Synthesis and characterization, studia, artykuły, PNIPAAm
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Colloid Polym Sci 277:1079±1082 (1999)
Ó
Springer-Verlag 1999
SHORT COMMUNICATION
X.-Z. Zhang
R.-X. Zhuo
Synthesis and characterization
of a novel thermosensitive gel
with fast response
Received: 17 March 1999
Accepted on revised form: 9 June 1999
Abstract A gel is a kind of water-
swellable but water-insoluble, cross-
linked polymer. Some kinds of gels
can respond to external temperature
changes. This speci®c property is
useful in biomedical and other tech-
nological ®elds. In this paper the
synthesis of a novel, thermosensitive
gel by the copolymerization of
N-isopropylacrylamide or N,N-
diethylacrylamide with 3-metha-
cryloxypropyltrimethoxy silane
(MPTMS) is reported. The forma-
tion of the gel is caused through the
interaction of MPTMS under acidic
condition. This thermosensitive gel
can deswell and reswell quickly in
response to the external temperature
changes, this behavior probably
being due to the heterogeneous
structure of the gel produced. This
fast-response gel may be useful both
in biomedical and biotechnological
®elds.
X.-Z. Zhang
á
R.-X. Zhuo (
&
)
Laboratory of Biomedical Polymer
Materials of the Ministry of Education
Chemistry Department
Wuhan University
Wuhan 430072, China
Key words Thermosensitive
á
Gel
á
Lower critical solution
temperature
á
Fast response
Introduction
Thermosensitive polymers show thermoreversible hy-
dration-dehydration changes in response to external
temperature changes and exhibit lower critical solution
temperatures (LCST) [1]. Correspondingly, crosslinked
gels are swellable below this temperature and undergo
abrupt changes in volume, precipitating from solution
suddenly as the temperature is increased above the
LCST. This property of discontinuous phase separation
of gels has been used in biomedical and biotechnological
®elds, for instance, controlled drug release [2], absorp-
tion of proteins [3], immobilization of enzymes [4] and
recyclable absorbents [5], etc.
The temperature response rate of a normal thermo-
sensitive gel is slow and this restricts some potential
applications of this kind of gel, such as on±o switches.
Several successful attempts have been made to increase
the response rate. For example, Hirasa et al. [6]
synthesized poly (vinyl methyl ether) gel by radiation
crosslinking. Kabra and Gehrke [7] synthesized poly(N-
isopropylacrylamide) (PNIPA) by using a phase-separa-
tion technique to create fast-temperature-response gels.
Wu et al. [8] used a new method, which is based on a
modi®cation of the original method used by Kabra and
Gehrke to prepare fast-response, macroporous PNIPA
gels. Recently, Kaneko and coworkers [9±11] prepared a
comb-type grafted PNIPA gel which showed a rapid
deswelling rate at high temperature (above its LCST).
This paper reports a new approach to preparing
a fast-temperature-response gel by using 3-met-
hacryloxypropyltrimethoxy silane (MPTMS) during
the polymerization/crosslinking reaction. PNIPA and
poly(N,N-diethylacrylamide) (PDEAA) gels are typical
thermosensitive gels, and N-isopropylacrylamide
(NIPA) and N,N-diethylacrylamide (DEAA) were cho-
sen as the monomers.
Experimental
Materials
NIPA and DEAA were synthesized and puri®ed according to Refs.
[4, 12], respectively. MPTMS was a gift from Wuhan University
Chemical Plant (Wuhan, China) and was used as supplied.
Ammonium persulfate (APS), sodium bisul®te (SBS) and glacial
acetic acid (HAc) were all analytical grade and were used as received.
1080
Synthesis of PNIPA and PDEAA gels
The monomers NIPA and MPTMS were dissolved in distilled
water/HAc (0.5 ml/0.5 ml) mixed solvent. APS and SBS were used
as a pair of redox initiators (5.0 wt% based on NIPA). The
polymerization/crosslinking was carried out at 26
°
C for 15 h in a
glass vessel. After the reaction, the gel was cut into discs (10 mm in
diameter and 3 mm in thickness) and then the gel discs were
immersed in ethanol at 5
°
C for 24 h. During this period, the
ethanol was replaced every 4±5 h with fresh ethanol in order to
leach out the chemical residues. The gel thus obtained was further
puri®ed by immersing it in distilled water at 5
°
C for 72 h.
Likewise, the distilled water was also refreshed at regular time
intervals as in the procedure described for ethanol in order to leach
out the ethanol. The gel thus obtained was de®ned as PNIPA gel.
The synthesis of the PDEAA gel was carried out at 20
°
C for
15 h and the other polymerization conditions and treatment
procedures were the same as those for the PNIPA gel. The feed
compositions of monomers and other reactants are listed in
Table 1.
blotting the excess surface water with moistened ®lter paper. The
weight changes of the gels were recorded during the course of the
reswelling process at regular time intervals. The water uptake (WU)
is de®ned as follows:
WU
100
W
t
ÿ W
d
=W
s
;
3
where the symbols W
t
, W
d
and W
s
are the same as de®ned
previously.
Results and discussion
Formation of the PNIPA and PDEAA gels
The formation of a gel is caused by the subsequent
reaction of MPTMS under acidic conditions. First, it is
quickly copolymerized with the monomers NIPA or
DEAA through redox radical polymerization. This
copolymerization process is quick and the MPTMS
reacted as a usual monomer at this moment. Then, the
trimethoxy silane groups of MPTMS hydrolyzed to give
silanol. Finally, silanol condensed to form a siloxane
linkage, thus causing the crosslinking of the polymer
chains. This process is complicated and the details of
it are under investigation. The formation of the gel is
shown in Fig. 1.
Measurement of the swelling ratios
of the PNIPA and PDEAA gels
The swelling ratios (SR) of these two kinds of gels were measured
gravimetrically after blotting the excess surface water with
moistened ®lter paper. The gels were incubated in distilled water
for at least 24 h at every particular temperature. The SR is de®ned
as follows:
SR
W
s
=W
d
;
1
where W
s
is the weight of water in the swollen gel after the
equilibrium in distilled water at a particular temperature and W
d
is
the dry weight of the gel dried in vacuum overnight.
Measurement of the deswelling kinetics
of the PNIPA and PDEAA gels
The deswelling kinetics of the PNIPA and the PDEAA gels were
measured gravimetrically at 50
°
C and 42
°
C, respectively, after
blotting the excess surface water with moistened ®lter paper. The
weight changes of the gels were recorded during the course of the
deswelling process at regular time intervals. The water retention
(WR) is de®ned as follows:
WR
100
W
t
ÿ W
d
=W
s
;
2
where W
t
is the weight of the gel at regular time intervals, W
s
is the
weight of the water in the swollen gel after reaching equilibrium in
distilled water at 5
°
C and W
d
is the dry weight of the gel.
Measurement of the reswelling kinetics
of the PNIPA and PDEAA gels
The reswelling kinetics of the PNIPA and the PDEAA gels were
measured gravimetrically at 20
°
C and 15
°
C, respectively, after
Fig. 1 The formation process of the poly(N-isopropylacrylamide)
(PNIPA) and poly(N,N-diethylacrylamide) (PDEAA) gels
Table 1 Feed composition for
the preparation of poly(N-iso-
propylacrylamide) (PNIPA)
and poly(N,N-diethylacryla-
mide) (PDEAA) gels
Component
Sample code
N1 N2 N3 N4 D1 D2 D3 D4
N-isopropylacrylamide (mg)
100 100 100 100
N,N-diethylacrylamide (mg)
100 100 100 100
3-methacryloxypropyltrimethoxy silane (ml)
6
8 12 20
8 10 14 20
1081
Table 2 Physical properties of the PNIPA and PDEAA gels
Physical property
Sample code
N1
N2
N3 N4 D1
D2
D3 D4
Appearance below/above lower
critical solution temperature
translucent/
opaque
translucent/
opaque
opaque/
opaque
opaque/
opaque
translucent/
opaque
translucent/
opaque
opaque/
opaque
opaque/
opaque
Mechanical strength
poor
normal
strong fragile poor
normal
strong fragile
Fig. 2 Temperature dependence of equilibrium swelling ratios of the
PNIPA gels (
h
-N1,
s
-N2,
D
-N3,
e
-N4)
Fig. 3 Temperature dependence of equilibrium swelling ratios of the
PDEAA gels (
h
-D1,
s
-D2,
D
-D3,
e
-D4)
Physical properties of the PNIPA and PDEAA gels
The appearance and the mechanical properties of
these two kinds of gels are summarized in Table 2. At
temperatures below the LCST of the corresponding
gel, when the content of MPTMS is increased, the gels
change from translucent to opaque. As the tempera-
ture is increased above their LCST, all the gels turn
opaque. At the same time, the mechanical properties
of the gels change according to the dierent MPTMS
contents. When the MPTMS content is very low, the
gel is too soft to take up a shape and the mechanical
strength is poor. As the MPTMS content is in-
creased, the mechanical strength of the gel is sucient
for the gel to be handled with forceps; however, if
the MPTMS content is too high, the gel becomes
fragile.
Fig. 4 Deswelling kinetics of the PNIPA gels at 50
°
C (
h
-N1,
s
-N2,
D
-N3,
e
-N4)
Deswelling kinetics of the PNIPA and PDEAA gels
Figures 4 and 5 show the deswelling kinetics of the
PNIPA and the PDEAA gels, respectively. It is evident
that all the gels can shrink and lose the water in them
quickly. The gels shrink to the equilibrated water
content in a few minutes and the equilibrated water
content is higher the greater the MPTMS content, with
the exception of D4.
Swelling ratio of the PNIPA and PDEAA gels
Figures 2 and 3 show the temperature dependence of the
equilibrium swelling ratio of the PNIPA and PDEAA
gels, respectively. These ®gures illustrate that the SR of
both the PNIPA and the PDEAA gels decrease as the
MPTMS content is increased at temperatures below the
LCST. However, the SR of the PNIPA gel is larger than
that of the corresponding PDEAA gel at low tempera-
ture. This is attributed to the dierent nature of
the isopropylamino and diethylamino substituent
groups.
Reswelling kinetics of the PNIPA and PDEAA gels
The reswelling kinetics of these two kinds of gels are
shown in Figs. 6 and 7. All the gels reswell and absorb
water quickly, but the equilibrated WU is dierent. This
shows that the reswelling rate is faster when the
1082
Fig. 5 Deswelling kinetics of the PDEAA gels at 42
°
C (
h
-D1,
s
-
D2,
D
-D3,
e
-D4)
Fig. 7 Reswelling kinetics of the PDEAA gels at 15
°
C (
h
-D1,
s
-D2,
D
-D3,
e
-D4)
structure of the gel obtained is heterogeneous, which
leads to the rapid response rate in response to external
temperature changes. During the deswelling process, the
trapped water molecules are easily freed and released
to the outer aqueous solution as the temperature is
increased above the LCST. As in the case of the
reswelling process, the polymer chains can relax/expand
immediately into the surrounding bulk aqueous medium
with hydration [15] and the fast reswelling rate is
achieved.
Fig. 6 Reswelling kinetics of the PNIPA gels at 20
°
C (
h
-N1,
s
-N2,
D
-N3,
e
-N4)
Conclusion
Through copolymerizing NIPA or DEAA with
MPTMS, we prepared a novel thermosensitive gel. This
gel was crosslinked through the condensation of silanols
under acidic condition, not by the usual crosslinker. This
gel exhibits a fast response rate to temperature changes
and may be useful both in biomedical and biotechno-
logical ®elds.
MPTMS content is increased, although this trend is not
always true. On comparing the reswelling kinetics and
deswelling kinetics, we found that the shrinkage process
is faster than the reswelling process.
Under the experimental conditions adopted, the gels
obtained are translucent or opaque at room temperature
and the reported dense, thick skin layer [13, 14], which is
believed to be impermeable to water, did not form
during the deswelling process. We infer that the network
Acknowledgements The authors are grateful for ®nancial support
from the National Natural Science Foundation of China.
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