Hydrogels copolymers N,N-dimethylacrylamide (DMA) and maleic acid (MA) were prepared by free-radical polymerization at 56°C in aqueous solution, using N,N-methylenebisacrylamide (NMBA) as cross-linking agent and potassium persulfate (KPS) as initiator. The effects of comonomer composition, cross-linker content, and variation of pH solutions on the swelling behavior of polymers were investigated. The obtained results showed an increase of the swelling of poly(N,N-dimethylacrylamide-
Hydrogels are composed of hydrophilic homopolymer or copolymer network and can swell in the presence of water or physiological fluids. Chemical cross-links (covalent bonds) or physical junctions (e.g., secondary forces, crystallite formation, and chain entanglements) provide the hydrogels, unique swelling behavior and three-dimensional structure [
The present work was aimed to synthesize the cross-linked copolymers of N,N-dimethylacrylamide (DMA) with maleic acid (MA) and to investigate the swelling behavior of the copolymer composed of a nonionic monomer DMA and an ionizable monomer MA. In order to study the effects of comonomer composition and the cross-linker content on the swelling of polymers, the equilibrium swelling values of DMA homopolymer and DMA-MA copolymers in buffer solutions with different pH values at room temperature were determined. The thermal stability of PDMA and P(DMA-MAx) was investigated by thermogravimetry.
N,N-dimethylacrylamide (DMA) was supplied from Merck and purified with vacuum distillation prior to usage in order to remove the inhibitor. Maleic acid (MA, 99%) purchased from Fluka, N,N-methylenebisacrylamide (NMBA) from Fluka as a cross-linking agent, and potassium persulfate (KPS) from Merk as an initiator were used as received. Distilled water was used for the polymerization reactions and swelling studies.
The following pH-buffered solutions were used in order to examine the swelling behavior [
Poly(N,N-dimethylacrylamide) (PDMA), poly(N,N-dimethylacrylamide-
Molar composition of the hydrogels.
DMA/MA |
DMA/MAa |
DMA |
MA |
KPSb |
NMBAb |
H2O |
---|---|---|---|---|---|---|
100/0 | 100/0 | 40.4 | 0 | 0.036 | 0.32 | 4 |
91/9 | 92/8 | 36.4 | 3.45 | 0.036 | 0.32 | 4 |
82/18 | 85/15 | 32.3 | 6.89 | 0.036 | 0.32 | 4 |
73/27 | 71/29 | 28.3 | 10.35 | 0.036 | 0.32 | 4 |
64/36 | 67/33 | 24.3 | 13.79 | 0.036 | 0.32 | 4 |
54/46 | 62/38 | 20.2 | 17.24 | 0.036 | 0.32 | 4 |
|
||||||
91/9 | 92/8 | 36.4 | 3.45 | 0.036 | 0.16 | 4 |
91/9 | 92/8 | 36.4 | 3.45 | 0.036 | 0.48 | 4 |
91/9 | 92/8 | 36.4 | 3.45 | 0.036 | 0.64 | 4 |
91/9 | 92/8 | 36.4 | 3.45 | 0.036 | 1.28 | 4 |
Polymerization reactions were performed in glass tubes with the inner diameters of 1 cm and lengths of 10 cm. DMA and MA monomers with different molar ratios and cross-linker concentration were dissolved in water. After nitrogen bubbles for 20 min through the solution, the initiator KPS was added. Then, the polymerization was carried out at 56°C for 15 min. At the end of the reaction, the glass tubes were broken carefully without destroying the cylindrical hydrogels. The resulting hydrogels were sliced into small cylinders with lengths of 4 mm and then immersed in distilled water for 5 days, and the water was changed every 24 hours in order to remove the residual unreacted monomers. The resulting swollen gels were dried at room temperature for several days and then in a vacuum oven at 40°C until constant weight.
The compositions of the formed copolymer hydrogels were determined by elemental analysis for nitrogen (Microanalizador CHNOS-932 LECO).
Hydrogels samples were prepared by grinding the dry hydrogels with KBr and compressing the mixture to form disks and then dried in vacuum oven at 70°C for several days. FTIR spectra of these hydrogels were recorded on a model Perkin Elmer spectrophotometer at room temperature with a resolution of 2 cm−1 and were averaged over 60 scans.
The glass transition temperatures (
A thermogravimetric analysis of the hydrogels investigated was carried out on a TG SETARAM under nitrogen atmosphere from 25°C to 500°C at a heating rate of 10°C/min.
Dried hydrogels were allowed to hydrate in buffer solutions with different pH values 0.9, 3.9, 6.9, and 8.7 at 16°C. After being fully hydrated, the samples were taken out and the excess water on their surface was gently removed by filter paper.
The weights of the hydrating samples were measured at timed intervals. The swelling ratio (SR) and equilibrium swelling ratio (
The effect of maleic acid composition on the swelling of the prepared P(DMA-MAx) hydrogels at different pHs (0.9, 3.9, 6.9, and 8.7) and at 16°C was investigated. For instance, the swelling curves of P(DMA-MAx) hydrogels with different compositions of MA at pH 6.9 are given in Figure
(a) Effect of maleic acid concentration on the dynamic swelling behavior of P(DMA-MAx) hydrogels in the media of pH 6.9 at 16°C (NMBA 0.32 mol% of monomers). (b) Effect of NMBA concentration on the dynamic swelling behavior of P(DMA-MA8) hydrogels in the media of pH 6.9 at 16°C.
It is well known that swelling is induced by the electrostatic repulsion of ionic charges present within the network. Thus, as the number of carboxylic groups increases on going from PDMA to P(DMA-MAx), the swelling increases too.
The values of equilibrium swelling ratio of DMA/MA hydrogels in different buffer solutions are reported in Table
(a) Equilibrium swelling ratios of P(DMA-MAx) hydrogels with different MA contents at different pH media. (b) Equilibrium swelling ratios of P(DMA-MA8) hydrogels with different NMBA contents at different pH media.
Hydrogels | Equilibrium swelling ratio (%) | |||
---|---|---|---|---|
pH = 0.9 | pH = 3.9 | pH = 6.9 | pH = 8.7 | |
PDMA | 833 | 831 | 852 | 844 |
P(DMA-MA8) | 870 | 901 | 1084 | 1526 |
P(DMA-MA15) | 1200 | 1391 | 1448 | 3173 |
P(DMA-MA29) | 1700 | 1836 | 2204 | 3799 |
P(DMA-MA33) | 1900 | 2034 | 2699 | 4556 |
P(DMA-MA38) | 2111 | 2514 | 3002 | 6961 |
Composition of NMBA (mol%) | Equilibrium swelling ratio (%) | |||
---|---|---|---|---|
pH = 0.9 | pH = 3.9 | pH = 6.9 | pH = 8.7 | |
0.16 | 1401 | 1535 | 1754 | 2637 |
0.32 | 870 | 901 | 1084 | 1526 |
0.48 | 685 | 781 | 1008 | 1436 |
0.64 | 600 | 697 | 938 | 1356 |
1.28 | 508 | 605 | 860 | 1288 |
The effect of increasing the amount of the cross-linking agent NMBA on the swelling capacity of the P(DMA-MA8) in different buffer solutions (pH 0.9, 3.9, 6.9, and 8.7) was investigated.
It is well known that the cross-link density directly affects the mechanical deformation of hydrogels. As the ratio of the cross-linking agent NMBA varied from 0.16 to 1.28 mol% in P(DMA-MA8), the degree of swelling was greatly reduced (Figure
The values of equilibrium swelling ratio of P(DMA-MA8) hydrogels with different NMBA contents at different pHs are given in Table
Figure
(a) Effect of pH on the equilibrium swelling ratio of P(DMA-MAx) hydrogels for different MA concentrations at 16°C (NMBA 0.32 mol% of monomers). (b) Effect of pH on the equilibrium swelling ratio of P(DMA-MA8) hydrogels for different NMBA compositions at 16°C.
The percentage equilibrium swelling of P(DMA-MA8) is decreased with increasing cross-linking agent concentration (Figure
Analysis of the mechanisms of water diffusion in swelling polymeric systems has received considerable attention in recent years, because of the important applications in biomedical, pharmaceutical, environmental, and agricultural engineering fields.
To determine the nature of water diffusion into hydrogels, kinetic modeling was conducted based on Fickian diffusion law for the onset stage of swelling (the model is valid only for the first 60% of the swelling) [
For example, Figures
(a) Swelling kinetic coefficients of P(DMA-MAx) hydrogels with different MA contents at different pH media. (b) Swelling kinetic coefficients of P(DMA-MA8) hydrogels with different NMBA contents at different pH media.
Medium | Hydrogels |
|
|
|
---|---|---|---|---|
pH = 0.9 | PDMA | 0.9997 | 0.576 | −3.428 |
P(DMA-MA8) | 0.9996 | 0.487 | −3.142 | |
P(DMA-MA15) | 0.9996 | 0.486 | −3.405 | |
P(DMA-MA29) | 0.9984 | 0.636 | −4.912 | |
P(DMA-MA33) | 0.9969 | 0.684 | −4.760 | |
P(DMA-MA38) | 0.9991 | 0.584 | −4.174 | |
|
||||
pH = 3.9 | PDMA | 0.9998 | 0.594 | −3.997 |
P(DMA-MA8) | 0.9993 | 0.582 | −3.762 | |
P(DMA-MA15) | 0.9989 | 0.586 | −4.209 | |
P(DMA-MA29) | 0.9978 | 0.685 | −5.263 | |
P(DMA-MA33) | 0.9986 | 0.768 | −5.242 | |
P(DMA-MA38) | 0.9992 | 0.661 | −4.615 | |
|
||||
pH = 6.9 | PDMA | 0.9997 | 0.559 | −3.883 |
P(DMA-MA8) | 0.9975 | 0.553 | −3.858 | |
P(DMA-MA15) | 0.9985 | 0.582 | −4.293 | |
P(DMA-MA29) | 0.9990 | 0.553 | −3.782 | |
P(DMA-MA33) | 0.9968 | 0.661 | −5.075 | |
P(DMA-MA38) | 0.9980 | 0.650 | −4.648 | |
|
||||
pH = 8.7 | PDMA | 0.9991 | 0.553 | −3.819 |
P(DMA-MA8) | 0.9991 | 0.564 | −3.711 | |
P(DMA-MA15) | 0.9988 | 0.668 | −4.748 | |
P(DMA-MA29) | 0.9991 | 0.738 | −5.745 | |
P(DMA-MA33) | 0.9991 | 0.734 | −5.680 | |
P(DMA-MA38) | 0.9981 | 0.796 | −5.316 |
Medium | Composition of NMBA (mol%) |
|
|
|
---|---|---|---|---|
pH = 0.9 | 0.16 | 0.9976 | 0.651 | −4.617 |
0.32 | 0.9996 | 0.487 | −3.142 | |
0.48 | 0.9976 | 0.553 | −3.665 | |
0.64 | 0.9964 | 0.564 | −4.021 | |
1.28 | 0.9979 | 0.523 | −3.555 | |
|
||||
pH = 3.9 | 0.16 | 0.9977 | 0.586 | −4.675 |
0.32 | 0.9993 | 0.582 | −3.762 | |
0.48 | 0.9981 | 0.589 | −4.081 | |
0.64 | 0.9985 | 0.575 | −4.064 | |
1.28 | 0.9968 | 0.608 | −4.410 | |
|
||||
pH = 6.9 | 0.16 | 0.9997 | 0.606 | −4.633 |
0.32 | 0.9975 | 0.553 | −3.858 | |
0.48 | 0.9974 | 0.566 | −3.858 | |
0.64 | 0.9989 | 0.571 | −3.932 | |
1.28 | 0.9975 | 0.571 | −4.191 | |
|
||||
pH = 8.7 | 0.16 | 0.9984 | 0.736 | −4.974 |
0.32 | 0.9991 | 0.564 | −3.711 | |
0.48 | 0.9991 | 0.658 | −4.333 | |
0.64 | 0.9971 | 0.631 | −4.178 | |
1.28 | 0.9985 | 0.687 | −4.429 |
(a) Dependence of ln
Figure
Scale-expanded infrared spectra of P(DMA-MAx) hydrogels in the hydroxyl region.
In the carbonyl stretching 1780–1560 cm−1 region, the infrared spectrum of PDMA recorded at room temperature shows a broad band at 1640 cm−1, assigned to the free carbonyl groups of the amide. With the incorporation of the MA groups within PDMA chain, a new band appears at 1735 cm−1, attributed to free carboxylic group
(a) Scale-expanded infrared spectra of P(DMA-MAx) hydrogels in the carbonyl region. (b) Scale-expanded infrared spectra of P(DMA-MA8) hydrogels with different compositions of NMBA in the carbonyl region.
The infrared spectra of P(DMA-MA8) with different NMBA compositions recorded at room temperature in the carbonyl region are illustrated in Figure
The thermogravimetric (TGA) and derivative thermogravimetric (DTGA) curves for PDMA and P(DMA-MAx) containing 8, 15, 29, 33, and 38 mol% of MA are represented in Figure
(a) TGA thermograms and DTGA curves of P(DMA-MAx) hydrogels. (b) TGA thermograms and DTGA curves of P(DMA-MA8) hydrogels with different compositions of NMBA.
Parameters, such as temperature of maximum degradation (determined considering the derivative curves) and percentage of mass loss in each stage of degradation for all studied systems, are summarized in Table
(a) Thermogravimetric parameters for P(DMA-MAx) hydrogels with different compositions of MA. (b) Thermogravimetric parameters for P(DMA-MA8) hydrogels with different compositions of NMBA.
Hydrogels | Stage 1 | Stage 2 | |||
---|---|---|---|---|---|
|
|
|
|
Residual weight (wt%) | |
PDMA | 55–170 | 4 | 366–475 | 447 | 0 |
P(DMA-MA8) | 55–219 | 5.7 | 312–488 | 440 | 4.38 |
P(DMA-MA15) | 53–220 | 9 | 281–485 | 428 | 6.44 |
P(DMA-MA29) | 55–230 | 11 | 262–471 | 409 | 7.72 |
P(DMA-MA33) | 56–224 | 13 | 254–489 | 406 | 9.42 |
P(DMA-MA38) | 54–223 | 16 | 241–472 | 397 | 11.6 |
Composition of NMBA (mol%) | Stage 1 | Stage 2 | |||
---|---|---|---|---|---|
|
|
|
|
Residual weight (wt%) | |
0.16 | 52–220 | 5 | 279–467 | 434 | 4.13 |
0.32 | 53–219 | 5.7 | 281–479 | 441 | 4.36 |
0.48 | 58–220 | 4.58 | 277–478 | 442 | 3.71 |
0.64 | 58–222 | 5.11 | 283–486 | 449 | 3.59 |
1.28 | 59–211 | 4.38 | 285–496 | 485 | 3.47 |
For P(DMA-MAx), the percentage of mass loss at the range 5–16% in stage 1 can be assigned to the elimination of water adsorbed by the hydrophilic groups and probably to the elimination of hydroxyl groups due to the development of cyclic anhydride involving the liberation of water. Data from the literature indicated that with temperature increase (above 200°C), poly(styrene-
The onset temperature of the second main degradation stage as well as temperature of maximum degradation for copolymers decreases with acid composition. The amount of residual weight at the end temperature of degradation increases with increasing acid content in P(DMA-MAx). The reason for incomplete degradation of copolymers is probably the thermal cross-linking induced by heating the sample during thermogravimetric analysis.
The thermogravimetric (TGA) and derivative thermogravimetric (DTGA) curves for the copolymers P(DMA-MA8) with different amounts of NMBA are shown in Figure
Figure
(a) DSC thermograms of P(DMA-MAx) hydrogels. (b) DSC thermograms of P(DAM-MA8) hydrogels with different compositions of NMBA.
The thermograms of P(DMA-MA8) with different amounts of NMBA are represented in Figure
A series of pH-sensitive P(DMA-MAx) hydrogels were synthesized by free-radical polymerization in water using NMBA as cross-linker.
The equilibrium swelling ratio of hydrogels increases with the increase of MA content but reduces with the increase of NMBA concentration. The hydrogels have apparent pH-sensitive character. With increase in pH value from 0.9 to 8.7, the swelling ratio increased accordingly. In the diffusion transport mechanism study, the results indicate that the swelling exponent
The authors declare that there is no conflict of interests regarding the publication of this paper.