shell structure of electrospun polycarbonate nanofibers

Accepted Manuscript Core/shell structure of electrospun polycarbonate nanofibers Xiaofeng Wang, Yiyang Xu, Yongchao Jiang, Jing Jiang, Lih-Sheng Turng, Qian Li

PII:

S0142-9418(18)30608-1

DOI:

10.1016/j.polymertesting.2018.08.009

Reference:

POTE 5571

To appear in:

Polymer Testing

Received Date: 12 April 2018 Revised Date:

20 July 2018

Accepted Date: 3 August 2018

Please cite this article as: X. Wang, Y. Xu, Y. Jiang, J. Jiang, L.-S. Turng, Q. Li, Core/shell structure of electrospun polycarbonate nanofibers, Polymer Testing (2018), doi: 10.1016/ j.polymertesting.2018.08.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Core/Shell Structure of Electrospun Polycarbonate Nanofibers Xiaofeng Wang,1 Yiyang Xu,1,2 Yongchao Jiang,1 Jing Jiang,3* Lih-Sheng Turng,2 and Qian Li1

National Center for International Research of Micro-Nano Molding Technology, School of

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Mechanics & Engineering Science of Zhengzhou University, 450001, China.

Wisconsin Institute for Discovery, University of Wisconsin-Madison, 53715, USA.

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School of Chemical Engineering and Energy of Zhengzhou University, 450001, China.

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Abstract

Internal structure is the key to tailoring the performance of electrospun (ES) nanofibers. However, it still remains very challenging to characterize the structures inside ES fibers. In this study, ES polycarbonate (PC) nanofibers were successfully cut open along and across the

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fiber axis by embedding. The characterization results revealed that these sections exhibited a hetereogeneous core layer structure was formed due to the phase separation. A clear

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core/shell-like structure consequently formed, whic is caused by the different evaporation behavior. The thickness of the shell layer slightly decreased with decreased fiber diameter,

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while the core layer showed a dramatical linear decrease. The dominant part was switched from the heterogeneous core layer to shell layer with high molecular orientation, which enables the production of nanofibers with superior properties.

Keywords: electrospinning, polycarbonate, nanofiber, core/shell structure

––––––––– Corresponding authors: [email protected]

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1. Introduction Electrospun (ES) polymeric nanofibers are a remarkably appealing 1D nanomaterial due to their huge specific area, excellent mechanical properties, easy production, and low cost [1-4].

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As such, they show great potential in air purification, energy storage, biomedicine, and tissue engineering applications [5-9]. In the ES process, a charged droplet of polymer solution undergoes stretching at a high strain rate and solvent evaporation induces solidification [10, 11], which yields polymer nanofibers. The most important factor that affects the performance

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of ES fibers is the configuration of polymer chains inside the fibers [12-16]. However, it still

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remains difficult to comprehensively understand the structure evolution of the chains due to limited methods for characterization. It would be of great value to be able to investigate the chain structure inside of ES nanofibers as it would enable the optimization of fiber properties and broaden their industrial application [17].

Orientated chain structure can be detected by atomic force microscopy (AFM), X-ray

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diffraction (XRD), Raman spectroscopy, and other methods [18-20]. However, direct morphological proof of this structure is rarely obtained. Furthermore, although the ES fibers

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theoretically have a core/shell structure due to varied chain orientation [21], this has seldom been demonstrated [22]. In our previous research, we successfully characterized the

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morphology of the orientated chain structure of amorphous ES nanofibers [23]. We found that there were some signs of a core/shell structure inside of the fibers, which could help fully understand the behavior of the polymer chains during the electrospinning process. In this study, ES polycarbonate (PC) nanofibers were embedded in a resin agent and cut using an ultramicrotome. The radial and axial cross sections were imaged using AFM, which supplied direct proof of the core/shell structure. In addition, the statistical results of transmission electron microscopy (TEM) characterization was conducted to investigate the variation of core and shell thickness with respect to the decrease of fiber diameter.

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2. Experimental Section 2.1 Material Polycarbonate (PC) 2720 (Mw = 42,000 g/mol) with a Tg of 155 oC was purchased from SABIC (USA). Tetrahydrofuran (THF), dimethyl formamide (DMF), and hexadecyl trimethyl

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ammonium bromide (CTAB) were obtained from Kermal Co (Tianjin, China). The embedding agent, Epon 812, was bought from Shell Chemical (USA). All of the materials were used as-received without further purification.

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2.2 Electrospinning

A high voltage supply was purchased from Gamma (USA). Electrospun nanofibers were

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fabricated from a solution with 16 wt. % PC that was dissolved in a mixed solvent of THF and DMF at a ratio of 7:3 (v/v). CTAB was added to the solution to increase the conductivity at a concentration of 0.2 wt. % PC. The PC nanofibers were electrospun at room temperature and ambient humidity (around 40%), with a solution feed rate of 0.6 mL/h, a receiving distance of

Supporting Information.

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20 cm and an applied voltage of 18 kV. More experimental details are provided in the

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3. Results and Discussion

PC nanofibers were electrospun from a solution composed of 16% PC and 0.2% CTAB by

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weight of PC. The solvent used was a mixture of tetrahydrofuran (THF) and dimethyl formamide (DMF) at a ratio of 7:3 by volume. As-spun PC nanofibers exhibited smooth surfaces and uniform diameters, as shown in Figure 1 (a). This differed from the beaded fibers reported by Shawon [24] due to the addition of CTAB, which increased the conductivity and decreased the viscosity of the PC solution, thereby eliminating the beads and leading to uniform PC fibers. The mean fiber diameter was 285 ± 69 nm, which was averaged from a population of more than 100 fibers. However, the phase contrast image using AFM (see Figure S1 (d)) showed a surface with a high quantity of irregular fibrils that were oriented along the fibers. These fibril-like structures were the aligned and aggregated PC chains that we reported [23]. -3-

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Figure 1. (a)–(c) The internal structure of as-spun PC nanofibers imaged using TEM. The scale bars measure 1 µm in (a), 500 nm in (b), and 100 nm in (c). (d) A possible mechanism for the formation of such structures.

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The TEM images revealed an heterogeneous structure inside of the fibers, as well as a core/shell-like structure (as shown in Figures 1 (b)–(c)). A possible mechanism for the

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formation of the core/shell structure is shown in Figure 1 (d). At the beginning (Stage I), surrounded by a large amount of solvent molecules, PC chains gain the ability to rotate and move [23]. Under the action of high voltage, the chains will stretch and orient along the fiber [19]. As the fibers move to the collector, the solvent molecules will constantly go into the air through the fiber surface [25], which inevitably leads to the solvent concentration gradient (Stage II). On the surface, the solvent concentration is lower than that inside the fibers, causing a decrease in chain mobility near the surface. As the solvent amount decreases to a certain degree, the fiber surface begins to solidify [25]. However, inside the fibers there is still -4-

ACCEPTED MANUSCRIPT enough solvent to allow the PC chains to keep stretching and form structures with chain orientation[21]. Meanwhile, phase separation might occur inside the fibers leading to the solvent-rich and polymer-rich domains [26-28]. As the solvent evaporated combined with high fiber stretching effect in the electrospinning process, the polymer-rich phase

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concentrated and then solidified into the matrix (Stage III). On the other hand, the solvent-rich phase eventually became to the less dense phase as solvent dried out. As seen in Figure 1 (c), there were many sharp dark area and light area in the core part of the fiber corresponding to

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the heterogeneous structure composed the matrix and less dense phase, respectively, which demonstrates the hypothesis above. As all of the solvents evaporated completely, the fibers

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solidified therefore the PC nanofiber exhibited two regions with different PC chain aggregation states along the radial direction: the core and the shell of the core/shell structure

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(as shown in Figure 1 (c)).

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ACCEPTED MANUSCRIPT Figure 2. (a) and (c): Phase contrast, and (b) and (d): Topography of the radial and axial sections of a single PC ES nanofiber. To further investigate the section of core/shell structure, as-spun PC nanofibers were embedded into resin. After solidification of liquid resin, the fibers were cut using an

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ultramicrotome. Detailed information of sample preparation and cut is provided in the supporting information (see Figure S2). As shown in Figure 2 (a), the phase contrast of the fiber radial section (as related to the topography shown in Figure 2 (b)) exhibited a clear

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core/shell structure. The area near the fiber surface was dark while the area near the fiber center was yellow. This difference of color (phase degree) represented a different mechanical

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response [29], which revealed the difference of the PC chain aggregation state in the core versus the shell of the core/shell structure. Besides the core/shell structure, irregular fibrils in the TEM image could also be found in the axial section along the fiber, as shown in Figures 2 (c) and (d). However, the phase contrast image using AFM showed a surface with a high

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quantity of irregular fibrils that were oriented along the fibers. These fibril-like structures

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were the aligned and aggregated PC chains that we mentioned above.

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Figure 3. Internal structure of a PC nanofiber: (a) before heat treatment, (b) annealed for 2 h at 120 oC, (c) annealed for 24 h at 120 oC, and (d) annealed for 2 h at 160 oC.

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Making a certain degree of change to the aggregated chain structure could help to understand chain behavior during processing [30, 31]. In this study, we investigated the chain behavior

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during ES processing by comparing the substructures within the fibers before and after annealing. A comparison of the substructures of as-spun PC nanofibers before and after heat treatment is shown in Figure 3. Before annealing, the core/shell structure was clear and the orientation structure (irregular fibrils in the core layer) exhibited a sharp outline. After annealing for 2 and 24 h at 120 oC, the core/shell structure was still clear. However, there was little difference in the orientation structure, which became less sharp after heat treatment. This was mainly because of the relaxation of the chain conformation. When the annealing temperature was raised to 160 oC—which was higher than the glass transition temperature of -7-

ACCEPTED MANUSCRIPT PC (155 oC)—after annealing for 2 h, the substructure vanished completely, including the core/shell structure and the chain orientation structure in the core layer. These results proved that chains near the fiber surface and in the center were in different aggregated states, and that the heterogeneous structure in the core became homogenous after annealing due to chain

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relaxation. The heterogeneity was highly forced and maintained due to rapid solidification of PC fibers. However, this heterogeneity was thermodynamic instable and once the PC chains regained the mobility by heating up to a temperature above PC’s glass transition temperature,

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highly heterogeneous substructure along fiber axis became homogeneous consequently.

Figure 4. The statistical and fitting results of the core layer and shell layer with respect to fiber diameter. Thshell and Thcore stand for the thickness of the shell and core layer, respectively. Pshell and Pcore represent the area percentage of shell layer and core layer to the cross section of fibers with different diameter. As shown in Figure 4, the shell thickness slightly increased with increased fiber diameter. However, the core thickness exhibited a linear increase with an increase in fiber diameter. The -8-

ACCEPTED MANUSCRIPT data in Figure 4 were measured from more than 40 isolated PC fibers with diameters ranging from 200 to 800 nm. Five different points for each single fiber were measured and averaged, and some of the statistical samples are shown in Figure S3. It appeared that the evaporation effect of the solvent determined the shell layer thickness. The reason might be that for fibers

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with different diameter, the formation mechanism might have been the same. Solvent on the fiber surface evaporated very fast due to the fast travelling polymer jet which resulted in a solidified shell layer. This solidified layer further restrained the solvent evaporation in the

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core area, and thus left a very different structure of shell layer from the core layer. Under certain circumstances, the solvent evaporation rates of different fibers were the same, which

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led to small differences in the shell thickness of different fibers. Meanwhile, the travelling path of the fibers with different diameter might have been similar to each other because of the constant collecting distance between the needle to the collector. The above two reasons determined that only slightly variations showed in shell thickness. However, the percentage of

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shell layer increased dramatically with the fiber diameter decreasing. Recalling that the structure of shell layer and core layer were different indicates a higher mechanical properties of the shell layer. The layer with a higher mechanical properties occupied the dominant

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section area which chorded to a dramatical increase in overall mechanical properties. In other words, the slightly variation of shell layer is the key of the size effect of ES PC nanofibers.

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It should be mentioned that the rate of solvent evaporation is affected by the intrinsic properties of the polymer and solvent, the electrospinning process, and environmental conditions [11]. Therefore, the size effect might be varied with respect to the fiber material and processing conditions. The thickness of the core/shell structure might be tuned by adjusting the solvent evaporation. This will favor optimization of the mechanical performance of ES fibers and enable their use in a broader range of applications such as bone tissue engineering and fine particle filtration.

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ACCEPTED MANUSCRIPT In summary, as-spun PC nanofibers exhibited a clear core/shell structure that formed due to a high strain rate and solvent-evaporation-induced phase separation. The thickness of the shell layer, regulated by the rate of solvent evaporation, showed less dependence on the fiber diameter and remained around 50 nm, while the thickness of the core layer increased linearly

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with fiber diameter. Phase contrast images of the section along the fiber revealed a clear heterogeneous sub-structure that was also confirmed by TEM results. The heterogeneity was caused by phase separation in the core part. Statistic result reveals that the thickness of shell

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layer decreases slightly with the fiber diameter decreasing, while the thickness of core layer shows a much higher slope of the decrease. The reason might be the evaporation behavior

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difference between shell and core. The evaporation in the formation of nanofiber might be tuned to optimize the structure of ES nanofibers, thereby greatly extending its application range.

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Supporting Information

Supporting Information Supplementary data related to this article can be found at http://dx.

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doi.org/10.1016/j.polymertesting.******

Acknowledgements: The authors acknowledge financial support from the International

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Science & Technology Cooperation Program of China [2015DFA30550] and the National Natural Science Foundation of China [11372286]. They are also grateful for the technical support from the Wisconsin Institute for Discovery at the University of Wisconsin–Madison.

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Highlights:

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1. Core-shell structure of polycarbonate nanofibers was derived based on the TEM and AFM results.

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2. Heterogeneous core substructure was formed due to the phase

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separation in the core area.

3. Shell layer only showed slightly decrease as the decrease of fiber

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diameter.