Effects of double photoreactions on polycoumarate photomechanics

Article abstract of DOI:10.1002/pola.24525

We established a polymerization condition of 3-hydroxycinnamic acid, 3HCA, having photoreactive units over all the main chains. The polymers showed two types of photoreaction, that is, intramolecular E-Z isomerization and interchain [2 + 2] cycloaddition. Spectroscopic studies showed that the rate of the isomerization was higher than that of the cycloaddition. Although the original fiber was very brittle, the short-time UV-irradiation made the fiber softer and more elastic because the chain bending by the isomerization preferentially occurred. On the other hand, the long-time irradiation made the fiber harder because of the cross-linking by the cycloaddition, and simultaneously enhanced the elasticity. Besides we found that the cast film of P3HCA showed a photo-bending behavior with the convex surface facing the UV-lamp by the initial E-Z isomerization.

Full text of DOI:10.1002/pola.24525

Effects of Double Photoreactions on Polycoumarate Photomechanics
KATSUAKI YASAKI,1 TAKUYA SUZUKI,1 KOJI YAZAWA,2 DAISAKU KANEKO,1 TATSUO KANEKO1
1Department of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai,
Nomi, Ishikawa 923-1292, Japan
2Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259-B-55 Nagatsuta-cho,
Midori-ku, Yokohama 226-8501, Japan
Received 12 October 2010; accepted 24 November 2010
DOI: 10.1002/pola.24525
Published online 3 January 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: We established a polymerization condition of 3-
hydroxycinnamic acid, 3HCA, having photoreactive units over
all the main chains. The polymers showed two types of photo-
reaction, that is, intramolecular E–Z isomerization and inter-
chain [2 þ 2] cycloaddition. Spectroscopic studies showed that
the rate of the isomerization was higher than that of the cyclo-
addition. Although the original fiber was very brittle, the short-
time UV-irradiation made the fiber softer and more elastic
because the chain bending by the isomerization preferentially
occurred. On the other hand, the long-time irradiation made
the fiber harder because of the cross-linking by the cycloaddi-
tion, and simultaneously enhanced the elasticity. Besides we
found that the cast film of P3HCA showed a photo-bending
behavior with the convex surface facing the UV-lamp by the
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initial E–Z isomerization. 2011 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 49: 1112–1118, 2011
KEYWORDS: biomimetics; cycloaddition; isomer; isomerization;
photoisomerization; photomechanics; polyarylates; polyesters
INTRODUCTION The cinnamate group exists as a photochro-
mic center of a photoactive yellow protein in photosynthetic
bacteria, and photoisomerization from an E to a Z coumarate
conformation changes the structure of the big protein
chains.1,2 On the other hand, cinnamates have been conven-
tionally used as a photocuring group showing [2 þ 2] cyclo-
addition to prepare photomechanical materials.3 These two
photoreactions simultaneously occur and have been con-
trolled by adjustment of intermolecular arrangements.4 Thus,
cinnamate derivatives are bioavailable, and photoreactive
components are useful to prepare the photomechanical poly-
mers. We have studied the photoreactive polymers where
the coumarates (hydroxyl cinnamates) were incorporated
into their main chains to create aromatic polyesters with
rigid backbones. The homopolymer oligo[4-hydroxycinnamic
acid (4HCA)] showed the [2 þ 2] cycloaddition responding
to the liquid crystalline alignment.5 The polyarylates
poly[4HCA-co-3,4-dihydroxy-cinnamic acid (DHCA)] showed
the cycloaddition to induce phototunable hydrolysis, which
may lead to timely degradable plastics.6,7 The photoreaction
of cinnamate groups existing in the rigid main chains of the
polyesters may strongly affect the mechanical properties of
the corresponding fibers and films.
materials to find a photoinduced increase in strain energy
and a photo-bending phenomenon.
RESULTS AND DISCUSSION
Polymerization
There are several bioderived coumarates such as 4HCA (p-
coumaric acid), 3-methoxy-4HCA (feluric acid), 3,5-dime-
thoxy-4HCA (sinapic acid), and 3HCA (m-coumaric acid). We
tried to homopolymerize these monomers by a previously
reported one-pot polymerization5–7 using an acetic anhy-
dride as well as a sodium acetate catalyst. The polymeriza-
tion of three former monomers with 4-substituted OH
groups did not proceed well to get only oligomers, presum-
ably due to too rigid chains showing no melting or solubility
in any solvents. On the other hand, the 3HCA polymerization
was proceeded at some extent to create poly(3HCA) P3HCA
although its number-average molecular weight, Mn ¼ 9600,
was not so high to process it. Then, we tried to establish
improved polymerization methods to prepare P3HCA from
acetylated 3HCA monomer, which was polymerized in bulk
at 200 ꢀC for 6 h in vacuo using various alkaline catalysts
[Scheme 1(a)]. In Table 1, the molecular weights (GPC; poly
(methyl methacrylate) standard) of P3HCA prepared using
various catalysts (1 mol % to monomers) were summarized.
Mn and the weight-average molecular weight (Mw) of P3HCA
reached 18,600 and 27,800, respectively; when the polymer-
ization was made with sodium phosphate, which might work
Here, we prepare a semiflexible polycoumarate to induce
double photoreactions of E–Z isomerization and [2 þ 2]
cycloaddition and investigate the influence of the photoreac-
tions on the mechanical properties of the polycoumarate
Additional Supporting Information may be found in the online version of this article. Correspondence to: T. Kaneko (E-mail: kaneko@jaist.ac.jp)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 1112–1118 (2011) 2011 Wiley Periodicals, Inc.
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SCHEME 1 (a) Polymerization of 3HCA to
create a polyester P3HCA. (b) Photoisome-
rization schemes of E–Z transformation
and photodimerization from two forms of
3HCA into its dimer linked by cyclobutane
group.
as an alkaline catalyst as well as a condensation reagent. IR
and 1H (nuclear magnetic resonance) NMR spectroscopy
demonstrated the formation of a targeted polyester structure
of P3HCA (Supporting Information Figs. S1 and S2). P3HCA
was dissolved in trifluoroacetic acid (TFA), N-methylpyrroli-
done, N,N0-dimethylformamide (DMF), and N,N0-dimethylace-
toamide to lead the formation of the cast films from their
solutions (Supporting Information Table S1); the film was
light yellow but transparent (Supporting Information Photo
S1). Figure 1(a) shows a differential scanning calorimetry
thermogram that showed a small but distinct flection point
typical of a glass transition (Tg) at around 121 ꢀC. On the
other hand, the polymers became viscous liquid over Tg to
create long fibers by melt spinning at a drawing rate of 5 cm
minꢁ1. Figure 1(b) shows a thermogravimetry (TG) diagram
indicating no distinct weight loss at less than 300 ꢀC. The
temperature where 10% weight loss (T10) was detected was
355 ꢀC. The values of Tg and T10 were high enough to keep
the original material property under the UV-irradiation con-
ditions from a high-pressure mercury lamp with an intensity
of 72.2 mW cmꢁ2 (with a wavelength cut filter giving a
wavelength range ofꢀk > 250 nm), which slightly heated the
samples at most 30 C.
Photoreaction
The photoreactivity of P3HCA was investigated by measuring
the UV–vis absorption spectra of the UV-irradiated speci-
mens, and the results are summarized in Figure 2(a). The
TABLE 1 Molecular Weights of P3HCA Prepared Under
Various Catalysts
Molecular Weights
Catalysls
Mn
9,800
Mw
Mw/Mn
(COONa)2
CH3COONa
Na2CO3
23,800
20,900
20,700
27,800
20,900
20,300
2.4
2.2
2.3
1.5
3.3
1.8
FIGURE 1 Thermodynamic properties of poly(3-hydroxycinnamic
acid). (a) Differential scanning calorimetry showed a glass transi-
tion temperature, Tg, at 121 ꢀC. (b) Thermogravimetry analysis
indicated a weight loss over 300 ꢀC and the 10%-weight loss
temperature, T10, was 355 ꢀC. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
9,600
8,800
Na3PO4
18,600
6,200
NaOH
CH3ONa
11,000
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
the P3HCA structure was disordered, presumably because
structural variation by E–Z isomerization and [2 þ 2]
cycloaddition.
Because it was difficult to estimate the degree of reaction
from the UV–vis or IR spectroscopy, 1H NMR studies were
performed. As shown in Supporting Information Figure S2,
P3HCA showed doublet peak at 6.80–6.88 ppm whose cou-
pling constants, J, ranged between 0.040 and 0.041 Hz, for
the b-proton of the E-form of the cinnamate group. The pho-
toreacted P3HCA only partially dissolved into TFA-d, and the
amount of the insoluble portion increased with an increase
in irradiation period. This phenomenon suggested that the
cross-linking of the [2 þ 2] cycloaddition occurred in the
solid state. After UV irradiation, the 1H NMR spectra of the
TFA-d-soluble parts of P3HCA were measured (Fig. 3). After
UV irradiation, doublet signals appeared at chemical shifts, d,
of 6.42–6.48 whose J value ranged between 0.030 and 0.032
Hz. They were assigned to the b-proton of the Z-form cinna-
mate group as shown in Scheme 1(b). Their peak intensities
increased with an increase in irradiation period (a-proton of
FIGURE 2 Spectroscopic studies of the photoreaction in
poly(3HCA). (a) Change in UV–vis spectra of P3HCA film irradiated
by UV light as a function of irradiation time. (b) IR/ATR spectra of
a polymer film UV-irradiated for 0, 1, 10, and 100 min.
peak appeared at kmax ¼ 295 nm in the UV–vis spectrum of
the P3HCA film before light irradiation. The peak intensity
was reduced by light irradiation, and shifted to a shorter
wavelength. From the basic reaction of the coumarates, one
can postulate that P3HCA mainly showed two kinds of pho-
toreactions as shown in Scheme 1(b). In this case, the peak
attenuation and shift may be due to the [2 þ 2] cycloaddi-
tion and isomerizations, respectively. The structural changes
of P3HCA were also confirmed using an ATR system. Figure
2(b) shows the IR/ATR spectra of the surface for P3HCA
films UV-irradiated for 0, 1, 10, and 100 min. The distinct IR
peaks assigned to vinylene bonds [AHC¼¼CHA] appeared at
1650, 1430, and 980 cmꢁ1 in the spectra of the nonirradi-
ated film. After 1 min of irradiation, the spectrum seemed to
show no intrinsic change as compared with the spectrum of
the nonirradiated film. On the other hand, the absorption
peak at 1430 cmꢁ1 shifted slightly to a higher wavenumber
and became weak. Furthermore, all of the peaks became
broadened after 10 min. of irradiation. With longer irradia-
tion times (100 min), the IR peaks became broader. These
peak changes indicate that the vinylene peaks reacted, and
FIGURE 3 1H NMR spectra of a polymer solution UV irradiated
for 0–100 min. Inset: close up spectrum in the 6.45–6.75 ppm
range. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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ARTICLE
FIGURE
4 (a)
Stress–strain
curves of P3HCA fiber recorded
by tensile tester. (b) Initial tensile
modulus, E, estimated from the
inclination of stress–strain curve
within
e
¼
0.015 and strain
energy per unit volume, U, from
the area under the stress–strain
curve.
Z-form also seemed to increase their intensities, but the
peak isolation from benzene peaks was difficult). These
results supported the E–Z photoisomerization of P3HCA by
UV irradiation. The degree of isomerization was estimated
from the ratio of the peak integral intensities for Z-form to
those of both forms to be 0.003 for 1 min of UV irradiation.
The isomerization degree increased to 0.011 with an
increase in the irradiation time up to 100 min. These results
suggested that the E–Z isomerization occurred in the solid
state, whereas the solid circumstance restricted the isomeri-
zation accompanied by a free volume change according to
the literature.8 The E–Z isomerization may be attributed to
the mobility of the P3HCA chains at room temperature as
seen in the local rotation of phenylester polymers.9 Although
the ratio of E–Z isomerization was very small, it is sufficient
to make a change of stable polymer chain conformation from
extended to bended one. On the other hand, two doublet sig-
nals at 5.08–5.15 and 4.84–4.88 ppm were observed in the
1H NMR spectra of P3HCA UV irradiated for 3 min and lon-
ger. These signals were assigned to cyclobutanes formed as a
result of UV dimerization. The degree of dimerization
degrees, which was estimated from the signal integral inten-
sities ratio of cyclobutane to vinylene plus cyclobutane, to be
0.001 for 3 min of irradiation and 0.021 for 100 min. The
peaks were extremely small and the photoreaction degree
reflected the reaction behavior of only soluble parts of
P3HCA were analyzed, but one can confirmed that their
intensities increased with an increase in irradiation time.
Furthermore, when irradiation time was more than 10 min,
the intensity ratio of cycloaddition was larger than that of
isomerization, due to dimerization not only from E-form but
also from Z-form. As a consequence, NMR studies suggested
that the speed of the cycloaddition was slower than that of
the isomerization reaction at a beginning of photoirradiation.
strain (S–S) curves. The nonirradiated fiber showed almost
straight S–S line (closed marks) and broke at a small e value
of about 0.03, which was a typical S–S curve of hard-rod
polymers. On the other hand, S–S curves of photoirradiated
fibers showed drastic rises at a range of e ¼ 0.02–0.04 after
initial straight lines. Especially, it is noteworthy that
increases in e after the rises were higher than e until the
rises. The S–S studies indicated that the fibers got stronger
after they were elongated at a certain value.
To study the S–S behavior more quantitatively, we estimated
initial tensile modulus, E, from the inclination of S–S curve
within e ¼ 0.015 and strain energy per unit volume, U, from
the area under the S–S curve [Fig. 4(b)]. Figure 4(b) shows
that a 1-min irradiation made the fiber softer and more
stretchable than the nonirradiated fiber. On the other hand,
the UV irradiation for more than 1 min made the fiber
harder, but the fiber still kept the improved stretchability.
Hardened fibers generally decrease their e value at break,10
but the P3HCA fibers became harder and more stretchable,
namely, more robust. Effects of UV irradiation on U values,
which were an index of robustness, were estimated to show
in Figure 4(b). U of the nonirradiated fiber was 7.59 ꢂ 105 J
mꢁ3 and, in the figure, the initial value was normalized as 1.
U values drastically increased to 3.30 ꢂ 106 J mꢁ3 by a
short-time UV irradiation for only 1 min (light energy; 0.173
J).11 In other words, the light imparted strain energy to the
fiber as high as 2.5 ꢂ 106 J mꢁ3 (difference between 3.30 ꢂ
106 and 7.59 ꢂ 105 J mꢁ3), which was translated as 0.00157
J in the fiber with a dimension of 0.02 m ꢂ 0.2 mmf. Then,
it can be considered that 0.9% of light energy was used for
increasing U of the fiber. The energy conversion degree was
not so high and decreased with an increase in irradiation
time. However, the effects of light irradiation on an increase
in e at break are significant.
Mechanical Behavior
We discuss these phenomena in the viewpoint of photochem-
ical reaction of P3HCA. Figure 5 shows the schematic illus-
tration of structural change by UV irradiation and/or tensile
force application. Because the vinylene group of P3HCA was
originally in the trans-form, the chains can adopt an almost
straight conformation as shown in Figure 5(a). If the tensile
We were aware of the increase in P3HCA fiber toughness fol-
lowing UV irradiation when handling them. Then, we made
the tensile test of the P3HCA fibers with a length 20 mm
and a radius of 0.1 mm, after Hg lamp was irradiated to
them for different periods. Figure 4(a) shows their stress–
EFFECTS OF DOUBLE PHOTOREACTIONS, YASAKI ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 Schematic illustration
of structural changes of P3HCA
fibers by UV-irradiation and/or
tensile force application.
force is applied to the fiber in this form, the fiber can be
easy to break because little chain entanglements was made,
and the chains easily slide one another [Fig. 5(b)]. On the
other hand, UV lamp was irradiated to the fiber to induce
the photoreaction such as trans-cis isomerization and [2 þ
2] cycloaddition. Because the isomerization can occur faster
than the cycloaddition, a short-time irradiation induced the
isomerization preferentially [Fig. 5(c)]. Then, the fiber
became softer by the 1-min irradiation as above-described.
The isomerization makes the chains bending to induce the
entanglements among them, which can render the fiber elas-
tic [Fig. 5(d)]. Longer UV irradiation increased the number
of cyclobutane units and Z-forms, namely, the formation of
cross-linked and bent chains were enhanced [Fig. 5(e)]. The
cross-linkage hardened the fiber possibly to decrease the
ability of stretch, but the increased Z-forms can keep the
elasticity to generate high U. The combination of two photo-
reactions may be significant for toughening the fiber and the
double photoreaction was a characteristic of coumarate com-
pounds. The P3HCA chain sliding might occur in the initial
straight region of S–S curve, but Z-form could make the
chain bending to induce their entanglement and to restrict
their sliding. After the rising of S–S curve shown in Figure 4,
the Z-forms might play a role of so-called ‘‘molecular
spring’’12–14 connecting hard-rod parts to each other, to
reduce the mechanical stress focusing. The coiled chains con-
taining Z-forms may be extended in the region of significant
increase, whereas the cyclobutane cross-linkage might play a
role of ‘‘clamp’’ bundling the polymer chains, to harden the
fibers. These double effects should change the shape of the
materials, but, unexpectedly, no shape changes in such very
hard fibers were observed by UV irradiation.
Photobending
We tried to observe the photodeformation of the P3HCA
materials using the cast films softer than the fibers. The film
was cut into rectangles (Supporting Information Photo S1,
size: 20 ꢂ 8 ꢂ 0.1 mm3), and the edge of the film was
attached at a supporter and hung as shown in Figure 6. The
mercury lamp was then shone onto the film surface. We
found that the film gradually bent in a direction away from
the lamp (Fig. 6 and Supporting Information Video S1). The
curvature of the film increased very rapidly to reach 20 mꢁ1
FIGURE 6 Photomechanical behavior of P3HCA films. Film was
hung by supporter and irradiated by a high-pressure Hg lamp
from left. The film was bent to face its convex surface to the
light. The small pictures at the bottom was taken from under-
neath of the films. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
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ARTICLE
FIGURE 7 IR spectra of a cross-section of
a polymer film UV irradiated for 100 min
taken in transmission mode. The spectra
changed depending on the depth from the
UV-irradiated surface. The inset illustration
schematically shows the surface reaction
within a depth of 20 lm. [Color figure can
be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
by 1 min of irradiation. Here, we confirmed no temperature
gradient and no solvent vaporization from the film. If the
mercury lamp was turned off, the bending motion stopped
immediately. To make a comparison, other plastic films of
poly(ethyleneterephthalate) and polystyrene were UV irradi-
ated by the same method with P3HCA film, but no photode-
formation was observed. Then, we concluded from these
findings that the photobending was induced by photoreac-
tion of P3HCA film. We additionally investigated the photo-
deformation of another polycoumarate films for poly(4HCA-
co-DHCA) with a liquid crystalline structure, which is pre-
pared in the previous study.7 The copolymer film also
showed a photobending but the bending direction was oppo-
site to the P3HCA film. The previous study showed the co-
polymer was photo-cross-linked very efficiently because the
liquid crystalline alignment enhanced the interchain cycload-
dition reaction.5,7 The surface photocuring of the film made
the photobending with the concave surface facing the UV
lamp.
ated film. These findings indicated that the photoreaction
occurred only in those parts of the film within a 20 lm
depth from the surface. The photobending is not reversible
in the current state presumably due to the first photoreac-
tion fixed the film shape too tightly, and, then, we are now
studying to make the photobending reversible.
EXPERIMENTAL
Monomer Syntheses
The phenol groups of coumarates do not have sufficient
reactivity to polymerize by the dehydration reaction with
carboxylic acid. Therefore, 3HCA (m-coumaric acid) was ace-
tylated to increase the reactivity to easily form 3HCA poly-
mers by transesterification with acetic acid elimination, as
follows. 3HCA (10 mmol) was dissolved in toluene (300 mL),
and then acetic anhydride (20 mmol) and pyridine (10 mL)
were added into the toluene solution. The solution was
refluxed under moderate agitation for 3 h. After the reaction,
pure water (100 mL)ꢀwas carefully poured into the resulting
solution at about 80 C to create two layers. During cooling,
many white needles and powders appeared around an
interface between the water and toluene. The needles and
powders were collected by filtration at room temperature,
and recrystallized in acetone/water solution, to create crys-
talline needles of 3-acetyloxycinnamic acid (Ac3HCA, yield:
86 wt %).
In P3HCA, the preferential photoreaction was the E–Z isom-
erization at the initial stage of the UV irradiation and the
rapid deformation occurred. Because the polymer chains
with rotatable ester bonds were distributed homogeneously,
the Z-forms were rapidly generated in the surface region of
the film. An increase of Z-form content increases the free
volume of the polymer chains around the film surface as
shown in a structural change from Figure 5(a) to 5(c). The
E–Z conversion generated the power high enough to push
away neighbored polymer chains just like as the coumarate
at the photoactive center of PYP1,2 and simultaneously made
the film surface expand to deform the film. [2 þ 2] Cycload-
dition followed the initial photoreaction of E–Z isomerization.
Namely, the photoreaction turned the P3HCA chains into the
zigzag architecture, which was successively cross-linked. The
photoreaction must preferentially occur at the surface of UV-
irradiated P3HCA film to form the gradient structure, and,
then, we took a map of the IR spectra depending on the film
depth (Fig. 7). The specimen was a thin cross-section (thick-
ness: 0.5–1 lm) of a film UV irradiated for 100 min. The
film surface irradiated by UV light showed very broad peaks
on its IR spectrum, but the spectrum of the film at a depth
of 10 lm from the surface showed sharper peaks than that
of the film surface. All spectra of the film at depths of 20–80
lm showed peaks as sharp as the IR peaks of the nonirradi-
Polymer Syntheses
The monomer Ac3HCA was polymerized by the following
procedure [Scheme 1(a)]. The monomer (10 g) and catalysts
such as sodium phosphate (1 mol % to monomer) were
mixed in a reaction vessel and then heated up to 150 ꢀC.
The monomer was melted, and the temperature was gradu-
ally increased up to 200 ꢀC. The reaction liquid was vac-
uumed to generate bubbles, presumably due to the acetic
acid eliminating as a result of the transesterification reaction.
These bubbles cannot be seen at about 30 min afterward.
The liquid became more viscous as the polymerization time
increased. After 6 h of polymerization, the reaction mixture
was cooled to room temperature and became hard and light
yellowish pellets. The pellets were milled into a powder and
reprecipitated from DMF solution into acetone. The dried
powders were then gathered by filtration (yields: 70–90%).
The molecular structures were analyzed by FTIR/ATR and
EFFECTS OF DOUBLE PHOTOREACTIONS, YASAKI ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
1H NMR spectroscopy to confirm the formation of 3HCA
polyesters (Supporting Information Figs. S1 and S2).
with keeping the elasticity. As a result, the fiber became
hard and strong to show a large strain energy. The double
photoreaction besides induced the photomechanical behavior
of the P3HCA film, to deform with facing its convex surface
to the UV lamp. This phenomenon may occur based on the
gradient structure formed through double photoreaction.
Thus, the polymers with a unique backbone composed of
coumarates may lead to a new field of photoresponsive bio-
base polymers.
Light Irradiation
The photoreaction of the coumarate groups was performed
by UV irradiation at k ¼ 250–450 nm. The fiber or film was
irradiated with a focused UV beam from a 100-W high-pres-
sure mercury lamp (Omni Cure S1000, EXFO Photonic Solu-
tions). In the photobending observation, the film was hung
and then irradiated with a focused UV beam, and the bend-
ing motion was recorded by a digital video (Supporting In-
formation Video). The distance between the fiber and the
lamp was 4 cm (72.2 mW cmꢁ2).
This research was financially supported from a Grant-in-Aid for
Comprehensive Support Programs for Creation of Regional
Innovation Science and Technology Incubation Program in
Advanced Regions ‘‘Practical Application Research’’ (Kaneko
project).
Spectroscopy
The ultraviolet–visible (UV–vis) absorption spectra oꢀf the
film supported on a quartz plate were measured at 25 C on
a Perkin Elmer Lambda 25 UV/VS spectrometer. The UV–vis
measurement was performed in 1-nm wavelength steps from
250 to 400 nm. Fourier-transformed infrared spectroscopy
(FTIR) spectra were recorded at room temperature with a
wavenumber range between 4000 cmꢁ1 and 900 cmꢁ1. The
spectrum change of the film surface as a function of light
irradiation time was recorded by the FTIR/attenuated total
reflection (ATR) method using a germanium-ATR (ATR30-
G45) accessory. A total of 512 scans with a resolution of 2
cmꢁ1 were accumulated for the spectra. The spectra at dif-
ferent film depths were recorded on a Shimadzu IMV4000
spectrometer equipped with a microsampling system (aper-
ture: 10 ꢂ 200 lm). A total of 64 scans with a resolution of
2 cmꢁ1 were accumulated for the spectra. A cross-sectional
specimen of the film with a thickness of 0.5–1.0 lm was pre-
pared by the microtome system LEICA ULTRACUT using a di-
amond knife (MICRO STAR DIAMOND KNIFE). The specimen
was dried on a BaF2 substrate for a day before measure-
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(AV400N, 400 MHz) with 16 accumulation scans, using the
proton resonance of tetramethylsilane as an internal stand-
ard (0.00 ppm). The UV-irradiated polymers were partially
dissolved in TFA-d, and, then, the supernatant was used as a
sample solution for the NMR measurement.
9 Boersma, A.; Turnhout, J. van; Wubbenhorst, M. Macromole-
¨
cules 1998, 31, 7453–7460.
10 Olayan, H. B.; Hami, H. S.; Owen, E. D. Polym Rev 1996, 36,
719–719.
11 When the Hg-lamp with a power of 0.0722 J sꢁ1 cmꢁ2 was
irradiated on the fiber surface with a area of 2 ꢂ 0.02 cm2
for 60 s, the irradiated energy to the fiber was calculated as
0.173 J.
CONCLUSIONS
We established a polymerization condition of a kind of cou-
marates 3HCA having photoreactive units over all the main
chains. The polymer showed double photoreaction of intra-
molecular E–Z isomerization and interchain [2 þ 2] cycload-
dition. The isomerization and cycloaddition can induce the
chain bending and cross-linking, respectively. The short-time
UV irradiation made the fiber softer and more stretchable,
whereas the long-time irradiation made the fiber harder
12 Chalmers, R.; Guhathakurta, A.; Benjamin, H.; Kleckner, N.
Cell 1998, 93, 897–908.
13 Frank, J.; Sengupta, J.; Gao, H.; Li, W.; Valle, M.; Zavialov,
A.; Ehrenberg, M. FEBS Lett 2005, 579, 959–962.
14 Stowell, M. H. B.; Marks, B.; Wigge, P.; McMahon, H. T. Nat
Cell Biol 1999, 1, 27–32.
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