ORIGINAL PAPER

Enhanced mechanical properties and pre-tension effectsof polyurethane (PU) nanofiber filaments preparedby electrospinning and dry twisting

Yujin Lee & Byoung-Suhk Kim & Joo Hyung Hong &

Soonjee Park & Hyungsup Kim & Ick-Soo Kim

Received: 21 August 2011 /Accepted: 29 September 2011 /Published online: 22 February 2012# Springer Science+Business Media B.V. 2012

Abstract We studied the mechanical properties of polyure-thane (PU) nanofiber filaments prepared by electrospinning anddry twisting, and compared them with those of thecorresponding nonwoven PU nanofiber webs. The morpholo-gies and mechanical properties of the nonwoven PU nanofiberwebs and the corresponding PU nanofiber filaments were in-vestigated by scanning electron microscopy (SEM) andthrough the use of a universal testing machine (UTM), respec-tively. The tensile strength and the Young’s modulus of thenanofiber filaments improved dramatically as the number oftwists or the width of the nanofiber webs was increased com-pared with the nonwoven PU nanofibers. Moreover, it wasfound that applying pre-tension was an effective method ofincreasing themechanical properties of PU nanofiber filaments.

Keywords Nanofiber . Electrospinning . Filaments . Drytwisting .Mechanical properties . Structure–propertyrelations

Introduction

Nanofibers have recently attracted a great deal of attentionbecause of their unique properties and promise for many

applications, such as in electrical, biomedical, and protec-tive products [1–4]. Several techniques, such as drawing [5],template synthesis [6, 7], phase separation [8], self-assembly [9–11], and electrospinning [12, 13], have beendeveloped to generate nanofibers. Among them, solutionelectrospinning is a simple, inexpensive, and straightfor-ward method for generating the polymer nanofibers, and itrepresents an important technological innovation for thenonwoven and textile industries. Numerous polymer sys-tems, including homopolymers, various kinds of copoly-mers, blends, and composites, have been successfullyelectrospun into nanofibers [14–18]. Polyurethane (PU)nanofiber webs exhibit good mechanical properties and arewidely held to be among the best nanofiber webs [19, 20].Nair et al. prepared electrospun PU nanofibers incorporatingsilver (Ag) nanoparticles using an electrospinning method toenhance the antibacterial as well as wound-healing proper-ties of wound dressings [17]. However, despite the potentialproperties mentioned above, the applications of variousnanofibers have been limited by their poor mechanicalproperties and the difficulties involved in mass producingthem (resulting from the unusually low production processthroughput). Therefore, many researchers have tried to im-prove the mechanical properties of yarns or fibers. Liu et al.[21] produced flexible, lightweight, high-strength, and high-ly conductive superaligned carbon nanotube (SACNT)/polyvinyl alcohol (PVA) composite yarns by using a con-tinuous twisting SACNT yarn as a conductive frameworkand inserting PVA into the intertube spaces of the SACNTframework using a PVA/DMSO solution, in order to en-hance the strength of the yarn. Park et al. [22] demonstratedthat the mechanical properties of polyelectrolyte multilayercoated random and aligned nylon 6 nanofiber mats wereenhanced by the surface nanocoating of polyelectrolytesonto nylon 6 nanofiber mats. Huang et al. [23] synthesizeda series of high molecular weight polyimide (PI) precursors:

Y. Lee :B.-S. Kim (*) : I.-S. Kim (*)Nano Fusion Technology Research Group,Faculty of Textile Science & Technology, Shinshu University,Ueda Nagano 386-0015, Japane-mail: kbsuhk@yahoo.come-mail: kim@shinshu-u.ac.jp

J. H. Hong :H. KimDepartment of Textile Engineering, Konkuk University,Seoul 143-701, South Korea

S. ParkSchool of Textiles, Yeungnam University,Gyeongsan, Gyeongbuk 712-749, South Korea

J Polym Res (2012) 19:9774DOI 10.1007/s10965-011-9774-4

poly(p-phenylene biphenyltetracarboxamide acid). Theyreported that high molecular weight 3,4,3′,4′-biphenyltetra-carboxulic dianyhydride (BPDA)/p-phenylenediamine(PDA) PI thin films and electrospun nanofiber sheets pos-sess excellent mechanical properties: tensile strengths of upto 900 MPa with elastic moduli of up to 18.0 GPa, andtensile strengths of up to 210 MPa with elastic moduli up to2.5 GPa, respectively.

In this paper, we report the mechanical properties and thepre-tension effects of polyurethane (PU) nanofiber filamentsprepared by electrospinning and dry twisting. Pre-tensioningis a method that is widely used to increase the tensilestrengths of materials [24, 25]. In general, the low loadingof a warp yarn system before the fabric weaving process is awell-known example of the pre-tensioning of woven struc-tures. This study develops the idea of using the pre-tensioning of nanofiber filaments to further improve qualityand strength properties.

Experimental

Materials

Polyurethane (PU, molecular weight~110,000 g/mol) waspurchased from Aldrich Chemical Co. (Milwaukee, WI,USA). A solution of PU dissolved in a mixture of N,N-dimethylformamide (DMF) and methyl ethyl ketone(MEK) (1:1 by volume ratio) was prepared for electrospin-ning. The solution concentration used in this work wasabout 12 wt%. In order to produce electrospun nanofibers,a high-voltage power supply (CPS-60 K022V1, ChunpaEMT Co.) capable of generating voltages up to 100 kVwas used to produce an electric field [26–28, 30]. The PUsolution was supplied through a plastic syringe attached to acapillary tip with an inner diameter of 0.6 mm and electro-spun onto a rotating metallic collector. The voltage appliedwas 20 kV. The distance between the capillary tip andcollector was 15 cm. As seen in Fig. 1, in general, electro-spun PU nanofibers are deposited as a randomly orientednanofiber web, forming a highly porous structure, which isheld together by interfiber bonding and crossing [2–4]. Thediameter and thickness of the fibers in the nanofiber matsobtained in this manner were 660±50 nm and ~15 μm,respectively.

Slit and dry twisting process

The fibrous electrospun PU webs were gently cut into ribbonshapes (Fig. 2b) using a knife. The widths of these PU webribbons were 3, 5, and 7 mm, respectively. The average lengthof each PU web used in this work was about 30 cm. In order toprocess filaments from the nonwoven nanofiber webs, a dry-

twisting method was used, which is also the method generallyused in the fiber manufacturing industry. Figure 2a shows aphotograph of the twisting machine used (MM-20, DaieiKagaku Seiki MFG. Co., Ltd., Kyoto, Japan). The distancebetween the chucks (fixed clamp and rotary clamp, Fig. 2c)was fixed at 25 cm. After the ribbon-shaped nonwoven PUnanofiber webs had been attached to each chuck, the right-sidechuck was automatically wound. The number of twists intro-duced into the ribbons, thus producing a nanofiber filament,was varied from 500 to 1000 T/m. This range was usedbecause <500 T/m led to irregular twisting and thus unevenlytwisted nanofiber filaments, whereas >1000 T/m resulted indouble-twisted nanofiber filaments; indeed, applying >2000 T/m sometimes caused the fiber to break. A detailed explanationsof this behavior is provided in our previous paper [29].

Characterization

The surface morphologies of the electrospun fibrous PUwebs and the resulting nanofiber filaments were observedusing a scanning electron microscope (SEM, VE-8800,Keyence, Osaka, Japan). The average fiber diameter andthe size distribution for each sample were characterizedusing an image analyzer (Image-J, National Institute ofMental Health, Bethesda, MD, USA). Mechanical proper-ties of the nonwoven PU nanofibers and the nanofiberfilaments were investigated using a universal testing ma-chine (UTM, AG-5000 G, Shimadzu Co., Kyoto, Japan)under a cross-head speed of 10 mm/min at room tempera-ture. In accordance with ASTM D-638, all nonwoven nano-fiber webs were prepared in the form of a standarddumbbell. The dimensions of the sample were 165×19×3.0 mm (length×width×thickness). The load cell capacity

Fig. 1 A typical SEM image of an electrospun PU nanofiber web.Inset shows an SEM image at a higher magnification

Page 2 of 5 Y. Lee et al.

was 50 N (Tensilon RTC 1250-A, A&D Company Ltd.,Tokyo, Japan).

Results and discussion

In this work, we successfully prepared PU nanofiber filamentsusing a recently developed technique that combines electro-spinning with twisting methods. Figure 3 shows the tensilestrength, pre-tension, and SEM images of the PU nanofiber

filaments prepared with different amounts of twisting, rangingfrom 500 T/m to 1000 T/m at constant nanofiber web width(7 mm). The diameter of the nanofiber filament rapidly de-creased with the amount of twisting, and the fiber densityincreased. The fibers were well aligned along the nanofiberfilament axes (SEM image, inset in Fig. 3). Moreover, thenanofiber filaments exhibited unique helical structures con-sisting of the stretched individual nanofibers. The tensilestrength of the PU nanofiber filaments increased linearly withthe amount of twisting, while the PU nanofiber filaments thatreceived pre-tension (20%) exhibited a significant increase intensile strength, to >850 T/m. The pre-tension was tensionartificially induced in the PU nanofiber filaments in additionto the weight of the filaments themselves and any loads thatthey carried. Pre-tension was applied to the nanofiber fila-ments by stretching them from their edges. In this work, thelevel of pre-tension applied was fixed at 20%. The tensilestrengths of PU nanofiber filaments prepared with 1000 T/mthat did and did not receive pre-tension (20%) were about72 MPa and 52 MPa, respectively. When the amount oftwisting was further increased above 1000 T/m, the PU nano-fiber filaments broke during twisting.

Figure 4 shows the Young’s moduli and tensile strengthsof PU nanofiber filaments prepared from nanofiber mats ofvarious widths but using a constant amount of twisting(750 T/m). It is apparent that both the Young’s modulusand the tensile strength of PU nanofiber filament increaseslinearly with nanofiber web width (in the range 3–7 mm).The maximum tensile strength (~ 52 MPa) of the PU nano-fiber filaments was obtained for a nanofiber mat width of7.0 mm at 750 T/m, which was more than five times higherthan that (~9 MPa) of the nonwoven PU nanofiber web.Each component of the twisted nanofiber filaments forms ahelix within the overall filament structure. Since there are

Fig. 2a–c Photographs of ourtwisting machine (a) andnanofiber strips (b), and aschematic of the dry-twistingmethod (c)

Fig. 3 The tensile strengths of PU nanofiber filaments prepared withdifferent amounts of twisting (ranging from 500 T/m from 1000 T/m)from constant-width nanofiber webs (7 mm) with (dotted line) orwithout (solid line) pre-tensioning (20%). Insets shows SEM imagesof the corresponding PU nanofiber filaments for twist numbers of 500,750, and 100 T/m

Enhanced mechanical properties and pre-tension effects of PU nanofiber filaments Page 3 of 5

many fibers and thus surfaces, friction is an extremelyimportant aspect of loading [31]. The strong interfacialinteractions between the nanofibers increased with the widthof the nanofiber web, which improved the load-transferefficiency, resulting in increased tensile strength andYoung’s modulus for the nanofiber filaments. Furthermore,we investigated the effect of applying pre-tension on themechanical properties of the nanofiber filaments. For in-stance, it has been know that yarn twisting reduces strengthvariability by increasing interfiber lateral interaction andfriction. Examination of the mechanical properties of the

dried, twisted yarns of electrospun polyacrylonitrile (PAN)nanofibers showed that the initial modulus and tensilestrength increased with twist angle [32].

Figure 5 shows the tensile strengths of pre-stretched (20%)PU nanofiber filaments prepared from nanofiber mats of dif-ferent widths (3, 5, and 7 mm), but using a constant amount oftwisting (1000 T/m). It was found that applying pre-tension isan effective method of improving the mechanical properties ofPU nanofiber filaments, since it increases the tensile strength byabout 30%. Figure 6 shows typical stress–strain curves of a PUnanofiber filament (width of the nanofiber mat: 7 mm, amountof twisting: 750 T/m) and a nonwoven PU nanofiber web.Compared with that of the nonwoven PU nanofiber web, thetensile strength of the PU nanofiber filament was significantlyhigher: 52 MPa. Breaking elongation of the PU nanofiberfilaments was ~250%, which was approximately double that(ca. 120%) of the nonwoven PU nanofiber web. These resultscan be attributed to the combined effects of decreasing voidsand increasing internal friction in the nanofiber filament as thewidth of nanofiber webs increases, as confirmed by SEManalysis (inset in Fig. 3). As a result, it was concluded that the

Fig. 5 Tensile strengths of PU nanofiber filaments prepared fromdifferent widths of nanofiber web (3, 5, and 7 mm), using a constantamount of twisting (1000 T/m), and by applying pre-tension

Fig. 6 Typical stress–strain curves of a nonwoven PU nanofiber web(a) and a PU nanofiber filament (b) (width of nanofiber web: 7 mm,amount of twisting: 750 T/m)

Table 1 Mechanical properties of nanofiber filaments prepared underdifferent twisting conditions

Processingconditions (T/m)

Tensilestrength(MPa)

Young’smodulus(MPa)

Elongationat break(%)

PU nonwovennanofiber web

0 9.0 7.2 120

PU nanofiber filamenta 500 41.2 59.9 220

750 52.0 63.2 250

1000 70.4 102.0 260

a The width of the PU nonwoven nanofiber web was 7 mm

Fig. 4 Young’s moduli and tensile strengths of PU nanofiber filamentsprepared from nanofiber webs of various widths, but using a constanttwist number (750 T/m)

Page 4 of 5 Y. Lee et al.

twisting process effectively improves the mechanical propertiesof nonwoven PU nanofibers. Data on the Young’s moduli,tensile strengths, and elongations at break for various nanofiberfilaments and a nanofiber web are summarized in Table 1.

Conclusions

In summary, we have developed PU nanofiber filamentsusing electrospinning and dry-twisting methods and charac-terized their mechanical properties. The width of the nano-fiber mat and the amount of twisting strongly affected themechanical properties of the resulting PU nanofiber fila-ments. That is, the tensile strength and Young’s modulusof the nanofiber filament increased dramatically with theamount of twisting or nanofiber web width, and were sig-nificantly better than those for nonwoven PVDF nanofibers(Young’s modulus: 23.4±1.1 MPa, tensile strength: 4.4±0.2 MPa, elongation at break: 105.1±1.8 %) [29]. More-over, applying pre-tension to the PU nanofiber filamentsstrongly affected their mechanical properties. Comparedwith the nonwoven PU nanofiber webs, the tensile strengthof the PU nanofiber filaments (about 52 MPa) was muchgreater, which was attributed to a combination of decreasingvoids and increasing internal friction in the nanofiber fila-ments as the nanofiber web width increases.

Acknowledgement This work was supported by grant-in-aid for theGlobal COE Program by the Ministry of Education, Culture SportsScience, and Technology, Japan.

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