Building ultramicropores within organic polymers based on a thermosetting cyanate ester resin

Article abstract of DOI:10.1039/b909424e

Ultramicropores with high surface areas (>530 m² g ^-1) and narrow micropore size distribution (4-6 ) were engineered within a new cyanate ester resin, extending the microporous concept (<20 ) to general thermosetting resins in the area of polymer chemistry.

Full text of DOI:10.1039/b909424e

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Building ultramicropores within organic polymers based
on a thermosetting cyanate ester resinw
Bufeng Zhang and Zhonggang Wang*
Received (in Cambridge, UK) 12th May 2009, Accepted 25th June 2009
First published as an Advance Article on the web 14th July 2009
DOI: 10.1039/b909424e
Ultramicropores with high surface areas (4530 m2
narrow micropore size distribution (4–6 A) were engineered
within a new cyanate ester resin, extending the microporous
g
À1) and
H2 and fast selective gas adsorption into account, ultra-
microporous materials have attracted much attention in H2
storage4b,4c,10 and molecule-size separation.11 Budd et al.4c
determined that micropores in their CTC-PIMs are centered
around 7 A. Also, Germain et al.6f most recently reported
hypercrosslinked polyanilines with pore sizes that range from
2.9–3.6 A. There are numerous examples of ultramicroporous
materials based on inorganic and organic–inorganic hybrid
materials.12 However, types of ultramicroporous organic
polymers with high surface areas are still limited to the above
examples.
˚
˚
concept (o20 A) to general thermosetting resins in the area of
polymer chemistry.
Research into microporous organic polymers, which have pore
sizes smaller than 20 A and high surface areas, is currently
driven by wide potential applications in catalysis, size-selective
absorbents, gas storage, etc.1 Compared with conventional
highly porous materials like zeolites which are usually
prepared through a template strategy, micropores in organic
polymers are directly engineered by their chain skeleton with
the character of highly rigid, bulky, nonplanar, and contorted
architectures.2 It is also identified that the pore structure can
be well defined via formation of rigid networks.3 Thus, efforts
to prepare these materials are mainly focused on several types
of special networks including polymers of intrinsic micro-
porosity (PIMs),2,4 covalent organic frameworks (COFs),5
hypercrosslinked polymers (HCPs)6 and conjugated microporous
polymers (CMPs).7
In this communication, we report a new facile approach to
prepare ultramicroporous polymers based on one of the most
common thermosetting networks—cyanate ester resins, which
are widely used in many fields such as electronics, aerospace
and adhesive industries.13 They exhibit high surface areas
(4530 m2 gÀ1), a median pore diameter of 4.7 A and narrow
micropore size distribution (4–6 A).
Usually cyanate esters undergo thermal polycyclotrimeriza-
tion in both bulk and solution to give polycyanurates, i.e.,
sym-triazine rings linked by aryl ether linkages, resulting in a
uniformly cross-linked structure. In this work,
a new
It is well known that thermosetting networks have provided
the broadest scope for synthetic variation of both chemical
functionality and structural topology. In the past decade, a
number of commercial thermosetting resins, such as epoxy and
cyanurate, have been successfully used to prepare macro-
porous polymers,8 but reports of microporous polymers based
on these resins are extremely rare. As a matter of fact, the
creation of microporosity within common thermosetting resins
is of fundamental significance. First, their widely accessible
monomer precursors not only make the preparation of micro-
porous polymers more convenient and cost-effective, but also
they provide numerous skeletal structures which should
facilitate fine control over the microporous properties. Second,
the mild curing conditions provide the possibility of introducing
functionalities into the porous materials. Third, good
processability of these thermosets should allow them to be
readily fabricated into useful forms. It is, therefore, strongly
desirable to expand the concept of microporosity to the
common thermosets in the area of polymer chemistry.
tetrafunctional cyanate ester tetrakis(4-cyanatophenyl)silane
(TCS) is used as the starting monomer for the highly efficient
Moreover, micropores are classified into ultramicropores
(pore widths o 7 A) and supermicropores (pore widths of 7 to
20 A).9 Taking advantage of high enthalpies of adsorption for
Department of Polymer Science and Materials, School of Chemical
Engineering, Dalian University of Technology, Dalian, 116012,
P. R. China. E-mail: zgwang@dlut.edu.cn; Tel: +86 411 39893859
w Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b909424e
Scheme
1 Cyclotrimerization reaction of TCS leading to the
formation of a polycyanurate network.
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network-building process (Scheme 1). TCS exhibits high
chemical reactivity owing to the high content of cyanate ester
groups per molecule, and the cure process actually occurs just
above the melting point of the monomer (ESIw, Fig. S1). To
ensure a uniform three-dimensional network, we employed the
following standard baking protocol designed to achieve near-
full cure of TCS: 170 1C for 4 h, 190 1C for 4 h, 230 1C for 8 h,
and lastly 270 1C for 20 h. Within a fully cured TCS network,
every three neighboring rigid tetrahedral units are held
together by a triazine ring. There is only a single oxygen-atom
bridge between the two types of branch points, i.e., tetrahedral
branchpoints and triazine branchpoints, which produces a
firm covalent crosslinked framework. Additionally, with
respect to the rigid framework whose connecting struts are
all identical in length, the higher density of branch points
means smaller and more uniform voids can be obtained in the
limited space constructed by the rigid tetraphenylsilane units.
However, it is rather difficult to obtain high conversions of
the cyanate groups of TCS to the triazine rings in bulk
polymerization even under a high post curing temperature
and long curing time. Although the curing mechanism of the
cyanate ester is not completely clear, the existence of a stable
curing intermediate unit [–OCONC(NH2)O–] in the early
stage of polymerization was confirmed.14 Fig. 1 presents the
possible network-building process. Because of the branchpoint
role of the monomer itself and the close packing of the reactive
groups in the bulk, the time taken to reach the gel point is so
short that an infinite network may have been formed before the
conversion of all intermediate segments to triazine rings—new
branchpoints. Then the steric and geometric constraints
imposed by the branch points and crosslink points act against
the approach of residual cyanate groups to intermediate
segments, resulting in a final defective triazine-based network
containing long flexible straight segments. In the IR spectrum
(Fig. 2), besides the characteristic triazine ring bands at
1560 cmÀ1 and 1362 cmÀ1, absorption peaks at 3482 cmÀ1
and 3392 cmÀ1 contributed by stretching of –NH2 and at
1719 cmÀ1 by CQO stretching were also observed, indicative
of the presence of [–OCONC(NH2)O–] intermediate segments.
The porosity of this cured polymer was probed by nitrogen
sorption at 77 K. As expected, these defects made the triazine-
based network less rigid, and the microporosity was reduced
Fig.
2 FT-IR spectra (A) TCS; (B) network cured in bulk;
(C) network cured in 20 wt% solution; (D) network cured in 10 wt%
solution.
due to pore collapse caused by the remaining flexible
intermediate segments. To achieve full conversion of
cyanate groups to triazine rings, we then investigated the
curing reaction in a thermodynamically preferred solvent,
diphenylsulfone.
In solution, the reduced concentration of functional groups
will effectively delay the time taken to reach the gel point and
consequently benefit the sufficient conversion of curing inter-
mediate segments to triazine rings. As shown in Fig. 2, with
the reduction in reaction concentration, the amount of
residual cyanate ester groups and intermediate segments in the
cured TCS was decreased, and almost completely disappeared
in
a 10 wt% solution. Thermogravimetric results also
demonstrate that the thermal stability of the cured products
in solution are much higher than that cured in bulk due to
the presence of intermediate segments in the networks
(ESIw, Fig. S2). The ideal network-building process in
dilute solution can be described as follows (Fig. 1): in the
initial stage of reaction, nucleation of growth occurs at
random through polytrimerization of the tetrahedral
monomers, resulting in large numbers of pre-gel branched
particles with micropores, then these branched particles meet
and further connect to one another on reactable sites to form
the final infinite network.
Nitrogen adsorption/desorption isotherms for cured
networks obtained from various solution concentrations are
given in Fig. 3a. All samples polymerized in dilution solution
(below 10 wt%) show a high uptake at very low relative
pressure, corresponding to gas sorption in micropores. Their
isotherms also exhibit weak hysteresis, the desorption curve lies
above the adsorption curve, and can be classified as Type 1B,15
Fig. 1 Schematic representation of the possible network-building
process in bulk and in solution.
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Notes and references
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The authors are grateful for the financial support of the
National Science Foundation of China (No. 50673014 and
No. 20874007) and the Program for New Century Excellent
Talents in University of China (No. NCET-06-0280).
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Chem. Commun., 2009, 5027–5029 | 5029