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Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer Melt-electrospinning of poly(ether ether ketone) bers to avoid sulfonation Nelaka Dilshan Govinna a , Thomas Keller a , Christoph Schick b,c , Peggy Cebe a,a Department of Physics and Astronomy, Center for Nanoscopic Physics, 574 Boston Ave., Tufts University, Medford, MA, 02155, USA b University of Rostock, Institute of Physics and Competence Centre CALOR°, Albert-Einstein-Str. 23-24, 18059, Rostock, Germany c Kazan Federal University, Institute of Chemistry, 18 Kremlyovskaya Street, Kazan, 420008, Russian Federation HIGHLIGHTS We successfully electrospun poly (ether ether ketone), PEEK, bers from the melt. As spun bers are chemically the same as native PEEK, and are non-crystalline. Fast scanning calorimetry was used to cool bers over a wide range of rates. PEEK bers shrink above T g but maintain some orientational birefringence. Dynamic fragility of bers is greater than that of melt-cooled PEEK. ARTICLE INFO Keywords: PEEK bers Melt-electrospinning Dynamic fragility ABSTRACT We have successfully electrospun un-sulfonated bers of poly(ether ether ketone), PEEK, from the molten state at 350 °C. Unlike solution electrospinning of PEEK, which produces sulfonated bers with reduced thermal stabi- lity, melt electrospinning produces chemically unaltered PEEK bers. These bers are smooth, defect-free, and round in cross-section. Most ber diameters range from 1.5 μm to 8.5 μm. A large interstitial ber with diameter reaching up to 100 μm is occasionally deposited at the end of the spin. As-spun oriented amorphous bers, having diameters less than 10 μm, were selected for fast scanning calorimetric studies of the glass transition and melting behavior. Dynamic fragilities of melt electrospun oriented amorphous PEEK bers and amorphous PEEK quenched from the molten state, were evaluated according to Moynihan's method of cooling at variable rates then reheating at a xed rate and were found to be 200 ± 5 and 150 ± 5, respectively. These results suggest that orientation in the amorphous state of electrospun bers plays a role in the dynamics of glass formation. 1. Introduction Poly(ether ether ketone) (PEEK) is a high-performance semi-crys- talline thermoplastic polymer that falls into the engineering polymer family of Poly(aryl ether ketone)s or PAEKs. Relative to other polymers, PEEK is highly resistant to most common solvents. PEEK has a high tensile strength of about 105 MPa, and high melting and degradation temperatures of 343 °C and 595 °C, respectively [1]. These properties make PEEK a desirable engineering material, but also make it dicult to process. Despite the challenges, the unique characteristics of PEEK, specically its thermal and chemical resilience, have garnered interest in developing its potential in a wide variety of applications. One area of interest is using PEEK as a candidate for ltration membranes as se- parators in fuel cell applications [24], or in gas separation [57]. PEEK membranes have also received attention in biomedical applications due to their mechanical properties and bio-inertness [810]. However, all this work is centered around PEEK-derivatives. These include, e.g., PEEK with β-tricalcium phosphate (β-TCP) [10]; sulfonated-PEEK (SPEEK) [11,12]; sulfonated PEEK with sodium as the counter ion (Na- SPEEK) [13]; zeolite 4 A incorporated SPEEK [6]; or, PEEK on highly cross-linked polyethylene (HXLPE) [8]. Due to its excellent chemical resistance, PEEK can only be dissolved at room temperature in highly concentrated sulfuric acid [14]. But in doing so, PEEK gains an SO 3 H + side chain. This modied version of PEEK is called sulfonated-PEEK, or SPEEK. The addition of this mole- cule to the side chain deteriorates some of PEEK's desirable character- istics, including lowering the thermal degradation temperature, at which signicant mass loss occurs, to values ranging between 345 °C [13] and 530 °C, depending on the counter-ion present. SPEEK does, however, show many characteristics that make it desirable in fuel cell membrane applications, including good proton conductivity [2,3,15]. Alternate research has been done to form a porous PEEK membrane https://doi.org/10.1016/j.polymer.2019.03.041 Received 1 February 2019; Received in revised form 14 March 2019; Accepted 17 March 2019 Corresponding author. E-mail address: [email protected] (P. Cebe). Polymer 171 (2019) 50–57 Available online 20 March 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved. T
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Transcript of Melt-electrospinning of poly(ether ether ketone) fibers to ...2.1.2. Melt electrospinning Melt...

  • Contents lists available at ScienceDirect

    Polymer

    journal homepage: www.elsevier.com/locate/polymer

    Melt-electrospinning of poly(ether ether ketone) fibers to avoid sulfonation

    Nelaka Dilshan Govinnaa, Thomas Kellera, Christoph Schickb,c, Peggy Cebea,∗

    a Department of Physics and Astronomy, Center for Nanoscopic Physics, 574 Boston Ave., Tufts University, Medford, MA, 02155, USAbUniversity of Rostock, Institute of Physics and Competence Centre CALOR°, Albert-Einstein-Str. 23-24, 18059, Rostock, Germanyc Kazan Federal University, Institute of Chemistry, 18 Kremlyovskaya Street, Kazan, 420008, Russian Federation

    H I G H L I G H T S

    • We successfully electrospun poly (ether ether ketone), PEEK, fibers from the melt.• As spun fibers are chemically the same as native PEEK, and are non-crystalline.• Fast scanning calorimetry was used to cool fibers over a wide range of rates.• PEEK fibers shrink above Tg but maintain some orientational birefringence.• Dynamic fragility of fibers is greater than that of melt-cooled PEEK.

    A R T I C L E I N F O

    Keywords:PEEK fibersMelt-electrospinningDynamic fragility

    A B S T R A C T

    We have successfully electrospun un-sulfonated fibers of poly(ether ether ketone), PEEK, from the molten state at350 °C. Unlike solution electrospinning of PEEK, which produces sulfonated fibers with reduced thermal stabi-lity, melt electrospinning produces chemically unaltered PEEK fibers. These fibers are smooth, defect-free, andround in cross-section. Most fiber diameters range from 1.5 μm to 8.5 μm. A large interstitial fiber with diameterreaching up to 100 μm is occasionally deposited at the end of the spin. As-spun oriented amorphous fibers,having diameters less than 10 μm, were selected for fast scanning calorimetric studies of the glass transition andmelting behavior. Dynamic fragilities of melt electrospun oriented amorphous PEEK fibers and amorphous PEEKquenched from the molten state, were evaluated according to Moynihan's method of cooling at variable ratesthen reheating at a fixed rate and were found to be 200 ± 5 and 150 ± 5, respectively. These results suggestthat orientation in the amorphous state of electrospun fibers plays a role in the dynamics of glass formation.

    1. Introduction

    Poly(ether ether ketone) (PEEK) is a high-performance semi-crys-talline thermoplastic polymer that falls into the engineering polymerfamily of Poly(aryl ether ketone)s or PAEKs. Relative to other polymers,PEEK is highly resistant to most common solvents. PEEK has a hightensile strength of about 105MPa, and high melting and degradationtemperatures of 343 °C and 595 °C, respectively [1]. These propertiesmake PEEK a desirable engineering material, but also make it difficultto process. Despite the challenges, the unique characteristics of PEEK,specifically its thermal and chemical resilience, have garnered interestin developing its potential in a wide variety of applications. One area ofinterest is using PEEK as a candidate for filtration membranes as se-parators in fuel cell applications [2–4], or in gas separation [5–7]. PEEKmembranes have also received attention in biomedical applications dueto their mechanical properties and bio-inertness [8–10]. However, all

    this work is centered around PEEK-derivatives. These include, e.g.,PEEK with β-tricalcium phosphate (β-TCP) [10]; sulfonated-PEEK(SPEEK) [11,12]; sulfonated PEEK with sodium as the counter ion (Na-SPEEK) [13]; zeolite 4 A incorporated SPEEK [6]; or, PEEK on highlycross-linked polyethylene (HXLPE) [8].

    Due to its excellent chemical resistance, PEEK can only be dissolvedat room temperature in highly concentrated sulfuric acid [14]. But indoing so, PEEK gains an SO3−H+ side chain. This modified version ofPEEK is called sulfonated-PEEK, or SPEEK. The addition of this mole-cule to the side chain deteriorates some of PEEK's desirable character-istics, including lowering the thermal degradation temperature, atwhich significant mass loss occurs, to values ranging between 345 °C[13] and 530 °C, depending on the counter-ion present. SPEEK does,however, show many characteristics that make it desirable in fuel cellmembrane applications, including good proton conductivity [2,3,15].

    Alternate research has been done to form a porous PEEK membrane

    https://doi.org/10.1016/j.polymer.2019.03.041Received 1 February 2019; Received in revised form 14 March 2019; Accepted 17 March 2019

    ∗ Corresponding author.E-mail address: [email protected] (P. Cebe).

    Polymer 171 (2019) 50–57

    Available online 20 March 20190032-3861/ © 2019 Elsevier Ltd. All rights reserved.

    T

    http://www.sciencedirect.com/science/journal/00323861https://www.elsevier.com/locate/polymerhttps://doi.org/10.1016/j.polymer.2019.03.041https://doi.org/10.1016/j.polymer.2019.03.041mailto:[email protected]://doi.org/10.1016/j.polymer.2019.03.041http://crossmark.crossref.org/dialog/?doi=10.1016/j.polymer.2019.03.041&domain=pdf

  • using Thermally Induced Phase Separation (TIPS). In this method, PEEKis combined with a diluent at a temperature where both are in a liquidstate. Upon cooling, phase separation occurs, and the diluent can beselectively removed using a solvent, leaving behind porous vacancies inthe PEEK structure [14,16]. Da Silva Burgal et al. [17] reported theproduction of PEEK nanofiltration membranes with very low sulfona-tion, approaching the level of native PEEK, where sulfonation is ameasure of the amount of sulfur groups added to the PEEK molecularchain. They report that their PEEK solutions had less than 3% sulfo-nation, but the final membranes they produced all had at least twice thedegree of sulfonation of the original solutions. Their work also in-dicated that increasing the degree of sulfonation decreased the che-mical resistance of SPEEK to several solvents.

    These solvent-based techniques all focus on forming membranesfrom porous films rather than by forming discrete fibers. To date, effortsat forming discrete fibers have been done with SPEEK. SPEEK fiberswith diameters around 150–200 nm were produced with ∼70% sulfo-nation by Boaretti et al. [18], and fibers with diameters of about100 nm have been manufactured and characterized by Sadrjahani et al.[19], where their fibers had a degree of sulfonation greater than 60%.

    The glass transition temperature (Tg), is the gradual and reversibletransition in amorphous materials at which the material undergoes atransition from the viscous rubbery state into the solid “glassy” state, astemperature is decreased. Since this is a material property that dependson the experimental time scale, studies that incorporate a variety ofheating and cooling rates allow us to investigate the variation of en-thalpy relaxation at Tg. In these cases, it is more convenient to use thefictive temperature, Tf. The fictive temperature is defined as the tem-perature at which the extrapolated enthalpy lines of the glass and of theliquid intersect [20–23]. The variation of Tf with cooling rate followsthe classic Williams–Landel–Ferry (WLF) [24] or Vogel–Ful-cher–Tammann (VFT) [25–27] relationships. Using these relationships,we can evaluate the dynamic fragility index (m) [20], which is ameasure of how rapidly the dynamics of a material slow down as it iscooled toward the glass transition.

    Polymers with large values of m (the usual case) are considered tobe very fragile [28–30], resulting mainly from chain rigidity [28] orother structural effects such as tacticity [31]. One exception is poly(oxyethylene) (POE), which exhibits a very low fragility index, m=23[32]. A few reports have presented estimates of the dynamic fragility ofPEEK [33–35]. Sanz et al. [33], present findings of dielectric relaxationand isothermal crystallization studies where the dynamic fragility indexof PEEK was determined to be 384, and they state that PEEK is veryfragile compared to other common polymers such as poly(ethyleneterephthalate) (PET, m=164), poly(dimethyl phenyl siloxane) (PDMS,m=175), and poly(L-lactic acid) (PLLA, m=127). Al Lafi [34] givesthe dynamic fragility index of PEEK to be 155 (from Dielectric Re-laxation Spectroscopy) and 280 (from Differential Scanning Calori-metry) demonstrating that the dynamic fragility can vary greatly, de-pending on the determination method used. In a comparative work[35], Goodwin et al., present findings of a fragility study on fluorinatedpolyethers and poly(ether ketone)s via dynamic mechanical thermalanalysis (DMTA) where they substitute some of the hydrogen moleculesin the polymer with fluorine molecules. Depending on the modification,they show that the fragility was in the range of 60–90. In this study, wepresent a direct comparison of the fragility of melt electrospun PEEKfibers (referred to as PEEK-F) and PEEK cooled from the melt afterheating up to 350 °C (referred to as PEEK-M) measured via fast scanningcalorimetry (FSC) [36–39]. The wide range of scanning rates madepossible by FSC has been employed to evaluate fragility of materials inprevious works such as Dotel et al. [40] (for poly(ethylene ter-ephthalate), PET), Tao et al. [41] (for ionic liquids) and Xavier et al.[42] (for poly(lactic acid), PLA). FSC has also been used to study thecrystalline nature of PEEK [43,44]. In the present work, our focus is onoriented amorphous PEEK fibers and quenched amorphous PEEK.

    To the best of our knowledge, our work is the first demonstration

    that PEEK polymer can be melt electrospun into fibers while main-taining its native chemical structure, i.e., without increasing the degreeof sulfonation of the molecular chain. We report on the electrospinningmethod and conditions used to create these fibers, and their char-acterization by microscopy, infrared analysis (FTIR), and thermalanalysis including thermogravimetry (TG), differential scanning ca-lorimetry (DSC), and fast scanning chip-based calorimetry (FSC). TGand FTIR analyses confirm that the melt electrospun fibers are chemi-cally un-altered from the as-received pellets used to fabricate them.Melt electrospun PEEK fibers and PEEK quenched from the molten stateexhibit amorphous states which differ from each other, and these statesdemonstrate very different fragility indices of 200 ± 5, and 150 ± 5,respectively.

    2. Experimental section

    2.1. Materials

    PEEK pellets with a range of molecular weights are commerciallyavailable, and these have melt viscosities ranging from 90 Pa s to475 Pa s at 400 °C. As shown in Supplementary Information, TableS1, they generally share the same properties, such as solvent resistance,tensile strength, and thermal resistance, which are largely unchangedfor different molecular weights [45–47]. Other properties such as ten-sile elongation at break, impact strength, and viscosity vary with mo-lecular weight.

    Previous studies of other polymers that have been successfully meltelectrospun give a reasonable range of target melt viscosities, as shownin Supplementary Information, Table S2. The lowest molecularweight PEEK, VICTREX™ 150G, was chosen for this study as since itsmelt viscosity (130 Pa s) lies within the range of viscosities wheresuccessful melt-electrospinning experiments have been performed onother polymers [48–51].

    2.1.1. Creation of SPEEKFor comparison with native PEEK, sulfonated PEEK (SPEEK) was

    made by dissolving PEEK pellets in 95–98% concentrated sulfuric acid(EM Science, Gibbstown, New Jersey). The solution was then spread ona glass plate until it was dry. Prior to testing, SPEEK films were washedthoroughly with DI water and dried once more to remove residualsulfuric acid.

    2.1.2. Melt electrospinningMelt electrospinning of PEEK was performed by heating the polymer

    into its molten state at 350–375 °C. An electrically grounded 20-gaugestainless steel wire was used as the “spinneret” and a high voltagecollector plate placed 3.6 cm away was maintained at a temperatureclose to room temperature. The electric field strength, E (whereE= voltage/distance) was maintained at 2–5 x 105 V/m to preventelectrical breakdown through the air and different electrical fieldstrengths were tested to assess the quality of fibers. A drop of moltenPEEK was created at the tip of the spinneret wire and melt-electro-spinning was performed. Detailed information on the melt electro-spinning procedure, and a diagram of the melt-electrospinning setup,can be found in the Supplementary Information, Section 2.

    2.2. Characterization

    2.2.1. MorphologyThe morphology of melt-electrospun fibers was studied using a Zeiss

    EVO MA10 scanning electron microscope (SEM) (Carl Zeiss,Oberkochen, Germany), operating at 5 kV. Samples were first coatedwith Au-Pd alloy for 90 s using a Cressington Sputter Coater 108(Cressington Scientific Instruments, Watford, UK). Software packageImageJ was used to analyze SEM images and to obtain statistics on fibersize.

    N.D. Govinna, et al. Polymer 171 (2019) 50–57

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  • 2.2.2. Infrared spectroscopyAbsorbance spectra of the samples were examined using attenuated

    total reflectance Fourier transform infrared (FTIR) spectroscopy on aJasco FTIR-6200 Spectrometer (Jasco Instruments, Tokyo, Japan).Spectra were obtained from 400 to 4000 cm−1 at 4 cm−1 resolutionwith 256 scans co-added. Air background was subtracted from allsample spectra.

    2.2.3. ThermogravimetryThermogravimetry (TG) was performed on a TA Instruments, Inc.

    (New Castle, Delaware, USA) Q500 series thermogravimetric analyzerfrom 25 °C to 1000 °C at 20 °C/min under 50ml/min N2 gas flow usingpolymer fiber samples of mass 5–15mg.

    2.2.4. Fast scanning calorimetryA Mettler Flash DSC1 (Mettler Toledo, Greifensee, Switzerland) was

    used to perform fast scanning calorimetric (FSC) experiments to mea-sure heat flow rate vs. temperature. The calorimeter was operatedunder nitrogen gas flow of 50ml/min. The empty sensor was condi-tioned using the manufacturer's procedure five times, repeatedlyheating and cooling to 450 °C at the same rate used for the samples. Theceramic sensor base temperature was set at −100 °C. Prior to experi-ments, Mettler Toledo UFSC1 sensors were cooled and reheated be-tween −100 °C and 470 °C at the rates used in the experiments, toobtain the empty sensor baseline signal which was subtracted from alldata scans. To ensure high quality and accurate data, a symmetrycorrection procedure was also carried out on all curves followingmethods developed previously [52,53].

    To test the crystalline nature of PEEK fibers, they were heated to400 °C at 2000 K/s, well past crystal melting temperature of 310 °C.Using different fiber samples from the same spin, experiments char-acterizing the glass transition were performed by first heating PEEK-Fas-spun fiber samples to 200 °C, just above the glass transition tem-perature (Tg), and then immediately cooling to −50 °C, followed by re-heating to 200 °C. The cool-heat ramp pair (i.e., first cool and secondheat) was repeated by using various cooling rates in the range from50 K/s to 8000 K/s, followed by heating at a fixed rate of 2000 K/s.Next, the fiber samples (PEEK-F) were then heated to 350 °C to removethe fibrous shape at which point the initial fibers have become PEEK-M.The cooling and reheating experiment is repeated on PEEK-M, and nowthe samples are heated to 350 °C during each heating cycle and thencooled at various rates. Each scan was carefully examined to determinewhether any crystallization had occurred during cooling. If any coolingscan exhibited crystallization of the material, the subsequent heatingramps were excluded from further analysis. No crystallization was ob-served for PEEK-F when cooled from 200 °C, at cooling rates of 3000,2000, 1000, 800, 600, 450, 300 and 200 K/s. No crystallization wasobserved for PEEK-M when cooled from 350 °C, at cooling rates of8000, 7000, 6000, 5000, 4000, 3000, 2000, 1000, 800, 600, 450, 300,200 and 140 K/s. Optical Images were taken of samples on the sensor,before and after heating, using an Olympus BX41 microscope (Tokyo,Japan) equipped with a ScopeTek DCM 510 camera (Hangzhou, China).

    Polarized hot stage microscopy was performed for as spun fibersPEEK-F and for PEEK-M using a Nikon Eclipse E600 POL (Minato,Tokyo, Japan) microscope equipped with a SPOT Insight 11.2 ColorMosaic camera (SPOT Imaging, Sterling Heights, Michigan, USA). AMettler Toledo FP82HT Hot stage (Columbus, Ohio, USA) was used toheat the samples at 20 K/min while observing either under bright fieldconditions, or through crossed polarizers (A⊥P). All samples were im-aged at high temperatures and after cooling to room temperature.

    3. Results and discussion

    Representative SEM images of melt electrospun PEEK fibers, spun atseveral different electric field strengths, are shown in Fig. 1. As shownby the SEM images in Fig. 1a–e, the obtained fibers were smooth and

    defect-free. Collections of fibers often showed a circular or complexlooping fiber orientation, shown in Fig. 1a, where the fibers tended tocoil up in a large spiral. This effect, known as “Liquid Rope Coiling”[54], is common in viscous fluids in the presence of a parallel forcevector field, such as gravity or the electrostatic force between thesource and the collector, when they suddenly encounter a rigid surface.The fluid assumes a direction of rotation and forms a corkscrew shapeof nearly uniform radius as the fluid rope piles up on top of itself. Nofibers were obtained with applied electric field strengths belowE=2.08×105 V/m, where the electric force was insufficient to drawmolten material toward the collector. At E=2.08×105 V/m, theaverage diameter of obtained fibers was about 4 μm. At higher electricfield strengths, fibers with smaller diameters could be seen among re-latively larger fibers. The amount of these smaller fibers increased asthe applied electric field strength increased. Since these smaller fiberswere numerous, at high electric fields, these dominated the fiber dia-meter distribution. Therefore, above E= 4.17× 105 V/m (i.e., at 15 kVand 3.6 cm), the average fiber diameter was less than 1 μm.

    In some cases, considerably larger fibers, with diameters> 50 μm,were also present among the smaller size fibers. This was more evidentin spins with larger applied electric fields, where a fiber larger than therest can be seen in the middle as shown in Fig. 1a and e. These largerfiber(s) were created towards the end time of the spin. When the PEEKdroplet is close to be fully consumed, the contact it has with the spin-neret is weakened due to the small amount of material left, and theelectrical force is able to pull most of the remaining molten PEEK ma-terial towards the collector creating a larger fiber which in some casescould reach 100 μm in diameter. Any residual molten PEEK can alsocontinue to create smaller diameter fibers which land on top of thelarger fiber, as shown in Fig. 1a. Existence of these large diameter fiberscould be avoided by stopping the spin prior to consumption of thesource PEEK at the spinneret. The best melt electrospinning parametersto acquire a dense fiber mat were found to be E=2.78×105 V/m (for10 kV applied voltage and 3.6 cm working distance between spinneretand collector).

    Thermogravimetric (TG) experiments confirmed that the thermalstability of the melt electrospun PEEK fibers was nearly identical to thatof native PEEK as-received pellets as shown by the blue and red curvesin Fig. 2a. Major mass loss events were seen around 550 °C corre-sponding well to literature values for native PEEK [1,12,55]. On theother hand, degradation onset of SPEEK (black) is around 350 °C, whichis also consistent with other experimental observations for highly sul-fonated PEEK [13,56].

    Using fast scanning calorimetry, the as-spun fibers were shown to benon-crystalline (Fig. 2b) from the heat flow rate traces for two differentfiber samples. The glass transition temperature (Tg) occurs around150 °C for all samples tested and an enthalpy recovery peak (en-dothermal peak at 165 °C) was also present for the first heating scans ofthe fibers. Small exothermic peaks were also seen very close to, andoverlapping with, Tg. These were a result of stress relaxation andshrinkage of fibers when they underwent the glass transition relaxationprocess for the first time. Fiber shrinkage was confirmed through op-tical imaging, as shown in Fig. 3a–c. The sample contains six cut piecesof melt electrospun PEEK fibers and they were imaged: 1. Before anytreatment (Fig. 3a); 2. after heating above Tg up to 200 °C and coolingdown (Fig. 3b); and, 3. after heating to 350 °C and cooling down(Fig. 3c) which resulted in loss of most of the fibrous shape. Fibers(Fig. 3a and b), or their once-melted counterparts (Fig. 3c) were alltransparent and amorphous when imaged on the sensors.

    The melt electrospun fibers underwent shrinkage upon heatingabove Tg but fibers were still able partially to maintain their fibrousshape. Molecular chains in as spun fibers are oriented parallel to thefiber axis due to the stresses imparted during the melt electrospinningprocess. Due to this preferred orientation, the refractive index parallelto the fiber axis is no longer equal to the refractive index perpendicularto this direction. This results in birefringence, which is a common

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  • experimental tool for the investigation of orientation effects in poly-mers [60]. As seen in Fig. 3g, these fibers are uniaxially birefringent,indicating molecular chain orientation. Upon heating to 200 °C, atemperature above Tg, entropic forces of elasticity are partially relaxedand the fibers shrink. As shown in Fig. 3h (and Supplementary In-formation, Fig. S2) birefringence remains after heating to 200 °C at20 K/min. Supplementary Information, Fig. S3 also confirms bi-refringence of fibers heated to 200 °C at 2000 K/s, the rate used in FSC.When these fibers were heated to very high temperatures, neither a coldcrystallization exotherm nor a melting endotherm were observed, in-dicating that the as-spun PEEK-F fibers were non-crystalline and did notcrystallize during heating at 2000 K/s. When heated to 350 °C, most of

    the fibrous shape was lost and no birefringence was observed (Fig. 3i),indicating randomization of molecular orientation within the material.

    To confirm that the electrospun fibers are chemically similar tonative PEEK, infrared spectroscopy was performed, and absorptionspectra are shown in Fig. 4. Comparing spectra from the native PEEKpellet (red curve) and the melt electrospun PEEK-F fibers (blue curve),it can be seen that these show all the characteristic infrared vibrationsof PEEK, which are listed in Supplementary Information, Table S3.

    In the infrared spectrum of SPEEK (Fig. 4), several characteristicpeaks can be observed at 1253 cm−1 (asymmetric O=S=O stretch),1078 cm−1 (symmetric O=S=O stretch) and 708 cm−1 (S-O stretch)[61,62]. As reported previously [19,61], with sulfonation, an increase

    Fig. 1. a) A representative SEM image from an obtained melt electrospun PEEK fiber mat showing the overall morphology. The scale bar is 500 μm. b-e) SEM imagesfor PEEK fibers spun at the indicated voltages and working distances. Scale bars are 20 μm. The electric field strengths are: 3.61×105 V/m (a, d); 2.08× 105 V/m(b); 2.78× 105 V/m (c); 4.44× 105 V/m (e). f-i) Histograms showing distribution of fiber diameter for the fiber mats shown in b-e. Fiber diameters were calculatedwith ImageJ using data from at least 150 fibers per spin condition.

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  • of the absorbance at 1472 cm−1 (1,2,4 three substituted C=C skeletalring vibration), and a decrease of absorbance at 1490 cm−1 (1,4 di-substituted C=C skeletal ring vibration) [61,63] can also be observed,resulting in a doublet of peaks. These features combined, generate acharacteristic spectrum of SPEEK. The spectrum of our SPEEK, blackline in Fig. 4, is in good agreement with observations of previous workon SPEEK [19,61].

    Results from experiments with different cooling rates are shown inFig. 5. Fig. 5a and b shows heating scans of PEEK-F and PEEK-M sam-ples, respectively. These experiments allow the dynamic fragility to bedetermined. The endothermic glass transition relaxation is clearly seenand the enthalpy relaxation peak increases in amplitude as the mag-nitude of the prior cooling rate decreases.

    To estimate the dynamic fragility index (m) [20], the fictive tem-perature Tf [20,21,23,64], was first determined using Moynihan'smethod [22]:

    ∫ ∫− = −≫

    Q Q dT Q Q dT( ˙ ˙ ) ( ˙ ˙ )T

    T T

    l gT T

    T T

    g

    f

    g

    g

    g

    (1)

    where Q̇ is the apparent heat flow rate of the sample, and Q̇l and Q̇gdenote the heat flow rates in the liquid and glass states, respectively.Sample mass was determined by comparing the observed heat capacityincrement at Tg with the literature value of 78.1 J/(mol K) [65].

    Fig. 5c shows the fictive temperature, Tf, calculated using Eqn. (1),as a function of logarithm of cooling rate for PEEK-F (filled symbols)and PEEK-M (open symbols). The dashed lines represent fits to thetemperature dependence of the data, commonly described by the Wil-liams-Landel-Ferry (WLF) model [24]:

    ⎝⎜

    ⎠⎟ =

    −+ −

    qq

    C T TC T T

    log( )

    ( )reff f ref

    f f ref10

    1 ,

    2 , (2)

    where q is the cooling rate, qref is a reference cooling rate (here, taken

    Fig. 2. a) TG curves showing weight remaining vs.temperature for as-received PEEK pellet (red), as-spun melt electrospun PEEK fibers, PEEK-F (blue),and SPEEK film (black). b) FSC heat flow rate tracesat 2000 K/s for two separate PEEK fiber sampleshaving different masses. Samples for FSC werechosen to have diameters smaller than 10 μm toprevent temperature gradients in the sample[57–59]. Only the glass transitions with an enthalpyrecovery peak were observed on heating, indicatingthe fibers are non-crystalline. No cold crystallizationor melting endotherm was observed for the PEEKfibers. Curves are shifted vertically for clarity. (Forinterpretation of the references to colour in thisfigure legend, the reader is referred to the Web ver-sion of this article.)

    Fig. 3. Microscopic images showing fiber shrinkage at Tg and birefringence properties of PEEK-F and PEEK-M. a-c) Optical images taken at room temperature, of sixcut pieces of PEEK fibers placed on a MultiSTAR UFS 1 sensor chip of the Mettler Flash DSC1. d-f) Optical bright field images of two fibers in the Mettler hot stage. g-i) Polarized images of the same samples. a, d, g) Fibers as-spun. Fibers are flared at their ends due to cutting. b,e,h) Fibers after being heated above Tg to 200 °C andthen cooled to room temperature. c, f, i) Fibers after being heated to 350 °C and then cooled to room temperature. All scale bars are 100 μm. The polarizer (P) andanalyzer (A) directions relevant for images g-i are shown with arrows.

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  • to be 200 K/s, the lowest common rate used for PEEK-F and PEEK-M.),Tf is the fictive temperature, Tf,ref is the fictive temperature determinedat qref, and C1 and C2 are constants. In this analysis, the dynamic fra-gility index (m) is given by Ref. [41] as:

    = − =md q

    d T TCC

    T(log )

    ( / )f ref ff ref

    10

    ,

    1

    2,

    (3)

    For PEEK-F, Tf,ref = 413.8 K, C1= 132.7, and C2=278.8. ForPEEK-M, Tf,ref = 410.7 K, C1= 108, and C2= 300.4. Dynamic fragilityindices were evaluated using equation (1), and we find that PEEK-M hasa dynamic fragility of m=150 ± 5 while melt-electrospun PEEK fi-bers (PEEK-F) have a dynamic fragility of m=200 ± 5, categorizingthem both as fragile systems [20,30].

    A few reports are found in the literature that explicitly mentiondynamic fragility of PEEK [33–35], but these reports are not in goodagreement. Our results are in the range of fragility values observed byprevious studies that report fragility indices for polymers with stiffbackbones and/or stiff side groups (e.g., polycarbonate, m=132 [32],and poly(ethylene terephthalate), m=156 [66]). It is expected thatpolymers containing rigid aromatic rings would have higher dynamicfragility indices [67]. The multiple aromatic rings present in the PEEKmonomer unit make its backbone quite rigid, resulting in its high glasstransition temperature (145 °C) [65] and high melting temperature (upto 335 °C) [68]. One very apt comparison can be made with poly(etherketone ketone), PEKK, a close chemical relative of PEEK in the poly(arylether ketone) family of semi-crystalline polymers [69]. It also has threearomatic rings in the monomer, but with two carbonyl groups com-pared to one carbonyl group in PEEK. Chemical similarity suggests thetwo polymers may exhibit similar fragility characteristics. Indeed, Ez-querra, et al. [70] using dielectric spectroscopy found the dynamicfragility of PEKK to be m=175, an intermediate value between ourPEEK dynamic fragility of m=200 ± 5 (PEEK-F) and m=150 ± 5(PEEK-M).

    Although both fragility results refer to non-crystalline samples, wefind that PEEK fibers have significantly larger fragility than the samematerial after it has been heated to a temperature above the meltingpoint, at which point most of the fibrous orientation is lost. This sug-gests that PEEK-F and PEEK-M have different non-crystalline states. Wemay understand this result by considering that in PEEK fibers, partialfiber structure remains in the fibers after heating above Tg to 200 °C. Wesuggest this residual fiber structure, seen in Fig. 3b,e,h, has molecularchains that retain some preferential orientation, and have not beencompletely randomized to an isotropic state. This has an impact on thefragility: the non-crystalline fibers constitute a one-dimensional

    partially oriented material having higher dynamic fragility towardsglass formation. This non-crystalline state of PEEK is unlike the stateachieved when the PEEK fiber material is heated to 350 °C. Here, thefibrous shape and the preferred orientation is completely lost, shown bythe lack of birefringence in Fig. 3i. The resulting material behavesdifferently with respect to its glass forming dynamics. Due to the nearlycomplete randomness of the molecular chain orientation of PEEK-M, itsdynamic fragility decreases substantially compared to more orientedPEEK-F.

    4. Conclusions

    We have successfully developed a method for the melt-electrospin-ning of native PEEK. This method is able to produce PEEK fibers havingthe same excellent thermal and chemical resistance, and the same glasstransition temperature, as the as-received native PEEK pellets. Up tonow, most of the effort in the field has been made towards reducing thedegree of sulfonation in SPEEK fibers to minimize the loss of thermaland chemical resistance. Using melt electrospinning, we have elimi-nated the need for aggressive solvents and avoided sulfonation which isdeleterious to PEEK's properties. By scaling production up, this processcan replace the current solution fabrication methods used in manyapplications for PEEK fiber membranes, which currently result in non-native, sulfonated PEEK.

    We then used fast scanning calorimetry, which provides a widerange of cooling rates, to examine the formation of the glassy state inboth PEEK fibers and melted PEEK. By cooling fibers and melted PEEKat different rates to form the glass, and then reheating at a constantrate, the enthalpic relaxation occurring in the vicinity of the glasstransition was analyzed using Moynihan's method. For both PEEK-F andPEEK-M, FSC experiments were conducted over a very similar range ofcooling rates allowing direct comparison of their fragility. The dynamicfragility index, m, was found to be 200 ± 5 for PEEK fibers (PEEK-F)heated to 200 °C and then cooled, compared to 150 ± 5 for meltedPEEK (PEEK-M) heated to 350 °C and then cooled. These fragility valuesagree with literature results of Lafi et al. [34]. While both of thesematerials are non-crystalline, their internal structures are not the same.Birefringence results show that PEEK-F retains some degree of mole-cular chain orientation along the fiber direction, whereas PEEK-M(heated to a much higher temperature) assumes a more random chainconformation. The one-dimensional polymer chain alignment in thePEEK-F fibers results in a statistically significant dynamic fragility valuethat is much larger compared to that of its molten PEEK-M counterpart.

    Fig. 4. Normalized FTIR absorbance vs. wavenumber foras-received PEEK pellets (red), melt electrospun PEEK-F(blue), and sulfonated PEEK, SPEEK (black). Melt elec-trospun oriented amorphous PEEK-F fibers show nearlyidentical features to as-received semi-crystalline PEEKpellets. Curves are shifted vertically for clarity. Arrowsmark the locations of peaks characteristic of SPEEK,which are not observed in as-received (unsulfonated)PEEK pellets. (For interpretation of the references tocolour in this figure legend, the reader is referred to theWeb version of this article.)

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  • Acknowledgements

    Support for this research was provided by the National ScienceFoundation, Polymers Program of the Division of Materials Research,under DMR-1608125. A portion of this work was performed at TuftsUniversity as part of the undergraduate thesis research of TK. The au-thors acknowledge help from Mr. Jonathan Minoff. CS acknowledgesfinancial support from the Ministry of Education and Science of theRussian Federation, grant 14.Y26.31.0019.

    Appendix A. Supplementary data

    Supplementary data to this article can be found online at https://doi.org/10.1016/j.polymer.2019.03.041.

    References

    [1] P. Patel, T.R. Hull, R.W. McCabe, D. Flath, J. Grasmeder, M. Percy, Mechanism ofthermal decomposition of poly(ether ether ketone) (PEEK) from a review of de-composition studies, Polym. Degrad. Stabil. 95 (5) (2010) 709–718.

    [2] M. Gil, X. Ji, X. Li, H. Na, J. Eric Hampsey, Y. Lu, Direct synthesis of sulfonatedaromatic poly(ether ether ketone) proton exchange membranes for fuel cell appli-cations, J. Membr. Sci. 234 (1) (2004) 75–81.

    [3] M. Rikukawa, K. Sanui, Proton-conducting polymer electrolyte membranes basedon hydrocarbon polymers, Prog. Polym. Sci. 25 (10) (2000) 1463–1502.

    [4] P. Xing, G.P. Robertson, M.D. Guiver, S.D. Mikhailenko, K. Wang, S. Kaliaguine,Synthesis and characterization of sulfonated poly(ether ether ketone) for protonexchange membranes, J. Membr. Sci. 229 (1) (2004) 95–106.

    [5] A.L. Khan, X. Li, I.F.J. Vankelecom, SPEEK/Matrimid blend membranes for CO2separation, J. Membr. Sci. 380 (1) (2011) 55–62.

    [6] Z. Tahir, A. Ilyas, X. Li, M.R. Bilad, I.F.J. Vankelecom, A.L. Khan, Tuning the gasseparation performance of fluorinated and sulfonated PEEK membranes by in-corporation of zeolite 4A, J. Appl. Polym. Sci. 135 (10) (2018) 45952.

    Fig. 5. FSC heating scans at 2000 K/s after coolingPEEK at different rates. Endothermic heat flow isrepresented by downward deflection from the base-line. a) Melt electrospun PEEK fibers (PEEK-F) whichhave been cooled from 200 °C. Cooling rates varyfrom 200 K/s to 3000 K/s; b) PEEK-M, generated byheating of the same sample shown in (a) to 350 °C.Cooling rates vary from 140 K/s to 8000 K/s. Samplesize was 122 ng. Heat flow signal from the emptysensor has been subtracted and symmetry correction[52,53] has been performed on all curves in (a) and(b). c) Fictive temperature as a function of logarithmof cooling rate. PEEK-F- filled symbols; PEEK-M -open symbols. The dashed lines represent the Wil-liams-Landel-Ferry (WLF) [24] fits to the two datasets.

    N.D. Govinna, et al. Polymer 171 (2019) 50–57

    56

    https://doi.org/10.1016/j.polymer.2019.03.041https://doi.org/10.1016/j.polymer.2019.03.041http://refhub.elsevier.com/S0032-3861(19)30265-4/sref1http://refhub.elsevier.com/S0032-3861(19)30265-4/sref1http://refhub.elsevier.com/S0032-3861(19)30265-4/sref1http://refhub.elsevier.com/S0032-3861(19)30265-4/sref2http://refhub.elsevier.com/S0032-3861(19)30265-4/sref2http://refhub.elsevier.com/S0032-3861(19)30265-4/sref2http://refhub.elsevier.com/S0032-3861(19)30265-4/sref3http://refhub.elsevier.com/S0032-3861(19)30265-4/sref3http://refhub.elsevier.com/S0032-3861(19)30265-4/sref4http://refhub.elsevier.com/S0032-3861(19)30265-4/sref4http://refhub.elsevier.com/S0032-3861(19)30265-4/sref4http://refhub.elsevier.com/S0032-3861(19)30265-4/sref5http://refhub.elsevier.com/S0032-3861(19)30265-4/sref5http://refhub.elsevier.com/S0032-3861(19)30265-4/sref6http://refhub.elsevier.com/S0032-3861(19)30265-4/sref6http://refhub.elsevier.com/S0032-3861(19)30265-4/sref6

  • [7] A. Iulianelli, C. Algieri, L. Donato, A. Garofalo, F. Galiano, G. Bagnato, A. Basile,A. Figoli, New PEEK-WC and PLA membranes for H2 separation, Int. J. HydrogenEnergy 42 (34) (2017) 22138–22148.

    [8] X. Meng, Z. Du, Y. Wang, Characteristics of wear particles and wear behavior ofretrieved PEEK-on-HXLPE total knee implants: a preliminary study, RSC Adv. 8 (53)(2018) 30330–30339.

    [9] S. Salerno, C. Campana, S. Morelli, E. Drioli, L. De Bartolo, Human hepatocytes andendothelial cells in organotypic membrane systems, Biomaterials 32 (34) (2011)8848–8859.

    [10] P. Feng, P. Wu, C. Gao, Y. Yang, W. Guo, W. Yang, C. Shuai, A multimaterialscaffold with tunable properties: toward bone tissue repair, Adv. Sci. 5 (6) (2018)1700817.

    [11] J.F.D. Montero, H.A. Tajiri, G.M.O. Barra, M.C. Fredel, C.A.M. Benfatti, R.S. Magini,A.L. Pimenta, J.C.M. Souza, Biofilm behavior on sulfonated poly(ether-ether-ke-tone) (sPEEK), Mater. Sci. Eng. C-Mater. Biol. Appl. 70 (2017) 456–460.

    [12] Y. Zhao, H.M. Wong, W.H. Wang, P.H. Li, Z.S. Xu, E.Y.W. Chong, C.H. Yan,K.W.K. Yeung, P.K. Chu, Cytocompatibility, osseointegration, and bioactivity ofthree-dimensional porous and nanostructured network on polyetheretherketone,Biomaterials 34 (37) (2013) 9264–9277.

    [13] B.T. Koziara, E.J. Kappert, W. Ogieglo, K. Nijmeijer, M.A. Hempenius, N.E. Benes,Thermal stability of sulfonated poly(ether ether ketone) films: on the role of pro-todesulfonation, Macromol. Mater. Eng. 301 (1) (2016) 71–80.

    [14] J.K. Fink, Chapter 6 - Poly(aryl Ether Ketone)s, High Performance Polymers, seconded., William Andrew Publishing, 2014, pp. 153–175.

    [15] S. Ramakrishna, K. Fujihara, W.-E. Teo, T. Yong, Z. Ma, R. Ramaseshan, Electrospunnanofibers: solving global issues, Mater. Today 9 (3) (2006) 40–50.

    [16] R.H. Mehta, D.A. Madsen, D.S. Kalika, Microporous membranes based on poly(etherether ketone) via thermally-induced phase-separation, J. Membr. Sci. 107 (1–2)(1995) 93–106.

    [17] J. da Silva Burgal, L.G. Peeva, S. Kumbharkar, A. Livingston, Organic solvent re-sistant poly(ether-ether-ketone) nanofiltration membranes, J. Membr. Sci. 479(2015) 105–116.

    [18] C. Boaretti, M. Roso, A. Lorenzetti, M. Modesti, Synthesis and process optimizationof electrospun PEEK-sulfonated nanofibers by response surface methodology,Materials 8 (7) (2015) 4096.

    [19] M. Sadrjahani, A.A. Gharehaghaji, M. Javanbakht, Aligned electrospun sulfonatedpolyetheretherketone nanofiber based proton exchange membranes for fuel cellapplications, Polym. Eng. Sci. 57 (8) (2017) 789–796.

    [20] C.A. Angell, Relaxation in liquids, polymers and plastic crystals — strong/fragilepatterns and problems, J. Non-Cryst. Solids 131–133 (1991) 13–31.

    [21] Y. Yue, R. Von der Ohe, S.L. Jensen, Fictive temperature, cooling rate, and viscosityof glasses, J. Chem. Phys. 120 (17) (2004) 8053–8059.

    [22] C.T. Moynihan, A.J. Easteal, M.A. Bolt, J. Tucker, Dependence of the fictive tem-perature of glass on cooling rate, J. Am. Ceram. Soc. 59 (1‐2) (1976) 12–16.

    [23] A.Q. Tool, Relation between inelastic deformability and thermal expansion of glassin its annealing range, J. Am. Ceram. Soc. 29 (9) (1946) 240–253.

    [24] M.L. Williams, R.F. Landel, J.D. Ferry, The temperature dependence of relaxationmechanisms in amorphous polymers and other glass-forming liquids, J. Am. Chem.Soc. 77 (14) (1955) 3701–3707.

    [25] H. Vogel, Temperaturabhangigkeitsgesetz der viskositaet von fluessigkeiten, Phys.Z. 22 (1921) 645–646.

    [26] G.S. Fulcher, Analysis of recent measurements of the viscosity of glasses, J. Am.Ceram. Soc. 8 (6) (1925) 339–355.

    [27] G. Tammann, W. Hesse, Die Abhängigkeit der Viscosität von der Temperature bieunterkühlten Flüssigkeiten, Z. Anorg. Allg. Chem. 156 (1) (1926) 245–257.

    [28] A.P. Sokolov, V.N. Novikov, Y. Ding, Why many polymers are so fragile, J. Phys.Condens. Matter 19 (20) (2007) 205116.

    [29] C.M. Evans, H. Deng, W.F. Jager, J.M. Torkelson, Fragility is a key parameter indetermining the magnitude of T g-confinement effects in polymer films,Macromolecules 46 (15) (2013) 6091–6103.

    [30] R. Böhmer, K.L. Ngai, C.A. Angell, D.J. Plazek, Nonexponential relaxations in strongand fragile glass formers, J. Chem. Phys. 99 (5) (1993) 4201–4209.

    [31] Y. Ding, V.N. Novikov, A.P. Sokolov, C. Dalle-Ferrier, C. Alba-Simionesco, B. Frick,Influence of molecular weight on fast dynamics and fragility of polymers,Macromolecules 37 (24) (2004) 9264–9272.

    [32] D. Huang, G.B. McKenna, New insights into the fragility dilemma in liquids, J.Chem. Phys. 114 (13) (2001) 5621–5630.

    [33] A. Sanz, A. Nogales, T.A. Ezquerra, Influence of fragility on polymer cold crystal-lization, Macromolecules 43 (1) (2010) 29–32.

    [34] A.G. Al Lafi, Structural development in ion-irradiated poly(ether ether ketone) asstudied by dielectric relaxation spectroscopy, J. Appl. Polym. Sci. 131 (6) (2014).

    [35] A.A. Goodwin, F.W. Mercer, M.T. McKenzie, Thermal behavior of fluorinated aro-matic polyethers and poly(ether ketone)s, Macromolecules 30 (9) (1997)2767–2774.

    [36] E. Zhuravlev, C. Schick, Fast scanning power compensated differential scanningnano-calorimeter: 1. The device, Thermochim. Acta 505 (1) (2010) 1–13.

    [37] E. Zhuravlev, C. Schick, Fast scanning power compensated differential scanningnano-calorimeter: 2. Heat capacity analysis, Thermochim. Acta 505 (1) (2010)14–21.

    [38] C. Schick, V. Mathot, Fast Scanning Calorimetry, Springer, Switzerland, 2016.[39] V. Mathot, M. Pyda, T. Pijpers, G. Vanden Poel, E. van de Kerkhof, S. van

    Herwaarden, F. van Herwaarden, A. Leenaers, The Flash DSC 1, a power compen-sation twin-type, chip-based fast scanning calorimeter (FSC): first findings onpolymers, Thermochim. Acta 522 (1) (2011) 36–45.

    [40] A. Dhotel, B. Rijal, L. Delbreilh, E. Dargent, A. Saiter, Combining Flash DSC, DSCand broadband dielectric spectroscopy to determine fragility, J. Therm. Anal.Calorim. 121 (1) (2015) 453–461.

    [41] R. Tao, E. Gurung, M.M. Cetin, M.F. Mayer, E.L. Quitevis, S.L. Simon, Fragility ofionic liquids measured by Flash differential scanning calorimetry, Thermochim.Acta 654 (2017) 121–129.

    [42] X. Monnier, A. Saiter, E. Dargent, Vitrification of PLA by fast scanning calorimetry:towards unique glass above critical cooling rate? Thermochim. Acta 658 (2017)47–54.

    [43] X. Tardif, B. Pignon, N. Boyard, J.W.P. Schmelzer, V. Sobotka, D. Delaunay,C. Schick, Experimental study of crystallization of PolyEtherEtherKetone (PEEK)over a large temperature range using a nano-calorimeter, Polym. Test. 36 (2014)10–19.

    [44] Y. Furushima, A. Toda, V. Rousseaux, C. Bailly, E. Zhuravlev, C. Schick,Quantitative understanding of two distinct melting kinetics of an isothermallycrystallized poly(ether ether ketone), Polymer 99 (2016) 97–104.

    [45] Victrex PLC, VICTREX™ 150G Datasheet, https://www.victrex.com/∼/media/datasheets/victrextds150g-151g.pdf , Accessed date: 1 September 2018.

    [46] Victrex PLC, VICTREX™ 450G Datasheet, https://www.victrex.com/∼/media/datasheets/victrex_tds_450g.pdf , Accessed date: 1 September 2018.

    [47] Victrex PLC, VICTREX™ 650G Datasheet, https://www.victrex.com/∼/media/datasheets/victrex_tds_650g.pdf , Accessed date: 1 September 2018.

    [48] H. Zhou, T.B. Green, Y.L. Joo, The thermal effects on electrospinning of polylacticacid melts, Polymer 47 (21) (2006) 7497–7505.

    [49] P.D. Dalton, D. Grafahrend, K. Klinkhammer, D. Klee, M. Möller, Electrospinning ofpolymer melts: phenomenological observations, Polymer 48 (23) (2007)6823–6833.

    [50] E. Zhmayev, D. Cho, Y.L. Joo, Modeling of melt electrospinning for semi-crystallinepolymers, Polymer 51 (1) (2010) 274–290.

    [51] J. Ko, N.K. Mohtaram, F. Ahmed, A. Montgomery, M. Carlson, P.C. Lee,S.M. Willerth, M.B. Jun, Fabrication of poly (-caprolactone) microfiber scaffoldswith varying topography and mechanical properties for stem cell-based tissue en-gineering applications, Journal of biomaterials science, Polymer edition 25 (1)(2014) 1–17.

    [52] P. Cebe, B.P. Partlow, D.L. Kaplan, A. Wurm, E. Zhuravlev, C. Schick, Using flashDSC for determining the liquid state heat capacity of silk fibroin, Thermochim. Acta615 (2015) 8–14.

    [53] D. Thomas, E. Zhuravlev, A. Wurm, C. Schick, P. Cebe, Fundamental thermalproperties of polyvinyl alcohol by fast scanning calorimetry, Polymer 137 (2018)145–155.

    [54] N.M. Ribe, Liquid rope coiling: a synoptic view, J. Fluid Mech. 812 (2016) R2.[55] S.X. Lu, P. Cebe, M. Capel, Thermal stability and thermal expansion studies of PEEK

    and related polyimides, Polymer 37 (14) (1996) 2999–3009.[56] P. Knauth, H. Hou, E. Bloch, E. Sgreccia, M.L. Di Vona, Thermogravimetric analysis

    of SPEEK membranes: thermal stability, degree of sulfonation and cross-linkingreaction, J. Anal. Appl. Pyrolysis 92 (2) (2011) 361–365.

    [57] J.E.K. Schawe, Influence of processing conditions on polymer crystallization mea-sured by fast scanning DSC, J. Therm. Anal. Calorim. 116 (3) (2014) 1165–1173.

    [58] A. Toda, R. Androsch, C. Schick, Insights into polymer crystallization and meltingfrom fast scanning chip calorimetry, Polymer 91 (2016) 239–263.

    [59] J.E.K. Schawe, Practical Aspects of the Flash DSC 1: Sample Prepataion forMeasurements of Polymers, Series 36 Mettler Toledo Thermal Analysis UserCom,2012, pp. 17–24 http://ch.mt.com/ch/en/home/supportive_content/usercom/TA_UserCom36.html , Accessed date: 12 June 2018.

    [60] B.R. Hahn, J.H. Wendorff, Compensation method for zero birefringence in orientedpolymers, Polymer 26 (11) (1985) 1619–1622.

    [61] A.G. Al Lafi, The sulfonation of poly(ether ether ketone) as investigated by two-dimensional FTIR correlation spectroscopy, J. Appl. Polym. Sci. 132 (2) (2015).

    [62] V.V. Lakshmi, V. Choudhary, I.K. Varma, Sulphonated poly(ether ether ketone):synthesis and characterisation, Macromol. Symp. 210 (2004) 21–29.

    [63] J.R. Scherer, Group vibrations of substituted benzenes—I: E1u bend-stretch modesand E2g ring deformations of the chlorobenzenes, Spectrochim. Acta 19 (3) (1963)601–610.

    [64] P. Badrinarayanan, W. Zheng, Q. Li, S.L. Simon, The glass transition temperatureversus the fictive temperature, J. Non-Cryst. Solids 353 (26) (2007) 2603–2612.

    [65] M. Pyda, Poly(oxy-1,4-phenylene-1,4-phenylene-carbonyl-1,4-phenylene) (PEEK)Heat Capacity, Enthalpy, Entropy, Gibbs Energy: Datasheet from “The AdvancedTHermal Analysis System (ATHAS) Databank – Polymer Thermodynamics,Springer-Verlag Berlin Heidelberg & Marek Pyda, 2018 Release 2014 inSpringerMaterials https://materials.springer.com/polymerthermodynamics/docs/athas_0108 , Accessed date: 14 December 2018.

    [66] C.M. Roland, P.G. Santangelo, K.L. Ngai, The application of the energy landscapemodel to polymers, J. Chem. Phys. 111 (12) (1999) 5593–5598.

    [67] R. Tao, S.L. Simon, Rheology of imidazolium-based ionic liquids with aromaticfunctionality, J. Phys. Chem. B 119 (35) (2015) 11953–11959.

    [68] P. Cebe, S.-D. Hong, Crystallization behaviour of poly(ether-ether-ketone), Polymer27 (8) (1986) 1183–1192.

    [69] D. Depeng, K. Jingxin, L. Yong, Properties of special engineering plastic polyetherketone ketone, Eng. Plast. Appl. 44 (9) (2016) 83–86.

    [70] T.A. Ezquerra, M. Zolotukhin, V.P. Privalko, F.J. Baltá-Calleja, G. Nequlqueo,C. Garcıá, J.G.d.l. Campa, J.d. Abajo, Relaxation behavior in model compounds ofpoly(aryl-ether-ketone-ketone) as revealed by dielectric spectroscopy, J. Chem.Phys. 110 (20) (1999) 10134–10140.

    N.D. Govinna, et al. Polymer 171 (2019) 50–57

    57

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    Melt-electrospinning of poly(ether ether ketone) fibers to avoid sulfonationIntroductionExperimental sectionMaterialsCreation of SPEEKMelt electrospinning

    CharacterizationMorphologyInfrared spectroscopyThermogravimetryFast scanning calorimetry

    Results and discussionConclusionsAcknowledgementsSupplementary dataReferences