Characteristic phase transition of aqueous solution of poly(N- isopropylacrylamide) functionalized with spirobenzopyran

Article abstract of DOI:10.1021/ma049661x

A novel functional copolymer was synthesized by modifying poly(N-isopropylacrylamide) with spirobenzopyran. The phase transition properties of the aqueous solution of this copolymer exhibited a logic-gate response to the light irradiation and to increased temperature, which has three different modes depending on the pH of the solution. Especially, a great increase in turbidity was observed even in dilute aqueous solution containing only 0.10 wt % of the copolymer although the copolymer contained spirobenzopyrans by only 1.1 mol%, It was confirmed also that the spirobenzopyran residues were isomerized by the influence of thermally induced phase transition of the same system. The main chain of thermoresponsive polymer and photoresponsive chromophores, which are linked closely together, affected each other.

Full text of DOI:10.1021/ma049661x

Macromolecules 2004, 37, 4949-4955
4949
Characteristic Phase Transition of Aqueous Solution of
Poly(N-isopropylacrylamide) Functionalized with Spirobenzopyran
Kim io Su m a r u ,*,† Mitsu yosh i Ka m ed a ,† Tosh iyu k i Ka n a m or i,† a n d Tosh io Sh in bo‡
Research Center of Advanced Bionics, National Institute of Advanced Science and Technology (AIST),
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, J apan, and Institute for Materials and
Chemical Process, National Institute of Advanced Science and Technology (AIST), Tsukuba Central 5,
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, J apan
Received February 19, 2004; Revised Manuscript Received April 16, 2004
ABSTRACT: A novel functional copolymer was synthesized by modifying poly(N-isopropylacrylamide)
with spirobenzopyran. The phase transition properties of the aqueous solution of this copolymer exhibited
a logic-gate response to the light irradiation and to increased temperature, which has three different
modes depending on the pH of the solution. Especially, a great increase in turbidity was observed even
in dilute aqueous solution containing only 0.10 wt % of the copolymer although the copolymer contained
spirobenzopyrans by only 1.1 mol %. It was confirmed also that the spirobenzopyran residues were
isomerized by the influence of thermally induced phase transition of the same system. The main chain
of thermoresponsive polymer and photoresponsive chromophores, which are linked closely together,
affected each other.
In tr od u ction
Recently, Desponds et al.10 proposed a new scheme
for synthesizing functional copolymers and synthesized
pNIPAAm partly modified with azobenzene chro-
mophores and carboxyl groups as an example. Although
they reported a photoinduced LCST shift at only one
pH value, they suggested that this polymer responds
to three different stimuli: light irradiation and changes
in temperature and pH. In this strategy of adding
various stimulus-responsive groups to a polymer, how-
ever, each kind of group acts on the polymer main chain
in a parallel manner, and only simple additive effects
are expected.
Instead of such a simple effect, we schemed to develop
a polymer that would show a cooperative response to
several stimuli, i.e., light irradiation and changes in
temperature and pH, and synthesized a novel functional
copolymer by modifying pNIPAAm with a spirobenzopy-
ran. This chromophore is considered to have four
different stable forms, and the proportion of each form
varies in response to both the pH of the solution and
whether it is irradiated with light.11 Especially the
photoisomerization of the chromophore resulted in a
large change in its chemical structure and physical
properties,11-14 and effective photosensitivity of the
polymer is expected even at small modification ratios.
On the basis of the detailed inspection of the spiroben-
zopyran residues, we systematically analyzed phase
transitions in aqueous solutions of this new copolymer
in the various conditions of light irradiation, tempera-
ture, and pH. We also discussed the isomerization of
the chromophores affected by the thermally induced
phase transition of the polymer main chain.
Modification of thermoresponsive polymers with sen-
sitive groups that respond to various stimuli1 has led
to the development of functional copolymers that re-
spond not only to temperature changes but also to other
physical and chemical stimuli. Above all, light irradia-
tion can be applied to the target in a local, contactless,
and immediate manner, and therefore, photocontrol of
the extent of polymer chain including its phase transi-
tion in solutions has been studied with various practical
applications. In a pioneering study in this field, Irie et
al.2 synthesized poly(N-isopropylacrylamide) (pNIPAAm)
partly modified with azobenzene chromophores and
showed that the lower critical solution temperature
(LCST) of its aqueous solution is shifted by light
irradiation.2 Later, Kro¨ger et al.3 found the photoin-
duced LCST shift of 20 °C for an aqueous solution of
azobenzene-modified poly(N,N-dimethylacrylamide).
The swelling and shrinking of hydrogels have also
been actively studied by using thermoresponsive poly-
mers partly modified with ionic groups. Ilavsky et al.4
reported that incorporating sodium methacrylate into
a hydrogel of poly(N,N-diethylacrylamide) increased the
phase-transition temperature. Brazel et al.5 analyzed
changes in the degree of swelling and the network mesh
size of a hydrogel of pNIPAAm containing sodium
methacrylate in response to changes in temperature and
pH.
In addition, by introducing benzo[18]crown-6 as a side
chain of pNIPAAm, Irie et al.6,7 synthesized a functional
polymer and obtained selective responses to the pres-
ence of specific ionic species. Later, by modifying a
porous base membrane with the same polymer by
plasma graft copolymerization, Yamaguchi et al.8 and
Ito et al.9 constructed a membrane device whose perme-
ability is controllable by both the presence of specific
ions and changes in temperature.
Exp er im en ta l Section
Syn th esis of p SP NIP AAm Cop olym er . We synthesized
the spirobenzopyran monomer by treating 1′,3′,3′-trimethyl-
6-hydroxyspiro(2H-1-benzopyran-2,2′-indoline) with acrylic
anhydride. The NIPAAm-based copolymer partly modified with
a spirobenzopyran (pSPNIPAAm) was synthesized in a solu-
tion of distilled tetrahydrofuran (THF) by free-radical polym-
erization, with 2,2′-azobis(isobutyronitrile) (AIBN) as an ini-
tiator. The initial molar concentration of NIPAAm monomer,
† Research Center of Advanced Bionics.
‡ Institute for Materials and Chemical Process.
* Corresponding author. E-mail: k.sumaru@aist.go.jp.
10.1021/ma049661x CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/25/2004

4950 Sumaru et al.
Macromolecules, Vol. 37, No. 13, 2004
F igu r e 1. Photoisomerization and protonation of spiroben-
zopyran chromophore in pSPNIPAAm.
acrylated spirobenzopyran monomer, and initiator in the
reaction solution were 2.0, 0.020, and 0.020 M, respectively.
After deaeration by freezing and thawing, the reaction solution
was polymerized by incubation at 60 °C for 12 h. The resulting
solution was poured into diethyl ether, and the precipitate was
collected and dried under vacuum to give pSPNIPAAm as a
white powder (Figure 1, top). The molecular weight was
determined by gel permeation chromatography (GPC; column,
Shodex KF-805) at 25 °C with THF as the eluant; the weight-
average and number-average molecular weights of pSP-
NIPAAm, estimated on the basis of polystyrene standards,
were 22 000 and 7800, respectively. NMR and UV-vis absor-
bance measurements revealed that the fraction of the spiroben-
zopyran units in the synthesized pSPNIPAAm was 1.1 mol
%, which agreed well with the molar ratio of the monomers in
the initial solution.
F igu r e 2. Absorption spectra of 0.20 wt % pSPNIPAAm in
aqueous solution containing various concentrations of [H+]added
.
(a) Full spectra and (b) expanded view of the spectra at the
isosbestic point.
used to irradiate visible light on a well-stirred sample solution
in a constant-temperature bath. The wavelength and intensity
of the irradiating light were 400-440 nm and 20 mW/cm2,
respectively.
Resu lt a n d Discu ssion
Ch a r a cter iza tion of p SP NIP AAm Aqu eou s Solu tion .
Absorbance/turbidity spectra of pSPNIPAAm in aqueous solu-
tion were measured with ∼2.5 mL samples containing HCl and
NaOH at various concentrations in a cuvette with an optical
path length of 10 mm in a UV-vis spectrometer having a
thermoregulated cell holder. To examine the proton dissocia-
tion of the chromophore introduced to pSPNIPAAm, NaOH
was added to the aqueous solution of pSPNIPAAm (0.20 wt
%) containing 0.50 mM HCl, which had been equilibrated at
25 °C. Then the solution was cooled to 0 °C, and the change of
absorption spectra was measured swiftly at the temperature
while NaOH was added in portions. Temperature dependence
measurements were carried out for well-stirred solutions
raising the temperature of the system stepwise. The UV-vis
absorption spectra were collected after the spectra stabilized
at each temperature. The typical rate of temperature increase
was 1 °C/h. Dynamic light scattering (DLS) measurements
were carried out with an Otsuka ELS-800 electrophoretic light
scattering spectrophotometer having a thermoregulated cell
holder. The light irradiation to the sample solutions were
carried out with a Hamamatsu Photonics LC6 Xe light source
and Ushio ModuleX ultrahigh-pressure Hg light source. In the
spectroscopic measurements and DLS measurements, light
from the Xe light source was guided to the well-stirred sample
solutions, in a thermoregulated cell holder in the spectrom-
eters, through a set of filters and a liquid-fiber light guide.
The wavelength and total intensity of the irradiating light
were 400-440 nm and 30 mW, respectively, for the irradiation
to the samples in acidic or neutral conditions and 480-520
nm and 60 mW for basic conditions. In the observation of the
appearance of the solution affected by light irradiation, the
ultrahigh-pressure Hg light source and a set of filters were
Ch a r a cter istics of Sp ir oben zop yr a n Resid u e. As
shown in Figure 1, four different stable states are
presumed for spirobenzopyran introduced for a side
chain of pSPNIPAAm: a colored merocyanine with an
open-ring structure (Mc) and a colorless spiro form with
a closed-ring structure (Sp) and the protonated forms
of each (McH and SpH). We found that the absorption
band of λmax ) 422 nm attributed to McH15 increased
gradually and reached equilibrium after HCl was added
to an aqueous solution of pSPNIPAAm in darkness at
25 °C. The absorbance value was hardly affected by
changes in [HCl] at concentrations greater than 0.2 mM
or by temperature changes at temperature in the range
from 0 to 25 °C, which strongly suggests that all the
chromophores are in the McH form under these condi-
tions.
In the examination of the proton dissociation of McH,
the absorbance attributed to McH decreased as NaOH
was added to the solution, which had initially contained
excess HCl (see Experimental Section), and another
absorbance of λmax ) 530 nm attributed to free Mc
increased. The experimentally obtained spectra at vari-
ous concentrations of added proton ([H+]added ) [con-
centration of added HCl] - [concentration of added
NaOH]) are shown in Figure 2. Under the condition of
[H+]added > 0 mM, an isosbestic point was found at 465
nm. This result indicated that McH was converted into
Mc, keeping the sum of the concentrations of McH and

Macromolecules, Vol. 37, No. 13, 2004
Phase Transition of pNIPAAm Solution 4951
F igu r e 3. Absorption spectra of 0.20 wt % pSPNIPAAm in
aqueous solution at [H+]added ) -0.32 mM: solid line, just after
addition of NaOH at 0 °C; dashed line, after allowing the
solution to remain at 0 °C for 30 min; dotted line, after allowing
the solution to remain at 0 °C for 1 day.
F igu r e 5. Absorbance/turbidity spectra of 0.10 wt % pSP-
NIPAAm aqueous solution at [H+]added ) 0.2 mM (pH ≈ 4):
(a) in the dark and (b) under light irradiation.
calculated from the concentration of contained chro-
mophore, the A530 value at [H+]added ) 0 mM was about
30% of its maximum value, indicating that the chro-
mophores in the merocyanine form were protonated to
a certain extent under this condition. We estimated pKa
of McH roughly to be 6-7 from the results of absorption
measurements in several buffer solutions, and pH
measurements at the point of color change supported
this estimation. We consider that this pKa value, which
is considerably small for phenol with ester group in the
para position, is due to the resonance stabilization of
phenolate ion16 and to the electrostatic repulsion of
proton by the positive charge of neighboring ammo-
nium.17 Therefore, Mc is not a strong base enough to
take proton away from water molecules. As a result, it
was suggested that our pSPNIPAAm prepared and
purified as described above contained some proton
source such as carboxyl groups by one per every 300
monomer units.
F igu r e 4. A530 plotted against [H+]added. A bar indicates the
theoretical [H+]added range in which titration of choromophore
contained in the solution should be completed.
Mc constant (see Supporting Information). When [H+]added
was negative, however, the absorbance at this wave-
length decreased as the amount of added NaOH in-
creased (Figure 2b).
While the sample remained at 0 °C at an [H+]added
value of -0.32 mM, the absorbance in the wavelength
range 300-650 nm decreased considerably as the time
proceeded (Figure 3). In an equilibrium state (1 day
after addition of NaOH), only 20% of choromophore
remained in the Mc form. This experimental result
suggested that Mc tends to isomerized thermally to Sp
in an alkaline environment, and this isomerization
caused small deviation of absorbance value at 465 nm,
which was observed for negative [H+]added value in
Figure 2 (see Supporting Information).
P h a se Tr a n sition of p SP NIP AAm Solu tion . Here,
we discuss the effects of irradiation with visible light
and of temperature changes on the phase transition of
0.10 wt % pSPNIPAAm in aqueous solution under
several conditions of [H+]added. Figure 5 shows the
absorbance/turbidity spectra of the pSPNIPAAm aque-
ous solution at [H+]added ) 0.2 mM (pH ≈ 4). As
temperature raised gradually from 0 °C in darkness
The absorbance values at 530 nm (A530) are plotted
against [H+]added in Figure 4. As NaOH was added to
the solution, the value of A530 increased only in the
[H+]added range from 0.1 to -0.2 mM, exhibiting the
typical tendency observed in acid-base titration. Upon
further addition of NaOH, however, A530 decreased
gradually due to the thermal isomerization of Mc to Sp
as discussed above. A bar in the figure indicates the
theoretical [H+]added range in which titration of choro-
mophore contained in the solution should be completed.
Although the [H+]added range in which A530 varied most
actually was in approximate agreement with that
(Figure 5a), the absorbance attributed to McH (λmax
)
422 nm) did not change at the temperatures lower than
25 °C and decreased remarkably only around the phase
separation temperature. This result suggests strongly
that a part of McH groups isomerized to the colorless
spiro form, which has a relatively hydrophobic closed-
ring structure, due to the thermally induced phase

4952 Sumaru et al.
Macromolecules, Vol. 37, No. 13, 2004
F igu r e 7. Photoinduced change in the absorbance/turbidity
spectra of 0.10 wt % pSPNIPAAm aqueous solution at [H+]added
) 0.2 mM (pH ≈ 4) accompanied by the irradiation with visible
light after standing in the dark at 31.5 °C.
F igu r e 6. Temperature-dependent turbidity at 700 nm and
DH values of 0.10 wt % pSPNIPAAm aqueous solution at
[H+]added ) 0.2 mM (pH ≈ 4): open symbols, under light
irradiation; closed symbols, in the dark.
with blue light at 31.5 °C. Before the irradiation (a),
the solution was transparent and was colored yellow due
to the light absorption by McH, and the yellow color was
lightened distinctly after light irradiation for 20 s (b).
After an additional 5 s irradiation (c), the yellow color
faded and the solution became slightly opaque. The
turbidity of the solution was increased rapidly on
further irradiation (d, e). When the stirring was stopped
after irradiation for 50 s in total (f), visually observable
precipitates were separated and suspended in the
solution, which were sedimented after leaving for 5 min
without stirring or irradiating (g). As described above,
also Kro¨ger et al. found copolymers whose aqueous
solutions exhibit a large photoinduced LCST shift.3 In
their systems, however, the precipitates obtained above
LCST were viscous and transparent phases containing
a lot of water. In contrast, the precipitate separated
from an irradiated pSPNIPAAm acidic solution we
found in this study was solid containing a little amount
of water, suggesting photoinduced dehydration of co-
polymer was much more remarkable than the system
reported by Kro¨ger et al. With the light irradiation with
larger intensity, the photoinduced phase transition of
pSPNIPAAm acidic solution was accelerated, indicating
that the photoisomerization of spirobenzopyran trig-
gered the dehydration of copolymer in an immediate
manner.
Next, we discuss the phase transition of a pSP-
NIPAAm aqueous solution at [H+]added ) 0 mM (pH ≈
7). Absorbance/turbidity spectra of the solution are
shown in Figure 9. In darkness (Figure 9a), two absor-
bance peaks attributed to Mc and McH at 422 and 530
nm, respectively, were observed at 25.0 °C, since pSP-
NIPAAm prepared and used in this study contained
some proton source as mentioned above. The absorbance
at both peaks was small, suggesting that many of
chromophores were in the colorless Sp state and pSP-
NIPAAm was not photochromic. Practically, the solution
exhibited similar behavior either in the dark or under
light irradiation of 400-440 nm wavelength (Figure 9b).
Figure 10 shows the temperature-dependent turbidity
at 700 nm of the solution under the same condition.
Whether the sample was irradiated with the light or
not, the solution greatly became turbid as the temper-
ature was increased above 30.0 °C at [H+]added ) 0 mM
(pH ≈ 7). This may have occurred because many of
chromphores were in the relatively hydrophobic Sp state
regardless of light irradiation. At temperature above
transition. Although the solution started to become
turbid at 32.0 °C, the turbidity did not increase much
even when the temperature increased up to 37.0 °C. The
chromophores, which remained in McH form even at the
temperature, were considered to stabilize the hydration
of pSPNIPAAm quite effectively. On the other hand,
under the irradiation with the light in the wavelength
range 400-440 nm (Figure 5b), the absorbance at 422
nm was reduced considerably, indicating that McH
groups were isomerized effectively to spiro form. The
solution started to become turbid at 29.0 °C, and the
turbidity increased sharply as temperature raised a few
more °C, in good contrast to the nonirradiated condition.
Figure 6 shows the temperature-dependent turbidity
at 700 nm of the solution under the same condition. The
tendency varied with the condition of light irradiation.
Although the value of turbidity measured for the ir-
radiated solution decreased drastically as the temper-
ature increased from 31.5 to 32.0 °C, this was due to
sedimentation of precipitated pSPNIPAAm as a result
of photoinduced dehydration, not to redissolution of the
copolymer. Also, the temperature-dependent data of
hydrodynamic diameter (DH) observed by means of DLS
are shown in Figure 6. DH, which corresponds to the
size of aggregates, is considered to reflect the aggrega-
tion state of the system18 and showed a good agreement
with the tendency of turbidity in both light and dark-
ness. This supports the analysis in this study monitor-
ing the phase transition through the turbidity change.
Figure 7 shows photoinduced changes in the absor-
bance/turbidity spectra when the same sample was
irradiated with visible light after standing in darkness
at 31.5 °C. The solution, which had been transparent
in the dark, became markedly turbid, and the absor-
bance at 422 nm disappeared after the light irradiation
for only a few minutes. The molar content of chro-
mophores in pSPNIPAAm was relatively small (1.1 mol
%) compared with previously reported photosensitive
copolymers containing more than 3 mol % of chro-
mophore.2,3 Nevertheless, the photoinduced increase in
turbidity was considerable, and the experimental data
in Figure 7 appear noisy because of the presence of large
precipitates.
Figure 8 shows the change in appearance of 0.10 wt
% pSPNIPAAm acidic solution brought about by light
irradiation. The solution at [H+]added ) 0.5 mM (pH ≈
3), which had been kept in the darkness, was irradiated

Macromolecules, Vol. 37, No. 13, 2004
Phase Transition of pNIPAAm Solution 4953
F igu r e 8. Photoinduced change in the appearance of 0.10 wt % pSPNIPAAm aqueous solution at [H+]added ) 0.5 mM (pH ≈ 3)
accompanied by the irradiation with visible light after standing in the dark at 31.5 °C. The wavelength and intensity of the
irradiating light were 400-440 nm and 20 mW/cm2, respectively.
F igu r e 10. Temperature-dependent turbidity at 700 nm of
0.10 wt % pSPNIPAAm aqueous solution at [H+]added ) 0 mM
(pH ≈ 7): open symbols, under light irradiation; closed
symbols, in the dark.
Similarly to the tendency found for [H+]added ) 0.2 mM
(pH ≈ 4), the absorption band at 530 nm, which is
attributed to the Mc form, decreased as the temperature
was increased to 30.0 °C (Figure 11b). In this case, a
temperature range in which the absorbance started to
decrease was far below the LCST of the solution, and
this is an interesting finding that we will discuss
separately.19 In the irradiated sample, this absorption
band, which had been suppressed by the light irradia-
tion at 0 °C, increased as the temperature was increased
(Figure 11b). This was not due to a shift in the
equilibrium of the isomerization between Mc and Sp,
but due to the increase in the rate constant for the
thermal recovery of Mc from Sp in darkness; in the
measurement, it took about 30 s to scan the wavelength
range around 530 nm after the light was turned off, and
the recovery of Mc from Sp during this period may have
affected the experimental results. We confirmed that the
recovery of Mc from Sp in darkness is faster at higher
temperatures. Putting the data shown in Figure 3
together, these experimental results indicated that the
activation energy in the isomerization from Mc to Sp is
not much larger than the thermal energy at 0 °C.
Figure 12 shows the temperature-dependent turbidity
at 700 nm of the solution under the same conditions.
Regardless of the light irradiation, the solution was hard
to become turbid, and a distinct increase of the turbidity
was not observed even at 36.0 °C. Although Figure 11
F igu r e 9. Absorbance/turbidity spectra of 0.10 wt % pSP-
NIPAAm aqueous solution at [H+]added ) 0 mM (pH ≈ 7): (a)
in the dark and (b) under light irradiation.
32.0 °C, the appearance of the solution varied depending
on whether it was irradiated with the light; while the
polymer aggregates remained suspended rather stably
in darkness, the optical density decreased due to
sedimentation of precipitated polymer during light
irradiation.
Subsequently, the absorbance/turbidity spectra ob-
tained for 0.10 wt % pSPNIPAAm aqueous solution at
[H+]added ) -0.3 mM (pH ≈ 10) are shown in Figure
11. As seen also in Figure 3, the spectra data at 0 °C
showed that only about 20% of all the chromophores
were in the Mc form, and many of chromophores were
in the Sp form having the closed-ring structure in an
alkaline environment even in the darkness (Figure 11a).

4954 Sumaru et al.
Macromolecules, Vol. 37, No. 13, 2004
F igu r e 13. Dependence of Tturb on [H+]added and the condition
of light irradiation (see the text for details): open symbols,
under light irradiation; closed symbols, in the dark.
Ta ble 1. Ch a r a cter istic Resp on se of P h a se Tr a n sition of
p SP NIP AAm Aqu eou s Solu tion to Ligh t Ir r a d ia tion a n d
Ch a n ge in Tem p er a tu r e a n d p H
T (turbidity)a
H (temperature)
L (irradiation) pH < 5 pH ≈ 7 pH > 9
1 (32-36 °C)
1 (32-36 °C)
0 (<30 °C)
1
0
1
0
1
0
0
0
1
1
0
0
H
0
0
0
0
0
0 (<30 °C)
logic expression of T using H and L HL
F igu r e 11. Absorbance/turbidity spectra of 0.10 wt % pSP-
NIPAAm aqueous solution at [H+]added ) -0.3 mM (pH ≈ 10):
(a) in the dark and (b) under light irradiation.
a
H, L, and T are defined as Boolean variables relating to
temperature, light irradiation, and the turbidity of the solution
that occurs in response to these stimuli, respectively. H is 1 when
the temperature is higher than 32 °C and 0 when it is lower than
30 °C; L is 1 when the solution is irradiated with light and 0 when
it is left in darkness; T is 1 when the solution is turbid and 0 when
it is transparent.
mophores may have unexpected effects on the phase
transition of the system.
To aid in discussing our findings relating to turbidity,
here we define Tturb as the temperature at which the
turbidity (in 10 mm optical path length at 700 nm) of a
0.10 wt % pSPNIPAAm aqueous solution is 0.05. Figure
13 shows the dependence of Tturb on [H+]added and on
light irradiation. At [H+]added ) -0.3 mM (pH ≈ 10),
the solution did not readily become turbid, and Tturb was
high (about 38 °C); at [H+]added ) 0 mM (pH ≈ 7), Tturb
was low (about 31 °C). At both values of [H+]added (0 and
-0.3 mM), the value of Tturb did not depend on whether
the sample was irradiated with the light or not. When
excess protons were contained (pH ) 3-4), however,
Tturb was changed largely in response to light irradia-
tion. Characteristic phase transitions (turbidity, T) of
pSPNIPAAm in aqueous solution in response to light
irradiation (L) and changes in temperature (H) are
summarized in logical expressions tabulated in Table
1. It is clearly shown that the phase transition has three
different modes corresponding to the pH of the solu-
tion: At high pH (>9), T is 0 regardless of the values of
H and L. When the solution is nearly neutral (pH ≈ 7),
T is the same as H regardless of the value of L. Under
acidic conditions (pH < 5), T is 1 only when both H and
L are 1 and otherwise is 0 (here T is the logical product
of H and L).
F igu r e 12. Temperature-dependent turbidity at 700 nm of
0.10 wt % pSPNIPAAm aqueous solution at [H+]added ) -0.3
mM (pH ≈ 10): open symbols, under light irradiation; closed
symbols, in the dark.
indicates that most of the chromophores were in the
hydrophobic Sp state as well as at [H+]added ) 0 mM
(pH ≈ 7), the solution at [H+]added ) -0.3 mM (pH ≈
10) did not become turbid easily. With respect to this
point, conversion of a very small portion of the monomer
units constituting pSPNIPAAm to acrylic acid in the
process of polymer synthesis may account for this
observation; carboxyl groups dissociated under alkaline
conditions, and the resultant net charge enhanced the
hydration of the polymer chains.1,20 As discussed in the
previous subsection, pSPNIPAAm prepared and purified
in this study was suggested to contain some proton
source by one per every 300 monomer units. Although
the influence of such a small amount of carboxyl groups
should not be great,4 synergistic effect with the chro-
Con clu sion
We synthesized a novel functional copolymer by
modifying pNIPAAm with spirobenzopyran moieties and

Macromolecules, Vol. 37, No. 13, 2004
Phase Transition of pNIPAAm Solution 4955
have investigated the cooperative effects of light ir-
radiation and changes in temperature and pH on the
phase transition of the aqueous copolymer solution. We
found that the copolymer solution exhibits a logic-gate
response to light irradiation and the temperature
increases, with three different modes depending on the
pH of the solution. This unique characteristic is due
mainly to interaction between the main chain of the
thermoresponsive pNIPAAm and spirobenzopyran moi-
eties, which respond to both light irradiation and pH
changes. Such a unique behavior was not found in the
earlier studies in which thermosensitive polymers were
modified with several different residues in an attempt
to obtain an additive effect.
On the other hand, the photoinduced phase transition
found in this study was drastic, and increases in
turbidity (at 700 nm, 10 mm optical path length) were
observed to be up to 0.7 for a dilute aqueous solution
containing only 0.10 wt % of the copolymer. It is
interesting that such a drastic dehydration of polymer
was brought about by the photoisomerization of small
proportion (only 1.1 mol %) of chromophores introduced
into the pNIPAAm. This was probably due to not only
the drastic change in the chemical structure of the
chromophores used in this study but also the short
distance between the polymer main chain and the
chromophores; the spirobenzopyrans were directly con-
nected to the polymer main chain through ester bonds,
and so we suggest that photoisomerization of the
chromophores directly affected the hydration of the
polymer main chain. In addition, we confirmed also that
the spirobenzopyran residues were isomerized in re-
sponse to the influence of the thermally induced phase
transition of the copolymer solution. Putting the pho-
toinduced phase transition together, we can conclude
that the main chain of the thermoresponsive polymer
and the photoresponsive chromophores, which were
linked closely together, affect one another. These ob-
servations obtained in this study were expected to
provide an important clue to design functional copoly-
mers containing various stimuli-responsive components.
Ack n ow led gm en t. This work was supported by
Industrial Technology Research Grant Program in 2002
from the New Energy Development Organization (NEDO)
of J apan. We thank Dr. Toshiyuki Takagi and Mr.
Tomonori Endo (AIST) for their helpful discussion about
the synthesis of the materials.
Su p p or tin g In for m a tion Ava ila ble: Isosbestic point
with respect to proton dissociation of McH. This material is
available free of charge via the Internet at http://pubs.acs.org.
Refer en ces a n d Notes
(1) Tanaka, T. Polymer 1979, 20, 1404.
(2) Irie, M.; Kungwatchakun, D. Macromolecules 1986, 19, 2476.
(3) Kro¨ger, R.; Menzel, H.; Hallensleben, M. L. Macromol. Chem.
Phys. 1994, 195, 2291.
(4) Ilavsky, M.; Hrouz, J .; Havlicek, I. Polymer 1985, 26, 1514.
(5) Brazel, C. S.; Peppas, A. Macromolecules 1995, 28, 8016.
(6) Irie, M.; Misumi, Y.; Tanaka, T. Polymer 1993, 34, 4531.
(7) Irie, M. Adv. Polym. Sci. 1993, 110, 49.
(8) Yamaguchi, T.; Ito, T.; Sato, T.; Shinbo, T.; Nakao, S. J . Am.
Chem. Soc. 1999, 121, 4078.
(9) Ito, T.; Hioki, T.; Yamaguchi, T.; Shinbo, T.; Nakao, S.;
Kimura, S. J . Am. Chem. Soc. 2002, 124, 7840.
(10) Desponds, A.; Freitag, R. Langmuir 2003, 19, 6261.
(11) Fissi, A.; Pieroni, O.; Angelini, N.; Lenci, F. Macromolecules
1999, 32, 7116.
(12) Shipway, A. N.; Willner, I. Acc. Chem. Res. 2001, 34, 421.
(13) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. Rev. 2000, 100,
1741.
(14) Rosario, R.; Gust, D.; Hayes, M.; J ahnke, F.; Springer, J .;
Garcia, A. A. Langmuir 2002, 18, 8062.
(15) Williams, R. M.; Klihm, G.; Braslavsky, S. E. Helv. Chim.
Acta 2001, 84, 2557.
(16) Rini, M.; Holm, A.-K.; Nibbering, E. T. J .; Fidder, H. J . Am.
Chem. Soc. 2003, 125, 3028.
(17) Sumaru, K.; Matsuoka, H.; Yamaoka, H. J . Phys. Chem. 1996,
100, 9000.
(18) Makhaeva, E. E.; Tenhu, H.; Khokhlov, A. R. Macromolecules
2002, 35, 1870.
(19) Kameda, M.; Sumaru, K.; Kanamori, T.; Shinbo, T., submitted
for publication.
(20) Ilavsky, M.; Hrouz, J .; Stejskal, J .; Bouchal, K. Macromol-
ecules 1984, 17, 2868.
MA049661X