versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.52 n.2 Concepción jun. 2007
J. Chil. Chem. Soc, 52, Nº 2 (2007) págs.: 1160-1162
STIMULI-RESPONSIVE HYDROGELS FROM ACRYLAMIDE WITH N-[3-(DIMETHYLAMINE)PROPYL] METACRYLAMIDE. SYNTHESIS AND PROPERTIES
S. AMALIA POOLEY*, BERNABÉ L. RIVAS, FRANCISCO J. RIQUELME
Polymer Department, Faculty of Chemistry, University of Concepción, Casilla 160-C, Concepción, Chile. firstname.lastname@example.org
Cross-linked copolymers of acrylamide (AAm) with N-[3-(dimethyl amine)propyl]methacrylamide (NDAPM) were prepared by solution free radical polymerization at different feed molar ratios. In this reaction, ammonium persulfate (APS) and N,N´-methylene-bis-acrylamide (MBA) were used as initiator and cross-linking reagents respectively. The effects of reaction parameters, including the concentration of crosslinking reagent and initiator, the monomer concentration, pH, temperature, the various salt solutions, and solvent polarity, on the water absorption have been investigated. The results indicate that the copolymers obtained are stimuli-responsive hydrogels that change their volume and elasticity in depending on the properties of the liquid phase.
Key words: copolymerization; gels; swelling; crosslinking; networks.
Polymeric hydrogels derived from poly(acrylamide), crosslinked by a small amount of a bifunctional compound have found wide applications in the fields of agriculture and medicine (1-4). Among water-containing polymeric gels, hydrogels derived from polyacrylamide are among the most widely investigated.
Hydrogel is a class of polymeric material with the ability to hold a substantial amount of water, presenting a soft, rubbery-like consistency, and low interfacial tension parameters (5). Product of a large amount of research during the last two decades, hydrogels are now recognized as a well established class of polymers with widespread applications in agriculture, medicine, food industry, biotechnology, environmental sciences, among others. The structural feature of these materials dominates its surface properties, permeselectivity, and permeability, giving hydrogels their unique, interesting properties and the similarity of their physical properties to those of living tissue (6-7).
Hydrogels are three-dimensional crosslinked hydrophilic polymeric structures that are able to swell in an aqueous environment (8). Due to their high water content, low water contact angle, high permeability, and low friction coefficient, hydrogels are studied extensively as a replacement for soft tissue (9).
Hydrogel properties mainly depend on the degree of crosslinking, the chemical composition of the polymeric chains, and the interaction between the network and surrounding liquids. Hydrophilicity or high water retention in hydrogels is attributed to the presence of hydrophilic groups, such as carboxylic acids, amides, alcohols and so on (10).
The use of acrylic acid based hydrogels to concentrate environmental samples by absorbing excessive amounts of water for pesticide residue analysis is a unique application in environmental monitoring (11-14).
In stimuli responsive hydrogels, the response of the functional group, depends on the type, changing according to the scale of external stimuli, such as pH, temperature, and salt concentration. These environmental variables are always found in controlled drug delivery, immobilized enzyme reactors, and separation processes (15-18).
The network density of the polymeric gels is also an important factor that is responsible for controlled release of active molecules. For example, many authors have reported diffusion of agrochemical drug release systems (19-24).
The aim of this paper is to synthesize copolymers of acrylamide with N-(3-dimethyl amine)propylmethacrylamide at different feed comonomer ratios with different degrees of crosslinking, and to study the swelling effects of these hydrogel systems in twice-distilled water at different pH, temperature, time, and salt concentration.
Acrylamide (AAm; Merck) was purified by recrystallization. All the other reagents were used as received without further purification.
Preparation of poly(acrylamideco-N-[3-(dimethylamine)propyl]methacr ylamide), P(AAm-co-NDAPM).
Crosslinked poly(AAm-co-NDAPM) hydrogels, containing 25, 33, and 50 mol-% of AAm were prepared by solution free radical polymerization. AAm was dissolved in water, NDAPM, MBA, and APS were added to the above AAm solution, the reaction solution was heated and polymerized for 24 hours at 70ºC in a Teflon tube of 40 mm diameter. The product was cut into small discs 5 mm x 20 mm, and dried until constant weight. Finally, the dried product was characterized and the swelling properties were determined.
The feed mole ratios of AAm and NDAPM are 1:1; 1:2; 2:1. The total weight percentage of both monomers in the solution is 15 %. The weight percentage of the cross-linking reagent respect to the monomers is 5 %. The weight percentage of the initiator respect to the monomers is 2 %.
The dried copolymers were ground to a suitably sized powder for FT-IR analysis. The FT-IR spectra of the copolymers were obtained with a Magna Nicolet IR-550 spectrophotometer.
The thermogravimetry analysis of the copolymers was performed using TGA (Polymer Laboratories, STA-625 thermobalance). Measurements on 5 mg of dry samples were carried out with a heating rate of 10 ºC/min from room temperature to 550 ºC under nitrogen atmosphere.
Copolymer morphology was examined by scanning electron microscopy (SEM) (Jeol, GSM-6380LV).
The sample of poly(AAm-co-NDAPM) (0.5 g) was immersed in 100 mL of distilled water for 12 h until equilibrium was reached at room temperature. The weight of the swollen gel was measured after the excess surface solution was removed by filter paper. Then, the swollen gel was weighed. The absorbency was calculated using the following equation:
Absorbency is expressed in grams of liquid retained in the gel per grams of dry copolymer. W2 and W1 are the weights of the swollen gel and the dry poly(AAm-co-NDAPM), respectively.
Effect of time, temperature, and pH on absorbency
The methods were the same as used for the swelling measurement in twice distilled water, saline solution, and ethanol. The pH values of the solution were adjusted with HCl or NaOH.
Water retention capacity
The hydrogels were placed in twice-distilled water for 12 hours. The swollen gels that reached equilibrium in water were drained in nylon bags for one hour, then the gels and the bags were put into an oven and heated at constant temperature. To investigate the weight variation, they were weighed at 1 hour intervals.
RESULTS AND DISCUSSION
Stimuli-responsive hydrogels change their volume and elasticity in response to a change in liquid phase properties, such as temperature, pH, solvent composition, and ionic strength. Depending on the chemical composition of gels and liquid in experimental conditions, the change in the swelling behavior can occur either continuously or discontinuously.
The relationship between the gels swelling behavior and the feed monomer ratios were studied at different temperatures and in solutions with various pHs. The transition temperature of the crosslinked gels changed according to the feed monomer ratio used in copolymerization. The pH value of the solution strongly affected the swelling ratio.
Synthesis and characterization
Three copolymerization reactions were performed in water at different feed compositions, while maintaining constant the total amount of comonomers (0.04 mol).
All hydrogels obtained in this study are transparent, smooth, and maintain their shape in the swollen state.
Figure 1 shows the FT-IR spectra of the three copolymers poly(AAm-co-NDAPM). The spectra show the typical absorption bands of both comonomers and the crosslinking reagent. Among the most characteristic absorption bands are the following (in cm-1): 3450-3100 (N-H stretching of amide group and N-H stretching of amine group); 2940, 2810 (C-H stretching, aliphatic.); 1643, 1550 (N-H deformation of amide group); 11464-1394 (stretching C-N tertiary amines).
The primary thermograms of all polymers show a typical sygmoidal shape. All the copolymers degrade in one step and they are stable until 250ºC with a weight-loss below 10% at 254ºC (see Figure 2).
The typical scanning electron microscopy (SEM) of the poly(AAm-co-NDAPM)1:1; 1:2; 2:1 are shown in Figure 3, where the three micrographs indicated a smooth morphology.
Time dependence swelling curves of hydrogels in distilled demineralized water and ethanol for three feed mol ratios are shown in Figure 4. They show that as time increases, the swelling percentage increases gradually until equilibrium. This result is due to the gradual diffusion of water molecules into the network of the hydrogel and the complete filling or occupation of the preexisting or dynamically formed spaces in the polymer chains.
The absorbency of poly(AAm-co-NDAPM) 1:2 copolymer is higher than the other copolymers in twice-distilled water as well as ethanol at room temperature. This result is due to the greater amount of amine groups incorporated into the backbone. These amine groups form a higher number of hydrogen bonds with water.
Effect of pH on absorbency
To investigate the influence of pH on the degree of gel swelling, the pH was adjusted with 1M HCl or 1M NaOH from pH 1 to pH 9. Figure 5 shows the effects of the solutions pH values on the swelling behavior for all the studied copolymers. The water absorption curve shows an optimal value at pH 2 when the feed monomer ratio is 1:1. The other two polymers show an optimal value at similar pH but with lower water absorption. The increase of water absorption at pH 2 is produced by protonation of amino groups coming from NDAPM, which increases the electrostatic repulsions between the polymer chains.
Effect of the temperature on absorbency
The effect of the temperature on the absorbency for poly(AAm-co-NDAPM) for three feed mol ratios is shown in Figure 6. It demonstrates that the absorbency increase as increases the temperature until 45 ºC.
Effect of the temperature on the maximum swelling degree Figure 7 shows that the swelling degree decreases as the temperature increases for three gels, and that this is a characteristic behavior. At higher temperature, the coil is contracted, limiting the water entry into the diffusion system.
Effect of salt solutions on absorbencies
Figure 8 shows the effect of the solution´s salt concentrations on the absorbency of the poly(AAm-co-NDAPM). The results indicate that this polymers absorbency depends in presence of the different NaCl concentrations, depends on the copolymer composition. Thus, for initial mole concentration 1:1 it is observed a decrease, an increase, slight increase, and finally a decrease. At higher NaCl concentration, only the ratio 1:2 shows a slight increase.
A novel hydrogel of AAm and NDAPM was prepared in aqueous solution, and the swelling properties were studied. The experimental results show that the poly(AAm-co-NDAPM) has a better absorbency in water with respect to ethanol and salt solutions. The absorbency of these polymers decrease as the solvent´s polarity decreases.
The effects of pH on the absorbency were studied. It was found that the absorbency of poly(AAm-co-NDAPM) increases as the pH increases from 1 to 2, and then decrease abruptly at higher pH. It was found that the absorbencies increase as increases the temperature. These results implies that poly(AAm-co-NDAPM) can be used in a wide temperature range.
The influence of the salt solutions on the absorbency was investigated, and the results indicate that absorbency is a variable function of the rising salt concentration.
The authors thank FONDECYT (Grant No 1050572 ) for financial support.
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