PI tough composites and their dielectric permittivity

Author’s Accepted Manuscript Electrospun nanofiber reinforced all-organic PVDF/PI tough composites and their dielectric permittivity Youliang Shen, Linlin Chen, Shaohua Jiang, Yichun Ding, Wenhui Xu, Haoqing Hou www.elsevier.com

PII: DOI: Reference:

S0167-577X(15)30374-8 http://dx.doi.org/10.1016/j.matlet.2015.08.019 MLBLUE19380

To appear in: Materials Letters Received date: 31 May 2015 Revised date: 29 July 2015 Accepted date: 2 August 2015 Cite this article as: Youliang Shen, Linlin Chen, Shaohua Jiang, Yichun Ding, Wenhui Xu and Haoqing Hou, Electrospun nanofiber reinforced all-organic PVDF/PI tough composites and their dielectric permittivity, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.08.019 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 galley proof before it is published in its final citable 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.

Electrospun Nanofiber Reinforced All-organic PVDF/PI Tough Composites and Their Dielectric Permittivity Youliang Shen,a, b, c Linlin Chen,b Shaohua Jiang, a,b* Yichun Ding,b Wenhui Xu,b Haoqing Hou, a,b* a

School of Materials Science and Engineering, Nanchang University, Nanchang, Jiangxi, 330031, People’s

Republic of China. b

College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi 330022,

People’s Republic of China. c

Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science and Technology Normal University, Nanchang,

Jiangxi, 330013, People’s Republic of China. *

Tel:

(+86)791-88120389;

Fax:

(+86)791-88120536;

E-mail:

[email protected]

and

[email protected] Abstract Flexible materials with high mechanical strength and good dielectric properties are highly desired for modern electronics. In this research, all-organic PVDF/PI nanofiber-reinforced composites were fabricated by layer-by-layer hot-pressing the randomly PVDF nanofibers and aligned co-PI nanofibers. Dielectric, mechanical and thermal properties of the PVDF/PI composites were investigated. Results showed that the mechanical properties was highly enhanced by the aligned co-PI nanofibers and the dielectric permittivity was inherited from the intrinsic good dielectric properties of PVDF. These all-organic PVDF/PI composites with excellent mechanical strength and good tunable permittivity can be used for flexible embedded capacitors and can be further employed as the polymer matrix for preparing high dielectric polymer-matrix composites. Keywords Polyimide nanofiber; electrospinning; composites; dielectric property; mechanical property; Introduction 1

All-organic composite materials with a high dielectric constant are attracting much attention due to their low density and flexibility [1]. It is well known that polyvinylidene fluoride (PVDF) possessed relatively high dielectric constant (10~14) [2, 3], low glass transition temperature (~40 oC) and low melting temperature (170 oC) [4, 5], which made PVDF as a good matrix to prepare all-organic high dielectric composites. Polyimides (PIs) are high performance polymers with excellent thermal stability and superior mechanical strength [6]. However, the low dielectric constant (2.5~3.5) of PIs greatly limits their applications in modern electronics [7, 8]. Recently, PI composites with improved dielectric properties and good mechanical properties were successfully fabricated by incorporating inorganic fillers, such as BaTiO3 nanoparticles [9], multi-walled carbon nanotubes (MWCNTs) [10], Zirconia nanocrystals [11], hybrid BaTiO3/MWCNTs [12], and so on. Nevertheless, due to the inhomogeneous dispersion of the fillers in the matrix, the increased loading of fillers significantly weakened the mechanical properties of the composites, especially the tensile strength and toughness. Previous work found that the electrospun PI nanofibers possessed excellent mechanical strength [13], superior thermal stability [14], and super high toughness [13], which made them excellent candidates as reinforcements [15, 16] and the layer-by-layer hot pressing can improve the uniformity and reduce the porosity of polymer-based composites [17-19]. Therefore, in this work, we choose high performance electrospun copolyimide (co-PI) nanofibers as reinforcement and high dielectric performance PVDF as matrix to prepare nanofiber reinforced all-organic composites. The mechanical and dielectric properties of the composites were investigated. Experimental Copoly(amic acid) solution (co-PAA, 20 wt%) was polymerized from 3,3’,4,4’-biphenyltetracarboxylic dianhydride (BPDA), biphenylamide (BPA) and 4,4’-oxydianiline (ODA) with molar ratio of 10/4/6) in N, N-dimethylacetamide (DMAc) at -5 oC for 24 h. As-prepared co-PAA solution was diluted to 10 wt% by acetone and then electrospun with 25 kV applied voltage, 20 cm collecting distance and 1.2 mL/h flow rate. The aligned 2

PAA nanofiber belt was collected by a collector with linear rotating speed of 16 m/s and imidized by heating up to 250 oC (10 oC min−1, 30 min annealing) and then 370 oC (2 oC min−1, 60 min annealing). PVDF/DMAc solution (20 wt%) was electrospun with 16~18 kV voltage, 20 cm collecting distance and 1.0 mL/h flow rate. The randomly nanofibers were collected by aluminum paper and then dried (vacuum, 100 oC, 10 h). PVDF/PI composite films (4 × 5 cm) were laminated from aligned co-PI nanofiber belt and PVDF nanofiber sheet by heat-pressing (200 oC, 3 MPa) with vacuum. The co-PI nanofiber amount were controlled as 10, 20, 30, 40 and 50 wt%, which were marked as S-1, S-2, S-3, S-4 and S-5. As comparison, the pure PVDF (S-0) and co-PI samples (S-6) with the same pressing conditions were also prepared. Scanning Electron Microscope (SEM, TESCAN vega3) was used to characterize the morphologies of samples. Dielectric properties were performed on a precision LCR meter (TH2819-A, Tonghui Electronic Co., Ltd., China) at the frequency ranges from 100 Hz to 100 kHz at ambient environment. All samples (8 × 8 × 0.1 mm) were adhered with silver conductive on the two sides as electrodes to avoid undesired capacitance and resistance. Differential Scanning Calorimetric (DSC, NETZCH-DSC 200F3) was performed from 25-300 oC (10 oC/min, N2). Mechanical properties were determined on an electromechanical testing machine (CMT-8102, Shenzhen, China) with 10 mm/min along the fiber alignment. The thickness were measured by screw micrometer. The mechanical properties of pure co-PI was measured by using the nanofiber belt without hot-pressing. Results and discussion Previous study showed that the aligned co-PI nanofiber belts had the largest tensile strength of about 1.1 GPa and the parameter of thickness for the test was calculated from the sample weight and the density of the corresponding co-PI [20]. In this work, the aligned co-PI nanofiber belts had the tensile strength of 179.5 MPa (Table 1), but the thickness was directly measured by the screw micrometer and composed of thickness from the co-PI solid and the pores in the samples. Therefore, the porosity of the as-prepared 3

co-PI nanofiber belt could be calculated by [(1-179.5/1100)*100%] = 83.6%, which is helpful for the infiltration of the PVDF (melting temperature of 164 oC, Fig. S3) into the co-PI nanofibers. Besides, the co-PI nanofibers possessed high Tg of 292 oC [20] and they would keep the fiber morphologies during the hot-pressing process. The well-retained co-PI nanofibers in the resulting PVDF/PI composites were preserved as skeletal framework in PVDF composites (Fig. 1). The co-PI nanofibers were homogenously embedded in the PVDF matrix due to the high porosity of co-PI nanofiber belt, which is a guarantee for the good reinforcing effect from co-PI nanofibers.

Figure 1. Cross sectional SEM images of the PVDF/PI composites with 20 wt% (a, b) and 50 wt% (c) aligned co-PI nanofibers. The samples were broken in liquid nitrogen along the longitudinal direction. The mechanical properties and the typical stress-strain curves of bulk PVDF film, co-PI and PVDF/PI composites are shown in Table 2 and Fig. 2. The bulk PVDF film (S-0) from hot-pressed random PVDF nanofibers showed a tensile strength of 43.7 MPa, modulus of 0.59 GPa and an elongation at break of 497.76%, which presented the excellent flexibility. By adding the aligned co-PI nanofibers, the mechanical strength of the composites was obviously increased. Compared to the bulk PVDF films, 10 wt% loading of the co-PI nanofibers in the composites led to a 230% and 120% increasing on the tensile strength and modulus, respectively. Further increasing the co-PI amounts resulted in gradual increase of tensile strength and modulus. Sample S-5 (50 wt% co-PI) showed the best reinforcement effect that the tensile 4

strength and modulus were 743% and 449% higher than those of bulk PVDF film and 105% and 248% higher than those of pure PI nanofiber belt. The remarkable reinforcement of the PVDF/PI composites could be due to the intrinsic superior mechanical strength of aligned co-PI nanofibers and the strong interfacial adhesion between co-PI nanofibers and PVDF matrix (Fig. 1) [15]. Toughness is a useful parameter to access the flexibility of the materials. It can be determined by dividing the integral area (MPa) under the stress-strain curves by the densities (g/cm3) of the materials.

Figure 2. Typical stress–strain curves of PVDF/PI composites with aligned co-PI nanofibers. Table 1. Mechanical properties of the PVDF/PI composites with different amount of co-PI nanofibers. Samples

Amount of co-PI nanofibers (wt %)

Tensile Strength (MPa)

Tensile Modulus (GPa)

Strain (%)

Toughness (J/g)

S-0 (bulk PVDF)

0

43.7

0.59

497.76

111.4

S-1

10

144.2

1.30

20.41

9.8

S-2

20

169.1

1.31

20.17

11.5

S-3

30

279.3

2.21

22.77

21.4

S-4

40

338.6

2.66

22.15

25.4

S-5

50

368.6

3.24

21.83

28.4

S-6

100

179.5

0.93

19.31

11.9

The toughness of bulk PVDF film, co-PI and co-PI/PVDF composites were listed in Table 1. PVDF’s toughness was 111.4 J/g, which is in the same level with another famous tough thermoplastic polyurethane (TPU, 111 J/g) [21]. In comparison, pure co-PI nanofibers showed a toughness of 11.9 J/g. When the amount of co-PI nanofibers was 10 5

and 20 wt%, the PI/PVDF composites (S-1 and S-2) exhibited toughness in the same level as the pure co-PI nanofiber belts. Further increasing the content of co-PI nanofibers (S-3, S-4 and S-5) in the composites led to an increase of toughness in the range of 20-30 J/g, which is even higher than that of PVA film (2.5 J/g), CNTs reinforced PVA electrospun nonwovens (16 J/g) and electrospun nylon-6 nanofibers (21.8 J/g) [22, 23].

Figure 3. Dependence of dielectric properties (dielectric permittivity and dielectric loss) of the PVDF/PI composites on the mass fraction of aligned co-PI nanofibers. The dependence of dielectric permittivity and dielectric loss of the PVDF/PI composites on the mass fraction of aligned co-PI nanofibers was illustrated in Fig. 3. At 1, 10 and 100 kHz, bulk PVDF film showed dielectric permittivity of 11.66, 11.42 and 10.95, while the casting co-PI films 3.45, 3.42 and 3.36, respectively. The dielectric permittivity of PVDF/PI composites was decreased with the increasing amount of co-PI nanofibers (Table S1). When the content of co-PI nanofibers was 10 wt%, the dielectric permittivity was 8.85 at 1 kHz, which was about 2.6 times of the pure co-PI and 1.45 times of the PI/Zirconia composites [11]. When the amount of co-PI nanofibers was as high as 50 wt%, the dielectric permittivity was 5.19, which was still 1.5 times of the pure co-PI. The dielectric loss of PVDF/PI composites showed almost no changes with different amount of co-PI nanofibers due to the low dielectric loss of PVDF and co-PI. The dielectric loss of bulk PVDF film (S-0), pure co-PI (S-6) and PVDF/PI composites with 50 wt% co-PI nanofibers (S-5) were 0.0186, 0.0043 and 0.0269, respectively at 1 kHz. 6

Besides, the frequency dependence of dielectric permittivity of PVDF/PI composites showed slight changes in dielectric constant and dielectric loss with the increasing frequency from100 Hz to 100 kHz (Fig. S5). Conclusion In summary, all-organic nanofiber-reinforced PVDF/PI composites with high mechanical strength and good tunable dielectric properties were fabricated by electrospinning and layer-by-layer hot-pressing. Aligned co-PI nanofibers led to superior reinforcement to the PVDF matrix. PVDF/PI (50 wt% PI) possessed the tensile strength as high as 368.6 MPa, which was 743% higher than that of bulk PVDF film. Except for the high mechanical strength, the PI/PVDF also had high toughness in the range of 20-30 J/g when loading 30-50 wt% of co-PI nanofibers. All these excellent mechanical properties were obtained without scarifying the good dielectric properties of PVDF, that the PVDF/PI composite with 10 and 50 wt% co-PI nanofibers still had the dielectric permittivity of 8.85 and 5.19, respectively. Within the testing frequency (100 Hz~100 kHz), the dielectric permittivity was stable and the composites had low dielectric loss. Hence, these all-organic PVDF/PI composites would be promising candidates for flexible embedded capacitors and as the polymer matrix for preparing high dielectric polymer-matrix composites. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grants No.: 21174058 & No.: 21374044), the Major Special Projects of Jiangxi Provincial Department of Science and Technology (Grant No.: 20114ABF05100) and the Technology Plan Landing Project of Jiangxi Provincial Department of Education. References [1] Zhang QM, Li H, Poh M, Xia F, Cheng ZY, Xu H, et al. Nature. 2002;419:284-7. [2] Gregorio R, Jr., Ueno EM. J Mater Sci. 1999;34:4489-500. 7

[3] Da Silva A, Wisniewski C, Esteves J, Gregorio R, Jr. J Mater Sci. 2010;45:4206-15. [4] El Mohajir B-E, Heymans N. Polymer. 2001;42:7017-23. [5] El Mohajir B-E, Heymans N. Polymer. 2001;42:5661-7. [6] Ding M. Isomeric polyimides. Prog Polym Sci. 2007;32:623-68. [7] Chen Z, Zhao J, Yan S, Yuan Y, Liu S. Mater Lett. 2015;157:201-4. [8] Simpson JO, St.Clair AK. Thin Solid Films. 1997;308–309:480-5. [9] Xu W, Ding Y, Jiang S, Ye W, Liao X, Hou H. Polym Compos. 2014;DOI: 10.1002/pc.23236. [10] Xu W, Ding Y, Jiang S, Zhu J, Ye W, Shen Y, et al. Eur Polym J. 2014;59:129-35. [11] Li X, Wang G, Huang L, Kang X, Cheng F, Zhao W, et al. Mater Lett. 2015;148:22-5. [12] Xu W, Ding Y, Jiang S, Chen L, Liao X, Hou H. Mater Lett. 2014;135:158-61. [13] He Y, Han D, Chen J, Ding Y, Jiang S, Hu C, et al. RSC Adv. 2014;4:59936-42. [14] Jiang S, Duan G, Chen L, Hu X, Hou H. Mater Lett. 2015;140:12-5. [15] Chen Y, Han D, Ouyang W, Chen S, Hou H, Zhao Y, et al. Composites Part B. 2012;43:2382-8. [16] Jiang S, Duan G, Schöbel J, Agarwal S, Greiner A. Compos Sci Technol. 2013;88:57-61. [17] Wu P, Zhang L, Shan X, Mater Lett. 2015;159:72-75. [18] Zhang L, Wang W, Wang XG, Bass P, Cheng ZY, Appl Phys Lett. 2013;103:232903. [19] Jiang S, Hou H, Greiner A, Agarwal S. ACS Appl Mat Interfaces. 2012;4:2597-603. [20] Chen S, Hu P, Greiner A, Cheng C, Cheng H, Chen F, et al. Nanotechnology. 2008;19:015604. [21] Jiang S, Greiner A, Agarwal S. Compos Sci Technol. 2013;87:164-9. [22] Jiang S, Duan G, Hou H, Greiner A, Agarwal S. ACS Appl Mat Interfaces. 2012;4:4366-72. [23] Blond D, Walshe W, Young K, Blighe FM, Khan U, Almecija D, et al. Adv Funct Mater. 2008;18:2618-24.

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Highlights All-organic tough PVDF/PI composites by electrospinning and layer-by-layer hot-pressing. Significantly enhanced tensile strength and tensile modulus. Comparable toughness to that of electrospun nylon-6 nanofibers. High dielectric constant of 8.85 and low dielectric loss of 0.015 at 1kHz.

Graphical Abstract

Electrospun nanofiber reinforced tough PVDF/PI composites with enhanced mechanical properties and tunable permittivity were highlighted.

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