Microstructured polyacrylamide hydrogels made with hydrophobic nanoparticles
Introduction
Hydrogels are materials that swell and hold high amounts of water without losing their shape [1]. Polymers and copolymers of acrylamide are the most commonly used hydrogels [2]. Polyacrylamide hydrogels have found applications in water purification and irrigation, in improving soil texture, and in pesticide formulations to limit spray drift [3]. They are also used in molecular biology laboratories as matrices for separating nucleic acid components during DNA sequence analysis and protein identification [4].
Polyacrylamide hydrogels (PAM) can absorb relatively high amounts of water and their swelling capacity is not very sensitive to pH or electrolytes [5]. For some applications, high water absorption capacity is desired because it increases the permeability and the biocompatibility of the hydrogels [6]. However, because water does not contribute to mechanical strength, these hydrogels present poor mechanical properties. To increase the range of applications it is necessary to improve the mechanical strength of these materials while maintaining their water absorption capacity.
To produce hydrogels with relatively high mechanical strength combined with high water content some procedures have been reported: (i) snake-cage polymerization to yield a cross-linked hydrophilic polymer (cage) in which a linear hydrophobic polymer (snake) with good mechanical properties is entrapped [7], [8]; (ii) micellar polymerization where a hydrophobic monomer is copolymerized with an hydrophilic monomer [9], [10]—in this case the hydrophobic polymer is used to reinforce the hydrogel. Recently our group reported a two-stage polymerization process where PAM nanosized particles, obtained by inverse (w/o) microemulsion polymerization, were incorporated into a PAM matrix giving as a result hydrogels with much higher swelling capacities and relatively high Young moduli [11].
In this work we examine the swelling kinetics, equilibrium swelling capacity and Young modulus at the equilibrium swelling of PAM hydrogels containing slightly cross-linked poly(methyl methacrylate) (PMMA) nanoparticles. These hydrophobic particles were prepared by direct (o/w) microemulsion polymerization. The equilibrium swelling and the mechanical properties at equilibrium swelling are compared with those of conventional hydrogels (i.e., without particles) with similar polyacrylamide content and degree of cross-linking.
Section snippets
Experimental
Acrylamide (AM) was 99% pure from Scientific Polymer Products. Methyl methacrylate (MMA) was 99% pure from Aldrich. Sodium bis (2-ethylhexyl)sulfosuccinate (Aerosol OT or AOT) was 98% pure from Fluka. Potassium persulfate (KPS) was 99.9% pure from Productos Quı́micos Monterrey. N,N′-methylenebisacrylamide (NMBA), from Scientific Polymer Products, was recrystallized from methanol. Doubly distilled and deionized water was used.
PMMA nanoparticles were synthesized by o/w microemulsion
Results
The appearances of the conventional and microstructured hydrogels are strikingly different. The conventional hydrogel is transparent and it is difficult to be detected by the naked eye when immersed in water. By contrast, the microstructured PAM hydrogels are bluish and slightly opaque and they can be clearly seen when immersed in water, even at maximum, swelling due to the presence of the PMMA particles dispersed in the matrix.
The sol fraction in these hydrogels was determined as described
Discussion and conclusions
The conventional PAM hydrogel is almost transparent and when immersed in water, it is very difficult to see it with the naked eye. This is because the refractive index of the swollen hydrogel (>90 wt% water) is very similar to that of water. By contrast, the hydrogels containing PMMA nanoparticles are bluish and they can be easily observed when immersed in water because the nanoparticles in the hydrogel scatter light. In fact, because the particles do not swell and their diameters are smaller
Acknowledgements
This work was supported by CONACYT (Ref. G38725-U).
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