A Degradable and Recyclable Photothermal Conversion Polymer

Article abstract of DOI:10.1002/chem.201801654

Decomposition and repolymerization of conjugated polymers offer great promise for developing recyclable photothermal conversion materials, which yet remain challenging. Herein, a crosslinked conjugated polymer based on a dynamic covalent bond of Schiff base is developed. This polymer possesses photothermal conversion efficiency as high as 90.4 %. Decomposition of the polymer under specialized conditions is corroborated by various characterizations. The kinetics study is also investigated to understand this degradation process. Furthermore, those decomposed species can be repolymerized back to conjugated polymers which possess the same photothermal conversion efficiency as the pristine polymer. Such a degradable and recyclable photothermal polymer is successfully used as a heat source for photothermal-electrical conversion to generate Seebeck voltage under either near infrared (NIR) irradiation or solar illumination.

Full text of DOI:10.1002/chem.201801654

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Accepted Article
Title: A degradable and recyclable photothermal conversion polymer
Authors: Xiao-Qi Xu, Zhen Wang, Ruiting Li, Yonglin He, and Yapei
Wang
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To be cited as: Chem. Eur. J. 10.1002/chem.201801654
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A degradable and recyclable photothermal conversion polymer
Xiao-Qi Xua, Zhen Wanga, Ruiting Lia, Yonglin Hea, and Yapei Wang a*
Abstract: Decomposition and repolymerization of conjugated
polymers offer great promise for developing recyclable
photothermal conversion materials, which yet remain
challenging. Herein, a crosslinked conjugated polymer based on
a dynamic covalent bond of Schiff base is developed. This
polymer possesses photothermal conversion efficiency as high
as 90.4%. Decomposition of the polymer under specialized
conditions is corroborated by various characterizations. The
kinetics study is also investigated to understand this degradation
process. Furthermore, those decomposed species can be
repolymerized back to conjugated polymers which possess the
same photothermal conversion efficiency as the pristine polymer.
Such a degradable and recyclable photothermal polymer is
successfully used as a heat source for photothermal-electrical
conversion to generate Seebeck voltage under either near
infrared (NIR) irradiation or solar illumination.
Photothermal conversion materials, which can convert light
energy directly into thermal heat, are arousing great attentions in
the fields of energy conversion and phototherapy.[1] Owing to the
advantages of tunable absorption,[2] easy modification[3] and
biological compatibility,[4] conjugated polymers have recently
been the subject of intense investigations for efficient
photothermal conversion materials.[1], [5] An inherent limitation of
conjugated polymers is the nature of difficult degradation even in
harsh conditions, which inevitably gives rise to unwanted wastes
in the practical applications. Of great promise is bridging π-
conjugated building units by cleavable bonds, in an effort to
tailoring the degradation of conjugated polymers.
Figure 1. (a) Schematic illustration of synthetic procedure. Inset:
the optical image of powder product. (b) Reflectance spectrum
of the conjugated polymer PACAT-TFB and UV-Vis-NIR
absorption spectra of ACAT. (c) FTIR spectra of the monomers
and the conjugated polymer PACAT-TFB.
Two building blocks, including amine capped aniline trimer
(ACAT) and 1,3,5-triformylbenzene (TFB), were copolymerized
into a A2B3-type hyperbranched network. The terminated amino
groups of ACAT are chemically active to react with aldehyde
groups of TFB. ACAT is a form of aniline trimer which already
has outstanding absorption in visible light window.[9] Extending
the π-conjugated length by linking it with TFB thereby not only
increases the stability but also enables the light absorption in
longer wavelength. The polymerization of ACAT and TFB was
completed in a mixed solvent of 1,2-dichlorobenzene (o-DCB)
and 1-butanol (n-BuOH) via acid catalysis. After heating at
120 °C for 3 days, a dark black powder was obtained as shown
in Figure 1a (See detailed synthetic procedures in the
Supporting Information). The reflectance spectrum shows that
this dark polymer has light absorption in a wide range, from UV
region to near infrared region (Figure 1b). The absorption in the
near infrared region reveals that the π-conjugated structure is
properly extended to narrow the energy bandgap between
HOMO and LUMO. The success of polymerization was further
verified by Fourier transform infrared spectroscopy (FTIR)
spectra, as shown in Figure 1c. The characteristic peaks at 3381
cm-1, 3313 cm-1 and 3207 cm-1 (blue line) assigned to amino
groups of ACAT are weakened and even disappeared after
polymerization. Meanwhile, the peak at 1701 cm-1 (red line)
assigned to C=O of TFB also became negligible in the
crosslinked polymer (PACAT-TFB). The solid state 13C NMR
spectrum of PACAT-TFB also confirms the formation of imine
groups (Figure S5). Additional DSC and PXRD studies illustrate
that PACAT-TFB is generally amorphous without evidence of
macroscopic crystalline structures (Figure S6).
Dynamic covalent bonds, breaking and reforming dynamically
under specific conditions, have been formulated as important
driving forces in supramolecular chemistry.[6] Superior to other
reversible single bonds, Schiff base is more appropriate for
establishing conjugated polymers because it can not only
provide double bonds in favour of extending π-conjugated length
but also be cleavable in a control manner.[7] Recently, a
degradable
semiconducting
polymer
of
PDPP-PD
(Polydiketopyrrolopyrrole-p-Phenylenediamine) convinced that
the Schiff base bonds are on the promising way of recyclable
utilization.[8] Nevertheless, conjugated polymers involved Schiff
base or other dynamic covalent bonds have barely acted as
degradable or even recyclable photothermal materials so far.
Herein, this work developed
a hyperbranched conjugated
polymer based on imine bonds, which could be depolymerized
and repolymerized. The polymer was successfully exploited as a
recyclable photothermal material.
[a]
X.-Q. Xu, Z. Wang, R. Li, Y. He, and Prof. Y. Wang
Department of Chemistry, Renmin University of China, Beijing,
100872 (PR China)
E-mail: yapeiwang@ruc.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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remains unchanged. Further study using thermal gravimetric
analysis (TGA) indicates that the hyperbranched polymer of
PACAT-TFB is thermally stable. Less than 5% weight loss was
observed at 360 oC (Figure S9).
Figure 2. (a) The surface temperature change of the PACAT-
TFB slice under light irradiation with different powers for 3 min.
Light wavelength is 808 nm. Inset: The optical image of a typical
compressed PACAT-TFB slice. (b) IR images of the light-
irradiated PACAT-TFB slice at different powers captured by IR
camera. (c) ON/OFF irradiation of the PACAT-TFB slice at 0.5 W
for ten cycles.
Figure 3. (a) Schematic illustration of degradation process. (b)
1H NMR spectra of degraded products (purple), TFB (red) and
ICAT (black), respectively. (c) Time-dependent UV-Vis
absorption spectra of the degraded products in the solution
phase. Inset: Fitting curve of time-dependent change of
absorption at 550 nm.
The photothermal performance of PACAT-TFB was
evaluated by monitoring the temperature change under light
irradiation. For the convenient manipulation, the powder-like
product was compressed into free-standing slices. As recorded
by IR camera in Figure 2a and 2b, the surface temperature was
rapidly increased once the slice was exposed to light irradiation
(808 nm). The temperature reaches a plateau in a few minutes
at a specific light power, as a balance between the light-induced
heating and the thermal dissipation towards the environment.
The temperature change is positively correlated with light power,
e.g. the temperature increases from room temperature (26 oC) to
46.9 oC and 180.3 oC at light power of 0.1 W and 0.9 W,
respectively. The photothermal effect is attributed to the exciton
relaxation of photoexcited polymer to ground states via internal
conversion and vibrational relaxation. The photothermal
conversion efficiency (ηPT) can be worked out by eq1:
In terms of the cleavable performance of imine bonds against
pH change, degradation of conjugated PACAT-TFB is expected
in acidic or basic environment. However, testing results illustrate
that PACAT-TFB is chemically stable in the aqueous media
containing of acids or bases. This abnormal phenomenon is
attributed to the hydrophobic nature as well as strong
intramolecular interaction of PACAT-TFB. As such, hydrophilic
agents are difficult to penetrate through the crosslinked network.
As shown in Figure 3a, an alternative strategy was proposed in
order to effectively break the conjugated structure. A competitor
of formaldehyde is involved to form new imine bonds with ACAT
in the degradation process. Unlike TFB with three aldehyde
groups, formaldehyde is able to terminate two ends of ACAT
and polymerization between them is not allowed. In other words,
decomposition of the crosslinked PACAT-TFB network should
take place once TFB is replaced by formaldehydes. Several
mixture solutions loaded with formaldehyde were investigated to
testify the possibility of PACAT-TFB degradation (From Figure
S11 to S15). It is confirmed that the polymer powder can be
completely degraded in a mixture solution consisting of acetic
ether, acetic acid and formalin under ultrasonic treatment. The
degraded products were then collected and examined by NMR
spectrum and FTIR spectrum. The characteristic peaks of TFB
Q
ηPT
=
eq1
P
Here, the following abbreviations apply: Q is the heat
generation rate of the slice under irradiation and P is the power
of the light. Taking the light power of 0.5 W as an example, ηPT is
calculated as 90.43%, indicative of high light use efficiency (See
detailed calculation of photothermal conversion efficiency in the
Supporting Information). It is worth noting that the mechanical
pressure for compressing PACAT-TFB into robust slices has
negligible influence on the photothermal conversion efficiency
(Figure S7). The photostability of PACAT-TFB was evaluated
through repeated light irradiation at a power of 0.5 W in view of
the potential application. After 10 cycles of ON/OFF irradiation at
0.5 W, the temperature change at a fixed irradiation time
1
at 10.22 ppm and 8.70 ppm are clearly appeared in the H NMR
spectra. The broad peak from 6.60 ppm to 7.20 ppm are well
matched with the signals of ICAT, which is a product of ACAT
terminated by two formaldehyde molecules. FTIR spectra also
supports the formation of TFB with the appearance of
characteristic peaks for stretching vibration of C=O bond.
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However, the characteristic peaks of amines at 3381 cm-1, 3313
cm-1 and 3207 cm-1 are not observed. Instead, new peaks at
2970 cm-1, 2922 cm-1 and 2853 cm-1 agree with the formation of
ICAT. It is noteworthy that acid and acetic ether are also
essential to ensure the effective degradation. The former
facilitates the dynamic breaking and reforming of imine bonds.
The latter is assumed to tailor the deep penetration of acid and
formaldehyde into the crosslinked network.
formaldehyde could be removed from ICAT at high temperature.
Meanwhile, ICAT was turned as ACAT. Thus the degraded
products, after the removal of formaldehyde and purification,
could be repolymerized into PACAT-TFB in a mixed solvent of o-
DCB, n-BuOH and acetic acid. As shown in Figure 4b, FTIR
spectra confirm the weakening of characteristic peaks at 1702
cm-1, 2970 cm-1, 2922 cm-1 and 2853 cm-1, and accordingly,
indicate the consumption of ICAT and TFB after the
repolymerization (Figure S16). Though the signals at 2930 cm-1
and 2875 cm-1 ascribed to NH=CH2 and C=O suggest an
incomplete repolymerizaiton, the photothermal conversion
efficiency of the repolymerized polymer is almost the same with
that of the pristine polymer (Figure S8).
Figure 4. (a) Schematic illustration of repolymerization process.
(b) FTIR spectra of repolymerization product (pink), degradation
products (purple), TFB (red) and ICAT (black).
Figure 5. (a) Evaluation of the NIR PTE converter under various
NIR irradiation powers, including the Seebeck voltage (V) and
maximum output power (Pmax). (b) 5 cycles of Seebeck voltage
output of NIR PTE converter under ON/OFF NIR irradiation with
different light powers. (c) Schematic illustration of a solar PTE
converter: PACAT-TFB slices (marked with black) are closely
attached on Peltier devices that are connected in series. The
Seebeck voltage is measured on an electrochemical workstation
under illumination at one sun. Inset: The optical image of a
typical compressed PACAT-TFB slice on a Peltier device. (d)
Seebeck voltage output of different number of PTE converters
under ON/OFF solar irradiation.
The degradation kinetics was further investigated by
monitoring the UV-Vis absorption of degraded products (Figure
3c). The absorbing peaks at 295 nm and 550 nm assigned to
TFB and ICAT, respectively, are gradually increased over time.
According to eq2, a kinetic curve is plotted between the degree
of degradation and the absorbance of ICAT at 550 nm based on
Lambert-Beer law,
t
Abs
= 1 − 0.947 ∗ e−
eq2
21.62
Absmax
Here Abs is the absorbance of ICAT at a specific degradation
time (t, min). The degradation lifetime (τ), at which the ratio of
degraded polymer to the initial polymer is 1/e, can be defined as
Mediated by the high photothermal conversion efficiency,
PACAT-TFB is proposed to convert light into other energy form
besides thermal energy. One feasible application is the
photothermal-electrical (PTE) conversion which relies on Peltier
devices. A piece of PACAT-TFB slice was adhered on a 15×15
mm Peltier device by thermal conductive silica glue. Being
irradiated by 808 nm NIR laser, the slice was able to convert
light to heat and subsequently warm up the upper surface,
meanwhile, the bottom surface of the Peltier device still
remained cold. Thus, the temperature gradient between two
surfaces of the Peltier device generated voltage based on
Seebeck effect. A considerable increase of the PTE voltage was
eq3,
1
Absτ = Absmax ∗ (1 − )
eq3
e
Thus the degradation lifetime of PACAT-TFB is estimated as
20.5 min.
In addition to the controllable degradation, it is speculated if
the degraded products, including ICAT and TFB, can be
recovered as the conjugated PACAT-TFB. Whereas ICAT is
deactivated by formaldehyde, its two amino groups can be
liberated once the dynamic imine bond breaks and
formaldehyde molecules are released. It was found that
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observed upon improving the NIR laser power according to
Figure 5a. We also calculated the maximum output power (Pmax
)
under NIR irradiation with different light power by eq4，
Pmax = (V)2/(4Ri)
eq 4
Here, Ri is the internal resistance of the Peltier device (Ri=2 Ω).
A considerable Seebeck voltage of 49.6 mV and an efficient
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illumination with a power of 0.9 W. The rapid rise and decline of
thermoelectric voltages were observed through alternatively
turning on and off light ranging from 0.9 W to 0.1 W with a 3 min
interval. Five heating and cooling cycles were tested in order to
illustrate the stability and repeatability of the PTE converter
(Figure 5b). Due to the ease of assembly, a number of Peltier
devices could be readily connected in series to improve the light-
absorbing area that may be practically used in sunlight (Figure
5c). As shown in Figure 5d, with the accumulation of number of
the Peltier devices, the Seebeck voltage undergoes a significant
increase, which notably reaches 0.57 mV when 9 Peltier devices
are connected and exposed to solar simulator. In this regard,
larger voltages are predicted if more Peltier devices are used.
This preliminary application suggests that PACAT-TFB may
function as a key part of thermo-related devices, thus satisfying
those demands on reasonable degradability and reuse.
In conclusion, we demonstrated a successful example of
degradable and recyclable photothermal conversion polymer. A
mild degradation condition was optimized to decompose the
crosslinked polymer in a short time. The well-characterized
degraded products could be repolymerized back to photothermal
conversion polymers. This line of research has laid the
groundwork for exploiting functional materials with low chemical
waste and sustainable utilization. In addition to the application of
photothermal-electrical conversion by the combination with
Peltier devices, it is envisioned that the photothermal conversion
polymer could be readily applied in other areas of photothermal
therapy, thermo-tailored catalysis, controllable CO2 capture, and
water purification.
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This work was financially supported by the National Natural
Science Foundation of China (21674127, 21422407, and
51373197). Prof. Long Chen at Tianjin University was
acknowledged for his helpful discussion.
Keywords: Photothermal conversion
Repolymerization Conjugated polymer
electrical conversion
•
Degradation
•
•
• Photothermal-
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COMMUNICATION
A
degradable
and
recyclable
Xiao-Qi Xu, Zhen Wang, Ruiting Li,
Yonglin He, and Yapei Wang *
conjugated polymer with excellent
photothermal property is designed
and synthesized. This polymer is
hoped to function as a key part of
Page No. 1– Page No.4
thermo-related
satisfying those
devices,
demands
thus
on
A degradable and recyclable
reasonable degradability and reuse.
photothermal conversion polymer
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