A methology is described for the synthesis of novel temperature-responsive interpenetrating polymer network (IPN) hydrogels with poly(2-acrylamido- 2-methylpropane sulfonic acid) (PAMPS) as a tightly crosslinked 1st network, temperature-responsive poly(acrylamide-co-N-(1,1-dimethyl-3-oxobutyl)- acrylamide) (P(AM-co-DAAM)) with low cost as a lossely crosslinked 2nd network. The structure and morphology of the IPN hydrogels were characterized by FTIR, TGA and SEM, and the results indicated that PAMPS network introduced P(AM-co-DAAM) hydrogels have large, thermally stable and interconnected porous network. The properties of the IPN hydrogels, which include: swelling capacity, equilibrium swelling/deswelling ratio, temperature- responsive behavior, and the dwelling kinetics as specific temperature, were investigated carefully. Results showed that the obtained IPN hydrogels displayed a controllable equilibrium swelling/deswelling behavior and possessed remarkable thermosensitivity. In addition, the results also indicate that the incorporation of the hydrophobic groups DAAM has a big effect on the LCST of the IPN hydrogels. Consequently, these novel temperature-responsive IPN hydrogels with low cost and slow-releasing performance would be promising for potential applications, such as environmental catalysis, water treatment, and agriculture.
Hydrogels are three-dimensional, hydrophilic, polymeric networks having a solid-like appearance that does not dissolve in water but can absorb large amounts of water and aqueous ingredients [
Temperature-responsive hydrogels demonstrate a good hydrophilicity in aqueous solutions at low temperature, and separate from the solution when the temperature is raised above the lower critical solution temperature (LCST). Poly(N-isopropyl acrylamide) (PNIPAAm) hydrogel is typical temperature-responsive polymeric network, which exhibits phase separation at its relative low LCST of 32˚C - 34˚C in aqueous solution [
From the viewpoint of applications, although the temperature-responsive hydrogels based on N-isopropylacrylamide (NIPAM) have been investigated for many biomedical and pharmaceutical applications [
Based on consideration of the above limitations, studies have shown that temperature-responsive polymers on acrylamide (AM)/N-(1,1-dimethyl-3-oxobutyl)-acrylamide) (DAAM), as a component of hydrogel, can effectively reduce the cost of temperature-responsive hydrogels [
In this report, novel temperature-responsive interpenetrating polymer network(IPN) hydrogels were prepared with a tightly crosslinked, highly negatively charged poly(2-acrylamide-2-methyl-propane sulfonic acid) (PAMPS) 1st network, and loosely crossliked, neutral temperature-responsive P(AM-co-DAAM) with low cost, which the LCST value of that can be controlled by varying the AM/DAAM mass ratio according to the application field, 2nd network. The structural characterizations and the thermal properties of these temperature-responsive IPN hydrogels were done by Fourier Transform Infrared Spectroscopy (FTIR), field emission scanning electron microscopy (SEM), thermogravimetric (TGA), and differential scanning calorimetric (DSC) analyses. These temperature-responsive IPN hydrogels with lowcost can have practical applications to environmental catalysis and water treatment.
2-acrylamido-2-methylpropane sulfonic acid (AMPS, 97%) was obtained from Sigma-Aldrich. Acrylamide (AM, 99.5%, ChangjiuAgri-Scientific Co. Ltd, Nanchang, China) and N-(1,1-dimethyl-3-oxobutyl)-acrylamide (DAAM, >98%, Liangxi Fine Chemicals Co. Ltd., Wuxi, China) were recrystallized twice from methanol and dried under vacuum prior to use. N,N-methylenebis(acrylamide) (MBA) (Sinopharm Chemical Reagent Co., Ltd., China) used as across-linking agent was recrystallized from ethanol. 2-oxoglutaric acid (Sinopharm Chemical Reagent Co., Ltd., China) was used as an initiator. The water used was doubly distilled in an all-glass apparatus, and the nitrogen gas was 99.999% in purity.
The temperature-responsive IPN hydrogel was prepared via a two-step strategy, as shown in Scheme 1. In the first step, the required masses of AMPS monomer, photoinitiator, crosslinker were dissolved in deionized water. Nitrogen was bubbled through the monomer/solvent mixture for 30 min toremove oxygen dissolved in the reaction mixture. The solution was cast on glass plates equipped with spacers, then photo-polymerized by UV lamp with full wavelength at 20˚C for 2 h. The hydrogel was then removed from the plates and immersed in deionized water to remove the unreacted monomers. The hydrogel was taken out and placed in fresh deionized water three times a day for 7 days before it was dried first in air and then dried in a vacuum oven. In the second step, the dried PAMPS hydrogel, of known weight, was immersed in 40 mL of aqueous containing desired amounts of AM, DAAM, 2-oxoglutaric acid (as photointiator) and MBA (crosslinker) for at least 3 days until the equilibrium was reached. The soft-swollen hydrogels were gently handled and kept between two glass plaques
Scheme 1. The synthetic procedure of temperature-responsive IPN hydrogel.
separated by a rubber gasket spacer. By irradiation with the UV lamp for 8 h (the distance between the lamp and the sample chamber was about 15 cm), the second network was subsequently synthesized in the presence of the first network. During the polymerization reaction, the temperature inside the chamber rose up to 40˚C - 50˚C and after 1 - 2 h the hydrogel became opalescent indicating the formation of temperature-responsive poly(acrylamide-co-N-(1,1-dimethyl-3-oxobutyl)-acrylamide) (P(AM-co-DAAM)) network. The as-formed IPN hydrogels, hereafter labeled as IPN1, IPN2, IPN3, IPN4 and IPN5 were dipped in distilled water for 7 days at room conditions for removing unreacted moieties. The water was renewed every 12 h. After, the hydrogels were dried at room temperature. The feed compositions of the hydrogels’ synthesis reaction are shown in
The FTIR spectra of the dried hydrogel samples were recorded by making KBrpellets on a Nicolet MX-1E FTIR spectrophotometer (USA). The FTIR spectra were recorded in the range of 400 - 4000 cm−1.
TGA was carried out with a NETZSCH STA 409 C/CD instrument under an oxygen free nitrogen atmosphere. Dry samples of 5 - 8 mg weight were used. A linear temperature heating rate of 10˚C・min−1 was maintained from 30˚C to 900˚C. TGA weight loss curves were recorded.
SEM was performed on hydrogels after freeze-dried to maintain the porous structure without any collapse. The samples were plunged in liquid nitrogen, and the vitrified samples were cut with a cold knife. They were mounted on the base plate and coated with gold. The morphology was imaged on a Hitachi S-570 SEM (Tokyo, Japan) using an accelerating voltage of 20 kV.
The swelling kinetics of the hydrogels was measured at 20˚C. After wiping off water or 0.9 wt% NaCl solution on the surface with filter paper, the SR of the hydrogel was recorded during the course of swelling at regular time intervals. The SR was calculated by Equation (1)
S R = W t − W d W d (1)
where Wt is the weight of wet hydrogel at regular time intervals and Wd is the weight of the dried hydrogel.
The kinetics of deswelling behavior of the hydrogels was measured at 50˚C. Before the measurement of deswelling kinetics, the hydrogels were reached swollen equilibrium in deionized water or 0.9 wt% NaCl solution at 20˚C. The weights of the hydrogels were recorded during the course of deswelling at regular time intervals after wiping off water or 0.9 wt% NaCl solution on the surface with filter paper. The deswelling ratio (WR) (%) is defined as follows:
W R ( % ) = W t − W d W S (2)
where Wt is the mass of hydrogels at time t, Wd is the mass of the dried hydrogels, and Ws is themass of water in the swollenhydrogels at 20˚C.
Sample | First network | Second Network | |
---|---|---|---|
AMPS (wt%) | (DAAM:AM) Molar ratio | (DAAM + AM) (wt%)* | |
IPN1 | 25% | 2:1 | 30 |
IPN2 | 25% | 3:2 | 30 |
IPN3 | 25% | 1:1 | 30 |
IPN4 | 25% | 1:1 | 20 |
IPN5 | 25% | 1:1 | 40 |
*The solution concentration of AM and DAAM used during polymerization of P(AM-co-DAAM)/PAMPS in Step 2.
The equilibrium SR of the hydrogel was measured after wiping off water or 0.9% NaCl solution on the surface with filter paper in the temperature range from 20˚C to 60˚C, hydrogel samples were immersed into excess deionized water or 0.9% NaCl solution for 24 h at every temperature. The SReq at different temperatures were calculated as Equation (1).
The DSC studies were performed on a Perkin-Elmer DSC7. The samples were heated from 8˚C to 60˚C, with a heating rate of 2˚C・min−1 in an inert condition.
Pre-weighted dried hydrogel samples were first immersed in deionized water at 20˚C to reach equilibrium, whereafter the oscillatory swelling behavior was observed in deionized water at alternate temperatures of 20˚C and 60˚C. After 30 min of de-swelling at 60˚C, the hydrogels were reimmersed in deionized water of 20˚C for another 30 min swelling. The measurement of the SR for the hydrogel was performed by repeating about steps for 330 min.
The FTIR analysis of the different hydrogel samples (
The thermal decomposition and weight loss profiles of PAMPS and IPN1 hydrogels were estimated from TGA thermogram as a function of temperature, as shown in
The interior morphology of IPN1 hydrogel is shown in
In order to determine the effects of the contents of DAAM and AM on the network
density of the IPN hydrogels which were prepared by the varying the DAAM/AM molar ratio and the concentration of DAAM and AM, the swelling studies of the IPN hydrogels were carried out at 20˚C in deionized water and 0.9 wt% NaCl solution. As shown in
do not swell appreciably in the presence of electrolytes due to the increase of movable counterions of asolution, which lead to a decrease in the osmotic pressure within the hydrogel, causing the hydrogel to shrink [
The investigation of deswelling kinetics is important for the temperature-responsive IPN hydrogels in measuring their water retention and deswelling rate.
The equilibrium swelling ratio (ESR) is one of the most important parameters for evaluating temperature-responsive hydrogels because it illustrates their LCST behavior. The effect of temperature on the ESR of the temperature-responsive IPN hydrogels in deionized water and 0.9 wt% NaCl solution at various temperature from 20˚C to 60˚C are shown in
and consequently the IPN hydrogels become much less hydrophilic. With the increasing DAAM/AM molar ratio or the contents of DAAM and AM in the INP hydrogels, the hydrogels became more hydrophobic and the swelling ratio of the hydrogels decreased more sharply.
The thermal behavior of the IPN hydrogels was investigated using DSC with the LCST reported as the peak temperature [
From the point of applications, the oscillating swelling-deswelling properties over a shorter time intervals with the small temperature cycles of the hydrogels are important, which would be stable for potential applicants. So it is necessary to investigate the oscillating swelling-deswelling kinetics in response to the temperature changes.
The effect of temperance on the appearance of IPN hydrogel is shown in
A series of novel temperature-responsive IPN hydrogels based on a copolymer of acrylamide and N-(1,1-dimethyl-3-oxobutyl)-acrylamide were successfully synthesized by a two-step method. Some conclusions can be drawn as follows. The temperature-response of the synthesized IPN hydrogels can be successfully endowed by immersing of P(AM-co-DAAM) solution into the first PAMPS network and the thermosensitivities of the IPN hydrogels are more obvious as the
DAAM/AM molar ratio or the content of P(AM-co-DAAM) moiety in the IPN hydrogels increased. The ESR of the IPN hydrogels decreases with increasing the DAAM/AM mass ratio or the content of P(AM-co-DAAM) moiety in the IPN hydrogels and the ESR in 0.9 wt% NaCl solution is lower than in deionized water. The physical properties of temperature-responsive IPN hydrogels, such as equilibrium swelling/deswellingratio, water retention, reversible response, and temperature dependence behaviors, could be effectively controlled by the internal chemical composition and external temperature. These novel temperature-responsive IPN hydrogels with low lost, thermally stable network, tunable swelling/deswelling characteristics, and distinct thermosensitivity are promising candidates for applications in environmental catalysis, water treatment and agriculture.
The authors gratefully acknowledge the financial supports from National Natural Science Foundation of China (No. 21464005, 201703157), Natural Science Foundation of Guizhou Province (No. J20181122), Youth Science and Technology Talent Development Project of the Education Department of Guizhou Province, as well as the Key Disciplines Construction Foundation of Applied Chemistry of Guizhou Province (No. 2012442).
The authors declare no conflicts of interest regarding the publication of this paper.
Wang, Y., Xia, H.F., Zhao, J., Cai, X.Q., Chen, S.K. and Li, B.X. (2018) A Novel Design Strategy for Temperature-Responsive IPN Hydrogels Based on a Copolymer of Acrylamide and N-(1,1-Dimethyl-3-Oxobutyl)-Acrylamide. Advances in Chemical Engineering and Science, 8, 255-270. https://doi.org/10.4236/aces.2018.84018