Boletín de la Sociedad Chilena de Química
versión impresa ISSN 0366-1644
Bol. Soc. Chil. Quím. v.45 n.2 Concepción jun. 2000
CHLORIDE HYDROGEL. SYNTHESIS AND WATER-ABSORPTION
Bernabé L. Rivas*, Guido S. Canessa, Esteban Martínez.
Department of Polymers, Faculty of Chemistry, University of Concepción, Casilla160-C,
Concepción, Chile. Fax: (56-41) 245974, E-mail: email@example.com
(Received: March 15, 2000 - Accepted: April 26, 2000)
In memoriam of Doctor Guido S. Canessa C.
The radical polymerization of [3-(methacryloylamino)propyl]trimethylammonium chloride by using ammonium persulfate as initiator and N,N-methylene-bis-acrylamide as crosslinker reagent was carried out. The polymers were completely insoluble in water and characterized by FT IR spectroscopy and thermal analysis. The effect of the crosslinker reagent on the water sorption capacity was investigated. The highest water-absorption (46.6 g of water/ g of xerogel) was observed with the lowest mol% of crosslinker reagent (2 mol%).
KEY WORDS: Hydrogel, crosslinking effect, water-absorption capacity.
Se realizó la polimerización radical del cloruro de [3-(metacriloilamino) propyl]trimetilamonio mediante persulfato de amonio como iniciador y N,N-metilen-bis-acrilamida como reactivo entrecruzador. Los polímeros son completamente insolubles en agua y fueron caracterizados por espectroscopía FT IR y análisis térmico. Se investigó el efecto de la concentración de reactivo entrecruzador sobre la capacidad de absorción de agua. La absorción mas alta de agua (46.6 g de agua/ g de xerogel) se observó con el hidrogel obtenido con la menor proporción de reactivo entrecruzador (2 mol%).
PALABRAS CLAVES: Hidrogel, efecto de entrecruzamiento, capacidad de absorción de agua.
The last few decades have witnessed an increase in importance of polymer hydrogel due to their specific properties which found wide practical applications (1,2). Hydrogels are three-dimensional network polymers that swell on contact with water but do not dissolve. These polymers may be prepared either by crosslinking the reactive functional groups of water-soluble linear polymers or by copolymerizing vinyl monomers with multifunctional cross-linking monomers. If a dry, hydrophilic cross-linked network is brought in contact with water, the macromolecular chains swell to the solvated network structure. The swelling of the hydrogel network is constrained by the cross-linked structure. When the thermodynamic swelling force is equal to the contractive force of the cross-linked network, equilibrium swelling is reached. One of the advantages of the hydrogels lies in their capability of undergoing a first-order phase transition (collapse) under the change of some external parameters (temperature, composition of solvent, electric field and the like) (3). At the collapse, the gel volume can change 10-100 times. The presence of changes on the chain (~1-10mole%) seems to be an important condition for the occurrence of a jumpwise change in gel volume (4). The collapse phenomenon and swelling behavior of hydrogels have been extensively studied in literature both theoretically and experimentally (5-10). Most experimental results were obtained with charged hydrogels of poly(acrylamide) (solvent-sensitive gels), and of poly(N-isopropylacrylamide)/ionic comonomers, and of poly(N,N-diethylacrylamide)/ionic comonomers (temperature-sensitive systems).
The aim of this paper is synthesize a crosslinked polymer hydrogel containing hydrophilic groups including amino and ammonium groups as potential nitrogen sources for future applications in agriculture. The effect of the crosslinker reagent amount on the water absorption capacity by the hydrogel was investigated.
Reagents: [3-(methacryloylamino)propyl] trimethyl ammonium chloride (MACPTA) (Aldrich Co.), N,N-methylene-bis-acrylamide (MBA) (Aldrich Co.), ammonium persulphate (APS) (Merck) were used without further purification.
Synthesis of the hydrogels: The four hydrogels were synthesized by radical polymerization. The initiator and crosslinker reagent were ammonium persulfate (1 mol%) and N,N-methylene-bis-acrylamide (2,3,4, and 5 mol%) respectively. In order to obtain more homogeneous gels the reaction was carried out between glass sheets. The polymerization mixture was kept for 24 h at 70°C. Subsequently, the polymers are removed and poured in deionized water (~ 5 L) and then in acetone. After the polymerization the entire resultant swollen gel was removed from the mould dried at room temperature for 48 h and then at 30°C until constant weight. As the mass fractions of all components of initial feed mixture were known, the weight of swollen gel allow the mass of monomer initially in the mould to be calculated. The swollen gel was washed thoroughly several times with deionized water and dried successively at room temperature, then at 45°C and finally to a constant weight in a vacuum oven at 45°C. The xerogel obtained was weighed. The ratio of this weight to that of total monomers initially indicated a very high conversion on a weight basis. Since the extracted dry gel is crosslinked polymer the value of conversion is, in principle, only an apparent one that does not allow for possible conversion of monomers to soluble linear polymer. We have not attempted to distinguish between tha latter and unreacted monomers in the washings. However, allowance for any linear polymer could give an actual fractional conversion that is greater than the apparent one and the sol fraction removed must be very small. Hence, the overall average copolymer composition can be approximated very well to the initial feed composition.
Water-absorption measurements: Each dried sample (ca. 1.00 g) was immersed in a large excess of bidistilled water (pH : 6.0, conductivity : 1.0 mS/cm2) at 25°C. It was confirmed that a period of 24 h was more adequate to ensure attainment of equilibrium. The measurements were done in duplicate.
Measurements: The FTIR and UV-vis spectra were recorded with a Magna Nicolet 550 Perkin Elmer Lambda 20 with integration sphere spectrophotometers respectively. The electron micrographs were recorded on electronic microscopy ETEC U1. The thermal analysis was carried under nitrogen atmosphere with a Thermal Analyzer Polymer Laboratories STA 625. The pH was measured with a Digital pH-Meter M. Jurgens and Co.
RESULTS AND DISCUSSION
Four hydrogels of [3-(methacryloylamino)propyl]trimethylammonium chloride by radical polymerization were synthesized. The hydrogels were completely insoluble in water.
|Table I. Experimental conditions and results of the radical polymerization of [3-(methacryloylamino)propyl]trimethylammonium chloride, MACPTA, with the crosslinker reagent N,N-methylene-bis-acrylamide, MBA, at 70°C for 24 h.|
The hydrogels were obtained at very high yield, independent of the crosslinker reagent mol %.
The elemental analysis of the hydrogels is summarized in Table II.
According to the elemetal analysis data it is possible assume that the crosslinking degree is very close to those corresponding in the feed.
All the hydrogels show basically the same absorption signals. Among the most characteristics are: 3440 cm-1 n(O-H), 2953 cm-1 n(Csp3-H), 1638cm-1 n( C=O). As an example, the figure 1 shows the FT IR of the sample 2.
|Fig.1. FTIR spectrum of hydrogel, sample 1.|
The figure 2 shows the UV-vis in solid state of the sample. It shows an important signal at 410.35 nm attributed to carbonyl group. This signal decreased and is broaded from 432.09 nm up to 686.7 nm.
Fig.2.UV-vis spectrum of hydrogel, sample 1. Scan speed: 480 nm/min; slit width: 1.00 nm; smooth bandwith: 3.000 nm.
The morphology of the hydrogels show not an important difference. All the surfaces are wrinkled, with holes and crevices. These properties allow the diffusion of the water molecules. As an example the figure 3 shows the morphology of the sample 2.
Fig.3. Electron micrograph (500x) of the hydrogel, sample 2.
All the hydrogels show a high thermal stability. Up to 100°C, only 3-6 % was lost attributed to the occluded water. Up to 300°C the weight-loss is lower than 20% due to probably CO2 and NH3 evolution. Above 300°C starts a strong weight loss involving degradation of the main and side chains with the highest values at 500°C. See table 3 and figure 4.
Fig.4. Thermogram of hydrogel, sample 1. Weight of sample: 3.460 mg; heating rate: 10°C/min under nitrogen.
|Weight-loss at different temperatures (°C)|
The determination of the gravimetric absorption ratio Q via eq. (1), in which wh and wx the weights of swollen and dry strips respectively, is a procedure that demands great care and attention to detail in order to obtain accurate and reproducible results. The procedure was repeated twice and an average value was used to calculate Q.
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The xerogel had previously been subjected to a rigorous washing process and indeed the absence of any sol fraction was comfirmed by deliberately drying to completeness and weighing the samples after swelling/deswelling. The final weight was the same value to wx. The pH of the water was measured only at the start at 25°C. As is customary in such work, no stirring or agitation was considred necessary during the swelling.
The water-absorption capacity of the four gels is shown in figure 5. The highest water- absorption value (46.6 g of water/ g of xerogel) was oberved with the hydrogel, sample 2. As increases the crosslinking degree decreases the capacity to absorb water The lowest value was of 8.7 g of water/g of xerogel for the sample 4 (5 mol% of crosslinking reagent). It may be attributed to the effect of an increase of the crosslinking density with increasing percentage of crosslinking agent and hence a decrease of the water-absorption. Because the hydrogels were very soft it was not possible to achieve stable discs to carry out other studies as effect of pH, temperature, and salt concentration on the water absorption capacity.
Figure 5. Dependency of the water absorption capacity on the crosslinker reagent amount.
The authors thank to FONDEF (Grant D-97-I-1062).
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