High swelling ability of polystyrene-based polyelectrolyte gels at low temperature

Article abstract of DOI:10.1039/c1sm05542a

Molecular design of polymer gels that exhibit high swelling degrees at low temperature was demonstrated with the aid of repulsive interaction among the polymer chains in the ionic polymer gels and the utility of organic solvents with relative low melting points. A small amount of tetraalkylammonium tetraphenylborate as ionic groups was incorporated into cross-linked polystyrene to yield novel ionic polymer gels. The swelling degrees at room temperature (23 °C) were about 100 times as much as their dried weights in various organic solvents. Moreover, they swelled and absorbed some organic solvents such as THF and dichloromethane at low temperature (-80 °C), and the swelling degrees were similar to those at room temperature. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1sm05542a

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Soft Matter
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COMMUNICATION
High swelling ability of polystyrene-based polyelectrolyte gels at low
temperature†
a
Kazuya Iseda, Masahiko Ohta, Toshikazu Ono and Kazuki Sada
a
b
a
*
Received 29th March 2011, Accepted 4th May 2011
DOI: 10.1039/c1sm05542a
(acrylic acid sodium salts), polyNIPAM, polystyrene and poly-
dimethylsiloxane were investigated at room temperature for super-
absorbent polymers, thermal stimuli-sensitive polymers,2–5 polymer
supports6–8 and microfluidic devices,9 respectively. However, the
swelling abilities under other extreme conditions such as high or low
temperature are still in their infancy. Watanabe et al. investigated the
swelling abilities and LCST behaviour of poly(benzyl methacrylate)
above 100 ꢀC by using non-volatile ionic liquids, as a pioneer work at
Molecular design of polymer gels that exhibit high swelling degrees
at low temperature was demonstrated with the aid of repulsive
interaction among the polymer chains in the ionic polymer gels and
the utility of organic solvents with relative low melting points. A
small amount of tetraalkylammonium tetraphenylborate as ionic
groups was incorporated into cross-linked polystyrene to yield novel
ionic polymer gels. The swelling degrees at room temperature
(23 ꢀC) were about 100 times as much as their dried weights in
various organic solvents. Moreover, they swelled and absorbed some
organic solvents such as THF and dichloromethane at low temper-
higher temperature (>100 C).10 On the other hand, swelling prop-
ꢀ
erties of polymer gels at low temperatures below 0 ꢀC have been
scarcely investigated, because polymer–polymer interactions should
be much stronger than at room temperature and suppress swelling
abilities in any media.
ꢀ
ature (ꢁ80 C), and the swelling degrees were similar to those at
room temperature.
As molecular design for the high swelling polymer gels at low
temperature, we focused on the utility of ionic groups. It is because
ion dissociation of electrolytes is generally insensitive to temperatures
of the media at low temperature,11–13 and dielectric constants of less
polar organic solvents increase with decreasing temperature.14 These
two features strongly suggest that dissociation of ionic polymer gels is
useful even at low temperature for large expansion of the network
against strong polymer–polymer interactions. Moreover, most of the
known ionic polymer gels are highly hydrophilic and their swelling
abilities have been investigated mainly ꢀin water, and the swelling
properties at low temperatures below 0 C have been never investi-
gated. Recently, we reported a new class of poly(alkyl acrylate) gels
with tetraalkylammonium tetraphenylborate as a lipophilic ionic
group, and they exhibited high swelling abilities in various non- or
less-polar organic solvents due to ion dissociation and we confirmed
that they act as polyelectrolyte gels in them.15,16 This result prompted
us to explore the ionic polymer gels that exhibit high swelling abilities
at relatively low temperatures (¼ꢁ20, ꢁ80 ꢀC) by similarly designed
polyelectrolyte gels and organic solvents with low melting points such
as THF, acetone and dichloromethane. In this report, we demon-
strate preparation of polystyrene-based polyelectrolyte gels bearing
a small amount of tetraalkylammonium tetraphenylborate as disso-
ciable ionic groups and swelling abilities at room temperature and at
low temperature in various organic solvents.
The most characteristic property of polymer gels as functional
materials is swelling, the drastic increase of their 3D network in
volume or weight due to absorption of solvent molecules. The
swelling properties of the polymer gels are generally evaluated by the
volume or weight ratios before and after soaking in them and should
be governed mainly by interaction of the polymer chains in the media
and rubber elasticity due to cross-linking of the polymer chains.
Good compatibility of the polymer chains to the media expands the
network and a poor one collapses. Densely cross-linked polymer gels
should be tough and their swelling abilities should be low, while
sparsely cross-linked ones should be soft and high. In addition to
these two factors, in the case of ionic polymer gels, osmotic pressure
and electrostatic repulsion generated by dissociation of the ionic
moieties in the polymer chains largely expand their network hugely,
and association of the ions and ion pairs collapses them. Therefore,
investigation of swelling abilities under the similar cross-linking
densities should be a good indicator for both the compatibility of the
polymer chain to the media and ionization of the ionic groups in the
media.1 The swelling abilities of the polymer gels have been mostly
studied under ambient conditions. The polymer gels based on poly-
aDepartment of Chemistry, Graduate School of Science, Hokkaido
University, Kita 10, Nishi 8, Kita, Sapporo, 060-0810, Japan. E-mail:
sadatcm@mail.sci.hokudai.ac.jp; Fax: +81 11 706 3473; Tel: +81 11 706
3473
bDepartment of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, 744 Motooka, Nishi, Fukuoka, 819-
0395, Japan
A quaternary alkylammonium salt with a styrene group was
prepared from alkylation of tri(n-hexyl)amine by 4-(chloromethyl)-
styrene. The resulting quaternary alkyl-ammonium halide was
treated with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate
(TFPB) to yield 1. Lipophilic polyelectrolyte gels (EG) were prepared
by radical polymerization initiated by AIBN in the presence of 1,
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1sm05542a
5938 | Soft Matter, 2011, 7, 5938–5940
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divinylbenzene (DVB) as a crosslinker, and styrene as a comonomer,
as shown in Scheme 1. As a reference, nonionic gels (NG) were
prepared by the same procedure in the absence of 1. The feed ratios
were adjusted to 1 : styrene : DVB 3 : 97 : 1 for EG, and 0 : 100 : 1
for NG, respectively.
The swelling abilities of polyelectrolyte gels EG(PS) and nonionic
gels NG(PS) in organic solvents with various polarities from toluene
(3 ¼ 2.4) to dimethylsulfoxide (DMSO; 3 ¼ 47) were investigated at
room temperature (23 ꢀC). The swelling degrees (Q) of EG(PS) and
NG(PS) were calculated as the following equation, and are summa-
rized in Fig. 1; Q ¼ (Wwet ꢁ Wdry)/Wdry (w/w). In extremely nonpolar
solvents (3 < 5), the swelling degrees of EG(PS) were smaller than
those of NG(PS). For example, the swelling degrees of EG(PS) in
toluene and in chloroform were 5 and 30, respectively, whereas those
of NG(PS) were 27 and 45, respectively. In these solvents, dissocia-
tion of the ionic groups was completely suppressed owing to their low
polarity, and they would act as tightly bound ion pairs or highly
aggregated species. As a result, collapse of the polymer network was
observed for EG(PS), and the swelling degrees were much lower than
NG(PS). On the other hand, in more polar solvents with dielectric
constants 6 < 3 < 40, except for a series of aliphatic alcohols, the
swelling degrees of EG(PS) were larger than those of NG(PS). In
these solvents, the polystyrene backbone had enough good miscibility
to expand the polymer chains, and the higher polarity promoted
dissociation of the ion-pairs, which enhanced significantly the
swelling degrees. For example, the swelling degrees for EG(PS) in
THF, dichloromethane and 1,2-dichloroethane were 68, 109 and 113,
and those of NG(PS) were 23, 35 and 32, respectively. The former
were several times as large as the latter. Moreover, it is noteworthy
that EG(PS) swelled in some ketones, such as 2-octanone (Q ¼ 75)
and acetone (Q ¼ 92), and in more polar solvents such as DMF, EG
(PS) took the largest swelling degree (Q ¼ 166), whereas NG(PS) did
not at all, 2-octanone (Q ¼ 9), acetone (Q ¼ 1). The enhancement
effect on the swelling degrees for EG(PS) tended to increase with
increasing solvent polarity owing to an increase in the number of
dissociated ionic groups. These results clearly indicated that EG(PS)
should act as the super-absorbent polymer for organic solvents.
We further investigated the swelling behaviours of EG(PS) at low
temperatures. Fig. 2 shows the swelling degrees at various low
temperatures in THF. The swelling degrees of EG(PS) after 48 h
incubation at 23 ꢀC, 2 ꢀC, ꢁ16 ꢀC, and ꢁ30 ꢀC were 71, 82, 67 and
74, respectively. At ꢁ80 ꢀC, the swelling degree was about half.
However, prolonged incubation for 96 h to the equilibrium swelling
Fig. 1 Swelling degrees of EG(PS) (grey bar) and NG(PS) (open bar) in
various organic solvents at room temperature (23 ꢀC). In parentheses,
their dielectric constants are shown.
Fig. 2 (Left) Swelling degrees of EG(PS) (grey bar) and NG(PS) (open
bar) in THF at various low temperatures after 48 h and at ꢁ80 ꢀC after 96
h incubation. (Right) Photographic images of (a) dried EG(PS) and (b)
wet EG(PS) in THF at ꢁ30 ꢀC.
ꢀ
degree increased the swelling degree similar to that at 23 C. They
were nearly constant against temperature. The rate of swelling
became much slower because of the slower kinetics of expansion of
the polymer network. Therefore, the swelling degrees of EG(PS) kept
high even at the low temperatures and were independent of the
incubation temperatures. Comparison of the swelling degrees
between EG(PS) and NG(PS) provided fruitful information for
understanding the role of the ionic groups for swelling. After the same
incubation time, the swelling degrees of NG(PS) at 23 ꢀC, 2 ꢀC, ꢁ16
ꢀ
ꢀ
ꢀC, ꢁ30 C and ꢁ80 C, were 23, 29, 27, 30 and 14, respectively.
They were all of a similar magnitude and were about one-third as
large as those of EG(PS). This decrease in the swelling degrees was
attributed simply to the absence of electrostatic repulsion and
osmotic pressure originated from the dissociated ionic groups in the
polymer chain. Even at ꢁ80 ꢀC, ion dissociation of EG(PS) played
a key role for the enhancement of the swelling ability. Moreover, this
result supported that the polystyrene polymer chains had enough
good compatibility to THF in this range of temperatures, due to the
absence of attractive force among them.
The swelling behaviour of EG(PS) in the other solvents, such as
dichloromethane and acetone, was investigated. The swelling degrees
are summarized in Fig. 3. In dichloromethane, the swelling degree at
Scheme 1 Synthesis of 1, EG(PS) and NG(PS).
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Combining these repulsive and attractive groups in the polymer gels
should enable us to develop various stimuli-sensitive polymer gels
working at low temperature. Development of super-absorbent
polymers for various environments is also under current
investigation.
Acknowledgements
We thank Prof. S. Shinkai and Prof. S. Ogo for fruitful discussions.
Financial support for this research was provided by the Grant-in-Aid
(B) No. 20350056, Nanotechnology Network Project (Kyushu-area
Nanotechnology Network) and the Grant-in-Aid for the Global
COE Program, ‘‘Science for Future Molecular Systems’’ from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan (MEXT). T.O. acknowledges the JSPS research fellowship for
young scientists.
Fig. 3 Swelling degrees of EG(PS) at ꢁ80 ꢀC (grey bar) and at room
temperature (23 ꢀC) (open bar) after 48 h incubation in THF, CH2Cl2 and
acetone. In parentheses, their dielectric constants are shown. *The
swelling degree at ꢁ80 ꢀC in THF was after 96 h incubation.
ꢁ80 ꢀC was similar to that at room temperature (23 ꢀC) like in THF.
In acetone, however, the swelling degree at ꢁ80 ꢀC was much lower
than at room temperature. In the latter solvent, although the ionic
groups could dissociate into free ions due to much higher dielectric
constant, compatibility of the polymer chains to the media became
much poorer, and the polymer–polymer interaction should be much
stronger at ꢁ80 ꢀC, which significantly suppressed the swelling
ability.
Notes and references
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In conclusion, we demonstrated a new class of polyelectrolyte gels
on the basis of polystyrene as a polymer backbone. This gel swelled
and absorbed various organic solvents from low dielectric media
(3 ¼ 6) to highly polar solvents (3 ¼ 40). High swelling degrees are
attributed to ionic dissociation of tetraalkylammonium tetraphe-
nylborate. Moreover, this polyelectrolyte gel could swell largely at
low temperature (ꢁ80 ꢀC) in some organic solvents. To the best of
our knowledge, this is the first example of super-absorbent polymers
working at low temperature, and this material should be the first step
for development of absorbent materials or actuators17 under extreme
conditions. Moreover, high swelling abilities in THF at ꢁ80 ꢀC
should provide a potent solid support for chemical reactions.18
Finally, our results clearly indicate that electrostatic repulsion by
ionic dissociation of polyelectrolytes should be a powerful tool to
generate a repulsive force among the polymer chains at low
temperature as well as at room temperature, although the other
intermolecular interactions such as hydrogen bonds, and p–p inter-
action should act as an attractive force at low temperature.
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5940 | Soft Matter, 2011, 7, 5938–5940
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