Electrospinning of a blend of a liquid crystalline polymer with poly ( ethylene oxide ) : Vectran nano fi ber mats and their mechanical properties

Vectran is a liquid crystalline polymer (LCP) with remarkable specific properties in its commercial microfiber form. Even though it has been widely studied in the last few decades, there have been no reports in the literature on Vectran nanofibers production. Due to the insufficient spinnability of Vectran, a “host–guest” method with poly(ethylene oxide) (PEO) as a host polymer is used in the present work to produce continuous and uniform nanofibers of Vectran–PEO blends. Subsequently, a heat treatment is applied and optimized to remove PEO and convert the amorphous Vectran–PEO nanofibers into more ordered and mechanically improved pure Vectran nanofibers. The conclusions are supported by scanning electron microscopy, thermal analysis, selected area electron diffraction (SAED) patterns and mechanical characterization of electrospun Vectran nanofiber mats after removal of PEO.


Introduction
Liquid crystalline polymers (LCPs) represent a class of polymers that are well known for their excellent mechanical properties, thermal and chemical resistance, and low density, which result in exceptional specic properties. 1 Unlike conventional polymers, they crystallize from an ordered and oriented molecular phase intermediate between an isotropic liquid phase and a crystalline solid, or amorphous glassy phase.
Ordered liquid crystalline phases are mainly classied as nematic, cholesteric and smectic.3][4] The nematic liquid crystal, N phase, possesses a longrange orientational order but only a short-range positional order.Nematics are the most important members of the liquid crystal family and are widely used in the display industry. 3,5,6he cholesteric phase is similar to the nematic phase on a local scale.Like the nematic phase, it can be described by a molecular alignment with respect to a direction towards which all the molecular orientations will be biased, at least locally (namely, a director).However, the direction in the cholesteric phase is twisted about an axis normal to the molecular orientation, following a helical path. 3,5,6Smectic phases are characterized by layered structures, with additional order being possible in each layer.Within the layers, the centers of molecules are arranged in equidistant planes.The planes are allowed to move perpendicular to the layers, and, within the layers, different arrangements of molecules are possible.The modications of the smectic phase are labeled according to the arrangement of the molecules within the layers. 3,5,6There are also other liquid crystalline phases (e.g.cubic, hexagonal, lamellar, columnar).Since they are not frequently present in thermotropic liquid crystals, they will not be detailed in this study. 4mong the main uses of LCPs, the following applications should be mentioned: production of high precision moldings for use in the electronics industry, multi-way electrical connectors, components of printers and disk drives, transformer bobbins and encapsulation for surface-mounted silicon chips. 3LCPs can be broadly classied into three classes: (a) aromatic polyamides, (b) aromatic heterocycles, and (c) aromatic copolyesters. 1Aromatic polyamide bers, commonly known as aramid bers, are obtained from polyamides containing aromatic rings along the main chain, and the most known commercial products are KevlarÒ and TwaronÒ.Aromatic heterocyclic polymers are lyotropic materials and are characterized by wholly aromatic molecular structures with fused heterocyclic rings along the main chain; PBI and ZylonÒ are two examples in this category.Aromatic copolyester polymers possess thermotropic behavior and are characterized by a molecular structure with a high degree of linearity and rigidity that allow formation of ordered phases over a wide temperature range; among them, three examples are VectraÒ, XydarÒ and EkonolÒ. 1 The melt-spun Vectra-based ber, commercially known as Vectran, is superior to aramid bers in several ways: it is highly resistant to creep, it resists ex or fold fatigue and abrasion, and it has better long-term resistance to UV degradation. 3,5hen diameters of polymer bers are reduced from micrometers to a few hundred nanometers, several attractive characteristics may be induced. 7Some of the characteristics area very large surface area to volume ratio (this ratio for nanobers can be 10 3 times larger than that of microbers), exibility in surface functionalities, change in the crystalline structure, 8 and superior mechanical performances (e.g.stiffness and tensile strength). 7There are several previous studies regarding nanobers forming from LCPs [e.g.0][11][12][13] On the other hand, to the best of our knowledge, there have been no reports on the formation of Vectran nanobers.Electrospinning 12,[14][15][16] seems to be one of the best possible routes to form such nanobers.In the electrospinning process, an electric eld of strength about 1 kV cm À1 is applied to a needle through which polymer solution is delivered.When the applied electric eld overcomes surface tension and the viscoelastic forces in the droplet pendent or sessile at the needle exit, a charged jet of the polymer solution is ejected.The jet undergoes the electricallydriven bending instability which stretches it dramatically, while the solvent evaporates. 16As a result, solidied nanobers are formed and are deposited on a solid collector, which is a grounded counter-electrode.][16] Electrospinning of liquid crystalline aromatic polyamide poly(p-phenylene terephthalamide) (PPTA), commercialized under the trade name Kevlar 49Ò (Dupont), was demonstrated in the seminal work of Reneker's group. 12They collected nanobers on the free surface of water in a grounded bath where PPTA solution in sulfuric acid precipitated, and solidied bers were formed.Acetoxypropyl cellulose (APC), a lyotropic cholesteric LCP, was successfully electrospun as microbers ($2.75 mm) using a standard electrospinning setup with a solid counter-electrode collector. 17Liquid crystal elastomers (LCE) were electrospun as microbers with an average diameter of 1.5 mm from a solution of photocross-linkable polymer using a conventional electrospinning apparatus. 13Only beaded bers were obtained, and cross-linking was conducted using UV light as a post-processing step.Also, the relationship between the diameter of electrospun LCP bers [BB-5(3-Me)] and their internal structure and molecular orientation was studied. 10he aim of this work is to electrospin liquid crystalline polyester (Vectran) or its blends with PEO as nanobers, and characterize their crystalline and mechanical properties prior to, and aer the removal of PEO.Vectran nanobers would open an avenue for applications of this popular liquid crystalline polymer in ultra-strong composites.It is emphasized that due to the above-mentioned reasons, none of the previously reported LCP nanobers and/or microbers can compete with Vectran in achieving ultra-strong nanobers.
In Section 2 the materials and methods to produce Vectran nanobers are described, as well as the instrumentation used and the thermal treatment procedure are detailed.In subsequent Section 3 the main properties of the resulting nanobers are analyzed and discussed.Also, the inuence of the optimized thermal treatment on the nanobers is investigated using various characterization methods, and their tensile strength is tested.In Section 4 the conclusions are drawn.

Materials
Kuraray supplied the as-spun Vectran LCP yarn used in this work under the trade name of Vectran NT having a linear density of 750 denier and 150 laments per yarn.Vectran is a copolymer of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphtonic acid (HNA) with a molar ratio of 73/27, respectively. 18,19Vectran NT has a molar mass of 290.27 Da and a molecular weight higher than 20 kDa. 20The bers possess an almost circular cross-section with an average diameter of 25.5 AE 2.1 mm. 21Poly(ethylene oxide) (PEO, molar mass 44.05 Da, M w ¼ 600 kDa), chloroform (>99.8%) and pentauorphenol (PFP, >98%) were purchased from Sigma-Aldrich.All materials were used as received without any further purication.

Sample preparation
Vectran solution (1 wt%) was prepared by dissolving 0.2 g of LCP in 20 g of a mixture of two solvents, chloroform and pen-tauorphenol, with a 70/30 ratio by weight, respectively.Stirring for 10 h at room temperature resulted in a yellow-clear solution.PEO solution (1.85 wt%) was prepared by dissolving 0.37 g of PEO in 19.63 g of chloroform under magnetic stirring for 5 h.Then, the Vectran solution was mixed with the PEO solution at a ratio of 15 : 1 by weight, respectively.The as-spun nanobers produced in the present work possess the Vectran-PEO weight ratio of 8.19 : 1, correspondingly.

Electrospinning process
To electrospin Vectran or its blends with PEO, a conventional electrospinning setup was used, with aluminum foil used as a collector. 22The controlled power generator employed was a Glassman High Voltage model EH, that can generate a DC voltage in the range 0-30 kV.A pump NE-1000 from New Era Pump Systems, Inc. was used to ensure a continuous supply of polymer solution during the process.The voltage was set at 10 kV at a solution ow rate of 1.5 mL h À1 , and the distance between the collector and the needle was kept equal to 10 cm.The samples obtained were dried for 24 h at room temperature.

Heat treatment
In the past two decades annealing on liquid crystalline polymers was widely studied, but never attempted on LCP nanobers.The annealing consisted of increasing the temperature of the nanobers close to the melting temperature, and keeping them under these conditions for a pre-determined amount of time.4][25][26][27] The annealing was used to enhance the mechanical properties of Vectran, as well as its melting temperature.The thermal treatment was performed in an oven in air in the temperature range 250-300 C, and the treatment duration was in the range 15-24 h.

Thermal analysis
Differential scanning calorimetry (DSC) measurements were performed using a Mettler DSC 30 Low Temperature Cell and a Mettler TC 15 TA Controller.The heating rate was 10 C min À1 under a nitrogen ux of 100 mL min À1 .
Thermogravimetric analysis (TGA) was applied, and measurements were performed using a modulated TGA Q5000IR by TA Instrument.The heating rate was 10 C min À1 under a nitrogen ux of 25 mL min À1 .

Mechanical analysis
Electrospun nanober mats were cut into rectangular specimens measuring 4 Â 30 mm 2 and mounted on window-like holders with a gage length of 20 mm.Tensile tests were conducted on 15 as-spun mat specimens.Also a sample (consisting of 13 heat-treated specimens) was subjected to the tensile test.The tensile tests were conducted using a universal testing machine (Instron, model 4502) with a 10 N load cell (Instron, model 2518-808, with a nominal accuracy of 0.025% of the load cell full scale) at a cross-head speed of 5 mm min À1 .

Observations
Scanning electron microscopy (SEM) images were obtained using JEOL JSM-6320F and Supra 40 Zeiss microscopes under high vacuum and secondary electron detector operating mode.Transmission electron microscopy (TEM) was performed using a JEOL JEM-3010 microscope.Optical observations were done using an Olympus BX51 microscope in refraction mode.
The selected area electron diffraction (SAED) method is similar to X-ray diffraction but uses electrons rather than X-rays.Because of that, the examined region can be as small as a few nanometers.SAED images are obtained by using an aperture in the virtual image plane of the microscope to select a region of interest such as an individual nanober.Only the electrons falling inside the dimensions of the aperture will be analyzed.The resulting scattered electrons are then imaged in the diffraction mode of the microscope.The circular pattern that appears is in essence a two-dimensional scattering pattern.This two-dimensional nature gives extra information about the orientation of the lattice scattering planes compared to a onedimensional X-ray diffraction pattern. 28

Results and discussion
Prior to trials of electrospinning of Vectran and PEO blends, electrospinning of pure Vectran solution was attempted.The results are shown in the SEM images of Fig. 1.It is seen that using electrospinning of pure Vectran solution, continuous and uniform nanobers could not be achieved.This result stems from different factors.The rigid molecular structure of Vectran results in a limited viscoelastic behavior, which is the primary condition for spinnability in electrospinning [14][15][16] The situation is worsened because of the tendency of LCP to form aggregates aer a fast evaporation of one of the solvents present in the Vectran solution (chloroform). 9The insufficient spinnability results in rupture of electrospun Vectran jet producing a mix of dried beads and non-continuous bers of varying diameters and shapes (cf.Fig. 1).Several different Vectran solutions with concentrations in the range 0.5 to 2 wt% have been used in these experiments, as well as different ratios of chloroform/PFP were attempted.However, in all cases only results similar to those shown in Fig. 1 were observed.
The use of spinnable additives, a procedure known as the host-guest approach, 29 became necessary in order to achieve good quality Vectran-based nanobers.PEO was chosen as a "host", since it is a exible high-molecular-weight polymer widely used to enhance viscoelasticity and spinnability of the solution.Moreover, PEO is also soluble in chloroform, one of the solvents used in the Vectran solution.Due to its high molecular weight, a sufficient spinnability was achieved even at relatively small concentrations of PEO (1.85 wt%).This facilitates the subsequent removal of PEO from nanobers without changing their morphology as shown in the next sections.
The PEO solution was added to the Vectran solution in different ratios, with 1 : 15 being the most effective, and the resulting nanobers had a Vectran-PEO ratio of 8.19 : 1.In Fig. 2(a) continuous nanobers electrospun from the PEO-Vectran blend are shown.Their average diameter is 195 nm and the molar ratio of Vectran-PEO is 1.25.
Aer electrospinning of PEO-Vectran solutions, the resulting nanober mats were dried to evaporate chloroform completely.Aer that, only PFP, PEO and Vectran remain in the bers.In previous works dealing with Vectran solutions, dichloromethane (DCM) was used to extract PFP from the lm produced. 30Due to the presence of PEO, DCM could not be used in the present work.
In order to remove PEO aer ber formation, the nanober mats were immersed for 24 h into a bath containing ethanol and water (cf.SEM images in Fig. 2).In Fig. 2(b) the Vectran bers seem to be swollen in comparison to Fig. 2(a), partly due to the expected dissolution of PEO aer the immersion into the water-ethanol bath, and partly due to the plasticizing effect stemming from water absorption.The latter means that water remains entrapped between the Vectran and the remnant PEO macromolecules in the nanobers causing an increase in the ber diameter.Despite the reduction of the PEO content in the nanobers, the immersion into the water-ethanol bath does not completely eliminate PEO from the nanobers.Therefore, instead of using a solvent-based PEO removal procedure, a heat treatment was used to ensure complete removal of PEO from nanobers, as well as to enhance their thermal stability and mechanical properties.In Fig. 2(c) the heat-treated nanobers, which were not immersed into the water-ethanol bath, are seen to be continuous and uniform.On the other hand, in Fig. 2(d) the nanobers, which were heat treated aer the immersion into the water-ethanol bath, look conglutinated.Fig. 2(b) demonstrates that the ber mat swells aer the immersion into the water-ethanol bath, and then, perhaps, partially melts during heat treatment.As a result, nanobers are additionally sintered, with the average diameter increasing to about 1 mm.
To demonstrate that swelling was due to the presence of PEO and not related to Vectran itself, the same immersion procedure was applied to electrospun mats aer heat treatment.Fig. 2(e) shows that, indeed, no swelling is visible in the heat-treated Vectran nanober mat.

Thermal analysis
To check whether the heat treatment resulted in a complete removal of PEO or not, both TGA and DSC techniques have been adopted.The partial dissolution of PEO present in the nano-bers aer the immersion into a water-ethanol bath was also conrmed by TGA analysis.Degradation of PEO used in this work is expected in the range from 210 C to 410 C. 31 TGA curves reported in Fig. 3 show that the non-treated nanobers that were not immersed into the water-ethanol bath lose 14.56% of their weight at 410 C, while non-treated nanobers immersed into the bath lose 6.76% in weight at the same temperature.Also, the heat-treated nanobers that were immersed into the water-ethanol bath lose 1.46%, and the heattreated nanobers that were not immersed into the bath lose 1.38%.This slight difference in the weight loss could be explained by the different morphologies of the two mats, namely, a conglutinated morphology aer the immersion and non-conglutinated nanobers without immersion.Conglutination results in different surface areas and different amounts of energy required for the Vectran degradation.The commercial Vectran NT shows a higher weight loss (3.31%) in comparison with the treated mats, as the results in Fig. 3 show.This higher loss is probably related to the size effect of the commercial bers, and also to the lower thermal stability of Vectran NT without the annealing treatment.Our results conrm that the immersion alone is not enough to completely remove PEO from LCP nanobers.However, aer the heat treatment there are no traces of residual PEO in the nanobers.It can be seen that immersion followed by heat treatment resulted in more conglutinated bers with a larger diameter, and requires additional treatment steps.That is why the immersion stage was completely excluded and the nanobers of the Vectran-PEO blend were only heat treated to eliminate PEO.
Vectran begins loosing weight at 460 C when the rst vibrations of the aromatic rings occur.These vibrations continue approximately up to 500 C, which is about 40 C  below the temperature of the maximum weight loss of Vectran.From 510 C onward there are C-O, O-H and C-H vibrations causing the actual degradation of Vectran.According to the existing scientic literature, the total degradation of Vectran completes at about 800 C. 6 Even though TGA analysis conclusively proves a complete removal of PEO from the blended bers, a question that still remains is how the heat treatment has inuenced the crystalline structure of the electrospun composite nanobers.The untreated commercially available bers show two wide endothermic peaks (Tp 1 , Tp 2 ) [Fig.][25] Aer heat treatment of commercial bers, a new endothermic peak (Tm 1 ) appears, which overlaps with the two endothermic peaks (Tp 1 , Tp 2 ) present in the untreated bers, and becomes a new melting temperature for such bers. 18,23,27As reported in the previous studies, Tm 1 could be related to the occurrence of inter-chain trans-esterication reactions. 18,23,27ote that initially the annealing of nanobers was performed in air at 300 C for 15 h to mimic a route previously used to anneal Vectran microbers. 27On a visual inspection, nano-bers showed a typical color change.However, the observations with an optical microscope revealed that the nanober mat has conglutinated and lost its brillar structure.This can be attributed to the fact that due to the smaller diameter of nanobers, they have a higher surface area/volume ratio in comparison with microbers.As a result, they melt at a temperature lower than the macro-and microscopic samples of the same polymer, as it was previously demonstrated with other nanobers. 26In order to avoid nanober melting and conglutination, the heat treatment temperature was reduced and its duration increased.This annealing procedure was conducted at 250 C during 24 h in air.The nanobers showed the expected color change, whereas the optical microscopy observations did not reveal any loss in their brillar structure.
In Fig. 4(b) DSC traces of Vectran-PEO electrospun nano-bers are shown for two cases, before and aer heat treatment.The DSC traces of the untreated Vectran nanobers show an endothermic peak at 61.4 C.This peak is related to the melting point of PEO (66 C), 31 conrming once again its presence in nanobers before heat treatment.The Tp 1 peak found in the untreated microbers is signicantly smoothened out, and the Tp 2 peak has disappeared.These peaks are related to a change in the crystallographic phase of Vectran, and their smoothening could be explained by the presence of PEO, which does not allow the orthorhombic to nematic transitions in the LCP.
Aer the heat treatment, the peak known as Tm 1 appears in the DSC traces of the nanobers.Also, one of the two endothermic peaks (Tp 2 ) characteristic of Vectran microbers appears, indicating the presence of a crystalline structure and a possible orthorhombic to nematic transition.The peak related to PEO is not present anymore, which conrms once again its complete elimination from the nanobers.
It is emphasized that the Tm 1 peak does not overlap with the two endothermic peaks as it happens in the commercially available microbers subjected to a similar heat treatment [Fig.4(a)].The temperature corresponding to the Tm 1 peak in the nanobers is 272.2C, which is lower by 38.7 C than the corresponding peak in the microbers.According to the previous studies, this phenomenon could be explained mainly by the following three factors: (i) a higher surface area/volume ratio in comparison with microbers; (ii) the plasticizing effect due to a residual solvent; and (iii) modication of the crystalline structure as a result of rapid solidication of polymer solutions in electrospinning. 26The hypothesis of residual solvent can be discarded since TGA and DSC analyses did not show any traces of it.Vectran is a liquid crystalline polymer, which means that it does not loose its crystalline structure in the liquid phase.From the SAED pattern, to be shown and discussed later, it could be seen that the presence of PEO, causes the loss of the Vectran crystalline structure before heat treatment.It will also be shown that although aer the heat treatment the nanobers demonstrate an increase in their crystalline orientation, their crystalline structure is still not absolutely identical to the commercial Vectran bers.The latter possess a higher order in their crystalline structure due to the production process.Therefore, the modication of the crystalline structure due to the fast evaporation of solvents stems from the higher surface area/ volume ratio in nanobers in comparison with microbers.Although the thermal analysis clearly indicates the change in the crystalline order, the nal clarication can only be obtained through the experimental evaluation of d-spacing in the nano-bers.It should be mentioned that instead of wide-angle X-ray scattering (WAXD) on nanober mats, selected area electron diffraction (SAED) on a single nanober was preferred.3][34][35] Only then it could be concluded that every single nanober underwent the lateral packing of crystalline structure.SAED was performed on both the untreated and heat-treated Vectran nanobers, and representative images are shown in Fig. 5.It can be seen from Fig. 5(a) that the SAED pattern of an untreated nanober does not show any sharp ring.The presence of a diffused halo clearly indicates that the existence of PEO had hampered lateral packing thus yielding an amorphous structure.][34][35] In Fig. 5(b) the SAED patterns show a strong meridional reection and one strong equatorial reection.From the calculation of the d-spacing it was found that the d-spacing corresponding to the meridional and equatorial reections was 2.2 Å and 4.7 Å, respectively.From ref. 25 it can be found that in Vectran there should be three meridional reection at the d-spacings $6.73 (m1), 3.06 (m2) and 2.07 (m3) Å.However, as ref. 25 shows, the intensity of m1 and m2 is quite low and if they were visible in the SAED pattern, they would have been near the center.The electron beam is so bright that in spite of using a beam stopper, the CCD camera was completely blinded near the center, which makes observation of m2 and m1 impossible.The d-spacing value clearly shows that the meridional reection corresponds to m3. 31 Besides the d-spacing, the equatorial reection also shows that it corresponds to the 110 plane. 31The latter, in conjunction with the thermal analysis, conclusively points out that annealing not only destroyed PEO but also improved the lateral order of the random crystal orientation.Also, the random sequences were crystallized into ordered crystals with higher melting temperatures.The increase in the thermal stability aer annealing is strongly related to the enhancement of structural order. 35The existence of an almost circular m3 ring (with a mix of bright and fading parts) in Fig. 5(b) instead of the arches seen in ref. 25, clearly shows that in our case the crystallites responsible for the m3 ring are not absolutely aligned with the nanober axis.However, it can be seen from Fig. 5 that m3 is brightest in the longitudinal direction of the nanobers, and the bright 110 plane is almost at the right angle to it, which appears to be the preferential direction of the crystallite orientation.

Mechanical analysis
Heat treatment is intended not only to remove residual PEO and improve thermal stability of Vectran, but also to increase mechanical properties of Vectran nanobers.According to the previous studies, the mechanical properties of Vectran micro-bers are markedly different before and aer heat treatment. 23,27,36Our data in Table 1 show that the heat treatment of Vectran nanober mats increased tensile strength by 334% and the elongation at break by 158%.On the other hand, the tensile modulus of the mats does not change signicantly, which is similar to the results obtained aer annealing of Vectran microbers. 23,27,36he improvement in the tensile properties of Vectran aer a heat treatment has generally been attributed to an increase in the molecular weight. 23,24g. 5 SAED pattern for (a) Vectran nanofibers before, and (b) after heat treatment.The camera distance was 40 cm in both cases.It is emphasized that PEO initially present in the nanobers before the heat treatment acts as a plasticizer and results in ductile behavior of the untreated mats (Fig. 6).However, aer the annealing when PEO has been completely removed, the nanober mats become much stiffer (Fig. 6), which was the main goal of this work.Although, the mat strength is smaller than that for individual heat-treated Vectran microbers [1.52 AE 0.12 GPa], 27 one should keep in mind that tensile strength of nanober mats is always much lower than that of an individual nanober. 14,37The main reasons are that (i) nanobers in electrospun mats are randomly oriented and cannot be deformed only along their main axis as is commonly done when single microbers are tested; (ii) the real crosssection of electrospun mats is difficult to be assessed and an average value is generally considered by measuring the external dimensions with a caliper; (iii) while measuring tensile strength of the nanober mats, we also inevitably measure effective strength of the inter-ber bonds, since in the electrospun mats bers are ill-entangled; (iv) electrospun nano-bers can possess signicant porosity due to solvent evaporation and PEO removal.Overall, the factors listed as (i) to (iv) yield mats with a much lower tensile strength than individual nano-and microbers.

Conclusions
Pure Vectran solutions possess insufficient spinnability to produce continuous and uniform nanobers using electrospinning.The use of a host-guest approach with the host being a high molecular weight PEO was employed to improve spinnability of Vectran.Then, continuous and uniform nanobers with an average diameter of 195 nm were formed.
In order to remove the remnant PEO, water-ethanol immersion was tried.It removed about 8% of the remaining PEO but led to nanober swelling.Therefore, the immersion stage was found to be inefficient and was avoided.Instead, PEO was eliminated by heat treatment.The heat treatment of Vectran microbers was optimized at 250 C for 24 h in air.TGA and DSC analyses conrmed the complete removal of the remaining PEO and solvents aer heat treatment.Also, DSC showed that PEO changes the crystalline structure of as-spun Vectran-PEO nanobers making them an almost amorphous material without crystalline orientation.However, aer heat treatment an endothermic peak (Tp 2 ) related to a formed crystalline structure was observed using DSC.Furthermore, SAED conrmed the absence of an ordered crystalline structure before heat treatment and a low degree of crystallinity and a welloriented structure in the Vectran nanobers aer heat treatment.
Finally, the annealing of Vectran nanober mats increases thermal stability due to the enhancement of the structural order.Correspondingly, tensile tests show a signicant improvement of the mechanical properties.The mechanical behavior in tensile tests changed from relatively so to stiff, and the elongation at break has also increased aer PEO was eliminated.

Fig. 3
Fig.3Comparison of TGA data of the Vectran-PEO mat, Vectran-PEO mat after immersion into a water-ethanol bath, Vectran-PEO mat after heat treatment, and Vectran NT (commercial).

Fig. 4
Fig. 4 DSC traces comparing (a) commercial Vectran microfibers before and after heat treatment, and (b) Vectran nanofibers before and after heat treatment.

Table 1
Tensile properties of heat-treated and untreated Vectran nanofiber mats