Synthesis, oxidation and crosslinking of tetramethyl bisphenol F (TMBPF)-based polymers for oxygen/nitrogen gas separations

Article abstract of DOI:10.1016/j.polymer.2014.09.010

Amorphous, high glass transition, crosslinkable poly(arylene ether)s for gas purification membranes have been synthesized. The polymers include a moiety capable of several oxidation reactions and UV crosslinking. Structural identification was confirmed by ¹H NMR and IR spectroscopy and molecular weights were determined by SEC. Two oxidation reactions of the polymers were identified, one by chemical treatment using Oxone and KBr and one by elevated thermal treatment in air. DSC and TGA were used for thermal characterization, and TGA, ¹H NMR and ATR-FTIR revealed the progress of the thermal oxidation reactions. Both polymers produced tough, ductile films and gas transport properties of the non-crosslinked linear polymers and crosslinked polymer are compared. The O₂ permeability of one exemplary non-crosslinked poly(arylene ether) was 2.8 Barrer, with an O₂/N₂ selectivity of 5.4. Following UV crosslinking, the O₂ permeability decreased to 1.8 Barrer, and the O₂/N₂ selectivity increased to 6.2.

Full text of DOI:10.1016/j.polymer.2014.09.010

Polymer 55 (2014) 5623e5634
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis, oxidation and crosslinking of tetramethyl bisphenol
F (TMBPF)-based polymers for oxygen/nitrogen gas separations
a
a
b
a
a
Benjamin J. Sundell , Andrew T. Shaver , Qiang Liu , Ali Nebipasagil , Priya Pisipati ,
a,
*
a
b
a
Sue J. Mecham , Judy S. Riffle , Benny D. Freeman , James E. McGrath
Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, VA 24061, USA
a
b
Department of Chemical Engineering, Center for Energy and Environmental Resources, The University of Texas at Austin, Austin, TX 78758, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 June 2014
Received in revised form
Amorphous, high glass transition, crosslinkable poly(arylene ether)s for gas purification membranes
have been synthesized. The polymers include a moiety capable of several oxidation reactions and UV
crosslinking. Structural identification was confirmed by 1H NMR and IR spectroscopy and molecular
30 August 2014
weights were determined by SEC. Two oxidation reactions of the polymers were identified, one by
chemical treatment using Oxone and KBr and one by elevated thermal treatment in air. DSC and TGA
were used for thermal characterization, and TGA, 1H NMR and ATR-FTIR revealed the progress of the
thermal oxidation reactions. Both polymers produced tough, ductile films and gas transport properties of
Accepted 5 September 2014
Available online 21 September 2014
Keywords:
2
the non-crosslinked linear polymers and crosslinked polymer are compared. The O permeability of one
Poly(arylene ether)
UV crosslinking
Gas separation
exemplary non-crosslinked poly(arylene ether) was 2.8 Barrer, with an O
UV crosslinking, the O permeability decreased to 1.8 Barrer, and the O
2
/N
/N
2
selectivity of 5.4. Following
selectivity increased to 6.2.
2
2
2
©
2014 Elsevier Ltd. All rights reserved.
1
. Introduction
solubility selectivity and diffusivity selectivity, the latter generally
being the dominant factor controlling overall selectivity of glassy
polymers [6]. Gas diffusion is very sensitive to the size of the
diffusing gases and the availability of free volume within the poly-
mer membrane [7,8]. A desirable gas separation membrane exhibits
both high permeability and selectivity; however, an inherent trade-
off relationship between these two properties has been identified.
Membranes with high gas permeability typically have lower selec-
tivity, and vice-versa. This observation led to the development of the
concept of the upper-bound by Robeson, which is typically repre-
sented as a logelog plot of selectivity versus permeability [5]. The
upper-bound plots were revised upwards once [9] based on the
development of new classes of polymer membrane materials and
have a mathematical and theoretical basis derived by Freeman [6].
Freeman, among others, noted that the most widely used approach
to prepare materials with properties above the upper bound was to
simultaneously increase polymer backbone stiffness and interrupt
interchain packing to increase diffusivity.
Polymeric membranes are important for the separation of gases
in a variety of industrial applications [1]. Several excellent reviews
have detailed industrial gas separations including the removal of
carbon dioxide from natural gas [2], nitrogen enrichment from air
[
[
3], hydrogen recovery and a variety of smaller membrane markets
1,4]. These reviews also discuss some of the challenges associated
with gas separation membranes such as achieving higher selec-
tivity to provide higher purity products at high yield and making
membranes more robust to harsh operating conditions and resis-
tant to plasticization.
Transportof gases in dense polymeric membranes is based on the
solution-diffusion model; the permeability of a gas across the
membrane is the product of diffusion and solubility coefficients
[3,5]. One measure of the ability of a membrane to separate a pair of
gases is the ratio of the permeability coefficient of the more
permeable gas tothatof the lesspermeable gas, knownas selectivity.
The selectivity may be further separated into the product of
Several families of glassy polymers have been investigated to
explore the influence of polymer structure on gas transport prop-
erties. For example, one early effort studied the effect of tacticity on
poly(methyl methacrylate) [10]. Polyimides typically have good gas
separation properties and are of significant interest in the field
because of their chemical robustness and low chain mobility
*
Corresponding author. 145 ICTAS, Virginia Polytechnic Institute and State Uni-
versity, USA.
E-mail addresses: sjmecham@vt.edu, bsundell@vt.edu (S.J. Mecham).
http://dx.doi.org/10.1016/j.polymer.2014.09.010
032-3861/© 2014 Elsevier Ltd. All rights reserved.
0

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B.J. Sundell et al. / Polymer 55 (2014) 5623e5634
0
[11,12]. Another candidate for gas separation polymers is poly-
2.2. Synthesis of 4,4 -methylenebis(2,6-dimethylphenol)
sulfones due to their chemical stability and excellent mechanical
properties, along with inherent rigidity due to in chain aromatic
rings. The effect of variations in backbone chemical structure on
permeability and selectivity of polysulfones has been studied
0
The synthesis of 4,4 -methylenebis(2,6-dimethylphenol), tetra-
methyl bisphenol F, hereafter referred to as TMBPF, was adapted
from a traditional synthesis of a phenol-formaldehyde resin [30].
Excess 2,6-xylenol (415.83 mmol, 50.80 g) was added to a 250-mL
three-necked flask equipped with a condenser, mechanical stirrer,
and addition funnel. The 2,6-xylenol was heated in a thermocouple
[13,14]. For example, changes to the isopropylidene group, which
influences bond rotation and intersegmental packing [15], the ef-
fect of symmetry of phenylene linkages, and methyl group place-
ment [16] all have significant influence on gas separation properties
of polysulfones. Several specialty polyimides have exceeded the
upper bound by undergoing a chemical transformation to form
carbon membranes at very high temperatures, such as that induced
ꢀ
regulated oil bath to 90 C and stirred as it began to melt. Sulfuric
acid (0.5 g) was added very slowly via the addition funnel, which
changed the reaction solution to a dark pink color. The addition
funnel was rinsed with DI water to ensure that all of the acid catalyst
was transferred into the reaction flask. Formalin (37% by mass
ꢀ
via pyrolysis between 550 and 800 C [17]. Additionally, thermal
rearrangement of ortho-positioned polyhydroxyimide precursors
produce crosslinked polybenzoxazoles with much narrower pore
size distributions than the precursor polyimides and, in some cases,
gas separation properties beyond the upper bound [18].
One intriguing approach for improving the properties of gas
separation membranes is to crosslink them, which is a well-known
method to decrease chain mobility. In the late 1980s, Hayes
demonstrated that UV crosslinked aromatic polyimides had a
significantly higher selectivity than their linear analogs [19]. The
effect of UV crosslinking on gas separation properties of the poly-
imides has been studied [20e23], and the effects of crosslinking
have also been decoupled from thermal annealing [24,25]. These
2
formaldehyde in H O, 15 mL) was added slowly via the addition
funnel over the course of several hours; during this time, the reac-
tion solution turned lighter in color and significantly more opaque.
As more product continued to form, the reaction mixture trans-
formed from a liquid to a solid. The crude product was removed from
the flask and filtered using an aspirator and washed copiously with
ꢀ
hot DI water. The crude product was dried at 70 C in a convection
oven and was recrystallized from MeOH to obtain a 90% yield.
2.3. Synthesis of bis-(2,6-xylenol)-F-DFB poly(arylene ether) ketone
(TMBPF-DFB)
crosslinked membranes are especially effective in CO
rations, because crosslinking greatly improves resistance to plas-
ticization by CO [26] or in some cases interrupts polymer
crystallization [27].
2
/CH
4
sepa-
The TMBPF-DFB polymer was synthesized using a nucleophilic
aromatic substitution procedure previously reported [31]. TMBPF
(39.01 mmol, 10.0000 g), DFB (39.01 mmol, 8.5121 g) and DMAc
(62 mL) were added to a 250-mL three necked flask. The reaction
flask was equipped with a mechanical stirrer, nitrogen inlet, and
DeaneStark trap filled with toluene. A stirring, thermocouple
2
In this paper, a poly(arylene ether sulfone) and poly(arylene
ether ketone) have been synthesized from inexpensive reagents
that contain moieties for UV crosslinking and an additional site for
chemical oxidation. The materials considered in this study contain,
ꢀ
regulated oil bath was heated to 155 C. After a homogeneous so-
ꢀ
lution was obtained and the oil bath was at 155 C, K
2
CO
3
0
in each repeat unit, a moiety derived from 4,4 -methylenebis(2,6-
(54.61 mmol, 7.5482 g) and toluene (31 mL) were added, which
immediately turned the light yellow solution a deep violet color.
dimethylphenol), which has been used previously in polymers for
forward osmosis [28] and as a self-cross-linked material for fuel
cells [29]. This benzylic methylene group may be oxidized to a
carbonyl by several routes and then UV crosslinked by benzylic
hydrogen abstraction. This structure also provides the ability to UV
crosslink a poly(arylene ether ketone) to increase backbone stiff-
ness and interrupt chain packing by oxidizing the crosslinked
polymeric backbone in the solid state. The synthesis and charac-
terization of these polymers are described, and investigations of
several oxidation routes are discussed. Initial film casting, UV
crosslinking, and gas transport properties are also discussed.
ꢀ
The reaction was stirred at 155 C for 3 h to azeotropically remove
ꢀ
any water, and then the bath was heated to 175 C. Toluene and
water were drained from the Dean Stark trap, and the reaction was
maintained at this final temperature overnight. After 16 h, the
viscous solution was diluted with additional DMAc (62 mL) and
filtered through celite using an aspirator. The polymer solution was
precipitated into rapidly stirring DI water to produce a fibrous
white solid, filtered using an aspirator, and then boiled several
times in DI water to remove any residual salt by-product. The solid
ꢀ
polymer was finally dried at 150 C in vacuo.
2.4. Synthesis of bis-(2,6-xylenol)-F-DCDPS poly(arylene ether)
2
. Experimental
sulfone (TMBPF-DCDPS)
2
.1. Materials
The TMBPF-DCDPS polymer was synthesized in the same
manner as the TMBPF-DBF polymer, except DCDPS (39.01 mmol,
11.2023 g) was used instead of DFB, and more DMAc (71 mL) was
used to obtain the same monomer concentration due to higher total
monomer mass.
2
,6-Dimethylphenol (2,6-xylenol, 99þ%), 37% formaldehyde in
H
2
O (formalin), phosphorous pentoxide (P ), lithium bromide
2 5
O
(
LiBr) and potassium bromide (KBr) were purchased from Sigma-
eAldrich. N-Methyl-2-pyrrolidone (NMP), methanol (MeOH) and
sulfuric acid were purchased from Spectrum Chemical. Chloroform
2.5. Oxidation of TMBPF-DCDPS polymer with Oxone/KBr
(
CHCl
were purchased from Fisher. DMAc used as a reaction solvent was
dried with calcium hydride (CaH ), distilled under reduced pres-
3 3
), N,N-dimethylacetamide (DMAc) and acetonitrile (CH CN)
Oxidation of the TMBPF-DCDPS polymer was adapted from a
literature procedure for the oxidation of small molecules with
benzylic methylene linkages [32]. TMBPF-DCDPS polymer (1.0 g)
2
sure and stored over 3 Å molecular sieves before use. Calcium hy-
dride (90e95%) and potassium peroxymonosulfate (Oxone) were
purchased from Alfa Aesar. Celite was purchased from EMD
3
was added to CH CN (26 mL) and DI water (2 mL) in a 100-mL
round bottom flask. Oxone (4.675 mmol, 0.712 g) and KBr
(1.063 mmol, 0.126 g) were added to the flask. The flask, under air,
was sealed with septa and stirred in a thermocouple regulated
0
chemicals. 4,4 -Difluorobenzophenone (DFB) was purchased from
0
TCI. 4,4 -Dichlorodiphenyl sulfone (DCDPS) was kindly provided by
ꢀ
Solvay and recrystallized from toluene before use.
water bath at 45 C. After several hours, the temperature was raised

B.J. Sundell et al. / Polymer 55 (2014) 5623e5634
5625
ꢀ
ꢀ
ꢁ1
ꢀ
to 60 C, and the reaction was stirred overnight. The heterogeneous
10 C min to 600 C to test the thermal stability in an inert at-
mosphere. The thermal oxidation reaction and consequent weight
reaction was poured directly into stirring DI water (250 mL), stirred
for several hours, filtered on an aspirator, and dried overnight at
0 C in a convection oven.
ꢀ ꢀ
ꢁ1
gain were performed in air by heating at 10 C min to 220 C, then
ꢀ
ꢀ
ꢁ1
ꢀ
ꢀ
ꢁ1
ꢀ
7
1 C min to 450 C, and finally 10 C min to 700 C.
2.6. Thermal oxidation of TMBPF polymers
2.9. Differential scanning calorimetry (DSC)
The TMBPF polymers were thermally treated in air in a Lind-
The glass transition temperatures (T ) of the polymers were
g
ꢀ
berg/Blue M 1200 C box furnace. Once the oven reached the
specified temperature, the polymer inside of an open scintillation
vial was placed into the oven. After a designated time, the vial was
removed and sealed for analysis.
investigated with a TA Instruments DSC Q200. The polymers were
ꢀ
ꢁ1
ꢁ1
ꢀ
heated under nitrogen at a rate of 10 C min to 350 C, cooled to
ꢀ
ꢀ
ꢀ
50 C, and heated again at a rate of 10 C min to 350 C. The DSC
thermograms shown are the second scan.
2.7. Structural characterization
2.10. Film preparation
1
Proton nuclear magnetic resonance ( H NMR) spectroscopy was
To prepare films, 0.8 g of polymer was added to 20 mL of chlo-
performed on a Varian Inova spectrometer operating at 400 MHz.
All spectra were obtained from 15% (w/v) 1 mL solutions in
roform (CHCl ) in a scintillation vial, and the mixture was stirred
until a homogeneous, transparent solution was obtained. The so-
3
deuterated chloroform (CDCl
3
). Fourier Transform Infrared Spec-
lution was syringe filtered through a 1.0 mm filter into a new vial.
troscopy with attenuated total reflectance (FTIR-ATR) was applied
to the polymers to observe the conversion of the methylene bridge
to the carbonyl group as a result of oxidation. The FTIR-ATR spectra
were recorded on an FTIR spectrometer (Varian 670 FTIR) equipped
with an ATR attachment with a diamond crystal. The spectral res-
The vial was sonicated 3 times for 60 min each to remove dissolved
gases. The solution was cast on a 10 ꢂ 15 cm clean, dry glass plate
on a level casting surface and then dried in air at ambient condi-
tions overnight. The following day the air dried film was removed,
ꢀ
the edges were trimmed, and the film was dried at 120 C under
ꢁ1
olution was 4 cm , and 32 background scans were performed. A
small amount of polymer was placed on the diamond crystal, and
the FTIR spectrum was measured with 32 scans. The spectra were
baseline corrected and normalized based on the initial C]O group
vacuum before gas transport experiments. TGA thermograms
showed that these drying conditions were suitable for the complete
removal of solvent.
ꢁ1
stretch at ca. 1525 cm . All measurements were performed at
ambient temperature. Intrinsic viscosities (IV) and molecular
weights of the polymers were obtained by size exclusion chroma-
tography (SEC). The SEC system consisted of an isocratic pump
2.11. UV crosslinking
Crosslinking was performed by irradiating polymer films in air
under a 100 W high intensity, long-wave UV lamp equipped with a
365-nm light filter (Blak-Ray B-100, UVP). The film was placed
about 3.5 cm from the UV lamp and irradiated for one hour on each
side. At this distance, the UV intensity was measured to be
(
Agilent 1260 infinity, Agilent Technologies, Santa Clara, CA) with
an online degasser (Agilent 1260, Agilent Technologies, Santa Clara,
CA), autosampler and column oven used for mobile phase delivery
ꢁ2
and sample injection, and three Agilent PLgel 10
mm Mixed B-LS
19.7 mW cm
.
columns 300 ꢂ 7.5 mm connected in series with a guard column as
the stationary phase. A system of multiple detectors connected in
series was used for the analysis. A multi-angle laser light scattering
2.12. Gel fractions
ꢀ
(
MALS) detector (DAWN-HELEOS II, Wyatt Technology Corporation,
Crosslinked films were dried at 120 C under vacuum overnight.
Then 0.1e0.2 g of the crosslinked film was placed in a 20 mL
scintillation vial filled with CHCl3 and stirred overnight. The
remaining solid was filtered, transferred to a pre-weighed vial, and
Goleta, CA), operating at a wavelength of 658 nm, a viscometer
detector (Viscostar, Wyatt Technology Corporation, Goleta, CA), and
a refractive index detector operating at a wavelength of 658 nm
ꢀ
(
Optilab T-rEX, Wyatt Technology Corporation, Goleta, CA) pro-
dried at 120 C under vacuum overnight to the final weight. Gel
vided online results. The system was corrected for interdetector
delay, band broadening, and the MALS signals were normalized
using a 21,720 g/mol polystyrene standard obtained from Agilent
Technologies or Varian. Data acquisition and analysis was con-
ducted using Astra 6 software (Wyatt Technology Corporation,
Goleta, CA). The mobile phase was NMP, which was vacuum
Fractions were calculated by equation (1) to determine the degree
of crosslinking.
Gel Fraction ð%Þ ¼ W
ꢂ 100
(1)
final
Winitial
distilled over P
2
O
5
before use. The salt, 0.05 M dried LiBr, was added
2.13. Density measurements
and dissolved in the NMP before the solvent was degassed and
filtered. The sample solutions were prepared in a concentration
range of 2~3 mg/mL and were filtered to remove any dust or
Density was measured using a Mettler Toledo balance equipped
with a density measurement kit. Ethanol was chosen as the refer-
ence liquid because the samples tested showed low ethanol uptake
over the time scale of the density measurement.
insoluble particles using 0.22
mm PTFE filters. Molecular weight
values were measured using light scattering, and the intrinsic vis-
cosity values were measured online. Specific refractive index
increment (dn/dc) values were calculated for each backbone type
based on 100% mass recovery using the Astra 6 software.
2.14. Gas permeation measurements
Gas permeation properties were measured using a constant-
volume/variable-pressure method [33]. The upstream portion of
the system was constructed from commercially available Swagelok
parts using Swagelok tube fittings. Welded joints and VCR con-
nections were used in the downstream portion to minimize leaks.
The membrane was housed in a stainless steel Millipore filter
2
.8. Thermogravimetric analysis (TGA)
The thermal stability and oxidation of the polymers were
investigated using a TA Instruments TGA Q5000. The polymers
were first heated under a nitrogen atmosphere at a rate of

5626
B.J. Sundell et al. / Polymer 55 (2014) 5623e5634
from a liquid to solid phase in addition to exhibiting a color change,
which indicates the formation of product as TMBPF has a signifi-
ꢀ
cantly higher melting point than 90 C and is insoluble in water.
Isolation of the monomer product was also very efficient. Washing
with near boiling water is effective at removing residual acid and
residual starting material. Upon drying, any residual 2,6-xylenol
impurity in the crude product was assessed qualitatively by the
presence of an orange/pink color, which is completely removed
when recrystallized from methanol to yield white crystals.
3.2. Characterization of TMBPF monomer
1
The H NMR spectrum ofTMBPF is shown in Fig. 2. All of the peaks
integrate quantitatively with regard to the molecular structure. The
1
H NMR also showed no organic side products or contaminants,
including starting material. The melting point of the recrystallized
product was measured via melting point apparatus and found to be
in good agreement with reported values (177 C) [35].
Fig. 1. Synthesis of TMBPF via electrophilic aromatic substitution.
ꢀ
holder (Millipore, Billerica, MA, USA) with an included support. A
Honeywell Super TJE 1500 psi (10.3 MPa) transducer (Honeywell
Sensotec, Columbus, Ohio, USA) was used to track upstream pres-
sure, and a MKS Baratron 626 transducer (MKS, Andover, MA, USA)
3.3. Synthesis and NMR of TMBPF polymers
Poly(arylene ether)s are potentially important candidates for
gas separation materials, and poly(arylene ether sulfone)s in
particular have already found wide application in the field [1,36].
The success of poly(arylene ether)s as gas separation membranes is
in part due to their mechanical robustness and high chemical sta-
bility [37,38].
was used to measure downstream pressure. The permeabilities of
ꢀ
N
2
and O
2
were measured at 35 C at 10 atm feed pressure.
3
. Results and discussion
3
.1. Synthesis of tetramethyl bisphenol F monomer (TMBPF)
The TMBPF containing poly(arylene ether ketone) (TMBPF-DFB)
and poly(arylene ether sulfone) (TMBPF-DCDPS) in this study were
synthesized by nucleophilic aromatic substitution with a weak base
[31,39]. The syntheses of the poly(arylene ether sulfone) and pol-
y(arylene ether ketone) are shown in Figs. 3 and 4 respectively.
An appealing feature of these polymers is the low cost of starting
materials. The TMBPF monomer is commercially available, or may
be easily synthesized from inexpensive 2,6-xylenol and formalin
via electrophilic aromatic substitution as shown in Fig. 1.
2 3
Upon addition of toluene and K CO , both reaction solutions turned
TMBPF is the key monomer in these polymers, because the
benzylic methyl groups are necessary for UV crosslinking and the
benzylic methylene group may undergo an oxidative chemical
transformation [32]. A slight excess of 2,6-xylenol was used to drive
the reaction to completion [34]. The reaction occurs very rapidly at
the conditions used; however a temperature of 90 C was necessary
to ensure that the 2,6-xylenol was melted and that homogeneous
stirring could occur. Upon formalin addition, the reaction shifted
a deep violet color immediately, which persisted throughout the
course of the reaction. In both reactions, toluene was used as an
azeotropic solvent to remove water formed by K CO reaction and
2 3
subsequent decomposition. Water could potentially react with the
activated dihalide monomer at higher temperatures and upset the
reaction stoichiometry, leading to low molecular weight polymers.
It was important to use a heat gun to dry the joints on the three-
necked flask during the azeotropic reflux, where water may
ꢀ
Fig. 2. 1H NMR spectrum of TMBPF monomer.

B.J. Sundell et al. / Polymer 55 (2014) 5623e5634
5627
Fig. 3. Synthesis and 1H NMR spectrum of TMBPF-DCDPS polymer.
become trapped. After the water was removed from the reaction,
toluene was drained, and the flask was brought to the final reaction
temperature. Difluorobenzophenone is a more reactive than
dichlorodiphenyl sulfone, because the carbon fluorine bond is more
polarized than the carbon chlorine bond, and therefore, more
efficiently stabilizes the Meisenheimer complex intermediate [40].
Thus, the final temperature of the poly(arylene ether ketone) re-
was performed to ensure polymer purity, structural identification
and removal of solvent. Figs. 3 and 4 demonstrate that all of these
objectives were achieved, as the integrations corresponded to the
expected structures and no impurities or endgroups were observed
in the spectra, an indication of high molecular weight.
3.4. SEC of TMBPF polymers
ꢀ
action was 20 C lower than that of the poly(arylene ether sulfone)
reaction. Both reactions exhibited high viscosity after 16 h and were
stopped at this time.
The polymers were sufficiently high in molecular weight to form
transparent, ductile films. SEC of the polymers quantitatively sub-
stantiated high molecular weight, and these results are shown in
Fig. 5 and Table 1. The polymers have a monomodal Gaussian
After polymer isolation and drying, the TMBPF polymers were
1
characterized by a variety of spectral and thermal methods. H NMR
Fig. 4. Synthesis and 1H NMR spectrum of TMBPF-DFB polymer.

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B.J. Sundell et al. / Polymer 55 (2014) 5623e5634
[41]. The methylene bridge of the TMBPF group in the backbone of
the polymer can be converted to a carbonyl via oxidation [42]. The
oxidation of the TMBPF containing poly(arylene ether sulfone) was
®
initially attempted using potassium peroxymonosulfate (Oxone )
and KBr in a procedure adapted from the oxidation of small mol-
ecules containing benzylic methylene linkages [32]. The poly(-
arylene ether sulfone) was initially tested for this oxidation route
because the un-oxidized form does not contain carbonyl linkages,
so the success of this reaction may be followed by tracking the
growth of this peak by FTIR spectroscopy.
The small molecule procedure called for the addition of Oxone
and KBr by molar equivalencies of 2.2 and 0.5 respectively. Because
each repeat unit contains one benzylic methylene group capable of
oxidation, the molecular weight of the repeat unit was used as the
molar mass to determine
considerations.
Addition of TMBPF-DCDPS polymer to aqueous acetonitrile
turned the liquid a light amber color, and addition of Oxone and KBr
produced a slight red tint that quickly disappeared upon stirring. It
was apparent that the polymer was mostly insoluble in the solvent
mixture, and gradually heating to 45 C or 60 C did not noticeably
improve the solubility. The solvent system of aqueous acetonitrile
was important for the success of oxidation in the small molecule
reactions [32]. Water is the source of oxygen in the oxidized product,
and changing the solvent to DMSO or DMF produced very low yields.
One reaction for oxidizing TMBPF-DCDPS was performed in chlo-
roform, a good solvent for this polymer, but this led to similar levels
of conversion with side reactions including crosslinking.
1 equivalent for stoichiometric
Fig. 5. SEC of TMBPF containing polymers.
Table 1
SEC of TMBPF containing polymers.
ꢀ
ꢀ
Sample
Mn (kDa)
Mw (kDa)
PDI
[
h
] (dL/g)
dn/dc (mL/g)
TMBPF-DCDPS
TMBPF-DFB
66.2
51.7
127.5
92.7
1.9
1.8
0.61
0.54
0.14
0.15
distribution and polydispersity index (PDI) that one would expect
from a step growth polymerization. The PDI should theoretically be
. However, some lower molecular weight material is likely lost
2
1
during polymer isolation, thereby decreasing the observed PDI.
Spectral verification of the oxidation reaction was done using H
NMR and FTIR-ATR spectroscopy. The H NMR spectra of TMBPF-
1
3
.5. Oxidation of TMBPF-DCDPS polymer with Oxone/KBr
DCDPS and its partially oxidized analog are shown in Fig. 6. Most
notably, the benzylic methylene peak c decreased to about 80% of
its initial value, indicating near 20% conversion to the oxidized
product. The peaks nearest to the benzylic methylene moiety, a and
To UV crosslink these polymers requires the presence of aro-
matic carbonyl groups in the backbone and benzylic methyl groups
Fig. 6. 1H NMR of TMBPF-DCDPS before and after oxidation with Oxone/KBr.

B.J. Sundell et al. / Polymer 55 (2014) 5623e5634
5629
Fig. 7. IR of TMBPF-DCDPS before and after oxidation with Oxone/KBr.
0
0
b, also decreased and new peaks, a and b appeared downfield
such as those involved in production of synthesis gas, which
potentially require membrane stability above 300 C [1], and pre-
ꢀ
relative to the initial control peaks. This downfield shift is consis-
tent with the electron withdrawing nature of the new carbonyl
linkage compared to the original methylene group. Peaks d and e
are less affected by this chemical transformation because of their
distance from the methylene or carbonyl group, but a broadening of
these peaks was still observed. FTIR-ATR spectra of the TMBPF-
DCDPS polymer and its partially oxidized analog are presented in
Fig. 7. Several signature peaks of the control polymer structure are
highlighted, including the sulfone linkage and the benzylic meth-
ylene linkage. Most importantly, the partially oxidized polymer had
combustion carbon capture, which might need membranes that
ꢀ
are stable at up to 150 C or higher [44]. The thermal stability of the
TMBPF polymers was initially tested under pure nitrogen, as shown
in Fig. 8. Both polymers had very high thermal stabilities, showing
ꢀ
ꢀ
no weight loss until 400 C and a 10% weight loss above 450 C.
3.8. TGA of TMBPF polymers in air
-
1
TGA of the TMBPF polymers was also performed in air, and the
a signature peak at 1656 cm , indicating the appearance of a
carbonyl group not observed in the control polymer.
results are presented in Fig. 9. The polymers both showed a very
ꢀ
interesting phenomenon, weight gain in the region of 250e325 C.
Initially, these polymers were only investigated for their ability to
chemically oxidize, but it became apparent that the benzylic
methylene linkage could also thermally oxidize at elevated
3
.6. UV crosslinking of oxidized TMBPF-DCDPS polymer
One important test on the partially oxidized TMBPF-DCDPS
polymer was the ability of the polymer to UV self-crosslink. The
UV crosslinking mechanism of interest was benzylic hydrogen
abstraction by a benzophenone moiety, which generated two rad-
icals that led to a crosslinked site upon recombination [43]. The
TMBPF-DCDPS control polymer lacked the benzophenone moiety
and was incapable of UV crosslinking. Two films were prepared to
test the self-crosslinking reaction, one of the TMBPF-DCDPS control
and one of the partially oxidized system. Both films were cast from
chloroform, dried, and then exposed to UV light. After irradiation,
both films were weighed and extracted with chloroform. The
TMBPF-DCDPS film quickly dissolved, indicating 0% gel fraction and
no crosslinking. However, the partially oxidized TMBPF-DCDPS film
was largely undissolved after one day and had a gel fraction of 80%,
demonstrating a high level of network formation.
3.7. TGA of TMBPF polymers under N
2
In addition to chemical and mechanical stability, high thermal
stability is also desired for gas separation membranes. Some
ꢀ
hydrogen separations are performed near 100 C today, and there is
discussion of performing separations at even higher temperatures,
2
Fig. 8. TGA of TMBPF containing polymers in N .

5630
B.J. Sundell et al. / Polymer 55 (2014) 5623e5634
temperatures in the presence of oxygen. The molecular mass dif-
ference between non-oxidized and oxidized polymer was calcu-
lated for both TMBPF containing poly(arylene ether ketone) and
poly(arylene ether sulfone). The theoretical weight gain required to
oxidize each methylene group to a carbonyl in the TMBPF-DFB
polymer was 3.2%, and it was 3.0% for the TMBPF-DCDPS poly-
mer. The theoretical weight percent increase was higher for the
ketone system because of its lower molecular weight repeat unit.
The actual weight gains shown in Fig. 9 were approximately 90% of
the theoretically possible weight gains, suggesting a high level of
ꢀ
ꢀ
oxidation. The temperature ramp above 220 C was done at 1 C/
min, because incomplete oxidation was observed at higher heating
ꢀ
rates (i.e., 10 C/min). The rates of thermal oxidation at various fixed
temperatures will be detailed in a subsequent publication.
Fig. 9. TGA of TMBPF containing polymers in air.
3.9. Thermal properties of TMBPF polymers under N
2
DSC was performed to identify the glass transition tempera-
tures (T 's) of the TMBPF polymers. The samples were heated to
g
ꢀ
3
50 C to erase their thermal history, cooled, and then heated
ꢀ
again to 350 C to produce the thermograms in Fig. 10. DSC was
performed under an inert atmosphere to eliminate the possibility
of thermal oxidation. Distinct endothermic transitions indicative
ꢀ
ꢀ
of the T
DCDPS. The T
ethers); notably, the T
ment with a prior literature study investigating the effect of pol-
ysulfone backbone structure on T [45]. The poly(arylene ether
more than 20 C higher than that of the poly(-
g
were found at 213 C for TMBPF-DFB and 235 C TMBPF-
's were in the range expected for poly(arylene
of TMBPF-DCDPS was in excellent agree-
g
g
g
ꢀ
sulfone) has a T
g
arylene ether ketone), likely due to the increased restrictions to
rotation about the sulfone linkage relative to the carbonyl linkage.
ꢀ
Results indicated that thermal oxidation in air began nearly 20 C
g
above the T of the polymers, as shown in Fig. 9 by weight gain.
These results seem to suggest that extensive chain motion is
needed for the oxidation process.
2
Fig. 10. DSC of TMBPF containing polymers in N .
Fig. 11. 1H NMR of thermally oxidized TMBPF-DFB.

B.J. Sundell et al. / Polymer 55 (2014) 5623e5634
5631
Fig. 12. 1H NMR of thermally oxidized TMBPF-DCDPS.
3.10. NMR of thermally oxidized TMBPF-DFB polymer
3.11. FTIR of the thermally oxidized TMBPF-DCDPS polymer
To further test the thermal oxidation reactions, the TMBPF-DFB
Fig. 13 shows the FTIR spectrum of the thermally oxidized
TMBPF-DCDPS compared to the untreated sample. The thermally
oxidized sample had significantly larger carbonyl peaks than the
Oxone/KBr oxidized product in Fig. 7. This comparison of the FTIR
ꢀ
polymer was heated in air to 260 C, well above T
0 min. The TMBPF-DCDPS polymer was heated to 280 C in air,
g
, and held for
ꢀ
1
g
well above T , and held for 120 min. The TMBPF-DCDPS polymer
was heated at a higher temperature and for a longer period of time
to account for the slower oxidation rates observed via TGA. During
these times, the polymers changed from white to a light yellow
1
color. The H NMR spectrum of the soluble fraction of the thermally
oxidized TMBPF-DFB is shown in Fig. 11, and the spectrum for
oxidized TMBPF-DCDPS is shown in Fig. 12.
Similar to the results obtained via partial Oxone/KBr oxidation,
the thermal oxidation reactions were analyzed by 1H NMR spec-
troscopy. The spectra show that the TMBPF-DFB polymer was
oxidized to 75% conversion, and the TMBPF-DCDPS polymer was
oxidized to 45% conversion, based on integration of the benzylic
methylene peak (i.e., peak c in Figs. 11 and 12) before and after
thermal oxidation. The signal-to-noise ratio was lower in the
thermal oxidation spectra compared to the Oxone/KBr oxidation
because of poor solubility in the deuterated solvent, presumably as
a result of thermal crosslinking concurrent with the oxidation re-
action. The growth of several small peaks (Figs. 11 and 12) may also
indicate increased structural complexity as the polymers began to
crosslink. The rate and exact temperature of these thermal oxida-
tion reactions and how they can be decoupled from crosslinking
will be further explored in a subsequent publication.
Fig. 13. IR of thermally oxidized TMBPF-DCDPS.

5632
B.J. Sundell et al. / Polymer 55 (2014) 5623e5634
Table 2
Initial pure gas permeability of linear and crosslinked TMBPF-DFB.
Gas permeability (Barrer) at 10 atm
ꢀ
35 C
&
O
2
N
2
CO
10
7.1
5.6
2
CH
4
H
2
TMBPF-DFB linear
2.8
1.8
1.4
5.6
0.52
0.29
0.25
1.06
0.54
0.25
0.25
0.95
29
23
14
32
TMBPF-DFB crosslinked
Bisphenol A polysulfonea
Tetramethyl bisphenol A polysulfone
a
21
a
ꢀ
ꢀ
Polysulfones [16]: CO
2
, CH
4
at 10 bar and 35 C; O
2 2
, N , H
2
at 1 bar and 35 C.
spectra supports the 1H NMR result that thermal oxidation pro-
ceeds to higher conversion than Oxone/KBr conversion.
3.12. Initial gas transport results
As the effect of UV crosslinking on gas permeation properties
was of interest for this research, gas transport results were obtained
to compare the linear TMBPF containing poly(arylene ether ketone)
and its UV crosslinked analog. A control film was cast from the
TMBPF-DFB polymer and then cut in half. Permeabilities and se-
lectivities of five different gases were directly tested on one half.
The other half was exposed to UV irradiation to induce photo-
chemical crosslinking. The mechanism for this photochemical re-
action is benzylic hydrogen abstraction by an excited state
benzophenone moiety [43,46], which has been shown to produce
polyimides with high selectivity [19]. The gel fraction of this UV
irradiated half was measured to be 100%, indicating that a highly
crosslinked network was obtained. The density of the linear
Fig. 14. Upper bound plot comparison of linear and crosslinked TMBPF-DFB [9].
publication, the effect of oxidation before and after UV crosslinking
will be explored for both the TMBPF-DFB and the TMBPF-DCDPS
polymers. The pure gas permeability was measured at five
different pressures, which is graphed in Figs. 15 and 16.
In Fig. 15, the permeability of CH
upstream pressure was increased, because of their condensability
49]. The effect of decreasing permeability in glassy polymers as a
4 2
and CO decreased slightly as
ꢁ1
TMBPF-DFB polymer was 1.116 ± 0.003 g mL and the density of
[
ꢁ1
the crosslinked TMBPF-DFB polymer was 1.146 ± 0.004 g mL . The
densification of the polymer membrane upon UV crosslinking was
expected based on prior literature, and reflects a decrease in
interchain spacing [47]. The pure gas permeabilities and selectiv-
ities of these two films are shown in Tables 2 and 3, where they are
compared with bisphenol A polysulfone (UDEL) and a tetramethyl
bisphenol A based polysulfone. The TMBPF-DFB linear polymer was
un-oxidized and non-crosslinked, whereas the TMBPF-DFB cross-
linked polymer was un-oxidized and crosslinked to afford a 100%
gel fraction. The transport properties of the TMBPF-DFB linear and
TMBPF-DFB crosslinked polymers are also shown in Fig. 14, where
they are graphed with a commercial polysulfone, polyimide
function of upstream pressure has been attributed to the dual-
sorption model [50]. An increase in upstream pressure effectively
reduces the Langmuir term and thus reduces permeability. Inter-
estingly this phenomenon was less pronounced in Fig. 16 for the
highly crosslinked TMBPF-DFB.
4. Conclusions
In this paper, an economical route was presented to synthesize
the TMBPF monomer and two TMBPF containing poly(arylene
(
Matrimid), and the 1991 and 2008 upper bounds.
The films demonstrate the expected relationship of crosslinking
decreasing gas permeability and improving selectivity. An addi-
tional benefit of crosslinking for thin films is a reduction in physical
aging [46]. The reduction of permeability after UV crosslinking has
also been attributed to membrane densification [48]. Notably, the
crosslinked TMBPF-DFB films had both higher permeability and
selectivity compared to UDEL polysulfone. One reason for the
greatly enhanced gas permeability of the TMBPF systems is the
presence of the numerous bulky methyl groups along the poly-
meric backbone. These two samples represent a minor set that was
obtainable from these TMBPF polymer series. In a subsequent
Table 3
Initial pure gas selectivity of linear and crosslinked TMBPF-DFB.
ꢀ
Gas selectivity at 10 atm & 35
C
O
/N
2 2
CO
2
/CH
4
2 2 2 4 2 2
H /N H /CH CO /N
TMBPF-DFB linear
5.4
6.2
5.6
19
28
22.4
22
75
79
56
30
72
92
56
34
19
24
22.4
19.8
TMBPF-DFB crosslinked
Bisphenol A polysulfonea
a
Tetramethyl bisphenol A polysulfone 5.3
a
ꢀ
ꢀ
Polysulfones [16]: CO
2
, CH
4
at 10 bar and 35 C; O
2 2
, N , H
2
at 1 bar and 35 C.
Fig. 15. Permeability as a function of feed pressure for linear TMBPF-DFB.

B.J. Sundell et al. / Polymer 55 (2014) 5623e5634
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