Tetraarylphosphonium perfluorocyclobutyl polyelectrolyte with low critical surface energy, high thermal stability, and high alkaline resistance

Article abstract of DOI:10.1002/pola.29525

Two tetraarylphosphonium polyelectrolytes having perfluorocyclobutyl units in their backbones have been prepared in which the counteranion is either bromide (PFP·Br) or bis(trifluoromethyl)sulfonimide (PFP·NTf₂). These polymers exhibit high thermal stability as assessed by thermogravimetric analysis, with a decomposition temperature of 460 °C for PFP·NTf₂. Even after heating at 300 °C for 72 h, PFP·NTf₂ shows no signs of degradation detectable by nuclear magnetic resonance spectrometry. As is typical for many tetraarylphosphonium species, films of these polymers can be quite resistant to degradation by alkaline solution. Upon alkaline challenge by exposure to 6 M NaOH at 65 °C for 24 h, for example, only 16% of the phosphonium centers in PFP·NTf₂ are degraded, making PFP·NTf₂ one of the most alkaline-stable phosphonium polymers to date. Despite having ionic backbones, PFP·Br and PFP·NTf₂ exhibit very low critical surface energies of 26.1 and 22.9 mJ m^?1, respectively. These values are on par with the values for poly(vinylene fluoride) and dimethylsiloxane. Such low surface energy polycations capable of high alkaline stability may find application as components of alkaline fuel cell membranes.

Full text of DOI:10.1002/pola.29525

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Tetraarylphosphonium Perfluorocyclobutyl Polyelectrolyte with Low
Critical Surface Energy, High Thermal Stability, and High Alkaline
Resistance
Wang Wan, Monte S. Bedford, Rhett C. Smith
Department of Chemistry and Center for Optical Materials Science and Engineering Technology, Clemson University, Clemson,
South Carolina 29634
Correspondence to: R. C. Smith (E-mail: rhett@clemson.edu)
Received 28 July 2019; Revised 27 September 2019; accepted 28 September 2019
DOI: 10.1002/pola.29525
ABSTRACT: Two tetraarylphosphonium polyelectrolytes having
perfluorocyclobutyl units in their backbones have been prepared
in which the counteranion is either bromide (PFPÁBr) or bis
(trifluoromethyl)sulfonimide (PFPÁNTf2). These polymers exhibit
high thermal stability as assessed by thermogravimetric analysis,
with a decomposition temperature of 460 ꢀC for PFPÁNTf2. Even
after heating at 300 ꢀC for 72 h, PFPÁNTf2 shows no signs of degra-
dation detectable by nuclear magnetic resonance spectrometry.
As is typical for many tetraarylphosphonium species, films of
these polymers can be quite resistant to degradation by alkaline
solution. Upon alkaline challenge by exposure to 6 M NaOH at
65 ꢀC for 24 h, for example, only 16% of the phosphonium centers
in PFPÁNTf2 are degraded, making PFPÁNTf2 one of the most
alkaline-stable phosphonium polymers to date. Despite having
ionic backbones, PFPÁBr and PFPÁNTf2 exhibit very low critical sur-
face energies of 26.1 and 22.9 mJ m−1, respectively. These values
are on par with the values for poly(vinylene fluoride) and
dimethylsiloxane. Such low surface energy polycations capable
of high alkaline stability may find application as components of
alkaline fuel cell membranes. © 2019 Wiley Periodicals, Inc.
J. Polym. Sci., Part A: Polym. Chem. 2019
KEYWORDS: degradation; polyelectrolyte; thermal stability;
surface energy
technique on Nafion substrates.12 In that case, critical surface
energy was estimated at 15–18 mJ m−2. Considering these obser-
vations, it was hypothesized that low critical surface energy, per-
fluorinated TPELs might likewise be of interest for alkaline fuel
cells.
INTRODUCTION Tetraarylphosphonium salts are among the
most stable ionic liquids known,1–5 and their incorporation into
polymer backbones has been targeted as a strategy to likewise
access polymers with high thermal stability.6–10 The resistance of
tetraarylphosphonium moieties to alkali degradation has also
garnered interest in utilizing tetraarylphosphonium polyelectro-
lytes (TPELs, Scheme 1) as membrane components of alkaline
fuel cells and related electrochemical energy technologies. Previ-
ously reported TPELs have had relatively high critical surface
energies and at least partial attendant solubility in polar solvents
such as water and methanol. Unfortunately, such solubility would
preclude the applicability of a polyelectrolyte in most fuel cell
applications. Some of the most successful membrane polymers
for proton-conducting fuel cells are perfluorinated polyelectro-
lytes having low critical surface energies such as Nafion that not
only resist dissolution under fuel cell operating conditions, but
also tend to form channels of polar versus nonpolar domains
through which ion percolation can occur.11 Concern for surface
wetting has driven the study of low critical surface energy mate-
rials in multiblock fuel cell ion-transport materials. For instance,
protein-functionalized proton-transport membranes for direct
methanol fuel cells were deposited by Langmuir–Blodgett
Some low surface energy phosphonium polymers and salts have
already been investigated. Ragogna and coworkers have pre-
pared photopolymerizable tetraalkylphosphonium salts, cleverly
designed to incorporate
C4F9 units rather than longer
perfluoroalkyl chains that are persistent ecological threats.13
Despite the ionic backbone constituents, photopolymerized coat-
ings of these fluorinated phosphonium polymers are quite
hydrophobic, attaining water contact angles of up to 101 ꢀC
(cf. Teflon, with a 115ꢀ water contact angle). The phosphonium
units in those polymers were alkylphosphonium salts and would
likely not be stable at operating conditions of alkaline fuel cells,
however. For this reason, TPELs having low critical surface ener-
gies are of interest, yet none have been reported to date.
TPELs require installation of four aryl groups onto the phos-
phorus center, and synthetic routes for accomplishing this are
limited. The preferred method for preparing TPELs is
Additional supporting information may be found in the online version of this article.
© 2019 Wiley Periodicals, Inc.
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SCHEME 1 Established route to prepare TPELs.
copolymerization of a bifunctional aryl halide or aryl triflate
with diphenylphosphine via a Pd-catalyzed P C bond-forming
reaction (Scheme 1).6,7,10 In selecting potential backbone com-
ponents for the current study, a perfluorinated moiety having
high thermal stability and which could be readily incorporated
into the requisite bifunctional aryl comonomer was targeted.
The perfluorocyclobutyl (PFCB) unit is an excellent candidate.
We previously reported the utility of the PFCB motif to pre-
pare thermally stable polymers supporting fluorescent back-
bone components within a hydrophobic environment,14 and
many other PFCB polymers having high thermal stability have
been reported.15–20
In the current work, we report on the preparation of the first
perfluorinated TPELs, PFPÁBr, and PFPÁNTf2. These PFCB-
incorporating TPELs were assessed for thermal stability, sta-
bility of films to alkaline challenge, thin film morphology, and
critical surface energy.
SCHEME 2 Preparation of PFPÁBr.
RESULTS AND DISCUSSION
The ionic nature of polyelectrolytes leads to high affinity to gel
permeation chromatography (GPC) columns. Although some suc-
cess has been observed in acquiring GPC data for polyelectrolytes
in solvents such as formic acid, a simpler method for determining
the Mn of TPELs has been reported.24–26 Specifically, since a slight
excess of diphenylphosphine monomer was used in the synthesis,
end groups will be comprised of phosphine groups that are subse-
quently oxidized to the phosphine oxides. NMR end-group analysis
comparing main chain phosphonium to end-group phosphine
oxide units was employed to determine a Mn of 31.8 kDa,
corresponding to a degree of polymerization of 52. This degree of
polymerization is similar to the reported values for TPELs pre-
pared by Pd-catalysis.6,7 Although 31P NMR integrations are not
accurate in a typically acquired spectrum, a 10 s delay between
scans was employed and triphenylphosphine was added as an
external standard after acquiring spectra of the polymer alone to
further validate the calculation following the reported procedure.6
Design and Synthesis
The PFCB unit was selected for incorporation into the target
tetraarylphosphonium polymer on the basis of its facile prepa-
ration and high thermal stability. The requisite monomer,
1,2-bis(4-bromophenoxy) hexafluorocyclobutane (2)21 was
thus prepared in two steps from p-bromophenol and
bromotrifluoroethylene22 (Scheme 2) via an established meth-
odology. Monomer 2 was reacted with diphenylphosphine via
the Pd-catalyzed P C bond-forming reaction6 to yield the per-
fluorinated polyelectrolyte PFPÁBr.
PFPÁBr was readily characterized by multinuclear nuclear mag-
netic resonance (NMR) spectrometry and elemental microanaly-
sis. The 31P NMR spectrum confirmed the complete consumption
of diphenylphosphine (occurring at −41.00 ppm)23 and the
appearance of a major resonance at 22 ppm characteristic of the
tetraarylphosphonium unit. The 19F NMR spectrum of the poly-
mer retains the characteristic pattern for the PFCB unit (Fig. 1),
confirming its retention under the reaction conditions and broad-
ening of the individual resonances commensurate with the
expectation for a polymer.
Somewhat surprisingly, PFPÁBr can be solubilized in methanol
despite the presence of PFCB units. To increase both the hydropho-
bicity and thermal stability of PFPÁBr, the bromide counterions
were exchanged for bis(trifluoromethyl) sulfonimide ([NTf2]−)
2
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as-synthesized material, suggesting that PFPÁNTf2 may be a
good candidate for practical high temperature applications.
PFPÁNTf2 was then tested for its stability to alkaline conditions.
The alkaline challenge conditions involved exposure of a thin film
of the polymer to a 6 M NaOH(aq) solution at either room temper-
ature (ca. 21 ꢀC) or 65 ꢀC. These conditions were selected to simu-
late potential operating conditions of an alkaline fuel cell. At room
temperature, PFPÁNTf2 shows no sign of degradation even after
4 days of exposure. At 65 ꢀC, however, 16% of the phosphonium
moieties are decomposed to phosphine oxides (presumably via
air oxidation of an intermediately formed phosphine) after 24 h.
The stability of PFPÁNTf2 is thus on par with the most alkaline-
resistant TPELs, and it is more alkaline-resistant under these
conditions than phosphonium polyelectrolytes that have been
successfully incorporated into operational alkaline fuel cells.27
Thin Film Surface Energy and Morphology
The critical surface energies (γc) for previously reported phospho-
nium polymer dip-cast films ranged from 35.0 to 73.6 mJ m−1. γc
values for PFPÁBr and PFPÁNTf2 were expected to be quite low by
merit of the PFCB moieties in the backbone. γc values were thus
determined by the method of Zisman28 whereby contact angles
(θc) are measured between the film and a series of test liquids hav-
ing known surface tension values. The Zisman plots of these data
are provided in Figure 2. γc values for PFPÁBr and PFPÁNTf2 from
this method were found to be 26.1 and 22.9 mJ m−1, respectively.
The contact angles with water for PFPÁBr and PFPÁNTf2 were cor-
respondingly high at 63 and 76ꢀ, respectively. These contact angles
with water are significantly lower than those achieved by the afore-
mentioned fluorinated phosphonium polymers reported by
Ragogna and coworkers (up to 101ꢀ).13 Despite their ionic nature,
FIGURE 1 The 19F NMR spectrum for monomer 2 (A) and
polymer PFPÁBr and (B) demonstrating the characteristic pattern
for the PFCB ring and the broadening of the signals in the
polymer.
counteranions. This was readily accomplished by dropwise addi-
tion of a LiNTf2 solution to a vigorously stirred solution of PFPÁBr,
upon which PFPÁNTf2 precipitated. Elemental analysis and inte-
gration of 19F NMR signals for [NTf2]− fluorine atoms versus those
in the polymer backbone PFCB unit confirmed the quantitative
exchange of bromide for [NTf2]−counteranions by this method.
Thermal and Alkaline Stability
Tetraarylphosphonium salts are among the most thermally
stable ionic liquids known, and the PFCB moiety is likewise
thermally robust. Polymers PFPÁBr and PFPÁNTf2 were thus
subjected to thermogravimetric analysis (TGA) to assess their
thermal stability, whereas the decomposition temperatures
(Td) for PFPÁBr is unimpressive at 235 ꢀC, Td for PFPÁNTf2
(460 ꢀC), is quite good and on par with reported values for
some of the more thermally stable ionic liquids. It has been
pointed out in the literature that TGA analysis under a nitro-
gen atmosphere is not necessarily a good indicator of long-
term thermal stability of a material as may be required in a
real-world application setting.5 A sample of PFPÁNTf2 was
thus held at a temperature of 300 ꢀC open to the air for 72 h.
Over this time, the sample color changed only slightly from
pale tan to slightly darker tan. After the sample was cooled, it
was fully soluble and its NMR spectra matched those of the
FIGURE 2 Zisman plots from contact angle data for PFPÁBr and
PFPÁNTf2 polymers.
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to the air. PFPÁNTf2 also resists degradation upon exposure to
6 M NaOH(aq) at 65 ꢀC for 24 h with 16% loss of phospho-
nium functional groups, making it one of the two most stable
of phosphonium polymers under these conditions. The low
surface energies of PFPÁBr and PFPÁNTf2 place them as the
most hydrophobic of TPELs for which surface energies are
reported. The combination of thermal/alkaline stability and
low critical surface energy suggest potential utility of these
and related perfluorinated TPELs in alkaline fuel cell and
related applications. Efforts are underway to prepare an
expanded suite of perfluorinated TPELs for testing in electro-
chemical energy generation contexts.
(A)
Height
20
0
–20
–40
–60
nm
8
4
μm
0
2
6
EXPERIMENTAL
(B)
General Considerations
Height
All air-sensitive reactions were performed in an MBRAUN
UNILab glovebox under nitrogen. Anhydrous solvents were
dried and degassed using an MBRAUN solvent purifier.
Chemicals were used without further purification after pur-
chased. All the NMR spectra were collected on a Joel ECX-
10
0
1
300 MHz spectrometer operating at 300 and 121.4 MHz for H
and 31P, respectively. TGA was performed on TA Instruments
SDT Q600 (Newcastle, Delaware, USA) from 25 to 800 ꢀC with
a heating rate of 20ꢀC min−1. To assure that the 31P nuclei
were fully relaxed between scans and thus provide accurate
integrations, the spectra with 2, 4, 8, and 10 s delay were com-
pared. Integration of resonances in spectra collected with a
10 s delay is identical to those collected with an 8 s delay. As a
result, 31P NMR spectra provided here were collected with a
10 s relaxation delay. To assure the accuracy of the chemical
shift values, PPh3 was added as an internal standard.
–10
–20
nm
8
4
μm
0
2
6
FIGURE 3 AFM height images for PFPÁBr (A) and PFPÁNTf2 and
(B) films cast from acetone. [Color figure can be viewed at
wileyonlinelibrary.com]
Synthesis of Compound 2
however, the critical surface energies of PFPÁBr and PFPÁNTf2 are
significantly lower than those of familiar polymers such as polysty-
rene (γc = 34 mJ m−2), poly(methyl methacrylate) (γc = 37.5
mJ m−2) and polyethylene terephthalate (γc = 39 mJ m−2) and on
par with the values for well-known hydrophobic polymers like
poly(vinylene fluoride) (25 mJ m−1) and polydimethylsiloxane
(23 mJ m−1).29–31
4-Bromo(trifluorovinyloxy)benzene (2.00 g, 7.90 mmol) was
heated up to 180 ꢀC in 25 mL round-bottom flask for 24 h under
nitrogen atmosphere. The reaction mixture was then purified via
silica-gel chromatograph by using n-hexane/ethyl acetate = 10:1
as fluent phase to yield colorless liquid (1.41 g, 2.79 mmol,
70.5%). 1H-NMR (300 MHz, CDCl3) δ: 7.40–7.50 (m, 4H),
6.96–7.09 (m, 4H), 19F NMR (282 MHz) δ: −131.35, −131.25,
−130.90, −130.10, −129.37, −128.68, −127.86.
Atomic force microscopy (AFM) was used to further analyze
the film morphology of PFPÁBr and PFPÁNTf2. Although both
polymers fully coat the glass slide upon dip casting from an
acetone solution, there are notable differences in the size of
nanoaggregates at the film surface (Fig. 3). The surface of the
PFPÁBr film is comprised of a dense layer of polydisperse
nanoaggregates with an average diameter of about 350 nm. In
contrast, the surface of the PFPÁNTf2 film is much more
sparsely populated with larger, still polydisperse, aggregates
having an average size of about 750 nm. The collapse of poly-
mers to form aggregates may be attributable to incompatibil-
ity of polar acetone solvent with the PFCB moieties.
Synthesis of PFPÁBr
In a glovebox, PFCBPPh2Br2 (0.500 g, 0.99 mmol), diphenylpho-
sphine (0.193 g, 1.04 mmol), and diisoproplyamine (1.05 g,
1.04 mmol) were mixed together in a 15 mL heavy-wall pressure
reaction tube. A sample of Pd2(dba)3 (0.010 g, 0.01 mmol) and
1.5 mL anhydrous ethylene glycol were then added, and the tube
was screwed on with a Teflon screw cap equipped with a Viton
O-ring. The sealed reaction system was heated at 145 ꢀC with stir-
ring for 24 h to yield dark yellow liquid. After cooling down to
room temperature, the reaction tube was opened to air and 50 mL
dichloromethane (DCM) was added to dissolve all components of
the reaction mixture. The DCM solution was washed by 25 mL 1 M
NaBr solution twice and dried over anhydrous Na2SO4. After con-
centrating to 10 mL by vacuum evaporation, the DCM solution
was added to 200 mL diethyl ether with vigorous stirring to yield
CONCLUSIONS
The first perfluorinated TPEL has been prepared. PFPÁNTf2 is
quite thermally stable, even to heating at 300 ꢀC for 72 h open
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white cloudy precipitation. The solid was separated by centrifuge
and washed with 20 mL diethyl ether twice, and then dried in vac-
uum oven overnight to obtain pure white powder product
(0.466 g, 75.0%). 1H-NMR (300 MHz, CD3CN) δ: 7.50–8.05 (br m,
18H). 19F NMR (282 MHz, CD3CN) δ: −134.11, −133.43, −130.74,
−130.67, −129.93, −129.87, −129.22, −129.11, −128.76,
−128.59, −128.44, −127.92, −127.72, −127.10, −126.89. 31P NMR
(121.4 MHz, CD3CN) δ: 22.01. anal. calcd for monomer formula
C28H18BrF12O2PS2 (ignoring end groups): C, 55.01; H, 2.97;
found: C, 57.17; H, 3.47%.
Scott M. Husson for access to the static contact angle equip-
ment, the Department of Chemical and Biomolecular Engineer-
ing for supplying purified water, and Igor Luzinov for access
to the film-casting facility.
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PFPÁBr (0.100 g, 0.16 mmol) solid was dissolved in 20 mL H2O/
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Alkaline Stability Testing
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ACKNOWLEDGMENTS
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