Mechanical Stability of aryl-ether polymeric materials for use in Proton Exchange Membranes
The current commercial and academic drive towards cheap and durable materials of low toxicity for
use in proton exchange membranes (PEM) has resulted in a range of materials being investigated.
Most prominent of these has been the use of polymeric aryl-ether-ketone materials which have
glass transition temperatures much higher than the operating temperatures of the PEMs. These
materials, such as PEEK (Victrex®), have been shown to retain, if not improve, their mechanical
properties after sulfonation due to increased ionic interaction creating a physical form of
crosslinking. 1 Increasing the nanophase separation between the ionic and non-ionic regions of the
ionomer materials has been further investigated through the use of block co-polymerisation. The
result of this provides a bi-phasic system where the hydrophilic domain provides the high physical
and mechanical properties required to withstand the operating conditions during PEM electrolysis.
The second domain provides the hydrophilic properties required for proton conductivity, which
crucially must provide a continuous network throughout the membrane morphology.
In our current work into the investigation into microblock polymer synthesis, poly aryl-ether systems
have been used for their properties in both mechanical and chemical stability. The novel design of a
7-aromatic ring monomer bound together via ketone linkages provides the high structural stability
required for PEM material.
The 7 aromatic-ring monomer has a high melting point (310 °C) and a highly crystalline nature arising
from the molecules rigidity, resulting from the ketone linkages, and the planar conjugated properties
of extended pi system. This crystalline nature will provide semi-crystalline properties within the
resulting polymer, providing it with greater mechanical stability, such as higher glass transition
temperatures and melting points. Through the tailoring of similar monomer units, specific
equivalent weights can be achieved.
O
Cl
O
AlCl3, DCB
100 OC
+
F
F
O
O
Cl
AlCl3,
DCB,140 oC
Cl
O
O
F
Molecular Weight: 682.71
F
O
O
Scheme 1, Synthesis of 7 aromatic –ring fluorine terminated monomer via Friedel-Crafts Acylation at both synthetic
steps.
In-corporation of the 7 aromatic-ring monomer with Hexafluoro Bisphenol A provides high molecular
weight polymeric systems, required for materials of high strength and toughness.
O
O
O
F3C
CF3
O
EW: 569.5 g/mol
O
O
n
Figure 1, 7 aromatic-ring monomer polymerised with Hexafluoro Bisphenol A via step growth condensation in DPS with
a K2CO3 initiator.
The polymer showed an initial high melting point at 210 °C which was not reproduced upon the
second heat, possibly showing a partially crystalline nature. The polymer also displayed a high glass
transition temperature of 194 °C making the material highly suited to the working conditions of PEM
electrolysis. A higher Tg could be achieved with the removal of the hexafluoro methyl side groups to
reduce steric hindrance, but this may also provide processing problems with reductions in solubility
and solvent interaction.
Figure 2, DSC Heat-Cool-Heat thermogram of polymer 7 aromatic-ring - Hexafluoro Bisphenol A displaying a Tm at 210°C
and Tg at 194 °C.
Conversion of the polymer to its ionomer form by the addition of sulfonic acid groups would be
expected to increase the Tg even with the addition of sterically large ionic groups. The ionic acid
groups provide additional non covalent bonding interactions causing an increase in the ionomers
glass transition state (238 °C) .
Figure 3, DSC thermogram of ionomer displaying an increase in Tg at 238 °C as a result of additional ionic interactions
from the bound sulfonic acid groups.
TGA analysis displays the thermal stability of the ionomer with the sulfonic acid groups, showing
resistance up to 300 °C and polymeric back bone, >450 °C. The ionomer also displays a water content
of ~5 % from atmospheric hydration of the hydrophilic domain.
Figure 4 TGA thermogram of Ionomer 7 aromatic-ring-Hexafluoro Bisphenol A displaying mechanical stability of sulfonic
acid groups up to 300 °C.
Once cast into membranes from aprotic organic solvents such as DMAc and NMP the material shows
a lower glass transition temperature, 156 °C NMP, 179 °C DMAc. Deviations in the Tg could be a
result of residual solvent interactions within the membrane.
Figure 5, DSC thermogram of ionomer membranes cast from DMAc and NMP displaying glass transitions at 179 °C and
156 °C respectively.
Mechanical and structural stability has also been displayed in the low swelling properties (18-23 %
increase in volume) during increased hydration temperature. The membranes also showed a high
level of water content (~14 % NMP and 5-11 % DMAc) over the hydration temperatures 20 °C -90 °C.
Membrane Volume / cm3
Volume vs Temperature
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
Membrane DMAc 1
Membrane DMAc 2
Membrane NMP 3
Membrane NMP 4
0
50
Hydration Temperature / °C
100
Figure 6, Membranes cast from DMAc and NMP displaying the volume increase with increase hydration over the
temperature range 20 °C to 90 °C.
Mass vs Temperature
Membrane Mass/ g
0.25
0.2
0.15
Membrane DMAc 1
Membrane DMAc 2
0.1
Membrane NMP 3
0.05
0
Membrane NMP 4
0
50
Hydration Temperature / °C
100
Figure 7, Membranes cast from DMAc and NMP displaying the mass increase with increased hydration over the
temperature range 20 °C to 90 °C.
Further analysis of the ionomer membrane via DMTA analysis displays consistent tensile strength
with reproduced material and with little fluctuation displayed over a range of thicknesses (20-80
µm).
E', E'' = f ( T )
10
E', E'' = f ( T )
10
10
1
10
Pa
10
10
10
1
Pa
9
10
E'
9
E'
E''
E''
10
10
8
10
7
-80
-60
-40
-20
0
20
60
40
Temperature T
80
100
120
140
160
180
10
200 °C 220
0
10
8
7
-80
-60
-40
-20
0
20
40
60
Temperature T
80
100
120
140
160
180
10
200 °C 220
0
Figure 8, DMTA analysis of sulfonated 7-aromatic-ring - Hexafluoro Bisphenol A cast from NMP. Left hand thermogram
shows reproducible data of the same sample. Right hand thermogram displays data for varying membrane thickness.
Blue 70-80 µm, Red 60-70 µm, Green 20-30 µm
From this data, it has been shown that the sulfonated 7-aromatic-ring - Hexafluoro Bisphenol A
ionomer has highly suitable mechanical and chemical properties for use in PEM electrolysis. The
membranes produced have glass transitions sufficiently higher than the working temperatures of a
PEM electrolysis cell (120 °C) and high mechanical stability under hydrated conditions. From this
analysis it has been shown that no additional strengthening through crosslinking or reinforcement is
required for efficient use in an electrolysis cell.
(1)
Xing, P.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Wang, K.; Kaliaguine, S. Journal of Membrane Science
2004, 229 95.