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HYDROGELS
THEORETICAL STUDY
Hydrogels as hydrophilic or
amphiphilic polymeric networks are fascinating materials of both fundamental and technological interest,
with a wide variety of applications based on their ability to absorb large amounts of water and to
influence the mass transfer of solutes. They do not dissolve, maintaining a defined shape due to a
permanent three dimensional structure. In recent years, the hydrogels are used as specific sorbents
and support carriers in biomedical engineering. They are key components of consumer products like
diapers, biochemical separation techniques, pharmaceutical delivery systems and medical devices such
as artificial organs. Applications of gels in pharmacy and biotechnolo
gy (drug delivery matrices, gel-based techniques for protein purification chromatography,
gel electrophoresis, and aqueous two-phase extraction) receive particular emphasis.
The networks are composed of homopolymers or copolymers, insoluble due to the presence of chemical
(covalent) or physical (ionic, hydrophobic interactions, entanglements) crosslinks. The crosslinks provide the network structure and physical integrity. The chains of the network are connected in such a fashion that pores exist and that a substantial fraction of these pores are of dimensions between 1 nm and 1000 nm. Hydrogels exhibit a thermodynamic compatibility with water that allows them to swell in aqueous media.
The most important property of a gel is the equilibrium degree of swelling , which is the ratio of swollen gel volume to that of the dry polymer. If a dry, hydrophilic crosslinked 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 crosslinked structure. When the thermodynamic swelling force is equal to the contractive force of the crosslinked 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 environmental stimulus (temperature, pH, solvent composition, ionic strength, light, mechanical stress, radiation, or other specific physical – chemical stimulus). At the collapse, the gel volume can change 10 – 100 times. The collapse phenomenon and swelling behaviour of hydrogels have been extensively studied both theoretically and experimentally, as controllable phenomenon applied for controlled drug delivery devices and chemical separation systems. a in literature. Such environmentally responsive gels (or stimulus – sensitive gels) have numerous applications including artificial muscles, chemical separations, controlled drug delivery, sensors and actuators.
Investigations of the acrylamide-based hydrogels have been reported in the last four decades. It is well known that the swelling behaviour of polymer gels depends on their network structure whereas the latter is closely related to the conditions under which the polymer gels are formed. Thus the understanding of the formation mechanism of polymer gels is of great interest in predicting their physical properties.
The formation of PAAm microgels occurs prior to the onset of macrogelation. Then, as the reaction proceeds, microgels are connected to a macrogel through their peripheral pendant vinyls and radical ends due to the high extent of cyclization reactions. At this stage, highly intramolecularly crosslinked microgel particles formed in the pregel period act as junction points. Schematized representation of the macrogelation is presented, as follows:
Schematic representation of the structure of PAAm gels prepared in dilute aqueous solutions
The microgels are presented as the junction points of the final inhomo-geneous network structure. Increasing the crosslinker content only increases the compactness or the concentration of the junction points.
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Supported by INVENT GRANT No. 131/28.09.2004 |
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