Effect of the Linking Group on the Thermoelectric Properties of Poly(Schiff Base)s and Their Metallopolymers

Article abstract of DOI:10.1002/asia.202100530

As polymer-based thermoelectric (TE) materials possess attractive features such as light weight, flexibility, low toxicity and ease of processibility, an increasing number of conducting polymers and their composites with high TE performances have been developed in recent years. Up to date, however, the research focusing on the structure-performance relationship remains rare. In this paper, two series of poly(Schiff base)s with either C=C or C≡C linker and their metallopolymers were synthesized and doped with single-walled carbon nanotubes to evaluate how the linking groups affected the TE properties of the resulting composites. Apart from the effect exerted by the morphology, experimental results suggested that the linkers played a key role in determining the band gaps, preferred molecular conformation and extent of conjugation of the polymers, which became key factors that influenced the TE properties of the resulting materials. Additionally, upon coordination with transition metal ions, the TE properties could be tuned readily.

Full text of DOI:10.1002/asia.202100530

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Effect of the Linking Group on the Thermoelectric Properties of
Poly(Schiff Base)s and Their Metallopolymers
Authors: Wai Yeung (Raymond) Wong, Jiahua Li, Zitong Wang, Zelin
Sun, and Linli Xu
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Effect of the Linking Group on the Thermoelectric Properties of
Poly(Schiff Base)s and Their Metallopolymers
Jiahua Li,[a,b] Zitong Wang,[a] Zelin Sun,[b] Linli Xu[a,b] and Wai-Yeung Wong*[a,b]
[a]
Jiahua Li, Zitong Wang, Dr. Linli Xu, Prof. Wai-Yeung Wong
Department of Applied Biology and Chemical Technology and Research Institute for Smart Energy
The Hong Kong Polytechnic University
Hung Hom, Kowloon, Hong Kong (P. R. China)
E-mail: wai-yeung.wong@polyu.edu.hk
[b]
Jiahua Li, Dr. Zelin Sun, Dr. Linli Xu, Prof. Wai-Yeung Wong
The Hong Kong Polytechnic University Shenzhen Research Institute
Shenzhen 518057 (P. R. China)
Supporting information for this article is given via a link at the end of the document.
Abstract: As the polymer-based thermoelectric (TE) materials are
highlighted by their attractive features such as light weight, flexibility,
low toxicity and ease of processibility, an increasing number of
conducting polymers and their composites with high TE performances
have been developed in recent years. Up to date, however, the
research focusing on the structure–performance relationship remains
rare. In this paper, two series of poly(Schiff base)s with either C=C or
C≡C linker and their metallopolymers were synthesized and doped
with single-walled carbon nanotubes to evaluate how the linking
groups affected the TE properties of the resulting composites. Apart
from the effect exerted by the morphology, experimental results
suggested that the linkers played a key role in determining the band
gaps, preferred molecular conformation and extent of conjugation of
the polymers, which became key factors that influenced the TE
properties of the resulting materials. Additionally, upon coordination
with transition metal ions, the TE properties could be tuned readily.
temperature coolers, mini power generators and especially the
devices that contact human body directly.[1a, 1b, 7] In view of the low
thermal conductivity of OTE polymers, power factor (PF) is
preferred to evaluate the TE performance of these materials,
where PF = S2σ.[5a, 8] With conjugated main chains acting as the
structural characteristic, a number of polymer-based TE materials
have been developed. However, other than some classical
structures
including
poly(3,4-ethylenedioxythiophene)
:
poly(styrenesulfonate) (PEDOT:PSS),[9] poly(3-hexylthiophene)
(P3HT),[10] 1,1,2,2-ethenetetrathiolate-based metallopolymers
(poly(M-ett)),[11] etc., most pure OTE polymers still suffer from low
TE performances[12] and remain to be explored. As an effective
solution, doping has been widely applied to significantly enhance
the electrical conductivity and the TE performance.[13] Among the
commonly used conductive nanofillers, carbon nanotubes (CNTs)
attracted the most intense research interest because of their
excellent electrical conductance, high charge carrier mobility and
mechanical properties.[14] For example, the PF of polypyrrole was
dramatically boosted up to over 300 times after SWCNTs were
introduced.[15] In light of the advantages of SWCNT-based doping,
a detailed comparison between two blending approaches (i.e. the
in situ preparation and physical mixing) were made to check the
difference in TE performance of a poly(Schiff base)-SWCNT
composite system.[16] The composites prepared in the physical
mixing fashion were demonstrated to yield higher PF with the top
value of 77.7 ± 5.8 μW·m-1·K-2 at a polymer/SWCNT ratio of 1/3.[16]
Introduction
Thermal power generation, as the dominant approach by far,
provides the vast majority of electrical energy for our daily life.
However, during this process, more than half of the energy is
dissipated in the form of heat which is regrettably released to the
air afterwards.[1] Emerging as a clean power generation solution,
thermoelectric (TE) materials realize the conversion between heat
and electricity, and thus have attracted increasing research
interest in recent years. The conversion efficiency of a TE material
is influenced by its electrical conductivity (σ, S·m-1), Seebeck
coefficient (S, V·K-1), thermal conductivity (κ, W·m-1·K-1) and
testing temperature (T, K) and is assessed by their product – a
dimensionless parameter named figure-of-merit (ZT) which is
defined as: ZT = S2σTκ-1. Therefore, materials with high σ and S,
but low κ are desirable. So far, some inorganics have been
demonstrated to show superior efficiency with ZT > 1.0,
represented by Bi-Te-based alloys,[2] lead chalcogenide[3] and
SiGe.[4] Nevertheless, their applications are quite restricted, as
these materials are generally hard to be processed and heavy or
rare metal elements are always included.[1c, 5]
Furthermore,
a few underlying studies looked into the
relationship between molecular structures and the TE properties
from various aspects. For example, it was found that the
heteroatoms in the poly(3-alkylchalcogenophene) system played
a key role in determining the TE properties.[17] The enhanced
electrical conductivity but weakened thermal power can be
observed by altering the chalcogen atoms from S to Se to Te. By
merging thiophene and carbazole moieties into the poly((9,9-
dioctylfluorene)-2,7-diyl-alt-benzothiadiazole)
structure,
the
polymers were demonstrated to exhibit optimized TE properties
with the peak PF value of 13.11 μW·m-1·K-2 at 90 °C.[18] In addition,
some studies investigated the roles of side chains played in
determining the TE properties of polymer-based OTE
materials.[19]
By contrast, organic TE (OTE) polymers are generally featured
with light weight, low toxicity, low κ, good flexibility, variable
structures and ease of processing,[1c, 5-6] and thus have shown
promising potential in niche applications such as room
There is no doubt that the way how functional units are
organized is another critical factor that determines the physical
properties of a polymer material. Hence, once the functional
1
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fragments have been decided, it is feasible to tune the
performance of polymers by integrating those segments with
various linkers. However, to the best of our knowledge, relevant
research is still rare.
P2 and P2-MCl2 (Figure S3) displayed similar patterns, in addition
to the emergence of a weak absorption band at 2206 cm-1 due to
the internal C≡C in the polymer chains.[20]
In light of the ease of synthesis and the potential coordination
capability, in this work, we selected poly(Schiff base) as the model
and fully addressed how the linking groups affected the TE
properties by comparing two structurally resembled 1,4-
diazabuta-1,3-diene (DAB)-linked poly(Schiff base)-SWCNT
composite systems. As for the poly(Schiff base)s (i.e. P1 and P2
shown in Scheme 1), similar main chains were built by employing
trans-stilbene and diphenylacetylene structures to bond with the
adjacent DAB fragments, so that the difference in performance
could be ascribed to the effects due to the linker type (C=C versus
C≡C). The polymers were synthesized through condensation
polymerization and the composites with stepwisely increased
SWCNT mass contents [fC, fC = (mSWCNT/mcomposite)×100%] were
prepared. The TE properties of the formed composites were
studied and further tuned by incorporating various transition metal
ions (Co2+, Ni2+ and Cu2+) to the DAB sections. Based on the
performance assessment, the effects of the linking groups on the
TE properties were discussed from the perspective of band
structures, molecular conformations and the contact modes
Figure 1. FTIR spectra of P1 and P1-MCl2.
Morphology study
The microstructure of the as-prepared composite films was
investigated by scanning electron microscopy (SEM). As shown
in Figure 2, at a low SWCNT doping ratio (fC = 15%), phase
separation behaviour was evidently observed. Nanotube bundles
are connected with each other, forming sparse and layered
network structures with aggregated P1 or P1-MCl2 particles sitting
atop or underneath. Interestingly, the continuous introduction of
SWCNT led to a significant morphology evolution. A condensed
CNT network was established and some polymer particles began
to fuse themselves with it when fC reached 45%. At this point, it is
hard to distinguish the boundaries between one layer of CNT
network and its adjacent ones. The interactions among CNTs
were dramatically improved and such an effect was additionally
enhanced by the coated poly(Schiff base), due to the increased
contact area. When the doping ratio fC climbed up to 90%, a
homogeneous and continuous surface was observed without the
abrupt appearance of individual particles any longer, implying that
all OTE polymers have been well dispersed in the composites. By
comparison, a similar morphology change could be found for P2-
SWCNT and P2-MCl2-SWCNT composite films (Figure S4),
except that severe phase separation was still retained even at the
medium doping level, which may imply inferior electrical
conductivities.
between the polymer chains and the SWCNT networks.
Glyoxal
H2N
X
NH2
X
N
N
CH2Cl
2, EtOH
n
1
P1
X = trans-HC=CH-,
≡
P2
X = -C C-,
X = trans-HC=CH-,
≡
2
X = -C C-,
MCl2
MeOH, THF
X
N
N
M
n
Cl
Cl
P1-MCl
X = trans-HC=CH-,
2
≡
P2-MCl
X = -C C-,
2
M = Co, Ni, Cu.
Scheme 1. Synthetic route of polymers P1 and P2 and their metallated products
P1-MCl2 and P2-MCl2.
Results and Discussion
Chemical structure determination
Considering the highly conjugated chain structures and the
resulting rigid molecular backbones, all these polymers are hardly
soluble in common organic solvents, and hence the attempts by
using nuclear magnetic resonance spectroscopy to determine the
structures were impractical. Therefore, the chemical structures of
the polymers were identified by Fourier-transform infrared (FTIR)
spectroscopy. As shown in Figure 1, the aligned absorption bands
at 960 cm-1 was induced by the bending vibration of the
disubstituted C=C bond, suggesting that the linker in P1 and P1-
MCl2 was in a trans-configuration.[20] Specifically, a characteristic
absorption band corresponding to the stretching vibration of the
imine bond[16] in P1 was detected at 1679 cm-1, suggesting the
successful synthesis of the target polymer. As expected, this band
was found to shift to the higher wavenumber region to a different
extent after coordination with metal ions. Besides, the strong
signals at 831 cm-1 is assigned to the C-H vibration on the p-
disubstituted phenyl rings.[20] By comparison, the FTIR spectra of
2
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TE properties
The TE performances (including the electrical conductivity,
Seebeck coefficient and PF) of the metallopolymer-SWCNT
composites were assessed at RT and are displayed in Figure 4.
As can be seen from Figures 4a and 4d, the electrical
conductivities of the composites were remarkably enhanced upon
the gradual addition of SWCNT. At the lowest SWCNT content,
P2-SWCNT composite showed a higher σ value than P1-SWCNT.
This may be benefited from the more uniform morphology: P2
particles were better dispersed in the mixture and a condensed
carbon nanotube network was developed (Figure S4a).
Additionally, those polymer particles combined more tightly with
the nanotube bundles. Under this circumstance, intermolecular
charge carrier transmission via CNTs and polymer chains,
especially that among the nanotube bundles in the inter-plane
fashion, was significantly promoted and thus a higher conductivity
property was endowed. Expectedly, thin films in both series gave
their highest electrical conductivity at fC = 90%, but P1-SWCNT
composites displayed superior values when the SWCNT content
was beyond 45%. Other than the effects brought about by the
morphologies, we proposed that such a distinctive phenomenon
may stem from the differences in the preferential molecular
conformations and the conjugation length, which will be discussed
in detail later.
Figure 2. SEM images of the (a-c) P1-SWCNT, (d-f) P1-CoCl2-SWCNT, (g-i)
P1-NiCl2-SWCNT and (j-l) P1-CuCl2-SWCNT composites films with the doping
ratio (fC) of 15% (left column), 45% (middle column) and 90% (right column).
Furthermore, we tried to tune the electrical conductivity of the
materials by binding metal ions to the poly(Schiff base) ligands. In
the case of P1-MCl2, the coordinated Co2+ and Ni2+ increased the
σ value by 26% and 45%, respectively, at fC = 15%, when
compared with their metal-free version. As the metallopolymers
were the dominate component at the initial doping ratio, we may
describe as well the possible factors mainly from the perspective
of conducting metallopolymers. According to previous reports,
such an enhancement would probably benefit from the
intramolecular redox matching mechanism[23] and the additional
interactions between metal centres and the SWCNT.[14a] In terms
of molecular structure, metal centres herein are bonded to the
conducting main chains directly through a d-π conjugation fashion,
which is a typical instance of the Wolf type II conducting
metallopolymer. Under such a condition, if redox-active metal
centres (Co2+ or Ni2+) were involved and their redox potential were
close to or matched with that of the organic backbone, the
conjugation and the charge carrier migration route could be
extended, resulting in an elevated electrical conductivity (Figure
5a). However, the incorporation of redox-inactive Cu2+ would not
change the charge transfer route, even if the d-π interaction is
retained (Figure 5b). Besides, considering the steric hindrance
effect brought about by Cu2+ and its auxiliary ligand, P1-CuCl2-
SWCNT exhibited a lower σ than its metal-free analogue. By
contrast, P2-MCl2-SWCNT failed to yield higher electrical
conductivity than P2-SWCNT at fC = 15%, which may result from
the redox mismatching consequences. The electrical conductivity
of these metallated composites also showed upward trends with
the increasing SWCNT content, although the advantages caused
by redox matching diminished. This was probably influenced by
the gradually homogenized morphology, increased charge carrier
mobility, elevated charge carrier concentration, and the
continuously improved charge carrier route. The highest σ value
of 9.25×104 S·m-1 was achieved by P1-NiCl2-SWCNT composite
at fC = 90%, nearly 20% higher than that of P1-SWCNT at the
same doping level.
Raman spectral analysis of the (metallo)polymer-SWCNT
composites
Based on the gradual evolution of the morphology, we speculated
that the interactions through polymer chains and SWCNT
networks may come into play during the blending process. To
prove this, Raman spectra were measured (Figures 3 and S5).
For pure P1 powder, two main absorption bands at 1165 and 1562
cm-1 were detected, which corresponded to the C-H bending and
imino stretching vibrations, respectively.[16] The Raman spectra of
the pure SWCNT film was composed of a radial breathing mode
(RBM) at 159 cm-1, an out-of-plane bending mode (D band, 1307
cm-1), a characteristic in-plane, anisotropic stretching vibration of
bonded sp2 C atoms (a G- band at 1571 cm-1 and a G+ band at
1592 cm-1) and a 2D band at 2594 cm-1.[21] By comparison, the
blending process brought about a blue shift at the G+ band, which
not only illustrated the effective π-π interactions between the
polymer chains and the SWCNT networks, but also indicated an
electron migration route from SWCNTs to the OTE polymer
chains, known as a typical p-doping behaviour.[22] Similar spectral
profiles and behaviour were detected from P2 and P2-derived
composite systems (Figure S5), in addition to the presence of the
new peak at 2214 cm-1 from neat P2 which was due to the C≡C
stretching.
Figure 3. Raman spectra of P1, SWCNT, P1-SWCNT and P1-MCl2-SWCNT
composites at fC = 45% under the excitation by a laser at 785 nm.
3
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Figure 4. Electrical conductivity, Seebeck coefficient and PF of (a-c) P1-SWCNT, P1-MCl2-SWCNT, (d-f) P2-SWCNT and P2-MCl2-SWCNT composites at various
fC.
Table 1. TE properties of the composites at fC = 90% and the neat SWCNT.
Polymer component
P1-SWCNT
σ (S·m-1)
S (μV·K-1)
PF (μW·m-1·K-2)
7.90 × 104
7.80 × 104
9.25 × 104
6.66 × 104
5.71 × 104
5.34 × 104
4.47 × 104
5.73 × 104
5.33 × 104
27.4
23.8
27.7
24.8
24.7
25.7
26.5
30.0
38.0
59.5
44.1
71.0
40.9
35.0
35.4
31.3
50.3
77.0
P1-CoCl2-SWCNT
P1-NiCl2-SWCNT
P1-CuCl2-SWCNT
P2-SWCNT
Figure 5. Mechanism of the charge carrier transfer in the Wolf type II conducting
metallopolymer, where the orbital energies of the metal ions (a) matched or (b)
mismatched that of the polymer chains.
P2-CoCl2-SWCNT
P2-NiCl2-SWCNT
P2-CuCl2-SWCNT
SWCNT
The variation trends of the Seebeck coefficient are displayed in
Figures 4b and 4e. All the composites gave positive S values,
suggesting their p-type semiconducting nature with the holes as
the predominant charge carriers. Beyond our expectation, the
Seebeck coefficient of these materials did not exhibit a sharp
declining trend. Instead, the S values in P1- and P2-derived
composites wandered at around 30 and 25 V·K-1, respectively.
Such an unusual phenomenon was possibly induced by the
energy filtering effect.[24] At the crystallite boundaries, charge
carriers with higher energy were selected and allowed to pass
by,[24] which possibly cancelled the side effect caused by the
elevated electrical conductivity and thus maintained the Seebeck
coefficient at a certain level.
Under the combined effects of electrical conductivity and
Seebeck coefficient, most P1-SWCNT and P1-MCl2-SWCNT (M
= Co and Ni) composites produced higher PFs when fC exceeded
45%, with the highest record of 71.0 μW·m-1·K-2 established by
P1-NiCl2-SWCNT at fC = 90% (Figure 4c). The TE properties of
the SWCNT and the metallopolymer-SWCNT composites at fC =
90% are summarized in Table 1.
Mechanism exploration
In order to gain more insight on how the linking patterns affect the
TE properties of the designed polymers, the band structures of P1
and P2 were first studied by solid state UV-Vis diffuse reflectance
spectroscopy. For analysis purpose, the reflectance spectra
(Figure 6a) were converted to the plots showing the correlation
between Kubelka-Munk function and photon energy hν (Figures
6b and 6c) so as to determine the band gaps of these two
polymers. Clearly, the energy gaps of P1 and P2 were 2.10 and
2.59 eV, respectively, implying that P1 exerted a narrower energy
gap between the Fermi level and the top of the valence band and
thus required a lower activation energy.
4
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Figure 6. (a) UV-Vis diffuse reflectance spectra of neat P1 and P2 powders. (b) The corresponding Kubelka-Munk function versus energy plots of P1 and (c) P2
powder. Insets: pictures of P1 (red) and P2 (dark yellow) powders taken under daylight.
Further research was conducted on the molecular level where
the simplified version of P1 and P2 was designed, synthesized
and labelled as model compounds MC-1 and MC-2 accordingly
(Figures 7a and 7b). Their UV-Vis absorption spectra (Figure S6)
displayed the absorption maxima at 397 and 378 nm in THF,
revealing the energy gaps of 2.62 eV and 2.77 eV for MC-1 and
MC-2, respectively. This result confirmed that with the same
molecular fragment, the C=C-linked molecule possessed a better
conjugation than the C≡C-featured version. Deeper investigations
were conducted by carrying out density functional theory
calculations on 6-31G* basis set. As illustrated in Figs. 8c and 8d,
two phenyl rings on the identical side of the di-imine moiety
adopted a coplanar arrangement in both compounds. It is worth
noting that, for MC-1 specifically, the coplanar property was
extended to all phenyl rings within one imine molecule, which
allowed for better conjugation and higher electrical conductivity.
By comparison, in molecule MC-2, a twisted structure with a
dihedral angle of about 80° between planes P1P2 and P3P4 was
observed, which reduced the conjugation to a large extent. As a
consequence, in the polymerized systems P1 and P2, we inferred
that such an effect induced by the conformational difference
would probably be amplified. In P1, each segment possessed
almost the same plane as its adjacent ones, rendering the entire
polymer chain a quasi-planar geometry. In the aggregated state,
such a structure was in favour of establishing more efficient
intramolecular interactions between the polymer molecules and
SWCNT by setting the planes lying on the SWCNT surfaces
(Figure S7a). Particularly, at the high SWCNT doping ratio, P1
particles were dispersed more thoroughly and coated on the
SWCNT network more uniformly, leading to a superior charge
carrier migration system in the in-plane, through-plane and
polymer-SWCNT fashions and thus better TE properties.
However, in P2-SWCNT, there may be a considerable number of
polymer segments standing on the SWCNT surfaces (Figure S7b),
which enlarged the contact distance and resulted in lowered TE
properties.
Figure 7. Molecular structures of (a) MC-1 and (b) MC-2. The optimized molecular conformations of (c) MC-1 and (d) MC-2. Note that the phenyl rings P1 and P2
are in the light purple plane, while phenyl rings P3 and P4 share the same magenta plane.
the Seebeck coefficients almost retained the original levels. By
coordinating with transition metal ions, the TE performances were
readily tuned. Interestingly, most P1-SWCNT, P1-CoCl2-SWCNT
and P1-NiCl2-SWCNT composites gave higher PF values when
Conclusion
compared with their triple bond versions at high doping ratios
exceeding 45%. Spectroscopic analysis suggested that P1 was of
narrower band gap than P2, which provided favourable condition
to yield higher electrical conductivity. Further research on the
model compounds indicated that MC-1 adopted an almost planar
conformation, while MC-2 preferred a highly twisted geometry.
Such structural difference may affect the contact mode with
SWCNT networks and thus becomes the key reasons that bring
about the significant distinction in the extent of conjugation, band
gap and TE properties. We believe this work provides a valuable
In conclusion, two types of conducting polymers were designed,
synthesized and blended with SWCNTs to assess how the linking
groups of the polymer segments, viz. C=C and C≡C units, exerted
influence on the TE properties. Morphology studies revealed that
a dense and uniform thin film with perfectly dispersed polymer
particles was in favour of yielding higher TE performance. With
the increase of the SWCNT doping ratio, the electrical
conductivities of all the composites were remarkably boosted, but
5
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and the other one gradually heated the composite up. Upon heating, the
voltage potential was recorded in real time. The entire process was
controlled automatically by the software and the highest temperature was
confined to below 60 °C. After completion of the measurement, the film
was allowed to cool to RT naturally before the next trial.
reference for the structural design of metallopolymer-based OTE
materials, and will thus speed up the future work of this field
effectively.
Experimental Section
Acknowledgements
Materials
We would like to acknowledge the financial support from the
Science, Technology and Innovation Committee of Shenzhen
Municipality (JCYJ20180507183413211), the National Natural
Science Foundation of China (51873176 and 21905241), Hong
Kong Research Grants Council (PolyU123384/16P), the Hong
Kong Polytechnic University (1-ZEIC), Research Institute for
Smart Energy (CDA2) and Ms. Clarea Au for the Endowed
Professorship in Energy (847S).
39% Glyoxal aqueous solution, NiCl2 and 1,2-dichloroethane were
purchased from TCI. Anhydrous CoCl2 and CuCl2 were purchased from
Acros Organics. SWCNTs (diameter: 1-2 nm, length: 5-30 μm, purity >
95%) were purchased from XFNANO. Other chemicals were purchased
from Energy Chemical and used as received.
Preparation of poly(Schiff base)s P1 and P2
The detailed synthetic routes are summarized in Fig. S1. As a general
protocol, under a nitrogen atmosphere, 2.0 mmol arylamine 1 or 2
(Scheme 1) was dissolved in a minimum amount of CH2Cl2 and the
resulting solution was diluted with 8 mL ethanol. Then 0.27 mL 39% glyoxal
was injected slowly. The resulting suspension was stirred at room
temperature (RT) overnight. After that, the precipitate was collected by
filtration and was washed repeatedly with CH2Cl2 and ethanol until the
dropping filtrate became colourless. Pure polymers P1 and P2 were
obtained as a dark red powder and a dark yellow powder, respectively,
after drying under reduced pressure.
Conflict of interest
The authors declare no conflict of interest.
Keywords: thermoelectric • polymer linker • metallopolymer •
single-walled carbon nanotube • composite
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Preparation of the metallated poly(Schiff base)s P1-MCl2 and P2-MCl2
In a glass vial, 23 mg poly(Schiff base) was thoroughly dispersed in a
mixture of 10 mL methanol and 5 mL THF by sonication. Then a solution
of 13 mg MCl2 (M = Co, Ni and Cu) in 4 mL methanol was added in a
dropwise manner with vigorous stirring. The reaction was maintained at
RT for 1 day. Finally, the solid was isolated, washed with copious methanol
and dried in vacuo, yielding the metallated products.
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Preparation of the metallopolymer-SWCNT composite films
In a round bottom flask, 100 mg SWCNT was added to 100 mL 1,2-
dichloroethane. The mixture was cooled with an ice bath and fully
dispersed in an ultrasonic homogenizer at a power of 95 W for 90 min.
Then 1.0 mL of the formed dispersion was transferred to a glass vial
containing the polymers and the vial was sonicated at 0 °C for 2 h. The
dosage of the polymer depends on the doping ratio. The resulting gel-like
mixtures were added to clean glass substrates in a dropwise manner until
the substrates were fully covered, and the thin films would be ready for
subsequent characterizations after solvent evaporation. The
abovementioned doping method could eliminate the undesired effects
resulting from the soluble impurities adsorbed by SWCNTs including
residual reactants, soluble oligomers or free metal ions. The length and
the width of the as-prepared films were both 15 mm and the thickness was
measured on a Bruker DektakXT surface profiler with a diamond-tipped
stylus whose radius was 2.5 μm.
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Entry for the Table of Contents
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In this paper, the role played by the linkers in affecting the thermoelectric (TE) properties of polymer-based TE materials was
highlighted. Besides, the TE performance was further tuned by coordination with various transition metal ions.
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