Dissolution of oligo(tetrafluoroethylene) and preparation of poly(tetrafluoroethylene)-based composites by using fluorinated ionic liquids

Article abstract of DOI:10.1039/c7cc08449h

Fluorophilic ionic liquids (ILs) showing enhanced compatibility with poly(tetrafluoroethylene) (PTFE) have been newly synthesised. The as-designed ILs contributed both to the dissolution of PTFE oligomers and to the preparation of composites with PTFE with no fear of bleed-out of the ILs.

Full text of DOI:10.1039/c7cc08449h

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DOI: 10.1039/C7CC08449H
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Dissolution of oligo(tetrafluoroethylene) and preparation of
poly(tetrafluoroethylene)-based composites by using fluorinated
ionic liquids
Received 00th January 20xx,
Accepted 00th January 20xx
Akiko Tsurumakia,b and Hiroyuki Ohno*,a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
Fluorophilic ionic liquids (ILs) showing enhanced compatibility solvents for polymer dissolution10 and for polymerisation.11
with poly(tetrafluoroethylene) (PTFE) have been newly These have been achieved by designing the ILs, taking the
synthesised. Thus designed ILs contributed both to the dissolution structure and property of polymers into consideration. For
of PTFE oligomer and to the preparation of composites with PTFE example, cellulose was believed to be insoluble in any kind of
with no fear of bleed-out of the ILs.
organic solvent under mild condition because of many
interchain hydrogen bonds. In spite of this, some ILs, designed
to show a high hydrogen bonding basicity, have succeeded in
dissolving cellulose.12 Subsequently, the utilisation of cellulose
such as ionogel, electrolyte, and bio-plastics has been
accelerated considerably with the aid of newly designed polar
ILs.13 Polymers, that were believed to be difficult to be used
because of their very poor solubility such as PTFE, have the
possibility of being utilised widely only by suitably designed ILs
as solvent and/or additives. Some composites composed of
PTFE and ILs have been reported.4,14,15 However, these
composites have been prepared by using the ILs, which are
solid at room temperature. In case of liquid ILs, binders
between PTFE and the ILs are required. This is because the
compatibility between PTFE and ILs was still low, and bleed-
out of ILs was still the remained subject to be clarified.
Poly(tetrafluoroethylene) (PTFE)1 is acknowledged as the most
resistive polymer against chemical-, thermal-, and
electrochemical-stresses. All of these properties are realised
by the fact that PTFE is composed of only C-C and C-F bonds
with high binding energy. PTFE is a vital material in several
industrial fields such as semiconductors, office automation
systems, and automobiles which require operation at high
temperature and with the use of corrosive fluids. On the other
hand, there are only a few reports of functionalised material
where PTFE is used as a matrix,2 although other kinds of
fluorinated polymers such as poly(vinylidene fluoride) (PVdF)
and its copolymer with hexafluoropropylene (PVdF-HFP) are
widely used as a base material for polymer electrolytes.3 This
is because PTFE cannot retain liquid additives within their
structure due to their infinite inertness. PTFE composites thus
have been prepared mainly through two processes: one is the
Here, we have attempted first to improve the compatibility
between ILs and PTFE by considering the fluorophilicity of the
ILs. As of now, some ILs composed of cation or anion with
fluorinated-alkyl chains have been reported, and these have
been applied as surfactant to stabilise emulsions of
perfluorohexane (PFH),16 as well as solvent for fluorinated
gases17-19 or alcohols.20, 21 However, even in those literatures,
it was also reported that no one has yet succeeded in the
complete dissolution of perfluoroalkane and of course that of
PTFE. With respect to fundamentals of fluorine chemistry,
there exist some empirical rules in achieving enhanced
fluorophilicity: the fluorophilic molecules are generally known
to have higher fluorine content (at least 60 wt%), longer
and/or many fluorinated-alkyl chains, etc.22 Also, the number
of functional moieties that induce attractive interactions such
as hydrogen bond and induction forces should be small to
obtain a higher fluorophilicity. These rules have never been
considered in designing ILs even though these should be useful
in improving their fluorophilicity. It should be possible to
incorporation of solid additives4, and the other is the
modification of PTFE polymer backbones.6, 7
5
In this study, a series of ionic liquids (ILs) is examined to
find a compatible partner with PTFE. ILs are molten salts at
ambient temperature which exhibit negligible volatility,
excellent thermal stability, and considerable ionic conductivity.
In addition to this, ILs exhibit a wide diversity of ion structure
because they are composed of organic ions. ILs have been
recognised as a unique partner for polymers and developed as
additive salts8 and/or plasticizer9 for polymers, as well as
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Table 1. %F and χPFH of salts examined in this study.
(a) [Tf2N] salts
Cations
(b) [C2mim] salts
%F
χPFH
5.2
Anions
BF4*
%F
χPFH
3.5
[C2OHmim]
[C4py]
28.0
27.4
29.1
26.3
26.1
27.0
23.7
14.9
33.6
40.1
61.5
13.0
29.1
38.7
49.5
19.8
41.7
5.4
PF6*
1.5
[C2mim]
6.1
[FAP]
18.5
2.3
[C4mmim]
7.8
[FSI]
[Pip14
]
7.9
[Tf2N]
6.1
[Pyr14
]
]
10.9
12.4
25.3
[Pf2N]
16.0
97.2
6.7
[N4441
[Nf2N]
[P666,14
]
CF3SO3*
C4F9SO3
*[C4mim] salts
29.6
(c) C8F17SO3 salts
Cations
(d) [P666F] salts
Anions
%F
χPFH
%F
χPFH
obtain ILs that have a higher compatibility with PTFE while
taking these rules into account.
[C2mim]
[P666,14
[P4446
52.9
32.9
41.1
solid
147.6
232.5
[Tf2N]
42.5
51.8
296.0
626.5a
]
C8F17SO3
]
a exhibited a lower critical soln. temp.
Structure and abbreviation of ILs used in this study are
summarised in Fig. 1, and their synthesis methods are
reported in the ESI. In an effort to seek ILs having better
compatibility with PTFE, we have evaluated the solubility of
perfluorohexane (PFH) in the IL, which was chosen as a model
compound of PTFE. Equivolume ILs and PFH were mixed with a
vortex mixer, and the mole fraction of perfluorohexane
compared to those having an α value higher than 0.5. In
conclusion, the ILs having apolar cations such as phosphonium
cation showed enhanced χPFH among the [Tf2N]-salts
investigated here.
dissolved in the IL phase (χPFH
)
was determined
The χPFH value for a series of [C2mim] salts was then
analysed (Table 1 (b)). When the spherical anions were
coupled with imidazolium cation, the χPFH value decreased in
the following order: [C4mim]PF6 < [C4mim]BF4 < [C2mim][FAP].
Even though [C4mim]PF6 had a higher %F than [C4mim]BF4, this
thermogravimetrically after the IL-rich phase containing a
trace of PFH was separated from the PFH-rich phase. Taking
into account the fact that the dissolution of fluoro-alkane, such
as PTFE and PFH, is known to be governed by the “like
dissolves like” principle,23 a higher χPFH indicates higher
fluorophilicity. The change in χPFH in relation to the polarity and
fluorine content (%F) of ILs was discussed.
-
IL exhibited a lower χPFH. Compared to the ILs containing BF4
-
and PF6 , the ILs having imide- and sulfonate-anions showed
higher χPFH except for [C2mim][FSI]. Due to this effect of anion
structure, these ILs are considered not to form a fluorine-rich
domain apart from charged domain. From the viewpoint of
the formation of the fluorine-rich domain, we have compared
the effect of fluorinated-alkyl chains in the anions. The ILs
dissolved higher amount of PFH when the anion contained
longer fluorinated-alkyl chain(s). We also found a positive
correlation between %F and χPFH value in the case of [C2mim]
salts as shown in Fig. S2. The most noticeable improvement in
Table 1 (a) summarises χPFH and %F in a series of [Tf2N]
salts. In contrast to the fact that the fluorophilicity of organic
molecules increases with an increase of their %F, a clear
correlation between χPFH and %F was not observed in the case
of [Tf2N] salts. On the contrary, the IL having the smallest %F,
[P666,14][Tf2N], exhibited the highest χPFH among a series of
[Tf2N] salts. When we focused on the cation structure of the
ILs, PFH was found to be dissolved into the ILs having less polar
cations. For example, [C2mim][Tf2N] dissolved 6.1 mmol mol-1
of PFH, and the χPFH decreased when we used the IL
[C2OHmim][Tf2N] that had polar hydroxide group on its cation
structure. On the other hand, the χPFH increased when the
polarity of the cation was decreased by replacing the proton
on the C(2)-position of the imidazolium-ring with a methyl
group (i.e., [C4mmim][Tf2N]). These two results are well
comprehended by the fact that PFH is apolar and exhibits
better compatibility with relatively apolar ILs. The presence of
orientation forces, hydrogen bonds, and induction forces in ILs
reduced the compatibility between PFH and ILs.22 The effect
of polarity on χPFH was also found when the α value of Kamlet-
Taft parameters, which denotes the hydrogen-bonding acidity
of ILs, was compared with χPFH (Fig. S1). The ILs with an α
value less than 0.5 dissolved a higher amount of PFH
χPFH was found when imide-anions were coupled with [C2mim].
The χPFH value was improved from 2.3 to 97.2 by replacing [FSI]
with [Nf2N] anions, in which the carbon number (n) of
fluorinated-alkyl chain was 0 and 4, respectively. The χPFH of
[C2mim][Nf2N] was found to be higher than that of
[C2mim][FAP], in spite of the fact that the %F of the former
was lower than the latter. This suggests the modification of
ions with longer fluorinated-alkyl chains is important more
than the improvement in the %F of ILs with short fluorinated-
alkyl chains. In fluorine chemistry, it is known that the
fluorophilicity of organic molecules is predominantly governed
by the overall %F.22, 24 However, in the case of ILs, not only the
%F but also the position of fluorine atoms was found to be
important in improving the fluorophilicity.
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Based on the above-mentioned results, we have
synthesised [C2mim]C8F17SO3 expecting the formation of
fluorine-rich domain. However, this IL was solid at room
temperature due to strong aggregation force of fluorinated-
alkyl chains.25,26 To cancel the self-aggregation of anions and to
induce amorphous domain, C8F17SO3 anion was combined with
a cation having longer alkyl chains and larger ion size. The ILs
composed of C8F17SO3 anion and phosphonium cations
([P666,14] and [P4446]) were liquid at room temperature. As
shown in Table 1 (c), these ILs showed considerably high χPFH
compared to the other ILs discussed above. The higher χPFH of
[P4446]C8F17SO3 is based on higher %F of the ILs. Also, a
fluorinated-alkyl chain was inserted into the cation structure
to further improve the fluorophilicity of ILs (see ESI for the film even after pressing were successfully obtained by using
synthesis of [P666F][Tf2N] and [P666F]C8F17SO3). When [P4446]C8F17SO3. Also fluoro-functionalised ILs, i.e., [P666F][Tf2N]
[P666F]C8F17SO3 and PFH were mixed at room temperature, χPFH and [P666F]C8F17SO3, enabled the preparation of homogeneous
was found to be 626.5 mmol mol-1 (Table 1 (d)). Interestingly, composites without bleed-out of ILs. The bleed-out of the ILs
when these were mixed at 0oC, the mixture resulted in a from these three composites was not observed during the
transparent homogeneous phase but gradually separated as it course of the observation for at least one month. The bleed-
reached room temperature. [P666F]C8F17SO3 exhibited lower out is considered to be controlled by the improved wettability
critical solution temperature (LCST)-type phase behaviour between these ILs and PTFE. Indeed, [P666F]C8F17SO3 exhibited
when PFH was added. The LCST was found to be around 14oC. the smallest contact angle with PTFE among those ever
Taking the fact that PFH is not compatible with charges nor reported (Table S1).29, 30 Also, differential scanning calorimetry
non-fluorinated-alkyl chains into account, the dissolution of of PTFE and PTFE/[P666F]C8F17SO3 composite was then
PFH in the IL is considered to be achieved through the measured (Figure S4). Though the melting point of PTFE was
formation of micelle-like environment of fluorophilic domain. found to be at similar temperatures, the endothermic drift
The micelle formation motivated by the aggregation of started at
a
lower temperature in the case of
fluorinated-alkyl chains is characterised by the triphilic PTFE/[P666F]C8F17SO3 compared to that of the pure PTFE. This
property that is found only in the ILs. Some ILs having a long suggests that in the case of 1:1 (weight) mixed systems, some
fluorinated-alkyl chain are known to form three domains PTFE chains are remained but other part of PTFE interact with
containing polar (ionic), non-polar (alkyl chain), and the ILs. This also shows high compatibility between PTFE and
fluorophilic (fluorinated-alkyl chain) moieties.27, 28 Similarly, [P666F]C8F17SO3.
[P666F]C8F17SO3 may form the domain concentrated with
Consequently, the ILs, functionalised with perfluorinated-
fluorinated-alkyl chains. With increasing temperature above octyl chains, were found to possess a high compatibility both
the LCST, PFH and the perfluorinated-octyl chains in with the PTFE oligomer and PTFE polymer. The homogeneous
[P666F]C8F17SO3 are considered to be exposed to the ionic and composites based on PTFE, which showed no bleed-out of ILs,
non-fluorinated-alkyl domain. Hence PFH molecules were were prepared by mixing PTFE with newly designed ILs.
aggregated to exclude polar groups so as to keep the entropy
high. For this reason, the dissolution of PFH in the IL was Research (KAKENHI) from the Japan Society for the Promotion
achieved only at temperatures below the LCST. of Science (JSPS). A.T. acknowledges to the financial support of
This study was supported by the Grant-in-Aid for Scientific
Selected ILs were then applied to prepare composites with JSPS Research Fellowship for Young Scientists (DC2).
PTFE. Equivalent weight of ILs and PTFE was mixed for this
experiment. Fig. 2 shows the pictures of thus prepared PTFE-
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