Materials Sciences and Applications, 2010, 1, 162-167
doi:10.4236/msa.2010.13026 Published Online August 2010 (http://www.SciRP.org/journal/msa)
Swelling Properties of New Hydrogels Based on the
Dimethyl Amino Ethyl Acrylate Methyl Chloride
Quaternary Salt with Acrylic Acid and
2-Methylene Butane-1,4-Dioic Acid
Monomers in Aqueous Solutions
Issa Katime1, Eduardo Mendizábal2
1Grupo de Nuevos Materialesy Espectroscopia Supramolecular, Departamento de Química Física, Facultad de Ciencia y Tecnología
(Campus de Leioa), Universidad del País Vasco, Bilbao, España; 2CUCEI, Universidad de Guadalajara, Guadalajara, México.
Received May 22nd, 2010; revised June 17th, 2010; accepted July 21st, 2010.
ABSTRACT
Hydrogels of dimethylaminoethyl acrylate methyl chloride quaternary salt (Q9) have been synthesized with different
monomer ratio by copolymerization of this poorly studied monomer either with acrylic acid or with 2-methylene bu-
tane-1,4-dioic acid. Hydrogel swelling was measured as a function of the composition of the hydrogel and of the
crosslinking agent ratio. High values of swelling have been obtained at very high crosslinking values (< 14 wt %) and
the equilibrium swelling was reached at very low time (less than 15 minutes). The swelling isotherms consisted of a
steep initial portion and then levelled off as asymptotically to the equilibrium swelling limit. The experimental data
suggest clearly that the swelling process obeys second-order kinetics. According to this, the kinetics rate constant and
the equilibrium water content were determined at different comonomer composition and crosslinker concentration. The
calculated kinetic constants ranged from 0.48 to 3.76 × 10-2 min-1 for poly (acrylic acid-co-Q9) hydrogels and from 0.68
to 4.0 × 10-2 min-1 for poly (2-methylene butane-1,4-dioic acid-co-Q9) hydrogels depending on the hydrogels composi-
tion. The diffusion process was evaluated for each hydrogel showing a non-Fickian type diffusion. In all cases was ob-
served a considerable increase in diffusion coefficient as Q9 content increases.
Keywords: Dimethyl Amino Ethyl Acrylate Methyl Chloride Quaternary Salt, 2-methylene butane-1,4-Dioic Acid,
Acrylic Acid, Swelling, Diffusion Coefficients, Kinetic Order
1. Introduction
A gel is a polymer network which swell when immersed
in solvent, but which is prevented from dissolving by the
presence of crosslinks which hold the structure intact. Ge-
ls which swell in aqueous solvents are called hydrogels
[1,2]. Equilibrium water content in hydrogels is one of
their basic properties. A hydrogel with high water con-
tent is generally more advantageous for medical applica-
tions because of its permeability and biocompability [3].
Usually a large swelling is accompanied by poor mech-
anical properties. There are several alternatives to find a
compromise between “large swelling” and “good mech-
anical behavior”. Increasing the crosslinking agent dens-
ity is a way to improve the mechanical properties but this
affects adversely the swelling. Copolymerization of hydr-
ophilic monomer (which favors swelling) with a less hy-
drophilic monomer results in a hydrogel with good water
absorbance and improved mechanical properties in the
resulting hydrogel [4].
It is now well established that some gels can reversibly
swell and shrink by as much as several hundred times in
response to small changes in environmental conditions su-
ch as temperature, solvent composition, pH, ionic streng-
th, electric field, and light. Knowledge of swelling kinet-
ics are important for designing controlled-released devic-
es for drugs and agriculture pesticides based on swellable
polymer matrices and for predicting the release rates of
the active ingredients [5].
In this paper, different hydrogels have been synthesi-

Swelling Properties of New Hydrogels Based on the Dimethyl Amino Ethyl Acrylate Methyl Chloride
163
Quaternary Salt with Acrylic Acid and 2-Methylene Butane-1,4-Dioic Acid Monomers in Aqueous Solutions
zed copolymerizing an unusual monomer, Dimethylam-
inoethyl acrylate methyl chloride quaternary salt (Q9)
with two other common monomers: acrylic acid (AA)
and 2-methylene butane-1,4-dioic acid (IA). The large
swelling of these hydrogels makes them suitable as water
reservoirs in agriculture. In previous studies made in our
laboratory we found a percentage degree of hydratation
H% ~ 4,000 for poliQ9 with 4 wt.% of crosslinking agent.
However its soft consistence could be a handicap to ma-
nipulate these polymers. For those reasons Q9 was co-
polymerized with others common and less hydrophilic
monomers 7], acrylic acid and methylenesuccinic, to
improve its mechanical properties. The influence of the
hydrogel composition and the crosslinking agent on the
swelling behavior of these hydrogels is reported here.
Kinetics parameters and diffusion data are also reported.
2. Experimental Part
Materials. Dimethylaminoethyl acrylate methyl chloride
quaternary salt Q9, Merck S.A), Acrylic acid (AA, Al-
drich Chemical Co.) and methylenesuccinic acid (MA,
Merck), analytical purity grade, were used as monomers.
Methylenesuccinic acid, or Itaconic acid is an organic
compound that is one of the three acids obtained by the
distillation of citric acid. Acrylic acid (AA), inhibited
with 200 ppm of hydroquinone monomethyl ether, with a
reported purity of 99%, was obtained from Aldrich Che-
mical Co. The acrylic acid monomer was purified by dis-
tillation at a reduced pressure of approximately 10 mm
Hg. The fraction having boiling point from 39-40˚C was
collected. N, N’-methylenbysacrylamide (Aldrich Chem-
ical) was employed as crosslinking agent and 2,2’-azobis
(2-amidinopropane) dihydrochloride, V-50 (Wako), as
initiator. Distilled water was employed in all the proc-
esses. All other chemicals reagents were analytical grade.
In Figure 1 the chemical structures of the reagents used
in this work are shown.
Hydrogel synthesis. Hydrogels were prepared by radi-
cal copolymerization at 50˚C of dimethylaminoethyl ac-
rylate methyl chloride quaernary salt and acrylic acid or
methylenesuccinic acid in cylindrical glass tubes. A ratio
of 25/75 total monomer/water in weight was used in all
cases. Comonomers ratios were varied to obtain different
hydrophilic properties. The initiator and crosslinking ag-
ent concentrations were 1% with respect to the monom-
ers in all cases. After bubbling nitrogen through the reac-
tion mixture, thermally-initiated polymerizations were
carried out for 4 hours. After reaction the hydrogels were
taken from the test tubes, cut into cylindrical samples im-
mersed in water for two weeks to remove any residual
monomers and uncrosslinked polymer and then dried
again to determine the conversion:
Structure (a)


 Cl)CH(NCHCHCOOCHCH 33222
Structure (b)
C
H
H
C
H
CO
2
H
Structure (c)
O
HO
CH
2
O
H
O
Structure (d)
CH2= CH – C – NH – CH2– NH –C = CH2
O

O

CH2= CH – C – NH – CH2– NH –C = CH2
O

O

O

O

CH2= CH – C – NH – CH2– NH –C = CH2
O

O

CH2= CH – C – NH – CH2– NH –C = CH2
O

O

O

O

CH2= CH – C – NH – CH2– NH –C = CH2
O

O

O

O

CH2= CH – C – NH – CH2– NH –C = CH2
O

O

O

O

O

O

O

O

O

O

CH2= CH – C – NH – CH2– NH –C = CH2
O

O

O

O

CH2= CH – C – NH – CH2– NH –C = CH2
O

O

O

O

O

O

O

O

O

O

Figure 1. Structures of the monomers and crosslinker agent:
(a) Dimethylaminoethyl acrylate methyl chloride quatern-
ary salt; (b) Acrylic acid; (c) Methylenesuccinic acid; (d) N-
N’-methylenbisacrylamide
g
o
m
Conversion(%)= 100
m (1)
Discs 1.5 mm diameter were obtained from the hydrog-
els rods. The discs were dried to constant weight and
then polished to get uniform and smooth surfaces (~ 1
mm thickness). Their dimensions were measured with a
micrometer.
Hydrogel Swelling. Dynamic swelling experiments
were performed by placing the discs in distilled water at
37.0  0.1˚C (in a thermostated bath) and measuring their
weight gain as a function of time. The discs were re-
moved from the water, dried quickly and carefully with
filter paper and weighted.
The degree of swelling at different times can be calcu-
lated from the following equation:
W(%) =
m
mm 0  × 100 (2)
were m0 and m are the weights of the initial dry sample
(xerogel) and of the hydrogel, respectively. W is the
water content when the equilibrium is reached. There is
another useful parameter to compare similar swellings,
H(%), the swelling degree, which can be expressed as:
H(%) =
0
0
m
mm × 100 (3)
3. Results and Discussion
Influence of gel composition and crosslinking ratio.
Figure 2 shows that around 80 minutes equilibrium is
reached. The studied hydrogels absorb huge amounts of

Swelling Properties of New Hydrogels Based on the Dimethyl Amino Ethyl Acrylate Methyl Chloride
164
Quaternary Salt with Acrylic Acid and 2-Methylene Butane-1,4-Dioic Acid Monomers in Aqueous Solutions
water, especially the hydrogels with a low content of
acrylic acid, as Figure 2 shows. This is a consequence of
the smaller acrylic acid-water interactions compared to
the Q9-water interactions. This behavior is in agreement
with most of the hydrogels that have been studied [6,7].
Figure 3 shows the variation of the percentage degree
of hydration as a function of swelling time for hydrogels
prepared from copolymers with the indicated crosslink-
ing agent ratio. An increase in the crosslinking agent
ratio had as expected a decrease in the amount of swell-
ing. As the crosslinking agent content increases, the re-
sultant structure of the hydrogel is less flexible, and con-
sequently the capacity of the hydrogels to deform them-
selves (and consequently to swell), decreases. The de-
crease of swelling when the crosslinking agent increases
had been extensively reported [8,9].
As can be seen in Figure 4, the water content at equil-
ibrium increases as the content of Q9 is increased, accor-
ding with the introduction of more hydrophilic monomer.
Swelling Kinetics. Considering second-order kinetics
the swelling rate at any time may be expressed as:
0
2000
4000
6000
8000
1 104
1,2 104
050100 150 200 250 300
H ( % )
Time ( min)
1.2 × 10
4
1.0 × 10
4
Figure 2. Swelling isotherms of hydrogels from different ra-
tio, acrylic acid/Q9 (1 wt.% crosslinker agent): () 10/90,
() 20/80, () 30/70, (×) 40/60
0
2000
4000
6000
8000
1 104
1,2 104
050100 150 200 250 300
H ( % )
Time
(
min.
)
1.2 × 10
4
1.0 × 10
4
Figure 3. Swelling isotherms of 90/10 poly(acrylic acid/dim-
ethylaminoethyl acrylate methyl chloride quaternary salt)
hydrogels with different crosslinker concentration () 1
wt%, () 2%, () 3 wt%
2
)WW(K
dt
dW   (4)
Upon integration between the limits (t = 0 and W = 0),
the following equation is obtained,
tKW1
tKW
W
2



 (5)
By rearranging the above equation, one gets the fol-
lowing expression,

 W
t
KW
1
W
t
2 (6)
According to this equation, the experimental data must
fit a straight line with a slope of  W1 with ordinate of
2
KW1 . Therefore, we can calculate the water content at
equilibrium (W) and the kinetics rate constants (K) from
a plot of t/W against time. The variation of t/W against
time is plotted in Figure 5 for the 90/10 hydrogel (Q9/
acrylic acid). In all cases the experimental data fit Equa-
tion (6) with r = 0.9995. Similar results were obtained for
70
75
80
85
90
95
100
55 60 65 70 75 80 85 90 95
W (%)
% Q9
Figure 4. Variation of the water content at equilibrium, W,
as a function of Q9 content at 37ºC for hydrogels with diff-
erent crosslinker concentration: () 1 wt%, (■) 2 wt%, ()
5 wt%
0
0,5
1
1,5
2
2,5
3
3,5
4
050 100 150 200 250 300
t / W
(min)
t (min)
Figure 5. Experimental data of water content W and time t
plotted according to Equation (1) (second order-kinetic) for
90/10 Q9/acrylic acid hydrogels from different crosslinking
agent ratio () 1 wt. %, () 2 wt. %, () 3 wt. %

Swelling Properties of New Hydrogels Based on the Dimethyl Amino Ethyl Acrylate Methyl Chloride
165
Quaternary Salt with Acrylic Acid and 2-Methylene Butane-1,4-Dioic Acid Monomers in Aqueous Solutions
the other hydrogels. Therefore, we could calculate the wa
ter content at equilibrium (W) and the kinetics rate con-
stants (K).
In Table 1 are summarized the experimental values of
swelling kinetic rate constants (K) as a function of the
Q9 and crosslinking percentage. An increase in the cro-
sslinking and a decrease in the more hydrophilic mono-
mer content, drive to a higher physical restriction in the
network, which causes that the swelling rate decreases.
Diffusion. When a glassy hydrogel is brought into con-
tact with water, water diffuses into the hydrogel and the
hydrogel swells [10]. Diffusion involves migration of wa-
ter into pre-existing or dynamically formed spaces bet-
ween hydrogel chains whereas swelling of the hydrogel
involves a larger scale segmental motion resulting in a
bigger separation between hydrogel chains [11]. The foll-
owing useful equation was used to determinate the nature
of diffusion of water into hydrogels:
n
tKt
M
M

(7)
where Mt and M denote the amount of solvent diffused
into the gel at time t and at infinite time respectively, K is
a constant related to the structure of the network and n is
a characteristic exponent of the transport mode of the
solvent 12. Depending on the relative rates of diffusion
and polymer relaxation three classes of diffusion mecha-
nisms are distinguished [13]: 1) Case I or Fickian diffu-
sion in which the rate of diffusion is much less than that
of relaxation (n = 0.50), 2) Case II diffusion, in which
diffusion is very rapid compared with the relaxation
processes (n = 1), and 3) Non-Fickian or anomalous dif-
fusion which occurs when the diffusion and relaxation
rates are comparables (0.50 < n < 1).
To elucidate the transport mechanisms, the swelling
curves were fitted to the following equation:
log tlognklog
M
Mt









(8)
The exponents n were calculated from the slopes and
values ranging from 0.66 to 1.24 were obtained (Table 2).
This indicates that diffusion of water to the interior of the
discs follows an anomalous diffusion mechanism, revea-
ling the existence of certain coupling between molecular
Table 1. Experimental values of kinetics rate constant (K)
as function of Q9 concentration and crosslinker concentra-
tion for poly(acrylic acid-co-Q9) hydrogels
K × 102 (min-1) (%) Q9
(%) NMBA 60 70 80 90
1 0.70 2.00 2.37 3.76
2 0.56 1.67 2.19 3.40
5 0.48 1.43 1.65 2.47
diffusion and tension relaxation which are developed du-
ring swelling of the xerogels. As Table 2 shows the hy-
drogel which swells the most is an unusual system with a
characteristic n exponent higher than 1. This table also
shows that as the Q9 percentage and crosslinker concent-
ration decreases the system deviates more from a Fickian
behavior. For an anomalous mechanism the coupling bet-
ween molecular transport and stress relaxation during sw-
elling causes deviations with respect to Fickian mechan-
ism. This becomes more important when Q9 content inc-
reases because it is a more hydrophilic monomer, and
when the crosslinker increases due to the decrease in free
volume.
The diffusion coefficients for non-Fickian sorption pro-
cesses can be obtained by mean of the following equation
14:
2
0.049
(4 )
Dt  (9)
where t is the time at which the swelling is one half the
equilibrium value and  is the radius of the cylindrical
hydrogel at this time. Figure 6 shows the variation of the
water diffusion coefficient of water i, D, for different
hydrogel compositions. Experimental values show that
when the Q9 content increases, the diffusion coefficient
of hydrogels, D, increases. The reason, is the higher sw-
elling degree at equilibrium when the more hydrophilic
component increases.
Hydrogels of methylenesuccinic acid/Q9. Swelling
of hydrogels of methylenesuccinic acid and Q9 were also
Table 2. Values of n (diffusion exponent), for poly(acrylic
acid-co-Q9) hydrogels as a function of Q9 composition and
crosslinker concentration
n % Q9
% NMBA 60 70 80 90
1 0.87 0.92 1.00 1.24
2 0.78 0.80 0.93 0.97
5 0.66 0.71 0.77 0.94
% Q9
55 60 65 70 75 80 85 90 95
D × 10
6
(
cm
2
/s
)
10
9
8
7
6
5
4
3
2
Figure 6. Variation of water diffusion coefficient at 37ºC , D,
of poly(acrylic acid-co-Q9) hydrogels

Swelling Properties of New Hydrogels Based on the Dimethyl Amino Ethyl Acrylate Methyl Chloride
166
Quaternary Salt with Acrylic Acid and 2-Methylene Butane-1,4-Dioic Acid Monomers in Aqueous Solutions
studied. Figures 7 and 8 show the variation of %H as a
function of swelling time for hydrogels prepared with
different Q9 concentration and crosslinker agent, respec-
tively. These hydrogels show a similar swelling depend-
ence on the content of the more hydrophilic monomer
(Q9) and on crosslinker agent, than the poly(acrylic
acid/Q9) hydrogels, that is a larger swelling degree is
obtained by using a larger concentration of Q9 and a
lower concentration of crosslinker.
0
200
400
600
800
1000
1200
1400
0 102030405
H ( % )
Time ( min )
0
Figure 7. Swelling isotherms of methylenesuccinic acid/Q9
hydrogels (6% crosslinker agent) for different Q9 concen-
tration: () 80%, (■) 85%, () 90 wt%, (●) 95 wt% and ()
100 wt%
0
500
1000
1500
2000
2500
3000
0 10203040
H ( % )
Time ( min )
50
Figure 8. Swelling isotherms of 5/95 methylenesuccinic ac-
id/Q9 hydrogels with different crosslinker concentration:
() 4 %wt, (●) 6 wt%, () 8 wt%, (■) 10 wt%, ()12 wt%
and (×) 14 wt%
Table 3. Kinetics rate constant of poly(methylenesuccinic
acid-co-Q9) hydrogels
K × 102 min-1 % NMBA
% Q9 4 6 8 10 12 14
80 0.68 2.83 2.30 3.26 2.79 4.00
85 2.14 2.06 1.92 3.72 3.65 3.46
90 2.75 2.56 2.32 2.26 2.21 2.17
95 2.61 2.46 2.33 2.25 2.06 2.12
100 2.90 2.89 2.38 2.62 2.17 2.46
Table 4. Values of n (diffusion exponent) for poly (methyl-
enesuccinic acid-co-Q9) hydrogels as a function of Q9 com-
position and crosslinker concentration
% NMBA
% Q9 4 6 8 10 12 14
80 0.97 0.73 0.82 0.63 0.56 0.54
85 0.90 0.82 0.66 0.68 0.63 0.59
90 0.88 0.74 0.84 0.66 0.67 0.68
95 0.87 0.86 0.71 0.77 0.58 0.84
100 0.97 0.86 0.67 0.73 0.83 0.93
By graphing the experimental data (not shown) using
Equation (3), it was found that the swelling process of
these hydrogels also obeys second-order kinetics. In Ta-
ble 3 are summarized the experimental values of kinetics
rate constants (K) as a function of Q9 and crosslinker
agent content. An increase in the crosslinking ratio and a
decrease in the more hydrophilic monomer content. The
values of n exponent are listed in Table 4. In all cases n
exponent is higher than 0.50. Therefore the water diffu-
sion into hydrogels displays also a non-Fickian character.
3. Conclusions
In this paper we have described the swelling kinetics of
poly(acrylic acid/dimethylaminoethyl acrylate methyl ch-
loride quaternary salt) and poly(methylenesuccinic acid/
dimethylaminoethyl acrylate methyl chloride quaternary
salt) hydrogels. These hydrogels absorb huge amounts of
water. The experimental data indicate that the swelling
process follows second–order kinetics for all the studied
systems. Hydrogels show large swelling properties as the
Q9 content increases and crosslinking agent ratio de-
creases.
Water diffusion into hydrogels is a non-Fickian type
diffusion. Diffusion coefficients increase with increasing
Q9 content .
The diffusion coefficients were determined by using
equation (9). In these copolymers is observed that the
crosslinking ratio has not an important effect on the wa-
ter transport, as we concluded with poly(acrylic acid-co-
dimethylaminoethyl acrylate methyl chloride quaternary)
hydrogels. For lower Q9 content hydrogels, the diffusion
coefficient remains constant with the composition, and in
these cases the determined value was 6.5 × 10-6 cm2/s. It
was only observed a considerable effect on diffusion
coefficient due to Q9 content on 0/100 hydrogel (itaconic
acid/Q9). The diffusion coefficient determined in this
case was 1.4 × 10-5 cm2/s.
4. Acknowledgements
The authors thanks the financial support for this work
was provided by Ministerio de Ciencia e Innovación (Pr-

Swelling Properties of New Hydrogels Based on the Dimethyl Amino Ethyl Acrylate Methyl Chloride
Quaternary Salt with Acrylic Acid and 2-Methylene Butane-1,4-Dioic Acid Monomers in Aqueous Solutions
167
oject Number: EUI2008-00178) and the Gobierno Vasco
for their financial support
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