ionic liquid blends

European Polymer Journal 71 (2015) 304–313

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Effect of ionic liquid anion and cation on the physico-chemical properties of poly(vinylidene fluoride)/ionic liquid blends R. Mejri a,b, J.C. Dias a,c, A.C. Lopes a,⇑, S. Bebes Hentati b, M.M. Silva c, G. Botelho c, A. Mão de Ferro d, J.M.S.S. Esperança d, A. Maceiras e, J.M. Laza e, J.L. Vilas f, L.M. León e,f, S. Lanceros-Mendez a,⇑ a

Centro/Departamento de Física, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal Laboratoire de Chimie des Matériaux, Faculté des Sciences de Bizerte, Université de Carthage, 7021 Zarzouna, Bizerte, Tunisia Centro/Departamento de Química, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal d Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, 2780-157 Oeiras, Portugal e Departamento de Química Física, Facultad de Ciencia y Tecnología, Universidad del País Vasco/EHU, Apdo. 644, Bilbao E-48080, Spain f Basque Center for Materials, Applications and Nanostructures (BCMaterials), Parque Tecnológico de Bizkaia, Ed. 500, Derio 48160, Spain b c

a r t i c l e

i n f o

Article history: Received 14 May 2015 Received in revised form 16 July 2015 Accepted 30 July 2015 Available online 7 August 2015 Keywords: Ionic liquids Polymer composites Electroactive polymers Poly(vinylidene fluoride)

a b s t r a c t Poly(vinylidene fluoride), PVDF, has been blended with different ionic liquids (IL) in order to evaluate the effect of the different IL anions and cations on the electroactive b-phase, thermal, mechanical and electrical properties of the polymer blend. [C2MIM][Cl], [C6MIM][Cl], [C10MIM][Cl], [C2MIM][NTf2], [C6MIM][NTf2], [C10MIM][NTf2] have been selected and were introduced in the polymer at a weight percentage of 40 wt%. It was found that the incorporation of ILs into the PVDF matrix leads to an increase of the b-phase content due to the strong electrostatic interactions between the dipolar moments of PVDF and the ILs. Further, the incorporation of ILs into PVDF strongly decreases the elastic modulus and increases the electrical conductivity of the blend with respect to the pure polymer matrix, all these effects being accompanied by a modification of the crystallization kinetics, as indicated by the modified spherulitic microstructure. Thus, novel PVDF/IL blends films with high transparency, excellent antistatic properties, and highly polar crystal form fraction were successfully achieved. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The lightness and flexibility of polymers and its possible conjugation with other organic and inorganic materials to form hybrid structures makes them very attractive for an increasing number of technological applications. Particularly, the development of ionic liquid/polymer based composites is continuously growing, mainly oriented toward the development of devices in the areas of energy [1] and actuators [2] but also for its use as plasticizers [3], solvents [4] or sensors [5,6]. The incorporation of ionic liquids (IL) in fuel cell electrolyte membranes allows them to be operational at temperatures above 100 °C under anhydrous conditions, since in IL the proton transport is independent of water content [7]. Further, the IL’s wide electrochemical stability window allows its use as high-voltage electrolytes in lithium-ion batteries, avoiding leakage and improving evaporation durability of electrolytes in dye-sensitive solar cells [8], resulting in an improvement of the ⇑ Corresponding authors. E-mail addresses: [email protected] (A.C. Lopes), [email protected] (S. Lanceros-Mendez). http://dx.doi.org/10.1016/j.eurpolymj.2015.07.058 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved.

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system lifetime. Still in the energy sector, IL/polymer composites are also used to develop safe and environmentally friendly supercapacitors [9] without aqueous and organic electrolytes. On the other hand, IL/polymer composites are also interesting in the area of electroactive actuators [2]. In this context, the incorporation of IL in the polymer matrix results in plasticizer and lubricant effects [10] and results in large bending displacement at low-voltages, caused by ion migration and accumulation on the electrode under an applied electric field [1]. The most interesting polymers for Electroactive Polymers (EAP) actuators applications are Nafion [11] and PVDF [12]. The last one is also known for its piezoelectric characteristics combined with a high thermal and chemical resistance. PVDF can crystallize in several polymorphs depending on the processing conditions method. The polar b-phase being the more interesting one for sensors and actuator applications due to its large piezoelectric response [12]. In this way, PVDF has been widely used in the production of composites for diverse applications [13]. The incorporation of IL in the PVDF matrix works as plasticizer [14], promotes the b crystalline phase in PVDF [15], has good miscibility [16,17] and increases the ionic conductivity, which is useful in the production of solid polymer-based electrolytes for batteries, fuel cells and supercapacitors [18]. The incorporation of the IL into the polymer matrix can be performed in the form of film [19], porous membranes [20] or fibers [21], including also the possible control of the hydrophobicity [22]. With respect to actuators, IL/PVDF polymer composites are also receiving increasing attention. Thus, composites have been developed for electrolyte in electrochemical actuators based in carbon nanotubes (CNTs) and ionic polymer-metal composites (IPMCs) [23,24]. Despite the variety of ionic liquids used in IL/PVDF composites production, e.g. [BMIM][BF4] [15,25], [C2MIM][NTf2] [24], [BMIM][PF6] [15], [BMIM][FeCl4] [15] and [BMIM][Cl] [15], and the promising results, there is no systematic study based on the effect of the type of cation/anion on the composite physico-chemical characteristics. Thus, this work reports on the preparation and characterization of IL/PVDF composites in the form of films with different anions and cation chain sizes, with large potential for sensor and actuator applications. 2. Experimental procedures 2.1. Materials Poly(vinylidene fluoride) (PVDF, Solef 6020) was obtained from Solvay. N,N-dimethylforamamide (DMF), 99.5%, was supplied by Merck. The ionic liquids, 1-ethyl-3-methylimidazolium chloride, [C2MIM][Cl], 1-hexyl-3-methylimidazolium chloride, [C6MIM] [Cl], 1-decyl-3-methylimidazolium chloride, [C10MIM][Cl], 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C2MIM][NTf2], 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C6MIM][NTf2] and 1-decyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide, [C10MIM][NTf2] were acquired from Iolitec (Germany) with stated purity of 99%. 2.2. Film preparation Composite films with thickness around 40–50 lm were prepared by solvent casting followed by melting and crystallization at room temperature. 40 wt% of IL was added to 6 mL of N,N-dimethylformamide (DMF) and mixed by 10 min under mechanical stirring. After that, 1 g of PVDF in powder form was added to the solution and dissolved with the help of a magnetic stirring since
3 h before spreading it into a glass. After these step, films were melted in an oven at a controlled temperature selected due to previous studies of 200 °C for 10 min [13]. Finally, the casted films were removed from the oven and cooled down at room temperature. Samples are transparent and no phase separation. The IL content of 40 wt% was selected in order to obtain large-enough variation of the physico-chemical properties of the composite for a suitable comparison among them. 2.3. Characterization Scanning electron micrographs (SEM) were collected with a Quanta 650 FEG (FEI) Scanning Microscope. The samples were previously coated with gold by magnetron sputtering with a Polaron Coater SC502. The size of polymer spherulites was determined by an average measurement of 100 spherulites, from the SEM images with the Image J software. Fourier Transform Infrared Spectroscopy (FTIR) was performed at room temperature using a JAS.CO FT/IR-4100 spectrometer in the attenuated total reflection (ATR) mode. Each sample was submitted to 32 scans with a resolution of 4 cm1. Different Scanning Calorimetry (DSC) was used to measure the crystalline fraction of the polymer within the composites and its melting temperature. These measurements were performed in a DSC 822e Mettler Toledo, using samples of 4 mg, under a nitrogen atmosphere in aluminum pans. Measurements were carried out from 25 to 200 °C at a heating rate of 10 °C min1. Thermogravimetric analysis (TGA) were performed with a TGA/SDTA 851e Mettler Toledo apparatus under a high purity nitrogen atmosphere (99.99% minimum purity) and a flow rate of 50 mL min1. Samples of approximately 4 mg were placed in an aluminum oxide crucible and heated from 25 to 850 °C at 10 °C min1. Measurements of the capacity and dielectric loss were performed with a Quadtech 1929 Precision LCR meter at room temperature, in the frequency range from 100 Hz to 1 MHz with an applied voltage of 0.5 V. Previously, the samples were

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prepared in the form of a parallel plate condenser with circular gold electrodes (5 mm diameter) deposited by magnetron sputtering with a Polaron Coater SC502. The a.c conductivity was calculated from r0 ¼ x  e0  e00 , where e0 (8.85  1012 Fm1) is the permittivity of free space, x = 2pf is the angular frequency and e00 ðxÞ is the frequency dependence of the imaginary part of the dielectric permittivity. The e00 ðxÞ was indirectly obtained by the dielectric measurements calculated by the application of C ¼ e0  A=d and tan d ¼ e00 =e0 , where C, A, d, tan d, e0 and e00 are the capacitance with the dielectric medium, the electrodes area, the separation between the electrodes, the dielectric loss and the real and imaginary parts of dielectric constant, respectively. Mechanical measurements were carried out in triplicate in samples with 15 mm length and 10 mm width, with AG-IS universal testing machine from Shimadzu with a load cell of 50 N. Tests were performed in tensile mode at room temperature (
23 °C) using a test velocity of 1 mm min1.

3. Results and discussion 3.1. Microstructural features The surface microstructure of the prepared samples, neat PVDF and IL/PVDF composites is shown in Fig. 1. All IL/PVDF composites, independently of the IL type, show a spherulitic microstructure similar to the one of neat PVDF, but with a better defined boundary between spherulites of smaller size that the ones form PVDF. This decrease of the spherulite size is attributed to the strong electrostatic interactions of the IL with the polymer chains, which show strong local dipole moments [26,27]. It is proven that strongly interactive fillers within the PVDF polymer matrix, not only act as nucleation centers for crystallization, but also modify the crystallization kinetic, leading to smaller spherulites [28]. The reduction of the average size of the spherulites thus dependent on the ion type present in the IL structure. There is an evident variation between the IL with [Cl] or [NTf2] as anion. The neat PVDF presents spherulites with an average size of 47 lm (Fig. 1A), while the ones presented by the composites [C2MIM][NTf2]/PVDF and [C2MIM][Cl]/PVDF show an average size of 18 and 6 lm (Fig. 1B and F), respectively. The ILs with [Cl] as anion are thus the most effective nucleation centers, increasing therefore the number of nucleation centers and giving rise to a larger number of smaller spherulites. Keeping the same anion, it can be observed, for both [CnMIM][NTf2]/PVDF and [CnMIM][Cl]/PVDF, an increase of the spherulite average size as the cation chain length increases from 2 to 10 carbon atoms. This fact is ascribed to the different number of the ionic units in the each film caused by the different weight of cation chain. This is, 40 wt% of [C2MIM][X] contains a higher number of ions units when compared with [C6MIM][X] and even a larger difference when compared with [C10MIM][X], leading, in the same way as referred before, to a higher number of crystallization centers and, as a result, to a larger number of smaller spherulites (Fig. 1B).

3.2. Polymer crystalline phase quantification Some studies showed that the presence of charged fillers within the PVDF matrix can acts as crystallization directors to promote the crystallization of specific polar b or c phases of the PVDF polymer from the melt [29,30]. The presence of IL in this polymer matrix could, in this way, induce the crystallization in one of the electroactive phases of PVDF. This effect has already been observed with different ionic liquids [27]. In this way, FTIR analysis was performed to determine the crystalline phase of the polymer within the composites films [13,31]. In the FTIR spectra, the characteristics vibration bands of the crystalline a-phase of PVDF are at 766, 792, 974, 1149, and 1383 cm1; and for the b-phase at 840 and 1279 cm1 and thus, can be used to identity and quantify the crystalline phases present in the samples [2,13,20]. Since the results for the composites with the IL presenting the same anion are very similar, only the ones of neat PVDF, [C2MIM][NTf2]/PVDF and [C2MIM][Cl]/PVDF are represented in Fig. 2A and of neat PVDF and [CnMIM][NTf2]/PVDF are presented in Fig. 2B, being representative of the rest of the samples. In the PVDF composites prepared with 40 wt%. [C2MIM][NTf2], [C6MIM][NTf2] and [C10MIM][NTf2], the characteristic absorption modes of the aphase are strongly reduced and completely disappears in the PVDF composites with [C2MIM][Cl], [C6MIM][Cl] and [C10MIM][Cl]. Simultaneously, the characteristic b-phase absorption bands appear very intensively. The neat PVDF polymer processed in the same way only presents absorption bands characteristics of the a-phase. Further, it is interesting to note the broad band around 1643 cm1 present in the sample with Cl (Fig. 2) that could indicate the presence of the water (m2 bending band), as we will report later. Further, the quantification of the b-phase content on the samples was performed from the FTIR spectra by applying Eq. (1) and the method presented previously by Martins et al. [13]:

FðbÞ ¼

Ab ðK b =K a ÞAa þ Ab

ð1Þ

In this equation, Aa and Ab represent the absorbance at 766 and 840 cm1, Ka and Kb are the absorption coefficients at the respective wave numbers, which values are 6.1  104 and 7.7  104 cm2 mol1, respectively.

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E

Spherulite size (μm)

α-PVDF6020 40

[NTf2]20

[Cl]-

0

[C2MIM]+

[C6MIM]+

[C10MIM]+

Fig. 1. SEM image of a-PVDF (A), [C2MIM][NTf2]/PVDF (B), [C6MIM][NTf2]/PVDF (C), [C10MIM][NTf2]/PVDF (D), [C2MIM][Cl]/PVDF (F), [C6MIM][Cl]/PVDF (G) and [C10MIM][Cl]/PVDF(H); spherulite size for the prepared samples (E).

The b-phase content for the different composites is shown in Table 1. Composites with [NTf2] as anion show a high percentage of b-crystalline phase, but a total crystallization in the b-phase is only observed in the composites with [Cl] as anion. As previously discussed, the electrostatic interaction with the IL is the main parameter responsible for the polymer nucleation in the electroactive phase. In the presence of these negative charges the positive part of the dipolar moments within

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A

α

α

β

B

β

α

α

β

β

[C2MIM][NTf2]

α-PVDF

700

800

900

1000 1100 1200 1300 1400 1500 1600 1700

[C10MIM][NTf2] Transmittance (a.u.)

Transmitance (a.u.)

[C2MIM][Cl]

[C6MIM][NTf2]

[C2MIM][NTf2] α-PVDF

700

-1

800

900

1000 1100 1200 1300 1400 1500 1600

1700

-1

Wavenumber (cm )

Wavenumber (cm )

Fig. 2. FTIR spectra of a-PVDF and PVDF with 40 wt% (w/w) of [C2MIM][NTf2] and [C2MIM][Cl] (A); FTIR spectra of a-PVDF and PVDF with 40 wt% (w/w) of [CnMIM][NTf2] (B).

Table 1 b-phase, melting temperature and crystalline content of a-PVDF, [CnMIM][NTf2]/PVDF and [CnMIM][Cl]/PVDF. Sample

b Phase (%)

Melting temperature (°C)

Crystallinity (%)

a-PVDF

0 83 ± 4 90 ± 4 87 ± 4 100 100 100

170 157 162 162 153 151 153

49.5 ± 5 56 ± 5 47 ± 4 58 ± 5 41 ± 4 37 ± 4 42 ± 5

[C2MIM][NTf2]/PVDF [C6MIM][NTf2]/PVDF [C10MIM][NTf2]/PVDF [C2MIM][Cl]/PVDF [C6MIM][Cl]/PVDF [C10MIM][Cl]/PVDF

the polymer chains, which are around the H atoms, tend to orient pointing toward the negative ions, inducing chain conformations corresponding to the b-phase [32]. The polymer phase contents for the pure polymer and the different composites are shown in Table 1. 3.3. Thermal properties The variation in the crystalline phase and spherulite size reflect a variation in the crystallization kinetics of the polymer that in turn can lead to variations in the crystallinity degree of the polymer. It is interesting, in this context, to evaluate the influence of the presence of the IL in the crystallinity degree of the polymer that in turn will influence the electromechanical response of the materials. Fig. 3A and B show the DSC scans for the neat PVDF as well as for the IL/PVDF composites, indicating both variations in the shape and temperature of the characteristic features. In all samples, the main melting peak of PVDF shifts to lower temperature. The remarkable difference is that [CnMIM][NTf2]/PVDF develop endothermic shoulders to the melting peak, which occur at about 140 °C, while wide endothermic phenomena are observed at lowest temperatures (80 °C), for [CnMIM][Cl]/PVDF. The decrease of the melting temperature of the polymer could be assigned to the strong electrostatic interactions between the IL and the crystalline polymer chains. Consequently, a lower thermal energy (lower melting temperature) is sufficient for the melt of the crystalline phase. Further, the lower thermal feature found in the [Cl] containing samples should be attributed to the imperfect crystals because of the high nucleation effect than in [CnMIM] [NTf2]/PVDF and to the water content of the IL/PVDF composites as it will be demonstrated by the TGA results presented later. The crystallinity degree (vc) of the films was determined from the DSC data using Eq. (2):

vc ¼

DH s xDHa þ yDHb

ð2Þ

where DHs is the melting enthalpy of the sample under consideration; DHa and DHb are the melting enthalpies of 100% crystalline sample in a and b-phase, respectively. x and y are the ratio of a and b phases within the films, obtained from FTIR measurements. Value of 93.07 and 103.4 Jg1 were used for the DHa and DHb, respectively [33]. The quantification of the crystallinity degree is presented in Table 1. Comparing the crystallinity degree of pristine PVDF with IL/PVDF composites, there is a slight decrease of the crystallinity degree in the [Cl] containing samples and no significant effect for the remaining ones, due to the previously described

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B

A [C10MIM][Cl]/PVDF

Heat flow Endo Up

Heat flow Endo Up

[C10MIM][Ntf2]/PVDF

[C6MIM][Ntf2]/PVDF

[C2MIM][Ntf2]/PVDF

[C6MIM][Cl]/PVDF

[C2MIM][Cl]/PVDF -1

0.4 W.g

-1

0.4 W.g α-PVDF

40

60

α-PVDF

80

100

120

140

160

180

40

200

60

80

100

120

140

160

180

200

Temperature (ºC)

Temperature (ºC)

Fig. 3. DSC heating scans of PVDF films with 40 wt% of [CnMIM][NTf2] (A) and [CnMIM][Cl] (B) samples.

interactions. Finally, the low temperature shoulders appearing in the [CnMIM][NTf2]/PVDF samples are related to illcrystallized regions [29] in the interface areas with the IL that are not detected in the [Cl] containing samples as the filler is degraded before the melting of the material. It has been shown that the crystallization kinetics depends on the nucleation effect of the fillers, which is particularly relevant for when IL are present due to their large size and strong electrostatic interactions. These facts lead to a hindering of the spherulite growth and ill-crystallization in the interface regions due to the electrostatic energy and defective crystallization in the proximity of the IL [34]. To further understand the thermal behavior of the samples and the polymer/IL interactions Fig. 4A shows the thermogravimetric curves of neat a-PVDF and the composites with 40 wt% of [C2MIM][NTf2] and [C2MIM][Cl]. The degradation process of neat a-PVDF, with an onset temperature close to 500 °C, is related with the carbon-hydrogen and carbon-fluoride scission and the formation of carbon–carbon double bond in parallel with the unzip HF molecules down from the polymer chain [35]. However, it is to notice the decrease of the degradation temperature of PVDF in the presence of IL both for the [C2MIM][NTf2]/PVDF and [C2MIM][Cl]/PVDF samples, with a reduction of about 50 °C on the onset temperature and with a significant decrease of the onset temperature to about half of the initial value, 270 °C, respectively. For the latter composites the occurrence of a previous weight loss is observed, at temperatures below 150 °C. This first step of weight loss is associated with the amount of water in the prepared IL/PVDF composites. Fig. 4B shows the existence of variations on the amount of weight loss in samples with different chain size in the cation of IL. It is shown that the longer the chain size of the cation the lower the percentage of weight loss, which is related with the high hydrophobicity of the ILs with longer alkyl chains and to

102 100

100

A

α-PVDF

B

α-PVDF

98

[C2MIM][Ntf2]/PVDF

60

[C2MIM][Cl]/PVDF

Weight (%)

Weight (%)

80 96

[C10MIM][Cl]/PVDF

94

[C6MIM][Cl]/PVDF

92

[C2MIM][Cl]/PVDF

40 90 20

88 86

0 0

200

400

Temperature (ºC)

600

800

50

100

150

200

250

Temperature (ºC)

Fig. 4. Thermogravimetric curves obtained for neat a-PVDF, [C2MIM] [NTf2]/PVDF and [C2MIM][Cl]/PVDF (A) and for neat a-PVDF and [CnMIM][Cl]/PVDF with different cation chain size (B).

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the mass (m) percentage of chloride ion in the respective ionic liquid (m Cl m½Cn MIM½Cl ) (Fig. 5). In this sense it is important to note that chloride ionic liquids handled in a glovebox (data not shown) do not present this weight loss at low temperature, confirming the presence of water in the films, as also suggested by the FTIR results (Fig. 2). It is to notice first the strong influence of the IL in the degradation temperature of the polymer due to the strong electrostatic interactions, which is further reinforced in the [Cl] containing samples due to the interaction of the polymer with the degradation products of the chloride ILs [36]. It has been reported that [CnMIM][NTf2] suffer decomposition at temperatures around 400 °C [37]. When chloride is the anion present in the IL, the onset temperature decrease to values around 250–300 °C [38], as in the present study. This behavior can be ascribed to the creation of HCl, which has been already described as one of the possible degradation products of [CnMIM][Cl] [38]. The production of this acid can be derived from the interaction of [Cl] with the hydrogen atoms, not only from the IL cation, but also from the PVDF polymer chain. This interaction with the hydrogen atoms of PVDF will anticipate the process of polymer degradation that begins with the loss of hydrogen atoms followed by the production of hydrogen fluoride, HF [7]. 3.4. Mechanical properties It has been reported that the presence of IL in polymer matrix affects the mechanical properties of the polymer [3]. With the aim to analyze the influence of different IL in the mechanical properties, quasi-static stress–strain measurements were carried out. Representative results are represented in Fig. 6A and the corresponding values of the elastic modulus in Fig. 6B. It is confirmed that the presence of IL acts as a plasticizer, reducing the elastic modulus of the material to less than 20% of the initial value. Keeping the same cation [C2MIM]+, the composites with [Cl] as anion show higher elastic modulus than the one with [NTf2], however the values are coincident for the composites with [C6MIM]+ and [C10MIM]+. In both cases, the plasticizer effect experiences a slight increase as the cation chain length increases. 3.5. Electrical properties The high electrical conductivity of IL can represent an important and useful characteristic for application of the composites in areas such as energy harvesting and storage and actuators. In this way, the a.c. electrical conductivity of the IL/PVDF composites was obtained and the results represented in Fig. 7. A strong increase of the electrical conductivity is observed with respect to the pristine polymer which in turn also depends on the type of anion and cation present in the ionic liquid. Thus, the composites with [NTf2] as anion show conductivity values with four orders of magnitude higher than neat PVDF and the samples with [Cl] as anion present values even higher, reaching values up to eight orders of magnitude higher when compared with neat PVDF. There is no marked effect of the size of the alkyl chain of the cation in the electrical conductivity of the IL/PVDF composites except for the [C10MIM][Cl] which present a value of electrical a.c. conductivity one order of magnitude higher. This increase in the electrical conductivity of the IL/polymer composites is fully ascribed to the number and mobility of the ionic species introduced within the polymer matrix by the IL [39]. The enhancement of the electrical conductivity when using [CnMIM][Cl] vs [CnMIM][NTf2] is most certainly related with the different amount of water content of the samples [40,41]. To the best of the authors knowledge, there is no explanation for the extraordinary increment of the conductivity of the [C10MIM][Cl]/PVDF matrix and this deserves some close attention in the near future.

7.5

st

Weight loss on the 1 step

7.0 [C2MIM][Cl]

6.5 6.0 5.5 [C6MIM][Cl]

5.0 4.5 4.0 3.5 [C10MIM][Cl]

3.0 12

14

16

18

20

22

24

mCl /m[CnmMIM][Cl] (%) -

Fig. 5. Relationship between the weight loss during the first degradation step of the [CnMIM][Cl]/PVDF composites and the mass of chloride anions in the total mass of the corresponding ionic liquid.

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[C2MIM][Ntf 2]

A

5x10 7

2400

[C10MIM][Ntf2]

Elastic Modulus (MPa)

Stress (Pa)

[C6MIM][Cl [C10MIM][Cl α-PVDF

3x10 7

α-PVDF 6020

2000

[C2MIM][Cl] 4x10 7

B

2800

[C6MIM][Ntf 2]

2x10 7 1x10 7

0

600

[Cl]

400

200

-

[Ntf 2]

-

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

α-PVDF 6020

Strain

[C2MIM]

+

[C6MIM]

+

[C10MIM]

+

Fig. 6. Stress/strain curves for PVDF/ILs composites (A) and respective elastic modulus (B).

0.01

B

1

A

0.1

1E-3

log (σ (s/m))

log (σ (s/m))

0.01 1E-4 1E-5 1E-6 [C2 MIM][Ntf2 ]

1E-8 1000

10000

100000

log (ω (rad/s))

1E-4 1E-5 1E-6

α-PVDF

1E-7

1E-3

[C6 MIM][Ntf2 ]

1E-7

[C10 MIM][Ntf2 ]

1E-8

1000000

1E7

α-PVDF

[C2 MIM][Cl] [C6 MIM][Cl] [C10 MIM][Cl]

1000

10000

100000

1000000

1E7

log (ω (rad/s))

Fig. 7. Log–log plot of the conductivity as a function of frequency for the neat a-PVDF and [CnMIM][NTf2]/PVDF (A) and [CnMIM][Cl]/PVDF (B) composites.

4. Conclusions Film samples of PVDF with IL containing different anion and cation chain sizes were successfully prepared by solvent casting and melting. The characteristics of the IL strongly influence the physico-chemical characteristics of the composites. Thus, [CnMIM][Cl]/PVDF composites present higher number of small spherulites due the higher electrostatic interactions of this IL with the polymer chains. This fact results also in the total crystallization of PVDF in the piezoelectric b-phase, comparatively with the [CnMIM][NTf2]/PVDF samples. No significant changes are observed in the crystallinity degree of the polymer (a decrease of
5% for the [Cl] containing samples), but, on the contrary, the presence of IL in the PVDF structure significantly affects the thermal degradation of the polymer. A reduction of 50 °C in the PVDF degradation temperature occurs in the presence of [CnMIM][NTf2] and an outstanding variation is observed in the samples with [CnMIM][Cl], where a reduction of 270 °C in the degradation temperature of the polymer is observed. The presence of IL also plays an important role in the mechanical properties, since IL works as a plasticizer, leading to a reduction of the elastic modulus up to 10% with respect to the value of the pristine polymer. Finally, the presence of IL results in an increase of composite a.c. conductivity that is of four orders of magnitude for [CnMIM][NTf2]/PVDF and up to eight orders of magnitude for [CnMIM][Cl]/PVDF samples. It this way, the proper selection of IL allows tailoring the material properties for applications in the areas such as energy harvesting and storage, sensors and actuators. Acknowledgements This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the projects PEST-C/QUI/UIO686/2013, PEST-C/FIS/UI607/2014, UID/Multi/04551/ 2013 and PTDC/QUI-QUI/117340/2010. The authors also thank funding from ‘‘Matepro – Optimizing Materials and

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Processes”, Ref. NORTE-07-0124-FEDER-000037, co-funded by the ‘‘Programa Operacional Regional do Norte” (ON.2 – O Novo Norte), under the ‘‘Quadro de Referência Estratégico Nacional” (QREN), through the ‘‘Fundo Europeu de Desenvolvimento Regional” (FEDER). The authors also thank Support by the project SIMPE-Interactive multitouch surfaces based on electroactive polymers – QREN-ADI-23122, through the COMPETE program. JCD thanks the FCT for the SFRH/ BD/90215/2012 grant.

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