Microporous polyacrylate matrix containing hydrogen bonded nanotubular assemblies

Article abstract of DOI:10.1016/j.polymer.2010.05.047

We report the straightforward photo-polymerization of polyacrylate films containing bis-urea based self-assembled nanotubes. The obtained materials are characterized by gas adsorption measurements, 129Xe NMR spectroscopy and WAXS. The presence of the bis-

Full text of DOI:10.1016/j.polymer.2010.05.047

Polymer 51 (2010) 3360e3364
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Microporous polyacrylate matrix containing hydrogen bonded
nanotubular assemblies
,
,
*
Farid Ouhib a b, Emmanuelle Bugnet a, Andrei Nossov b, Jean-Luc Bonardet b, Laurent Bouteiller a
a UPMC Univ Paris 06, UMR 7610, Chimie des Polymères, F-75005 Paris, France, and CNRS, UMR 7610, Chimie des Polymères, F-75005 Paris, France
b UPMC Univ Paris 06, UMR 7197, Laboratoire de Réactivité des Surfaces (LRS), F-75005 Paris, France
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the straightforward photo-polymerization of polyacrylate films containing bis-urea based self-
assembled nanotubes. The obtained materials are characterized by gas adsorption measurements, 129Xe
NMR spectroscopy and WAXS. The presence of the bis-ureas is shown by butane adsorption (at 273 K and
ambient pressure) to be responsible for the formation of a significant microporosity. This porosity is
however not detected by the classical argon adsorption procedure (at 77 K and low pressure). This effect
is attributed to the contraction of the material at low temperature and pressure, and may be of general
concern for other organic porous materials. One of the potential advantages of the present materials is
that the porosity results from the self-assembled nanotubes and should therefore be independent of the
matrix mechanical properties. It should in particular be possible to adjust the flexibility of the matrix by
changing the monomer composition.
Received 22 March 2010
Received in revised form
21 May 2010
Accepted 21 May 2010
Available online 1 June 2010
Keywords:
Self-assembly
Ultramicroporous
Nanotube
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
and the chemical cross-linking steps are decoupled. The template
can either remain in the final membrane (if the template presents
Most porous materials currently used in commercial applica-
tions are inorganic (zeolites, activated carbon, or clays, for
example), however porous organic materials provide new oppor-
tunities in numerous technological areas, such as gas storage,
separation processes or catalysis [1]. Their main potential advan-
tages over inorganic materials are their lower specific weight,
dielectric constant, and refractive index, and the virtually unlimited
possibilities to functionalize their surface. In this context, several
kinds of organic materials have been reported: on one hand, the
assembly of well defined molecular building blocks can yield
Covalent Organic Frameworks (COF) [2], MetaleOrganic Frame-
works (MOF) [3e5], or porous organic crystals [6e11], which
enable both an exquisite control of the pore dimensions and
a possible stimuli responsiveness. On the other hand, microporous
materials can be obtained from rigidly crosslinked polymers
[12e16]. In this case, a high surface area together with good
mechanical properties can be obtained. In an attempt to combine
a better control of the pore dimensions and good mechanical
properties, it is also possible to polymerize a matrix in the presence
of a self-assembled template. This approach has the additional
advantage of an easier processability, because the pore formation
an intrinsic porosity) [17e23], or it can be removed by dissolution
or chemical degradation [24e26]. In the field of ultramicroporous
materials (pore width <1 nm), only the former approach is feasible,
and therefore, most systems reported consist of microchannels or
nanotubes formed by assembly of macrocyclic compounds, where
the macrocycle ensures the control of the pore width. In this case,
the rigidity of the matrix is not essential, so that elastomeric
ultramicroporous membranes can be envisaged.
In fact, self-assembled nanotubes can also be obtained from
non-macrocyclic monomers [27e36], with the added advantage of
a
more direct synthetic accessibility than for macrocyclic
compounds. In particular, bis-urea based monomers (Scheme 1) are
extremely simple compounds known to form micrometer long
nanotubes in non polar solvents [35e40]. We therefore decided to
investigate the possibility to use these bis-ureas as template to
form ultramicroporous materials.
2. Experimental
2.1. Synthesis of bis-ureas
Bis-urea 1 was prepared from racemic 2-ethylhexylamine and
2,4-toluenediisocyanate as described previously [41]. The synthesis
of bis-urea 2 from racemic 2-aminobutanol will be reported later.
Synthesis of bis-urea 3: to a stirred solution of bis-urea 2 (429.5 mg,
* Corresponding author.
E-mail address: laurent.bouteiller@upmc.fr (L. Bouteiller).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.05.047

F. Ouhib et al. / Polymer 51 (2010) 3360e3364
3361
placed in a cell. After treatment under vacuum at 60 ꢀC, the cell is
connected to a home-made volumetric apparatus. Xenon is intro-
duced in the manifold and condensed in the cell at liquid nitrogen
temperature. The cell is then sealed and brought back to the
ambient temperature. The pressure of xenon in the manifold is
determined so that the pressure in the cell after sealing is about
10 bar. Spectra are recorded with an AMX 300 Bruker spectrometer
at the frequency of 83 MHz. The correct value of the pressure in the
cell can be obtained from the chemical shift of the gas signal, which
varies linearly with the xenon gas pressure.
O
O
O
O
O
O
N
H
N
H
N
H
N
H
1
2
3
O
O
O
O
N
H
N
H
N
H
N
H
3. Results and discussion
3.1. Design of the system
N
N
N
N
CH3 CH3
CH3 CH3
Due to strong hydrogen bonding between urea groups, bis-urea
1 spontaneously forms long and rigid nanotubes in a wide
temperature and concentration range. The inner diameter of the
nanotubes is fixed by the supramolecular architecture and has been
shown by host-guest studies and molecular simulations to be close
Scheme 1. Structure of hydrogen bonding bis-ureas 1 and 2, and reference bis-urea 3.
0.963 mmol) in 5 mL of anhydrous dimethylformamide was added
NaH (143.2 mg, 5.778 mmol) at 0 ꢀC. After 30 min, iodomethane
(820.1 mg, 5.778 mmol) was added to the solution under nitrogen,
at 0 ꢀC. The mixture was stirred at room temperature for 14 h. The
solution was diluted with 10 mL of dichloromethane, washed with
water, dried over magnesium sulphate and evaporated to dryness.
The product was purified by silica gel column chromatography
(n-hexane/ethylacetate, 3/1) to give bis-urea 3 as a viscous oil (54%
ꢀ
to 7 A [36,38]. Our aim is to dissolve the bis-urea in a suitable
monomer where the nanotubes can self-assemble, and then to
polymerize the monomer. Our assumption is that the monomers
present inside the cavity of the nanotubes do not have enough
conformational freedom to polymerize, so that it should be possible
to evaporate them after polymerization of the matrix, and thus to
ꢀ
obtain porous channels of 7 A diameter.
In low polarity solvents, bis-urea 1 nanotubes are isotropically
dispersed and entangled, thus yielding gels. The viscoelastic nature
of these gels proves that the nanotubes can break and recombine on
the time-scale of seconds [44e46]. The dynamic character of the
nanotubes has also been confirmed by isothermal titration calo-
rimetry experiments, which show that dilution of the solution is
responsible for a fast shortening of the nanotubes [37,47]. In some
cases, the fast dynamics of this system is an advantage, because it
ensures that a thermodynamic equilibrium is reached and that the
self-assembled structures are stable over unlimited periods of time.
In the present case however, the fast dynamics may allow the
assemblies to reorganize during polymerization. To limit this
potential issue, we need to use a fast polymerization process.
Moreover, because a high polymerization temperature may desta-
bilize the nanotubes, we selected a photo-polymerization process.
Photo-initiated free radical polymerization has been widely used to
yield). 1H NMR (200 MHz, DMSO):
d
(ppm) ¼ 0.87 (t, 6H, CH3); 1.34
(m, 4H, CH2); 2.05 (s, 6H, Ar-CH3); 2.46 (s, 6H, NeCH3); 3.05 (s, 6H,
NeCH3); 3.29 (m, 4H, CH2eO); 3.65 (m, 2H, CH); 3.96 (d, 4H,
CH2eO); 5.14 (m, 4H, CH2]CH); 5.82 (m, 2H, CH]CH2); 6.83 (s, 1H,
Ar-H); 8.03 (s, 1H, Ar-H).
2.2. Polymerization
The bis-urea (1, 2 or 3) was dissolved under stirring in a mixture
of isobornylacrylate (Aldrich) and divinylbenzene (Aldrich) (95/5
by weight) at 60 ꢀC. In the case of bis-ureas 1 and 2, a viscoelastic
gel was obtained. The photoinitiator 2,2-dimethoxy-2-phenyl-
acetophenone (DMPA, Ciba) (1% by weight/isobornylacrylate) was
added to the mixture. The solution was placed between two glass
slides separated by a 1 mm thick spacer, and then photo-poly-
merized under a DYNAX UV light curing system (2000 Flood Model,
400 W) for 5 min. The obtained film was dried under vacuum for
several days.
polymerize
a matrix containing non-dynamic self-assemblies
[25,48e51]. We therefore sought to investigate if the same
approach can be used in the case of a more dynamic system.
A second bis-urea (2) was also considered, to test the versatility
of our approach. The xylene spacer in 2 (compared to the toluene
spacer in 1) is expected to stabilize the nanotube structure due to
a better preorganization of the monomer [39]. Moreover, the ally-
lether side-chain was introduced to improve the solubility in
moderately polar monomers. The solubility of both bis-ureas was
then tested in a range of monomers (Table 1). Unexpectedly, bis-
urea 1 does not form viscoelastic gels in any styrene-based
monomers, although it was previously shown that 1 forms visco-
elastic gels in a wide range of aromatic solvents (such as toluene,
xylenes or ethylbenzene) [36]. In fact, 1 is insoluble in most
styrene-based monomers; the only exceptions being 2-substituted
styrene derivatives. Unfortunately, these monomers are too large to
fit inside the nanotubes and therefore destabilize them [36].
Among the acrylic monomers tested, isobornylacrylate is the only
monomer, which yields viscoelastic gels for both bis-ureas. This
monomer was thus selected for further study. In particular, it was
checked by FTIR that the characteristic signature of nanotubes is
indeed obtained at room temperature (see Fig. S1 in Supporting
2.3. Porosity measurements
The polymer samples were first ground for 10 min, while cooled
with liquid nitrogen, to yield a fine powder. The surface area and
the porosity were measured by volumetry, with an ASAP 2020
Micromeritics apparatus using either argon at 77 K or butane at
273 K as adsorbent. The surface areas were calculated following the
Brunauer, Emmet and Teller method (BET surfaces) and the
microporous volumes were estimated from the point B, point from
which the adsorbed quantity of adsorbent begins to vary linearly
with the relative pressure of the gas. It corresponds to the filling of
micropores [42].
2.4. Xe NMR
It has been proved that xenon NMR of adsorbed xenon is a very
good tool to probe the micro and ultra-microporosity of porous
materials [43]. Typically 300e500 mg of powdered polymer are

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F. Ouhib et al. / Polymer 51 (2010) 3360e3364
Table 1
1
Solubility at room temperature of bis-ureas 1 and 2 in vinylic monomers (1% solu-
tions) (F: fluid solution; G: viscoelastic gel; P: precipitates at room temperature after
dissolution at 70 ꢀC; I: insoluble; nt: not tested).
P1 adsorption
P1 desorption
P2 adsorption
P2 desorption
0.8
0.6
0.4
0.2
0
Monomer
Bis-urea 1
Bis-urea 2
Styrene
I
I
Divinylbenzene
I
I
2-Methylstyrene
3-Methylstyrene
4-Methylstyrene
2,5-Dimethylstyrene
2,4-Dimethylstyrene
2,4,6-Trimethylstyrene
4-Isopropylstyrene
4-Tert-butylstyrene
4-Chlorostyrene
4-Fluorostyrene
F
P
P
F
P
F
P
P
I
P
I
G
I
nt
nt
nt
nt
nt
nt
nt
nt
nt
nt
nt
nt
G
0
0.2
0.4
0.6
0.8
1
4-Chloromethylstyrene
Vinylcyclohexane
Methylmethacrylate
Butylacrylate
Relative pressure (P/P°)
Fig. 1. Argon adsorption and desorption isotherms at 77 K, for P1 and P2 samples.
I
G
Isobornylacrylate
Butanedioldiacrylate
G
I
G
I
Butane is ideally suited for this, because it can be condensed at
273 K under atmospheric pressure, and it is characterized by a small
enough encumbering surface area (0.47 nm2). The adsorption
isotherms of butane at 273 K and argon at 77 K of P1 and P2 are
compared in Fig. 2. Remarkably, the adsorbed quantity of butane by
P1 and P2 is more than 20 times higher than that for argon (in the
relative pressure range 0 < P/P0 < 0.4). The BET surface areas for P1
and P2, calculated using butane as adsorbate at 273 K, are respec-
tively 65.4 m2/g and 111 m2/g. These values of surface area are
much higher than that obtained with argon as adsorbate at 77 K.
These results show clearly the high sensitivity of the microporous
properties of P1 and P2 to the measuring conditions. Moreover, if
we consider that the volume adsorbed at point B (Fig. 2) corre-
sponds to the filling of the micropores, we find microporous
volumes of 0.0095 and 0.015 cm3 gꢁ1, for P1 and P2 respectively. If
Information) [35]. The thermal stability of the nanotubes was then
checked by high sensitivity DSC: for 1 in isobornylacrylate, a tran-
sition temperature of 38 ꢀC was detected (see Fig. S2 in Supporting
Information), whereas no transition was detected for 2 in iso-
bornylacrylate (up to 80 ꢀC), thus confirming the improved stability
of the nanotubes in the case of bis-urea 2.
Finally, samples were prepared by photo-polymerization of
mixtures containing a bis-urea, isobornylacrylate, divinylbenzene
(as cross-linker) and DMPA (as photoinitiator). Here, two samples
are described, the first (P1) is composed of 10% 1 and 90% monomer
(isobornylacrylate þ divinylbenzene) (95/5). The second sample
(P2) is composed of 10%
2 and 90% monomer (isobornyl-
acrylate þ divinylbenzene) (95/5). It was checked by WAXS on the
final samples that no phase separation occurred during polymeri-
zation (see Fig. S3 in Supporting Information).
ꢀ
we assume that all bis-ureas in the samples form nanotubes of 7 A
inner diameter, we can estimate a maximum microporous volume
of 0.01 cm3 gꢁ1 (see Supporting information). The experimental
values are therefore of the correct order of magnitude. The higher
microporous volume and surface area of sample P2 compared to
sample P1 may be related to the better stability of nanotubes in the
case of bis-urea 2. It is indeed possible that a larger fraction of bis-
ureas are involved in the nanotubes in the case of bis-urea 2.
To confirm that the large quantity of butane adsorbed in the
samples is due to the presence of porosity, and not to swelling of
3.2. Characterization of porosity by gas sorption
In order to evaluate the microporosity, argon adsorption and
desorption of P1 and P2 were measured at 77 K, and the results are
shown in Fig. 1. In both cases, the obtained isotherm can be clas-
sified as type II and is consistent with a non-microporous material
[42]. The apparent BrunauereEmmetteTeller (BET) surface areas
for P1 (1.1 m2/g) and P2 (0.73 m2/g) are very low. Moreover, the
model of Horvath-Kawazoe [52] indicates a low microporous
volume (0.0005 cm3/g for P1 and 0.0004 cm3/g for P2) and the
complete absence of micropores with diameter lower than 1 nm.
This negative result obtained with conditions classically used to
characterize inorganic materials may in fact be an artefact due to
the experimental conditions of the adsorption and desorption
measurement. Indeed, during the experiment the sample is placed
under vacuum and at a very low temperature (77 K), which may
cause a change in the supramolecular structure of the nanotubes.
Knowing that the hydrogen bonds that maintain the tubular
structure of bis-ureas are very sensitive to temperature and pres-
sure, it is possible that if nanopores are present at room tempera-
ture and ambient pressure, they may reorganize and get obstructed
at low temperature and low pressure. A similar contraction
phenomenon has been observed in the case of nanoporous poly-
amides [15].
15
P1 butane adsorption
P2 butane adsorption
P1 argon adsorption
12
P2 argon adsorption
Series5
9
B point
6
3
0
0
0.1
0.2
0.3
0.4
Relative pressure (P/P°)
To confirm this hypothesis, we conducted new measurements of
adsorption and desorption, but with a gas that can be condensed at
a temperature close to room temperature and ambient pressure.
Fig. 2. Comparison between argon adsorption isotherm (obtained at 77 K) and butane
adsorption isotherm (obtained at 273 K), for P1 and P2 samples.

F. Ouhib et al. / Polymer 51 (2010) 3360e3364
3363
15
12
9
equilibrium pressure of xenon over the solid) and a second strongly
shifted near 220 ppm that we attribute to xenon in the polymer, in
agreement with the literature [43]. Unfortunately the limited shift
difference between the high field peaks in P2 and R3 does not allow
to distinguish xenon adsorbed in the bis-urea nanotubes from
xenon adsorbed in the polymer matrix. The reason is probably due
to the similar chemical nature of the walls of the nanotubes and the
polymer matrix and to the low microporous volume of the mate-
rials. However, the moderately larger peak width for P2 (680 Hz),
compared to R3 (550 Hz) may be an indication of a more hetero-
geneous environment in the case of P2, i.e. with xenon adsorbed
either in the bis-urea pores or in the polymer matrix.
P1
P2
R3
R4
6
3
0
4. Conclusion
0
50
100
150
200
250
300
Pressure (mmHg)
We report the straightforward photo-polymerization of poly-
acrylate films containing bis-urea based self-assembled nanotubes.
The presence of the bis-ureas is shown by butane adsorption (at
273 K and ambient pressure) to be responsible for the formation of
significant microporosity. This porosity is however not detected by
the classical argon adsorption procedure (at 77 K and low pressure).
This effect, attributed to the contraction of the material at low
temperature and pressure, may be of general concern for other
organic porous materials. One of the potential advantages of the
present materials is that the porosity results from the self-assem-
bled nanotubes and should therefore be independent of the matrix
mechanical properties. It should in particular be possible to adjust
the flexibility of the matrix by changing the monomer composition.
Fig. 3. Butane adsorption isotherms at 273 K, for P1, P2, R3 and R4 samples.
the polymer matrix by butane, the following blank experiments
were performed. The first reference sample (R3) contains the
methylated bis-urea 3 (Scheme 1), which cannot self-assemble into
nanotubes, and the second reference sample (R4) is the crosslinked
polyisobornylacrylate matrix containing no bis-urea. The four
samples (P1, P2, R3 and R4) were prepared strictly under the same
conditions. The reference samples were also analyzed by BET with
butane as adsorbent at 273 K. The obtained isotherms of adsorption
are compared in Fig. 3, and show that R3 and R4 adsorb much less
butane than P1 and P2. The calculated BET surface area for R3 and
R4 are 3.1 m2/g and 13 m2/g, respectively. The origin for the
difference in butane adsoption between R3 and R4 is presently
unknown, but the results clearly show that swelling of the matrix
by butane cannot account for the large adsorption measured for P1
and P2. This therefore confirms the porosity of P1 and P2 samples.
Acknowledgements
Financial support from the French ANR program (ANR-06-BLAN-
0159 zeoliplastic) is acknowledged.
Appendix. Supporting information
3.3. Xenon NMR measurements
Supplementary data associated with this article can be found in
the on-line version, at doi:10.1016/j.polymer.2010.05.047.
It is well known that 129Xe NMR of adsorbed xenon is a useful
technique to probe the microporosity of solids. Initially, this tech-
nique has been successfully used in the case of zeolites and then
extended to others microporous materials (such as porous silica,
pillared clays, activated carbons, coals, or polymers.) [43]. In this
latter case, it is generally necessary to work at fairly high xenon
pressures (e10 bar) and with pulse delays of several seconds. We
tried to use xenon NMR to confirm the existence of a microporosity
in the samples. Fig. 4 presents xenon spectra adsorbed under about
10 bar on P2 and its R3 reference. In both cases, we observe two
well defined signals: one near 4 ppm which is due to the xenon gas
(the value of the chemical shift allows us to determine the
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