Investigation of the Effect of Nanosilica on Rheological, Thermal

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Investigation of the Effect of Nanosilica on Rheological, Thermal, Mechanical, Structural, and Piezoelectric Properties of PVDF Nanofibers Fabricated Using Electrospinning Technique Seyyed Arash Haddadi, Ahmad Ramazani S.A., Soroush Talebi, Seyyedfaridoddin Fattahpour, and Masoud Hasany Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02622 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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Industrial & Engineering Chemistry Research

Investigation of the Effect of Nanosilica on Rheological, Thermal, Mechanical, Structural, and Piezoelectric Properties of PVDF Nanofibers Fabricated Using Electrospinning Technique Seyyed Arash Haddadi†, Ahmad Ramazani S.A.†,, Soroush Talebi†, Seyyedfaridoddin Fattahpour†, Masoud Hasany†, †

Chemical and Petroleum Engineering Department, Sharif University of Technology, Azadi Avenue, P.O. Box 11365-9465, Tehran, Iran

ABSTRACT In this study, effects of different nano-SiO2 contents on the rheological properties of poly vinylidene fluoride (PVDF) solution and mechanical, thermal, structural, and piezoelectric properties of composite nanofibers were investigated. Results showed increase in fiber diameter (~125 to 350 nm) and ~450 % increase in tensile strength as content of nano-SiO2 particles increased. Degree of crystallinity decreased by 19 % as nano-SiO2 content increased by 2 % (w/w). Further investigation demonstrated that silica could significantly improve piezoelectric properties of PVDF nanofibers as output voltage showed an increase in presence of silica attributed to change in crystalline structure of PVDF. Keywords: Poly(vinylidene fluoride); Nanofiber; Solution electrospinning; Nano-SiO2; Piezoelectric behavior



Corresponding author: [email protected], Tel: +9866166405, Fax: +9866166404

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1. INTRODUCTION In recent decades, electrospun nanofibers have attracted a plenty of attentions by researchers and different industrial sectors owing to its wide advanced application such as affinity membrane and filtration 1–3, release control and drug delivery 4, catalyst and enzyme carriers, sensors batteries

5,6 9

, energy storage 7, scaffolds used in tissue engineering and reinforced nanocomposite

10–12

4,8

, separators of Li-ion

. These nano range materials are characterized

by a very high surface-to-volume ratio, low basis weight, small pore size and high permeability. However, electrospun nanofibers generally show rather low mechanical properties, which can limit their utilization in many applications 13,14. Recently, it has been confirmed that nanocomposite materials could combine basic properties of organic and inorganic materials and offer specific properties, in proportion to the desired application. For example, higher mechanical performance, proper pore size, enhanced surface interactions with biological entities, and other desired characteristics were added to collagenbased hydrogels. These characteristics are because of the incorporation of particulate nanosilicates resulted in promoting osteogenesis in the absence of any osteoinductive factors 15

. On the other hand, preparation of artificial membranes with excellent separation

performance, good thermal and chemical stability and adaptability to harsh environments was achieved through electrospinning of PVDF-trimethylsilyl oxime (TMSO) and subsequent thermal treatment for the sol-gel process of TMSO 16. Poly(vinylidene fluoride) (PVDF), a semi-crystalline ferroelectric polymer

17,18

, is one of the

most extensively applied materials in the industry and biomedicine for outstanding antioxidation, superior thermal and hydrolytic stabilities, high chemical resistance, excellent mechanical properties

16,17

, maintenance of cellular functions with excellent adhesion

functionality 19,20 and piezoelectric properties 21. PVDF and its copolymers have the best allaround electroactive properties in comparison with other polymers with piezo, pyro and ferroelectricity characterization such as nylon 11 and polylactic acid (PLLA). The strong electrical dipole moment of the PVDF monomer unit makes it suitable for sensor, actuator, biomaterials, biomedical fields and tissue engineering applications. Differences in electronegativity of fluorine atoms as compared to hydrogen and carbon atoms are responsible for these remarkable electroactive properties

22

. The monomer units and dipolar

moments are packed in a morphology which can demonstrate an overall dipolar contribution per unit cell in terms of α, β and γ phases

23

. Through these phases, β-phase has the highest 2

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dipolar moment per unit cell (~8×1030 C m) compared to the other phases. The α-phase is non-polar because of antiparallel packing of the dipoles within the unit cell, while, β and γ phases are the most electrically inducing phases in PVDF crystalline structure. The similarity of β and γ phases led to the contradictory identification and quantification

22,24

. It is recently

accepted that the peak at 840 cm-1 is the strong band just for the β-phase, whereas, for the γphase, the characteristic peak appears as a shoulder of 833 cm-1 band. A more recent approach to increase β-phase in PVDF structure is use of electrospinning method. This method is efficient and appropriate when the electrospun fibers have sub-micron to nanoscale diameter and relies on an electrostatically driven jet of polymer melt or solution

21,25

.

Furthermore, addition of some fillers with special surface charge such as clay, carbon nanotube, hydrated ionic salts, TiO2, palladium and gold nanoparticles can enhance the nucleation of β-phase and increase piezoelectric characterization of PVDF 21,22. PVDF fibers are loaded with SiO2 nanoparticles to extend potential applications of PVDF in membrane and biomedical engineering by improving specific properties of PVDF, such as its hydrophilicity, antifungal, piezoelectric, mechanical characteristics, biocompatibility, cell differentiation in a growth-free approach, ionic conductivity, etc.

26–29

. It should be noticed

that in spite of promising properties of silica filled PVDF nanofiber nanocomposites for medical and membrane applications such as polymer electrolyte separator in batteries

30,31

,a

comprehensive study on the effects of silica concentration on electrospinability, thermal, mechanical, structural and piezoelectric properties of these materials are not reported yet. So, in this study, the effect of nanosilica concentration on the morphology, porosity, mechanical properties, hydrophobicity, and piezoelectric behavior of the PVDF-SiO2 electrospun nanofibers are comprehensively investigated. Rheological characteristics of silica filled PVDF solutions would be also determined to investigate connections between their rheological properties and electrospinability of suspensions.

2. EXPERIMENTAL 2.1. Raw materials PVDF and dimethylformamide (DMF) were purchased respectively from Sigma-Aldrich and Merck. Average molecular weight and density of purchased PVDF were around Mw=270000 g mol-1 and 1.78 g ml-1, respectively. A hydrophilic grade of silica nanoparticles (SiO2) was 3 ACS Paragon Plus Environment


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purchased from Nano-Sany Company of Iran with the properties tabulated in Table 1. All raw materials were used without further treatment for making a composite solution. Table 1: Properties of nano silica. Material

Density/

Particle

Specific surface

Purity/

powder

g cm-3

size/ nm

area/ m2 g-1

%

SiO2

0.11

20-30

190-685

95-98

2.2. Preparation of electrospun nanofibers The hydrophilic SiO2 nanoparticles have higher chemical compatibility with PVDF due to the existence of hydroxyl groups onto the surface of silica nanoparticles. According to literature 32

, in order to disperse nanoparticles in polymer matrix, both polymer and nanoparticles are

added into the solvent under stirring. But, in this work, dispersion of silica nanoparticles was performed firstly in DMF as solvent using high power probe sonication and then, PVDF pellets were added slowly to the previous solutions. Sonication of DMF solutions containing SiO2 nanoparticles could certainly leads to the elimination of silica agglomeration and makes almost a uniform solution. So, in the first step, silica nanoparticles were dispersed into DMF at a concentration of 0.5, 1 and 2 % (w/w), based on the PVDF weight, using a probe sonicator (FAN AZMA, Iran) for 45 min at 100 Watts, 25 kHz frequency (1 min pulse on and 2 min pulse off). Ice-water bath was used for prevention of temperature rising during ultrasonication process. Then, PVDF pellets were added slowly to the stirring DMF-SiO2 solutions using a high shear mixer (4000 rpm) at room temperature (24±1 ºC and 20±5 RH) for 1 h. Finally, after complete dissolving of PVDF pellets, the solutions were remained under stirring for 5 h resulted in a milky solution with PVDF content of 10 % (w/v). In order to carry out electrospinning of the composite solution (PVDF-SiO2), an electrospinning setup (SISTANA ES LAB SBS, Nano Azma, Iran), equipped with two syringes and needles (ID=0.8 mm), a ground electrode, a rotating and traversing drum wrapped with aluminum sheet, and a high voltage supply, was utilized. As sketched in Figure 1, due to faster operation, the composite solutions were sucked into two syringes where needles were connected to the high voltage of 13 kV and then were pumped and rotated on the drum (rotation speed=2000 rpm, traversing oscillation frequency=25 min-1) with a mass flow rate of 0.5 ml h-1 for each needle. The distance between the drum and both 4 ACS Paragon Plus Environment

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needle tips were adjusted to 14 cm. Also, in order to measure the effect of SiO2 nanoparticles on the surface chemistry of PVDF, the prepared solutions were separately coated on the surface of the clean glass plates using a 30 μm film applicator. In addition, In order to control the environmental conditions during the electrospinning process, the electrospinning chamber was placed in a clean room with temperature range of 24±1 ºC and relative humidity of 20±5 and the electrospinning of all solutions was carried out at the mentioned environmental conditions.

Figure 1: A schematic picture of the setup used for electrospinning of PVDF and PVDF-SiO2 solutions. 2.3. Characterization of electrospun nanofiber 2.3.1. FE-SEM. To investigate the morphology of the electrospun fibers, images from different regions of each membrane were taken by a field-emission scanning electronic microscope (Mira 3-XMU, TESCAN, Czech Republic). Prior to FE-SEM observation, all membranes were double-coated with Gold. Average diameter and porosity of the electrospun nanofibers were acquired by analyzing the taken pictures using an open source image processing software (Fiji). The spectrum of energy dispersive X-ray (EDX) was applied to the same samples used in FE-SEM, for mapping of elements (C and Si) on the surface of the electrospun nanofibers. 2.3.2. ATR-FTIR spectroscopy. To evaluate the effects of SiO2 nanoparticles on the 5 ACS Paragon Plus Environment


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structure of PVDF and surface characterization of the composite fibers, Fourier transform infrared spectroscopy (FTIR, Tensor 27 (Bruker, Germany) was employed in the wavenumber range of 400-4000 cm-1 using KBr pellet. Also, the ATR accessory contained a ZnSe crystal (25 mm×5 mm×2 mm) was used to evaluate the different crystalline forms of the electrospun PVDF nanofibers (α and β phases) and calculation of β-phase content (F(β)) according to Eq. 1 obtained by Gregorio et al 33. 𝐹(𝛽) =

𝐴𝛽 𝐾 ( 𝛽⁄𝐾 ) 𝐴𝛼 + 𝐴𝛽

(1)

𝛼

where, Aα and Aβ are the intensity of absorption peaks at 766 and 840 cm-1, Kα and Kβ are the absorbance coefficient at respective wavenumber, which values are 6.1×10 4 and 7.7×104 cm2, respectively. 2.3.3. Piezoelectric characterization The voltage outputs of the electrospun nanofibers were evaluated using a digital oscilloscope (ROHDE & SCHWARZ, HMO-3522, Germany) under constant loading force of 12±2 N. The tests were conducted 3 times for each membrane and the mean values were reported. 2.3.4. Wettability. Contact angles of double-distilled water on the surface of the nanocomposite electrospun membranes and coated films of the neat and nanocomposite PVDF were measured using a contact angle analyzer (SHARIF AZMA, CA-1, Iran). The neat and nanocomposite PVDF membranes (5×5 cm2) were cut and contact angle measurements were carried out on the surface of membranes and the contact angle values of water droplets were calculated via ImageJ open source software. In order to investigate the effects of SiO2 nanoparticles on the wettability, surface free energy (γsv) and work of adhesion (Wa) of PVDF, contact angle measurements were performed on the surface of the neat and nanocomposite PVDF films. Values of surface free energy and work of adhesion were quantified using Young and Neumann equations, respectively, based on a previous study by Motamedi et al. 34. It should be noted that results of surface characterization obtained by the mentioned equations are valid just for the nonporous surfaces. Thus, the coated films of the neat and nanocomposite PVDF were also prepared to determine the exact values of surface characterization. Triplicates were prepared for each sample for the contact angle measurements and all contact angle measurements were carried out at room temperature (24±1 ºC and 20±5 RH). 6 ACS Paragon Plus Environment

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2.3.5. Rheological measurements. The rheological characteristics of the neat PVDF and the composite solutions were measured using a rheometric mechanical spectrometer (RMS, Model MCR 301, Anton Paar, Austria) at 27 °C. All the measurements were carried out in parallel plate geometry (gap distance=0.8 mm). 2.3.6. Tensile test. A material testing system (H10KS, HOUNSFIELD, Germany) by a force load cell of 500 N capacity was used to investigate mechanical characteristics of the electrospun nanocomposite. Three strip-shaped specimens (2×10 cm2) were tested for each membrane at a loading velocity of 1 mm min-1 at 27 ºC. 2.3.7. DSC measurement. The thermal properties of electrospun nanofibers were evaluated using differential scanning calorimetry (DSC, Q100, TA, USA) under pure nitrogen atmosphere at a heating rate of 10 °C min-1, from room temperature to 220 °C, under pure nitrogen atmosphere. The crystallinity of the samples was calculated by Eq. 2 16,35: 𝐶𝑟𝑦𝑠𝑡𝑎𝑙𝑙𝑖𝑛𝑖𝑡𝑦 (%) =

𝑠𝑎𝑚𝑝𝑙𝑒 ∆𝐻𝑚 × 100 % ∗ ∆𝐻𝑚

(2)

∗ in which ∆𝐻𝑚 is the melting enthalpy for totally crystalline PVDF. Assuming the same 𝑠𝑎𝑚𝑝𝑙𝑒 ∗ melting enthalpy for all crystalline forms of PVDF, ∆𝐻𝑚 will be 102.5 J g-1 36. ∆𝐻𝑚 is

the enthalpy change through crystallization in DCS curves and calculated by the trapezoidal method in Excel 2013. 2.3.8. Porosity and water uptake measurements The apparent porosity of the electrospun nanofibers was described as the pore volume divided by the total volume of the porous nanofibers, determined from the following equation at room temperature 37,38: 𝐴𝑝𝑝𝑎𝑟𝑒𝑛𝑡 𝑝𝑜𝑟𝑜𝑠𝑖𝑡𝑦 (%) = {1 −

𝜌𝐹 } × 100 % 𝜌𝑝

(3)

in which, ρF and ρp are the nanofiber and polymer density, respectively. Water uptake of the electrospun nanofibers was computed by socking the nanofibers (2.5×2.5 cm2) in the deionized water at room temperature inside a glove box. The globe box was used to keep the electrospun membranes clean and away from any dirt and also to determine the porosity and water uptake percentages of the electrospun nanofibers under the same environmental conditions (24±1 ºC and 20±5 RH). The weight of the wetted nanofibers was 7 ACS Paragon Plus Environment


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measured at different intervals. The excess water remaining on the surface of the nanofibers was wiped using a tissue paper. The water uptake was calculated using the Eq. 4 39,40: 𝑊𝑎𝑡𝑒𝑟 𝑢𝑝𝑡𝑎𝑘𝑒 (%) =

𝑀𝑤𝑒𝑡 − 𝑀𝑑𝑟𝑦 × 100 % 𝑀𝑑𝑟𝑦

(4)

where, Mwet and Mdry are the mass of the wet and dry nanofiber, respectively.

3. RESULTS AND DISCUSSION Nanocomposite fibers were prepared from PVDF-SiO2 blend solution at different loadings of SiO2 including 0.5 (PVDFSI-0.5, red), 1 (PVDFSI-1, green) and 2 wt. % (PVDFSI-2, orange). For comparison purpose, neat PVDF nanofibers (PPVDF, navy blue) were also prepared. 3.1. FE-SEM results Figure 2 shows the FE-SEM images of the electrospun PVDF nanofibers containing various SiO2 contents.

Figure 2: The FE-SEM images of the electrospun nanofibers. More SiO2 nanoparticle content resulted in higher diameter and roughness of the electrospun 8 ACS Paragon Plus Environment

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nanofibers, which is due to increase in viscosity (See Section 3-4) of the PVDF-SiO2 solutions. The average diameter of the electrospun nanofibers has been depicted in Figure 3. Solvent evaporation of composite nanofibers decreased in presence of SiO2 nanoparticles, which led to more beads in the electrospun composite nanofibers 41. The reduction of solvent evaporation in presence of silica nanoparticles can be strongly related to the increase in viscosity of the solutions, formation of Si-O-Si structures in the polymer matrix and physical adsorption of solvent molecules onto the surface of the nanoparticles. Consequently, these reasons can decrease the evaporation rate of the solvent from the nanofibers

41,42

. Elemental

mapping of nanofibers for carbon, Silicon and oxygen elements are presented in Figure 4. According to Figure 4, one can find that SiO2 nanoparticles are uniformly distributed on the surface of nanofibers. Also, Figure 4 demonstrates that existence of SiO2 nanoparticles in PVDF matrix changed the surface roughness and affected the average diameter of the electrospun composite nanofibers 16.



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Figure 3: Average fiber diameter for the electrospun nanocomposite with different SiO2 contents. Micrographs were taken by FE-SEM and the average diameter of fibers acquired using image processing software (Fiji). 10 ACS Paragon Plus Environment

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In addition, silicon elemental analysis presented in Table 2 showed that the concentration of silicon is much more than expected values. So, one could conclude that concentration of SiO2 on the external surface of fibers should be much more than that of the internal layers of nanofibers.

Figure 4: EDX diagram of PVDFSI-2 fibers and SEM-EDX elemental mapping of carbon, silicon and oxygen. Table 2: EDX analysis results of PVDFSI-2 fibers shown in Figure 3. Element

Wt.%

Atom %

C

20.99

44.85

F

10.19

26.34

Si

17.84

24.09



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O

10.3

9.41

Au

40.68

5.3

Total

100

100

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This phenomenon leads to changes in surface roughness and hydrophilicity 16 even more than the expected values.

3.2. FTIR results The effect of SiO2 nanoparticles on the chemical structure of PVDF and its composite nanofibers were investigated using FTIR spectroscopy. The FTIR spectra of silica powder, PPVDF and PVDFSI-2 fibers have been illustrated in Figure 5. Also, the main FTIR adsorption peaks are listed in Table 3.

Figure 5: FTIR absorbance spectra of silica powder, PPVDF and PVDFSI-2 fibers. Table 3: Characteristic absorption peaks observed in the FTIR spectrum of silica powder, PPVDF and PVDFSI-2 fibers.

No.

Functionality

1

O-H stretching

Wavenumber/ cm-1

Wavenumber/ cm-1

Wavenumber/ cm-1

(silica powder)

(PPVDF)

(PVDFSI-2)

3447

3420

3460


Ref. 43

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2


Si-O-Si asymmetric stretching

486, 1118

-

529

41,44

3

Si-O-Si network

834

866

873

41,44

4

C-H stretching

-

2835, 2951

2861, 2930

45,46

5

C-F stretching

-

860, 1475

882, 1452

45,46

6

C-C

-

1110

1130

44

7

CF2 bending

-

480, 548

478, 529

45,46

According to Figure 5, it can be seen that the stretching vibration of O-H bond observed at 3400-3500 cm-1 has been intensified in presence of hydrophilic SiO2 nanoparticles for PVDFSI-2 fibers

41

. This result clearly shows that the loading of hydrophobic SiO2

nanoparticles into PVDF matrix can increase the hydrophilicity of the composite electrospun nanofibers 47. In the case of composite fibers, the characteristic peaks at around 480 and 860 cm-1 are assigned to the bending of CF2 and stretching of C-F bonds

3,48

which were overlapped with

the asymmetric stretching and network peaks of Si-O-Si bond, respectively. Furthermore, other characteristic peaks of SiO2 nanoparticles were significantly overlapped with the absorption peaks of PVDF

16

. Hereby, the FTIR spectra of composite fibers could not be

clearly distinguished. That is why FTIR spectra of PVDFSI-0.5 and PVDFSI-1 nanofibers have not been presented in Figure 5.

3.3. Piezoelectric characterization of the electrospun nanofibers Depending on condition of PVDF electrospun nanofiber preparation, the prepared PVDF membrane can contain one or more crystalline phase structures 22. The ATR-FTIR spectra of the electrospun nanofibers are illustrated in Figure 6a. According to Figure 6a, the adsorption peaks at 760 cm-1 was related to α-phase crystalline structure. Also, the observed peaks at 840 and 1276 cm-1 were associated with β-phase in PVDF and PVDF-SiO2 electrospun nanofibers.



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Figure 6: Typical ATR-FTIR spectra of the electrospun nanofibers (a) and schematic representation of β-phase formation mechanism in presence of SiO2 nanoparticles (b). According to Figure 6a, the intensity of absorbance peaks at wavenumber of 766 cm-1 which is attributed to the α-phase of the electrospun nanofibers decreased in presence of SiO2 nanoparticles. Also, the intensity of the characteristic peaks of β-phase for the electrospun nanofibers at 840 cm-1 increased due to nucleation of β-phase in the electrospun nanofibers 21. The detailed data obtained from Figure 6a and the β-phase contents of the electrospun nanofibers calculated by Eq. 1 are tabulated in Table 4. Table 4: The detailed data obtained from ATR-FTIR analysis.

Sample

Intensity peak at -1

Intensity peak at

F(β)/ %

766 cm (Aα)

840 cm-1 (Aβ)

PPVDF

0.023

0.031

51.62±3

PVDFSI-0.5

0.027

0.179

83.99±4.5

PVDFSI-1

0.048

0.148

70.94±3.5

PVDFSI-2

0.037

0.077

62.23±3.2

As it can be seen in Figure 6a and Table 4, the addition of SiO2 nanoparticles into the PVDF matrix can increase the content of β-phase and enhance electroactive properties of the electrospun nanocomposite fibers, considerably. The nucleation of β-phase in such nanocomposites can be attributed to an effective interaction between CH2 groups of the polymer chains, having a positive charge density and surface of nanoparticles with negative 14 ACS Paragon Plus Environment

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charge density surface charge

49 50

. Among various nanofillers, SiO2 nanoparticles have naturally negative

which can interact with CH2 groups of PVDF chains. Figure 6b shows the

conformation of α and β phases and interaction between SiO2 nanoparticles and the CH2 groups of PVDF chains. According to Table 4, the addition of SiO2 nanoparticles into the PVDF matrix more than 0.5 wt. %, led to decrease in β-phase contents. Moreover, these values were higher than those of the neat PVDF nanofiber. It seems that in spite of the literature

51

, the nucleation of

electroactive β-PVDF depends on both size and content of the filler in PVDF matrix. At high contents of the fillers, rearrangements and placements of the polymer chains into the crystalline structures are not efficient and complete. Consequently, both crystallinity and electroactive properties of the polymer matrix can be decreased. Output voltages of the electrospun nanofibers are tabulated in Table 5. Table 5: Output voltages of the electrospun nanofibers. Sample

Applied force/ N

Output Voltage/ V

Sensibility/ V/N

PPVDF

12.75

14.3±0.8

1.122±0.063

PVDFSI-0.5

13.9

24.6±1

1.771±0.072

PVDFSI-1

12.75

20.1±0.9

1.576±0.070

PVDFSI-2

11.2

16.8±0.5

1.5±0.045

As it can be observed in Table 5, addition of SiO2 nanoparticles into the PVDF matrix more than 0.5 wt. % decreased the output voltages of the electrospun nanofibers. However, total output voltages of the electrospun nanocomposite fibers were higher than the neat PVDF nanofibers. These results are in good agreement with the β-phase contents of the electrospun nanofibers obtained from ATR-FTIR spectra. 3.4. Contact angle measurement The surface hydrophobicity of produced materials plays an essential role regarding the application, ranging from tissue engineering

52

to different membrane

16

. In general, the

hydrophobicity is evaluated by water contact angle, and higher hydrophobic surface exhibits the larger contact angles 16,53. The contact angle data of the electrospun nanofibers and coated films were shown in Table 6. The PPVDF nanofibers showed a water contact angle of 79.8 º which is highly lower than the same nanofiber in other works 16,53. This difference is ascribed 15 ACS Paragon Plus Environment


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to change in the surface properties of fibers surface such as roughness, porosity and the randomly interwoven structure among fibers 54. Table 6: Contact angle, work of adhesion, and surface energy data for the electrospun nanofibers and coated PVDF-SiO2 films. θL/ º

θR/ º

(Left side)

(Right side)

PPVDF

77.5

PVDFSI-0.5

Average θ/ º

Wa/ mJ m-2

γsv/ mJ m-2

79.8

78.65±1.15

-

-

55.1

56

55.55±0.5

-

-

PVDFSI-1

51.8

52.5

52.15±0.35

-

-

PVDFSI-2

47.2

48.1

47.65±0.45

-

-

PPVDF

97.7

99.4

98.55±0.85

62

23.9

PVDFSI-0.5

91.8

93.1

92.45±0.74

69.7

27.7

PVDFSI-1

82.3

80.4

81.35±0.55

83.7

34.6

PVDFSI-2

76.6

77.5

77.05±0.43

89.1

37.3

Sample type

Electrospun

Coated

According to Table 6, it can be inferred that the addition of SiO2 nanoparticles decreased the water contact angle of the nanocomposite membranes due to the increment in the average diameter and decrease in the porosity. The contact angle between water droplet and the external surface of sample was decreased in samples with the higher SiO2 content and reached to 47.2 ° for PVDFSI-2 membrane. This is because of the physical/chemical change in the nature of the electrospun nanofiber surface and roughness or increase in the surface heterogeneity in the samples with the higher SiO2 contents. The increase in the surface heterogeneity in the higher SiO2 contents is also obvious in Figure 3. The contact angle of a liquid on a certain surface can be influenced by any changes in the surface topology and porosity. Consequently, the contact angle measurement was also performed on the surface of the coated PVDF samples in order to ignore the effects of surface topology and porosity on the surface characterization of the PVDF samples. The work of adhesion and surface free energy data were calculated by Young and Neumann (Eq. 5 and 6, respectively) equations 55–57. 𝑊𝑎 = 𝛾𝐿𝑉 (1 + cos 𝜃)

(5)


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𝑊𝑎 = 2(𝛾𝐿𝑉 𝛾𝑆𝑉 )0.5 𝑒𝑥𝑝[−𝛽(𝛾𝐿𝑉 − 𝛾𝑆𝑉 )2 ]

(6)

In these equations, γlv and θ are water surface tension and water contact angle of the electrospun nanofibers, respectively. The values of γlv and β are 72.8 mJ m-2 and 0.0001247±0.000010 (mJ m-2)-2, respectively

55

. In the first step of calculation, Wa was

obtained from Eq. 5, and afterward, the γsv was computed by Eq. 6. Results disclosed that the addition of SiO2 caused an increase in the hydrophilicity or wettability of the PVDF films 16. The surface free energy increase along with the surface polarity increment of the composite films could be the result of the higher wettability of the PVDF-SiO2 fibers compared with the neat PVDF film 16. 3.5. RMS results 3.5.1. Steady rheological characteristics of the PVDF-SiO2 solution It has been previously approved that rheological properties have shown significant role in electrospinning 58–60. Figure 7 illustrates the dependence of the steady viscosity (η) and shear stress on the shear rate (𝛾̇ ) for the neat PVDF and PVDF containing various contents of SiO2 nanoparticles. It can be obtained that PVDF solutions show a pseudoplastic behavior. According to Figure 7, at low shear rates, the apparent viscosity of PVDF increase gradually with increasing SiO2 loadings due to the flow-impeding effect caused by the presence of the SiO2 nanoparticles 61.

Figure 7: Steady viscosity (colored plots) and shear stress (black plots) as a function of shear 17 ACS Paragon Plus Environment


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rate for the neat PVDF and PVDF containing various SiO2 contents at 27 ºC. The presence of SiO2 nanoparticles restrains the shear flow of PVDF matrix due to the formation of Si-O-Si bridges acting as a physical framework in matrix

41

. The formation of

such physical frameworks in PVDF matrix could decrease the segmental mobility of PVDF chains in the shear flow, resulting in an increase in both steady viscosity and shear stress. Moreover, the interactions of PVDF chains and SiO2 nanoparticles are enhanced with increasing in SiO2 loading. These interactions are far weaker than van der Waals interactions in the chain coins

41

. Consequently, such interactions are destroyed preferentially due to

disentanglement of chain coins at higher shear rates. As a result, both neat and SiO2 filled PVDF showed a reduced linear flow region and shear thinning behavior, especially at higher SiO2 loading levels 62. Power law model was used to obtain the deviation from the Newtonian flow by a non-Newtonian index according to the Eq. 7 62,63: 𝜂 = 𝐾𝛾̇ 𝑛−1

(7)

where, K is the consistency coefficient and n is the power law index. Plotting ln(η) vs. ln(𝛾̇ ) in the shear thinning flow region led to a linear relationship, from which K and n were calculated from the slope and intercept, respectively 63. The rheological parameters obtained by fitting the Power law model were tabulated in Table 7. Table 7: Rheological parameters of the neat and SiO2 filled PVDF solution. Sample

K/ Pa.sn

n

R2

PPVDF

1.22

0.715

0.955

PVDFSI-0.5

1.43

0.688

0.971

PVDFSI-1

2.25

0.655

0.970

PVDFSI-2

3.13

0.600

0.965

According to the summarized data in Table 7, it can be found that the power law model fits the data well with the correlation coefficient (R2) being more than 0.95. It could be mentioned that significant increase in consistency index and reduction in power-law index should be results of appropriate dispersion of nanoparticles in PVDF matrix and the high interaction between SiO2 nanoparticles and PVDF chains. Decreasing in power-law index and increasing the shear thinning behavior of PVDF-SiO2 solutions with increasing in SiO2 concentration could help the flowability of these solutions to have a good electrospinnability even at the 18 ACS Paragon Plus Environment

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high spinning rates. 3.5.2. Dynamic rheological characteristics of the PVDF-SiO2 solution The steady rheological properties of both neat and SiO2 filled PVDF solutions obtained in the previous section cannot give detailed data on the intermolecular interactions of nanoparticles and polymer chains due to nonlinear behavior at high shear rate

63

. Hereby, the dynamic

frequency sweep was conducted to evaluate rheological properties in a common linear flow for all PVDF solutions. The plot of complex viscosity (η=η-iη) and complex modulus (G=G+iG) as a function of angular frequency () are shown in Figure 8.

Figure 8: Complex viscosity (colored plots) and complex modulus (black plots) as a function of angular frequency for the neat PVDF and PVDF containing various SiO2 contents at 27 ºC. According to Figure 8, it can be observed that the complex viscosity increases significantly with increasing SiO2 loading at low frequency which could be due to Si-O-Si physical bridges formation through PVDF matrix 41. It is believed that the nanoparticle fillers can play a role to restrict polymer segmental motion

64

. The interaction of polymer chains with SiO2

nanoparticles reduces the mobility of the polymer chains and leads to the formation of restricted mobility regions around the SiO2 nanoparticles according to Eisenberg’s model 64,65 shown schematically in Figure 9.



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Figure 9: Schematic illustration of the segmental mobility of PVDF chains around the SiO2 nanoparticle. As one might expect, the dynamic complex modulus of the PVDF solutions increased with increasing SiO2 nanoparticles loading and angular frequencies

66

. These results indicate that

the elastic properties of PVDF might be improved which could be due to the formation of SiO-Si physical framework in PVDF matrix 41. 3.6. DSC results The effects of SiO2 nanoparticles loading on the thermal behavior of PVDF were investigated by using DSC method. In Figures 10.a and 10.b, endothermic and exothermic sections of DSC plots related to the melting and crystallization of the electrospun membranes are illustrated, respectively.


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Figure 10: (a): endothermic and (b): exothermic sections of DSC plots of the electrospun membranes. The obtained results from both endothermic and exothermic sections of DSC plots for the electrospun membranes have been tabulated in Table 8. Table 8: DSC results of the electrospun membranes. Melting

Melting

temperature/

enthalpy/

ºC

J g-1

PPVDF

163.8

36.4

139.8

33.5

32.7

PVDFSI-0.5

163.6

30.5

142.9

29

28.3

PVDFSI-1

162.7

25.4

143.1

22.6

22.1

PVDFSI-2

160.8

16.4

145.3

14.4

13.7

Sample

Crystallization temperature/ ºC

Crystallizati on enthalpy/

Crystallinity/ %

J g-1

According to Table 8, the neat PVDF displays an endothermic melting peak at 163.8 ºC and exothermic peak of crystallization centered at 139.8 ºC. The small difference between the values of the melting and crystallization enthalpy could be due to the electrospinning effects on the crystallization of PVDF. The addition of SiO2 nanoparticles into PVDF matrix led to a slight decrease in the melting temperature of the electrospun fibers

67

. However, the melting

enthalpy decreased considerably. In fact, the changes in the melting a point, melting enthalpy 21 ACS Paragon Plus Environment


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and crystalline phase decrements and increment of the crystallization point in the presence of SiO2 nanoparticles might be due to more nucleation effects and chain hindrance of silica nanoparticles which may be more highlighted by increasing in SiO2 concentration. However, it seems that in spite of the reduction in the total crystallinity of PVDF-SiO2, the percentage of the β-phase which is responsible for piezoelectric properties is not reduced. According to ATR-FTIR and piezoelectric results presented in Section 3.3, it is inferred that at low SiO2 concentration, the β-phase is not reduced. In addition, the hindrance effects of SiO2 might increase with the increment in SiO2 concentration and so at the highest concentration of SiO2 (2 wt. %), the percentage of β-phase also become less than that of the neat PVDF. It should be noted that even at this concentration as presented in Table 4, the proportion of β-phase (F(β)) to the total crystalline phase is increased comparing to that of the neat PVDF. Furthermore, the amorphous phases of the polymers can be boosted in the presence of inorganic nanoparticles with surface charges due to the higher nucleation rate through the solidification process

16

. The increase in the amorphous regions in the polymer makes it

proper for purposes with higher ionic conductivity such as tissue engineering applications 16. 3.7. Tensile test results The mechanical properties of the electrospun nanofibers were studied using tensile test. The typical stress-strain curves and values of the ultimate tensile strength and toughness for the neat and SiO2 filled PVDF electrospun nanofibers are shown in Figure 11.


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Figure 11: Typical stress-strain curves (a) and values of the ultimate tensile strength and toughness for the electrospun nanofibers (b). The results shown in Figure 11a clearly illustrate that the addition of SiO2 nanoparticles led to increase in tensile strength and the elongation at break reduction (mm) from about 2 and 115 to 11 and 40 for the neat PVDF and PVDF fibers containing 2 wt. % SiO2 nanoparticles, respectively. The incorporation of inorganic nanoparticles in the polymer usually leads to reinforcement of the polymer. This clearly suggests that SiO2 nanoparticles can act as a temporary cross-linker between PVDF chains. This phenomenon provides localized regions of enhanced strength through PVDF matrix

16

. In addition, the inorganic nanoparticles make

the polymer slightly less flexible and so lead to a decrease in elongation at break for composite fibers

16

. Hence, elongation at break of the PVDF fibers containing SiO2

nanoparticles enormously decreased with SiO2 content (about 65 % lower than that of neat PVDF fibers). This means that these membranes are too brittle for polymer electrolytes applications, while, the higher mechanical performance of the electrospun nanofibers adapts them to be used in other applications such as battery separators and fabrication of scaffolds for cell culture. Despite the very low elongation at break for the PVDF fibers containing SiO2 nanoparticles, the tensile strength and toughness of these fibers increase with increasing in SiO2 nanoparticle loading. According to Figure 11b, it can be clearly seen that the tensile strength and toughness of the fibers increase gradually with the addition of SiO2 nanoparticles into PVDF matrix. This is an important result as it shows that the PVDF fibers containing SiO2 nanoparticles can considerabely adsorb more energy before failure compared with the neat PVDF fiber 68. 3.8. Results of Porosity and water uptake measurements The porosity percentages of the neat and SiO2 filled PVDF nanofibers are demonstrated in Table 9. Table 9: The porosity percentages of the electrospun nanofibers. Sample

PPVDF

PVDFSI-0.5

PVDFSI-1

PVDFSI-2

Porosity/ %

81.5±3.5

77.6±4.1

73±2.5

66.2±3

It is evident that the porosity percentages of the electrospun nonofibers decreased with the 23 ACS Paragon Plus Environment


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increasing of SiO2 nanoparticles loading from 81.5 % to 66.2 % for PPVDF and PVDFSI-2, respectively. The porosity results clearly show that the increase in the average fiber diameter leads to the reduction in the porosity percentage (See Figure 2). Figure 12a presents a comparison of the water uptake percentages of the electrospun nanofibers at different intervals, which indicates the high water preservation of the PVDF nanofiber in presence of SiO2 nanoparticles. The results clearly illustrate that the water uptake of the nanofibers increases with increasing in SiO2 nanoparticles loading. In the case of PVDFSI-2, the water uptake reached to about 450 %. According to the results of contact angle measurements, the loading of hydrophilic SiO2 nanoparticles into the PVDF matrix can increases the hydrophilicity of the nanofibers

41

. The high porosity of the electrospun

nanofibers and the high amorphous regions of PVDF can be responsible for the higher water uptake values 16.

Figure 12: Water uptake percentages of the electrospun nanofibers (a) and schematic illustration of H-bond formation between SiO2 nanoparticles and water molecules (b). Also, in the case of SiO2 filled PVDF nanofibers, the formation of H-bonds between hydroxyl groups of SiO2 nanoparticles and water molecules can increase the adsorption of water by the nanofibers

41

shown schematically in Figure 12b. Furthermore, the formation of the

interconnected pore structure during the electrospinning process led to the faster water penetration into the nanofibers 16.


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4. CONCLUSION This work presented fabrication and characterization of the electrospun PVDF nanofibers containing various contents of SiO2 nanoparticles. The obtained results can be summarized as follow: 

FE-SEM results showed that the loading of SiO2 nanoparticles led to increase in average fiber diameter. Also, the elemental mapping analysis proved uniform dispersion of SiO2 nanoparticles in the PVDF matrix.



Contact angle measurement and FTIR revealed an increase in the hydrophilicity of both electrospun nanofibers and coated films with increasing of SiO2 nanoparticles due to the changes in the surface topology, roughness and surface characterization of the PVDF nanofibers and films.



Piezoelectric characterization of the electrospun composite nanofibers revealed that addition of SiO2 nanoparticles into PVDF matrix can increase β-phase content and piezo-voltages. Also, electroactive properties of the electrospun nanofibers were related to both size and contents of SiO2 particles in PVDF matrix.



Steady rheological characteristics of the PVDF containing SiO2 nanoparticles showed higher apparent viscosity and shear stress compared with the neat PVDF and Power law model fitted well the shear thinning behavior of the neat and SiO2 filled PVDF.



DSC results from the electrospun nanofibers showed that the loading of SiO2 nanoparticles created amorphous regions and decreased the degree of crystallinity due to the interaction between the SiO2 nanoparticle surfaces and the PVDF chains.



The PVDF nanofibers with higher SiO2 contents illustrated the superior tensile strength and toughness and the lower elongation at break because of the SiO2 network formation in PVDF matrix.



The porosity percentages of the electrospun nanofibers decreased with increasing of the average fiber diameter. The high water uptake percentages of the SiO2 filled PVDF nanofibers were attributed to H-bond formation between SiO2 nanoparticles and water molecules as well as the well-interconnected pore structures.



Improving Piezoelectric properties and increasing hydrophilicity of PVDF electrospun nanofibers with introducing silica nanoparticles could extend PVDF applications especially in the biomedical, robotics, membrane, and sensor fields.



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Page 26 of 31

ACKNOWLEDGMENT The Authors would like to thank the Iran National Science Foundation (INSF) for partially supporting of this research under Proposal number 88000967.

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