Effect of Induction on the Dispersion of Semiconducting and Metallic Single-Walled Carbon Nanotubes Using Conjugated Polymers

Article abstract of DOI:10.1021/acs.macromol.5b00631

Despite significant advances in single-walled carbon nanotube (SWNT) synthesis and purification strategies, the separation of metallic and semiconducting SWNTs on a large scale remains a barrier to the realization of many commercial applications. Selective extraction of specific SWNT types by wrapping and dispersion with conjugated polymers has been found effective for semiconducting SWNTs, but structural parameters that dictate selectivity are poorly understood. Here, we report nanotube dispersions with two structurally similar conjugated copolymers, both being poly(fluorene-co-phenylene) derivatives, having comparable degrees of polymerization but differing in the extent of electron donation from functional groups on the phenylene comonomers. It is found that copolymers decorated with electron donating methoxy functionalities lead to predominant dispersion of semiconducting SWNTs, while copolymers decorated with electron withdrawing nitro functionalities bias the dispersion toward metallic SWNTs. Differentiation of semiconducting and metallic SWNT populations was carried out by a combination of UV-vis-NIR absorption spectroscopy, Raman spectroscopy using multiple excitation wavelengths, and fluorescence spectroscopy. These results provide new insight into polymer design features that dictate preferential dispersion of specific SWNT types.

Full text of DOI:10.1021/acs.macromol.5b00631

Article
pubs.acs.org/Macromolecules
Effect of Induction on the Dispersion of Semiconducting and Metallic
Single-Walled Carbon Nanotubes Using Conjugated Polymers
Nicole A. Rice, Ayyagari V. Subrahmanyam, Brian R. Coleman, and Alex Adronov*
Department of Chemistry, McMaster University, Hamilton, Ontario L8S 4L8, Canada
S
* Supporting Information
ABSTRACT: Despite significant advances in single-walled
carbon nanotube (SWNT) synthesis and purification strat-
egies, the separation of metallic and semiconducting SWNTs
on a large scale remains a barrier to the realization of many
commercial applications. Selective extraction of specific SWNT
types by wrapping and dispersion with conjugated polymers
has been found effective for semiconducting SWNTs, but
structural parameters that dictate selectivity are poorly
understood. Here, we report nanotube dispersions with two
structurally similar conjugated copolymers, both being poly-
(fluorene-co-phenylene) derivatives, having comparable de-
grees of polymerization but differing in the extent of electron
donation from functional groups on the phenylene comonomers. It is found that copolymers decorated with electron donating
methoxy functionalities lead to predominant dispersion of semiconducting SWNTs, while copolymers decorated with electron
withdrawing nitro functionalities bias the dispersion toward metallic SWNTs. Differentiation of semiconducting and metallic
SWNT populations was carried out by a combination of UV−vis−NIR absorption spectroscopy, Raman spectroscopy using
multiple excitation wavelengths, and fluorescence spectroscopy. These results provide new insight into polymer design features
that dictate preferential dispersion of specific SWNT types.
either m-SWNTs (electrodes, interconnects, etc.) or sc-SWNTs
(transistors, sensors, etc.), their separation into pure samples is
imperative. Several methods for separating and purifying
SWNTs have recently been developed, including density-
gradient ultracentrifugation (DGU),35 agarose gel filtration,36
electrophoresis,37 and selective dispersion using conjugated
polymers.38 Of these, selective dispersion with conjugated
polymers is promising as it is a low-cost and scalable process.
While it has recently been shown that some conjugated
polymers, such as commercially available polyfluorenes, can
selectively disperse sc-SWNTs in toluene, the resulting
dispersions are extremely dilute, precluding isolation of bulk
quantities of purified SWNTs.38 More importantly, selectivity
for metallic SWNTs has not been reported and remains an
elusive goal.
The selectivity of the interaction between conjugated
polymers and SWNTs arises from a complex mixture of
polymer features, including polymer structure, conformation,
molecular weight, and the nature of the side chains.39,40
Modification of some or all of these parameters has been
demonstrated to have significant impact on the stability and
selectivity of SWNT dispersions.41,42 In addition to the
polyfluorenes mentioned above, interactions of SWNTs with
INTRODUCTION
■
Among the known nanoscale materials, single-walled carbon
nanotubes (SWNTs) have attracted a tremendous amount of
research attention since their discovery.1−5 Their unique
properties, including high tensile strength,6 high aspect ratio,7
thermal and electrical conductivity,8−11 and extraordinary
optical characteristics,12−14 make them potentially valuable
components of advanced materials with a wide range of
applications. Indeed, SWNTs have been incorporated in field-
effect transistors (FETs),7,15 sensors,16−19 photodetectors,20
photovoltaics,21−23 flexible printed circuits,24 electrode materi-
als for flexible electronics,25 touch screens,26 and micro-
electronic interconnects,27 among other devices.28 In these
applications, the molecular nature, resilience, and amenity to
chemical modification of SWNTs make them decisively
advantageous over many other nanoscale materials. However,
despite recent progress in nanotube commercialization,27
applications that require controlled electrical and optical
properties have not kept pace with expectations. This lag is a
consequence of the inability to industrially prepare SWNTs
that are pure in terms of their electrical properties. All known
SWNT synthesis methods, such as high-pressure carbon
monoxide disproportionation (HiPCO),29 carbon vapor
deposition (CVD),30 arc discharge,31 laser ablation,32 and
plasma torch growth,33 result in the production of mixtures of
metallic SWNTs (m-SWNTs) and semiconducting SWNTs
(sc-SWNTs).34 Since components of electronic devices require
Received: March 25, 2015
Revised: July 8, 2015
© XXXX American Chemical Society
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Scheme 1. Synthesis of Monomers
polythiophenes,43 polycarbazoles,44 poly(phenylenevinylene)-
s,45 poly(phenyl acetylenes),46 and a number of other
structures and derivatives have been investigated.47 Some of
these structures have shown significant selectivity in their
interactions with specific chiralities, diameters, and conductivity
types, rivaling that of polyfluorenes. Despite this progress, a
fundamental understanding of the interaction selectivity, from
the perspective of controllable polymer characteristics such as
backbone structure, side-chain structure, and electronic proper-
ties, has been elusive. One of the difficulties in determining the
effect of structural variability on interaction selectivity is the fact
that changing monomer structure often affects polymerization
kinetics, solubility, and overall reactivity toward polymerization
catalysts, which results in significant variability in the polymer
chain length. It has been well documented that chain length, in
addition to other structural elements, can have a significant
impact on the types of SWNTs being dispersed by a particular
polymer.48,49 Thus, maintaining constant chain length is
imperative when comparing polymers of different structure in
their ability to disperse SWNTs, as this decreases the chance of
inaccurate interpretation of observations.
In this report, we investigate how inductive effects of
polymer side chains impact the electronic types of SWNTs that
are dispersed by conjugated polymers composed of fluorene
and phenylene units. We demonstrate that when other variables
(such as molecular weight and size of side chains) are kept
constant, the electronic nature of the polymer backbone has a
significant effect on which species of SWNTs are dispersed. In
particular, we observe that by changing from a simple electron-
rich comonomer (p-dimethoxyphenyl) to an electron-poor
comonomer (p-dinitrophenyl), it is possible to switch from
dispersing solely sc-SWNTs to dispersing a mixture that
contains significant amounts of m-SWNTs.
obtained.53 Drastic changes in the size of the functionalities on
the polymer backbone could change how the polymer wraps
the SWNT surface, potentially interfering with the polymer−
nanotube interaction. It was therefore desirable to keep side-
chain size identical between different polymer structures. It was
also important to obtain polymers with the same degree of
polymerization, as polymer length has been previously
demonstrated to significantly affect the quality of SWNT
dispersions.48,49
A fluorene-containing polymer backbone was chosen because
polyfluorenes have been shown to interact well with SWNTs
and can exhibit selectivity toward small subsets of SWNTs
under appropriate solvent conditions.54 1,4-Dibromo-2,5-
dinitrobenzene and 1,4-diiodo-2,5-dimethoxybenzene were
chosen as comonomers to produce the electron-poor and
electron-rich polymer structures, respectively. The nitro and
methoxy functionalities are comparable in size, allowing the two
polymers to adopt similar conformations on the surface of
SWNTs. The choice of small electronically inductive
functionalities also avoids any negative steric interactions that
could arise from the presence of bulky or drastically different
side chains. Synthesis of the 9,9-bis(dodecyl)fluorene-2,7-
diboronate ester monomer55 (3) and the two comonomers,
1,4-dibromo-2,5-dinitrobenzene56 (5) and 1,4-diiodo-2,5-dime-
thoxybenzene57 (7), was accomplished according to literature
procedures (Scheme 1).
The two poly(fluorene-co-phenylene) copolymers were
prepared using Suzuki polycondensation conditions by reacting
the 9,9-bis(dodecyl)fluorene-2,7-diboronate ester (3) with the
corresponding electron-deficient and electron-rich monomers
(5 and 7, respectively), as shown in Scheme 2. The nitro-
containing copolymer (PFdNB) was prepared using PEPPSI-
IPr58 as the catalyst in a toluene/K2CO3(aq) (2.5:1 v/v)
cosolvent mixture. The crude polymer was purified by
precipitation in methanol, followed by Soxhlet extraction in
hexanes and acetone to yield a yellow solid in 81% yield. Gel
permeation chromatography (GPC) indicated that the polymer
had a number-average molecular weight (Mn) of 26.0 kDa,
corresponding to a degree of polymerization (DP) of 39 repeat
units (Table 1). The electron-rich methoxy-containing
copolymer (PFdOMB) was prepared using a Pd(PPh3)4
catalyst in a DMF/toluene (3:1 v/v) cosolvent mixture. The
PFdOMB polymer was also precipitated into methanol and
purified using Soxhlet extraction with hexanes and acetone and
RESULTS AND DISCUSSION
■
Polymer Design and Synthesis. The polymer structures
were carefully designed in an effort to minimize the various
parameters that can affect the nature of the polymer−SWNT
interaction and prevent unambiguous investigation of the effect
of changing the electronic character of the conjugated polymer
backbone on the polymer’s ability to disperse SWNTs. In
particular, polymer architecture has been shown to play an
important role in the types of SWNTs that are dispersed50−52
as well as on the stability of the polymer−SWNT complexes
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the expense of overall SWNT concentration.60 A convenient
compromise was found with a 1:1 THF/toluene (v/v)
cosolvent mixture. This solvent system displayed almost
identical selectivity results to those obtained in neat toluene,
but with a concentration of SWNTs that was comparable to
what is achieved in THF. In addition, nanotube suspensions
remained stable for at least several months in this mixed solvent
system, showing no difference in stability relative to either of
the single solvents. This cosolvent mixture was used for all
polymer−SWNT samples except PFdOMB60−SWNT, for
which THF was found to yield the best results. The
PFdOMB21−, PFdOMB27−, and PFdOMB60−SWNT samples
were all clear, green dispersions, while the PFdNB−SWNT
suspension exhibited an orange-brown color (see Supporting
Information, Figure S3).
Polymer−SWNT Complex Characterization. To inves-
tigate the degree of nanotube bundling that is present in
samples of the polymer-dispersed SWNTs, atomic force
microscopy (AFM) studies were performed. Polymer−SWNT
complex samples were prepared by spin-coating dilute
dispersions on freshly cleaved mica. Tapping-mode AFM
analysis was performed on these samples, and representative
images from these studies are shown in Figure 1. Long,
Scheme 2. Synthesis of PFdNB and PFdOMB Copolymers
by Suzuki Polycondensation
Table 1. Yield and Molecular Weight Data for Dinitro
(PFdNB) and Dimethoxy (PFdOMB) Copolymers
a
polymer
yield (%)
Mn (kDa)
Mw (kDa)
PDI
DP
PFdNB
81
74
55
62
26.0
20.7
26.9
59.8
49.8
54.5
1.92
2.64
2.20
3.29
39
32
42
94
PFdOMB21
PFdOMB27
PFdOMB60
59.2
196.6
a
Calculated from Mn values.
was isolated as a pale yellow solid. Several polymerization
attempts were necessary to produce a polymer with Mn similar
enough to PFdNB to allow for a valid comparison. The
polymer fraction with the closest molecular weight to PFdNB
had an Mn of 26.9 kDa (PFdOMB27), which corresponds to a
DP of 42 (Table 1). Two other polymerizations yielded
significant amounts of the dimethoxy polymer, one with a
slightly lower molecular weight (PFdOMB21, Mn = 20.7 kDa)
and one with a much higher molecular weight (PFdOMB60, Mn
= 59.8 kDa). These polymers were also used to prepare
complexes with SWNTs (see Supporting Information) to
investigate the effect of molecular weight on selectivity toward
specific SWNT types. All the polymers used in this study
exhibited excellent solubility in various organic solvents at room
temperature, including THF, toluene, dichloromethane, and
chloroform.
Polymer−SWNT Complexes. Supramolecular polymer−
SWNT complexes were prepared using the different copoly-
mers and raw HiPCO SWNTs following previously reported
procedures.59 Briefly, 5 mg of SWNTs was added to a polymer
solution consisting of 10 mg of polymer dissolved in 20 mL of
solvent. The mixture was sonicated for 1 h in a bath sonicator
chilled with ice, followed by centrifugation at 8346g for 30 min.
The supernatant was carefully removed from the centrifuge
tube, filtered, and continuously washed with solvent to remove
excess polymer. After removal of excess polymer, the polymer−
SWNT buckypaper was redispersed in 15 mL of solvent by
sonication, followed by a second centrifugation. A SWNT
suspension using sodium dodecylbenzenesulfonate (SDBS) as
the surfactant for nanotube dispersion was also prepared
following literature procedures and used for comparison.60
Two different solvents, THF and toluene, were used in the
initial studies. Consistent with previous results using poly(9,9-
dioctylfluorene) (PFO),60 we obtained relatively concentrated
SWNT suspensions in THF using PFdNB−SWNT and the
three PFdOMB−SWNT samples; however, with the exception
of PFdOMB60, these samples showed very little selectivity in
the chiralities dispersed. In toluene, greater selectivity toward a
smaller subset of chiralities was obtained, but this occurred at
Figure 1. Representative AFM images for (a) PFdOMB27−SWNT and
(b) PFdNB−SWNT, with corresponding height profiles shown below.
The dashed white lines represent the location of the height profile, and
the black scale bars correspond to 500 nm.
filamentous structures were observed in both samples, with
heights ranging from 1 to 5 nm. The smallest diameter features
correspond to individual polymer-coated SWNTs, while the
larger diameter features could arise from small polymer−
SWNT bundles, which were either present in the dispersions or
formed upon removal of the solvent during the spin-coating
process. These observed height profiles indicate that significant
exfoliation of nanotube bundles occurred upon sonication with
both polymer types, and there was no appreciable difference in
the degree of nanotube exfoliation by PFdOMB and PFdNB.
UV−vis−NIR absorption spectroscopy was performed on
the polymer−SWNT supramolecular complexes (Figure 2).
The absorption features for SWNTs arise from the interband
transitions of the van Hove singularities, with different
chiralities of SWNTs having different transition energies
between the various allowed levels, enabling the electronic
transitions to be organized as a function of diameter and
chirality.61 The absorbance features can be grouped into three
categories in the observed absorbance range: two semi-
conducting regions, S11 (830−1600 nm) and S22 (600−800
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identification of different SWNT species within a given sample
as both m- and sc-SWNTs can be examined.65 Resonance
enhancement occurs when the excitation wavelength is tuned
to overlap with the van Hove singularities present in the 1D
density of states for a particular nanotube.66 As these electronic
transitions depend on nanotube diameter and chirality, only a
subset of the total nanotube population will be enhanced and
detected for any given excitation wavelength.67 The low-
frequency radial breathing mode (RBM, 100−400 cm−1) is a
particularly useful region for study as the peak locations have an
inverse dependence on SWNT diameter.68
Thin film samples were prepared from the polymer−SWNT
complexes by drop-casting the solutions onto silicon wafers and
allowing the solvent to evaporate. A reference SWNT sample
was also prepared by sonicating a small amount of the SWNT
starting material in CHCl3 and using this suspension to prepare
a solid film using the same drop-casting method. Raman scans
were obtained at three excitation wavelengths, including 514,
633, and 785 nm. It has previously been shown that these three
excitation wavelengths are adequate for characterizing the
electronic nature of a HiPCO SWNT sample, as both metallic
and semiconducting species can be separately probed.69 Figure
3 shows the RBM regions from the three samples at each
excitation wavelength (full-scale Raman scans are provided in
the Supporting Information, Figure S5). All Raman spectra
were normalized to the G band at approximately 1590 cm−1
and offset for clarity. Upon excitation at 514 nm, two dominant
RBM features are observed in the SWNT spectrum (Figure 3a,
black curve): a broad feature arising from sc-SWNTs centered
at 180 cm−1, and several sharp peaks from 225 to 290 cm−1,
which are attributed to m-SWNTs.70 Both the semiconducting
and metallic features remain present in the PFdNB-SWNT
sample (Figure 3a, red curve), while PFdOMB27−SWNT only
shows a single peak in the sc-SWNT region (Figure 3a, blue
curve), suggesting that m-SWNTs are not present in this
sample. This fact is corroborated by analysis of the G band
region at this excitation wavelength, which is shown in the inset
in Figure 3a. The G band is composed of two peaks: a lower
frequency G− and a higher frequency G+. For sc-SWNTs, both
the G− and G+ have Lorentzian line shapes, but for m-SWNTs
the G− exhibits a broader Breit−Wigner−Fano (BWF) line
shape.71 A very broad G− band is observed for both the SWNT
and PFdNB−SWNT samples but not for PFdOMB27−SWNT,
confirming that m-SWNTs are largely absent in the latter
sample.
Figure 2. UV−vis−NIR absorption spectra for PFdOMB27−SWNT
(blue) and PFdNB−SWNT (red) in a 1:1 mixture of THF/toluene.
nm), and a metallic M11 (440−645 nm) region.62 Despite
spectral congestion in these regions, qualitative assignment
based on peak location and shape can be made for the
PFdOMB27−SWNT complex in the S11 region (see Supporting
Information, Table S1).
The absorption spectrum for PFdOMB27−SWNT displays
multiple intense, sharp features in both the S11 and S22 regions.
This absorption spectrum suggests that the electron-rich
PFdOMB effectively exfoliates sc-SWNTs in the THF/toluene
cosolvent mixture. The location of the various peak maxima is
red-shifted compared to a reference SDBS−SWNT dispersion
(see Supporting Information, Figure S6), consistent with
previous observations for conjugated polymer−SWNT supra-
molecular dispersions.63 The PFdNB−SWNT absorption
spectrum also contains multiple red-shifted peaks in the S11
and S22 regions; however, the peaks are broader and lack the
fine features displayed by the PFdOMB27−SWNT sample. The
relatively intense, featureless absorption background of the
PFdNB−SWNT spectrum is indicative of the presence of m-
SWNTs.64 However, detailed analysis of the M11 region is
complicated for these samples as the polymer absorption
overlaps in this region.
To further investigate the differences in nanotube
populations dispersed by the two different polymers, resonance
Raman spectroscopy was performed. This technique allows for
Figure 3. RBM regions of the Raman spectra using (a) 514, (b) 633, and (c) 785 nm excitation wavelengths. The gray regions represent locations of
signals arising from semiconducting nanotubes, while the pink regions represent the locations of signals from metallic nanotubes. The inset in (a)
shows the G-band region, located at ∼1590 cm−1, upon excitation at 514 nm.
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Figure 4. Photoluminescence maps (a) and (b) corresponding to PFdOMB27−SWNT and PFdNB−SWNT at a similar concentration and plotted
on the same scale. (c) is a more concentrated sample of PFdNB−SWNT, with the scale adjusted to show weak fluorescence.
Figure 5. Absorption spectra (a) and PL spectra recorded using λex = 650 (b) and 725 nm (c) upon addition of 50 μL aliquots of a 2 mg/mL
solution of PFdNB to PFdOMB27−SWNT. Plotted spectra represent samples after every second aliquot for clarity.
Both m- and sc-SWNTs are also in resonance when the 633
nm excitation wavelength is used. For HiPCO nanotube
samples at this excitation wavelength, m-SWNT features are
found at approximately 175−230 cm−1, while sc-SWNTs give
rise to peaks at approximately 230−300 cm−1.62,72 Both m- and
sc-SWNT features are observed in the SWNT and PFdNB−
SWNT samples, while only sc-SWNT features are observed for
PFdOMB27−SWNT, confirming the interpretation of results
from 514 nm excitation.
SWNTs, while PFdNB disperses a higher proportion of m-
SWNTs, along with some amount of sc-SWNTs. In addition,
we found that the two higher molecular-weight electron-rich
polymers, PFdOMB27 and PFdOMB60, exhibit higher selectivity
for sc-SWNTs relative to the lower molecular weight
PFdOMB21 (complete Raman analysis of these samples is
provided in the Supporting Information, Figure S10 and
preceding paragraphs).
Photoluminescence (PL) Mapping. PL maps were
recorded for the polymer−SWNT samples (Figure 4) as well
as for the SDBS−SWNT suspension (Supporting Information,
Figure S6). The locations of the various SWNT fluorescence
maxima were assigned according to previously published data.13
High-intensity PL signals were observed for the PFdOMB27−
SWNT sample, with the most intense peak arising from the (8,
7) chirality (consistent with the major UV−vis−NIR
absorption peak at 1287 nm, Figure 2). Both the PL map
and absorption spectrum for PFdOMB27−SWNT also show
removal of some smaller diameter semiconducting species, such
as the (6, 5) and (7, 5) chiralities. Additionally, intense PL
signals were observed for the PFdOMB21−SWNT and
PFdOMB60−SWNT dispersions (Supporting Information,
Figure S11), but a slightly different distribution of chiralities
was found. Overall, the PL and absorption spectra for all three
PFdOMB−SWNT dispersions showed intense signals that can
be mainly attributed to the (8, 7) chirality along with significant
amounts of the (9, 4) and (7, 6) nanotube species.
While mainly semiconducting HiPCO SWNTs are in
resonance with the 785 nm excitation wavelength, it has been
reported that a few large diameter metallic species, most
notably the (16, 7) and (12, 9) chiralities, may be observed in
the low-frequency region.68,70 Several small peaks were
observed in the SWNT and PFdNB−SWNT spectra from
149 to 168 cm−1 and were assigned to m-SWNTs, while no
features were observed below 200 cm−1 for the PFdOMB27−
SWNT sample (Figure 3c). The most intense peak in the
SWNT spectrum occurs at 265 cm−1, which arises from (10, 2)
SWNTs that come into resonance with this excitation
wavelength when they are bundled.73 This peak is often
referred to as the “bundling peak” and can be a useful handle to
qualitatively identify the presence of bundling in a SWNT
sample treated with various dispersants.74 Figure 3c shows that
a significant decrease in the bundling peak occurs when
SWNTs are dispersed with PFdNB, suggesting that nanotube
bundles are being efficiently exfoliated by this polymer.
Furthermore, there is no peak present at 265 cm−1 for the
PFdOMB27−SWNT sample, which indicates either that the
SWNTs are completely debundled or that the (10,2) chirality is
not dispersed by PFdOMB27 (see Figure 4a). On the basis of
these results, it is clear that PFdOMB27 disperses only sc-
The PL map for the PFdNB−SWNT sample is strikingly
different. We initially recorded the map with a sample
concentration that was very similar to that of PFdOMB27−
SWNT, by matching the absorption intensity for the (8, 7)
chirality at 1287 nm (see Figure 2). Despite evidence from the
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absorption and Raman spectra for the presence of sc-SWNTs in
the PFdNB−SWNT sample, the PL map did not display any
fluorescence peaks of significant intensity (Figure 4b). After
repeated attempts at various concentrations, a low-intensity PL
map (Figure 4c) was recorded, with weak fluorescence from
some low diameter SWNTs, such as (6, 5), (7, 5), and (7, 6),
and very weak PL intensity from the (8, 7) chirality. Since m-
SWNTs do not exhibit fluorescence,75 and are known to
quench the fluorescence of sc-SWNTs,76 this result, in
combination with the Raman data, indicates that PFdNB is
much more selective for m-SWNTs than PFdOMB.
To further support the selectivity of PFdNB for m-SWNTs,
it is necessary to rule out the possibility that the electron-poor
polymer itself is responsible for quenching nanotube
fluorescence. We therefore performed a polymer exchange
experiment to ascertain whether quenching by the polymer was
possible. A 2 mg/mL solution of PFdNB in the THF/toluene
cosolvent mixture was prepared and added to a dilute sample of
the PFdOMB27−SWNT dispersion in small aliquots (50 μL
increments, corresponding to 0.1 mg of PFdNB). The sample
was sonicated for 10 min after each addition, and an absorption
and two PL spectra (at λex = 650 and 725 nm) were recorded.
The results of this experiment (Figure 5a−c) show an increase
in the absorption intensity from 600 to 1100 nm, attributed to
the tail end of the PFdNB polymer absorbance (which is
present in large excess). Initially, as the PFdNB polymer was
added there was no change in the SWNT PL intensity.
However, upon the sixth addition a slight decrease in the PL
intensity was noticed for both excitation wavelengths. A
continual decrease in PL intensity was observed upon addition
of subsequent aliquots. However, this decrease is attributed to
the dilution of SWNTs, rather than to quenching, as a similar
decrease in SWNT PL intensity was observed in a control
experiment where identical volumes of pure solvent were added
to the same starting concentration of PFdOMB27−SWNT (see
Supporting Information, Figure S7).
ASSOCIATED CONTENT
■
S
* Supporting Information
Full experimental details, full characterization of the polymers,
photographs of polymer−SWNT dispersions, spectral data for
an SDBS-dispersed SWNT reference sample, details of
quenching studies, and spectral data for PFdOMB21−SWNT
and PFdOMB60−SWNT samples. The Supporting Information
is available free of charge on the ACS Publications website at
DOI: 10.1021/acs.macromol.5b00631.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail adronov@mcmaster.ca; Tel (905) 525-9140 x23514;
Fax (905) 521-2773 (A.A.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support for this work was provided by the National
Science and Engineering Research Council of Canada
(NSERC) and the Canada Foundation for Innovation (CFI).
N. Rice is grateful for support through an NSERC-PGS
scholarship.
REFERENCES
■
(1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603−605.
(2) Ajayan, P. M. Chem. Rev. 1999, 99, 1787−1800.
(3) Carbon Nanotubes and Related Structures: Synthesis, Character-
ization, Functionalization, and Applications; Guldi, D. M., Martín, N.,
Eds.; Wiley-VCH: Weinheim, 2010.
(4) Harris, P. J. F. Carbon Nanotube Science: Synthesis, Properties and
Applications; Cambridge University Press: Cambridge, 2009.
(5) Saito, R.; Dresselhaus, M. S.; Dresselhaus, G. Physical Properties of
Carbon Nanotubes; Imperial College Press: London, 1998.
(6) Yu, M.-F.; Files, B. S.; Arepalli, S.; Ruoff, R. S. Phys. Rev. Lett.
2000, 84, 5552−5555.
(7) Terrones, M. Annu. Rev. Mater. Res. 2003, 33, 419−501.
(8) Han, Z.; Fina, A. Prog. Polym. Sci. 2011, 36, 914−944.
(9) Avouris, P.; Chen, Z.; Perebeinos, V. Nat. Nanotechnol. 2007, 2,
605−615.
(10) Avouris, P. Acc. Chem. Res. 2002, 35, 1026−1034.
(11) Collins, P. G.; Avouris, P. Sci. Am. 2000, 283, 62−69.
(12) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.;
Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.;
Kittrell, C.; Ma, J.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science
2002, 297, 593−596.
(13) Weisman, R. B.; Bachilo, S. M. Nano Lett. 2003, 3, 1235−1238.
(14) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.;
Ohtsuka, Y.; Achiba, Y. Synth. Met. 1999, 103, 2555−2558.
(15) Durkop, T.; Getty, S. A.; Cobas, E.; Fuhrer, M. S. Nano Lett.
2004, 4, 35−39.
(16) Qi, P.; Vermesh, O.; Grecu, M.; Javey, A.; Wang, Q.; Dai, H.;
Peng, S.; Cho, K. J. Nano Lett. 2003, 3, 347−351.
CONCLUSIONS
■
In the design of next-generation polymers for selective
dispersion and purification of SWNTs, it is imperative to
understand the key parameters that dictate polymer selectivity
toward specific SWNT types. We have demonstrated that
modification of the poly(fluorene-co-phenylene) backbone with
electron-donating or electron-withdrawing functionalities can
have a significant impact on the electronic nature of the
SWNTs dispersed by the polymer. The copolymer bearing
electron-donating methoxy functional groups interacts prefer-
entially with semiconducting SWNTs, while the nearly identical
copolymer bearing electron-withdrawing nitro functionalities
produces dispersions that are much more enriched in metallic
SWNTs (though some semiconducting SWNTs are still
present). Considering that the fluorene unit is relatively
electron rich itself, and preferentially interacts with semi-
conducting SWNTs, it is not surprising that exclusive
dispersion of metallic SWNTs was not achieved in this case.
Nevertheless, the electron-rich copolymer shows a clear
preference for semiconducting SWNTs, while the electron-
poor copolymer shows a bias toward metallic SWNTs. This
work justifies further investigation of more electron-poor
conjugated polymers for their ability to exhibit greater
selectivity for metallic SWNTs.
(17) Dionisio, M.; Schnorr, J. M.; Michaelis, V. K.; Griffin, R. G.;
Swager, T. M.; Dalcanale, E. J. Am. Chem. Soc. 2012, 134, 6540−6543.
(18) Rivas, G. A.; Rubianes, M. D.; Rodriguez, M. C.; Ferreyra, N. F.;
Luque, G. L.; Pedano, M. L.; Miscoria, S. A.; Parrado, C. Talanta 2007,
74, 291−307.
(19) Pang, X.; Imin, P.; Zhitomirsky, I.; Adronov, A. Macromolecules
2010, 43, 10376−10381.
́
(20) Nanot, S.; Haroz, E. H.; Kim, J.-H.; Hauge, R. H.; Kono, J. Adv.
Mater. 2012, 24, 4977−4994.
(21) Rowell, M. W.; Topinka, M. A.; McGehee, M. D.; Prall, H.-J.;
Dennler, G.; Sariciftci, N. S.; Hu, L.; Gruner, G. Appl. Phys. Lett. 2006,
̈
88, 233506.
F
DOI: 10.1021/acs.macromol.5b00631
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
(22) Bindl, D. J.; Safron, N. S.; Arnold, M. S. ACS Nano 2010, 4,
5657−5664.
(54) Ding, J.; Li, Z.; Lefebvre, J.; Cheng, F.; Dubey, G.; Zou, S.;
Finnie, P.; Hrdina, A.; Scoles, L.; Lopinski, G. P.; Kingston, C. T.;
Simard, B.; Malenfant, P. R. L. Nanoscale 2014, 6, 2328−2339.
(23) Keru, G.; Ndungu, P. G.; Nyamori, V. O. Int. J. Energy Res. 2014,
38, 1635−1653.
(55) Jo, J.; Chi, C.; Hoger, S.; Wegner, G.; Yoon, D. Y. Chem. - Eur. J.
̈
2004, 10, 2681−2688.
(24) Sun, D.-M.; Liu, C.; Ren, W.-C.; Cheng, H.-M. Small 2013, 9,
1188−1205.
(56) Jackson, C. L.; Calhane, D. F. Am. Chem. J. 1902, 28, 451−474.
(57) Wariishi, K.; Morishima, S.; Inagaki, Y. Org. Process Res. Dev.
2003, 7, 98−100.
(25) Green, A. A.; Hersam, M. C. Nano Lett. 2008, 8, 1417−1422.
(26) Hecht, D. S.; Thomas, D.; Hu, L.; Ladous, C.; Lam, T.; Park, Y.;
Irvin, G.; Drzaic, P. J. Soc. Inf. Dispersion 2009, 17, 941−946.
(27) De Volder, M. F. L.; Tawfick, S. H.; Baughman, R. H.; Hart, A. J.
Science 2013, 339, 535−539.
(58) O’Brien, C. J.; Kantchev, E. A. B.; Valente, C.; Hadei, N.; Chass,
G. A.; Lough, A.; Hopkinson, A. C.; Organ, M. G. Chem. - Eur. J. 2006,
12, 4743−4748.
(59) Imin, P.; Imit, M.; Adronov, A. Macromolecules 2012, 45, 5045−
5050.
(28) Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam,
M. C. Chem. Soc. Rev. 2013, 42, 2824−2860.
(60) Hwang, J.-Y.; Nish, A.; Doig, J.; Douven, S.; Chen, C.-W.; Chen,
L.-C.; Nicholas, R. J. J. Am. Chem. Soc. 2008, 130, 3543−3553.
(61) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley,
R. E.; Weisman, R. B. Science 2002, 298, 2361−2366.
(62) Strano, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen,
M. J.; Shan, H.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E.
Science 2003, 301, 1519−1522.
(29) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.;
Colbert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1999, 313,
91−97.
(30) Kong, J.; Cassell, A. M.; Dai, H. J. Chem. Phys. Lett. 1998, 292,
567−574.
(31) Journet, C.; Maser, W. K.; Bernier, P.; Loiseau, A.; Lamy de la
Chapelle, M.; Lefrant, S.; Deniard, P.; Lee, R.; Fischer, J. E. Nature
1997, 388, 756−758.
(63) Furmanchuk, A.; Leszczynski, J.; Tretiak, S.; Kilina, S. V. J. Phys.
Chem. C 2012, 116, 6831−6840.
(32) Guo, T.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E.
Chem. Phys. Lett. 1995, 243, 49−54.
(64) Naumov, A. V.; Ghosh, S.; Tsyboulski, D. A.; Bachilo, S. M.;
Weisman, R. B. ACS Nano 2011, 5, 1639−1648.
(33) Kim, K. S.; Cota-Sanchez, G.; Kingston, C. T.; Imris, M.;
Simard, B.; Soucy, G. J. Phys. D: Appl. Phys. 2007, 40, 2375−2387.
(34) Rafique, M. M. A.; Iqbal, J. J. Encapsulation Adsorpt. Sci. 2011, 1,
29−34.
(65) Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.;
Saito, R. Nano Lett. 2010, 10, 751−758.
(66) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Phys. Rep.
2005, 409, 47−99.
(35) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam,
M. C. Nat. Nanotechnol. 2006, 1, 60−65.
(67) Doorn, S. K. J. Nanosci. Nanotechnol. 2005, 5, 1023−1034.
(68) Doorn, S. K.; Heller, D. A.; Barone, P. W.; Usrey, M. L.; Strano,
M. S. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 1147−1155.
(69) Strano, M. S.; Zheng, M.; Jagota, A.; Onoa, G. B.; Heller, D. A.;
Barone, P. W.; Usrey, M. L. Nano Lett. 2004, 4, 543−550.
(70) Strano, M. S.; Doorn, S. K.; Haroz, E. H.; Kittrell, C.; Hauge, R.
H.; Smalley, R. E. Nano Lett. 2003, 3, 1091−1096.
(71) Brown, S. D. M.; Jorio, A.; Corio, P.; Dresselhaus, M. S.;
Dresselhaus, G.; Saito, R.; Kneipp, K. Phys. Rev. B: Condens. Matter
Mater. Phys. 2001, 63, 155414.
(36) Liu, H.; Feng, Y.; Tanaka, T.; Urabe, Y.; Kataura, H. J. Phys.
Chem. C 2010, 114, 9270−9276.
(37) Tanaka, T.; Jin, H.; Miyata, Y.; Kataura, H. Appl. Phys. Express
2008, 1, 114001.
(38) Nish, A.; Hwang, J.-Y.; Doig, J.; Nicholas, R. J. Nat. Nanotechnol.
2007, 2, 640−646.
(39) Gao, J.; Kwak, M.; Wildeman, J.; Herrmann, A.; Loi, M. A.
Carbon 2011, 49, 333−338.
(40) Berton, N.; Lemasson, F.; Hennrich, F.; Kappes, M. M.; Mayor,
M. Chem. Commun. 2012, 48, 2516−2518.
(72) Strano, M. S.; Zheng, M.; Jagota, A.; Onoa, G. B.; Heller, D. A.;
Barone, P. W.; Usrey, M. L. Nano Lett. 2004, 4, 543−550.
(73) Heller, D. A.; Barone, P. W.; Swanson, J. P.; Mayrhofer, R. M.;
Strano, M. S. J. Phys. Chem. B 2004, 108, 6905−6909.
(74) O’Connell, M. J.; Sivaram, S.; Doorn, S. K. Phys. Rev. B: Condens.
Matter Mater. Phys. 2004, 69, 235415.
(75) Haroz, E. H.; Duque, J. G.; Tu, X.; Zheng, M.; Walker, A. R. H.;
́
Hauge, R. H.; Doorn, S. K.; Kono, J. Nanoscale 2013, 5, 1411−1439.
(76) Zhang, D.; Yang, J.; Li, Y. Small 2013, 9, 1284−1304.
(41) Lemasson, F.; Berton, N.; Tittmann, J.; Hennrich, F.; Kappes,
M. M.; Mayor, M. Macromolecules 2012, 45, 713−722.
(42) Berton, N.; Lemasson, F.; Tittmann, J.; Sturzl, N.; Hennrich, F.;
̈
Kappes, M. M.; Mayor, M. Chem. Mater. 2011, 23, 2237−2249.
(43) Lee, H. W.; Yoon, Y.; Park, S.; Oh, J. H.; Hong, S.; Liyanage, L.
S.; Wang, H.; Morishita, S.; Patil, N.; Park, Y. J.; Park, J. J.; Spakowitz,
A.; Galli, G.; Gygi, F.; Wong, P. H. S.; Tok, J. B.-H.; Kim, J. M.; Bao, Z.
Nat. Commun. 2011, 2, 541−548.
(44) Lemasson, F. A.; Strunk, T.; Gerstel, P.; Hennrich, F.; Lebedkin,
S.; Barner-Kowollik, C.; Wenzel, W.; Kappes, M. M.; Mayor, M. J. Am.
Chem. Soc. 2011, 133, 652−655.
(45) Keogh, S. M.; Hedderman, T. G.; Gregan, E.; Farrell, G.;
Chambers, G.; Byrne, H. J. J. Phys. Chem. B 2004, 108, 6233−6241.
(46) Liang, S.; Chen, G.; Peddle, J.; Zhao, Y. Chem. Commun. 2012,
48, 3100−3102.
(47) Tuncel, D. D. Nanoscale 2011, 3, 3545−3554.
(48) Imin, P.; Cheng, F.; Adronov, A. Polym. Chem. 2011, 2, 1404−
1408.
(49) Jakubka, F.; Schießl, S. P.; Martin, S.; Englert, J. M.; Hauke, F.;
Hirsch, A.; Zaumseil, J. ACS Macro Lett. 2012, 1, 815−819.
(50) Gao, J.; Loi, M. A.; de Carvalho, E. J. F.; dos Santos, M. C. ACS
Nano 2011, 5, 3993−3999.
(51) Akazaki, K.; Toshimitsu, F.; Ozawa, H.; Fujigaya, T.;
Nakashima, N. J. Am. Chem. Soc. 2012, 134, 12700−12707.
(52) Ozawa, H. H.; Fujigaya, T. T.; Niidome, Y. Y.; Hotta, N. N.;
Fujiki, M. M.; Nakashima, N. N. J. Am. Chem. Soc. 2011, 133, 2651−
2657.
(53) Liu, J.; Moo-Young, J.; McInnis, M.; Pasquinelli, M. A.; Zhai, L.
Macromolecules 2014, 47, 705−712.
G
DOI: 10.1021/acs.macromol.5b00631
Macromolecules XXXX, XXX, XXX−XXX