaped stress–strain curves were observed in most of the ex-periments when nanofibers were collected at 5m/s.However,the nanofibers collected at 20m/s did not reveal an initial plateau,apparently due to the prestretching by the wheel duringcollection.Figure 4.SAXS pattern recorded from a bundle of the electrospun PA66nanofibers.Figure 5.Typical stress–strain curve of the individual PA66nanofibers collected at take-up velocities of 5and 20m/s,and a commercial microfiber that was prepared by melt extrusion processing followed by cold drawing.TENSILE PROPERTIES OF PA66ELECTROSPUN NANOFIBERS 1487Journal of Polymer Science:Part B:Polymer Physics DOI 10.1002/polbWhen observing nanofibers using SEM after thetensile tests,local failures were not observedaside from a ductile fracture(Fig.6).The aver-age elongation-at-break was found to be e f¼61 and66%for the nanofibers collected at20and5m/s,respectively,and is reflective of the highductility of thesefibers.For comparison,we analyzed microfibersmade from PA66(which is identical to the mate-rial that was used in the electrospinning proc-ess)with a diameter of38l m(SD¼0.5l m),prepared by melt extrusion followed by colddrawing.The average maximum tensile stressof thesefibers was r max¼590MPa(SD¼55 MPa),with the average elongation at break of $50%and the average Young’s modulus E¼1.2 GPa(SD¼0.13GPa);compare Figure5.It is emphasized that the mechanical properties of the electrospun nanofibers in the present work were measured right after the electrospinning process,without any postprocessing.Hence,thecomparison of the mechanical properties of thepostprocessed microfibers with those of the as-spun nanofibers is rather problematic.From autilitarian perspective the as-spun polyamide-6,6nanofibers can be employed for various ap-plications in tissue engineering,where the nano-meter-range size is of advantage,whereas themechanical properties are comparable to thoseof live tissues.Note that the experiments were carried out atroom temperature(258C),which is below theglass transition temperature,T g¼448C,of extruded PA66samples.28However,when work-ing with nanoscale materials29andfibers withlow crystallinity,30the glass transition tempera-ture may be lower than normal.In which caseour measurements could be characterized asbeing conducted close to the glass transitiontemperature.This might have resulted in ther-mally activated relaxation processes,whichwould have increased thermal mobility of thechains and consequently enhanced nanofiberductility.In addition,flow-induced crystalliza-tion of the stretched chains may have occurred,which is equivalent to an effective reduction of T g.It is emphasized that these explanations are some-what speculative,albeit they may explain the rela-tively high maximal stress of the nanofibers. CONCLUSIONSThe present study characterizes the intrinsicstructure and the overall stress–strain behaviorof PA66electrospun nanofibers.The intrinsicstructure of nanofibers with diameters down to100nm displays an a-crystalline phase(with tri-clinic cell structure).Similar structures in mi-crofibers obtained by melt extrusion have beenreported in the literature.Various parameters ofthe electrospinning process such as the rate ofstructural formation(which is on a millisecondscale)and the high strain rate(which is of theTable2.Young’s Modulus,Maximal Stress,and Elongation at Break of the Electrospun NanofibersFiberTake-up Velocity(m/s)AverageDiameter(nm)(Standard Deviation)Average Young’sModulus(E;MPa)(Standard Deviation)Average MaximalStress(r max;MPa)(Standard Deviation)Average Elongationat Break(e f;%)(Standard Deviation)5570(40)453(150)110(14)66(15)20550(120)950(390)150(49)61(19)Figure 6.Fracture surface of a PA66electrospunnanofiber.1488ZUSSMAN ET AL.Journal of Polymer Science:Part B:Polymer PhysicsDOI10.1002/polborder of1000sÀ1)are observed to affect signifi-cantly the crystallite orientation and the crystal-linity level of the electrospun nanofibers.How-ever,no lamellar superstructure was detected during SAXS analysis.The scatter of the me-chanical properties of the nanofibers was rather significant,as evidenced by the rather high standard deviations in the measured values of Young’s modulus and the maximal stress.These findings may be attributed to an increase in lon-gitudinal nonuniformity of the nanofibers,which is a typical result of the electrospinning process. Furthermore,the high ductility of the nanofib-ers was probably sufficient to induce a signifi-cant degree of the orientation of the crystallites along thefiber axis,resulting in high maximal stress values when performing tensile tests. This explanation is validated by the observation that increasing the take-up velocity of the sharp wheel resulted in a higher orientation uniform-ity of the electrospun nanofibers.The authors acknowledge the support of this research by the Israel Science Foundation—The Israel Acad-emy of Sciences,grant number26/03. 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