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DOI: 10.1039/C5RA01366F (Paper) RSC Adv., 2015, 5, 30514-30521

Facile access to poly(DMAEMA-co-AA) hydrogels via infrared laser-ignited frontal polymerization and their polymerization in the horizontal direction

Yan Liu, Cai-Feng Wang and Su Chen*
State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Chemical Engineering, Nanjing Tech University (formerly: Nanjing University of Technology), Nanjing, 210009, P. R. China. E-mail: chensu@njtech.edu.cn; Fax: +86-25-83172258

Received 23rd January 2015 , Accepted 17th March 2015

First published on 17th March 2015

Abstract

A series of poly(DMAEMA-co-AA) hydrogels (DMAEMA = dimethylaminoethyl methacrylate, AA = acrylic acid) were quickly produced via infrared laser ignited frontal polymerization (LIFP), and LIFP in the horizontal direction was performed successfully for the first time. The dependence of the front velocity and front temperature of LIFP on the molar ratios of AA/DMAEMA and the concentrations of initiator was investigated, along with the characterization of the swelling capacity and morphology properties of the as-prepared hydrogels. The as-prepared hydrogels are sensitive to pH values ranging from 2 to 12, and their maximum equilibrium swelling ratio can reach 2497% in pH = 7. Moreover, the hydrogels are able to absorb an anionic dye (orange G) and a cationic dye (methylene blue) through electrostatic interaction, offering potential as dye adsorbents for water purification. Additionally, LIFP was employed horizontally to seal dyestuff solutions, expanding the scope of LIFP applications on spilled toxic substances without touching the location of the reaction.

1. Introduction

Frontal polymerization1 (FP) develops the coupling of the Arrhenius kinetics and thermal diffusion of the exothermic polymerization that converts the monomer into polymer via the propagation of a localized reaction zone through the whole system. No further energy is required in the coming reaction process, as long as a brief excitation with an external stimulus is given. Because of its less energy-consuming, time-saving and convenient operation, FP has extracted extensive research interest in the preparation of various polymers, especially in functional hydrogels. In this respect, Washington and Steinbock2 first fabricated temperature-sensitive poly(N-isopropylacrylamide) hydrogels via FP in 2001. Afterwards, starch-graft-poly(acrylic acid) hydrogels,3,4 porous polyacrylamide hydrogels,5,6 poly(N,N-dimethylacrylamide) hydrogels,7 superabsorbent hydrogels,8–11 graphene-containing stimuli-responsive polymer hydrogels,12,13 functional hybrid composites,14–18 amphiphilic hydrogels,19,20 N-vinylimidazole-based hydrogels,21 and interpenetrating polymer networks hydrogels22,23 have been developed using FP. Owing to their unique properties, these hydrogels are widely applied in diverse applications, such as scaffolds for tissue engineering,24,25 cell-encapsulating devices for drug delivery,26–29 media for electrophoresis,30,31 extracellular matrices for biological tissue studies,32–34 soft contact lenses,35 actuators and sensors in soft machines,36,37 packers in oil fields,38 and absorbents in waste management.39,40

Recently, Chen et al. developed a plasma-ignited frontal polymerization for the synthesis of white light-emitting fluorescent polymer nanocomposites14,41 and amphiphilic hydrogels,19,20 a magnetically induced frontal polymerization for constructing robust self-healing host–guest hydrogels,42 as well as laser-ignited frontal polymerization (LIFP) for the preparation of poly(NMA-co-VCL) hydrogels43 and 4-vinylpyridine-based hydrogels.44 Among the above methods, laser-ignited frontal polymerization as a new FP model has been demonstrated to treat waste with remote control and convenience.

In this work, poly(DMAEMA-co-AA) hydrogels were successfully synthesized via a simple LIFP route, along with the first investigation of its horizontal direction LIFP. The dependence of the front velocity and front temperature on the AA/DMAEMA molar ratios and initiator concentrations, as well as the chemical structures and swelling behaviour of poly(DMAEMA-co-AA) hydrogels, were thoroughly investigated in detail. The as-prepared hydrogels are sensitive to pH values ranging from 2 to 12, and their maximum equilibrium swelling ratio can reach 2497% in pH = 7. Horizontal direction LIFP could easily dispose of spilling of toxic substances without close contact to the harmful substances, such as the sealing of dye waste liquids through horizontal LIFP. Subsequently, the adsorption of as-prepared hydrogels towards an anionic dye and a cationic dye was also investigated. The results showed that these hydrogels confer high adsorption performance towards dyes through electrostatic interaction.

2. Experimental

2.1. Materials

Dimethylaminoethyl methacrylate, acrylic acid, N-methyl-2-pyrrolidone (NMP), N,N-methylenebisacrylamide (MBAA), the redox couple ammonium persulfate (APS)/N,N,N′N′-tetramethylethylenediamine (TMEDA), and the dyes orange G (OG) and methylene blue (MB) were all commercially available at analytical reagent-grade and used without further purification.

2.2. LIFP of poly(DMAEMA-co-AA) hydrogels

The anionic monomer AA and the cationic monomer DMAEMA were polymerized by LIFP to obtain poly(DMAEMA-co-AA) hydrogels (see Scheme 1). Aiming at decreasing the triggering temperature and front temperature, we chose the APS/TMEDA couple as the redox initiator to obtain transparent hydrogels without bubbles. First of all, the appropriate amounts of DMAEMA and AA were dissolved in NMP in a beaker with the assistance of ultrasound. Then, APS and MBAA were added and shaken vigorously at ambient temperature to obtain a homogeneous mixture. Finally, the homogeneous solution was obtained by mixing the reductant TMEDA and poured into a 10 mL (D = 15 mm) glass test tube. A typical composition was AA/DMAEMA = 8[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol, (DMAEMA + AA) = 8 g, NMP = 40 wt%, APS = 0.8 wt%, [APS]/[TMEDA] = 1[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol, and MBAA = 0.6 wt%. The reaction mixture was kept at ambient temperature to slow down bulk polymerization. Subsequently, an infrared laser (C-60) was utilized to irradiate the upper side of the mixture until the formation of a stable front. The whole laser treatment process was 67 s, and the current was kept at 4 mA.
image file: c5ra01366f-s1.tif
Scheme 1 Schematic illustration of the fabrication route to prepare poly(DMAEMA-co-AA) hydrogels and their application in adsorption.

2.3. Horizontal LIFP of poly(DMAEMA-co-AA) hydrogels

In a typical run for the synthesis of poly (DMAEMA-co-AA) hydrogels by horizontal LIFP, the appropriate amounts of DMAEMA and AA were dissolved in NMP in a beaker with the assistance of ultrasound. Then, APS and MBAA were added and shaken vigorously at ambient temperature to obtain a homogeneous mixture. Finally, the homogeneous solution was obtained by mixing the reductant TMEDA and poured into a railboat (70 mm × 1.5 mm) which could be filled with 5 g of the monomer mixture. In this case, LIFP was triggered horizontally at one end of the railboat vessel by the laser irradiation until a horizontal hot traveling front formed, under the conditions of a triggering current at 4 mA while the reaction mixture was kept at ambient temperature (27 °C). The whole laser treatment process was 45 s. Initial conditions: AA/DMAEMA = 8[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol, NMP = 40 wt%, MBAA = 0.6 wt%, APS = 0.8 wt%, and [APS]/[TMEDA] = 1[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol.

2.4. Batch polymerization (BP)

Several batch runs were performed in order to compare the resultant samples with the corresponding ones obtained by LIFP. In a typical run, the same amounts of each component as quoted above were mixed with vigorous stirring until homogenization. The final resultant solution was poured into a reaction vessel and the reaction temperature was set at 60 °C for 2 h.

2.5. Velocity and front temperature measurements

The front velocity was confirmed from the slope of line obtained by recording the front position and its corresponding reaction time. A constant front velocity is strong evidence of the occurrence of a pure free-radical FP. Temperature profiles were determined by measuring the temperature as a function of time at a fixed point, using a K-type thermocouple, and then were converted to spatial ones using the front velocities. After the propagating front came to an end, the end-products were cooled to room temperature and then removed from the vessel for further investigation.

2.6. Material characterization

2.6.1. Fourier-transform infrared analysis. To further investigate the chemical structure of the polymeric compound, Fourier-transform infrared (FTIR) analysis was performed using a Nicolet-6700 spectrometer from Thermo Electron at room temperature.
2.6.2. Thermogravimetric analysis. The thermal properties of the polymeric compounds were determined with a thermogravimetric apparatus (Shimadzu-TGA 50) in a nitrogen atmosphere with a heating rate of 10 °C min−1 from 40 to 1000 °C. The samples produced by LIFP were prepared by drying in a vacuum oven at 90 °C for 2 days at a pressure of 70 kPa.
2.6.3. Swelling measurement. The swelling properties of the as-prepared hydrogels were demonstrated by gravimetric analysis. Briefly, the dehydrated samples were immersed in Britton–Robinson (B–R) buffer solution at room temperature. At intervals, the hydrogels were drawn out from the medium, weighed after wiping off the excess solution with filter paper, and then taken back to the original solution. The measurements were taken again and again until the weight of hydrogels remained stable at a constant value. The swelling ratio (SR, %) was calculated using the following equation:
 
SR = (Wi − W0)/W0 × 100% (1)

Here, Wi and W0 are the weights of the swollen and dry hydrogels, respectively.

2.6.4. SEM measurement. The morphology of the resultant samples obtained was investigated by SEM with a QUANTA 200 (Philips-FEI, Holland) at 20.0 kV. The sliced samples were immersed in excess deionized water at room temperature for a week. The water was changed daily to get rid of the water-soluble materials and then the samples were put in a vacuum oven at 60 °C for complete drying. The dried samples were then immersed in solution until they swelled to the maximum extent. Afterwards, the samples were freeze-dried for 20 h. The treated samples used for SEM measurement were cut to expose their inner structure.
2.6.5. Adsorption equilibrium experiments. The capability of the LIFP prepared hydrogels with different pH values ranging from 2 to 12 to adsorb the anionic dye OG and the cationic dye MB was investigated. The dyes were adsorbed onto the poly(DMAEMA-co-AA) in batch experiments, in which 100 mL dye solution with an initial concentration of 60 mg L−1 was placed in a sealed vessel, and 0.10 (±0.0050) g of hydrogel was added to it. At various intervals, approximately 0.5 mL of the dye solution was removed for analysis. The dye concentration was analysed using a Gold Spectrumlab 54 Ultraviolet-visible (UV-vis) spectrophotometer. Predetermined calibration curves were used to convert the absorbance values at the wavelength corresponding to the maximum absorbance (λmax) into dye concentrations. All of the experiments were triplicated. The amounts of dye adsorbed on the hydrogel were determined according to the following equation:
 
qe = (Co − Ce)V/m (2)
where m (g) represents the mass of the sample, Co and Ce (mg L−1) are the initial and equilibrium dye concentrations, similarly, V (L) is the volume of the solution.

3. Results and discussion

3.1. LIFP of poly(DMAEMA-co-AA) hydrogels

Our preliminary experiments aimed to obtain poly(DMAEMA-co-AA) hydrogels by LIFP along with a stable front, and to prevent the occurrence of spontaneous polymerization (SP). Firstly, the pot life of the reagents which were put in the container at room temperature until SP occurred was measured. The result shows that the homogeneous reactant mixture still remained inert after two hours at ambient temperature, while it became reactive when it was treated for only several seconds with the infrared laser. As described in the schematic of the preparation of poly(DMAEMA-co-AA) hydrogels by LIFP, the upper layer of the solution was initiated via infrared laser for about 67 s, whereas the other end of the tube remained open to atmospheric pressure. After initiation, the exothermic nature of polymerization promotes an increase of the temperature in the adjacent portion of the reactor, which triggers the polymerization of the monomers located there.

If the polymerization front propagates along the entire reactor at a constant velocity, we can infer the occurrence of pure FP, excluding SP. Fig. 1a shows typical data for the position of the thermal propagation front as a function of time. The experimental data are well fitted by a straight line, indicating that a constant-velocity, self-sustaining front is achieved. This is a strong proof that reveals that no undesired SP takes place; otherwise simultaneous SP can bring about a deviation from linearity which would be obviously observed in Fig. 1a. Another key feature to confirm the above judgment is given by the analysis of the temperature profile. As shown in Fig. 1a and b, the constant part at the left side of curve means that the temperature is not variable until the front arrives at a fixed point, which is a sign of pure LIFP without SP. Additionally, in less than 0.93 cm, the temperature begins to increase obviously. The increasing tendency of the temperature can be observed by the IR thermal imager due to the exothermic reaction, and Tmax reaches 87 °C.


image file: c5ra01366f-f1.tif
Fig. 1 (a) Front position versus time for poly(DMAEMA-co-AA) hydrogels prepared by LIFP. (b) Typical temperature profile of poly(DMAEMA-co-AA) hydrogels prepared by LIFP. Initial conditions: AA/DMAEMA = 7[thin space (1/6-em)]:[thin space (1/6-em)]3 mol/mol, NMP = 40 wt%, APS = 0.8 wt%, [APS]/[TMEDA] = 1[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol.

3.2. Effects of the AA/DMAEMA molar ratio on LIFP

To investigate effects of the AA/DMAEMA molar ratio on the features of the LIFP, a series of molar ratios of AA to DMAEMA ranging from 6[thin space (1/6-em)]:[thin space (1/6-em)]4 to 9[thin space (1/6-em)]:[thin space (1/6-em)]3 (mol/mol) were used in the LIFP method for producing poly(DMAEMA-co-AA) hydrogels. As shown in Fig. 2a, all the relationships between front position and time are perfectly in line with straight lines, which indicates that the propagation of the thermal fronts moved at constant velocities. Fig. 2b shows the Vf and Tmax of poly(DMAEMA-co-AA) as a function of several AA/DMAEMA molar ratios. In terms of the LIFP run, the Vf at AA/DMAEMA = 6[thin space (1/6-em)]:[thin space (1/6-em)]4, 8[thin space (1/6-em)]:[thin space (1/6-em)]4, 7[thin space (1/6-em)]:[thin space (1/6-em)]3, and 9[thin space (1/6-em)]:[thin space (1/6-em)]3 mol/mol are 0.25, 0.38, 0.60, and 0.88 cm min−1 respectively. Similar to this velocity trend, increasing AA/DMAEMA from 6[thin space (1/6-em)]:[thin space (1/6-em)]4 to 9[thin space (1/6-em)]:[thin space (1/6-em)]3 mol/mol induces Tmax to increase from 65 to 96 °C. From Fig. 2b, we can easily conclude that Tmax and Vf both increase as the proportion of AA is elevated. Indeed, at AA/DMAEMA = 9[thin space (1/6-em)]:[thin space (1/6-em)]3 mol/mol, the two parameters (Tmax and Vf) reach their maximum values, which could be ascribed to the higher reactivity of AA toward radical polymerization. In other words, the Vf and Tmax values can be adjusted by regulating the molar ratios of AA/DMAEMA in this system.
image file: c5ra01366f-f2.tif
Fig. 2 (a) Front position versus time of poly(DMAEMA-co-AA) hydrogels prepared by LIFP with different molar ratios of AA/DMAEMA. (b) Vf and Tmax of poly(DMAEMA-co-AA) hydrogels versus molar ratios. Initial conditions: NMP = 40 wt%, APS = 0.8 wt%, MBAA = 0.6 wt% and [APS]/[TMEDA] = 1[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol.

3.3. Effect of the initiator concentration on LIFP

The initiator concentration turns out to be an important factor in LIFP. Generally, with a too low initiator concentration, the front will suspend because of heat loss. On the contrary, with a too high initiator concentration, the pot life will be too short to achieve FP, which will finally result in the heterogeneous polymer . In order to optimize the initiator concentration, several runs at different APS concentrations varying from 0.80 wt% to 1.4 wt% with [APS]/[TMEDA] = 1[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol were performed. We have found that the front can’t be propagated in the case of an APS concentration of less than 0.80 wt%; on the contrary, many bubbles will occur in the system when APS concentration is more than 1.4 wt%. Therefore, all of the runs were performed with APS concentrations ranging from 0.8 to 1.4%. Vf and Tmax as a function of APS concentration are shown in Fig. 3b. As expected, the front velocity monotonically increases from 0.37 to 0.56 cm min−1 when the initiator concentration is increased gradually. It is observed that Tmax can be obviously changed by altering the initiator concentration. Over the entire range of APS concentrations from 0.8 to 1.4 wt%, Tmax increases from 76 to 96 °C. The dependence of Tmax on the initiator concentration is parallel to the corresponding Vf trend. Additionally, it should be noted that our runs were performed under nonadiabatic conditions, so the increase in velocity versus the time was reduced due to heat loss.
image file: c5ra01366f-f3.tif
Fig. 3 (a) Front position versus time for poly(DMAEMA-co-AA) prepared by LIFP at different initiator concentrations. (b)Vf and Tmax versus the initiator concentration. Initial conditions: AA/DMAEMA = 8[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol, NMP = 40 wt%, MBAA = 0.6 wt% and [APS]/[TMEDA] = 1[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol.

3.4. Swelling behaviour of poly(DMAEMA-co-AA) hydrogels by LIFP

The swelling capability of AA/DMAEMA hydrogels obtained via LIFP in water (pH = 7.0) at different molar ratios of AA/DMAEMA was investigated by using gravimetric analysis (Fig. 4). The overall trend is that the SRs of all the samples gradually increase with the swelling time and finally reach an equilibrium swelling ratio (ESR). As shown in Fig. 4, the swelling ratios of the hydrogels reach equilibrium states after 80 h, and the ESRs are 2497%, 1427%, 734%, and 159%, corresponding to monomer molar ratios of AA/DMAEMA = 6[thin space (1/6-em)]:[thin space (1/6-em)]4, 8[thin space (1/6-em)]:[thin space (1/6-em)]4, 7[thin space (1/6-em)]:[thin space (1/6-em)]3, 9[thin space (1/6-em)]:[thin space (1/6-em)]3, respectively. It has been found that higher AA concentration gives rise to lower water SRs. For comparison, a BP reaction at the same composition as that of LIFP was performed. As seen in Fig. 4e, the ESR of the sample synthesized by BP is 977%, whereas the ESR of the one obtained by LIFP is 1427%, indicating that the swelling capacity of the hydrogel synthesized via the LIFP is superior to that obtained via the BP.
image file: c5ra01366f-f4.tif
Fig. 4 Swelling kinetics of poly(DMAEMA-co-AA) hydrogel prepared by LIFP with NMP = 40 wt%, MBAA = 0.6 wt%, APS = 0.8 wt%, and [APS]/[TMEDA] = 1[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol at different AA/DMAEMA molar ratios: (a) 6[thin space (1/6-em)]:[thin space (1/6-em)]4, (b) 8[thin space (1/6-em)]:[thin space (1/6-em)]4, (c) 7[thin space (1/6-em)]:[thin space (1/6-em)]3, (d) 9[thin space (1/6-em)]:[thin space (1/6-em)]3 mol/mol. (e) Swelling kinetics of poly((DMAEMA-co-AA) hydrogel prepared by BP with a typical composition.

3.5. Characterization of poly(DMAEMA-co-AA) hydrogels

To verify the success of the polymerization, the ATR FT-IR of AA, poly(DMAEMA-co-AA), and DMAEMA was performed, along with TGA measurement of the resultant polymer. The sliced hydrogels were immersed in deionized water at room temperature for several days to remove residual soluble impurities, changing the water daily during the immersion. Then, the samples were dried in a vacuum oven at 60 °C until the weight of the samples remained constant. Finally, the dried samples were ground into powders for FTIR and TGA characterization. As shown in Fig. 5a curve 1, it can notably be seen that the absorption peaks at 1620 and 1618 cm−1, which are assigned to the C[double bond, length as m-dash]C in DMAEMA and AA segments, disappeared after polymerization. The aliphatic C–N stretching band and the symmetric stretching band of C–O in esters with the C–O–C structure (DMAEMA) are observed at 1384 cm−1 and 1168 cm−1, respectively. Additionally, Fig. 5b is the TGA spectrum of as-prepared hydrogel. There is a single degradation step in the curve, signifying that the as-prepared resultant hydrogel is a copolymer of AA and DMAEMA; otherwise, there should be more than one degradation stage in the TGA curve. All the results indicate that the copolymerization has occurred and the monomers such as DMAEMA and AA have been incorporated into the resultant hydrogel.
image file: c5ra01366f-f5.tif
Fig. 5 FTIR spectra recorded at room temperature of (a) (1) AA, (2) DMAEMA and (3) poly(DMAEMA-co-AA); (b) TGA curve of poly(DMAEMA-co-AA) prepared by LIFP.

3.6. Morphology of poly(DMAEMA-co-AA) hydrogels

The morphological characteristics of the obtained poly(DMAEMA-co-AA) hydrogels with different AA/DMAEMA molar ratios were investigated by SEM. As shown in Fig. 6, the morphological structure of poly(DMAEMA-co-AA) hydrogels can distinctly change from sponge-like microporous to honeycomb micropores with increasing AA/DMAEMA molar ratios. Specifically, the pore diameters of poly(DMAEMA-co-AA) hydrogels at AA/DMAEMA = 6[thin space (1/6-em)]:[thin space (1/6-em)]4, 8[thin space (1/6-em)]:[thin space (1/6-em)]4, 7[thin space (1/6-em)]:[thin space (1/6-em)]3, and 9[thin space (1/6-em)]:[thin space (1/6-em)]3 mol/mol are around 68.30, 46.27, 39.92, and 18.94 μm, respectively, which are in agreement with those of the ESR results. That is, the larger pore diameters are, the larger the ESR value is.
image file: c5ra01366f-f6.tif
Fig. 6 SEM micrographs of poly(DMAEMA-co-AA) hydrogels prepared by LIFP with MBAA = 0.6 wt%, APS = 0.8 wt%, [APS]/[TMEDA] = 1[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol at different AA/DMAEMA molar ratios: (a) 6[thin space (1/6-em)]:[thin space (1/6-em)]4, (b) 8[thin space (1/6-em)]:[thin space (1/6-em)]4, (c) 7[thin space (1/6-em)]:[thin space (1/6-em)]3 and (d) 9[thin space (1/6-em)]:[thin space (1/6-em)]3 (mol/mol).

3.7. Swelling capability of poly(DMAEMA-co-AA) hydrogels with different pH values

The pH-dependent equilibrium swelling studies were carried out in buffer solutions with pH values ranging from 2 to 12 at room temperature. The swelling equilibrium degrees of hydrogels with changing pH values are shown in Fig. 7. As shown in Fig. 7a, the swelling degree of all samples increases with increased swelling time and finally tend to constant values, namely, the ESRs. It has been found that the as-prepared hydrogels are sensitive to pH values. Fig. 7b shows the pH dependence of the equilibrium swelling ratio for poly(DMAEMA-co-AA) hydrogels. As the pH values of the solution increase from 2 to 6, the poly(DMAEMA-co-AA) hydrogels remain in the swollen state; then it exhibits a first-order transition and attains a collapsed state when the pH value of the solution is 6. In the pH range between 6 and 8, the poly(DMAEMA-co-AA) hydrogels remain in the collapsed state, while they show an abrupt swelling above pH = 8. In terms of the resulting structure of the poly(DMAEMA-co-AA) hydrogels, the appropriate balance of repulsion and attraction between the carboxyl groups of the AA chains and the tertiary amine side groups of the DMAEMA chains is assumed to be the key factor in the pH-dependent transition behaviour. In this pH range, the carboxyl–carboxyl and ammonium–ammonium repulsion lead to a high swelling capacity. The attraction between the ammonium and carboxyl groups restricts further swelling, and thus, the swelling capability is highly dependant on pH values. These results are consistent with those observed for the swelling capability of an amphoteric polymer, poly(acrylic acid-co-diallyldimethyl ammonium chloride)45 and the swelling capability of an ampholytic polymer, poly(aspartic acid).46 Typically, between pH 6–8, the coulombic repulsion and attraction between the negative charges of carboxylate anions and the positive charges of ammonium cations are balanced at a level which results in a collapsed state.
image file: c5ra01366f-f7.tif
Fig. 7 (a) Swelling kinetics of poly(DMAEMA-co-AA) hydrogels versus pH value (2, 4, 6, 8, 10, and 12) of the medium. (b) pH dependence of equilibrium swelling ratio for poly(DMAEMA-co-AA) hydrogels. (AA/DMAEMA = 8[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol, NMP = 40 wt%, MBAA = 0.6 wt%, APS = 0.8 wt%, and [APS]/[TMEDA] = 1[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol).

3.8. Horizontal LIFP of poly(DMAEMA-co-AA) hydrogels

In order to make the best of LIFP technology, the polymerization of poly(DMAEMA-co-AA) hydrogels by LIFP horizontally was investigated. Fig. 8 shows the LIFP process in the horizontal direction. During the entire polymerization process, the temperature variation of a fixed point with time could be observed by an IR thermal imager. As shown in Fig. 8a, we can clearly observe the hot traveling front moving at a constant velocity. Fig. 8c shows typical data for the position of the thermal propagation front as a function of time. The experimental data are well fitted by a straight line and we calculate that the frontal velocity is 1.14 cm min−1. This is a strong evidence that pure FP has occurred. Fig. 8b presents the IR thermal images. We can observe that the temperature starts to rise from room temperature, and then gradually increases to the highest point of 82.2 °C in a few minutes. Finally, it has a tendency of slowly decreasing. This phenomenon demonstrates another representative feature of pure LIFP. Therefore, we can utilize LIFP horizontally to treat dye solutions effectively, and the resulting hydrogels are easily peeled off from the vessel without any waste residue. The dye solution was composed of an aqueous solution containing 200 mg L−1 OG (as shown in Fig. 8d and e).
image file: c5ra01366f-f8.tif
Fig. 8 (a) Photos of the propagating front of the horizontal LIFP; (b) corresponding photos taken using a Fluke Ti30 IR thermal imager; (c) front position versus time for hydrogels prepared by horizontal LIFP. (d–e) Dye solution sealing: 200 mg L−1 OG solution. Hydrogel compositions: AA/DMAEMA = 8[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol, NMP = 40 wt%, MBAA = 0.6 wt%, APS = 0.8 wt%, and [APS]/[TMEDA] = 1[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol.

3.9. Adsorption of poly(DMAEMA-co-AA) hydrogels towards dyes at different pH values

We further investigated the adsorption capabilities of poly(DMAEMA-co-AA) hydrogels containing carboxyl/amino segments towards MB and OG dyes at different pH values ranging from 2 to 12. Hydrogels with the same composition were employed in all runs, with different pH values. After the hydrogels absorbed the dyes, the reduction in the concentration of MB and OG with time is shown in Fig. 9a and b, respectively. Furthermore, the dye adsorption capacities of the hydrogels at different pH values is clearly indicated in Fig. 9c. After hydrogels were immersed in dye-loaded B–R buffer solutions at pH = 2, 4, 6, 8, 10, and 12 for about 90 hours, the adsorption capacities of hydrogels for MB were determined to be 7.92, 11.92, 13.01, 45.16, 51.08, 55.03 mg g−1, respectively. Likewise, the adsorption capacities of hydrogels for OG are 53.53, 51.02, 50.01, 36.04, 32.42 and 15.31 mg g−1, respectively. By comparing the adsorption capacities of hydrogels in an acidic medium with those in a basic medium, it is evident that there is no notable difference in the hydrogel adsorption capacities towards MB dye when the pH values range between 2 and 6, revealing that the acid medium could not remarkably affect the hydrogel adsorption capacities towards MB. However, the hydrogel adsorption capacities towards MB are highly enhanced when pH values range between 8 and 12. The reason could be explained as the acid medium allowing the hydrogels containing amino segments to be protonated. MB is a cationic dye. This causes a strong electrostatic repulsion between the hydrogels and cationic MB dye. On the contrary, the alkaline environment favours the absorption of the MB dye due to the electrostatic attraction. Similarly, Fig. 9b indicates that the hydrogels absorb more OG dye in an acidic medium since OG is a typical anionic dye.
image file: c5ra01366f-f9.tif
Fig. 9 Variations in the concentrations of (a) MB and (b) OG with time at different pH values for poly (DMAEMA-co-AA) with AA/DMAEMA = 8[thin space (1/6-em)]:[thin space (1/6-em)]4 mol/mol. (c) The adsorption capacities of poly(DMAEMA-co-AA) hydrogels for MB (left) and OG (right) in solutions of pH = 2, 4, 6, 8, 10, 12.

4. Conclusions

In this work, a series of poly(DMAEMA-co-AA) hydrogels were facilely produced by laser ignited frontal polymerization (LIFP). More importantly, the hydrogels were synthesized for the first time using a horizontal LIFP model, which confers several advantages, such as rapid remote synthesis away from dangerous sites, and easy sealing of toxic wastes. The parameters which affect the frontal polymerization of the as-prepared hydrogels, such as the molar ratio of AA/DMAEMA and the APS concentration were thoroughly investigated. The as-prepared hydrogels are sensitive to pH values ranging from 2 to 12, and their maximum equilibrium swelling ratio could reach 2497% at pH = 7. Swelling studies show that the SRs of the poly(DMAEMA-co-AA) hydrogels produced via LIFP are superior to those obtained by BP. Also, the as-prepared hydrogels could effectively absorb MB and OG dyes, which occur in a basic medium and an acidic medium, respectively. That is, they would provide potential as dye water waste scavengers. In addition, horizontal LIFP was employed to seal dyestuff solutions. This model would expand the scope of LIFP applications to spilled toxic substances without touching the location of the reaction, even in a radiation danger zone.

Acknowledgements

This work was supported by the National High Technology Research and Development Program of China (863 Program) (2012AA030313), National Natural Science Foundation of China (21006046 and 21474052), Natural Science Foundation of Jiangsu Province (BK20131408), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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