polymers

Article Melt-Electrospun Polyethylene Nanofiber Obtained from Polyethylene/Polyvinyl Butyral Blend Film

Mohammad Zakaria 1,2 , Kanta Shibahara 1 and Koji Nakane 1,*

1 Frontier Fiber Technology and Science, University of Fukui, Fukui 910-8507, Japan; [email protected] (M.Z.); [email protected] (K.S.) 2 Department of Textile Engineering, Dhaka University of Engineering & Technology, Gazipur 1700, Bangladesh * Correspondence: [email protected]



Received: 13 December 2019; Accepted: 12 February 2020; Published: 16 February 2020


Abstract: We prepared low-density polyethylene (LDPE) nanofiber, a few hundred nanometers in diameter, using polyvinyl butyral (PVB) and a laser melt-electrospinning (M-ESP) device. We blended PVB with LDPE via an internal melt mixer, removed the PVB after M-ESP by ethanol treatment, and studied the influence of PVB on fiber diameter. A substantial diameter reduction with improved crystallinity of LDPE fiber was observed with increased PVB content in the blend. PVB inclusion also increased the polarity of the LDPE/PVB blend, resulting in better spinnability. The removal of PVB from LDPE/PVB blend fiber caused a massive drop in the LDPE fiber diameter, due to fiber splitting, particularly in PVB-rich samples. Fourier transform infrared (FTIR) spectroscopy of fibers confirmed that the prepared nanofiber was the same as pure LDPE fiber.

Keywords: polyethylene; nanofiber; melt-electrospinning; crystallinity; blends

1. Introduction Electrospinning is an electrohydrodynamic process that draws super-fine continuous fibers using electrostatic force to collect the liquid jet from a polymer solution or melt. This process is widely used to fabricate nanofibers from 50 or more varieties of polymers, due to its simplicity and suitability [1,2]. It has two primary divisions, i.e., solution and melt-electrospinning [2]. Solution electrospinning is an effective and versatile technique [3]. However, the melt-electrospinning (M-ESP) system ensures a safer environment and avoids the use of solvents, including their removal or recycling. M-ESP also produces an extreme loss of mass during solvent evaporation, which ensures better surface smoothness and mechanical properties of the fiber [4]. Moreover, polyolefins such as polypropylene (PP) and polyethylene (PE) have no appropriate solvents at room temperatures [5]. Thus, M-ESP is considered to be a more ecological, productive, cost-effective, and safer alternative to solution electrospinning [5–7]. However, relatively few studies have been conducted on electrospinning directly from polymer melts [4–9], owing to the high-temperature setup needed for M-ESP systems and the low conductivity and high viscosity of polymer melts. In addition, M-ESP manufactures relatively coarser fiber than solution electrospinning [4]. Using a CO2 laser heating source, we introduce a new electrospinning technique that simplifies the high-temperature setup of M-ESP [10]. Our laser melting system eliminates the need for a conventional reservoir for molten polymer, resulting in nozzle-free efficient electrospinning with lower energy consumption [11–14]. The thermal degradation of the polymer melt caused by long-term heating is also avoided by highly controlled and localized laser heating [15]. Furthermore, to improve polymer conductivity and reduce melt viscosity, multi-component systems such as blends [13,16] and additives [9,17] have been employed for M-ESP with limited success.

Polymers 2020, 12, 457; doi:10.3390/polym12020457 www.mdpi.com/journal/polymers Polymers 2020, 12, 457 2 of 12

Polyethylene (PE) has some outstanding characteristics, including chemical resistance, ease of processing, near-zero moisture absorption, toughness, electrical properties, low friction coefficient, and abrasion resistance, thus ensuring that it constitutes the world’s largest polymer volume consumption [18][19,20]. However, the low strength and thermal conductivity of bulk PE greatly hinder its functionality [18]. Instead, a significant enrichment of the mechanical and thermal resistivity of polymers has been reported by the proper alignment of polymer chains [21,22]. Therefore, the stretching of PE from bulk into thin films, or micro- or nanofibers, improves the polymer chain arrangement and results in improved mechanical and thermal properties [23,24]. Moreover, in the case of polymeric fiber, diameter conversion from micrometers to nanometers tremendously enhances the specific surface area, producing an excellent smooth covering against ultra-fine particles and microorganisms [2]. Sheng et al. have reported ultra-drawn high-quality PE nanofibers with a diameter of only 50–500 nm, using the two-stage heating of polymer gel [18]. Moreover, PE nanofibers around 300 nm in diameter have been produced by solution-type electrospinning via a high-temperature solution and rotating collector [25]. Li et al. [26] introduced a two-stage solution-gel heating method and developed single PE nanofibers only a hundred nanometers in diameter. Again, however, there have been few attempts to produce PE nanofiber using M-ESP. Rongjian et al. demonstrated a simple M-ESP device but were unable to produce PE fiber less than 5 µm [4] in diameter. No PE fiber less than 1 µm in average diameter has yet been reported, using M-ESP. In our laboratory, nanofibers were successfully obtained using the laser M-ESP system from various polymers with polar groups, namely ethylene-vinyl alcohol copolymer (EVOH) [11,13,14], polylactic acid (PLA) [10], and nylon 6/12 [14]. However, nanofiber could not be obtained from a single polymer substance without a polar group, such as PP, PE, etc. [17], although we have succeeded in fabricating PP nanofiber from PP/EVOH [27] and PP/PVB [28] blends. Polymer density is the main classifier of PE grades, and the first PE grade developed was low-density polyethylene (LDPE). LDPE is an excellent amorphous polymer with a relatively low melting point and good flow behavior, due to long side-chain branching [20]. As a result, it has advantages in the fabrication of a thin fiber using the laser melting process. Polyvinyl butyral (PVB) is a non-toxic, odorless, and environmentally friendly random amorphous copolymer [29–31], which is a very useful material in the electrospinning process [32]. PVB has been applied to potentially improve the spinnability of polymer solutions [33–35] and melts [28]. Accordingly, PVB could be a fruitful aid for PE nanofiber preparation that uses a laser melting device. The present work aims to produce low-density PE nanofibers from LDPE/PVB blended films using our line-like CO2 laser beam M-ESP device. In particular, we investigated the effect of PVB content on fiber spinnability, crystallinity, and diameter.

2. Experimental

2.1. Materials A powder form of the blend PVB component, grade B60HH, comprised of 80–87% vinyl butyral, 3 12–16% vinyl alcohol, and 1–4% vinyl acetate, with a density (ρ) of 1.12 g/cm , melting point (Tm) of 140 ◦C, and a melt flow rate (MFR) of 14 g/10 min, was a gift from the Kuraray Co. Ltd., Tokyo, Japan. 3 LDPE with MFR: 13 g/10 min, Tm: 105 ◦C and ρ: 0.919 g/cm was the kind gift of the Tosoh Corp., Japan. Ethanol (99.5%) was also procured from the Nacalai Tesque, Inc., Kyoto, Japan.

2.2. Preparation of Blend Film Various specimen blends were prepared with a kneading extruder (IMC-A300, Imoto Machinery Co., Ltd., Kyoto, Japan), by adding 10, 30, 50, 70, and 90 weight-% (wt.%) PVB to LDPE. Melt blending was conducted under N2 gas flow with a screw speed of 50 rpm at 190 ◦C for 10 min. Hereafter, film blend will be abbreviated as follows: a blended film comprising 50 wt.% PVB will be designated, Polymers 2020, 12, 457 3 of 12

PVB-50. A hot-press (Gonno Hydraulic Press Manufacturing Co., Osaka, Japan) was employed at 190 C with a pressure of 25 MPa to fabricate LDPE/PVB sheets of 120 mm 60 mm 0.2 mm for ◦ × × preparation of nanofibers by laser M-ESP. Pure PVB and LDPE films were also made for comparison.

2.3. Formation of Fibers Using Laser M-ESP From LDPE/PVB blended film, fibers were fabricated by the M-ESP system equipped with a line-like CO laser melting device, at a room temperature of 25 2 C, and 68% 2% relative humidity; 2 ± ◦ ± further details regarding the equipment were provided in our previous paper [27]. The blended film was automatically fed at a speed of 4 mm/min into the laser melting zone, where it was melted at around 200 ◦C by the laser beam. A high voltage was supplied to the copper electrode slit and laser power of 4.0 kV/cm and 40 W, respectively. A copper anode plate (150 mm 150 mm 5 mm) × × was placed 100 mm away from the electrode, and all electrospun fibers were collected on its surface. PVB was removed from as-spun LDPE/PVB blended fibers by ethanol treatment for 6.0 h at room temperature. For comparison, pure LDPE and PVB fibers were also prepared from the pure films following the same procedure.

2.4. Characterization Differential scanning calorimetry (DSC) of produced films and fibers was carried out using DSC-60 (Shimadzu Corp., Kyoto, Japan). Samples were heated from 30 to 200 ◦C, held for 5 min, and cooled to 1 30 C at a rate of (+) 10 C and ( ) 4 C min , respectively. The crystallinity (Xc(%)) of LDPE was ◦ ◦ − ◦ − calculated from the DSC results using the following equation [36]:

∆H ( ) = m Xc % o 100. (1) ∆Hm ωPE × × o where ∆Hm is the experiential melting enthalpy of LDPE in the samples, ∆Hm is the melting enthalpy 1 of 100% crystallized PE (288 J g− [37]), and ωPE is the weight fraction of LDPE in the compounds. The rheological properties of samples were measured using a modular compact rheometer (MCR 302, Anton Paar, Graz, Austria). A parallel plate, 25 mm in diameter and with a gap of 1 mm was used. To determine the linear viscoelastic region, strain sweep experiments were conducted at a frequency 1 1 of 1 rad s− . Then, frequency sweep tests were carried out between 0.1 and 100 rad s− at a strain amplitude of 5%. All tests were performed at 190 ◦C under a nitrogen atmosphere. Disc polymer samples, 25 mm in diameter and 1-mm thick, were prepared using a hot press for tests. Scanning electron microscopy (SEM) of fibers and film surfaces was carried out using a Keyence microscope (VE-9800; Keyence Co. Ltd., Osaka, Japan). An ion coater (SC-701; Sanyu Electron Co., Ltd., Tokyo, Japan) was used to gold-sputter coat the sample surface before SEM observation. Image analysis software (Photo Ruler) was used to measure the average and standard deviation of the PVB particle size in blend films and fiber diameters (indicated by D and σ), respectively, from the SEM images. Measurements were conducted for 3–5 SEM images of different areas of a sample, and more than 100 fibers were employed. Additionally, histograms were prepared for all individual sets of evaluations. The Fourier transform infrared (FTIR) spectrum was measured at room temperature within a range 1 of 400–4000 cm− using an IR spectrometer (IR Affinity-1; Shimadzu Corp., Kyoto, Japan) equipped with a diamond/ZnSe crystal embedded single reflection ATR accessory (MIRacle 10; Shimadzu Corp., Kyoto, Japan). The characteristic band for pure PVB, pure LDPE, blended LDPE/PVB, and PVB-removed LDPE fibers were recorded. Polymers 2020, 12, 457 4 of 12

3. Results and Discussion

3.1. Structural Analysis of LDPE/PVB Blend Films DSC measurements of the LDPE/PVB blend films verified the melting behavior of the compounds. Figure1a contains DSC heating scans of LDPE /PVB blend films. Incremental increases of PVB fraction Polymers 2020, 12, x FOR PEER REVIEW 4 of 12 in blends caused a reduction of endothermic enthalpy (∆Hm) of LDPE. Throughout the heat scans of theLDPE heat/PVB scans blends, of LDPE/PVB this influence blends, is only this frominfluence the crystalline is only from polymer the crystalline LDPE. As polymer a result, LDPE. the low As ratios a of LDPE in the blends ensured the decline of ∆Hm. In other words, relatively less energy was required result, the low ratios of LDPE in the blends ensured the decline of ∆𝐻. In other words, relatively less energyto melt was the compoundrequired to withmelt the higher compound PVB content. with hi Thus,gher PVB the blending content. ofThus, PVB the with blending LDPE resultedof PVB with in the LDPEeasy internal resulted dispersion in the easy and internal thermal dispersi processingon and ofthermal the compounds. processing of the compounds.

FigureFigure 1. 1. (a()a )Differential Differential scanning scanning calorimetry calorimetry (DSC) (DSC) heating heating curve curve of oflow low density density polyethylene/ polyethylene / polyvinylpolyvinyl butyral butyral (LDPE/PVB) (LDPE/PVB) films; films; and and (b (b) )crystallinity crystallinity (%) (%) of of LDPE LDPE in in LDPE/PVB LDPE/PVB blends. blends.

FromFrom the the DSC DSC results, results, the the crystallinity crystallinity (%) (%)of LDPE of LDPE was wascalculated, calculated, as shown as shown in Figure in Figure1b. The1 b. additionThe addition of PVB of PVBgreatly greatly enhanced enhanced the crystallinity the crystallinity of LDPE. of LDPE. Without Without PVB, PVB, the the observed observed crystallinity crystallinity ofof LDPE LDPE was was 26%; 26%; the the inclusion inclusion of of PVB PVB raised raised this this percentage, percentage, and and the the highest highest LDPE LDPE crystallinity crystallinity (35%)(35%) waswas foundfound for for PVB-70. PVB-70. However, However, the the maximum maximum PVB PVB content content (90%) (90%) resulted resulted in a slight in a reduction slight reductionof crystallinity of crystallinity (27%). Reza (27%). Zanjanijam Reza Zanjanijam et al. [et36 al.] found [37] found the surface the surface of the of the PVB PVB domain domain acts acts as a asnucleating a nucleating agent agent for PP, for which PP, which improved improved the structure the structure of the PPof crystalthe PP andcrystal resulted and resulted in an increase in an of increasecrystallinity. of crystallinity. PP and PE arePP membersand PE are of members the same polyolefinof the same family polyolefin and show family a similar and show interaction a similar with interactionPVB. Consequently, with PVB. theConsequently, crystallinity the of crystallinity LDPE was increasedof LDPE was by theincreased inclusion by the of PVBinclusion in LDPE of PVB/PVB inblends. LDPE/PVB In contrast, blends. as In discussed contrast, in as the discussed SEM images in the of SEM blend images films, of the blend structure films, of the the structure LDPE/PVB of blendthe LDPE/PVBwith 90% PVB blend was with damaged 90% PVB in was ethanol damaged solution in ethano due tol solution the elimination due to the of elimination the large amount of the large of PVB amountfrom the of compound PVB from (Supplementarythe compound (Supplementary Materials, Section Materials, 1, Figure Sectio S1). Therefore,n 1, Figure a S1). little Therefore, deterioration a littleand pudeteriorationffiness of the and LDPE puffiness crystal of the occurred LDPE crystal for large occurred PVB particles for large inside, PVB particles resulting inside, in the resulting reduction inof the LDPE reduction crystallinity of LDPE increments crystallinity for increments PVB-90 blends. for PVB-90 Even so,blends. the highEven PVB-mixed so, the high LDPE PVB-mixed showed LDPEslightly showed higher slightly crystallinity higher than crystallinity pure LDPE, than due pure to theLDPE, nucleating due to the effect nucleati of PVB.ng Theeffect inclusion of PVB. ofThe PVB inclusionalso improved of PVB the also tensile improved strength the of tensile LDPE /PVBstrength film of (Supplementary LDPE/PVB film Materials, (Supplementary Section 2,Materials, Figure S2), Sectionwhich indicates2, Figure theS2), betterwhich molecularindicates the arrangement better molecular of LDPE. arrangemen It can thust of beLDPE. said It that can the thus structure be said of thatLDPE the crystal structure in LDPE of LDPE/PVB crystal blends in isLDPE/PVB undoubtedly blends improved is undoubtedly by PVB. improved by PVB.

3.2.3.2. Rheological Rheological Properties Properties of of Blends Blends FigureFigure 2 illustrates the linear viscoelastic viscoelastic prop propertieserties of of the the LDPE/PVB LDPE /PVB blends, blends, including including pure pure LDPELDPE and PVB.PVB. TheThe complexcomplex viscosity, viscosity, storage, storage, and and loss loss moduli moduli of pureof pure PVB PVB were were significantly significantly higher higher than those of pure LDPE. As a result, the addition of PVB increased the complex viscosity of LDPE/PVB blends. For the storage and loss modulus of blends, similar characteristics were apparent. However, the viscoelastic behavior of PVB-10 and PVB-30 were very close to pure LDPE, due to the low amount of PVB in the blend. In contrast, the blend with 90% PVB (PVB-90) showed viscosity close to that of pure PVB. LDPE and PVB are totally immiscible, so adding PVB enhances the viscosity of the blends proportionally.

Polymers 2020, 12, 457 5 of 12 than those of pure LDPE. As a result, the addition of PVB increased the complex viscosity of LDPE/PVB blends. For the storage and loss modulus of blends, similar characteristics were apparent. However, the viscoelastic behavior of PVB-10 and PVB-30 were very close to pure LDPE, due to the low amount of PVB in the blend. In contrast, the blend with 90% PVB (PVB-90) showed viscosity close to that of pure PVB. LDPE and PVB are totally immiscible, so adding PVB enhances the viscosity of the Polymersblends 2020 proportionally., 12, x FOR PEER REVIEW 5 of 12

FigureFigure 2. 2. (a(a) )Complex Complex viscosity; viscosity; (b (b) )storage; storage; and and (c (c) )loss loss moduli moduli as as a a function function of of angular angular frequency frequency for for purepure LDPE, LDPE, pure pure PVB PVB and and LDPE/PVB LDPE/PVB blends. blends.

3.3.3.3. Structure Structure of of As-S As-Spunpun LDPE/PVB LDPE/PVB Blend Blend Fibers Fibers TheThe SEM SEM images images of of as-spun as-spun pure pure LDPE, LDPE, pure pure PV PVB,B, and and LDPE/PVB LDPE/PVB blend blend fibers fibers are are shown shown with with histogramshistograms in in Figure 3 3.. PVBPVB hadhad a a substantial substantial impact impact on on easing easing the the M-ESP M-ESP process process and and minimizing minimizing the theas-spun as-spun fiber fiber diameter. diameter. The The electrospinning electrospinning of pure of pure LDPE LDPE by the by laser the laser melting melting device device was very was tough, very tough,and a fewandof a Taylorfew of conesTaylor were cones developed were developed with a larger with dimension.a larger dimension. Hence, a Hence, very large a very (D = large19.75 (Dµm) = 19.75diameter µm) withdiameter a higher with standard a higher deviation standard ( σdeviation= 11.46 µ (m)σ = was 11.46 found µm) forwas pure found LDPE for fiber.pure TheLDPE addition fiber. Theof PVB addition increased of PVB the increased number ofthe Taylor number cones of Taylor per cm cones (Supplementary per cm (Supplementary Materials, Section Materials, 3, Figure Section S3) 3,and Figure decreased S3) and the decreased fiber diameter the fiber with diameter increasing with uniformity.increasing uniformity. Even the additionEven the ofaddition only 10% of only PVB 10%with PVB LDPE with resulted LDPE in aresulted smoother in electrospinninga smoother electrospinning process and fiber process with aand 7.86- fiberµm diameter.with a 7.86-µm Greater diameter.inclusion ofGreater PVB resulted inclusion in aof further PVB resulted decrease ofin fibera further diameter decrease and better of fiber uniformity. diameter Consequently, and better uniformity.the highest Consequently, PVB dosing (90%) the resultedhighest PVB in the dosing lowest (90%) diameter resulted (2.89 inµ m)the oflowest LDPE diameter/PVB blend (2.89 fiber, µm) very of LDPE/PVBclose to the blend fiber producedfiber, very from close pure to the PVB fiber (1.37 producedµm). from pure PVB (1.37 µm). AsAs mentioned inin the the DSC DSC section, section, PVB PVB inclusion inclusion eased theeased thermodynamic the thermodynamic process byprocess prompting by promptinginterior molecular interior molecular dispersion dispersion in blends. in In blends addition. In toaddition having to an having outstanding an outstanding surface adhesion surface adhesionproperty property [38,39], PVB [39,40], molecules PVB molecules encourage encour the heterogeneousage the heterogeneous nucleating nucleating activity for activity polyolefins for polyolefinsduring high-temperature during high-temperature processing processing [36]. As a [37] result,. As a the result, LDPE the molecules LDPE molecules were easier were to easier handle to handleduring during M-ESP, M-ESP, enabling enabling well-drawn well-drawn long polymer long polymer chains. Thus,chains. the Thus, blended the blended polymer polymer jet was regularly jet was regularlydischarged discharged from the Taylorfrom the cone Taylor with acone thinner with dimension. a thinner Indimension. Section 3.2 In on Section rheological 3.2 on properties, rheological we properties,observed that we theobserved complex that viscosity the complex of blends viscosit increasedy of proportionallyblends increased with pr PVBoportionally dosing. Whereas with PVB the dosing.higher viscosityWhereas of the polyolefins higher viscosity is considered of polyolefin the majors barrieris considered to its fiber the diameter major barrier minimization, to its fiber here, diameterthe polarity minimization, of PVB played here, an the important polarity role of inPVB the played reduction an ofimportant as-spun fiberrole in diameter the reduction by preventing of as- spunthe complications fiber diameter of by higher preventing viscosity the blends.complications The polarity of higher and viscosity the immiscibility blends. The of polarity PVB with and LDPE the immiscibilityincreased the of electrostatic PVB with LDPE attraction increased of the the grounded electrostatic collector attraction to the of the melted grounded polymer collector compound, to the meltedresulting polymer in a significant compound, reduction resulting in fiber in diameter a significant with better reduction spinnability in fiber (Supplementary diameter with Materials, better spinnabilitySection 3). (Supplementary Materials, Section 3).

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Pure LDPE PVB-10 PVB-50

D = 19.75 μm D = 7.85 μm D = 4.89 μm σ = 11.46 μm σ = 3.10 μm σ = 2.60 μm

PVB-70 PVB-90 Pure PVB

D = 1.37 μm D = 3.43 μm D = 2.89 μm σ = 0.39 μm σ = 1.42 μm σ = 1.26 μm

Figure 3. SEM images and histograms of as-spun pure LDPE and LDPELDPE/PVB/PVB blend fibers.fibers.

3.4. Structure of PVB-Removed LDPE Fibers SEM micrographsmicrographs of of PVB-removed PVB-removed LDPE LDPE fibers fibers obtained obtained from from LDPE-rich LDPE-rich LDPE LDPE/PVB/PVB blend blend fibers arefibers shown are shown in Figure in Figure4. Parallel 4. Parallel grooves grooves from where from thewhere PVB the was PVB removed was removed were visible were onvisible the LDPEon the fiberLDPE surface fiber surface for PVB-30 for PVB-30 & 50, while & 50, some while holes some with holes irregular with irregular channels channels were also were found also in found PVB-10. in ThePVB-10. parallel The arrangementparallel arrangement of clear PVB-removalof clear PVB-re groovesmoval confirmedgrooves confirmed the excellent the surfaceexcellent adhesion surface propertiesadhesion properties of PVB [38 of,39 PVB], including [39,40], including the improved the improved drawing drawing effect during effect M-ESP. during AsM-ESP. may beAs seen,may thebe seen, PVB the removal PVB removal channels channels became deeperbecame anddeeper more and pronounced more pronounced with increasing with increasing increments increments of PVB inof blends.PVB in blends. In addition, In addition, PVB elimination PVB elimination from LDPE-rich from LDPE-rich samples samples also decreased also decreased the ultimate the ultimate fiber diameter.fiber diameter. However, However, the deep the channels deep channels on the fiberon th surfacee fiber leftsurface by PVB left removal, by PVB rambledremoval, the rambled geometric the shapegeometric of the shape cross-section. of the cross-section.

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PVB-10 D = 6.22 μm σ = 2.92 μm

PVB-30 D = 5.09 μm σ = 1.72 μm

PVB-50 D = 4.52 μm σ = 1.17 μm

FigureFigure 4. 4. SEMSEM images images and and histograms histograms of of PVB-removed PVB-removed LDPE/PVB LDPE/PVB blend fibers. fibers.

TheThe crucialcrucial outcomeoutcome of of this this study study was was revealed reveal aftered after PVB wasPVB removed was removed from the from PVB-rich the PVB-rich samples samplesshown in shown Figure in5. Figure A massive 5. A dropmassive in the drop LDPE in th fibere LDPE diameter fiber diameter was observed, was observed, due to PVB due removal to PVB removalfrom PVB-rich from PVB-rich specimens, specimens, particularly particularly PVB-70 and PVB-70 -90 blend and fibers.-90 blend Eliminating fibers. Eliminating PVB from LDPE-richPVB from LDPE-richsamples produced samples theproduced multiple the parallel multiple grooves parallel on grooves the fiber on surfacethe fiber that surface are shownthat are in shown Figure in4. FigureFor PVB-rich 4. For PVB-rich samples, thesesamples, grooves these were grooves deeper, were finally deeper, resulting finally in resulting the splitting in the of splitting single or of multiple single orfibers. multiple Consequently, fibers. Consequently, the LDPE nanofibers the LDPE nanofibers with a diameter with a of diameter 349 259 of nm349 and± 259 235 nm and141 235 nm ± were 141 ± ± nmfound were for found PVB-70 for and PVB-70 90, respectively. and 90, respectively. Although Al thethough 90% PVBthe 90% blend PVB fiber blend was fiber confirmed was confirmed to produce to producemore delicate more LDPEdelicate nanofiber LDPE nanofiber with better with consistency, better co PVB-70nsistency, also PVB-70 created also LDPE created fibers LDPE on a nanoscalefibers on adimension nanoscale whiledimension using while less PVB. using less PVB. FigureFigure 66 comparescompares the fiber fiber diameter diameter of of LDPE LDPE/PVB/PVB blends blends and and PVB-removed PVB-removed LDPE LDPE fibers. fibers. A two-stepA two-step fiber fiber diameter diameter reductio reductionn was was observed, observed, first first during during M-ESP M-ESP with with PVB PVB and and LDPE LDPE mixtures; mixtures; andand second when PVB was removed from LDPE/PVB LDPE/PVB blend fibers. fibers. Due Due to to the the addition addition of of PVB, PVB, a massivemassive diameter diameter drop drop of of blend blend fiber fiber was was observed observed for for PVB-10, PVB-10, which which then then continued continued linearly linearly up upto theto the PVB-90. PVB-90. Slight Slight and and huge huge fiber fiber diameter diameter dr dropsops were were observed observed when when PVB PVB was was removed removed from LDPE-LDPE- and PVB-richPVB-rich samples,samples, respectively.respectively. A A significant significant reduction reduction in in fiber fiber diameter diameter was was found found for for the thePVB-90 PVB-90 blend blend fiber, fiber, from from 2.89 2.891.26 ± 1.26µm toµm 235 to 235141 ±nm 141 due nm todue the to multiple the multiple splitting splitting of LDPE of LDPE fibers. ± ± fibers.The histograms The histograms and standard and standard deviations deviations also confirmed also thatconfirmed more consistent that more fibers consistent were obtained fibers were with obtainedhigher PVB with content. higher MostPVB ofcontent. the PVB-rich Most of samples the PVB-rich were formed samples by were PVB. Hence,formed ethanolby PVB. removed Hence, ethanola substantial removed portion a substantial of the fiber, portion leaving of the only fiber, 10% leaving LDPE, only which 10% demonstrated LDPE, which moredemonstrated delicate LDPEmore delicatenanofibers. LDPE Furthermore, nanofibers. the Furthermore, development the of development multiple single of nanofibers,multiple single due tonanofibers, fiber splitting, due ensuredto fiber splitting,a huge drop ensured in fiber a huge diameter. drop Therefore,in fiber diameter. nanofibers Therefore, with minimal nanofibers diameter with wereminimal also diameter obtained; were this, alsohowever, obtained; caused this, an however, extended caused standard an deviation.extended standard deviation.

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PVB-70 D = 349 nm PVB-70 D = 349 nm σ = 259 nm σ = 259 nm

PVB-90 D = 235 nm PVB-90 D = 235 nm σ = 141 nm σ = 141 nm

FigureFigure 5. 5. SEMSEM images images and and histograms histograms of of PVB-removed PVB-removed LDPE/PVB LDPE/PVB blend fibers. fibers. Figure 5. SEM images and histograms of PVB-removed LDPE/PVB blend fibers.

FigureFigure 6. 6. ComparisonComparison of of the the diameters diameters of of LDPE/PVB LDPE/PVB blends blends and and PVB-removed PVB-removed LDPE fibers. fibers. Figure 6. Comparison of the diameters of LDPE/PVB blends and PVB-removed LDPE fibers. 3.5.3.5. DSC DSC Analysis Analysis of of Fibers Fibers 3.5. DSC Analysis of Fibers FigureFigure 7 7shows shows DSC DSC heat heat values values of PVB-removed of PVB-removed LDPE andLDPE pure and LDPE pure fibers LDPE and thefibers crystallinity and the Figure 7 shows DSC heat values of PVB-removed LDPE and pure LDPE fibers and the crystallinityof as-spun fibers of as-spun before fibers and after before ethanol and after treatment. ethanol PVB-removed treatment. PVB-removed LDPE fibers LDPE exposed fibers the exposed melting crystallinity of as-spun fibers before and after ethanol treatment. PVB-removed LDPE fibers exposed thecurve melting like pure curve LDPE like fiber,pure LDPE but exhibited fiber, but extended exhibited endothermic extended endothermic slope magnitude. slope magnitude. In Section 3.1 In, the melting curve like pure LDPE fiber, but exhibited extended endothermic slope magnitude. In SectionFigure1 b3.1, showed Figure that 1b includingshowed that PVB including truly improved PVB truly both improved the crystal both structure the crystal and crystallinity structure and of Section 3.1, Figure 1b showed that including PVB truly improved both the crystal structure and crystallinityLDPE. However, of LDPE. the laser However, M-ESP the process laser had M-ESP a detrimental process had eff ecta detrimental on the crystallinity effect on of the pure crystallinity LDPE and crystallinity of LDPE. However, the laser M-ESP process had a detrimental effect on the crystallinity ofblend pure polymers. LDPE and The crystallinityblend polymers. of LDPE The was crystall drasticallyinity decreasedof LDPE forwas electrospinning drastically decreased in blends withfor of pure LDPE and blend polymers. The crystallinity of LDPE was drastically decreased for electrospinninghigher PVB content. in blends Generally, with thehigher crystallinity PVB content. of a polymer Generally, is a functionthe crystallinity of the supercooling of a polymer [40 is,41 a] electrospinning in blends with higher PVB content. Generally, the crystallinity of a polymer is a functionand is significantly of the supercooling lower for electrospun [41,42] and fibers is signif comparedicantly tolower samples for electrospun prepared by fibers common compared processing to function of the supercooling [41,42] and is significantly lower for electrospun fibers compared to samplestechnologies prepared (molding by common or film casting) processing [42]. technologies Here, M-ESP (molding was conducted or film at casting) room temperature, [43]. Here, M-ESP where samples prepared by common processing technologies (molding or film casting) [43]. Here, M-ESP wasthe polymerconducted was at melted room temperature, at around 200 where◦C by laser,the polymer and as-spun was melted fibers were at around collected 200 immediately°C by laser, byand a was conducted at room temperature, where the polymer was melted at around 200 °C by laser, and as-spuncollector. fibers As a were result, collected the melted immediately electrospun by blenda collector. fiber wasAs a quenched result, the and melted solidified electrospun very quickly blend as-spun fibers were collected immediately by a collector. As a result, the melted electrospun blend fiberbefore was the quenched crystal formed and properly,solidified degradingvery quickly crystallinity. before the The crystal thinner formed fibers properly, cooled more degrading rapidly, fiber was quenched and solidified very quickly before the crystal formed properly, degrading crystallinity.and resulted inThe a greaterthinner reduction fibers cooled of crystallinity more rapidly, for PVB-rich and resulted blend fibers.in a greater Instead, reduction after ethanol of crystallinity. The thinner fibers cooled more rapidly, and resulted in a greater reduction of crystallinitytreatment, the for crystallinity PVB-rich blend of as-spun fibers. fiber Instead, was regainedafter ethanol and reachedtreatment, near the the crystallinity value of blend of as-spun films. crystallinity for PVB-rich blend fibers. Instead, after ethanol treatment, the crystallinity of as-spun fiber was regained and reached near the value of blend films. fiber was regained and reached near the value of blend films. The highest improvement was observed after the ethanol treatment for PVB-rich samples. For The highest improvement was observed after the ethanol treatment for PVB-rich samples. For PVB-90, crystallinity was recouped from 18% to 27%. The ethanol treatment was applied to as-spun PVB-90, crystallinity was recouped from 18% to 27%. The ethanol treatment was applied to as-spun

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LDPE/PVB blend fibers at room temperature for 6 h. To remove PVB from closely associated LDPE, ethanol enters the core of the LDPE fiber and enhances the arrangement of polymer chains after the PVB matrix dissolves. Polaskova et al. [44] also observed the improved molecular orientation and crystallinity of electrospun fiber with accompany of methanol solution. Like LDPE/PVB film, the crystallinity of PVB-removed LDPE fiber was improved by the increase of PVB content in blend fibers. The highest crystallinity (30%) was found for PVB-70, while only 24% of crystallinity was recorded for pure LDPE fiber. We found the finest LDPE nanofiber from the PVB-90 blend demonstrated crystallinity of around 28%, which is also higher than the crystallinity of pure LDPE Polymersfiber. Thus,2020, 12 we, 457 confirmed that LDPE fiber prepared from LDPE/PVB blends showed better9 of 12 crystallinity than that made from pure LDPE.

Figure 7. ((a)) DSC DSC heat heat inspections inspections of of PVB PVB removed removed LDPE LDPE and and pure pure LDPE fibers; fibers; and ( b) crystallinity (%) of melt electrospun fibersfibers beforebefore andand afterafter ethanolethanol treatment.treatment.

3.6. FTIRThe Analysis highest of improvement Fibers was observed after the ethanol treatment for PVB-rich samples. For PVB-90, crystallinity was recouped from 18% to 27%. The ethanol treatment was applied to as-spun Figure 8 shows the FTIR spectra of pure PVB, pure LDPE, PVB-removed LDPE, and LDPE/PVB LDPE/PVB blend fibers at room temperature for 6 h. To remove PVB from closely associated LDPE, blend fibers. Peaks at 1132 cm−1 and 995 cm−1 corresponding to a C-O-C butyral ring and C-O ethanol enters the core of the LDPE fiber and enhances the arrangement of polymer chains after the stretching [45], respectively, are the typical bands of PVB. Both the LDPE/PVB blended and pure PVB PVB matrix dissolves. Polaskova et al. [43] also observed the improved molecular orientation and fibers spectrums strongly revealed those characteristic peaks. However, after ethanol treatment, the crystallinity of electrospun fiber with accompany of methanol solution. Like LDPE/PVB film, the peaks disappeared from the LDPE/PVB blended fiber. This confirmed the complete elimination of crystallinity of PVB-removed LDPE fiber was improved by the increase of PVB content in blend fibers. PVB from blended fiber by ethanol treatment. Other typical bands at 1377 cm−1 (distinguishing band The highest crystallinity (30%) was found for PVB-70, while only 24% of crystallinity was recorded for for polyolefin [46]), 1463 cm−1, and 2915 cm−1 ascribed to -CH3 symmetric deformation vibrations, pure LDPE fiber. We found the finest LDPE nanofiber from the PVB-90 blend demonstrated crystallinity bending deformation, and -CH2 asymmetric [47], respectively, are firmly visible for pure LDPE and of around 28%, which is also higher than the crystallinity of pure LDPE fiber. Thus, we confirmed PVB-removed LDPE fiber. This supported the notion that the characteristics of PVB-removed LDPE that LDPE fiber prepared from LDPE/PVB blends showed better crystallinity than that made from and pure LDPE fiber are similar. We concluded that the melt electrospun LDPE fiber from LDPE/PVB pure LDPE. blend film is a pure LDPE fiber. 3.6. FTIR Analysis of Fibers Figure8 shows the FTIR spectra of pure PVB, pure LDPE, PVB-removed LDPE, and LDPE /PVB 1 1 blend fibers. Peaks at 1132 cm− and 995 cm− corresponding to a C-O-C butyral ring and C-O stretching [44], respectively, are the typical bands of PVB. Both the LDPE/PVB blended and pure PVB fibers spectrums strongly revealed those characteristic peaks. However, after ethanol treatment, the peaks disappeared from the LDPE/PVB blended fiber. This confirmed the complete elimination of 1 PVB from blended fiber by ethanol treatment. Other typical bands at 1377 cm− (distinguishing band 1 1 for polyolefin [45]), 1463 cm− , and 2915 cm− ascribed to -CH3 symmetric deformation vibrations, bending deformation, and -CH2 asymmetric [46], respectively, are firmly visible for pure LDPE and PVB-removed LDPE fiber. This supported the notion that the characteristics of PVB-removed LDPE and pure LDPE fiber are similar. We concluded that the melt electrospun LDPE fiber from LDPE/PVB blend film is a pure LDPE fiber.

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Figure 8. FourierFourier transform transform infrared infrared (FTIR) (FTIR) spectrum spectrum of of pure pure PVB, PVB, pure pure LDPE, LDPE, PVB-removed PVB-removed LDPE, LDPE, andand LDPE/PVB LDPE/PVB blend fibers. fibers.

4. Conclusions Conclusions We successfully minimized the fiber fiber diameter of LDPE nanofibers, nanofibers, producing producing fibers fibers that that are are a fewfew hundred hundred nanometers nanometers in diamet diameterer by laser M-ESP M-ESP system, system, using using PVB PVB as as a a blend blend component. component. Here, Here, thethe process process involved two-step fiber fiber diameter redu reduction:ction: during during M-ESP M-ESP via via the the inclusion of PVB and by subsequently removing removing PVB PVB from from LDPE/PVB LDPE/PVB bl blendend fiber. fiber. A A massive massive blend blend fiber fiber diameter drop drop was observed observed with with even even a alow low 10% 10% inclusion inclusion of PV of PVB,B, and and the thetrend trend continued continued linearly linearly up to up 90% to PVB 90% content.PVB content. In addition, In addition, PVB improved PVB improved the polarity the polarity and drawing and drawing effect of e ffLDPE/PVBect of LDPE blend/PVB films blend during films laserduring melting, laser melting, which whichfacilitated facilitated the production the production of a more of a moredelicate delicate fiberfiber with with better better spinnability. spinnability. A furtherA further drop drop of ofLDPE LDPE fiber fiber diameter diameter was was achiev achieveded after after PVB PVB was was removed removed from from LDPE/PVB LDPE/PVB blend fibers,fibers, due due to to the the fiber fiber splitting, splitting, mostly mostly in in PVB-ri PVB-richch samples. samples. Consequently, Consequently, LDPE LDPE nanofibers nanofibers with with a a diameter of 235 141 nm were obtained from blended fiber using 90% PVB. DSC results also diameter of 235 ± 141± nm were obtained from blended fiber using 90% PVB. DSC results also revealed thatrevealed the improved that the improved crystallinity crystallinity of LDPE of in LDPE the film in the was film decreased was decreased for M-ESP for M-ESP but regained but regained after ethanolafter ethanol treatment. treatment. These These superfine superfine polyethylene polyethylene nanofibers nanofibers constitute constitute a apromising promising material material for for battery separators and filtration filtration media for waste-water treatment.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/12/2/457/s1, SupplementaryFigure S1: SEM micrographsMaterials: The of blend following films surfaceare available after removing online at PVB www.mdpi.com/xxx/s1, from (a) PVB-10, (b) PVB-30, Figure (c) S1: PVB-50, SEM micrographs(d) PVB-70, (e) of relation blend films of particle surface size after of theremoving PVB domains, PVB from Figure (a) PVB-10, S2: Tensile (b) stress-strainPVB-30, (c) PVB-50, curve of (d) (a) PVB-70, pure LDPE, (e) relationpure PVB, of andparticle (b) LDPEsize of/PVB the blendPVB domains, films, Figure Figure S3: S2 Images: Tensile and stress-strain the number curve of Taylor of (a) cones pure during LDPE, laser pure M-ESP PVB, andfrom (b) di ffLDPE/PVBerent blend blend films. films, Figure S3: Images and the number of Taylor cones during laser M-ESP from differentAuthor Contributions: blend films. M.Z. performed the experiment, analyzed the data, and prepared the manuscript. K.S. collected the materials and performed the experiment. K.N. supervised the work and revised the manuscript. AuthorAll authors Contributions: have read and M.Z. agreed performed to the publishedthe experiment, version analyzed of the manuscript. the data, and prepared the manuscript. K.S. collected the materials and performed the experiment. K.N. supervised the work and revised the manuscript. Funding: This research received no external funding. Funding: This research received no external funding. Acknowledgments: M. Zakaria is grateful to the Japan Government for the scholarship (MEXT) support towards Acknowledgments:his Ph.D. study. M. Zakaria is grateful to the Japan Government for the scholarship (MEXT) support towards hisConflicts Ph.D. study. of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References

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