Materials Letters 172 (2016) 149–152

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Direct microscopic observation of shish-kebab structure inhigh-temperature electrospun iPP fibers

Shuangyang Liu a, Feifei Zhang a, Guoqiang Zheng a,n, Kun Dai a, Chuntai Liu a,n,Changyu Shen a, John Zhanhu Guo b

a College of Materials Science and Engineering, the Key Laboratory of Material Processing and Mold of Ministry of Education, Zhengzhou University,Zhengzhou 450001, PR Chinab Integrated Composites Laboratory, University of Tennessee, Knoxville, TN 37996, USA

a r t i c l e i n f o

Article history:Received 5 January 2016Received in revised form10 February 2016Accepted 21 February 2016Available online 27 February 2016

Keywords:PolymersShish-kebab structureIsotactic polypropyleneElectrospinningCrystal growth

x.doi.org/10.1016/j.matlet.2016.02.1117X/& 2016 Elsevier B.V. All rights reserved.

esponding authors.ail addresses: gqzheng@zzu.edu.cn (G. Zheng)

a b s t r a c t

Electrospinning is not only a technique to fabricate macroscopic fiber but also a method to control themicrostructure. In this study, high-temperature solution electrospinning was used to prepare isotacticpolypropylene (iPP) fibers. Scanning electron microscope (SEM) observation indicates that a singleelectrospun iPP fiber consists of many nano-fibrils. Based on SEM observation, these nano-fibrils areshish-kebab structure. Electrostatic elongational force involved in the electrospinning process is con-sidered to be responsible for the development of shish-kebab structure.

& 2016 Elsevier B.V. All rights reserved.

1. Introduction

In the 1960s, typical shish-kebab structure developed in thesheared polyethylene (PE) solution was first observed by Penningsand Kiel [1]. It has been well established that the extended chainsconstitute the shish and the kebabs consisting of folded chainsgrow perpendicularly to the shish [2]. Importantly, shish-kebabformed in flow fields (elongation, shear, mixed flows and so on)generally endows semicrystalline polymers with excellent me-chanical properties [3]. As a result, such fascinating structure hasdrawn more and more attentions for both academic interests andpractical applications. To date, shish-kebab developed under flowfield of polyolefin melt has been reported [4–8]. Electrospinning isnot only a versatile technique to fabricate polymeric fibers but alsoa method to control the microstructure of as-electrospun fibers.Consequently, electrospinning is envisioned to be capable of in-ducing the formation of shish-kebab structure in the electrospunfibers. With respect to microstructure tuning, more and more at-tentions have been paid to the electrospun polyolefin fibers [9–13].Shish-kebab structure in the electrospun PE fibers was first re-ported in 2010 [14]. However, with respect to isotactic poly-propylene (iPP), the study on its microstructural development is

, ctliu@zzu.edu.cn (C. Liu).

seldom found. To date, only the macroscopical surface and someindexes of the crystal structure in the electrospun iPP fibers havebeen investigated [15–17]. Moreover, in our previous study, al-though the oriented crystal structure was formed in electrospuniPP fibers [17], direct evidence of shish-kebab was not given.

In this paper, iPP fibers were fabricated by high-temperatureelectrospinning and then washed with hot xylene. SEM was em-ployed to directly observe the inner morphology of the fibers.Mechanism of the shish-kebab under the confined electrospinningenvironments is proposed.

2. Experimental

Commercial iPP (T30S) was purchased from Dushanzi Petro-leum Chemical Co., China. Its melt flow index (MFI) and weight-average molecular weight (Mw) were 3.0 g/10 min (230 °C, 21.6 N)and 39.9�104 g/mol, respectively. Xylene (analytical reagent, 99%)was bought from Tianjin Chemical Reagents Plant. All the chemi-cals were used as received without any treatment.

A home-made electrospinning apparatus with an infrared (IR)heating system was shown in Fig. S1 in the Supplementary ma-terial, which can be also found in Ref. [17]. The iPP granules werefirst dissolved completely in xylene at 135 °C for 3 h with drasti-cally mechanical stirring to obtain homogeneous solution. And

S. Liu et al. / Materials Letters 172 (2016) 149–152150

then, electrospinning was carried out at an applied voltage of20 kV over a distance of 20 cm from the tip of spinneret to col-lector. Temperature around the metal spinneret of the glass syr-inge and collector was 12072 and 7072 °C, respectively. More-over, a rotating ferrum frame collector was adopted to produce themacroscopically aligned fibers [17].

SEM (JEOL JSM 7500F) was employed to observe the fiber'smorphology. A number of macroscopically aligned fibers were firstpiled up to prepare a bundle of electrospun iPP fibers. Such fiberbundle was then washed in hot xylene (ca. 100 °C) for 10 min toremove amorphous outer iPP coating and thus the inner crystalmorphology was revealed [8]. Of note, hot xylene was renewedeach 5 min. Before observation, the samples were sputtered withgold for better imaging. Polarized Fourier Transform InfraredSpectroscopy (FTIR) equipped with a polarizer (NICOLET 6700)was used to examine the orientation level of a single electrospuniPP fiber. The spectra were recorded from 400 to 4000 cm�1 with aresolution of 4 cm�1.

Fig. 2. The polarized FTIR spectra of the electrospun iPP fibers.

3. Results and discussion

As shown in Fig. 1a, an obvious groove is observed on thesurface of fiber, which can be regarded as the boundary betweentwo adjacent fibrils (the fibril is marked by “1” in Fig. 1a). Ourprevious study [17] has demonstrated that such fibrils are firstlyformed upon electrostatic elongational force and then con-glutinate with each other by micellar junctions to compose themacroscopically electrospun fibers during the gradual solventevaporation. Moreover, many nano-fibrils parallel to the long-itudinal axis of electrospun iPP fibers are clearly observed in eachfibril (solid arrow in Fig. 1a). Additionally, each nano-fibril consistsof two parts (see Fig. 1b and c), viz., one is streak-like entityaligning along the electrospun fiber's longitudinal axis (“A” inFig. 1c), and the other is parallel rod-like entities perpendicular tothese streak-like ones (“B” in Fig. 1c). Interestingly, such orientednano-fibrils are extremely identical to the shish-kebab structure,which are always found in the flow induced crystallization [2].Therefore, in view of the electrostatic elongational force duringelectrospinning, these oriented crystals are envisioned to be shish-kebab structure.

FTIR spectroscope equipped with a polarizer was employed toexamine the orientation level of molecular chains and crystallinephase in a single electrospun iPP fiber (the single fiber and samplepreparation process for FTIR measurement can be found in Sup-plementary Material). The polarized FTIR spectra are shown inFig. 2. The absorbance intensity of each band in the parallel- andperpendicular-polarized FTIR spectra is substantially differentfrom each other, indicating the remarkable uniaxial orientation inthe electrospun fiber. This agrees well with the SEM observation

Fig. 1. SEM images of the

that many nano-fibrils orient along the longitudinal axis of theelectrospun iPP fiber (see Fig. 1). Moreover, the bands at 841 and1220 cm�1 belong to the characteristic of α-form crystal [18], in-dicating that the electrospun fiber is of α-form. In order to quan-titatively evaluate the orientation degree (fc) in the electrospun iPPfiber, the following expression was used [19]:

=( )⊥

RA

A 1//

= −+

+− ( )

⎛⎝⎜

⎞⎠⎟⎛⎝⎜

⎞⎠⎟f

RR

RR

12

21 2c

0

0

where R is the dichroic ratio, A// is the parallel-polarized infrared-absorbance intensity and ⊥A is the perpendicular-polarized one fora particular vibration in the observed FTIR spectra, R0 is the di-chroic ratio value when the macromolecules are completelyaligned. If the polarization moment vector is completely alignedwith the molecular chain for these bands, → ∞R0 [20]. In this case,

=+−

1RR

21

0

0. Because the minimum critical length of 1220 cm�1 band

is larger than that of 841 cm�1 band [21], the orientation degreebased on R of 1220 cm�1 band was calculated. The calculated or-ientation degree of crystalline phase is 0.83 in the electrospun iPPfiber [19]. To further reveal the oriented crystals, these electrospuniPP fibers were washed by hot xylene.

To remove the outer amorphous coating of iPP, a bundle ofelectrospun fibers was washed with hot xylene and therefore or-iented crystals could be exposed. As shown in Fig. 3a and b, only

electrospun iPP fibers.

Fig. 3. SEM images of the electrospun iPP fibers washed with hot xylene.

S. Liu et al. / Materials Letters 172 (2016) 149–152 151

the outer micellar junctions conglutinating nano-fibrils to formfibril can be removed after washing for 10 min in hot xylene.Moreover, nano-fibrils can be easily distinguished (the typicalnano-fibrils are shown by arrow in Fig. 3c). Interestingly, suchnano-fibrils not only orient along the electrospun fiber's long-itudinal axis but also maintain a certain distance between eachother. Meanwhile, some immature lamellae perpendicular to thefiber axis are observed (see the arrows in Fig. 3d). It is undoubtedlyconcluded that such nano-fibrils are shish-kebab structures. This isthe first time to confirm, via SEM observation, that shish-kebabstructure can be developed in electrospun iPP fiber.

Based on the aforementioned experimental results, the for-mation mechanism of the observed shish-kebab structure can beexplained as follows. During the electrospinning process, the longrandom molecular chains in the solution will be stretched due tothe strong elongational force of electrostatic field. Once subjectedto electrostatic field, the bundle of oriented chains forms the shishstructures that can act as stable nuclei for subsequent crystal-lization. That is, the remaining crystallizable free chains of the gelnetwork will crystallize epitaxially or “soft epitaxially” on theshish [22–24].

4. Conclusions

In conclusion, iPP fibers were successfully fabricated via high-temperature solution electrospinning. After being washed with

hot xylene, the inner crystal morphology of electrospun fiber wasexposed and directly observed via SEM. Interestingly, a singleelectrospun fiber consists of many nano-fibrils which are domi-nant by shish-kebab structures. Elongational force is considered tobe responsible for the development of such oriented crystals.Moreover, owing to the confined crystallization environmentsduring electrospinning, shish-kebab structures are imperfect.Shish-kebab structure may influence the fiber's mechanical prop-erties, which will naturally be our future work.

Acknowledgment

We express our great thanks to the Major State Basic ResearchProjects (2012CB025904), HASTIT (2012HASTIT002) and Plan forScientific Innovation Talent of Henan Province for financial support(144100510003).

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.matlet.2016.02.111.

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