Nanostructuration of ionic liquids in fluorinated matrix: Influence on the mechanical properties

Article abstract of DOI:10.1016/j.polymer.2011.01.052

In this work, a new method to prepare fluorinated coatings with mechanical properties enhanced has been developed. Pyridinium, imidazolium, and phosphonium ionic liquids have been synthesized and used as new synthetic building blocks in a polytetrafluoroe

Full text of DOI:10.1016/j.polymer.2011.01.052

Polymer 52 (2011) 1523e1531
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Nanostructuration of ionic liquids in fluorinated matrix: Influence
on the mechanical properties
,
b
,
c
,
a
,
b
,
,b,c
Sébastien Livi a
, Jannick Duchet-Rumeau c, Jean-François Gérard a
*
a Université de Lyon, F-69003 Lyon, France
b INSA Lyon, F-69621 Villeurbanne, France
c CNRS, UMR 5223, Ingénierie des Matériaux Polymères, France
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, a new method to prepare fluorinated coatings with mechanical properties enhanced has
been developed. Pyridinium, imidazolium, and phosphonium ionic liquids have been synthesized and
used as new synthetic building blocks in a polytetrafluoroethylene matrix. The strategy demonstrated
using long alkyl chain cations provides an opportunity to prepare nanomaterials with a nanoscale
structuration. The design of these new ionic and nanostructured materials is very dependent on the
cationeanion combination of ILs. The morphology analyzed by transmission electronic microscopy (TEM)
shows that it is clearly tuned by the chemical nature of ILs. The finest structuration leads to a dramatic
compromise between stiffness and deformation of material. The small-angle X-Ray scattering (SAXS)
shows the evolution of the ionic networks during the mechanical sollicitation.
Received 7 October 2010
Received in revised form
13 January 2011
Accepted 29 January 2011
Available online 24 February 2011
Keywords:
Ionic liquid
Building block
Nanostructuration
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
obstacles, our research has been directed towards organic com-
pounds with intrinsic and unique properties: Ionic Liquids (ILs)
In the world of nanotechnology, the goal is to create new mate-
rials with significantly improved physical properties from designing
matter structuration at nanoscale. For many years, several app-
roaches have been described for designing new structures from
organic molecules, polymers, or organiceinorganic hybrids. One of
the well-known approach consists in the design of nanostructured
thermosets obtained by the use of block copolymers with amphi-
philic block [1e3]. Another way for the development of nano-
structured polymers is the introduction of inorganic nano-objects
having nanometre-scale dimensions such as silica [4], layered sili-
cates [5e10], or carbon nanotubes [11e13]. The problem with these
different methods mentioned above is that they require several
preparation steps before achieving improved materials. In fact, in the
field of the polymer nanocomposites, it is often necessary to modify
the surface of the inorganic nanoparticles in order to improve the
dispersion as well as the final properties of materials. In the case of
thermosets, the synthesis of block copolymers can be long and
difficult. Moreover, it is necessary to use large amounts of copoly-
mers to improve properties of the material. To overcome these
especially known for their excellent thermal stability, non-flamma-
bility, low saturated vapour pressure, good electrical, and thermal
conductivity and commonly used as green solvents in inorganic
synthesis [14] and synthesis of nanoparticles [15], homogeneous and
heterogeneous catalysis [16], in polymer science as plasticizers [17],
or lubricants [18].
They are also commonly used as surfactants in the lamellar sili-
cate nanocomposites. The most frequently used are ammonium
salts [19e21]. In recent years, pyridinium, imidazolium, and phos-
phonium ionic liquids known for their excellent thermal stabilityare
increasingly used [22,23]. And recently, a new approach of ILs has
been studied by Wathier and Grinstaff [24] who have suggested that
ionic liquids may play an important role in the formation of ionic
networks based on coordinating ion pairs. They have demonstrated
that Coulomb interactions are governed by pairwise interactions
between cation and anion and the extended structure of the ionic
liquid may lead to a supramolecular ionic network. Thus, a combi-
nation of the mechanical properties linked to ionomers with the
homogeneity and the high charge densities typical of ionic liquids
could lead to a new range of optimized materials. However, from our
knowledge, no paper described the use of ionic liquids within
a polymer matrix to design nanostructured phase. In this paper
the synthesis of ionic liquids based on various cation: pyridinium,
imidazolium and phosphonium but also on the different anions
(hexafluorophosphate, iodide and bromide) has been described.
* Corresponding author. Cornell University, College of Engineering Materials Science
& Engineering, 416 Bard Hall, Ithaca, NY 14853-1501, USA. Tel.: þ33 607 255 6684;
fax: þ33 607 255 2365.
E-mail address: sebastien.livi@gmail.com (S. Livi).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.01.052

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S. Livi et al. / Polymer 52 (2011) 1523e1531
Then, their influences in a fluorinated matrix, chosen for coating
applications as well as the consequences of this structuration at
nanoscale on the material physical behaviour such as mechanical
properties have been studied.
evaporation under vacuum, the beige coloured powder was filtered,
washed repeatedly with pentane and dried. Purification of the
resulting imidazolium salts was accomplished by crystallization
from ethyl acetate/acetonitrile: 75/25 mixture. After drying, alkyl
imidazolium salt was fully characterized by spectroscopy and
thermogravimetric analysis (TGA). The assignment of 13C NMR sp-
ectroscopy resonance peaks is an evidence of the success of the
ionic liquid synthesis.
2. Experimental
2.1. Materials
All chemicals used for to the synthesis of ionic liquids, i.e. tri-
phenylphosphine (95%), imidazole (99.5%), pyridine (99%), iodooc-
tadecyl (95%) and all the solvents (toluene, sodium methanoate,
pentane and acetonitrile) were supplied from Aldrich and used as
received. The polytetrafluoroethylene used in this study, denoted as
PTFE, is an aqueous dispersion of PTFE from Solvay. The composition
is as follows: PTFE (60 wt %), water (32e33 wt %), octylphenol pol-
yethoxylates TritonÒ (7 wt %) and ammonium perfluorooctanoate
(0,1 wt %). The pH of aqueous dispersion is 10 and the PTFE particle
average size is about 220 nm.
2.2.2.1. 1-Octadecyl-3-octadecylimidazolium iodide. 13C NMR (CDCl3):
d
14.10 (2CH3); 22.67 (2CH2Me); 26.23; 28.97; 29.35e29.69; 30.24;
31.91 (CH2); 50.10; (CH2N]); 50.32 (CH2Ne); 121.69; 122.48 (]CN);
136.88 (NeC]N). Yield ¼ 94%, melting temperature (ꢁC): 67 ꢁC. The
degradation temperature of the C18C18Im Iꢀ determined for a weight
loss of 50 wt % is 310 ꢁC.
2.2.3. Synthesis of octadecyltriphenylphosphonium iodide, bromide
(C18P Iꢀ, C18P Brꢀ)
In a 100 mL flask were placed under a positive nitrogen pressure,
triphenylphosphine and octadecyl iodide or octadecyl bromide. The
stirred suspensions were allowed to react for 24 h at 120 ꢁC in
toluene (20 mL), a yellow precipitate was formed. The reaction
mixture was then filtered, washed repeatedly with pentane. Most
of the solvent was removed under vacuum and the product was
dried to a constant weight to give a white solid. These salts were
characterized by 13C NMR spectroscopy and thermogravimetric
analysis (TGA).
2.2. Synthesis of pyridinium, imidazolium, and phosphonium ionic
liquids
The synthesis of 1-octadecyl-3-octadecylimidazolium and octa-
decyltriphenylphosphonium iodide was obtained from a route similar
to that described by Livi et al. [25]. Octadecyltriphenylphospho-
nium hexafluorophosphate and octadecylpyridinium iodide are not
commercially available.
2.2.3.1. Octadecyltriphenylphosphonium iodide. 13C NMR (CDCl3):
d 14.00 (CH3); 22.67 (CH2Me); 23.2; 29.37e29.66; 30.24; 31.85
2.2.1. Synthesis of octadecylpyridinium iodide (C18Py Iꢀ)
These compounds are not commercially available and their
synthesis was not reported before. In a 100 mL flask was placed
under a nitrogen pressure, 10 mmol of octadecyl iodide (C18H37I)
and distilled pyridine (1.5 equiv.). The stirred suspension was
allowed to react for 24 h at room temperature. A yellow precipitate
was formed. The reaction mixture was then filtered, and washed
repeatedly with pentane. Most of the solvent was removed under
vacuum. A white solid was obtained. After drying, alkyl phospho-
nium salt was fully characterized by spectroscopy 13C NMR and
thermogravimetric analysis (TGA).
(PCH2); 118.45; 130.43; 133.70; 135.15 (P-Carom.). Yield ¼ 97%,
melting temperature (ꢁC): 86 ꢁC.
2.2.3.2. Octadecyltriphenylphosphonium bromide. 13C NMR (CDCl3):
d
14.00 (CH3); 22.52 (CH2Me); 23.0; 28.90e29.66; 30.20; 32.00
(PCH2); 118.85; 130.80; 133.50; 135.45 (P-Carom.). Yield ¼ 88%,
melting temperature (ꢁC): 86 ꢁC. The degradation temperature of
the C18P Iꢀ and C18P Brꢀ determined for a weight loss of 50 wt % is
320 ꢁC.
13C NMR (CDCl3)
d: 14.04 (CH3); 22.58; 25.91; 28.99; 29.25e29.60;
2.2.4. Synthesis of octadecyltriphenylphosphonium
31.81e31.84 (CH2); 62.02 (CH2N]); 128.57 (C]C); 144.82; 145.50 (]CN).
Yield ¼ 90%, melting temperature (ꢁC): 102 ꢁC. The degradation
temperature of the C18Py Iꢀ determined for a weight loss of 50 wt % is
270 ꢁC.
hexafluorophosphate (C18P PFꢀ6 )
In a 100 mL flask, octadecyl iodide (C18H37I) (5.0 g, 1 equiv.) was
dissolved into dichloromethane (25 mL). The mixture was stirred
for 30 min at room temperature. A solution of hydrogen hexa-
fluorophosphate (HPF6) (3.8 g, 2 equiv.) diluted in water (25 mL)
was stirred for 30 min and added to the octadecyl iodide solution.
The stirred suspension was allowed to react for 24 h at room
temperature. The reaction mixture was then introduced in a sepa-
ratory funnel and the organic layer was washed repeatedly with
distilled water (4 ꢂ 25 mL). The mixture was dried over anhydrous
magnesium sulphate and concentrated under reduced pressure.
The solvent was removed by evaporation under vacuum and the
product was dried to a constant weight to give a white solid. The
salt was characterized by 13C NMR spectroscopy and thermogra-
vimetric analysis (TGA).
2.2.2. Synthesis of 1-octadecyl-3-octadecylimidazolium iodide
(C18C18Im Iꢀ)
A solution of sodium methoxide was prepared from sodium
(1 equiv.) in dry freshly distilled methyl alcohol (10 mL) in a sealed
septum, 100 mL round-bottomed, three necked flask equipped with
a condenser, under nitrogen atmosphere and magnetic stirring.
Imidazole (1 equiv.) diluted in acetonitrile (10 mL) was then added
into the stirred mixture of sodium methoxide previously cooled at
room temperature. After 15 min, a white precipitate was formed.
The suspension was then concentrated under reduced pressure for
1 h. The dried white powder was dissolved in acetonitrile and
a powder of alkyl iodide (1 equiv.) diluted in acetonitrile (10 mL)
was then added under an inert atmosphere of nitrogen at room
temperature. The mixture was stirred for 1 h, then heated under
reflux at 85 ꢁC for about 24 h. A powder of alkyl iodide (1 equiv.)
diluted in acetonitrile (10 mL) was added to the mixture at room
temperature. The stirred suspension was heated under reflux at
85 ꢁC for about 24 h leaving a brownish viscous oil in each case.
After cooling to room temperature, the solvent was removed by
2.2.4.1. Octadecyltriphenylphosphonium hexafluorophosphate. 13C
NMR (CDCl3):
d 14.00 (CH3); 22.35 (CH2Me); 23.5; 29.12e29.74;
30.35; 31.75 (PCH2); 118.75; 130.22; 133.50; 135.05 (P-Carom.).
Yield ¼ 80%, melting temperature (ꢁC): 80 ꢁC. The degradation
temperature of the C18P PFꢀ6 determined for a weight loss of 50 wt
% is 450 ꢁC.
In this work, different combinations cation/anion are proposed
and are summarized in Fig. 1.

S. Livi et al. / Polymer 52 (2011) 1523e1531
1525
2.3. Processing and characterization of the IL/PTFE films
The aqueous suspensions of PTFE containing 1 wt % of pyridinium
or imidazolium or phosphonium ionic liquids were agitated by
a RayneriÒ disperser with a speed of 1500 rpm for 15 min. Then the
suspension was spread on stainless steel plates with a bar coater
equipped with a bar of 50 microns in order to get 20 microns-thick
dried films. A thermal treatment at 400 ꢁC for 10 min was necessary
to obtain the polymer film.
Thermogravimetric analysis (TGA) performed on the ionic liquids
and on PTFE/IL nanocomposites were performed by using Q500
thermogravimetric analyser (TA instruments). The samples were
heated to 700 ꢁC at a rate of 20 K minꢀ1 under nitrogen flow of
90 mL/min.
DSC measurements were carried out by using Q20 (TA instru-
ments) in the range of ꢀ20 ꢁC to 400 ꢁC. The samples were kept for
3 min at 400 ꢁC to erase the thermal history before being heated or
cooled at a rate of 10 K minꢀ1 under nitrogen flow of 50 mL/min. For
calculations of enthalpy of melting and crystallization, the enthalpy
of reference used for PTFE is 82 J/g [19,20].
Wide angle X-ray diffraction spectra (WAXD) were carried out on
a Bruker D8 Advance X-ray diffractometer at the H. Longchambon
diffractometry centre at room temperature. A bent quartz mono-
chromator was used to select theCuKa1 radiation(¼0.15406 nm)and
run under operating conditions of 45 mA and 33 kV in Bragge
Fig. 1. Structure of the ionic liquids.
Brentano geometry. The angle range analyzed was from 1 to 30ꢁ 2
q.
Small-Angle X-ray Scattering was carried out on the D2AM beam-
line at the European synchrotron Radiation Facility (ESRF, Grenoble,
France). The incident photon energy was 16 keV. A bidimensional
Fig. 2. TEM micrographs of neat PTFE (a) and blends PTFE/C18C18Im Iꢀ (b), PTFE/C18Py Iꢀ (c), PTFE/C18P Iꢀ (d).

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S. Livi et al. / Polymer 52 (2011) 1523e1531
Fig. 3. TEM micrographs of blends PTFE/C18P Iꢀ (a), PTFE/C18P Brꢀ (b), PTFE/C18P PF6ꢀ (c).
detector (CCD camera from Ropper Scientific) was used to collect the
scattered radiation. The contribution of the emptycell was subtracted
from the scattering images of the studied samples.
X-ray Photoelectron Spectroscopy experiments were carried out
in a KRATOS AXIS Ultra DLD spectrometer using a hemispherical
analyzer and working at a vacuum better than 10ꢀ9 mbar. All the
data were acquired using monochromated Al Ka X-rays (1486.6 eV,
150 W), a pass energy of 40 eV, and a hybrid lens mode. The area
the first one corresponds to the formation of aggregates of ionic
clusters while the second one is similar to a co-continuous mor-
phology. A less achieved co-continuous morphology is also ob-
tained with pyridinium ionic liquids (C18Py Iꢀ). In the case of
phosphonium ionic liquid, an excellent dispersion is achieved since
a structuration at nanoscale is obtained in the polymer matrix.
The use of the different counteranions halide (Iꢀ, Brꢀ) or fluori-
nated (PFꢀ6 ) associated to phosphonium cation significantly influ-
ences the final morphology as reported in Fig. 3. A poor distribution
of important aggregates can be observed with bromide anion
whereas a coarse morphology resulting from a good distribution of
aggregates is also obtained with the fluorinated anion. Both of these
microscale structurations display a large contrast in comparison to
the nanoscale morphology reached with the iodide anion.
Due to the hydrophobic nature of polytetrafluoroethylene and
strong interactions between ionic compounds in PTFE/IL blends,
a phase-separated morphology is generated spontaneously. In fact,
in the present case, whatever the ionic liquid considered, their
miscibility remains very poor even with the presence of the oca-
decyl chain. Nevertheless, such morphologies could be compared to
the ones observed in ionomers for which ion-containing polymers
provide a mean of generation of various types of morphologies and
subsequent properties especially in polymer blends [26,27]. In such
materials, the ionic species introduce specific interactions between
components and could lead to miscible or partially miscible blends
of two immiscible polymers from the control of the type of ionic
acid group and/or counteranion. It is well known that the clustering
of ion pair in a low dielectric constant medium is responsible for
different nano- and microstructures which can be predicted theo-
rically [28,29]. The main parameter controlling the microphase
separation in a non-polar media is the dipoleedipole interactions
between pairs leading to formation of multiplet structures [30,31],
i.e. ionic aggregates. In this study, the same phenomena could be
evoked with the formation of ionic liquid aggregates which from
different types of morphologies depending on the balance of the
interactions between polymer medium and anionecation pairs.
In conclusion, the ionic liquids have a similar behaviour to
ionomers which are well described in the literature [29,30]. As
reported, ionic liquids form ionic clusters of varied morphologies
from nanoscale up to microscale, easily tuned by the wide variety of
salts achieved by a great number of combinations possible between
cation and anion.
analysed is 700
Ce(C,F) components of the C1s band at 284.6 eV.
m
m ꢂ 300
mm. The peaks were referenced to the
The Transmission Electron microscopy (TEM) was collected at the
Centre of Microstructures (Université de Lyon) on a Philips CM 120
field emission scanning electron microscope with an accelerating
voltage of 80 kV. The samples were cut using an ultramicrotome
equipped with a diamond knife, to obtain 60 nm thick ultrathin
sections. Then, the sections were set on copper grids for observation.
Tensile property tests were carried out on an MTS 2/M electro-
mechanical testing system at 22 ꢃ 1 ꢁC and 50 ꢃ 5% relative
humidity at crosshead speed of 0.004 and 0.2 sꢀ1. A cookie cutter
was used to obtain dumbbell-shaped specimens.
Dynamic Mechanical Thermal Analysis was carried out on a Rheo-
metrics Solid Analyzer RSA II at 0.01% tensile strain and a frequency
of 1 Hz. The heating rate was 3 K/min for the temperature range
from ꢀ130 ꢁC to 320 ꢁC.
3. Results and discussion
3.1. Effect of ionic liquids on the structuration of fluorinated
polymer films
3.1.1. Bulk nanostructures evidenced by transmission electronic
microscopy (TEM)
Transmission electron microscopy is the suitable tool to reveal
the existence of ILs as a separated phase into the fluorinated matrix
according to the difference in electronic densities. With only 1 wt %
of ILs, a structuration is reached (Fig. 2). In this study, the chemical
nature of the cation and halide anions plays a key role on the
different morphologies obtained. The dispersion of imidazolium
ionic liquid (C18C18Im Iꢀ) generates two types of nanostructuration:
Table 1
Atomic percentage of species detected on the surface of PTFE films.
Table 2
Atomic %
CeF
CeC
Influence of organic cation on the physical properties of PTFE films (Std, cooling rate
10 K minꢀ1).
PTFE
59
97
30
3
PTFE 1%
Tm (ꢁC)
Tc (ꢁC)
DHm (J/g)
Xc (%)
C18P Iꢀ
Samples
PTFE 5%
C18P Iꢀ
93
88
5
PTFE
326
328
328
329
310
307
307
308
32
28
29
36
39
34
35
44
PTFE/C18P Iꢀ
PTFE/C18C18Im Iꢀ
PTFE/C18Py Iꢀ
PTFE 1%
12
C18C18Im Iꢀ

S. Livi et al. / Polymer 52 (2011) 1523e1531
1527
Table 3
Table 4
Influence of anion on the physical properties of PTFE films (Std, cooling rate
Dynamical mechanical analysis of IL-modified PTFE: Change of storage moduli at
various temperatures at 1 Hz.
10 K minꢀ1).
Samples
Tm (ꢁC)
Tc (ꢁC)
D
Hm (J/g)
Xc (%)
Sample
E0 (*106 MPa)
E0 (*106 MPa)
E0 (*106 MPa)
25 ꢁC
150 ꢁC
250 ꢁC
PTFE
326
328
327
329
310
307
307
308
32
28
28
31
39
34
34
38
PTFE/C18P Iꢀ
PTFE/C18P Brꢀ
PTFE/C18P PFꢀ6
PTFE
505
405
425
547
594
522
84
139
138
193
136
141
42
82
66
99
68
57
PTFE/C18P Iꢀ
PTFE/C18P Brꢀ
PTFE/C18P PFꢀ6
PTFE/C18C18Im Iꢀ
PTFE/C18Py Iꢀ
3.1.2. Surface analysis by X-ray Photoelectron Spectroscopy (XPS)
According to the chemical nature of the two components, i.e.
PTFE and ILs, a surface segregation could occur. In order to evaluate
the interactions between ILs and fluorinated matrix and a possible
enrichment of one of the components at the surface of the films,
X-ray photoelectron spectrometry was used to analyze the surface
of the neat PTFE and ILs/PTFE films. The Table 1 shows the
percentage of CeF and CeC bonds detected on the sample surface.
The neat PTFE composed only of (eCF2eCF2e) monomer units
displays a ratio of two between the CeF (60%) and CeC (30%) bonds.
As the ionic liquid is added to the polymer matrix, a significant
decrease of CeC bonds in the presence of ionic liquids C18C18Im Iꢀ,
C18P Iꢀ is observed. With C18P Iꢀ, only 3% of CeC bonds were
measured instead of 30% on neat PTFE film which cannot be
explained by a surface segregation. These results are an evidence of
the influence of ionic liquids on the chain scission due to the
formation of HF, HI, HBr acids [32,33], favoured by the fluorinated
aqueous suspension. Thus, an increase of CeF bonds during the
post-treatment at high temperature is observed. Moreover, the
migration of IL takes place in the bulk of the material since no trace
of iodine, phosphorous, or nitrogen has been found at the surface
even at higher ILs contents, i.e. 5 wt %.
enthalpies of melting
fluorinated matrix and with the addition of 1 wt % of the different
types of IL.
D
Hm, and crystallization,
D
Hc, for the neat
The IL incorporation into polymer matrix has a minor effect on
the thermal transitions. The melting temperatures of neat PTFE or
PTFE modified with pyridinium, imidazolium, or phosphonium
ionic liquids are included between 326 ꢁC (PTFE) and 329 ꢁC (PTFE/
C18Py Iꢀ). Regarding the crystallization temperatures, no significant
difference is observed since the values are nearly in the same
temperature range, i.e. from 307 ꢁC to 310 ꢁC. Only, the chemical
nature of organic cation induces differences in the crystallinity of
the PTFE matrix. Indeed, the addition of phosphonium and imida-
zolium ionic liquids results in a slight decrease in crystallinity. This
decrease could be attributed to the steric hindrance of cations
caused by the presence of two long alkyl chains on imidazolium ion
and the presence of three benzyl groups and a long alkyl chain on
phosphonium ion. On the other side, octadecylpyridinium iodide
that is less hindered has a light nucleating effect on the crystalli-
zation of fluoropolymer with an increase of 5%.
3.2.2. Influence of halide or fluorinated anions associated to
phosphonium cation
3.2. Effect of ionic liquids on the PTFE crystallinity
The effect of halide (Brꢀ, Iꢀ) or fluorinated (PFꢀ6 ) anions
combined to phosphonium cation on the crystallinity of polymer
films was studied by DSC (Table 3).
3.2.1. Influence of the organic cation
The effect of IL on the polymer physical characteristics such as
crystallinity was studied (the samples were analyzed at a heating
and a cooling rate of 10 K minꢀ1). Table 2 gathers melting (Tm) and
crystallisation (Tc) temperatures as well as the corresponding
The melting and crystallization temperatures are not affected by
the nature of the anion since the DSC measurements are the same
whatever the anion used. In terms of the melting enthalpies, the
Fig. 4. Storage moduli E0, main and secondary relaxations of PTFE, PTFE C18P Iꢀ evidenced on tan
d spectra recorded at 1 Hz.

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S. Livi et al. / Polymer 52 (2011) 1523e1531
Table 5
Table 7
Effect of ionic liquids on tensile properties of polymer films (1 wt %) (crosshead
Influence of the chemical nature of the anion coupled to the strain rate (0.004 sꢀ1
)
speed: 0.004 sꢀ1).
on the percentage of crystallinity (Std, cooling rate 10 K minꢀ1).
Sample
Tensile modulus
Strain at break
Samples
Tm (ꢁC)
Tc (ꢁC)
D
Hm (J/g)
Xc (%)
(MPa) (ꢃ5e10 MPa)
(%) (ꢃ10%)
PTFE
326
327
327
327
310
309
307
307
32
32
28
34
39
39
34
41
PTFE
65
170
120
140
110
90
180
522
140
70
160
160
PTFE 0.004 sꢀ1
PTFE/C18P Brꢀ
PTFE/C18P Brꢀ
0.004 sꢀ1
PTFE/C18P Iꢀ
PTFE/C18P Brꢀ
PTFE/C18P PFꢀ6
PTFE/C18C18Im Iꢀ
PTFE/C18Py Iꢀ
PTFE/C18P PFꢀ6
PTFE/C18P PFꢀ6
0.004 sꢀ1
329
329
308
308
31
39
38
48
PTFE/C18P Iꢀ
PTFE/C18P Iꢀ
0.004 sꢀ1
328
328
307
307
28
42
34
51
anion has a predominant effect on the halide or fluorinated anions.
This phenomenon could be explained by the fact that the fluori-
nated nature of the anion improves the compatibility with the
matrix and contributes to the matrix crystallization.
20% except for PTFE/C18P PF6ꢀ that remains in the same order of
magnitude as the neat matrix. On the other side, as the temperature
increases, the moduli values for the PTFE/IL materials decrease
slightly but remain always higher than ones measured on neat PTFE.
This improvement is more significant when the fluorinated anion is
used. In fact, the hexafluorophosphate anion contributes to higher
thermomechanical behaviour of IL-modified PTFE. This phenomenon
is even more pronounced at higher temperature.
These changes of storage moduli can be explained by the fact that
ionic liquids form in the PTFE medium a separated phase which
displays a strong cohesion due to the ionic dipoleedipole interac-
tions. According to the temperature dependence of ionic interac-
tions, the multiplet aggregates which exist at low temperature could
exist in a larger range of temperature, i.e. to higher temperatures,
before reaching the temperature at which ionic forces become too
weak to contribute to the stiffness of the material. This hypothesis
could be similar to the temperature dependence of storage modulus
of ionomers with temperature [26].
3.3. Effect of ionic liquids on the mechanical properties of
fluorinated polymer
3.3.1. Dynamical mechanical analysis of fluorinated polymer-IL
blends
As reported in Fig. 4, the dynamic mechanical spectrum of PTFE
and PTFE/C18P Iꢀ display the storage moduli E0 and the main
relaxation peaks. These peaks were previously assigned by McCrum
to the long range motions i.e.
a
relaxation at 120 ꢁC, to the
b
relaxation attributed to the combination of a transition in crystal
form and at ꢀ97 ꢁC, the random chain motions is attributed to the
g
relaxation [34].
The thermomechanical properties can be significantly improved
thanks to the high thermal stability of ionic liquids. The storage
moduli, E0, obtained by DMA at different temperatures are sum-
marized in Table 4.
3.3.1.1. Influence of cations on dynamical mechanical analysis. The
chemical nature of the organic cation plays a key role on the ther-
momechanical properties of polytetrafluoroethylene. At 25 ꢁC, the
film with the imidazolium ion shows at room temperature the
highest in the elastic moduli E0 with an increase of 17% compared to
unfilled film and 47% compared to the film filled with phosphonium
ion. Then at 150 ꢁC, whatever the organic cation used, similar moduli
but always higher than PTFE are obtained. At higher temperatures
like 250 ꢁC, the highest values in moduli are measured on the PTFE
films filled with the phosphonium ionic liquid because of its better
thermal stability than the imidazolium salt one.
3.3.2. Mechanical properties of fluorinated films
The mechanical properties determined on the fluorinated films
with 1 wt % of ionic liquids are gathered in Table 5.
3.3.2.1. Influence of cations on static mechanical analysis. The mech-
anical performance is very dependent on the nature of ionic liquids
used. In fact, for 1 wt% of pyridinium and imidazolium ions, the
mechanical behaviours analysed in uniaxial tension mode are similar.
An increase of the modulus of 38% and 41% respectively is obtained
with a slight decrease of 11% for the elongation at break. In the
opposite, with the phosphonium ionic liquid, surprising results are
observed since an increase of 160% of the stiffness and 190% for the
strain at break are achieved. The differences of strain at break have
been attributed to the distributions of the ionic liquid phase in
a polymer film. Thus, the co-continuous morphologies of pyridinium
and imidazolium salts lead to a decrease of the elongation at break
while the ‘spider web’ morphology obtained with the phospho-
nium ionic liquid amplifies the plasticization of the polymer matrix.
To explain the increases in Young’s modulus of the different systems,
3.3.1.2. Influence ofanions on dynamical mechanical analysis. At room
temperature, the addition of phosphonium ionic liquids, whatever
the nature of the anion used, causes a decrease in modulus of about
Table 6
Influence of the nature of the ionic liquid coupled to the strain rate (0.004 sꢀ1) on the
percentage of crystallinity (Std, cooling rate 10 K minꢀ1).
Samples
Tm (ꢁC)
Tc (ꢁC)
D
Hm (J/g)
Xc (%)
PTFE
326
327
329
328
310
309
308
308
32
32
36
38
39
39
44
46
Table 8
PTFE 0.004 sꢀ1
PTFE/C18Py Iꢀ
PTFE/C18Py Iꢀ
0.004 sꢀ1
DSC data on fluorinated films strained at different strain rate (0.004 and 0.2 sꢀ1) (Std,
cooling rate 10 K minꢀ1).
Samples
Tm (ꢁC)
Tc (ꢁC)
D
Hm (J/g)
Xc (%)
PTFE/C18C18Im Iꢀ
PTFE/C18C18Im Iꢀ
0.004 sꢀ1
328
328
307
308
29
36
35
44
PTFE without strain
PTFE 0.004 sꢀ1
326
327
329
328
328
329
310
309
308
307
307
308
32
32
40
28
42
49
39
39
49
34
51
60
PTFE 0.2 sꢀ1
PTFE/C18P Iꢀ
PTFE/C18P Iꢀ
0.004 sꢀ1
328
328
307
307
28
42
34
51
PTFE C18P Iꢀ without strain
PTFE C18P Iꢀ 0.004 sꢀ1
PTFE C18P Iꢀ 0.2 sꢀ1

S. Livi et al. / Polymer 52 (2011) 1523e1531
1529
Table 9
fine structuration and the largest increase in the percentage of
crystallinity lead to better mechanical results. Whereas for C18P Brꢀ
and C18P PFꢀ6 , the presence of many well-dispersed ionic aggregates
in the matrix explain the higher decrease at the strain at break as
observed in ionomers [35].
Effect of the strain rate on tensile properties of the polymer films.
Sample
Tensile modulus (MPa)
Strain at break (%)
PTFE 0.004 sꢀ1
65
170
170
200
180
450
522
715
PTFE 0.2 sꢀ1
PTFE/C 18 P Iꢀ 0.004 sꢀ1
PTFE/C 18 P Iꢀ 0.2 sꢀ1
3.3.3. Effect of strain rate on the uniaxial tension behaviour of
polymer/IL films
3.3.3.1. Effect of phosphonium ionic liquid on the mechanical and the
crystallinity properties. Due to the fine dispersion of the phospho-
nium ionic liquid in the fluorinated film and the excellent mechanical
compromise achieved, the effects of strain rate were investigated on
the Young’s modulus and ultimate properties as well as on the
morphologychanges after uniaxial stretching at a given strain. In fact,
DSC measurements after deformation at different strain rates are
listed in Table 8 and the mechanical properties are summarized in
Table 9.
First of all, one can remember that the PTFE films are not
oriented due to the processing method, i.e. drying from a water-
based solution followed by heating at 400 ꢁC. At low strain rates,
the melting enthalpy of the neat PTFE is unmodified, whereas as the
fluoropolymer is strained at high strain rates, crystallization under
strain takes place. Indeed, the high strain rate applied promotes
chain extension which leads to an increase of crystallinity in the
polymer. This increase in crystallinity rate could be associated to an
increase of Young’s modulus from 65 at 170 MPa.
the evolution of the percentage of crystallinity has been studied
under mechanical strain (Table 6).
These results clearly demonstrate that when the films are
submitted to a strain rate of 0.004 sꢀ1, a significant increase of the
crystallinity ratio is obtained whatever the chemical nature of the
organic cation used. In conclusion, the presence of ionic phase in
the polytetrafluoroethylene matrix generates a phenomenon of
crystallization under strain. However, better interactions seem to
take place between the phosphonium ionic liquid functionalized
with long alkyl chains and the fluorinated matrix.
3.3.2.2. Influence of anion on static mechanical analysis. The mech-
anical properties can be also tailored from the chemical nature of
the anion. By using the phosphonium ionic liquid, differences are
observed for Young’s modulus and the strain at break as a function
of the anion used. For the three anions, a strong increase of modulus
is obtained of 160% with C18P Iꢀ, 84% with C18P Brꢀ and 115% with
C18P PFꢀ6 . On the other hand, for the strain at break, the phospho-
nium ionic liquid containing iodide anion has a plasticizing effect
with a large increase (190%) whereas for the C18P Brꢀ and C18P PF6ꢀ,
the addition of these salts reduces the fracture behaviour of the
fluorinated matrix with a decrease of 22% and 84% respectively.
These results are consistent with the morphologies shown
previously and the DSC data presented in Table 7. In fact, in the case
of phosphonium ionic liquid combined with the iodide anion, a very
The addition of phosphonium ionic liquid in the fluoropolymer
enhances the crystallization under strain with an increase of 10%
compared to the neat polymer film. This increase could be due to
a rearrangement of the ionic liquid aggregates in the polymer
matrix. In fact, the ionic interactions have a reversible character
and the ionic liquid phase based on multiplet aggregates could be
reorganized continuously during strain. Regarding mechanical
Fig. 5. SAXS patterns of neat PTFE and PTFE/C18P Iꢀ under different strain rates. (arrows indicate the axe of uniaxial tension).

1530
S. Livi et al. / Polymer 52 (2011) 1523e1531
Fig. 6. TEM micrographs of PTFE/C18P Iꢀ under different strain rates (arrows indicate tension axis).
properties, this slight increase in the modulus at high strain rate
could reflect the effect of competition between relaxation for the
re-organization of ionic liquid phase and deformation.
coupled to different mechanical behaviours observed. Thus, we have
clearly demonstrated that the effects of the chemical nature of ionic
liquid determined by a choice of the cation: pyridinium, imidazolium
versus phosphonium and a choice of the anion halide versus fluori-
nated play a significant role on the structuration and the physical
properties of the polymer. Typically, a co-continuous morphology as
is the case for the pyridinium and imidazolium salts generates
a decrease in strain at break. In opposite, the spider web morphology
obtained for the phosphonium ionic liquid with iodide anion lead to
a dramatic plasticization (þ190%) of the fluorinated matrix. We have
also demonstrated that the chain scission caused by the formation of
HBr, HI, HF acids during the heat treatment of fluorinated coatings
induced crystallization under strain which is responsible of the
Young’s modulus increase. To conclude this study, we have showed
that it is possible to achieve polymer film with an unprecedented
flexibilityanda stiffness dramaticimprovementbyusing averysmall
amount of ionic liquid (1 wt %) and that moreover does not really
require any special conditions for their synthesis. These new building
blocks must open a new alternative in material field for various
applications and will be the subject of many studies in the future.
3.3.4. Effect of ionic liquids on the morphology after deformation
For a better understanding of the material change during uniaxial
tensile test, SAXS analysis were performed on the PTFE films with
and without IL after deformation reached at different deformation
rates as reported by Visser and al [36] for ionomers.
The SAXS images performed on unfilled PTFE and PTFE/C18P Iꢀ
at different strain rates, i.e. 0, 0.004, 0.2 sꢀ1 are shown in Fig. 5.
Without deformation, the isotropic character of the PTFE film
due to the processing method used is observed. As a strain rate
(even low) is applied, a configuration with four leaf clover is
observed and in presence of ionic liquid, a different organization is
noticed. Indeed, without strain, the PTFE/C18P Iꢀ sample shows an
anisotropic behaviour that is kept at low strain rates. At a strain rate
of 0.2 sꢀ1, this phenomenon disappears in favour of an anisotropic
behaviour oriented in six directions. As a consequence, the pres-
ence of ionic liquid as a dispersed nanophase which is based on
multiplet clusters could be used to modify the deformation
phenomena of PTFE due to the dynamic character of this phase
under strain.
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