Polymer 171 (2019) 50–57

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Polymer

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Melt-electrospinning of poly(ether ether ketone) fibers to avoid sulfonation T ∗ 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, 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, 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. fi • PEEK bers shrink above Tg but maintain some orientational birefringence. • Dynamic fragility of fibers is greater than that of melt-cooled PEEK.

ARTICLE INFO ABSTRACT

Keywords: We have successfully electrospun un-sulfonated fibers of poly(ether ether ketone), PEEK, from the molten state at PEEK fibers 350 °C. Unlike solution electrospinning of PEEK, which produces sulfonated fibers with reduced thermal stabi- Melt-electrospinning lity, melt electrospinning produces chemically unaltered PEEK fibers. These fibers are smooth, defect-free, and Dynamic fragility round in cross-section. Most fiber diameters range from 1.5 μm to 8.5 μm. A large interstitial fiber with diameter reaching 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 and melting behavior. Dynamic fragilities of melt electrospun oriented amorphous PEEK fibers and amorphous PEEK quenched from the molten state, were evaluated according to Moynihan's method of cooling at variable rates then reheating at a fixed rate and were found to be 200 ± 5 and 150 ± 5, respectively. These results suggest that orientation in the amorphous state of electrospun fibers plays a role in the dynamics of glass formation.

1. Introduction this work is centered around PEEK-derivatives. These include, e.g., PEEK with β-tricalcium phosphate (β-TCP) [10]; sulfonated-PEEK Poly(ether ether ketone) (PEEK) is a high-performance semi-crys- (SPEEK) [11,12]; sulfonated PEEK with sodium as the counter ion (Na- talline thermoplastic polymer that falls into the engineering polymer SPEEK) [13]; zeolite 4 A incorporated SPEEK [6]; or, PEEK on highly family of Poly(aryl ether ketone)s or PAEKs. Relative to other polymers, cross-linked polyethylene (HXLPE) [8]. PEEK is highly resistant to most common solvents. PEEK has a high Due to its excellent chemical resistance, PEEK can only be dissolved tensile strength of about 105 MPa, and high melting and degradation at room temperature in highly concentrated sulfuric acid [14]. But in − + temperatures of 343 °C and 595 °C, respectively [1]. These properties doing so, PEEK gains an SO3 H side chain. This modified version of make PEEK a desirable engineering material, but also make it difficult PEEK is called sulfonated-PEEK, or SPEEK. The addition of this mole- to process. Despite the challenges, the unique characteristics of PEEK, cule to the side chain deteriorates some of PEEK's desirable character- specifically its thermal and chemical resilience, have garnered interest istics, including lowering the thermal degradation temperature, at in developing its potential in a wide variety of applications. One area of which significant mass loss occurs, to values ranging between 345 °C interest is using PEEK as a candidate for filtration membranes as se- [13] and 530 °C, depending on the counter-ion present. SPEEK does, parators in fuel cell applications [2–4], or in gas separation [5–7]. PEEK however, show many characteristics that make it desirable in fuel cell membranes have also received attention in biomedical applications due membrane applications, including good proton conductivity [2,3,15]. to their mechanical properties and bio-inertness [8–10]. However, all Alternate research has been done to form a porous PEEK membrane

∗ Corresponding author. E-mail address: [email protected] (P. Cebe). 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 Available online 20 March 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved. N.D. Govinna, et al. Polymer 171 (2019) 50–57 using Thermally Induced Phase Separation (TIPS). In this method, PEEK that PEEK polymer can be melt electrospun into fibers while main- is combined with a diluent at a temperature where both are in a liquid taining its native chemical structure, i.e., without increasing the degree state. Upon cooling, phase separation occurs, and the diluent can be of sulfonation of the molecular chain. We report on the electrospinning selectively removed using a solvent, leaving behind porous vacancies in method and conditions used to create these fibers, and their char- the PEEK structure [14,16]. Da Silva Burgal et al. [17] reported the acterization by microscopy, infrared analysis (FTIR), and thermal production of PEEK nanofiltration membranes with very low sulfona- analysis including thermogravimetry (TG), differential scanning ca- tion, approaching the level of native PEEK, where sulfonation is a lorimetry (DSC), and fast scanning chip-based calorimetry (FSC). TG measure of the amount of sulfur groups added to the PEEK molecular and FTIR analyses confirm that the melt electrospun fibers are chemi- chain. They report that their PEEK solutions had less than 3% sulfo- cally un-altered from the as-received pellets used to fabricate them. nation, but the final membranes they produced all had at least twice the Melt electrospun PEEK fibers and PEEK quenched from the molten state degree of sulfonation of the original solutions. Their work also in- exhibit amorphous states which differ from each other, and these states dicated that increasing the degree of sulfonation decreased the che- demonstrate very different fragility indices of 200 ± 5, and 150 ± 5, mical resistance of SPEEK to several solvents. respectively. These solvent-based techniques all focus on forming membranes from porous films rather than by forming discrete fibers. To date, efforts 2. Experimental section at forming discrete fibers have been done with SPEEK. SPEEK fibers with diameters around 150–200 nm were produced with ∼70% sulfo- 2.1. Materials nation by Boaretti et al. [18], and fibers with diameters of about 100 nm have been manufactured and characterized by Sadrjahani et al. PEEK pellets with a range of molecular weights are commercially [19], where their fibers had a degree of sulfonation greater than 60%. available, and these have melt viscosities ranging from 90 Pa s to

The glass transition temperature (Tg), is the gradual and reversible 475 Pa s at 400 °C. As shown in Supplementary Information, Table transition in amorphous materials at which the material undergoes a S1, they generally share the same properties, such as solvent resistance, transition from the viscous rubbery state into the solid “glassy” state, as tensile strength, and thermal resistance, which are largely unchanged temperature is decreased. Since this is a material property that depends for different molecular weights [45–47]. Other properties such as ten- on the experimental time scale, studies that incorporate a variety of sile elongation at break, impact strength, and viscosity vary with mo- heating and cooling rates allow us to investigate the variation of en- lecular weight. thalpy relaxation at Tg. In these cases, it is more convenient to use the Previous studies of other polymers that have been successfully melt fictive temperature, Tf. The fictive temperature is defined as the tem- electrospun give a reasonable range of target melt viscosities, as shown perature at which the extrapolated enthalpy lines of the glass and of the in Supplementary Information, Table S2. The lowest molecular liquid intersect [20–23]. The variation of Tf with cooling rate follows weight PEEK, VICTREX™ 150G, was chosen for this study as since its the classic Williams–Landel–Ferry (WLF) [24] or Vogel–Ful- melt viscosity (130 Pa s) lies within the range of viscosities where cher–Tammann (VFT) [25–27] relationships. Using these relationships, successful melt-electrospinning experiments have been performed on we can evaluate the dynamic fragility index (m) [20], which is a other polymers [48–51]. measure of how rapidly the dynamics of a material slow down as it is cooled toward the glass transition. 2.1.1. Creation of SPEEK Polymers with large values of m (the usual case) are considered to For comparison with native PEEK, sulfonated PEEK (SPEEK) was be very fragile [28–30], resulting mainly from chain rigidity [28]or made by dissolving PEEK pellets in 95–98% concentrated sulfuric acid other structural effects such as tacticity [31]. One exception is poly (EM Science, Gibbstown, New Jersey). The solution was then spread on (oxyethylene) (POE), which exhibits a very low fragility index, m = 23 a glass plate until it was dry. Prior to testing, SPEEK films were washed [32]. A few reports have presented estimates of the dynamic fragility of thoroughly with DI water and dried once more to remove residual PEEK [33–35]. Sanz et al. [33], present findings of dielectric relaxation sulfuric acid. and isothermal crystallization studies where the dynamic fragility index of PEEK was determined to be 384, and they state that PEEK is very 2.1.2. Melt electrospinning fragile compared to other common polymers such as poly(ethylene Melt electrospinning of PEEK was performed by heating the polymer terephthalate) (PET, m = 164), poly(dimethyl phenyl siloxane) (PDMS, into its molten state at 350–375 °C. An electrically grounded 20-gauge m = 175), and poly(L-lactic acid) (PLLA, m = 127). Al Lafi [34] gives stainless steel wire was used as the “spinneret” and a high voltage the dynamic fragility index of PEEK to be 155 (from Dielectric Re- collector plate placed 3.6 cm away was maintained at a temperature laxation Spectroscopy) and 280 (from Differential Scanning Calori- close to room temperature. The electric field strength, E (where metry) demonstrating that the dynamic fragility can vary greatly, de- E = voltage/distance) was maintained at 2–5x105 V/m to prevent pending on the determination method used. In a comparative work electrical breakdown through the air and different electrical field [35], Goodwin et al., present findings of a fragility study on fluorinated strengths were tested to assess the quality of fibers. A drop of molten polyethers and poly(ether ketone)s via dynamic mechanical thermal PEEK was created at the tip of the spinneret wire and melt-electro- analysis (DMTA) where they substitute some of the hydrogen molecules spinning was performed. Detailed information on the melt electro- in the polymer with fluorine molecules. Depending on the modification, spinning procedure, and a diagram of the melt-electrospinning setup, they show that the fragility was in the range of 60–90. In this study, we can be found in the Supplementary Information, Section 2. present a direct comparison of the fragility of melt electrospun PEEK fibers (referred to as PEEK-F) and PEEK cooled from the melt after 2.2. Characterization heating up to 350 °C (referred to as PEEK-M) measured via fast scanning calorimetry (FSC) [36–39]. The wide range of scanning rates made 2.2.1. Morphology possible by FSC has been employed to evaluate fragility of materials in The morphology of melt-electrospun fibers was studied using a Zeiss previous works such as Dotel et al. [40] (for poly(ethylene ter- EVO MA10 scanning electron microscope (SEM) (Carl Zeiss, ephthalate), PET), Tao et al. [41] (for ionic liquids) and Xavier et al. Oberkochen, Germany), operating at 5 kV. Samples were first coated [42] (for poly(lactic acid), PLA). FSC has also been used to study the with Au-Pd alloy for 90 s using a Cressington Sputter Coater 108 crystalline nature of PEEK [43, 44]. In the present work, our focus is on (Cressington Scientific Instruments, Watford, UK). Software package oriented amorphous PEEK fibers and quenched amorphous PEEK. ImageJ was used to analyze SEM images and to obtain statistics on fiber To the best of our knowledge, our work is the first demonstration size.

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

2.2.2. Infrared spectroscopy defect-free. Collections of fibers often showed a circular or complex Absorbance spectra of the samples were examined using attenuated looping fiber orientation, shown in Fig. 1a, where the fibers tended to total reflectance Fourier transform infrared (FTIR) spectroscopy on a coil up in a large spiral. This effect, known as “Liquid Rope Coiling” Jasco FTIR-6200 Spectrometer (Jasco Instruments, Tokyo, Japan). [54], is common in viscous fluids in the presence of a parallel force − − Spectra were obtained from 400 to 4000 cm 1 at 4 cm 1 resolution vector field, such as gravity or the electrostatic force between the with 256 scans co-added. Air background was subtracted from all source and the collector, when they suddenly encounter a rigid surface. sample spectra. The fluid assumes a direction of rotation and forms a corkscrew shape of nearly uniform radius as the fluid rope piles up on top of itself. No 2.2.3. Thermogravimetry fibers were obtained with applied electric field strengths below Thermogravimetry (TG) was performed on a TA Instruments, Inc. E = 2.08 × 105 V/m, where the electric force was insufficient to draw (New Castle, Delaware, USA) Q500 series thermogravimetric analyzer molten material toward the collector. At E = 2.08 × 105 V/m, the from 25 °C to 1000 °C at 20 °C/min under 50 ml/min N2 gas flow using average diameter of obtained fibers was about 4 μm. At higher electric polymer fiber samples of mass 5–15 mg. field strengths, fibers with smaller diameters could be seen among re- latively larger fibers. The amount of these smaller fibers increased as 2.2.4. Fast scanning calorimetry the applied electric field strength increased. Since these smaller fibers A Mettler Flash DSC1 (Mettler Toledo, Greifensee, Switzerland) was were numerous, at high electric fields, these dominated the fiber dia- used to perform fast scanning calorimetric (FSC) experiments to mea- meter distribution. Therefore, above E = 4.17 × 105 V/m (i.e., at 15 kV sure heat flow rate vs. temperature. The calorimeter was operated and 3.6 cm), the average fiber diameter was less than 1 μm. under nitrogen gas flow of 50 ml/min. The empty sensor was condi- In some cases, considerably larger fibers, with diameters > 50 μm, tioned using the manufacturer's procedure five times, repeatedly were also present among the smaller size fibers. This was more evident heating and cooling to 450 °C at the same rate used for the samples. The in spins with larger applied electric fields, where a fiber larger than the ceramic sensor base temperature was set at −100 °C. Prior to experi- rest can be seen in the middle as shown in Fig. 1a and e. These larger ments, Mettler Toledo UFSC1 sensors were cooled and reheated be- fiber(s) were created towards the end time of the spin. When the PEEK tween −100 °C and 470 °C at the rates used in the experiments, to droplet is close to be fully consumed, the contact it has with the spin- obtain the empty sensor baseline signal which was subtracted from all neret is weakened due to the small amount of material left, and the data scans. To ensure high quality and accurate data, a symmetry electrical force is able to pull most of the remaining molten PEEK ma- correction procedure was also carried out on all curves following terial towards the collector creating a larger fiber which in some cases methods developed previously [52,53]. could reach 100 μm in diameter. Any residual molten PEEK can also To test the crystalline nature of PEEK fibers, they were heated to continue to create smaller diameter fibers which land on top of the 400 °C at 2000 K/s, well past crystal melting temperature of 310 °C. larger fiber, as shown in Fig. 1a. Existence of these large diameter fibers Using different fiber samples from the same spin, experiments char- could be avoided by stopping the spin prior to consumption of the acterizing the glass transition were performed by first heating PEEK-F source PEEK at the spinneret. The best melt electrospinning parameters as-spun fiber samples to 200 °C, just above the glass transition tem- to acquire a dense fiber mat were found to be E = 2.78 × 105 V/m (for perature (Tg), and then immediately cooling to −50 °C, followed by re- 10 kV applied voltage and 3.6 cm working distance between spinneret heating to 200 °C. The cool-heat ramp pair (i.e., first cool and second and collector). heat) was repeated by using various cooling rates in the range from Thermogravimetric (TG) experiments confirmed that the thermal 50 K/s to 8000 K/s, followed by heating at a fixed rate of 2000 K/s. stability of the melt electrospun PEEK fibers was nearly identical to that Next, the fiber samples (PEEK-F) were then heated to 350 °C to remove of native PEEK as-received pellets as shown by the blue and red curves the fibrous shape at which point the initial fibers have become PEEK-M. in Fig. 2a. Major mass loss events were seen around 550 °C corre- The cooling and reheating experiment is repeated on PEEK-M, and now sponding well to literature values for native PEEK [1,12,55]. On the the samples are heated to 350 °C during each heating cycle and then other hand, degradation onset of SPEEK (black) is around 350 °C, which cooled at various rates. Each scan was carefully examined to determine is also consistent with other experimental observations for highly sul- whether any crystallization had occurred during cooling. If any cooling fonated PEEK [13,56]. scan exhibited crystallization of the material, the subsequent heating Using fast scanning calorimetry, the as-spun fibers were shown to be ramps were excluded from further analysis. No crystallization was ob- non-crystalline (Fig. 2b) from the heat flow rate traces for two different served for PEEK-F when cooled from 200 °C, at cooling rates of 3000, fiber samples. The glass transition temperature (Tg) occurs around 2000, 1000, 800, 600, 450, 300 and 200 K/s. No crystallization was 150 °C for all samples tested and an enthalpy recovery peak (en- observed for PEEK-M when cooled from 350 °C, at cooling rates of dothermal peak at 165 °C) was also present for the first heating scans of 8000, 7000, 6000, 5000, 4000, 3000, 2000, 1000, 800, 600, 450, 300, the fibers. Small exothermic peaks were also seen very close to, and

200 and 140 K/s. Optical Images were taken of samples on the sensor, overlapping with, Tg. These were a result of stress relaxation and before and after heating, using an Olympus BX41 microscope (Tokyo, shrinkage of fibers when they underwent the glass transition relaxation Japan) equipped with a ScopeTek DCM 510 camera (Hangzhou, China). process for the first time. Fiber shrinkage was confirmed through op- Polarized hot stage microscopy was performed for as spun fibers tical imaging, as shown in Fig. 3a–c. The sample contains six cut pieces PEEK-F and for PEEK-M using a Nikon Eclipse E600 POL (Minato, of melt electrospun PEEK fibers and they were imaged: 1. Before any

Tokyo, Japan) microscope equipped with a SPOT Insight 11.2 Color treatment (Fig. 3a); 2. after heating above Tg up to 200 °C and cooling Mosaic camera (SPOT Imaging, Sterling Heights, Michigan, USA). A down (Fig. 3b); and, 3. after heating to 350 °C and cooling down Mettler Toledo FP82HT Hot stage (Columbus, Ohio, USA) was used to (Fig. 3c) which resulted in loss of most of the fibrous shape. Fibers heat the samples at 20 K/min while observing either under bright field (Fig. 3a and b), or their once-melted counterparts (Fig. 3c) were all conditions, or through crossed polarizers (A⊥P). All samples were im- transparent and amorphous when imaged on the sensors. aged at high temperatures and after cooling to room temperature. The melt electrospun fibers underwent shrinkage upon heating

above Tg but fibers were still able partially to maintain their fibrous 3. Results and discussion shape. Molecular chains in as spun fibers are oriented parallel to the fiber axis due to the stresses imparted during the melt electrospinning Representative SEM images of melt electrospun PEEK fibers, spun at process. Due to this preferred orientation, the refractive index parallel several different electric field strengths, are shown in Fig. 1. As shown to the fiber axis is no longer equal to the refractive index perpendicular by the SEM images in Fig. 1a–e, the obtained fibers were smooth and to this direction. This results in birefringence, which is a common

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

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 images for 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 calculated with ImageJ using data from at least 150 fibers per spin condition. experimental tool for the investigation of orientation effects in poly- the fibrous shape was lost and no birefringence was observed (Fig. 3i), mers [60]. As seen in Fig. 3g, these fibers are uniaxially birefringent, indicating randomization of molecular orientation within the material. indicating molecular chain orientation. Upon heating to 200 °C, a To confirm that the electrospun fibers are chemically similar to temperature above Tg, entropic forces of elasticity are partially relaxed native PEEK, infrared spectroscopy was performed, and absorption and the fibers shrink. As shown in Fig. 3h (and Supplementary In- spectra are shown in Fig. 4. Comparing spectra from the native PEEK formation, Fig. S2) birefringence remains after heating to 200 °C at pellet (red curve) and the melt electrospun PEEK-F fibers (blue curve), 20 K/min. Supplementary Information, Fig. S3 also confirms bi- it can be seen that these show all the characteristic infrared vibrations refringence of fibers heated to 200 °C at 2000 K/s, the rate used in FSC. of PEEK, which are listed in Supplementary Information, Table S3. When these fibers were heated to very high temperatures, neither a cold In the infrared spectrum of SPEEK (Fig. 4), several characteristic − crystallization exotherm nor a melting endotherm were observed, in- peaks can be observed at 1253 cm 1 (asymmetric O=S=O stretch), − − dicating that the as-spun PEEK-F fibers were non-crystalline and did not 1078 cm 1 (symmetric O=S=O stretch) and 708 cm 1 (S-O stretch) crystallize during heating at 2000 K/s. When heated to 350 °C, most of [61,62]. As reported previously [19,61], with sulfonation, an increase

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

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 traces at 2000 K/s for two separate PEEK fiber samples having different masses. Samples for FSC were chosen to have diameters smaller than 10 μmto prevent temperature gradients in the sample [57–59]. Only the glass transitions with an enthalpy recovery peak were observed on heating, indicating the fibers are non-crystalline. No cold crystallization or melting endotherm was observed for the PEEK fibers. Curves are shifted vertically for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web ver- sion of this article.)

− of the absorbance at 1472 cm 1 (1,2,4 three substituted C=C skeletal T≫Tg TT≫ g − ˙˙ ˙˙ ring vibration), and a decrease of absorbance at 1490 cm 1 (1,4 di- ∫∫(Qlg−= Q)( dT Q − Qg) dT substituted C=C skeletal ring vibration) [61,63] can also be observed, Tf TT≪ g (1) resulting in a doublet of peaks. These features combined, generate a where Q˙ is the apparent heat flow rate of the sample, and Q˙ l and Q˙ g characteristic spectrum of SPEEK. The spectrum of our SPEEK, black denote the heat flow rates in the liquid and glass states, respectively. line in Fig. 4, is in good agreement with observations of previous work Sample mass was determined by comparing the observed heat capacity on SPEEK [19,61]. increment at Tg with the literature value of 78.1 J/(mol K) [65]. Results from experiments with different cooling rates are shown in Fig. 5c shows the fictive temperature, Tf, calculated using Eqn. (1), Fig. 5. Fig. 5a and b shows heating scans of PEEK-F and PEEK-M sam- as a function of logarithm of cooling rate for PEEK-F (filled symbols) ples, respectively. These experiments allow the dynamic fragility to be and PEEK-M (open symbols). The dashed lines represent fits to the determined. The endothermic glass transition relaxation is clearly seen temperature dependence of the data, commonly described by the Wil- and the enthalpy relaxation peak increases in amplitude as the mag- liams-Landel-Ferry (WLF) model [24]: nitude of the prior cooling rate decreases. fi To estimate the dynamic fragility index (m) [20], the ctive tem- ⎛ q ⎞ CT1,()ffref− T fi log10 = perature Tf [20,21,23,64], was rst determined using Moynihan's ⎜q ⎟ CTT+−() ⎝ ref ⎠ 2,ffref (2) method [22]:

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

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 six cut 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 and then 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) and analyzer (A) directions relevant for images g-i are shown with arrows.

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

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

to be 200 K/s, the lowest common rate used for PEEK-F and PEEK-M.), partially oriented material having higher dynamic fragility towards

Tf is the fictive temperature, Tf,ref is the fictive temperature determined glass formation. This non-crystalline state of PEEK is unlike the state at qref, and C1 and C2 are constants. In this analysis, the dynamic fra- achieved when the PEEK fiber material is heated to 350 °C. Here, the gility index (m) is given by Ref. [41] as: fibrous shape and the preferred orientation is completely lost, shown by the lack of birefringence in Fig. 3i. The resulting material behaves dq(log10 ) C1 ff m =− = Tfre, f di erently with respect to its glass forming dynamics. Due to the nearly dT(/)fref, T f C2 (3) complete randomness of the molecular chain orientation of PEEK-M, its dynamic fragility decreases substantially compared to more oriented For PEEK-F, T = 413.8 K, C = 132.7, and C = 278.8. For f,ref 1 2 PEEK-F. PEEK-M, Tf,ref = 410.7 K, C1 = 108, and C2 = 300.4. Dynamic fragility indices were evaluated using equation (1), and we find that PEEK-M has a dynamic fragility of m = 150 ± 5 while melt-electrospun PEEK fi- 4. Conclusions bers (PEEK-F) have a dynamic fragility of m = 200 ± 5, categorizing them both as fragile systems [20,30]. We have successfully developed a method for the melt-electrospin- A few reports are found in the literature that explicitly mention ning of native PEEK. This method is able to produce PEEK fibers having dynamic fragility of PEEK [33–35], but these reports are not in good the same excellent thermal and chemical resistance, and the same glass agreement. Our results are in the range of fragility values observed by transition temperature, as the as-received native PEEK pellets. Up to previous studies that report fragility indices for polymers with stiff now, most of the effort in the field has been made towards reducing the backbones and/or stiff side groups (e.g., polycarbonate, m = 132 [32], degree of sulfonation in SPEEK fibers to minimize the loss of thermal and poly(ethylene terephthalate), m = 156 [66]). It is expected that and chemical resistance. Using melt electrospinning, we have elimi- polymers containing rigid aromatic rings would have higher dynamic nated the need for aggressive solvents and avoided sulfonation which is fragility indices [67]. The multiple aromatic rings present in the PEEK deleterious to PEEK's properties. By scaling production up, this process monomer unit make its backbone quite rigid, resulting in its high glass can replace the current solution fabrication methods used in many transition temperature (145 °C) [65] and high melting temperature (up applications for PEEK fiber membranes, which currently result in non- to 335 °C) [68]. One very apt comparison can be made with poly(ether native, sulfonated PEEK. ketone ketone), PEKK, a close chemical relative of PEEK in the poly(aryl We then used fast scanning calorimetry, which provides a wide ether ketone) family of semi-crystalline polymers [69]. It also has three range of cooling rates, to examine the formation of the glassy state in aromatic rings in the monomer, but with two carbonyl groups com- both PEEK fibers and melted PEEK. By cooling fibers and melted PEEK pared to one carbonyl group in PEEK. Chemical similarity suggests the at different rates to form the glass, and then reheating at a constant two polymers may exhibit similar fragility characteristics. Indeed, Ez- rate, the enthalpic relaxation occurring in the vicinity of the glass querra, et al. [70] using dielectric spectroscopy found the dynamic transition was analyzed using Moynihan's method. For both PEEK-F and fragility of PEKK to be m = 175, an intermediate value between our PEEK-M, FSC experiments were conducted over a very similar range of PEEK dynamic fragility of m = 200 ± 5 (PEEK-F) and m = 150 ± 5 cooling rates allowing direct comparison of their fragility. The dynamic (PEEK-M). fragility index, m, was found to be 200 ± 5 for PEEK fibers (PEEK-F) Although both fragility results refer to non-crystalline samples, we heated to 200 °C and then cooled, compared to 150 ± 5 for melted find that PEEK fibers have significantly larger fragility than the same PEEK (PEEK-M) heated to 350 °C and then cooled. These fragility values material after it has been heated to a temperature above the melting agree with literature results of Lafi et al. [34]. While both of these point, at which point most of the fibrous orientation is lost. This sug- materials are non-crystalline, their internal structures are not the same. gests that PEEK-F and PEEK-M have different non-crystalline states. We Birefringence results show that PEEK-F retains some degree of mole- may understand this result by considering that in PEEK fibers, partial cular chain orientation along the fiber direction, whereas PEEK-M fiber structure remains in the fibers after heating above T to 200 °C. We g (heated to a much higher temperature) assumes a more random chain suggest this residual fiber structure, seen in Fig. 3b,e,h, has molecular conformation. The one-dimensional polymer chain alignment in the chains that retain some preferential orientation, and have not been PEEK-F fibers results in a statistically significant dynamic fragility value completely randomized to an isotropic state. This has an impact on the that is much larger compared to that of its molten PEEK-M counterpart. fragility: the non-crystalline fibers constitute a one-dimensional

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

Fig. 5. FSC heating scans at 2000 K/s after cooling PEEK at different rates. Endothermic heat flow is represented by downward deflection from the base- line. a) Melt electrospun PEEK fibers (PEEK-F) which have been cooled from 200 °C. Cooling rates vary from 200 K/s to 3000 K/s; b) PEEK-M, generated by heating of the same sample shown in (a) to 350 °C. Cooling rates vary from 140 K/s to 8000 K/s. Sample size was 122 ng. Heat flow signal from the empty sensor 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 logarithm of 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 data sets.

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