This invention relates to novel gel compositions which are capable of discontinuous volume change of several hundred times induced by exposure to light.
Gel is a form of material between the liquid and the solid state. It consists of a crosslinked network of long polymer molecules with liquid molecules trapped within the network. Gels play important roles in various aspects of our everyday life.
In chemistry and biochemistry, gels are used extensively as matrices for chromatography and electrophoreses analytical methods that separate molecules according to their molecular weights and charges. In these techniques, the pore size of the crosslinked polymer network plays an essential role in its sieving effects.
Recently, with the help of light scattering spectroscopy, very interesting phenomena in permanently crosslinked gels have been found: as the temperature is lowered, the polymer network becomes increasingly compressible, and at a certain temperature, it becomes infinitely compressible. At the same time, the effective pore size of the network increases and diverges. It is also observed that the volume of gels changes reversibly by a factor as large as several hundred by an infinitesimal change in external conditions such as solvent composition or temperature. Tanaka, Physical Review Letters, Vol. 40, no.12, pp. 820-823, 1978 and Tanaka et al, Physical Review Letters, Vol. 38, No. 14, pp 771-774, 1977; Tanaka et al Physical Review Letters 5, Vol. 45, pg. 1636, 1980; Ilovsky, Macromolecules, Vol. 15, pg. 782, 1982; Hrouz et al, Europ. Polym. J., Vol. 17, pg. 361, 1981; Ohmine et al, J. Chem. Physics, Vol.8, pg. 6379, 1984; Tanaka et al, Science, Vol. 218, pg. 462, 1972 and Ilavsky et al, Polm, Bull, Vol. 7, pg. 107, 1982.
It is known that copolymers of acrylamide and sodium acrylate in gel form are capable of drastic volume change and it is also known that the gel showed continuous volume change when the concentration of the sodium component, which was an ionizable group, was smaller than the critical value, while the change became discontinuous when the concentration of the acrylic acid component was greater than the critical value.
According to the conventional knowledge as described above, in order to prepare a polymer gel having the phase transition function with large discontinuous volume change when the concentration of the sodium acrylate component was greater than the critical value.
According to the conventional knowledge as described above, in order to prepare a polymer gel having the phase transition function with discontinuous volume change, it was believed necessary that the polymer contained a considerably large amount of the ionizable group.
The volume change of the conventional gels having an ionic group described above significantly depends on hydrogen ion concentration (pH) of the liquid medium to be used. Therefore, the conventional ionic gel could not be used when pH of the liquid medium could not be controlled sufficiently.
It is also known that polymers containing no ionizable group formed from a monomer containing a predominant amount of a N-isopropylacrylamide and a crosslinking agent exhibit a drastic volume transition in water or mixtures of a solvent and water in response to change in liquid solvent composition. These gels do not depend upon hydrogen ion concentration of the liquid solvent.
It is disclosed in U.S. Pat. No. 4,732,930 that an ionic gel based upon N-isopropylacrylamide and an ion containing monomer which is capable of drastic volume in respond to changes of temperature, pH or ion composition.
Prior to this invention no gel have been known which are capable of drastic volume change in response to exposure to photons, e.g. visible or ultraviolet light. It would be desirable to provide such gel composition since photons provide a means for quickly changing the physical and/or chemical environments which promote a drastic volume change in the gel.