Exploration of the versatility of ring opening metathesis polymerization: An approach for gaining access to low density polymeric aerogels

Article abstract of DOI:10.1039/c2ra21214e

We report the preparation of low density polymeric aerogels using the ring opening metathesis polymerization (ROMP) approach to copolymerize dicyclopentadiene (DCPD) with norbornene-based monomers (NB-R) employing a first generation Grubbs' ruthenium-base

Full text of DOI:10.1039/c2ra21214e

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PAPER
Exploration of the versatility of ring opening metathesis polymerization: an
approach for gaining access to low density polymeric aerogels{
Sung Ho Kim,*a Marcus A. Worsley,a Carlos A. Valdez,a Swanee J. Shin,b Christoph Dawedeit,b Tom Braun,b
Theodore F. Baumann,a Stephan A. Letts,b Sergei O. Kucheyev,b Kuang Jen J. Wu,b Juergen Biener,b Joe H.
Satcher Jr.a and Alex V. Hamzab
Received 19th June 2012, Accepted 19th July 2012
DOI: 10.1039/c2ra21214e
We report the preparation of low density polymeric aerogels using the ring opening metathesis
polymerization (ROMP) approach to copolymerize dicyclopentadiene (DCPD) with norbornene-
based monomers (NB-R) employing a first generation Grubbs’ ruthenium-based catalyst. The ROMP
approach offers an attractive synthetic method that enables the fabrication of low-density (0.02–
0.05 g cm23), uniform thickness aerogel coatings on non-planar substrates. First, we explore the effect
of crosslinking in the polymer backbone on the uniformity of the gel coatings formed under shear by
either adding a multi-norbornene based crosslinker (NBn-R) to increase, or linear NB-R comonomers
to decrease, the degree of crosslinking, respectively. We observed that adding linear monomers
dramatically improved the uniformity of the gel films which we attribute to cross-linking induced
changes in the rheological properties at the gel point. Second, the copolymerization of DCPD and
NB-R with a different pendant group also causes a significant change in the morphology of the
PDCPD-based aerogels by modifying the lengths of the strands in the fibrous polymer network. The
effect of NB-R addition on the pore structure of the aerogels is discussed in the context of a molecular
and interparticle crosslinking model. Finally, (bis)iodo-norbornene was synthesized to demonstrate
the feasibility to fabricate functionalized aerogels by using our copolymerization approach. Our
results highlight the potential of the ROMP-based copolymerization approach as a facile and versatile
route to functionalized low density polymeric aerogels.
their tenuous solid network. In a similar fashion, the preparation
1. Introduction
of organic and polymeric aerogels has been achieved by either (i)
Aerogel materials with large intrinsic surface areas and ultralow
physical phase inversion (or coagulation) of polymers from the
densities have become attractive candidates for a wide range of
liquid phase to solids by changing the solvent/non-solvent pairs or
applications including supercapacitors and batteries, thermal/
(ii) polymerization of multi-functional organic species into three-
dimensional cross-linked polymer networks.5–9 While most of the
electrical insulators, advanced catalyst supports, absorbents and
carriers, and hydrogen storage.1–4 Organic and polymeric aerogels
previous work has focused on the utilization of bulk aerogels, we
were first reported as the counterparts of inorganic aerogels.5–9
are currently exploring the fabrication of low-density, uniform
The sol–gel synthesis of inorganic aerogels typically involves the
thickness (from 10 mm to 100 mm) aerogel coatings on the inside of
cylindrical and spherical substrates.10,11 Such porous polymer
transformation of molecular precursors into highly cross-linked
inorganic gels that are subsequently dried using special techniques
coatings open the door to many interesting new applications such
(i.e., supercritical drying, freeze drying, etc.) in order to preserve
as superhydrophobic surfaces, anti-reflection coatings, column
chromatography, and porous polymer membranes.12 Our specific
aChemical Sciences Division, Lawrence Livermore National Laboratory,
P. O. Box 808, L-235, 7000 East Avenue, Livermore, CA 94550, USA.
interest is in the fabrication of inertial confinement fusion (ICF)
targets.11 These targets consist of a small spherical capsule, whose
E-mail: kim61@llnl.gov; Tel: +1-925-423-2072
bNanoscale Synthesis and Characterization Laboratory, Lawrence
Livermore National Laboratory, P. O. Box 808, L-235, 7000 East Avenue,
Livermore, CA 94550, USA
inside wall is coated with a thin layer of a low density foam that is
used (i) to support and define the shape of the cryogenic liquid or
solid hydrogen fuel for the fusion reaction and/or (ii) as a scaffold
to support dopants for diagnostics. Briefly, our strategy to
fabricate aerogel coatings consists of filling a prefabricated shell
with the required amount of the sol precursor followed by the
continuous rotation of the shell to form a smooth layer of uniform
thickness during the gel formation.11 Cross-linking provides the
{ Electronic supplementary information (ESI) available: Details on the
experimental procedure for the synthesis of cyclic olefin monomers,
rheological measurements of G9, G99, and g* during the gelation of
P(DCPD-r-NB) (100/x) samples, the procedure to determine the molar
ratios of NB-R to DCPD in copolymer gels from the RBS results, and the
gel time, photos, and morphologies of P(DCPD-r-NB-R) with a different
composition. See DOI: 10.1039/c2ra21214e
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required control over the viscosity of the precursor solution at the
sol–gel transition.
coating. The 4 MV ion accelerator (NEC, model 4UH) at
Lawrence Livermore National Laboratory was used to perform
Rutherford backscattering spectrometry (RBS) measurements
Here, we use the ring opening metathesis polymerization
(ROMP) approach for the synthesis of polymeric aerogels. To
our knowledge, there have only been a few previous examples of
applying the ROMP approach to prepare bulk porous aerogel
materials.13,14 Lee and Gould first demonstrated the feasibility of
polydicyclopentadiene (PDCPD) aerogels as new insulation
materials.13 Leventis et al. reported a novel low temperature
synthetic route to extremely robust polyimide aerogels via ROMP
of a crosslinkable norbornene end-capped diimide monomer.14a
Although these works demonstrate the ability to produce
monolithic aerogels, our goal of forming uniform aerogel coatings
adds another layer of complexity. For example, the coating of
non-planar surfaces with thin and homogeneous aerogel films
requires continuous rotation of the object to be coated. Without
proper control over viscosity and gel time, shear forces can
damage the growing gel networks and prevent the formation of
uniform coating.10 Using the example of polydicyclopentadiene-
based gels, we demonstrate that both the moduli and viscosity at a
gel point can be tuned by modifying the degree of cross-linking
through the copolymerization of dicyclopentadiene (DCPD) with
a linear norbornene comonomer. The effect of monomer addition
on the morphology of the resulting aerogels was investigated by
using substituted norbornene monomers (NB-R) with various
pendant groups R. Finally, a (bis)iodo-norbornene monomer was
added to demonstrate the fabrication of functionalized aerogels
and to give further quantitative understanding on this copolymer-
ization process. The goal of our study is to provide a simple and
versatile synthetic route to tailor the gelation behavior, pore
structure, and the morphology of the final polymer network
expanding the applicability of low density polymeric aerogels.
4
with 1.5 MeV He+ ions incident normal to the sample surface
and backscattered into a detector at 164u relative to the incident
beam direction. During the ion-beam analysis, samples were
neutralized by coirradiation with a 500 eV electron beam.
Analysis of the RBS spectra was done with stopping powers and
scattering cross sections from the RUMP code15 with the
assumption of a specimen composition of C10H12Ix. BET surface
areas, pore volumes and pore size distributions were measured
by nitrogen absorption porosimetry with
a Micromeritics
Instrument ASAP 2000 after degassing at 70 uC for 12 h. Bulk
densities were calculated from the weight and the physical
dimensions of the samples.
2.2 Synthesis of norbornene-based cyclic olefin monomers
Scheme 1 summarizes the synthesis of the norbornene-based cyclic
olefin crosslinker and comonomers used in this study. First, the
norbornene-based crosslinker, 1,4-di(exo-bicyclo[2.2.1]hept-5-en-2-yl)
benzene (2), was prepared as previously reported (Scheme 1a).16
Purification was carried out by column chromatography (ethyl
acetate/hexanes) followed by re-crystallization in ethyl acetate.
Next, several norbornene monomers with different lengths of the
alkyl groups were prepared similarly to the synthesis of 5-methox-
ymethylbicyclo[2.2.1]hept-2-ene (4) (Scheme 1b).17,18 Other alkyl
substituted norbornene-based monomers 5–7 were synthesized by
reacting 5-norbornene-2-methanol (3) with corresponding alkyl
halides such as hexyl bromide, dodecyl bromide, and benzyl
bromide in the presence of sodium hydride. The purification of the
monomers was carried out by extraction followed by silica gel flash
column chromatography. Finally, iodo-norbornene monomer 9
was prepared and used as a monomer with a strong electronic
interaction. The synthesis of the iodo-norbornene monomers was
accomplished by initially tosylating the corresponding norbornene
alcohol 3 in the presence of 4-(dimethylamino)pyridine (DMAP)
followed by tosyl displacement using sodium iodide (Scheme 1c).19–21
As a practical way to double the number of chemical function-
alities in one monomer, 5-norbornene-2,3-dimethanol 10 was used
to prepare the bis-tosylated norbornene monomer 11 and bisiodo-
norbornene monomer 12 as a typical example of functional
monomers for the monitoring of gel formation. Full details of the
synthesis and characterization of the norbornene-based mono-
mers are described in the ESI.{
2. Experimental
2.1 Materials and general characterization
Dicyclopentadiene (DCPD, Aldrich), norbornene (NB, 99%,
Aldrich), 5-norbornene-2-methanol, (3, 98%, Aldrich), 5-nor-
bornene-2-exo,3-exo-dimethanol (10, 97%, Aldrich) and a first
generation Grubbs’ catalyst, bis(tricyclohexylphosphine)benzyli-
dene ruthenium dichloride (+97%, Aldrich), and other chemicals
were used as received unless noted otherwise. As supplied, the
DCPD and monomer 3 are a mixture of mostly endo and exo
isomers. They were used without further purification. Toluene
(anhydrous, 99.8%, Aldrich) was degassed by bubbling with high
purity nitrogen prior to use.
2.3 Preparation of poly(dicyclopentadiene-random-substituted
norbornene) P(DCPD-r-NB-R) aerogels
1H NMR spectra were obtained with a Bruker Avance 600
instrument at 600 MHz. Rheological properties of precursor
Low-density (0.02–0.05 g cc21) P(DCPD-r-NB-R) gels were
prepared from a ring opening metathesis polymerization (ROMP)
of DCPD and NB-R in toluene employing a first generation
Grubbs’ catalyst as the catalyst/initiator (Scheme 2). Here, NB-R is
a generic term for norbornene (NB) and other NB derivatives.
Different P(DCPD-r-NB-R) (100/x) (wt/wt) copolymer wet-gels
were obtained by controlling the feed ratios of DCPD and NB-R
by weight. For example, to fabricate 50 mg cc21 P(DCPD-r-NB-R)
(100/20) (wt/wt) aerogels, separate 50 mg cc21solutions of DCPD
and NB-R monomers in toluene were prepared and then mixed as
needed (0.83 mL of the DCPD and 0.17 mL of the NB-R solution
solutions during the gelation were characterized with
a
Rheometrics Fluids Spectrometer RFS 8400 in a dynamic-
time-sweep mode with a Couette geometry (a 17 mm radius cup
and a 16 mm radius bob). Measurements were carried out at
room temperature with a strain amplitude of 5% of the 1 mm gap
between the cup and the bob and a frequency of 1 radian per
second. To minimize solvent evaporation, the cup was covered
with a custom built lid. The morphology of the monolithic
aerogels was investigated with a Jeol JSM-7401F scanning
electron microscope (SEM) at an acceleration voltage of 2–3 kV
in a lower secondary electron image (LEI) mode with no sputter
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Scheme 1 The synthesis of cyclic olefin monomers.
for a 100/20 wt/wt aerogel). After adding an aliquot of the catalyst
(0.1 mg) dissolved in toluene, the mixture was gelled in a sealed vial
under ambient conditions.
synthesis of the norbornene-based crosslinker 2 from a carbon–
carbon coupling reaction of norbornadiene 1 with diiodobenzene,
as previously reported.16 Second, various cyclic-olefin monomers,
NB-R, were prepared from 5-norbornene-2-methanol (3) and
5-norbornene-2,3-dimethanol (10) in order to investigate the effect
of a pendant group on the morphology of gels (Scheme 1b and 1c).
The alcohol functionalized norbornene monomers can be used to
form derivatives having various functional groups, as previously
reported.17,18 Norbornene-based monomers were selected for their
mild polymerization conditions and ability to form statistical
copolymers with dicyclopentadiene rings. In this study, we
prepared alkyl norbornene monomers (4–7) by protecting a
pendant alcohol group with various alkyl halides including methyl
iodide, hexyl bromide, dodecyl bromide, and benzyl bromide,
respectively (Scheme 1b). Based on the difference in the polarity of
reactant 3 and the protected adducts 4–7, thin layer chromato-
graphy (TLC) and column chromatography were used for the
2.4 Supercritical CO2 drying
Wet gels were first placed into an agitated acetone bath for
2–3 days and then transferred to a Polaron critical point dryer.
Acetone-filled gels were exchanged with a continuous flow of
liquid CO2 at 850 psi. Once the solvent was completely removed,
the temperature and pressure in the dryer were increased to the
supercritical regime (50 uC and y1500 psi) and kept for 2–4 h.
The pressure was then allowed to slowly decrease to atmospheric
pressure while keeping the temperature constant. Dry aerogels
were subsequently recovered by slowly cooling down to room
temperature.
3. Results and discussion
1
analysis and purification of the crude product. H NMR spectra
before and after the protection reaction show the characteristic
shift of the peaks assigned to the –C(2)H–, –C(2)H–CH2O–R, and
–C(5)HLC(6)H– bonds. Finally, a norbornene monomer 8 with a
good leaving group was prepared from the reaction of 3 with
p-toluenesulfonyl chloride (TsCl) in the presence of a base such as
triethylamine (TEA), pyridine, or 4-(dimethylamino)pyridine
(DMAP) (Scheme 1c). The use of DMAP or pyridine was found
to be suitable to promote the reaction and led to a quantitative
Considering that pure PDCPD polymers have a partially cross-
linked structure,22–29 our approach was to control the degree of
crosslinking in the polymer backbone through either (i) the use of a
more reactive crosslinker to increase the crosslinking density or (ii)
the copolymerization of DCPD with a linear monomer to decrease
the degree of crosslinking. The norbornene-based crosslinker 2
was first used as a control to overcome the inherent limitation in
the crosslinking of DCPD molecules. Scheme 1a shows the
Scheme 2 Ring opening metathesis polymerization of DCPD and NB-R.
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tosylation, while TEA resulted only in a partial conversion. This
1
reaction was monitored by H NMR spectroscopy, in which the
on the uniformity of PDCPD films.10 Here, we extend our study
to higher NB concentrations and to other NB-based monomers,
NB-R. For comparative purposes, we include our previous
results in the following discussion. The effect of NB-R additions
was first evaluated by studying the formation of thin films in
rotating horizontal glass vials (y20 mL) containing a precursor
solution (1 mL) on a compact roller system, commonly used in
cell culture studies (Fig. 1). Fig. 1b through 1d show the results
of gel coating experiments with pure PDCPD in conjunction
with a norbornene based crosslinker and P(DCPD-r-NB-R)
copolymer gels. These figures reveal that pure PDCPD gels do
not form monolithic thin films under these conditions, instead,
the vial was coated with irregularly-shaped small lumps of the gel
and contained a significant amount of fluid suggesting that the
lumps consisted of a higher density gel. The norbornene based
crosslinker 2 does not show the superfluous fluid, but does not
form a film probably due to its extremely fast gelation behavior.
In sharp contrast, a uniform coating of a transparent wet-gel
layer on the inside wall of the glass vial was observed from
P(DCPD-r-NB-R) (100/20) at 50 mg cc21, regardless of the
structure of the –R substituents. In order to understand this
unique change in gelation behavior under rotation and shear, we
investigate the effect of norbornene addition on the rheological
properties of P(DCPD-r-NB) gels, as shown in Fig. 1e through
1g (Fig. S1 in the ESI{). Isothermal dynamic time tests at a given
frequency and strain show that both shear moduli and complex
viscosity increase over time. A sudden increase in moduli and
viscosity in a short period of time is a typical change seen in
gelation. In this study, we determined the gel point, or the initial
formation of an infinite network, to be the time when the storage
shear modulus, G9, exceeds the loss shear modulus, G99,24
although several different protocols have been reported in the
literature.30 Our rheological measurements reveal that the time at
which G9 and G99 cross over drastically increases with increasing
NB concentration up to 10 wt % and then levels off even after
further NB addition. Both moduli and viscosity at the crossover
point increase almost linearly up to 40 wt %, as summarized in
Table 1. The higher viscosity of P(DCPD-r-NB) near the
gelation point reduces the shear experienced by the growing
polymer network and makes it possible to fabricate a uniformly
coated wet-gel layer on specific substrates, as reported in our
previous work.10 The BET surface areas and bulk densities of the
aerogel samples are also summarized in Table 1. It is worthy to
mention that norbornene has a much higher ROMP reactivity
(kobs of 0.42 s21) than endo-DCPD (kobs of 0.019 s21) under the
same conditions.22 Surprisingly, the gel time for PDCPD (491 s)
is shorter than for P(DCPD-NB) gels (y2100 s). Although
PDCPD aerogels show no volume change during the gelation
and supercritical CO2 drying process, we observe that the
measured density (y37 mg cm23) is lower than the target density
(y50 mg cm23). This suggests the incomplete conversion of
monomers to gels, for example due to the early precipitation of
highly crosslinked PDCPD networks caused by their limited
solubility in toluene. Some amounts of shrinkage of the gels
during the acetone exchange process and the CO2 drying process
were observed in the P(DCPD-r-NB) samples with more than
20 wt% NB, causing a sudden decrease in the BET surface areas
and a deviation from the target bulk density (see Fig. S2 in the
ESI{). Preventing volume shrinkage at higher NB concentrations
resonances at 5.90 ppm, 3.75–3.30 ppm and 2.30–1.60 ppm,
corresponding to –C(5)HLC(6)H–, –C(2)H–CH2OH, and
–
C(2)H– in 3, respectively, clearly shifted to 5.69 ppm, 4.08–
3.57 ppm, and 2.40–1.73 ppm without overlapping those of the
starting material. Norbornene activated with a good leaving group
provides us with a useful platform for the introduction of
additional chemical functionalities in the polymer. For example,
iodo-norbornene monomer 9 was prepared by reacting monomer 8
with sodium iodide. This reaction was confirmed from the change
in the chemical shift and the disappearance of –OTs at 7.78–
7.34 ppm. Norbornene-diol monomer 10 was also selected as a
starting material to double the number of functionalities in one
monomer. Direct reaction of monomer 10 with TsCl and sub-
sequent precipitation of the mixture in diethyl ether gave bis-
tosylated norbornene 11 as a white powder. Further purification of
the residual solution by column chromatography isolated mono-
substituted tosyl norbornene and cyclic ether norbornene bypro-
1
ducts, as identified by H NMR. Then monomer 11 was reacted
with excess NaI and transformed into bisiodo-norbornene
monomer 12 in quantitative yield.
Scheme 2 shows the synthetic scheme of P(DCPD-r-NB-R)
copolymer gels from the ROMP reaction of a mixture of DCPD
and NB-R in toluene. In proposing the chemical structures of the
copolymers, it is assumed that the strained norbornene-derived
double bond in the DCPD monomer is much more reactive than
the cyclopentene-derived double bond, and the olefin addition
contributes to the formation of crosslinked PDCPD.27 Modified
PDCPD-based aerogels were prepared by controlling the compo-
sition of the mixture of cyclic-olefin monomers including DCPD
and NB-R. The gelation behavior of the DCPD–NB-R mixture
showed a dependency on the ratio of NB-R to DCPD as well as
the concentration of the olefin monomers and the catalyst,
consistent with a previous study of pure PDCPD aerogels.10,13
Bulk PDCPD and P(DCPD-r-NB-R) copolymer wet-gels were
stable even after a few weeks, and pure NB solutions in the same
conditions did not form gels. In the present study, the ratio of
the catalyst to monomers and the total monomer concentration
were fixed as 0.002 (wt/wt) and 50 mg cc21, respectively, unless
otherwise noted. Both pure DCPD and mixtures of DCPD and
NB-R were gelled at room temperature, and the catalyst did not
appear to have any acute air or moisture sensitivity. It is worthy to
mention that the use of a first generation Grubbs’ catalyst over a
second generation Grubbs’ catalyst is due to its shorter gelation
time in our typical experimental conditions (not in a glove box).
Typically, the addition of NB-R monomers increases the gelation
time from y10 min for pure DCPD to a few hours up to a few
days. The addition of the NB-based crosslinker 2, on the other
hand, reduces the gelation time to y1 min (see Table S1 in the
ESI{). During the gelation of pure DCPD or 2, there was a phase
transition from a transparent solution to an opaque gel due to
the light scattering by polymerized wet-gels. However, wet-gels
prepared from a higher loading of the NB-R monomer remained
transparent after gelation. All the wet-gels were supercritically
dried to form white monolithic aerogels.
The ability to form low-density thin film aerogel coatings with
a thickness of y10 mm to 100 mm is crucial to our intended
application. Recently, we reported on the effect of NB addition
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Fig. 1 (a) Schematic representation of the coating experiment of the precursor solution. (b–d) Typical examples of the formation of wet-gels and
supercritical CO2 dried aerogel films prepared from pure DCPD, a multi-norbornene (nNB)-based crosslinker, and a mixture of DCPD and NB (or
NB-R). (e–g) The influence of NB addition on the formation of P(DCPD-r-NB) copolymer gels under shear and the rheological properties of the shear
moduli and complex viscosity at the gel point.
is the subject of current work that will be described in a future
publication.
Copolymerization with norbornene (NB) decreases the cross-
linking density of pure PDCPD-based gels. Fig. 2d through 2f
show the effect of the NB concentration (10 to 40 wt %) on the
morphologies of dried 50 mg cc21 P(DCPD-r-NB). P(DCPD-
r-NB) copolymer aerogels have a fibrous three-dimensional
porous network similar to that of pure PDCPD. However, the
length of the individual fiber strands decreases from a few mm in
the case of pure PDCPD to hundreds of nm in P(DCPD-r-NB)
gels with higher concentrations of the NB monomer. This is
consistent with the observation that NB addition results in the
formation of clear wet-gels.
In addition to modifying the rheological behavior, the
copolymerization also impacted on the morphologies of the
PDCPD-based aerogels. Fig. 2a through 2c are SEM images of
dried PDCPD aerogels with different DCPD concentrations of
50, 30, and 20 mg cc21. At a concentration lower than 10 mg cc21
,
the reaction was too slow to form gels within 7 days. Regardless of
the target concentrations, pure PDCPD aerogels have a similar
three-dimensional porous network of randomly oriented fibrous-
shape structures with a non-uniform irregular pore structure.13
Table 1 General characteristics of P(DCPD-r-NB) aerogels
Feed ratio of DCPD to NBa
b
b
b
tgel
(s)
G9gel
(Pa)
g*gel
(Pa s)
Surface areac
Total pore volumec
Bulk densityd
Sample
by weight
by moles
(m2 g21
)
(m2 g21
)
(mg cm23
)
P(DCPD-r-NB)
100 : 0
100 : 0
491
0.07
7.45
11.22
16.45
20.97
0.08
10.53
15.81
23.23
29.59
103
114
102
57
0.31
0.21
0.19
0.07
0.02
37 ¡ 4
48 ¡ 8
82 ¡ 13
102 ¡ 37
244 ¡ 48
b
100 : 10
100 : 20
100 : 30
100 : 40
100 : 14
100 : 28
100 : 42
100 : 56
2086
2110
2148
2055
22
DCPD and NB monomers were separately dissolved to have a concentrations of 50 mg cc21 in toluene and mixed to a final composition. The
a
gel point of the system was determined from the crossover of the storage shear modulus, G9, and the loss shear modulus, G99, measured by
c
d
rheometry. The surface area and total pore volume of the aerogels measured by BET. Bulk densities were calculated from the weight and
physical dimensions and averaged with the multiple sample measurement
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Fig. 2 Effect of the composition of precursor solutions on aerogel morphologies. (a–c) Pure PDCPD aerogels prepared from (a) 50, (b) 30, and (c)
20 mg cc21 solutions and (d–f) P(DCPD-r-NB) copolymer aerogels prepared from mixtures with different DCPD–NB ratios of (d) (100/10), (e) (100/
20), and (f) (100/40) (wt/wt) at 50 mg cc21
.
The effect of monomer addition on the morphologies of
aerogels was further explored by using various norbornene
monomers (NB-R) containing substituted groups with different
steric and electronic structures. Fig. 3a through 3c are typical
SEM images of aerogels prepared from mixtures of DCPD and
NB-R with different lengths of alkyl groups derived from methyl
iodide, hexyl bromide, and dodecyl bromide. P(DCPD-r-NB-R)
aerogels prepared using monomer 4 (R = methyl) show the
characteristic fibrous network morphology observed in pure
PDCPD and P(DCPD-r-NB) at low NB concentrations, while
monomers 5 and 6, which contain a longer alkyl chain group,
created aerogels with a shorter and more entangled network.
Pure PDCPD formed a number of clusters of long linear
ligaments, whose entanglement led to the development of large,
irregular shaped pores. However, the addition of comonomers
such as NB and monomers 4–6 appears to have either reduced
the length of the ligaments or caused more entanglement between
them, the result of which is drastically different pore shape and
Fig. 3 (a–f) Characteristic morphologies of P(DCPD-r-NB-R) aerogels prepared from a mixture of DCPD with a different NB-R comonomer: (a)
DCPD+4, (b) DCPD+5, (c) DCPD+6, (d) DCPD+7, (e) DCPD+8, and (f) DCPD+9. (g) The proposed model for representing the effect of NB-R
monomers on the formation of P(DCPD-r-NB-R) aerogels with different morphologies.
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size. As shown in Fig. 3d to 3f, more drastic changes in not only
the pore size but also the pore morphology of the aerogels were
observed from the addition of comonomers 7–9 that contain
strong hydrophobic or electronic interactions. Monomers 7 and
8 with benzyl and tosyl groups resulted in spherical pores with
sizes of y50 nm, while the addition of iodo-monomer 9
produced extended long fibers reminiscent of pure PDCPD
(see Fig. S3 in the ESI{). It is worthy to mention that the
norbornene-based crosslinker 2 with a similar reactivity of two
norbornene rings does not accompany the unique fiber-like
morphologies of PDCPD-based aerogels. It formed a fine
particulate morphology, often observed in silica aerogels (see
Fig. S4 in the ESI{).13 The effect of NB-R (or NB) monomer
addition on the morphologies is schematically represented in
Fig. 3g. The distinct features in P(DCPD-r-NB-R) copolymer
networks are changes in the length of the fiber strands, and pore
sizes and morphologies surrounded by the strands. Leventis and
coworkers31–33 explained the formation of organic (polymer)
aerogels by (i) crosslinking at the molecular level resulting in
early phase separation of small surface-reactive particles and (ii)
subsequent inter-particle crosslinking that improves the mechan-
ical properties of the materials. Based on those principles, long
PDCPD strands with lengths of hundreds of nm up to a few mm
would be the combination of primary and secondary particles
like in polyurea aerogels.32 In this study, the incorporation of
NB-R with a different substituent groups might distort further
growth of the primary PDCPD fibrils by either (i) decreasing the
number of reactive sites (i.e., the crosslinking) at the surface or
(ii) sterically hindering inter-particle crosslinking, as has been
proposed for polyimide aerogels from anhydride and isocya-
nates.33 The formation of many shorter fiber strands with good
solubility and/or better chain mobility makes it possible to form
more entangled morphologies, and it could explain the
transformation in the pore size and morphologies as well as
the higher moduli and viscosity at the gel point in copolymer gel
systems. Considering the effect of different crosslinkers, the
unique fibril structure of primary particles seems to be attributed
to the molecular structure of PDCPD and its inter-molecular
interaction. In this regard, the effect of different DCPD stereo-
isomers on morphologies is currently being investigated and
will be a subject of further release. This result indicates that
additional tuning over pore morphologies of polymeric aerogels
could be gained by the copolymerization approach in conjunc-
tion with a careful design of comonomers and/or cross-linkers.
Full details on the morphologies of P(DCPD-r-NB-R) aerogels
with different compositions are included in the ESI{.
The versatility of the copolymer networks derived from ring
opening metathesis polymerization (ROMP) allows one to
introduce chemical functionality by taking advantage of the
tunability of the NB-R comonomer. For our specific application,
a millimetre-sized spherical shell with a thin aerogel layer on the
inside of the shell, the availability of a non-destructive imaging
technique such as X-ray radiography is essential to judge the
success of the coating experiment. This, however, requires
increasing the weak X-ray absorption contrast of the CH-based
foam by introducing a high-Z element. This motivated our effort
to synthesize the bisiodo-norbornene monomer 12 (Fig. 4a).
Fig. 4b shows a photo of P(DCPD-r-12) aerogels with different wt
percentages, a, of monomer 12. Monolithic aerogels were formed
from the supercritical CO2 drying process of P(DCPD-r-12). They
did not show any serious shrinkage during the polymerization and
drying process and retain the original shapes of PDCPD although
a slight shrinkage occurred for high concentrations of monomer
12. A typical SEM image of the dried aerogels indicates that they
have a fibrous-shape morphology, similar to other P(DCPD-
r-NB-R) aerogels (Fig. 4c). The incorporation of iodine was
Fig. 4 Introduction of chemical functionality into aerogels: (a) bisiodo norbornene monomer 12 as an example of a functional monomer, (b) a photo
of dried P(DCPD-r-12) (100/a) (wt/wt) monoliths, (c–f) typical examples of (c) an SEM image and (d) energy-dispersive X-ray spectrum (EDS), (e)
C-mapping and (f) I-mapping images of the P(DCPD-r-12) (100/100) aerogel.
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Fig. 5 Rutherford backscattering spectrometry (RBS) analysis of P(DCPD-r-12) (100/a) (wt/wt) aerogels: (a) A typical RBS spectrum (symbols) and a
RUMP-code simulation (solid line), (b) the atomic concentration of iodine in P(DCPD-r-12) samples as a function of the dose of the analyzing ion
beam (1.5 MeV He), and (c) the chemical structure of P(DCPD-r-12) used for the determination of the composition of the samples.
verified with energy-dispersive X-ray spectroscopy (EDS) and the
result is shown in Fig. 4d. Characteristic carbon and iodine EDS
mapping images in Fig. 4e and 4f reveal a uniform distribution of
iodine through the aerogels.
(DCPD) with various substituted norbornene (NB-R) monomers
highlights the versatility of the ROMP approach. Significant
enhancements in the coating behavior observed for P(DCPD-
r-NB-R) copolymer gels make it possible to fabricate uniform low
density polymeric aerogel coatings on various substrates. The
copolymerization approach can also be used to modify the pore
structure and morphology of aerogels as well as to introduce
chemical functionality. The approach described herein could
potentially expand the use of hydrocarbon aerogel materials in a
wide range of applications in the fields of material, environmental,
and industrial science.
Further quantitative information on the material composition
was obtained from Rutherford backscattering spectrometry
(RBS) because of the unique ability of RBS to deliver highly
accurate film compositions without the use of standards. Fig. 5a
shows a typical RBS spectrum of P(DCPD-r-12) (100/100)
acquired with an analyzing ion dose of y8 6 1012 cm22. We
found that the iodine concentration in the aerogels increased
with the concentration of monomer 12 in the copolymer, as
anticipated. However, we also found that He+ ion irradiation
during the RBS analysis resulted in composition changes, as
shown in Fig. 5b. Thus, the initial sample composition at zero
dose of the analyzing beam for each sample was determined by
extrapolating dose dependencies. The iodine concentrations in
P(DCPD-r-12) (100/20) and (100/100) samples were determined
to be about 0.63 and 1.90 (at. %), respectively. From the
assumption of the chemical structure of P(DCPD-r-12) (Fig. 5c),
the RBS result is used to determine the molar ratios of NB-R to
DCPD in copolymer gels (see details in the ESI{). These values
correspond to (100/21) and (100/75) by weight, respectively. This
shows that the comonomers were uniformly incorporated
throughout the polymer gels with a relatively good yield from
feed ratios.
Acknowledgements
This work was performed under the auspices of the U.S.
Department of Energy by the Lawrence Livermore National
Laboratory under contract DE-AC52-07NA27344. We thank Dr
Brian P. Mayer and Dr Ian D. Hutcheon for their valuable
assistance in the execution of the NMR and SEM experiments.
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