From dense monomer salt crystals to CO2 selective microporous polyimides via solid-state polymerization

Article abstract of DOI:10.1039/c3cc47674j

Fully aromatic polyimides are synthesized via solid-state polymerization of the corresponding monomer salts. The crystal structure of salts shows strong hydrogen bonding of the reactive groups and thereby paves the way for solid-state transformations. The polycondensation yields copies of the initial salt crystallite habits, accompanied by the development of a porosity especially suited for CO₂.

Full text of DOI:10.1039/c3cc47674j

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From dense monomer salt crystals to CO2
selective microporous polyimides via
solid-state polymerization†
Miriam M. Unterlass,*ab Franziska Emmerling,c Markus Antoniettia and Jens Webera
Cite this:DOI: 10.1039/c3cc47674j
Received 6th October 2013,
Accepted 29th October 2013
DOI: 10.1039/c3cc47674j
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Fully aromatic polyimides are synthesized via solid-state polymeriza- intermediates.8 These monomer salts (3 in Scheme 1) are composed of
tion of the corresponding monomer salts. The crystal structure of salts the ionic forms of the comonomers and are fundamentally different
shows strong hydrogen bonding of the reactive groups and thereby from salt-type monomers such as sodium styrene-sulfonate, where only
paves the way for solid-state transformations. The polycondensation one ion, here styrene-sulfonate, is polymerized. Monomer salts 3 are
yields copies of the initial salt crystallite habits, accompanied by the well-known in polyamide synthesis, where they are often referred to as
development of a porosity especially suited for CO2.
‘‘nylon-salts’’,9 but far less investigated in the field of polyimides.
Thermogravimetric analysis (TGA),8 as well as the promising results
presented earlier by Imai et al., indicated that PIs can be synthesized in
the solid-state directly from the monomer species.10 The aim of the
present work is to shine more light on the processes involved in such
solid-state polymerization (SSP) and to evaluate the potential of the PI
materials for application in gas separation.
We focused on a monomer system that allows for the synthesis
of intrinsically microporous polymers,11 namely a combination of
9,90-bis(4-aminophenyl)fluorene (BAPF, 1) with either pyromellitic
acid dianhydride (PMDA, 2a) or 3,30,4,40-biphenyl-tetracarboxylic
acid dianhydride (BPDA, 2b). Monomer salts 3 were prepared by
precipitation from water, as depicted in Scheme 1 (for experi-
mental details see ESI†). The salt 3 is composed of a diamminium
cation (denoted H2BAPF2+) and a diacid–dicarboxylate anion
(denoted PMA2ꢀ and BPTCA2ꢀ, respectively). SEM images of the
salts show cuboid crystallites for 3a and platelet-like crystallites
for 3b, both are polydisperse (Fig. 1 and ESI†).
The purity and composition of the studied monomer salts were
ensured by FTIR spectroscopy, elemental analysis (EA) and TGA, as
reported previously.8 Both monomer salts were polycrystalline and of
high crystallinity, as verified by powder X-ray diffraction (PXRD) under
ambient conditions (see ESI†). Crystallite densities were determined
via analytical density-gradient ultra-centrifugation (AUC).12 A binary
solvent mixture of toluene and bromoform (ratio 0.71/0.29) was used
in combination with absorption optics. The density gradient of the
pure solvent mixture upon ultracentrifugation over the cell radius was
calculated according to a conventional protocol.13 In both cases we
find a single sharp peak, allowing for the exclusion of eventual
polymorphism (see ESI†). The obtained densities (1.39 g cmꢀ3 for
3a and 1.41 g cmꢀ3 for 3b) are in the typical range for organic
crystals.14 The structure of 3a (PMA-H2BAPF) was elucidated from
High performance polymers (HPPs) are an important class of
materials, mainly used for technological applications such as
microelectronics and aeronautics, but also for gas separation
applications.1 They combine extraordinary mechanical and/or
electronic properties with lightweight, solvent resistivity and
high temperature stability.2–4 These properties arise from mole-
cular features: HPPs consist of stiff, usually aromatic, monomeric
units, resulting in highly stable polymers.
Polyimides (PIs), such as the well known poly(4,40-oxydiphenylene-
pyromellitimide) aka Kaptont, are HPPs obtained by step-growth
polymerization. Thus, according to Carothers’ law,5 equimolarity of
the comonomers is essential in order to obtain a considerable degree of
polymerization and conversion. PIs are usually synthesized in solution
from dianhydrides and diamines (e.g. 1 and 2, Scheme 1), typically
using toxic solvents and catalysts (e.g. m-cresol and isoquinoline).
Clearly, alternative pathways minimizing environmental danger are
highly desirable,6,7 and options such as water and supercritical solvents,
or entirely solvent-free pathways find increasing use in numerous
synthesis fields. We recently investigated the hydrothermal synthesis
of PIs and found isolable diamminium dicarboxylic acid–dicarboxylate
a Max Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry,
Science Park Golm, D-14424 Potsdam, Germany
b Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt
9/BC/2, A-1060 Vienna, Austria. E-mail: miriam.unterlass@tuwien.ac.at
c
¨
¨
¨
Bundesanstalt fur Materialprufung und -forschung, Richard- Willstatter- Str. 11,
D-12489 Berlin, Germany
† Electronic supplementary information (ESI) available: Experimental procedures,
diffractograms of monomer salts, SEM, in situ high-temperature PXRD, structure
refinement details, AUC data, buoyancy measurements, FT-IR data, IAST calcula-
tions and gas selectivity measurements. CCDC 955505. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3cc47674j
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Scheme 1 Solid-state synthesis of PIs. (A) The diamine BAPF 1 forms a monomer salt 3 with an aromatic dianhydride 2 in water. Upon heating, solid-state
polycondensation takes place and the aromatic PI 4 is obtained (depicted as 4a). (B) Dianhydrides used in this study; pyromellitic acid dianhydride (PMDA) 2a and
3,30,4,40-biphenyltetracarboxylic acid dianhydride (BPDA) 2b. (C) Solid-state polymerization: monomer salt crystallites 3 are transformed to PI copies 4 by heating,
undergoing amorphization. Green inset: tetrameric arrangement of comonomers within the crystal structure of 3; orange inset: chain structure of the resulting PI 4a.
Fig. 1 Exemplary SEM images of 3a (PMA-H2BAPF) and the corresponding
polymer 4a (P)-PMDA-BAPF.
PXRD data as no monocrystals could be grown, using the program
FOX.15 To reduce the total number of degrees of freedom, the phenyl
rings were refined as a rigid group. The crystal structure of 3a was
solved by a simulated annealing procedure and further subjected
to a Rietveld refinement, which converged to Rwp = 5.873% (RBragg
=
4.658%) and agrees well with the measured powder patterns (Fig. 2a).
2ꢀ
3a crystallizes in the triclinic space group P1. PMA and H2BAPF2+
%
Fig. 2 Crystal structure of PMA-H2BAPF. (a) Superposition of observed (grey)
and calculated patterns (red); their difference in light grey. (b) Packing of PMA2ꢀ
and H2BAPF2+ molecules viewed along the c axis; (c) detailed view of a hydrogen
bonded tetramer. (b and c) Colour code: C-atoms: grey, O-atoms: red; N-atom:
blue; H-atom: omitted for visibility (see ESI† for larger representation).
molecules form tetramers (Fig. 2c) connected via two hydrogen bonds
between the amminium group and the carboxylate group (N–HꢁꢁꢁO,
2.8125 Å, 1171, N–HꢁꢁꢁO, 2.9832 Å, 1061). The tetramers are arranged
parallel to the a–b plane and form stacks along the c-axis (Fig. 2b). The
solution confirms the ammonium–dicarboxylate dicarboxylic acid
structure suggested by FTIR spectroscopy (see ESI†), and reveals a complete transformation of both monomer salts: –NH3+, –CO2H and
complex hydrogen-bonding pattern. Similar hydrogen bonding motifs –CO2ꢀ modes disappear entirely, and imide modes appear (see ESI†).
have been reported for amminium carboxylate salts of fumaric and Additional indication of the successful transition into PIs is provided
succinic acid.16 Unfortunately, the structure of 3b could not be solved, by the products’ aspect: in both cases, the colour changes from white
as the simulation did not converge to a minimum. However, we (3a and 3b) to bright orange PI 4a and greenish yellow PI 4b,
expect equally complex H-bonding. Clearly, the comonomers arranged respectively. SEM images (Fig. 1) of monomer salt 3a and PI 4a
within this tetramer have to move considerably to form the final PI clearly show that the external crystal habit is preserved without
chain (cf. 4a in Scheme 1).
melting phenomena: identical copies are obtained. Additionally,
Polyreactions were carried out by heating 3a and 3b to 270 1C and the density of 4a decreases to 1.274 g cmꢀ3 (obtained by buoyancy
250 1C, respectively, for 2 hours; neither an inert atmosphere nor measurements, see ESI†). The mass loss corresponds very well to 4
reduced pressure was necessary, since the elimination byproduct equivalents of water per monomeric unit, for a constant volume given
H2O phase separated and was further removed by evaporation. The by the obtained external copies. The obtained polymers are soluble
polymerization points of 3a and 3b are at 260 1C and 245 1C, in NMP after stirring at 80 1C for 2 hours (see ESI† for SEC analysis),
respectively, as determined by TGA.8 FT-IR analysis confirms the and rather high weight average molecular weights are observed.
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The solubility is expected to be a consequence of the fluorenyl units selectivities could arise as a consequence of a ‘‘molecular sieving
within the polymer backbone, i.e. the cardo principle.17
effect’’ due to very small pore sizes as observed previously in modified
The thermal transformation of the salts into PIs was monitored by zeolites.21 To see whether the as-made microporous polyimides are
HT-XRD using synchrotron irradiation. The recorded patterns (see indeed capable of separating CO2 and N2, column breakthrough tests
ESI†) reflect a transition from highly crystalline salts to amorphous were conducted. A gas mixture composed of CO2/N2 was allowed to
products. The overall amorphization occurs in both cases between flow through a column filled with 4a particles. The gas composition
250 1C and 275 1C, and has three main reasons. First, from the initial was analyzed by mass-spectrometry straight after the gas left the
arrangement of the two monomers in the salt’s crystal structure column (see ESI†). We found evidence for the successful separation of
(cf. Scheme 1 and Fig. 2b and c) a considerable molecular movement CO2/N2 gas mixtures, i.e. pure N2 left the column before the elution of
is necessary to react to the cyclic imide. Second, two equivalents of CO2. Desorption of N2 and CO2 by He purge could be achieved
water are liberated per imide ring formation. As water molecules move proving the physisorptive nature of the process.
considerably at these temperatures and also tend to phase separate
In summary, the successful solid-state transformations of monomer
from the formed PI chains, we believe that they play a major role in salts to aromatic microporous PIs at ambient pressure have been
the global amorphization. Third, the kinked structure of BAPF 1 evidenced. The crystal structure of 3a has been solved for the very first
should allow for a variety of energetically comparable conformations, time. The polymerization yields structural amorphization. The necessary
additionally disturbing chain-to-chain packing. While the internal internal structural rearrangements do not affect the external crystal
structure amorphizes, the macroscopic crystal habit persists, empha- habit and identical copies of the monomer salt crystallites can be
sizing the solid-state nature of the transformation. The amorphization obtained. The obtained microporous PIs are able to separate CO2 from
goes along with the development of a special type of microporosity, as N2, indicating that these amorphous polymeric materials are an inter-
evidenced by CO2 adsorption experiments conducted at 273.15 K and esting addition to the field of classic crystalline microporous materials. It
298.15 K. The porosity of the PIs synthesized via SSP shows almost can be expected that the presented synthesis routine will have an impact
equal gas uptake (B1.2 mmol gꢀ1 at 273.15 K; see ESI†) as chemically on the sustainable synthesis of a variety of functional HPPs opening
equivalent PIs that have been synthesized using solution-based thereby new possibilities in the area of science and technology.
routines previously, proving the intrinsic nature of the micropores.11,18
This work was supported by the ERC Senior Excellence Grant
Note that the PI particles are apparently non-porous in nitrogen ‘‘HYDRAChem’’, project no. 227639, and the Max Planck Society.
adsorption experiments at 77.4 K, i.e. the developing intrinsic porosity The authors are indebted to Dr Antje Wilke, Martin Blu¨mke and
is very specifically suited for CO2. Consequently, we explored the Dr Anthony Bell (B2 beamline, DESY Hamburg) for experimental
versatility of the materials for gas separation applications. We calcu- support at the synchrotron. Jessica Brandt, Marlies Graewert and
¨
lated the separation selectivity a for the gas pair CO2/N2 based on CO2 Antje Volkel are acknowledged for synthetical support, SEC measure-
and N2 single gas adsorption isotherms obtained at 273.15 K and ment and AUC measurement and data treatment, respectively.
298.15 K, respectively, using the ideal adsorbed solution theory (IAST)
methodology (see ESI†).19 The PIs showed a high a(CO2/N2) of >1000
at 273.15 K, and a a(CO2/N2) of >300 at 298.15 K (gas composition of
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Fig. 3 (a) IAST selectivity of 4a for a gas mixture (0.15/0.85 of CO2/N2) at
298.15 K; (b) single gas adsorption isotherms of CO2 and N2 on 4a together with
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