polydimethylsiloxane composites

Accepted Manuscript Uniformly dispersed polymeric nanofiber composites by electrospinning: Poly(vinyl alcohol) nanofibers/polydimethylsiloxane composites Kentaro Watanabe, Tomoki Maeda, Atsushi Hotta PII:

S0266-3538(17)32770-7

DOI:

10.1016/j.compscitech.2018.06.007

Reference:

CSTE 7263

To appear in:

Composites Science and Technology

Received Date: 14 November 2017 Revised Date:

20 April 2018

Accepted Date: 7 June 2018

Please cite this article as: Watanabe K, Maeda T, Hotta A, Uniformly dispersed polymeric nanofiber composites by electrospinning: Poly(vinyl alcohol) nanofibers/polydimethylsiloxane composites, Composites Science and Technology (2018), doi: 10.1016/j.compscitech.2018.06.007. 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 Uniformly dispersed polymeric nanofiber composites by electrospinning: poly(vinyl alcohol) nanofibers/polydimethylsiloxane composites Kentaro Watanabe, Tomoki Maeda, and Atsushi Hotta

Corresponding Author: Professor Atsushi Hotta E-mail: [email protected]

SC

Yokohama 223-8522, Japan

RI PT

Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku,

Postal address: Department of Mechanical Engineering, Keio University, 3-14-1

M AN U

Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan

Abstract

A method for the fabrication of homogeneous and well-dispersed polymeric nanofiber composites was investigated. Nanofiber fillers can be used to produce

TE D

polymeric nanocomposites by mixing the fillers to base polymers, eventually enhancing the mechanical property of the matrix polymers. To produce such composites, nanofibers were usually sandwiched by molten matrix polymers at high temperature

EP

before molding. The traditional so-called sandwich method, however, was found to produce rather biased and inhomogeneous composites due largely to the solid

AC C

entanglement of the nanofibers. In this work, unwoven polymer nanofibers were synthesized through electrospinning by controlling the electrostatic repulsion of the nanofibers. We modified the electrospinning apparatus for the direct synthesis of homogenous composites: nanofibers were electrospun and directly ejected from the electrospinning syringe to the matrix polymer solution (not in a solid state), where a regular metal electrode plate was replaced by an optimized metal container containing the base polymer solution. It was found that this new fabrication method could realize homogeneous mixing of the nanofibers that were eventually uniformly dispersed in the

1

ACCEPTED MANUSCRIPT polymer solution. Poly(vinyl alcohol) (PVA) was used for nanofibers and polydimethylsiloxane (PDMS) was used for polymer matrix. The field emission scanning electron microscopy (FE-SEM) revealed the homogeneous and well-dispersed PVA nanofibers in the resulting PDMS composites. The composites also presented

RI PT

higher mechanical properties as compared with the composites fabricated by the traditional sandwich method.

Keywords: Fibres, Nano composites, Electro-spinning, Polymer-matrix composites

SC

(PMCs), Mechanical properties

M AN U

1. Introduction

Polymeric nanofiber composites were nanocomposites, possibly providing higher mechanical properties with superior structural characteristics including high transparency due to their nanoscale structures, as compared with traditional fiber composites by carbon, glass, and Kevlar fibers [1-7]. To produce polymeric nanofibers

TE D

for composite fillers, electrospinning has been widely used and recognized as an efficient way [8]. Various polymers have recently been successfully electrospun into ultrafine fibers [9-11]. Fine fibers with a very large surface area could create unique

EP

properties such as the flexibility in surface functionalities, the superior mechanical performance, and the good interfacial adhesion [12-17]. In fact, Liao et al. reported a

AC C

cellulose acetate (CA) nanofiber-reinforced epoxy composite film via electrospinning and solution-impregnation methods [13]. Moreover, it was found that the strong interfacial adhesion of the CA/epoxy composites resulted in the high mechanical strength.

As for the fabrication methods for composites, there have been quite a few researches on the manufacturing of nanofiber composites using electrospun nanofibers. The composites were generally obtained by sandwiching electrospun nanofibers by base polymer films using a hot press, and the sandwich process was carried out after the

2

ACCEPTED MANUSCRIPT fabrication of nanofiber sheets by electrospinning [18-20]. For the sandwich method, the glass-transition temperatures (Tg) and the melting points (Tm) of polymeric fibers as well as matrix polymer are both important: a composite should be molded below Tg or Tm of the fibers, but above Tg or Tm of the base polymer, so that the molten base

RI PT

polymer could effectively penetrate and fill the voids of the nanofiber sheets when blended [19]. The resulting sandwiched composites were reported to present enhanced mechanical properties for highly functionalized polymers: controlled biodegradability

SC

by polyvinylpyrrolidone fibers/polycaprolactone and enhanced mechanical properties by CA nanofibers/ poly(butylene succinate) [18, 20]. These composites made by the

M AN U

hot-press method, however, were relatively anisotropic and inhomogeneous, containing nanofibers with insufficient dispersion in the base polymer especially in the depth (thickness) direction. In particular, the inhomogeneity of the nanofiber composites in the thickness direction could become prominent as the amount of the fibers decreased, while the uniformity in the thickness direction of the composite could be obtained when

TE D

the concentration of the fibers reached as high as 40-70%. This was mainly due to the firm entanglements of the nanofibers established during the synthesis through electrospinning.

EP

The main purpose of this work is to modify the existing electrospinning method for the direct synthesis of nanofiber composites to realize uniform nanofiber

AC C

dispersion. Our challenge in this work particularly lies in improving the inhomogeneity of the nanofiber composites due to the lower concentration of fibers, as well as studying the nano-effects of the PVA/PDMS composites at the low concentrations of the PVA fibers (1-6 wt%). Instead of using an existing earthed metal plate, polymer solution was used as a new earthed collector that was filled up in a newly designed grounded metal container (Fig. 1). After crosslinking, the dispersibility of poly(vinyl alcohol) (PVA) nanofibers was studied using the field emission scanning electron microscopy (FE-SEM) through the ruptured composite surfaces. The mechanical properties of the

3

ACCEPTED MANUSCRIPT composites were also investigated by analyzing the stress-strain curves obtained by a tensile tester.

2. Materials and Method

RI PT

2.1 Materials

For the nanofibers, PVA was used (from Junsei Chemical Co., Ltd.), which has the molecular weight of Mw~66,000 measured by the gel permeation chromatography

SC

(GPC), Tm of 228°C measured by the differential scanning calorimetry (DSC), and the Young’s modulus of 2.5 GPa measured by a tensile tester. Distilled water and ethanol

M AN U

(both from Wako Pure Chemical Industries, Ltd.) were used to dissolve PVA. PDMS (SILPOT 184 from Dow Corning Toray Co., Ltd.) was used as the matrix polymer with the Young’s modulus of 1.4 MPa.

2.2 Modified electrospinning setting for the synthesis of the fiber composites A new collector for the electrospinning was designed to obtain the aimed

TE D

composites in which nanofibers were well dispersed. In an ordinary electrospinning apparatus, the existing metal collector (plate) was simply grounded to gather fabricated nanofibers as indicated in Fig. 1 (a). For the new setting, the base polymer solution was

EP

poured into a grounded metal container instead of the ordinary metal plate. Fig. 1 (b) shows the new optimized setting of the electrospinning apparatus presenting a

AC C

cylindrical doughnut-shaped silicone rubber packing being laid on to an aluminum tub to contain liquid base polymer. The dimensions of the insulating rubber packing and the conducting aluminum were both pre-examined and optimized: for the rubber packing, 55 mm for the internal diameter, 92 mm for the external diameter, and 0.5 mm for the thickness were selected and for the aluminum container, 100 mm for the external diameter. With these optimized experimental settings, the electrospun nanofibers could be neatly synthesized and ejected into the polymer solution. 2.3 Fabrication of well-dispersed PVA/PDMS composites (PVA/PDMS-d)

4

ACCEPTED MANUSCRIPT PVA pellets were dissolved in water/ethanol (10/1) to make 8 wt% of PVA solution. The solution was stirred at 500 rpm for 24 hrs at 90˚C for the complete dissolution. 0.3-1.5 g of the base PDMS prepolymer solution with a crosslinking agent (PDMS prepolymer/crosslinking agent=10/1) was prepared and installed in the collector

RI PT

mentioned above. PVA nanofibers were then fabricated using an electrospinning apparatus (IMC-1639 from Imoto Machinery Co., Ltd.) with a 5 mL syringe (Model 1005 LT from Hamilton Co.) and a needle of 0.53 mm in diameter, where the applied

SC

voltage was set at 7.2 kV, the feed rate was set at 0.3 ml/hour, and the tip-target distance was set at 8 cm. The homogeneous nanofiber morphology of the electrospun PVA was

M AN U

confirmed by the SEM micrograph as shown in Fig. S1, which should make significant contributions to the mechanical property of the resulting PVA/PDMS composites. After blending the PVA nanofibers with the PDMS prepolymer, the composites were crosslinked at 50°C under the pressure of 1 atm for ~24 hours. For comparison, the traditional sandwiched PVA/PDMS composites (PVA/PDMS-s) were also made by the electrospinning

method.

Other

electrospinning

conditions

for

TE D

conventional

PVA/PDMS-s were the same as those for PVA/PDMS-d. The PVA concentration in the PVA/PDMS composite was calculated as

EP

follows:

AC C


     % =

 −  × 100 

where WComposite was the weight of the PVA/PDMS composite and WPDMS was the weight of the PDMS used for the fabrication of the PVA/PDMS composite. 2.4 Dispersibility of the PVA nanofibers in the composites To evaluate the dispersibility of the PVA nanofibers in the PDMS matrix, microstructural observations were carried out by SEM. After the tensile test, the fracture surface was scanned using the FE-SEM to study the fiber morphology. The composite

5

ACCEPTED MANUSCRIPT specimens were coated with osmium to prevent charging during the SEM measurements. 13 pictures were taken down to the depth of 70 µm from the surface of the composites, and the detection frequency of PVA nanofibers was set and measured over the 6 µm× 14 µm square area of the SEM micrographs.

RI PT

2.5 Transparency of the PVA/PDMS composites

The light transmittance was investigated by the spectral photometer (U2810 from Hitachi High-Technologies Co.) from 350 nm to 750 nm in wavelength for the

SC

measurement of the transparency of the composites. 2.6 Young’s modulus and fracture strain measurements

M AN U

Tensile testing was performed to examine the mechanical properties of PVA/PDMS-d and PVA/PDMS-s. A dog-bone shaped specimen of 16.5 mm × 3 mm was prepared and used for the mechanical testing. The appropriate tensile load was applied to the samples with the crosshead speed of 3 mm/min using a universal testing machine (AG-IS, Shimadzu Co.). The gauge length, the distance between the two jaws,

TE D

was set at 22 mm, and at least 4 specimens were tested for the determination of the mechanical properties of each sample.

EP

3. Results and discussions

3.1 Dispersibility of PVA nanofibers

AC C

As shown in Fig. 2, a partially coarse surface was observed in the fractured surface of PVA/PDMS-s at the PVA concentrations of 1 wt% and 4 wt%. Comparing the fractured surface of the PVA/PDMS-s with that of pure PDMS base polymer (Fig. S1), the coarse parts of the PVA/PDMS-s were produced by PVA nanofibers near the surface of the composite film. In fact, the aggregations of the PVA nanofibers with 150 nm in diameter only existed near the surface of the composite and the nanofibers were not found relatively deep inside the composite. The experimental results indicated that the PVA nanofibers had already been well entangled with each fiber before mixing using

6

ACCEPTED MANUSCRIPT the conventional sandwich method, where the fibers were firmly fixed and immobile during the mixing with liquid uncrosslinked PDMS. The entangled PVA nanofibers could never be untied and loosened and consequently, and an inhomogeneous PVA/PDMS-s composite was obtained by the traditional sandwich method.

RI PT

Fig. 3 shows the SEM micrographs of the fractured surface of PVA/PDMS-d fabricated by the modified ES method. In contrast to the PVA/PDMS-s, the PVA/PDMS-d had PVA nanofibers not only at the surface but also deep inside the

SC

composite. It was therefore concluded that the new ES method of using liquid polymer as a grounded collector could readily produce a relatively homogeneous composite with

M AN U

well-dispersed PVA nanofibers in PDMS.

Fig. 4 shows the evaluation of the dispersibility of PVA nanofibers by the SEM micrographs regarding (a) PVA/PDMS-s and (b) PVA/PDMS-d. For the quantitative evaluation of the dispersibility, the cross-sectional micrograph of the composites was divided into small rectangles of 6 µm×14 µm, as was presented in Fig. 4 from the

TE D

surface to the bottom of the composite. By counting the number of the nanofibers penetrating the sides of the rectangles, the dispersibility of the nanofibers could be analyzed by presenting the frequency of the fibers shown in Fig. 5. Y-axis shows the

EP

frequency of the nanofibers and X-axis shows the distance from the surface (i.e. the depth: 0 µm for the surface and 80 µm for the bottom of the composite). As was

AC C

mentioned above, it was confirmed that PVA/PDMS-s indeed presented inhomogeneity exhibiting aggregations of the PVA nanofibers near the surface. In contrast, as for the PVA/PDMS-d, independent nanofibers were evenly scattered from the surface to the bottom of the PVA/PDMS-d specimens. In more detail, ~95% of the nanofibers preferably existed within the depth of 20 µm for the PVA/PDMS-s. The average frequency for the PVA/PDMS-d specimens was about 8% almost uniformly dispersed from 0 µm to 80 µm in depth. This is because, as was discussed, the PVA nanofibers in the PVA/PDMS-s were already too entangled to diffuse when the PDMS prepolymer

7

ACCEPTED MANUSCRIPT was added to the electrospun PVA nanofiber sheets. PVA nanofibers in the PVA/PDMS-d on the other hand, could be widely dispersed throughout the composite, since PVA nanofibers were electrospun and, at the same time, directly ejected into the liquid PDMS prepolymer before the hard entanglement could be established, eventually

RI PT

exhibiting higher dispersibility in the resulting composites. It was expected that the colloidal-dispersion effects were realized in our nanofiber composites due to the nanoscale size of the PVA nanofibers. Macroscopically, it could also be due to the larger

SC

density of the PVA as compared with that of the PDMS: the density of the PVA was ~1.3 g/cm3 and the density of the PDMS was ~0.97 g/cm3, while the thickness of the

M AN U

PDMS was ~0.5 mm. Hence the settling of the PVA nanofibers and the viscous resistance driven by the viscous PDMS prepolymer could well balance to fabricate a homogeneous composite with good dispersion in our experiments. 3.2 Transparency of the composites

It was generally known that when different types of materials were mixed

TE D

inhomogeneously, the transparency of the resulting composite may be low, becoming translucent or rather turbid due to the dispersibility, the filler aggregations, the micro-voids, and/or the difference in the refraction index between the matrix and the

EP

fillers. Thus, the transparency of the newly synthesized well-dispersed nanofiber composites was investigated. Fig. 6 shows the light transmittance of the PVA/PDMS

AC C

composites measured by a UV-VIS spectral photometer. The thickness of the base film thickness was calculated as 0.1 mm by the Lambert-Beer law [21]. By increasing the PVA concentration, the average light transmittance of the PVA/PDMS-s composites became ~28% lower than that of pure PDMS. The average light transmittance of the PVA/PDMS-d, on the other hand, became ~15% lower than that of pure PDMS even at the highest concentration of 4 wt%. In fact, the light transmittance of the PVA/PDMS-d was 13% higher than that of the PVA/PDMS-s. The fact that the light transparency of the PVA/PDMS-d film was significantly higher than that of the PVA/PDMS-s film was

8

ACCEPTED MANUSCRIPT also confirmed from the macroscopic view (Fig. S2). Therefore, it was confirmed that increasing the PVA-nanofiber concentration decreased the light transmittance of the resulting composites, and that the composites with well-dispersed nanofibers retained higher transparency than the composites with aggregated nanofibers synthesized by the

RI PT

conventional sandwich method due to the light scattering. As compared with the previous methods, our newly modified method could generate highly transparent

complex process for the synthesis. 3.3 Mechanical properties of the composites

SC

polymeric composites by a relatively simple system requiring neither vacuuming nor

M AN U

The effects of the fiber concentrations on the mechanical properties of the PVA/PDMS composites were studied. Fig. 7 shows the results of the tensile testing on the PVA/PDMS-d films with various PVA concentrations (0 wt% (pure PDMS), 0.1 wt%, 1 wt%, 2 wt%, 4 wt%, and 6 wt%). The pure PVA-nanofiber film showed a typical stress-strain curve of a brittle material with a sharp inclination and the low ductility in

TE D

its stress-strain curve. The pure PDMS film presented a distinctive stress-strain curve of a rubber material with relatively the low gradient and the high ductility in its stress-strain curve. In fact, the well-dispersed isotropic PVA/PDMS-d composites

EP

almost fully retained the highly elastic characteristics up to 10% of the PVA-fiber concentration. The observed changes in the stress-strain curves by varying the fiber

AC C

concentrations were more drastic than the changes observed for the PVA/PDMS-s (Fig. S3).

The Young’s moduli and the fracture strains of all the composites measured by

the tensile testing were presented in Fig. 8. It was found that both the PVA/PDMS-d and the PVA/PDMS-s films had the higher Young’s moduli than the neat PDMS film with the Young’s modulus of 1.4 MPa. In more detail, at 6 wt% of the fiber concentration, the Young’s modulus of the sandwiched PVA/PDMS-s composite showed approximately 6 times higher than that of the pure PDMS. The Young’s modulus of PVA/PDMS-d was

9

ACCEPTED MANUSCRIPT getting much higher, reaching approximately 14 times higher than that of the pure PDMS and even about 2.3 times higher than that of the PVA/PDMS-s film with the same fiber concentration. The difference in the Young’s moduli between PVA/PDMS-s and PVA/PDMS-d may highly be due to the unavoidable micro-voids and the fiber

RI PT

aggregations caused by the hard fiber entanglement that was intrinsic to the sandwiched composites. According to Fig. 8 (b), the fracture strains of the PVA/PDMS-d were almost constant regardless of the fiber concentrations. It was also found that the fracture

SC

strains of the PVA/PDMS-s were lower than those of PVA/PDMS-d.

M AN U

4. Conclusions

A homogeneous composite with well-dispersed nanofibers was made using a base polymer in its liquid state, either in melt or in solution, during the electrospinning, and the liquid base polymer was also used as an earth collector for the electrospinning. Without using an ordinary metal plate as an electrospinning collector, electrospun

TE D

nanofibers could be properly fabricated, simultaneously being ejected into the liquid polymer matrix to fabricate a homogeneous composite with well-dispersed fibers. The cross-sectional SEM micrographs of the composites revealed the uniformly dispersed

EP

PVA-fiber structures in the PDMS matrix. Thus, it was concluded that PVA nanofibers could be well dispersed into PDMS matrix by using this new method, unlike

AC C

PVA/PDMS-s that was fabricated by the conventional method. The enhancement of the mechanical property was confirmed for a newly made PVA/PDMS-d composite which also remained highly transparent due to the nanoscale fiber dispersion. Therefore, the suggested method in this work may provide a new way to improve the inhomogeneity of the composite materials, simultaneously enhancing the mechanical properties of the composite just by adding fibers at low concentrations. Further studies on the use of the various types of polymers are expected as this method could be currently used only when the fiber polymer is not soluble to the matrix polymer in solution or in melt.

10

ACCEPTED MANUSCRIPT

Acknowledgements This work was supported in part by a Grant-in-aid for Science Research (A) and by a Fund for the Promotion of Joint International Research (Fostering Joint International

RI PT

Research) (No. 15H02298 and No. 15KK0244, respectively to A.H.) from Japan Society for the Promotion of Science (JSPS: “KAKENHI”). A.H. is grateful to Prof

stimulating suggestions and encouragements.

M AN U

Appendices A. Supplemental Information

SC

Glenn H. Fredrickson from the University of California, Santa Barbara for offering

Supplementary data related to this chapter can be found at ---.

References

[1] Z.M. Huang, Y.Z. Zhang, M. Kotaki, S. Ramakrishna, A review on polymer

TE D

nanofibers by electrospinning and their applications in nanocomposites, Composites Science and Technology 63(15) (2003) 2223-2253. [2] M.M. Bergshoef, G.J. Vancso, Transparent nanocomposites with ultrathin,

EP

electrospun nylon-4,6 fiber reinforcement, Advanced Materials 11(16) (1999) 1362-1365.

AC C

[3] D.A. Stone, N.D. Wanasekara, D.H. Jones, N.R. Wheeler, E. Wilusz, W. Zukas, G.E. Wnek, L.T.J. Korley, All-Organic, Stimuli-Responsive Polymer Composites with Electrospun Fiber Fillers, Acs Macro Letters 1(1) (2012) 80-83.

[4] G.L. Wang, D.M. Yu, R.V. Mohan, S. Gbewonyo, L.F. Zhang, A comparative study of nanoscale glass filler reinforced epoxy composites: Electrospun nanofiber vs nanoparticle, Composites Science and Technology 129 (2016) 19-29. [5] K. Liu, B.N. Zhu, T. Duan, C.L. Du, Y. Tan, S. Sheng, Transparency of chrysotile nanofiber-reinforced nanocomposite films: Effect of Rayleigh scattering,

11

ACCEPTED MANUSCRIPT Composites Science and Technology 119 (2015) 68-74. [6] B.Y. Li, H.H. Yuan, Y.Z. Zhang, Transparent PMMA-based nanocomposite using electrospun graphene-incorporated PA-6 nanofibers as the reinforcement, Composites Science and Technology 89 (2013) 134-141.

RI PT

[7] D. Chen, R.Y. Wang, W.W. Tjiu, T.X. Liu, High performance polyimide composite films prepared by homogeneity reinforcement of electrospun nanofibers, Composites Science and Technology 71(13) (2011) 1556-1562.

SC

[8] C.H. Park, C.H. Kim, L.D. Tijing, D.H. Lee, M.H. Yu, H.R. Pant, Y. Kim, C.S. Kim, Preparation and Characterization of (polyurethane/nylon-6) Nanofiber/(silicone)

(2012) 339-345.

M AN U

Film Composites via Electrospinning and Dip-coating, Fibers and Polymers 13(3)

[9] T. Maeda, K. Hagiwara, S. Yoshida, T. Hasebe, A. Hotta, Preparation and Characterization

of

2-Methacryloyloxyethyl

Phosphorylcholine

Polymer

Nanofibers Prepared via Electrospinning for Biomedical Materials, Journal of

TE D

Applied Polymer Science 131(14) (2014) 40606.

[10] T. Maeda, K. Takaesu, A. Hotta, Syndiotactic polypropylene nanofibers obtained from solution electrospinning process at ambient temperature, Journal of Applied

EP

Polymer Science 133(13) (2016) 43238. [11] K. Bito, T. Maeda, K. Hagiwara, S. Yoshida, T. Hasebe, A. Hotta,

AC C

Poly(2-methacryloyloxyethyl phosphorylcholine) (MPC) nanofibers coated with micro-patterned diamond-like carbon (DLC) for the controlled drug release, Journal of Biorheology 29 (2015) 51-55.

[12] E.P.S. Tan, C.T. Lim, Mechanical characterization of nanofibers - A review, Composites Science and Technology 66(9) (2006) 1102-1111. [13] H.Q. Liao, Y.Q. Wu, M.Y. Wu, H.Q. Liu, Effects of Fiber Surface Chemistry and Roughness on Interfacial Structures of Electrospun Fiber Reinforced Epoxy Composite Films, Polymer Composites 32(5) (2011) 837-845.

12

ACCEPTED MANUSCRIPT [14] Y. Guan, W. Li, Y.L. Zhang, Z.Q. Shi, J. Tan, F. Wang, Y.H. Wang, Aramid nanofibers and poly (vinyl alcohol) nanocomposites for ideal combination of strength and toughness via hydrogen bonding interactions, Composites Science and Technology 144 (2017) 193-201.

RI PT

[15] C. Wang, Y.J. Wu, C.Y. Fang, C.W. Tsai, Electrospun nanofiber-reinforced polypropylene composites: Nucleating ability of nanofibers, Composites Science and Technology 126 (2016) 1-8.

SC

[16] S.H. Jiang, G.G. Duan, J. Schobel, S. Agarwal, A. Greiner, Short electrospun polymeric nanofibers reinforced polyimide nanocomposites, Composites Science

M AN U

and Technology 88 (2013) 57-61.

[17] S. Lin, Q. Cai, J.Y. Ji, G. Sui, Y.H. Yu, X.P. Yang, Q. Ma, Y. Wei, X.L. Deng, Electrospun nanofiber reinforced and toughened composites through in situ nano-interface formation, Composites Science and Technology 68(15-16) (2008) 3322-3329.

TE D

[18] R. Neppalli, C. Marega, A. Marigo, M.P. Bajgai, H.Y. Kim, V. Causin, Improvement of tensile properties and tuning of the biodegradation behavior of polycaprolactone by addition of electrospun fibers, Polymer 52(18) (2011)

EP

4054-4060.

[19] R. Neppalli, C. Marega, A. Marigo, M.P. Bajgai, H.Y. Kim, V. Causin,

AC C

Poly(epsilon-caprolactone) filled with electrospun nylon fibres: A model for a facile composite fabrication, European Polymer Journal 46(5) (2010) 968-976.

[20] N. Kurokawa, S. Kimura, A. Hotta, Mechanical properties of poly(butylene succinate) composites with aligned cellulose-acetate nanofibers, Journal of Applied Polymer Science 134 (2017) 45429. [21] A. Larena, G. Pinto, F. Millan, Using the kambert-beer law for thickness evaluation of photoconductor coatings for recording holograms, Applied Surface Science 84(4) (1995) 407-411.

13

ACCEPTED MANUSCRIPT

Figure captions Fig. 1. (a) A traditional way of electrospinning and (b) a modified way of electrospinning with an assemble collector to fabricate nanofiber composites for

RI PT

uniformly dispersed nanofiber composites

Fig. 2. SEM micrographs of the fracture surfaces of PVA/PDMS-s at the fiber concentrations of (a) 1 wt% and (b) 4 wt% under several magnifications.

SC

Fig. 3. SEM micrographs of the fracture surfaces of PVA/PDMS-d at the fiber concentrations of (a) 1 wt% and (b) 4 wt% under several magnifications.

M AN U

Fig. 4. Evaluating the spatial distribution of PVA nanofibers in: (a) PVA/PDMS-s and (b) PVA/PDMS-d by SEM cross-sectional micrographs. To facilitate visualization, fibers were slightly colored brown.

Fig. 5. Frequency of PVA nanofibers against the depth of the composites. Fig. 6. Light transmittance of PVA/PDMS composites by the UV-VIS spectral

TE D

photometer

Fig. 7. Tensile stress-strain curves of pure PVA nanofiber, pure PDMS, and PVA/PDMS-d films with various PVA fiber concentrations

EP

Fig. 8. (a) Young’s moduli and (b) fracture strains against PVA nanofiber concentrations

AC C

of PVA/PDMS-d and PVA/PDMS-s

14

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT