Thermosensitive properties of semi-IPN gel composed of amphiphilic gel and zwitterionic thermosensitive polymer in buffer solutions containing high concentration salt

Article abstract of DOI:10.1016/j.polymer.2011.06.014

In this study, a semi-interpenetrating polymer network (semi-IPN) gel, which consists of an amphiphilic N,N-dimethylacrylamide-co-N-isopropylacrylamide (DMAA-co-NIPAM) gel and an interpenetrating zwitterionic thermosensitive poly(N-isopropylacrylamide-co-N,N-dimethyl(acrylamidopropyl)ammonium propane sulfonate) (poly(NIPAM-co-DMAAPS)) was prepared. The thermosensitive behavior of the semi-IPN gel was investigated in a buffer solution composed of a relatively high concentration of sodium chloride and sodium citrate as salts, and sodium dodecyl sulfate as a surfactant, which are generally used as a buffer solution in biochips. At low temperatures, the semi-IPN gel in the buffer solution was absolutely transparent; however, when the gel was heated, the gel became milky white or opaque without a large change in the gel size. The network of the transparent gel is homogeneous, whereas that of the opaque gel consists of coarse and dense parts. Such a structural change in the gel network was confirmed by the temperature dependence of the permeability of the buffer solution through the semi-IPN gel membrane. The permeability increased drastically when the gel became opaque because of heating.

Full text of DOI:10.1016/j.polymer.2011.06.014

Polymer 52 (2011) 3791e3799
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Thermosensitive properties of semi-IPN gel composed of amphiphilic gel
and zwitterionic thermosensitive polymer in buffer solutions containing
high concentration salt
,
b
,
a
a
a
Atsushi Takahashi a , Kenta Hamai , Yuko Okada , Shuji Sakohara
*
a Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan
b Yokohama Corporate Research Laboratories, Mitsubishi Rayon Co., Ltd., 10-1 Daikoku-cho, Tsurumi-ku, Yokohama 230-0053, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, a semi-interpenetrating polymer network (semi-IPN) gel, which consists of an amphiphilic
N,N-dimethylacrylamide-co-N-isopropylacrylamide (DMAA-co-NIPAM) gel and an interpenetrating
zwitterionic thermosensitive poly(N-isopropylacrylamide-co-N,N-dimethyl(acrylamidopropyl)ammo-
nium propane sulfonate) (poly(NIPAM-co-DMAAPS)) was prepared. The thermosensitive behavior of the
semi-IPN gel was investigated in a buffer solution composed of a relatively high concentration of sodium
chloride and sodium citrate as salts, and sodium dodecyl sulfate as a surfactant, which are generally used
as a buffer solution in biochips. At low temperatures, the semi-IPN gel in the buffer solution was
absolutely transparent; however, when the gel was heated, the gel became milky white or opaque
without a large change in the gel size. The network of the transparent gel is homogeneous, whereas that
of the opaque gel consists of coarse and dense parts. Such a structural change in the gel network was
confirmed by the temperature dependence of the permeability of the buffer solution through the semi-
IPN gel membrane. The permeability increased drastically when the gel became opaque because of
heating.
Received 28 February 2011
Received in revised form
3 June 2011
Accepted 8 June 2011
Available online 15 June 2011
Keywords:
Semi-IPN gel
Zwitterionic polymer
Thermosensitive property
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
reaction is carried out at relatively high temperature, i.e.
50e70 ꢀC.
Recently, the application of polymeric hydrogels to biochips
such as DNA chips and protein chips has attracted special attention
in the field of biotechnology [1e5]. In hydrogel-type biochips, the
capture probes are immobilized on the gel network, and the target
probes diffuse through the gel network and react with the capture
probes. The hydrogel-type biochip offers the advantage of having
a larger immobilization capacity than that of conventional biochips,
in which the capture probes are immobilized two-dimensionally on
a flat plate. Furthermore, in the hydrogel-type biochip, the gel
network, which serves as a matrix that holds water, prevents the
chips from drying.
(b) High transparency of the gel at room temperature. The
hybridization reaction is usually detected by means of a fluo-
rescence technique, and the detection is carried out at room
temperature. The target probes are previously labeled with
a fluorescent material. The hybridization is detected by the
fluorescence emitted from the target probes, which react with
the capture probes in the gel. Therefore, transparency of the gel
at room temperature is a requisite for easy detection of the
fluorescence response.
The existing hydrogel-type biochips satisfy requirement (b).
However, the diffusivity or permeability of the target probes
through the gel network is low, and thus, a long analysis time is
required.
The important requirements for the hydrogel-type biochip are
as follows:
(a) High diffusivity or permeability of the capture probes such as
DNA through the gel network in the reaction process. The
In order to alleviate this problem, we proposed a semi-
interpenetrating polymer network (semi-IPN) gel, which consists
of an amphiphilic gel and an interpenetrating thermosensitive
polymer [6]. Thermosensitive polymers are soluble in water below
their lower critical solution temperature (LCST) and gel is trans-
parent; however, they are insoluble in water above the LCST.
* Corresponding author. Yokohama Corporate Research Laboratories, Mitsubishi
Rayon Co., Ltd., 10-1 Daikoku-cho, Tsurumi-ku, Yokohama 230-0053, Japan.
Fax: þ81 45 504 1139.
E-mail address: takahashi_at@mrc.co.jp (A. Takahashi).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.06.014

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A. Takahashi et al. / Polymer 52 (2011) 3791e3799
Fig. 2. Chemical structure of DMAAPS.
motion of the polymer molecules overcomes the intrachain and/or
interchain interactions [12]. Furthermore, the intrachain and/or
interchain interactions are disrupted by the addition of salts, and
the resulting expansion of the polymer chain leads to a decreased
UCST [12e16]. In other words, the solubility of the polymer is
enhanced by dissolved salts. In addition, the LCST of poly(NIPAM) in
aqueous salt solutions is increased by copolymerization with the
zwitterionic monomer [17e21]. Based on these properties of
zwitterionic polymers, it is expected that the LCST of poly(NIPAM)
in buffer solutions containing a high concentration of SSC can be
controlled by copolymerization with DMAAPS.
In this study, the effects of the preparation conditions of the
semi-IPN gel, such as the copolymerization ratio of NIPAM in DMAA
gel, the content of poly(NIPAM-co-DMAAPS), and the copolymeri-
zation ratio of DMAAPS on the thermosensitive behavior of the
semi-IPN gel in buffer solutions containing various concentrations
of SSC were examined experimentally. The thermosensitive
behaviors were examined by measuring the changes in the trans-
mittance through the semi-IPN gel and the changes in the
permeability of the buffer solution through the semi-IPN gel
membrane with temperature.
Fig. 1. Conceptual representation of structural changes in the semi-IPN gel networks.
Therefore, on heating, the interpenetrating thermosensitive poly-
mers of the semi-IPN gel become entangled in the gel network
resulting in shrinkage of a part of the gel network without a large
volume change, as shown in Fig. 1. This structural change results in
enhanced permeability of materials through the gel. This
phenomenon was confirmed in our previous study [6]. In that
study, the DMAA gel was used as the base gel and poly(N-iso-
propylacrylamide) (poly(NIPAM)) was used as the interpenetrating
thermosensitive polymer. The DMAA gel is amphiphilic and its
swelling degree is almost independent of temperature and the
composition of the solution such as the salt concentration. Poly(-
NIPAM), on the other hand, is well known as a typical thermo-
sensitive polymer with an LCST of approximately 32 ꢀC in water.
There have been several reports on semi-IPN gels composed of
a hydrophilic gel and an interpenetrating thermosensitive polymer;
these reports state that the responsiveness, mechanical strength, and
partition of hydrophobic materials in these gels have been improved
by controlling the temperature [7e10]. The thermosensitive proper-
ties of these semi-IPN gels have been examined only in water.
However, biochips are usually used in buffer solutions, which are
composed of a relatively high concentration of salts and a surfactant.
Nonetheless, there is a dearth of research of the thermosensitive
behavior of the semi-IPN gel composed of an amphiphilic gel and
interpenetrating thermosensitive polymer in buffer solutions.
Typical buffer solutions used in biochips are composed of
sodium chloride (NaCl), sodium citrate, and sodium dodecyl sulfate
(SDS). The composition of the buffer solution is usually expressed in
terms of SDS and a mixture of sodium chloride and sodium citrate
(SSC). The solution represented as 1 ꢁ SSC consists of a mixture of
0.15-mol/l sodium chloride and 0.015-mol/l sodium citrate. In
biochips, various concentrations of SSC up to 6 ꢁ SSC (0.9-mol/l
NaCl and 0.09-mol/l sodium citrate) are used as the buffer solution.
In our previous study [6], the thermosensitive behavior of the
semi-IPN gel composed of DMAA-co-NIPAM gel and inter-
penetrating poly(NIPAM) was examined in the buffer solution
containing 2 ꢁ SSC and 0.2% SDS. The LCST of poly(NIPAM)
decreases with increase in the concentration of salt, and at high salt
concentrations poly(NIPAM) is insoluble at room temperature [11].
Therefore, the semi-IPN gel proposed in our previous study could
not be at SSC concentrations higher than 3 ꢁ SSC.
The main purpose of this study is to confirm the structural
chang
es in the gel network shown in Fig. 1 in a buffer solution
containing salts of relatively high concentrations. To promote the
permeation or diffusion of materials through the gel network, the
effective pore size of the gel network should be large, or in other
words, the cross-linking density should be small. However, in such
a case, the resultant mechanical strength of the gel decreases, and it
is difficult to use these gels for biochips. On the other hand, it is
considered that the mechanical strength is maintained by inter-
penetrating polymer in the gel network, even if the cross-linking
density is low. In this study, the effect of the concentration of
cross-linker in the preparation of the gel on the permeability was
not investigated; in other words, the effective pore size of the
network of the DMAA-co-NIPAM gel was not considered. Our
attention is focused on the structural changes in the gel network.
2. Experimental section
2.1. Synthesis
2.1.1. Synthesis of DMAAPS
DMAAPS was synthesized by the ring-opening reaction of 1,3-
propanesultone with N,N-dimethylaminopropylacrylamide (DMA-
PAA) in the presence of acetonitrile, at 30 ꢀC for 1.5 h [22]. DMAPAA
and 1,3-propanesultone were kindly supplied by Kohjin Co. Ltd.,
and were used without further purification. A mixture of 1,3-
propanesultone (75 g) and acetonitrile (75 g) was added drop-
wise to the mixture of DMAPAA (100 g) and acetonitrile (200 g)
under continuous stirring with a magnetic stirrer over a period of
1.5 h at 30 ꢀC, and the mixing was continued for 16 h at the same
temperature. The resulting white precipitate was filtered, washed
with acetone, and dried under vacuum.
In order to obviate this problem, we paid attention to zwitter-
ionic polymers such as poly(N,N-dimethyl(acrylamidopropyl)
ammonium propane sulfonate) (poly(DMAAPS)), the chemical
structure of which is shown in Fig. 2. Poly(DMAAPS) is insoluble in
water at lower temperatures, and is soluble at high temperatures;
in other words, the poly(DMAAPS) has an upper critical solution
temperature (UCST). The zwitterionic polymer is considered to be
in a collapsed-coil state in water below the UCST due to the intra-
chain and/or interchain interactions; however, above the UCST, the
polymer adopts an extended conformation because the thermal
2.1.2. Synthesis of poly(NIPAM), poly(DMAAPS), and poly(NIPAM-
co-DMAAPS)
NIPAM was also kindly supplied by Kohjin Co. Ltd. and was
purified by recrystallization from hexane before use. Poly(NIPAM)
was prepared by radical polymerization in the same manner as that

A. Takahashi et al. / Polymer 52 (2011) 3791e3799
3793
Table 1
Synthesis conditions of semi-IPN gels.
[mol/m3]
NP0
NP10
NP20
NP30
Monomer
Co-monomer
: N,N-dimethylacrylamide (DMAA)
: N-isopropylacrylamide (NIPAM)
1000
0
900
100
800
200
700
300
(Copolymerization ratio of NIPAM)
0 mol%
4
10
10 mol%
20 mol%
30 mol%)
Linker
Accelerator
Initiator
: N,N0-methylenebisacrylamide (MBAA)
: N,N,N0,N0-tetramethylethylenediamine (TEMED)
: Ammonium peroxodisulfate (APS)
0.5
Solvent: aqueous poly(NIPAM-co-DMAAPS) solution: 0.1, 0.3, 0.5 wt%.
Copolymerization ratio of DMAAPS: 10 mol%.
Temperature: 20 ꢀC.
Reaction time: 6 h.
reported in our previous paper [23]. N,N,N0,N0-tetramethylethyle-
nediamine (TEMED) and ammonium peroxodisulfate (APS) were
used as an accelerator and an initiator, respectively. The prepared
poly(NIPAM) was purified by dialysis over a period of one week,
using the membrane Cellu Step T3, Nominal MWCO: 12000-14000,
(Membrane Filtration Products, Inc.). The average molecular weight
of poly(NIPAM) was estimated from the intrinsic viscosity as
previously reported [23]. Intrinsic viscosity measurement was
performed using the polymer solution dissolved in tetrahydrofuran
at 27 ꢀC, and the following equation that was proposed by Fujish-
ige [24] was used to calculate the number average molecular
weight Mn:
is transparent because, in this state, the polymer is soluble in the
solution. However, when the change to the hydrophobic state is
induced by heating or cooling, the solution becomes opaque or
milky white, because the hydrophobic polymer is insoluble in the
aqueous solution. Therefore, the transition temperature (LCST or
UCST) can be estimated by measuring the change in the trans-
mittance through the solution as a function of temperature. The
transmittance was measured at 600 nm using a spectrophotometer
equipped with a temperature-control system (V-530, Japan Spec-
troscopy Co., Ltd.) and the measurements were performed for the
solutions with various concentrations of NaCl, sodium citrate, and
SDS.
The temperature dependence of the transmittance through the
semi-IPN gels immersed in the buffer solution was also measured in
order to examine their thermosensitive behaviors in the buffer
solution. The measurements were performed at 600 nm using the
aforementioned spectrophotometer. Measurements were carried
out using a quartz cell with a path length of 10 mm, and the gel was
placed perpendicular to the light path and gently pressed on the
inner wall of the quartz cell filled with the buffer solution.
0:65
½
h
ꢂ ¼ 9:59 ꢁ 10ꢃ3Mn
(1)
The Mn of poly(NIPAM) prepared in this study was about 7.42 ꢁ 106.
Poly(DMAAPS) and poly(NIPAM-co-DMAAPS) were also
prepared by radical polymerization in the same manner as that
mentioned above. In the preparation of poly(NIPAM-co-DMAAPS),
the copolymerization ratio of DMAAPS was 5, 10, and 20 mol%. The
molecular weights of these polymers were not evaluated.
2.2.2. Measurement of the swelling properties of semi-IPN gels
The effect of the temperature on the degree of swelling of the
semi-IPN gels in the buffer solution was examined using the
cylindrical gels. The dried gels were immersed in the buffer solu-
tion at 25 ꢀC until the swelling of the gel reached equilibrium, and
the swelling diameter of the gel was measured with a cathetometer.
The temperature was then increased to the desired temperature,
and the equilibrium swelling diameter was measured again. This
procedure was repeated up to 70 ꢀC.
2.1.3. Preparation of semi-IPN gels
The semi-IPN gels were prepared in an aqueous solution con-
taining poly(NIPAM-co-DMAAPS) by copolymerization using
DMAA as the primary monomer, NIPAM as a co-monomer, and
N,N0-methylenebisacrylamide (MBAA) as a cross-linker. The prep-
aration was performed at 20 ꢀC by radical polymerization using
TEMED and APS. The synthesis conditions of the semi-IPN gels are
listed in Table 1.
In order to measure the transmittance through the semi-IPN
gels, plate-type gels of 3 mm thickness were prepared. The
synthesis was performed between two glass plates separated by
a 3-mm-thick spacer. The gels were cut into dimensions of
10 ꢁ 20 mm. Cylindrical gels were also synthesized for use in
swelling ratio measurements. The synthesis was performed in
a glass tube with an inner diameter of 6 mm. The gels were cut into
lengths equal to the diameter.
2.2.3. Measurement of the temperature dependence of the
permeability of the buffer solution through the semi-IPN gel
membrane
The semi-IPN gel membrane was synthesized in the hole of the
gel holder composed of three glass plates with a total thickness of
3.6 mm as shown in Fig. 3(a). The diameter of the hole of the center
plate was 30 mm, and a cut was made in a part of the center plate to
introduce the reactant solution. The diameter of the holes on the
glass plates on both sides was 20 mm. The surface of the glass plate
containing holes was treated with chlorodimethylvinylsilane to
introduce double bonds on the wall surface [25]. The synthesis of
the gel was performed by radical polymerization. The preparation
procedure was as follows. The hole was covered by two slide
glasses, and the reactant solution, which was previously degassed,
was poured into the hole through a cut in the center plate. The
reactant solution was composed of two aqueous solutions. The first
was the monomer solution containing DMAA, NIPAM, MBAA,
TEMED, and poly(NIPAM), and the other was the APS solution. After
2.2. Measurements
2.2.1. Measurement of transmittance through the buffer solution
containing the polymer and through the semi-IPN gel immersed in
the buffer solution
The transition behavior of poly(DMAAPS), poly(NIPAM), and
poly(NIPAM-co-DMAAPS) was investigated by measuring the
change in the transmittance as a function of temperature in an
aqueous solution containing SSC and SDS, which constitute the
buffer solution. In the hydrophilic state of the polymer, the solution

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A. Takahashi et al. / Polymer 52 (2011) 3791e3799
Fig. 3. (a) Gel holder and (b) apparatus for permeability measurements.
completion of the polymerization of the gel, the slide glasses
covering the holes were removed, and the surface of the gel
membrane was reinforced by a stainless steel net, which was
attached to the glass plate by quick-drying glue as shown in
Fig. 3(a). Then, the membrane was washed by immersing it into
distilled water to remove unreacted monomers and other reagents.
The DMAA-co-NIPAM gel membrane without poly(NIPAM-co-
DMAAPS) was also prepared for comparison. The gel was prepared
using the following composition: DMAA/NIPAM/MBAA: 700/300/
4 mol/m3 (NP30). The poly(NIPAM-co-DMAAPS) concentration was
0.5 wt%.
The permeability of the buffer solution through the gel
membrane was measured with the apparatus shown in Fig. 3(b).
The apparatus was composed of two permeation cells made of
glass, a 500 mm long glass column was used to generate the
hydrostatic pressure and a calibrated glass pipette was used to
measure the flow rate. The gel holder shown Fig. 3(a) was placed
and fixed between two permeation cells. The apparatus was
immersed in the water bath. Then, the buffer solution was poured
into both cells, and the apparatus was left overnight to reach an
equilibrium swelling state and the steady permeation rate at 35 ꢀC,
after which the position of the meniscus in the pipette was
measured as a function of time. Then, the temperature was raised to
the desired temperature and the apparatus was again left over-
night, and the measurement was repeated. The procedure was
repeated until the temperature reached 50 ꢀC.
where
n
is the velocity of the buffer solution; A is the cross-sectional
P is the pressure applied to the gel
area of the gel membrane;
membrane; d is the thickness of the gel membrane and
viscosity of the buffer solution.
D
m is the
3. Results and discussion
3.1. Thermosensitive behavior of poly(DMAAPS) in water
Fig. 4 shows the temperature dependence of the trans-
mittance through the aqueous solution of poly(DMAAPS). At
lower temperatures, the solution was opaque or milky white, and
the transmittance was very low. Increasing the temperature
resulted in a change in the solution from opaque to completely
transparent, and the transmittance increased to near 100%. In
other words, the poly(DMAAPS) has an UCST. Below the UCST, the
zwitterionic polymer forms aggregates because the intrachain
and/or interchain interactions between the side chains of the
polymer are larger than the force of molecular motion, whereas
above the UCST, the polymer is soluble in water because thermal
motion becomes larger than the interactions between the side
chains of the polymer [11]. As the zwitterionic polymer
concentration was increased, there was an increase in the UCST.
When the polymer concentration increase, the intrachain and/or
interchain parings also increase, and more thermal energy is
required to overcome the intrachain and/or interchain interac-
tions. Therefore, the UCST also increases. On the other hand,
when the polymer concentration was low, the change in the
transmittance was broad. This is considered due to the fact that
the interchain and/or intrachain parings decreased and are
distributed inhomogeneously.
The permeation coefficient K was estimated by the following
equation:
V
A
D
P
m
v ¼
¼ K$
d$
(2)
3.2. Thermosensitive behavior of poly(NIPAM), poly(DMAAPS), and
poly(NIPAM-co-DMAAPS) in the buffer solution
Fig. 5 shows the effect of the concentration of NaCl in the
solution containing the poly(DMAAPS) on the transmittance of the
solution. The concentration of poly(DMAAPS) and the measure-
ment temperature were fixed at 10 g/l and 25 ꢀC, respectively. The
transmittance increased sharply at approximately 1.2 g/l of NaCl.
This observation implies that the zwitterionic polymer may be
soluble in the solution at a high NaCl concentration.
Poly(NIPAM), on the other hand, is a representative polymer
with an LCST of approximately 32 ꢀC in water, and it is well known
that the LCST decreases as the concentration of the salt is
increased [11]. Fig. 6 shows the effect of NaCl concentration in the
solution on the transmittance of the solution containing poly(-
NIPAM). These data were obtained in this study. From the figure it
Fig. 4. Transition behavior of poly(DMAAPS) in water.

A. Takahashi et al. / Polymer 52 (2011) 3791e3799
3795
Fig. 5. Transition behavior of poly(DMAAPS) in sodium chloride (NaCl) solution.
Fig. 7. Transition behavior of poly(NIPAM-co-DMAAPS) in sodium chloride (NaCl)
solution.
can be seen that the LCST decreases with increase in NaCl
concentration. These phenomena are attributed to the fact that the
poly(NIPAM) molecules become more hydrophobic in salt solu-
tions because the salt molecules destroy the hydration layer
around the poly(NIPAM) molecules. Therefore, poly(NIPAM) is
insoluble in solutions having a high salt concentration at room
temperature.
It is expected that the thermosensitive properties of poly(-
NIPAM) in salt solution can be easily changed by copolymerizing
DMAAPS with NIPAM. Fig. 7 shows the effect of NaCl concentration
in the solution containing poly(NIPAM-co-DMAAPS) on the LCST.
The copolymerization ratio of DMAAPS was fixed at 10 mol%. From
the figure it can be seen that the LCST of poly(NIPAM-co-DMAAPS)
increased compared with that of poly(NIPAM) at the same NaCl
concentration. This result is thought to be due to the fact that the
interaction between the zwitterionic polymer and salt inhibits the
dehydration of poly(NIPAM).
the ions in the solution or, in other words, by the salt in the
solution.
Fig. 9 shows the temperature dependence of the transmittance
of the poly(NIPAM) (Fig. 9(a)) and poly(NIPAM-co-DMAAPS)
(Fig. 9(b)) solutions containing various concentrations of SDS. The
LCST of poly(NIPAM) increased as the concentration of SDS was
increased. Furthermore, the decrease in the transmittance became
less as the concentration of SDS increased. These results are
different from those in the solution containing NaCl or sodium
citrate. These phenomena are well known and explained by the
interaction between the hydrophobic groups of SDS and the iso-
propyl groups of poly(NIPAM) [26e28]. A similar tendency was
observed for the solution containing the poly(NIPAM-co-DMAAPS);
however, the LCST of poly(NIPAM-co-DMAAPS) was higher than
Fig. 8 shows the temperature dependence of the transmittance
of the poly(NIPAM) (Fig. 8(a)) and poly(NIPAM-co-DMAAPS)
(Fig. 8(b)) solutions containing various concentrations of sodium
citrate. The LCST of poly(NIPAM) decreased as the concentration of
sodium citrate increased; however, copolymerization with
DMAAPS results in an increase of the LCST compared with that of
poly(NIPAM) in addition to the effect of the NaCl concentration as
shown in Fig. 7. In Figs. 7 and 8, it can be seen that the effect of the
salt on the LCST of the poly(NIPAM-co-DMAAPS) is larger than that
on the LCST of the poly(NIPAM). This reason is attribute to the fact
that LCST of an ionic thermosensitive polymer is largely affected by
that of the poly(NIPAM) solution at
concentration.
a
comparable SDS
Fig.10 shows the effect of the copolymerization ratio of DMAAPS
on the transition behavior of poly(NIPAM-co-DMAAPS) in water
(Fig. 10(a)) and in the buffer solutions composed of mixtures of SSC
and SDS (Fig. 10(b)e(d)). The SDS concentration was fixed at 0.2 wt
% and the SSC concentrations were 2ꢁ, 4ꢁ, and 6 ꢁ SSC. The
solutions represented as 2 ꢁ SSC, 4 ꢁ SSC and 6 ꢁ SSC consist of
a mixture of 0.30-mol/l NaCl and 0.030-mol/l sodium citrate, 0.60-
mol/l NaCl and 0.060-mol/l sodium citrate and 0.90-mol/l NaCl and
0.090-mol/l sodium citrate, respectively. In water, the LCST
increased as the copolymerization ratio of DMAAPS increased, and
the sharpness of the transition was lost. In the buffer solution
containing 2 ꢁ SSC (Fig. 10(b)), the transition of poly(NIPAM) was
broad, and the LCSTs of the poly(NIPAM-co-DMAAPS) with varying
copolymerization ratios increased as the copolymerization ratio of
DMAAPS increased; however, the transitions were not clear. These
results are attributed to the fact that the interaction of poly(NIPAM)
and SDS, which inhibits the transition of poly(NIPAM), is stronger
than that of poly(NIPAM) and SSC, which promotes the transition of
poly(NIPAM). In the buffer solution containing 4 ꢁ SSC (Fig. 10(c)),
poly(NIPAM) did not dissolve. The LCSTs of poly(NIPAM-co-
DMAAPS) in the buffer solution containing 4 ꢁ SSC were lower than
those in the buffer solution containing 2 ꢁ SSC. Furthermore, in the
buffer solution containing 6 ꢁ SSC (Fig. 10(d)), the poly(NIPAM-co-
DMAAPS) copolymerized using 5 mol% DMAAPS also did not
dissolve at room temperature, and therefore the result was not
shown; the transition of the polymer copolymerized using 10 mol%
DMAAPS was observed between 30 and 40 ꢀC; and the transition of
the polymer copolymerized using 20 mol% DMAAPS was not
observed in the experimental temperature range. These results
Fig. 6. Transition behavior of poly(NIPAM) in sodium chloride (NaCl) solution.

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A. Takahashi et al. / Polymer 52 (2011) 3791e3799
Fig. 8. Transition behavior of (a) poly(NIPAM) (b) poly(NIPAM-co-DMAAPS) in sodium citrate solution.
imply that the transition of poly(NIPAM) in the buffer solution
composed of various concentrations of salt can be controlled by
copolymerization with the zwitterionic DMAAPS at various ratios.
The change in the transmittance, as the temperature was
decreased, was also measured in the solution of 4 ꢁ SSC using pol-
y(NIPAM-co-DMAAPS) copolymerized 10 mol% DMAAPS. Hysteresis
was observed. However, the temperature difference was small. The
curve of the transmittance shifted to lower temperature of approxi-
mately 3 ꢀC as compared with that through the upward process.
was almost 0% for the NP30 semi-IPN gel, which was opaque or
milky white.
It is well known that the network distribution of the transparent
gel is homogeneous macroscopically, whereas that of the opaque
gel consists of dense and coarse networks, in other words, the
structure is inhomogeneous [29]. The formation of the inhomoge-
neous network structure can be attributed to the fact that the
interpenetrating poly(NIPAM-co-DMAAPS) molecules become
entangled in the gel network resulting in shrinkage of a part of the
gel network, as shown in Fig. 1.
The transmittance through the NP10 semi-IPN gel increased
slightly with time. This increase in transmittance implies that there
was outflow of a part of the interpenetrating poly(NIPAM-co-
DMAAPS) molecules in the semi-IPN gel from the gel. On the other
hand, the transmittances through the NP20 and NP30 semi-IPN gels
remained constant during the course of the experiment, which
implies that the interpenetrating poly(NIPAM-co-DMAAPS) mole-
cules were held stably in the gel network. This stability was
attributed to the fact that the interaction between the poly(NIPAM)
molecules and the NIPAM components in the gel network
suppresses the outflow of the interpenetrating polymer molecules
from the gel.
3.3. Stability of semi-IPN gels composed of DMAA-co-NIPAM gel
and interpenetrating poly(NIPAM-co-DMAAPS)
Fig. 11 shows the change in the transmittance through the
semi-IPN gels with time. Three kinds of gels were used: NP10,
NP20 and NP30, which were prepared in the poly(NIPAM-co-
DMAAPS) solution of 0.5 wt%. The semi-IPN gels were immersed
in a buffer solution composed of 6 ꢁ SSC and 0.2% SDS at 30 ꢀC,
and the transmittance through the semi-IPN gels was measured
at 50 ꢀC and at the desired time interval. The initial trans-
mittance through the semi-IPN gels decreased as the copoly-
merization ratio of NIPAM in the gel network was increased, and
Fig. 9. Transition behavior of (a) poly(NIPAM) (b) poly(NIPAM-co-DMAAPS) in sodium dodecyl sulfate (SDS) solution.

A. Takahashi et al. / Polymer 52 (2011) 3791e3799
3797
Fig. 10. Transition behavior of poly(NIPAM-co-DMAAPS) in buffer solution consisting of SSC and SDS. The SDS concentration was fixed at 0.2 wt%, while that of SSC was varied from
2 ꢁ SSC to 6 ꢁ SSC. The polymerization ratio of DMAAPS was varied from 0 to 20 mol%. The concentration of poly(NIPAM-co-DMAAPS) was 0.5 wt%.
3.4. Thermosensitive properties of the semi-IPN gel in the buffer
solution
3.5. Swelling properties of the semi-IPN gels in the buffer solution
Fig. 13 shows the temperature dependence of the swelling
diameter of the cylindrical semi-IPN gels in the buffer solution
comprised of 6 ꢁ SSC and 0.2% SDS. The NP10, NP20, and NP30 gels
were used, and the content of poly(NIPAM-co-DMAAPS) was fixed
at 0.5 wt%. The gel diameter decreased slightly with increasing
temperature. Furthermore, the change in the gel diameter was
dependent on the copolymerization ratio of NIPAM in the gel
Fig. 12 shows the effect of the content of the interpenetrating
poly(NIPAM-co-DMAAPS) in the semi-IPN gel on the thermo-
sensitive properties of the semi-IPN gel in the buffer solution,
which is evaluated by the change in the transmittance through the
semi-IPN gel with temperature. The NP20 gel was used. The
copolymerization ratio of DMAAPS in the interpenetrating poly(-
NIPAM-co-DMAAPS) was 10 mol%. The measurement of the
transmittance was performed in the buffer solution comprised of
6 ꢁ SSC and 0.2% SDS. The transmittance through the gel without
the interpenetrating polymer did not decrease in the experimental
temperature range; in other words, the network structure of the
gel did not change. The transmittance through the semi-IPN gel
containing 0.5 wt% interpenetrating poly(NIPAM-co-DMAAPS)
began to decrease at about 30 ꢀC and reached 20% at 50 ꢀC. The
transition behavior of this semi-IPN gel was similar to that of
poly(NIPAM-co-DMAAPS) shown in Fig. 10(d). Based on these
results, it is suggested that the gel network changed from
a
homogeneous structure to an inhomogeneous structure
composed of the coarse and dense networks because of the
transition of the interpenetrating poly(NIPAM-co-DMAAPS). On
the other hand, the transmittance through the semi-IPN gel con-
taining 1.0 wt% interpenetrating poly(NIPAM-co-DMAAPS) was
low even at 30 ꢀC. These results suggest that in the presence of
a higher content of interpenetrating poly(NIPAM-co-DMAAPS), the
inhomogeneous network structure forms in the preparation of the
semi-IPN gel.
Fig. 11. The transmittance change through the semi-IPN gels with time. The gel was
prepared with various copolymerization ratios of NIPAM. The polymerization ratio of
DMAAPS is 10 mol%. The concentration of poly(NIPAM-co-DMAAPS) is 0.5 wt%.

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A. Takahashi et al. / Polymer 52 (2011) 3791e3799
Fig. 12. Temperature dependence of the transmittance through NP20 semi-IPN gels.
The gel was prepared in the solution containing various concentrations of poly(NIPAM-
co-DMAAPS).
Fig. 14. Temperature dependence of the permeability of the buffer solution through
NP30 gel without poly(NIPAM-co-DMAAPS) and NP20 semi-IPN gel.
network, and decreased as the copolymerization ratio of NIPAM
was increased. On the other hand, the gel diameter was hardly
affected by the content of the interpenetrating poly(NIPAM-co-
DMAAPS) in the content range of 0e0.5 wt%. These results indicate
that the change in the network structure of the semi-IPN gel
occurred without a large volume change. The change in the gel size
observed here is sufficiently small to enable the use of the semi-IPN
gel as a base gel for biochips.
through the coarse network. On the basis of these results, the
structural change of the semi-IPN gel network due to the transition
of the interpenetrating poly(NIPAM-co-DMAAPS) was confirmed.
4. Conclusion
The transition behaviors of the poly(NIPAM-co-DMAAPS) and
the semi-IPN gel composed of the DMAA-co-NIPAM gel and the
interpenetrating thermosensitive poly(NIPAM-co-DMAAPS) were
examined in buffer solutions containing relatively high concen-
trations of SSC. The LCST of poly(NIPAM-co-DMAAPS), in which
zwitterionic DMAAPS is copolymerized with NIPAM, increased as
the polymerization ratio of DMAAPS increased and decreased with
increasing SSC concentration. The LCST of poly(NIPAM-co-
DMAAPS) could be controlled by selecting the polymerization ratio
of DMAAPS in the desired temperature range even at an SSC
concentration of 6 ꢁ SSC, which is a quite high salt concentration.
The change in the semi-IPN gel from transparent to opaque or
milky white corresponding to the transition of poly(NIPAM-co-
DMAAPS) occurred without a large volume change. The develop-
ment of opacity implies that the gel network changed from
3.6. Temperature dependence of the permeability of the buffer
solution through the semi-IPN gel
Fig. 14 shows the temperature dependence of the permeability
of the buffer solution containing 6 ꢁ SSC and 0.2% SDS through the
semi-IPN gel membrane. The NP30 semi-IPN gel prepared in the
0.5 wt% poly(NIPAM-co-DMAAPS) solution was used. The copoly-
merization ratio of DMAAPS in the interpenetrating poly(NIPAM-
co-DMAAPS) was 10 mol%. The results for the gel membrane
without poly(NIPAM-co-DMAAPS) are also shown for comparison.
The permeability of the buffer solution through the gel membrane
without poly(NIPAM-co-DMAAPS) decreased slightly as the
temperature increased. The decrease in permeability with
increasing temperature can be attributed to the fact that the
volume of the gel decreased slightly as the temperature increased;
in other words, the effective pore size of the gel network decreased
slightly. On the other hand, the permeability through the semi-IPN
gel increased drastically between 35 and 45 ꢀC. This temperature
range corresponds to the transition behavior of the poly(NIPAM-co-
DMAAPS) shown in Fig. 10(d) and the semi-IPN gel shown in Fig. 12.
It is considered that the buffer solution could permeate easily
a
homogeneous structure to an inhomogeneous structure
composed of the coarse and dense networks due to the transition of
the interpenetrating poly(NIPAM-co-DMAAPS). This structural
change in the network was confirmed by the fact that the perme-
ability of the buffer solution through the gel membrane increased
drastically in the temperature range corresponding to the transi-
tion of the poly(NIPAM-co-DMAAPS) and the semi-IPN gel.
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