Effects of Spacers on Photoinduced Reversible Solid-to-Liquid Transitions of Azobenzene-Containing Polymers

Article abstract of DOI:10.1002/chem.201902273

Photoisomerization in some azobenzene-containing polymers (azopolymers) results in reversible solid-to-liquid transitions because trans- and cis-azopolymers have different glass transition temperatures. This property enables photoinduced healing and proce

Full text of DOI:10.1002/chem.201902273

A Journal of
Accepted Article
Title: Effects of Spacers on Photoinduced Reversible Solid-to-Liquid
Transitions of Azobenzene-Containing Polymers
Authors: Philipp Weis, Andreas Hess, Gunnar Kircher, Shilin Huang,
Günter Auernhammer, Kaloian Koynov, Hans-Jürgen Butt,
and Si Wu
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To be cited as: Chem. Eur. J. 10.1002/chem.201902273
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Effects of Spacers on Photoinduced Reversible Solid-to-
Liquid Transitions of Azobenzene-Containing Polymers
Philipp Weis,[b] Andreas Hess,[b] Gunnar Kircher,[b] Shilin Huang,[b] Günter K. Auernhammer,[b]
Kaloian Koynov,[b] Hans-Jürgen Butt,[b] Si Wu*[a,b]
Abstract: Photoisomerization in some azobenzene-containing
polymers (azopolymers) results in reversible solid-to-liquid transitions
because trans and cis azopolymers have different glass transition
temperature (Tg) values. This property enables photoinduced healing
and processing of azopolymers with high spatiotemporal resolution.
However, a general lack in knowledge about the influence of the
polymer structure on photoinduced reversible solid-to-liquid
transitions hinders the design of such novel polymers. Here, we
demonstrate the synthesis and photoresponsive behaviors of new
azopolymers with different lengths of spacers between the polymer
backbone and the azobenzene group on the side chain. The
azopolymers with no and 20 methylene spacer (P-0-Azo and P-20-
Azo) did not show photoinduced solid-to-liquid transition.
Azopolymers with 6 or 12 methylene spacer (P-6-Azo and P-12-Azo)
showed photoinduced solid-to-liquid transition. This study
demonstrated that spacer is essential for azopolymers for
photoinduced reversible solid-to-liquid transitions and thus gives
insight into how to design azopolymers for photoinduced healing and
processing.
material is typically heated by rising the temperature. In some
cases, it is advantageous to transfer a material from a solid to a
liquid and vice versa isothermally using another external stimulus
but not heat. The chemical structure of stimuli-responsive
polymers can be changed using an external stimulus that can be
exploited to change the materials properties like phase transition
temperatures. External stimuli such as light have been used to
switch polymer structures.[2] Light is a non-contact stimulus. It is
clean and can be switched on and off fast. Photoswitchable
polymers typically exhibit two states with different absorption
spectra.[3] In this way, they can be transformed into each other by
utilizing light with different wavelengths. Compared to heat
induced solid-to-liquid transitions, photoinduced solid-to-liquid
transitions have higher spatiotemporal resolution.
One of the most studied photoswitchable molecules is
azobenzene.[4] Azobenzene has a thermodynamically stable
trans-isomer with a rod-like shape. The metastable cis-isomer has
a bent shape. Trans-to-cis isomerization is induced by UV
irradiation. The cis-to-trans back isomerization can be induced by
either visible light irradiation or heating. Azobenzene-containing
small molecules show reversible photoinduced solid-to-liquid
transition based on an isothermal crystal-to-melt transition.[5]
However, most azobenzene-containing small molecules tend to
crystallize, which restricts photoisomerization. In contrast, many
Introduction
Glass transition temperature (Tg) or melting temperature (Tm) of a
chemical compound is highly dependent on its chemical
structure.[1] To change a material from a solid to a liquid, the
azobenzene-containing
polymers
(azopolymers)
show
photoisomerization in solid state because coil structure of
polymers give more free volumes for photoisomerization.[6]
Photoirradiation of azopolymers has been exploited to fabricate
healable
coatings,[1a]
photoswitchable
adhesives,[7]
[a]
Prof. S. Wu
photoactuators,[8] patterned surfaces,[9] deformable colloids[10]
and solar thermal fuels.[11] Photoisomerization of some
azopolymers leads to an isothermal solid-to-liquid transition of
some azopolymers.[1a, 7, 12] Here, instead of heating the polymers
above their Tg values, light was used to switch Tg of the
polymers.[1a, 13] If Tg is lowered below the surrounding temperature,
the material is liquefied. The adhesion of ABA-type triblock
copolymers with azopolymer termini[12c] and azopolymers with
different alkyl substitutions has been investigated.[7] It was found
that the length of the alkyl chain has only minor influence on the
adhesion strength. However, the solid-to-liquid transition based
on photoswitchable Tg and other properties have not been
investigated.
CAS Key Laboratory of Soft Matter Chemistry, Hefei National
Laboratory for Physical Sciences at the Microscale, Department of
Polymer Science and Engineering, University of Science and
Technology of China, Jinzhai Road 96, Hefei 230026, China
E-mail: siwu@ustc.edu.cn
Dr. P. Weis , A. Hess, G. Kircher, Dr. S. Huang, Dr. G. K.
Auernhammer, Dr. K. Koynov, Prof. H.-J. Butt, Prof. S. Wu
Max Planck Institute for Polymer Research
[b]
Ackermannweg 10, 55128 Mainz, Germany
E-mail: wusi@mpip-mainz.mpg.de
Present Addresses: A.Hess: University of Potsdam, Institute of
Chemistry, Karl-Liebknecht-Straße 24-25, 14476 Potsdam, Germany
Dr. S. Huang.: School of Materials Science and Engineering，Sun
Yat-sen University, No. 135, Xingang Xi Road, Guangzhou, 510275,
P. R. China
Dr. G.K.Auernhammer.: Leibniz-Institut für Polymerforschung, Hohe
Str. 6, 01069 Dresden, Germany
Supporting information for this article is given via a link at the end of
the document.
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While our previous work has focused on demonstrating the
proof-of-concept of photoswitchable Tg of azopolymers,[1a] here
we focus on understanding the influence of the spacer between
azopolymer backbone and azobenzene groups on the side chain.
This variation of the spacer length is an important step towards a
deeper understanding of photoinduced reversible solid-to-liquid
transitions and the possibility for future applications such as
healing and reprocessing hard polymers with light.
lengths between the polymer backbone and the azobenzene unit
(no spacer, 2, 6, 12 or 20 methylene units, Scheme 1). Number
averaged molecular weights of P-n-Azo were 7.6, 8.9, 9.9, 8.6 and
10.6 × 103 g mol-1, respectively (Table 1).
Photoinduced solid-to-liquid transition
All P-n-Azo in this study showed reversible trans-to-cis
photoisomerization in solid state, which was demonstrated by UV-
vis absorption spectroscopy of P-n-Azo films before and after light
irradiation (Figure 1, left). Before irradiation all P-n-Azo exhibit a
strong π-π* absorption band in the UV range and a weak n-π*
band in the visible light range. UV light irradiation of P-n-Azo films
decreased the π-π* band and increased the n-π* band,
demonstrating trans-to-cis isomerization of P-n-Azo which is well
known for azo chromophores. The cis-to-trans back isomerization
was induced by visible light irradiation of the cis n-π* absorption
band, enabling reversible photoisomerization. This reversible
trans-to-cis isomerization was induced by light irradiation in
several cycles (Figure 1, center). The absorbance of P-6-Azo and
P-12-Azo films was lower before irradiation than after UV and
visible light irradiation (Figure 1 and 2) because stacking and
orientation of the chromophores changed.[1a,14] Azo groups
preferentially orient perpendicular to the substrate before
irradiation.[14] The degree of perpendicular oriented azo groups is
reduced after UV and visible light irradiation and the absorption
maxima and shape of P-12-Azo in THF solution and in film were
different. The absorption band of the film was blue-shifted,
indicating H-aggregation of azo groups.[15]
Scheme 1. Photoisomerization and chemical structures of P-n-Azo
azopolymers with different spacer lengths ranging from 0 to 20.
Table 1. Some properties of P-n-Azo.
Spacer
Length
(number of
methylene
units)
Mn
/
PDI
(GPC)
DP[a]
Photoinduced
Solid-to-
Liquid
103 g mol-
1 (GPC)
While photoinduced solid-to-liquid transition of P-0-Azo, P-2-
Azo and P-20-Azo was not observed, P-6-Azo and P-12-Azo
showed photoinduced solid-to-liquid transitions (Figure 1, right).
P-2-Azo powders showed no distinct change under optical
microscope and therefore P-2-Azo was not further investigated.
Transition
P-0-Azo
P-2-Azo
P-6-Azo
P-12-Azo
P-20-Azo
0
7.6
8.9
9.9
8.6
10.6
1.63
1.60
1.32
1.38
1.38
29
30
28
20
20
No
2
No
Photoinduced reversible solid-to-liquid transitions of P-12-
Azo
6
Yes
Yes
No
12
20
First, we investigated why P-12-Azo showed photoinduced
reversible solid-to-liquid transitions. Photoswitching trans P-12-
Azo to cis P-12-Azo liquefied the sample. The liquid cis P-12-Azo
was switched back to a solid P-12-Azo using green light (Figure
3a). While cis P-12-Azo was soft and stuck to a needle, trans P-
12-Azo was hard and could be removed from the surface by
scratching with a needle and blowing away with a stream of N2.
Accordingly, in piezo rheology measurements the moduli of P-12-
Azo changed from low values for cis P-12-Azo to high values for
trans P-12-Azo. (Figure 3b, c). For all tested frequencies, the fact
that the storage modulus (G’) of cis P-12-Azo was higher than its
loss modulus (G’’) evidenced that cis P-12-Azo was in a liquefied
state (Figure 3 and S21).
To better understand why P-12-Azo was liquefied at room
temperature by UV light irradiation, we performed differential
scanning calorimetry (DSC) measurements (Figure 3d, e). P-12-
Azo was stable at least up to 200 °C (Figure S22). DSC curve of
trans P-12-Azo was measured and compared to the DSC curve
of cis P-12-Azo. Trans P-12-Azo showed a Tg at 45 °C (Figure 3e
and S23b) and a phase transition at 120 °C. P-12-Azo was
[a] Degree of polymerization calculated from GPC results.
Results and Discussion
Synthesis of P-n-Azo
First, the new azopolymers P-n-Azo were synthesized and
characterized using NMR and GPC (Scheme S1 and Figures S1-
S20). The synthesis of the azopolymers was according to our
previous work with some modifications.[1a] In particular, we
synthesized azopolymers with long and no spacers, which were
not investigated in the previous work. After synthesizing the
azobenzene molecule, methylene spacers of different sizes were
attached to the azobenzene molecule, followed by an acrylate unit.
The monomers were then polymerized by radical polymerization
to get P-n-Azo. P-n-Azo are polyacrylates with different spacer
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Figure 1. Photoisomerization of P-n-Azo (n = 0, 2, 6, 12, 20). Left: UV-vis absorption spectra of P-n-Azo films before irradiation (black line), after UV irradiation (365
nm, 10 min, 45 mW cm-2) (red line) and after subsequent green light irradiation (blue line) (530 nm, 2 min, 50 mW cm-2). Center: Value of absorbance maximum
after repeated UV and green light irradiation demonstrating switching cycles. Right: Optical microscopy images of P-n-Azo powders before and after UV light
irradiation (365 nm, 8 min, 20 mW cm-2). Image size: 260 µm × 130 µm.
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isotropic above 120 °C and anisotropic below 120 °C (Figure S24).
After UV light irradiation, trans P-12-Azo was transformed into cis
P-12-Azo (Figure S25). Cis P-12-Azo showed a Tg below room
temperature (-9 °C) and thus acted as a liquid at room
temperature. During the first DSC heating, cis P-12-Azo started to
thermally transform back to trans P-12-Azo at 50 °C and was
completely switched to trans P-12-Azo after the first heating. In
the second heating the DSC curve was the same as that for the
solid trans P-12-Azo. It can be concluded that the Tg difference of
cis and trans P-12-Azo leads to the photoinduced solid-to-liquid
transition.
irradiation. Measurements were conducted at 18 °C. d) DSC first and second
heating curves of cis P-12-Azo. During first heating the cis form switched back
to the trans form thermally, demonstrated by the exothermic peak around 100 °C.
The second heating curve was similar to the trans heating curves in e),
demonstrating the cis form was completely switched back to trans by heating.
LC: liquid crystalline phase; I: isotropic phase.
Figure 2. UV-vis absorption spectra of P-12-Azo in THF solution (black) in
comparison to P-12-Azo film before irradiation (red) and after UV (365 nm, 45
mW cm-2, 10 min) and visible light irradiation (530 nm, 88 mW cm-2, 2 min) (blue).
The absorption spectra of P-12-Azo film before and after irradiation were
different because orientation and stacking of azo groups in the film were
different before and after irradiation.
Figure 3. Photoinduced liquid-to-solid transition of P-12-Azo. a) Photographs of
liquid cis P-12-Azo and solid trans P-12-Azo. First, the liquid stuck to a needle.
After visible light irradiation, the liquid turned into a solid and was scratched from
the substrate with a needle (dashed circle). The powder was removed by
blowing with a nitrogen gun. b) G’ = storage modulus; G” = loss modulus of cis
P-12-Azo and c) after it was switched back to trans P-12-Azo using green light
Figure 4. DMA data of cis P-12-Azo. G’ = storage modulus; G” = loss modulus.
From the maximum of G” the glass transition temperatures were determined. a)
Cis P-12-Azo was cooled from room temperature to -20 °C. Tg was -14 °C. b)
Cis P-12-Azo was then heated from -20 to 120 °C. Tg was -14 °C. During the
heating, the cis P-12-Azo was switched to trans P-12-Azo thermally. c) Then,
the sample was cooled towards room temperature. Tg was 48 °C, which
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confirms that cis P-12-Azo was a liquid at room temperature, while trans P-12-
Azo was a solid at room temperature.
Photoswitchable Tg of P-12-Azo was confirmed by dynamic
mechanical analysis (DMA) measurements (Figure 4): Cis P-12-
Azo was cooled from room temperature to -20 °C where G” had a
maximum at -14 °C. During heating G” still had a maximum
at -14 °C. This showed that Tg (-14 °C) of Cis P-12-Azo is below
room temperature. Cis P-12-Azo thermally switched back to trans
P-12-Azo during the first heating. During the second cooling G”
had a maximum at 48 °C, which was in accordance with Tg of
trans P-12-Azo measured by DSC. This experiment confirmed
that cis P-12-Azo was a liquid and trans P-12-Azo was a solid. P-
12-Azo has switchable Tg because of trans-cis isomerization.
In the trans state, P-12-Azo showed anisotropic structures. UV
light irradiation changed the anisotropic trans state to an isotropic
cis state (Figure 5 and S26). However, the loss of the anisotropic
structure was not the reason for the solid-to-liquid transition. By
quenching trans P-12-Azo from the isotropic state at high
temperature, amorphous trans P-12-Azo was prepared, which
also showed the photoinduced solid-to-liquid transition (Figure
S27). P-12-Azo, which had a slightly longer methylene spacer
between the backbone and the chromophore than P-6-Azo,
showed similar behavior to P-6-Azo.[1a]
Figure 6. First and second heating DSC curves of trans and cis P-0-Azo. Note:
the y-axis of the trans and cis DSC curves have different ranges because of the
large exothermic cis-to-trans peak in the first heating of the cis DSC curve. The
zoomed-in cis DSC curve has the same y axis range as the trans curve to
demonstrate similarity of second heating curves of P-0-Azo. Dotted lines are
guides for the eye to see the step which corresponds to Tg. Photographs show
solid trans (yellow) and cis (red) P-0-Azo powders.
Photoresponsiveness of P-20-Azo
Photoinduced solid-to-liquid transition of P-20-Azo was not
observed at room temperature (Figure 1). Trans P-20-Azo
showed two phase transitions at ~80 °C and ~110 °C (Figure 7a
and S24). Above 125 °C, P-20-Azo was isotropic; below 125 °C,
P-20-Azo was anisotropic. After UV irradiation, trans P-20-Azo
was transformed to cis P-20-Azo and showed a phase transition
at ~40 °C. Even though cis P-20-Azo had a lower phase transition
than trans P-20-Azo, cis P-20-Azo was not a liquid at room
temperature because the phase transition temperature was
above room temperature. However, when trans P-20-Azo was
irradiated using UV light at 50 °C, liquefaction occurred (Figure
7b). These results showed that cis P-20-Azo was liquid at 50 °C
and trans P-20-Azo was solid at 50 °C. Photoinduced solid-to-
liquid transition was achieved at 50 °C. Also, cis P-20-Azo was
isotropic at 60 °C, while it was anisotropic at 36 °C and could be
reversibly switched between the two states (Figure 7c).
Figure 5. Polarized optical microscopy (POM) images of P-12-Azo before
irradiation, after UV irradiation (365 nm, 65 mW cm-2) through a mask, green
light irradiation (530 nm, 12 mW cm-2) and annealing at 150°C. The scale bar is
250 μm. P: polarizer; A: analyzer. Trans azo groups exhibited liquid crystalline
order. UV irradiation transformed azo groups from an anisotropic trans to an
isotropic cis state. Visible light irradiation switches cis azo groups back to trans
azo groups. The resulting trans state was amorphous. The anisotropic order
was regenerated by annealing at 150 °C and cooling down to room temperature.
Photoresponsiveness of P-0-Azo
Photoinduced solid-to-liquid transition of P-0-Azo was not
observed (Figure 1). Trans P-0-Azo has a Tg at ~100 °C (Figure
6 and Figure S23a). Anisotropic phases of P-0-Azo could not be
observed because the temperature for P-0-Azo to become
isotropic is close to the decomposition temperature of P-0-Azo.
When the polymer was switched to cis, a Tg shift was not
observed. Tg of trans P-0-Azo is in the same temperature range
of the broad exothermic peak of the thermal cis-to-trans back
reaction. Tg of cis P-0-Azo could overlap with the cis-to-trans peak.
Additionally, the step in DSC which is considered as Tg was small.
After preparation for DSC measurements, cis P-0-Azo was a fine
powder (Figure 6), in contrast to cis P-12-Azo, which was a liquid
(Figure 3a). Typically, a spacer is used between the azo
chromophore and the polymer backbone to provide enough space
and freedom for the motions of azo chromophores.[15a, 16] Our
results demonstrated that the spacer is essential for photoinduced
solid-to-liquid transition of azopolymers.
Figure 7. a) DSC first and second heating curves of trans and cis P-20-Azo.
b) Optical microscopy of trans P-20-Azo powders at 20 °C, after heating to 50 °C
and after UV light irradiation at 50 °C. c) POM images of a cis P-20-Azo film
losing anisotropy after heating to 60 °C and recovers it after cooling to 36 °C.
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Azopolymers typically exhibit liquid crystalline phases.[17]
Switching trans azobenzene groups in polymer side chains to cis
azobenzene disrupts the anisotropic phase because in contrast to
the rigid trans azobenzene, cis azobenzene is a bent molecule.[18]
Longer spacer increases the interaction of polymer side chains.
When the azobenzene at the end of the long side chain is
switched from trans to cis, there are interactions between the long
alkyl side chains. Crystallization of P-20-Azo is similar to
polyethylene or polyacrylates with long side chains but without
azobenzene,[19] which explains why a phase transition at ~40 °C
was observed for cis P-20-Azo.
was in a cis rich state and the film was liquid. Then, the film
surface was scratched with an AFM tip attached to
a
micromanipulator. Immediately the polymer flowed to fill the gap
of the scratch after 10 s (Figure 8b and Movie 1).
To neglect any heating effect during irradiation several control
experiments were performed.
1. There were 30 min between irradiation and scratching. The
film was still healed.
2. A thermal imager was used to detect the temperature of the
polymer film during irradiation. The temperature of the film did not
rise above 25 °C (Figure S28), which was below Tg of the trans
polymer.
3. Irradiating the film with visible light for 30 s to transform the
liquid cis film back to the solid trans film immediately stopped the
flow (Figure S29a and Movie 2).
4. Heating the film to 35 °C for 10 min did not liquefy the film
(Figure S29b and Movie 3). After that the same film was irradiated
with UV light again (365 nm, 65 mW cm-2, 6 min) to obtain a cis
rich film and the scratches on it was immediately healed again
(Figure S30 and Movie 4).
All these experiments clearly showed that the flow and healing
were because of photoisomerization but not a photothermal effect.
Conclusions
Side-chain type azopolymers with different spacer lengths were
synthesized and their photoresponsive properties were studied.
For P-6-Azo and P-12-Azo, photoswitchable Tg and photoinduced
reversible solid-to-liquid transitions were observed. For
azopolymers with a short or long spacer between the azobenzene
group and the polymer backbone, photoinduced solid-to-liquid
transitions at room temperature were not observed. P-0-Azo did
not show photoinduced solid-to-liquid transition because its Tg did
not show significant decrease after irradiation. P-20-Azo did not
show photoinduced solid-to-liquid transition at room temperature,
but it showed the transition at 50 °C because the phase transition
temperature was decreased to ~40 °C. Its side chain is so long
that interactions among the side chains enable crystallization of
the polymer in solid state. However, the phase transition of cis P-
20-Azo was above room temperature and photoinduced solid-to-
liquid transitions cannot be observed at room temperature. When
cis P-20-Azo was heated to 50 °C, a photoinduced solid-to-liquid
transition was achieved. P-12-Azo showed similar behavior to P-
6-Azo. Using P-12-Azo, we demonstrated photoinduced healing
at real time for the first time. We expect that this kind of
photoresponsive polymers can be used as smart materials for
printing and microstructure preparation. By UV light irradiation,
the polymer can flow or be processed. After the desired structure
is formed, the polymer can be solidified by visible light irradiation
to obtain a tough material for different applications.
Figure 8. a) Scheme and optical microscopy images of a trans P-12-Azo film
before scratching, after scratching and healed after UV and visible light
irradiation. b) Scheme and optical microscopy images of a cis rich film before
scratching, after scratching and healed after 10 s. Characteristic areas of the
films were used to find exactly the same position on the films after irradiation.
Flowing and healing of P-12-Azo induced by light
Inspired by the photoinduced reversible solid-to-liquid transitions
of P-12-Azo, the polymer was used for photo-healable coatings.
Spin-coated films of P-12-Azo were scratched with an AFM tip
attached to a micromanipulator at characteristic spots to
demonstrate the same area is used for all measurements. After
UV light irradiation and subsequent visible light irradiation, the
scratch was healed because cis P-12-Azo was a liquid and can
flow to fill the gap of the scratch (Figure 8). Visible light switched
back the liquid cis to a solid trans film.
The flowing of the material on a cis P-12-Azo surface was also
studied under an optical microscope. To demonstrate healing of
cis P-12-Azo a film was irradiated with UV light (6 min,
65 mW cm-2) to obtain a cis rich film. After irradiation, P-12-Azo
Experimental Section
Materials: Phenol (>99%), p-toluidine (99%), acryloyl chloride (97%),
reactants and solvents were purchased from Sigma Aldrich and used
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without further purification. 12-Bromo-1-dodecanol (>98%) was purchased
from TCI. Dimethyl icosanedioate (>95%) was purchased from TCI. 2,2’-
Azobisisobutyronitrile (AIBN) (98%) was purchased from Sigma Aldrich
and was recrystallized from ethanol before use. Milli-Q water was provided
by a Sartorius Arium 611 VF Purification System.
Keywords: azobenzene • photoisomerization • photoresponsive
• polymers • self-healing
[1]
a) H. Zhou, C. Xue, P. Weis, Y. Suzuki, S. Huang, K. Koynov, G. K.
Auernhammer, R. Berger, H.-J. Butt, S. Wu, Nat. Chem. 2017, 9, 145-
151; b) A. Pipertzis, A. Hess, P. Weis, G. Papamokos, K. Koynov, S. Wu,
G. Floudas, ACS Macro Letters 2018, 7, 11-15.
Synthesis: The synthesis route of the P-n-Azo polymers is shown in
Scheme S1. Details of syntheses and characterization of the azopolymers
are provided in the Supporting Information (Figures S1-S20).
[2]
[3]
[4]
G. S. Kumar, D. C. Neckers, Chem. Rev. 1989, 89, 1915-1925.
F. Ercole, T. P. Davis, R. A. Evans, Polym. Chem. 2010, 1, 37-54.
H. Rau, in Photochemistry and Photophysics, Vol. 2 (Ed.: J. Rabeck),
Boca Raton, FL, 1990, pp. 119-141.
Methods: Proton and carbon nuclear magnetic resonance (NMR) spectra
were recorded on a Bruker Avance 250 MHz, 300 MHz or 500 MHz
spectrometer. Mass spectra (MS) were obtained using a VG instrument
ZAB 2-SE-FPD. The molecular weights and molecular weight distributions
of P-n-Azo were determined using an Agilent Technologies 1260 Infinity
PSS SECurity GPC (pump: 1260 IsoPump) equipped with UV and RI
detectors running in tetrahydrofuran (THF) at 30 °C and a PLgel MIXED-B
column (dimension: 0.8 × 30 cm, particle size: 10 µm) with a polystyrene
standard. UV-vis absorption spectra were recorded on a Perkin Elmer
Lambda 900 spectrometer. Baselines were corrected and spectra were
normalized using OriginPro software. Optical microscopy images were
captured on a Zeiss optical microscope equipped with a CCD camera.
Thermogravimetric analysis (TGA) was carried out on a Mettler Toledo
[5]
a) Y. Okui, M. Han, Chem. Commun. 2012, 48, 11763-11765; b) H.
Akiyama, M. Yoshida, Adv. Mater. 2012, 24, 2353-2356; c) Y. Norikane,
E. Uchida, S. Tanaka, K. Fujiwara, H. Nagai, H. Akiyama, J. Photopolym.
Sci. Tec. 2016, 29, 149-157; d) E. Uchida, R. Azumi, Y. Norikane, Nat.
Commun. 2015, 6, 7310; e) M. Hoshino, E. Uchida, Y. Norikane, R.
Azumi, S. Nozawa, A. Tomita, T. Sato, S.-i. Adachi, S.-y. Koshihara, J.
Am. Chem. Soc. 2014, 136, 9158-9164; f) Y. Norikane, E. Uchida, S.
Tanaka, K. Fujiwara, E. Koyama, R. Azumi, H. Akiyama, H. Kihara, M.
Yoshida, Org. Lett. 2014, 16, 5012-5015; g) T. Yamamoto, Y. Norikane,
H. Akiyama, Polym. J. 2018, 50, 551-562; h) M. Baroncini, S. d'Agostino,
G. Bergamini, P. Ceroni, A. Comotti, P. Sozzani, I. Bassanetti, F.
Grepioni, T. M. Hernandez, S. Silvi, M. Venturi, A. Credi, Nat. Chem.
2015, 7, 634-640.
TGA-851 under nitrogen atmosphere with a heating rate of 10 °C min−1
.
DSC data were collected using a Mettler Toledo DSC-822 under nitrogen
atmosphere. The polymers were measured with a heating or cooling rate
of 10 °C min-1. The shear moduli were measured using a homemade
piezo-rheometer in shear mode at 18 °C.[20] A cis azopolymer sample with
a thickness of 100 μm was sandwiched between two quartz plates that
were used as transparent sample holders. Firstly, the shear moduli of cis
azopolymers were measured in a dark environment to prevent possible
light-induced transitions. Subsequently, cis azopolymers between the
quartz plates were converted to trans azopolymers by 530 nm light
irradiation. Trans azopolymers were measured by the piezo-rheometer
under the same conditions. DMA was conducted on an Advanced
Rheometric Expansion System (ARES, Rheometric Scientific Company).
Shear deformation was applied under conditions of controlled deformation
amplitude, which was kept in the range of the linear viscoelastic response
of the studied samples. Plate-plate geometry was used with plate
diameters of 6 mm. The experiments were conducted under dry nitrogen
atmosphere at a heating rate of 3 °C min-1 and a constant deformation
frequency of 1 or 10 rad s-1. Photoisomerization was induced by LEDs with
wavelengths of 365 nm, 470 nm and 530 nm (Mightex Systems, device
types LCS-0365-07-22, LCS-0470-03-22 and LCS-0530-15-22,
respectively). The output intensities were controlled by an LED controller
(Mightex Systems, device type SLC-MA04-MU), calibrated by an optical
powermeter (Spectra-Physics Corporation, Model 407A). Preparation of
cis polymers for DSC and DMA measurements was described in our
previous paper.[1a] An MMO-203 hydraulic micromanipulator (Narishige,
Tokyo, Japan) equipped with an AFM cantilever was used to scratch
polymer films.
[6]
a) J.-I. Anzai, T. Osa, Tetrahedron 1994, 50, 4039-4070; b) J. Stumpe,
T. Fischer, H. Menzel, Macromolecules 1996, 29, 2831-2842; c) J. M.
Kuiper, J. B. F. N. Engberts, Langmuir 2004, 20, 1152-1160; d) S. Wu, L.
Wang, A. Kroeger, Y. Wu, Q. Zhang, C. Bubeck, Soft Matter 2011, 7,
11535-11545; e) P. Rochon, J. Gosselin, A. Natansohn, S. Xie, Appl.
Phys. Lett. 1992, 60, 4-5.
[7]
[8]
H. Akiyama, T. Fukata, A. Yamashita, M. Yoshida, H. Kihara, J. Adhesion
2017, 93, 823-830.
a) L. Zhang, H. Liang, J. Jacob, P. Naumov, Nat. Commun. 2015, 6,
7429; b) Y. Yu, M. Nakano, T. Ikeda, Nature 2003, 425, 145-145; c) W.
Wu, L. Yao, T. Yang, R. Yin, F. Li, Y. Yu, J. Am. Chem. Soc. 2011, 133,
15810-15813; d) H. Yu, T. Ikeda, Adv. Mater. 2011, 23, 2149-2180; e) J.
Hu, X. Li, Y. Ni, S. Ma, H. Yu, J. Mater. Chem. C 2018, 6, 10815-10821.
a) A. Kravchenko, A. Shevchenko, V. Ovchinnikov, A. Priimagi, M.
Kaivola, Adv. Mater. 2011, 23, 4174-4177; b) W. Wang, C. Du, X. Wang,
X. He, J. Lin, L. Li, S. Lin, Angew. Chem. Int. Ed. 2014, 53, 12116-12119;
c) X. Kong, X. Wang, T. Luo, Y. Yao, L. Li, S. Lin, ACS Appl. Mater. &
Interfaces 2017, 9, 19345-19353; S. Huang, Y. Chen, S. Ma, H. Yu,
Angew. Chem. Int. Ed. 2018, 57, 12524-12528
[9]
[10] a) R. Wei, J. Ma, H. Zhang, Y. He, J. Appl. Polym. Sci. 2016, 133; b) J.
Wang, S. Wang, Y. Zhou, X. Wang, Y. He, ACS Appl. Mater. Interfaces
2015, 7, 16889-16895; c) Y. Li, Y. He, X. Tong, X. Wang, J. Am. Chem.
Soc. 2005, 127, 2402-2403.
[11] T. J. Kucharski, N. Ferralis, A. M. Kolpak, J. O. Zheng, D. G. Nocera, J.
C. Grossman, Nat. Chem. 2014, 6, 441-447.
[12] a) P. Weis, W. Tian, S. Wu, Chem. Eur. J. 2018, 24, 6494-6505; b) S. Ito,
A. Yamashita, H. Akiyama, H. Kihara, M. Yoshida, Macromolecules 2018,
51, 3243-3253; c) S. Ito, H. Akiyama, R. Sekizawa, M. Mori, M. Yoshida,
H. Kihara, ACS Appl. Mater. Interfaces 2018, 10, 32649-32658; d) R. H.
Zha, G. Vantomme, J. A. Berrocal, R. Gosens, B. de Waal, S. Meskers,
E. W. Meijer, Adv. Funct. Mater. 2018, 28, 1703952; e) W.-C. Xu, S. Sun,
S. Wu, Angew. Chem. Int. Ed. 2019, in press, 10.1002/anie.201814441.
[13] a) C. Appiah, G. Woltersdorf, R. A. Pérez-Camargo, A. J. Müller, W. H.
Binder, Eur. Polym. J. 2017, 97, 299-307; b) Y. Yue, Y. Norikane, R.
Azumi, E. Koyama, Nat. Commun. 2018, 9, 3234.
Acknowledgements
We acknowledge technical support from J. Thiel, A. Best, A.
Hanewald, Dr. M. Kappl and helpful discussions with Prof. G.
Floudas. G. K. A. and S. H. acknowledge financial support by
DFG through the SPP 1681 (grant number AU 321/3). This work
was supported by the Deutsche Forschungsgemeinschaft (DFG,
WU 787/2-1 and WU 787/8-1), the Thousand Talents Plan, and
the Fonds der Chemischen Industrie (FCI, No. 661548).
[14] K. Nishizawa, S. Nagano, T. Seki, Chem. Mater. 2009, 21, 2624-2631.
[15] a) H. Menzel, B. Weichart, A. Schmidt, S. Paul, W. Knoll, J. Stumpe, T.
Fischer, Langmuir 1994, 10, 1926-1933; b) X. Tong, L. Cui, Y. Zhao,
Macromolecules 2004, 37, 3101-3112.
This article is protected by copyright. All rights reserved.

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[16] a) T. Manouras, M. Vamvakaki, Polym. Chem. 2017, 8, 74-96; b) G. D.
Han, S. S. Park, Y. Liu, D. Zhitomirsky, E. Cho, M. Dinca, J. C. Grossman,
J. Mater. Chem. A 2016, 4, 16157-16165.
[17] T. Ikeda, T. Sasaki, S. Kurihara, Proc. Jpn. Acad., Ser. B 1993, 69, 7-12.
[18] J. M. Robertson, J. Chem. Soc. 1939, 232-236.
[19] S. D. Baruah, N. C. Laskar, B. Subrahmanyam, J. Appl. Polym. Sci. 1994,
51, 1701-1707.
[20] M. Roth, M. D’Acunzi, D. Vollmer, G. K. Auernhammer, J. Chem. Phys.
2010, 132, 124702.
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Entry for the Table of Contents
FULL PAPER
The influence of the spacer (R in the
TOC figure) on photoinduced solid-
to-liquid transitions of azopolymers is
investigated. While azopolymers with
moderate spacer length exhibit
photoinduced solid-to-liquid
Philipp Weis, Andreas Hess, Gunnar
Kircher, Shilin Huang, Günter K.
Auernhammer, Kaloian Koynov, Hans-
Jürgen Butt, Si Wu*
Page No. – Page No.
transitions, azopolymers with short
and long spacer do not. Healing
scratches by light is demonstrated on
hard coatings made from
azopolymers and followed at real
time.
Effects of Spacers on Photoinduced
Reversible Solid-to-Liquid
Transitions of Azobenzene-
Containing Polymers
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