Ionic Liquid Cation Size-Dependent ... - ACS Publications

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Ionic liquid Cation Size Dependent Electromechanical Response of Ionic Liquid/poly(vinylidene fluoride)-based Soft Actuators Daniela M. Correia, João Barbosa, Carlos M. Costa, Patricia Reis, José M. S. S. Esperança, Verónica de Zea Bermudez, and Senentxu Lanceros-Mendez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00868 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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The Journal of Physical Chemistry

Ionic Liquid Cation Size Dependent Electromechanical Response of Ionic Liquid/Poly(vinylidene fluoride)-based Soft Actuators

D. M. Correia1,2*, J. C. Barbosa1,2, C. M. Costa2,3, P. M. Reis4, J.M.S.S. Esperança4, V. de Zea Bermudez1, S. Lanceros-Méndez5,6,*

1Department

of Chemistry and CQ-VR, Universidade de Trás -os -Montes e Alto Douro,

5000-801 Vila Real, Portugal 2Centro

de Física, Universidade do Minho, 4710-057 Braga, Portugal

3Centro

de Química, Universidade do Minho, 4710-057 Braga, Portugal

4LAQV,

REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia

Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal 5BCMaterials,

Basque Center for Materials, Applications and Nanostructures, UPV/EHU

Science Park, 48940 Leioa, Spain 6Ikerbasque,

Basque Foundation for Science, 48013 Bilbao, Spain

*Corresponding authors [email protected] [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract This work reports on the development of ionic liquid (IL)/poly(vinylidene fluoride) (PVDF)

actuator

composites.

ILs

sharing

the

same

anion

(bis(trifluoromethylsulfonyl)imide, [TFSI]-) and different cations belonging to the families of pyridinium, imidazolium and ammonium ions, comprising alkyl side chains with variable length were used. The physical-chemical properties, thermal behavior, mechanical, electrical and bending responses of the PVDF/IL composites were evaluated. The incorporation of ILs into the PVDF matrix leads to an increase of the electroactive β phase content. Moreover, the β phase content increases with the increase of the IL alkyl chain length. The degree of crystallinity depends on the IL chain length, as well as on the mechanical properties, revealing the plasticizing behavior exerted by the IL. The electrical conductivity decreased with the increase of the cation alkyl chain size. The highest bending response was observed for the [Pmim][ TFSI]/PVDF and [Pmpip][TFSI] composites

(where

Pmim

and

Pmpip

represent

propylimidazolium

and

propylmethylpiperidinium), being 5.7 and 6.0 mm, respectively, at 5 V and 100 mHz.


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1. Introduction Smart materials, defined as materials with the ability to change their properties through the application of external physical stimuli (e.g. electrical and magnetic fields, heat or light) and with high functionality in simplified structures, are gaining increasing interest to act as sensors and actuators 1-2. In this context, smart materials based on electroactive polymers (EAPs) have emerged as particularly attractive candidates, as they exhibit suitable characteristics as self-activated smart materials 3. EAPs have the ability to deform in the presence of an electric field

3

because their

operation relies on the application of an external voltage, EAPs actuators present high performance and a wide range of potential applications 4. These materials are able to produce large strains, making them suitable for sensors and actuators applications in areas, such as robotic, prosthesis and rehabilitation 3, environment 5, and tissue engineering 6, among others 7. Among all the EAPs, poly(vinylidene fluoride) (PVDF) and its co-polymers are the most widely used polymers for actuator applications, being a piezoelectric material, its actuator performance is characterized by low deformation and high frequency response

8-9.

The

great interest in PVDF is mainly due to its remarkable set of properties, in particular large dielectric constant, high polarity, biocompatibility, easy processing, high piezoelectric coefficients, high mechanical strength and ionic conductivity

7, 9-10.

biocompatible polymer suitable for biomedical applications

Moreover, it is a

7, 11.

At least, five

polymorphic modifications can be found in the PVDF crystalline phase (identified as α, β, γ, δ and ε), the β phase is the main responsible for its electroactive properties 10, 12.

In the last years research in the area has been essentially focused on the development of actuators based on ionic EAP composites, allowing much larger deformations at lower



voltage than piezoelectric actuators

13.

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Different ionic EAP actuators based on ionic

polymer−metal composites, conducting polymers and carbon nanotubes (CNTs) have been proposed due to the large deformations they exhibit even upon low voltage applications 4. The major drawbacks of these ionic actuators are their low durability under dry conditions and their low electrochemical stability, which leads to a decrease of the actuator performance 4. To overcome these limitations, attention has been increasingly paid to the development of ionic liquid (IL)-based ionic EAPs, obtained through the inclusion of ILs into the polymer matrix. ILs are salts, usually composed by an organic cation and an organic/inorganic anion, which display a myriad of interesting characteristics, such as negligible vapor pressure, high chemical and thermal stabilities, flame retardancy, high ionic conductivity and broad electrochemical window some cases, they have an electrochemical stability window of 4 V or more

14-15.

16-17.

In

The

development of ionic polymer composites based on ILs will promote the development of low voltage large deformation actuators 9. A large set of different ILs have been used to develop IL-based PVDF and PVDF copolymer composites as actuators. Studies reveal that the IL anion and cation, the length of the IL alkyl chain and the IL content in the polymer matrix, influence the ionic mobility of the composite and, in this sense, the actuator performance 13, 18. The influence of the IL anion size on the performance of an actuator incorporating 1-hexyl-3methylimidazolium

chloride

([Hmim][Cl])

and

1-hexyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide ([Hmim][TFSI]) allowed to conclude that a bending response of 0.53% was found for the PVDF/IL composite with [Hmim][Cl] at 10 V square signal 18. It was also reported that the bending response of PVDF with two different ILs (N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium

bis(trifluoromethylsulfonyl)imide

([N1112OH][TFSI]) and 1-ethyl-3-methylimidazolium ethylsulfate ([Emim][C2SO4]))


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depended more of the IL content than on IL type. The highest strain bending response was found for the PVDF/IL composite with 25 wt% of [N1112OH][TFSI] at 5.0 V 13. The effect of the alkyl chain length for the imidazolium cations [Emim]+, [Hmim]+ and [Dmim]+ was evaluated, with a maximum value of 0.3% bending response obtained for an PVDF/IL composite with 40 wt% of [Emim][TFSI] 9. Further, the electromechanical properties of Nafion/poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) fabricated using a gel electrode based on CNT–IL indicated that the ionic conductivity of the Nafion-P(VDF-HFP)–IL gel electrolyte was lower comparatively to the P(VDFHFP)–IL gel electrolyte

16

. Other works based on polymers/ILs were performed

19-22.

However, to the best of our knowledge, no studies in the literature have explored the cation type effect on the performance of the actuator and consequently on its bending actuation 16. In the present work, PVDF/IL composite films were prepared by solvent casting using several ILs with a common anion, [TFSI]-, and distinct cations. The influence of the cation type of the composites on the physical-chemical, electrical and mechanical properties of the composites was evaluated. Moreover, the effect of the ionic mobility of the cation on the bending actuation and on the performance of the actuator will be highlighted.

2. Experimental section

2.1.

Materials

PVDF (Solef 6020) and N, N-dimethyl formamide (DMF, 99.5%) were purchased from Solvay and Merck, respectively. Eight different ILs sharing the same anion were used in this

work.

[Pmpyr][TFSI],

[Pmpip][TFSI],

[Emim][TFSI],

[Edmim][TFSI],

[Pmim][TFSI] were purchased from iolitec with purities >99%. [N2113OH][TFSI], [N211



201][TFSI]

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and [N2114][TFSI] were synthesized according to the procedure described in

reference 23. Their chemical structures and their main properties are given in Figure 1 and Table 1, respectively. The properties of the TFSI--based ILs were obtained from the manufacturer. The properties of the synthetized ILs were measured as reported in

24.

Coulometric Karl Fischer titrations revealed water contents below 500 ppm and the negative results on the AgNO3 test showed the absence of chloride anions above the detection limit of this technique for all the synthesised ammonium based ionic liquids.

a)

b) O F3C

O S

N

S

O

CF3 O

Figure 1. Chemical structures of the different cations (a) and of the [TFSI]- anion (b) of the ILs used in this work.


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Table 1. Properties of the TFSI--based ILs used in this work (data obtained from the manufacturer and from 24).

Cation [Emim]+ [Edmim] + [Pmim] + [Pmpyr] + [Pmpip] + [N211201]+ [N2114]+ [N2113OH]+

2.2.

Molecular weight (g/mol) 391.31 405.34 405.30 408.39 422.41 132.23 130.25 132.23

Density (g/m3)

Ionic conductivity (mS/cm)

Viscosity (mPa/s)

1.52 (20 ºC) 1.49 (25 ºC) 1.48 (18 ºC) 1.43 (29 ºC) 1.41 (23 ºC) 1.43 (25 ºC) 1.37 (25 ºC) 1.46 (25 ºC)

6.63 (20 ºC) 3.18 (20 ºC) 4.40 (20 ºC) 4.92 (30 ºC) 2.12 (30 ºC) 2.85 (25 ºC) 1.82 (25 ºC) 1.25 (25 ºC)

39.4 (20 ºC) 68.5 (25 ºC) 43.8 (25 ºC) 58.7 (25 ºC) 176 (25 ºC) 61.5 (25 ºC) 101 (25 ºC) 166 (25 ºC)

Sample Preparation

Before PVDF dissolution in DMF in a ratio of 15/85 % w/w, the IL (40% wt. (IL/PVDF)) was dissolved in DMF. After the complete dissolution of the polymer, the films were obtained by casting the solution onto a glass substrate followed by the solvent evaporation in an oven (P-Selecta) at 210 °C for 10 min (Figure 2) 25.

Figure 2. Schematic representation of the PVDF/IL composites preparation. 2.3.

Characterization

Attenuated Total Reflectance (ATR)/Fourier Transform infrared (FTIR) measurements were performed with a Jasco FT/IR-4100 apparatus at room temperature in ATR mode ACS Paragon Plus Environment


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from 4000 to 600 cm-1 using 64 scans at a resolution of 4 cm-1. From ATR/FTIR data it is possible to calculate the content of the electroactive (EA) phase, F (EA), using Equation (1) 10, 26: 𝐹(𝐸𝐴) =

𝐴𝐸𝐴

(𝐾

840

(1)

)

𝐾766 (𝐴766 + 𝐴𝐸𝐴)

where A766 and AEA are the absorbances at 766 and 840 cm-1, corresponding to the α- and the electroactive phases ( and/or ), respectively, and K and K are the corresponding absorption coefficients (6.1 × 104 and 7.7 × 104 cm2 mol-1). Differential scanning calorimetry (DSC) measurements were performed with a Mettler Toledo 821e equipment at 10 ºC min-1 under nitrogen atmosphere. The degree of crystallinity (χ) was calculated from the DSC scans, using Equation 2 10:



H xH   yH 

(2)

where ΔH is the melting enthalpy of PVDF, ΔHα and ΔHβ are the melting enthalpies of α (93.07 J g-1) and β phases (103.4 J g-1) of PVDF, respectively (no melting feature corresponding to the  phase was detected), and x and y are α and β phase proportions of each sample, respectively. The thermal analysis was performed with a Thermal Gravimetric Analyser (TGA) model Perkin-Elmer instrument, Pyris1TGA. The samples were heated under argon atmosphere between 25 and 900 °C at a heating rate of 10 °C min-1.


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The mechanical analysis was performed in a Linkam Scientific Instruments TST 360, Temperature Controlled, Tensile Stress Testing Stage. A constant axial tension of 10 µm s-1 was applied to the samples. Dielectric measurements were performed through the parallel plate capacitor using a Quadtech 1920 LCR precision meter. The real (ε´) and imaginary (ε´´) part of the dielectric function were obtained from the measurement of the capacity (C) and the dielectric losses (tan ) at room temperature in the frequency range of 100 Hz to 1 MHz with an applied voltage of 0.5 V. ε´ and ε´´ were calculated using the following equations, respectively 27: 𝐶.𝑑

𝜀´ = 𝜀0.𝐴

(3)

and 𝜀′′ = 𝑡𝑎𝑛𝛿.𝜀´

(4)

where C is the capacitance, εo is the permittivity of vacuum (8.85×10-12 F m-1), A is the electrode area and d is the thickness of samples. For these measurements, circular gold electrodes of 5 mm diameter were deposited by a magnetron sputtering with a Polaron Coater SC502 onto both sides of each sample. The error associated to the dielectric measurements is ~2%. The AC electrical conductivity was calculated using the following equation 28:

 '     0 ''  

where ε0 is the permittivity of free space,   2f is the angular frequency.


(5)


Page 10 of 28

The bending (ε) response was evaluated for all the samples by equation 6 9, 13, where L is the sample free length, d the thickness and δ the displacement along the x axes. 𝜀=

2𝑑𝛿 2

(6) 2

𝐿 +𝛿

Gold covered electrodes with dimensions of 12 mm × 2 mm were deposited on both sides of the samples by magnetron sputtering (Polaron SC502). The needles of the sample-holder (that can be seen in Figure 8, contacting the sample) were connected to an Agilent 33220A function generator and an oscilloscope PicoScope 4227. The samples were analysed by a square wave signal with different peak voltages (2.0, 5.0 and 10.0 V) and frequencies between 200 and 700 mHz.

3. Results and discussion

3.1.

Chemical and thermal characterization

The possible chemical modifications that might have occurred after the processing method and by the IL filler incorporation were evaluated by ATR/FTIR measurements. Figure 3 shows the ATR/FTIR spectra obtained for the PVDF/IL composites. Figure 3a shows that the vibrational absorption bands of the chemical groups of the PVDF polymer chain (-CH2-CF2- monomeric units) are present in all the ATR/FTIR spectra of the IL-doped samples. The bands observed at approximately 795, 766, 678 and 976 cm-1 are characteristic of the stretching vibration of CF2 and CH2 groups and are attributed to the PVDF α phase

29-30.

The main absorption band characteristic of the electroactive β

phase is found at 840 cm-1 and is associated with the stretching vibration of the CH2 group 29-30.

Other absorption bands at 1071, 1176 and 1402 cm-1 are assigned to β PVDF. The


Page 11 of 28

presence of weak absorption bands at 813, 1232 and 1275 cm-1 in PVDF/IL composites are attributed to small contents of γ-PVDF 10, 31. The three bands at approximately 1349, 1132 and 740 cm-1, present in the spectra of the PVDF/IL samples and absent in the spectrum of PVDF are due to the TFSI anion. While the band at 740 cm-1 is ascribed to the cis [TFSI]- conformation, the bands at 1132 and 1349 cm-1 are assigned to the asymmetric and symmetric stretching vibration of SO2 group 33-34. Other absorption bands associated with the [TFSI]- anion are overlapped with those of PVDF. It is the case of the 612 cm-1 mode, attributed to the stretching vibration mode of SO2 group, of the band at 1051 cm-1, ascribed to the asymmetrical S–N–S stretching mode, and of the event at 1479 cm-1 attributed to the C–H bending of methyl group 33-34. The quantification of the amount of electroactive β phase was performed using equation 1. The results of this calculation are shown in Table 2.

a)

+

[Pmpip]

 

 



b)

[Pmpip]+

Endo

+

[Pmpyr]

+

[N211201]

+

[Pmpyr]

Heat Flow /(Wg )

+

-1

Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[N2113OH] +

[N2114]

+

[Pmim]

+

[Edmim] +

[Emim] PVDF

[N211201]+ [N2113OH]+ + [N2114]

[Pmim]+ [Etdmim]+ [Emim]+ PVDF

0,5 W/g

1440

1200

960

-1

720

30

Wavenumber (cm )

31

32

33

34

70

105

Temperature /(°C)

140

Figure 3. a) ATR/FTIR spectra and b) DSC curves for PVDF and the PVDF/IL composites. After the evaporation of the solvent at 210 ºC, pristine PVDF exhibits the TGTG (transgauche–trans-gauche) conformation characteristic of the α phase 10, 35. However, as Table


175


2 evidences, the introduction of the ILs into the PVDF polymer matrix led to a β phase content increase from 33 to 89%. It should be highlighted that the β phase nucleation depends both on the cation type and on the length of the alkyl side chain of the cation. An increase of the β phase content is observed upon increase of the alkyl chain length of the cation. The highest β phase content is observed for the [Pmim]+ and [N2114]+ cations (89 and 86%, respectively). Depending on the cation nature, the incorporation of the IL into the polymer matrix induced the transformation of the polymer chain to an all-trans (TTT) planar zigzag conformation ascribed to the β phase. This fact is attributed to the interaction of the ILs with the PVDF polymer chain (cation-polymer and anion-polymer interactions), mainly by the interaction between the CH2 groups of PVDF holding a positive charge density with the negative charge of the anion, and the interaction that can occur between the CF2 dipoles of PVDF with the cation 36. Taking into account the chemical structure and molecular weight of the cations, it is possible to draw several conclusions. For the ammonium cations, which present alkyl chains with similar size, the presence of distinct functional groups does not influence significantly the amount of β phase produced. As notice in Table 2, for the ammonium cations, the β phase content increases in the order [N2113OH]+ < [N211201]+ < [N2114]+ indicating that the presence of the polar groups in the cation reduces the electrostatic interactions between the polymer and the IL. This trend is similar to that observed for the molar volume of these ILs. For the imidazolium-based ILs, the β phase content increases with the increase of the length of the alkyl chain. As shown in Table 2, the β phase increases in the following order [Emim]+

Ionic Liquid Cation Size-Dependent ... - ACS Publications

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