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Articles you may be interested inEffect of ionization on the temperature-and pressure-induced phase transitions of poly(Nisopropylacrylamide) gels Reentrant volume-phase transitions are observed in N-isopropyoacrylamide gels in the methanol-water mixtures. When the solvent composition is varied systematically, the gel undergoes two transitions: a discontinuous collapsing followed by a discontinuous swelling. The reentrant transition defines a closed-loop instability phase boundary having both upper and lower critical points. The closed-loop phase boundary depends on temperature and diminishes to a point at approximately 0 DC. A simple mean field theory is presented to describe the phenomenon, which reveals an alteration of free energy of alcohol-water interaction by presence of polymer network. In the case of ethanol-water mixtures, there appear two closed-loop phase boundaries, whose physico-chemical basis are not yet clear.

When an electric field is applied to an ionic gel in a buffer solution, the anode side of the gel shrinks if the gel is contacting with the electrode, while it swells if the gel is placed with sufficient separation from the electrode. We calculate the changes of ion concentration profiles under an electric field, taking account of ion transports and electrochemical reactions. Combining these results with Flory's theory for the swelling of ionic gels, we explain the above phenomena. We predict that the swelling behavior is governed by the concentration of the dominant ions and that the swelling speed is proportional to the square of the electric current.

SYNOPSISBending of poly( vinyl alcohol) hydrogel mixed with poly (sodium acrylate) chains, PVA-PAA gel, under the influence of dc electric fields was studied. The PVA-PAA gel was prepared by repeatedly freezing and thawing a mixture of PVA and polyacrylic acid aqueous solutions. The PVA-PAA gel was a hydrogel with the PAA chains, which were entangled with the PVA polymer network and were fixed in the gel. The PVA-PAA gel bent toward the negative electrode in electrolyte solutions under dc electric fields as did the polyelectrolyte gel with negatively charged polyions. The PVA gel, free of PAA, was insensitive to dc electric fields. The deflection of the bending and the bending speed were influenced by the field intensity, the concentration of the polyion in the gel, and the thickness of the gel.The bending of the PVA-PAA gel was qualitatively explained by a bending theory of polyelectrolyte gel, based upon the change of the osmotic pressure due to the ion concentration difference between the inside and the outside of the gel.

A phase transition of spheres of ionized N-isopropylacrylamide gels with diameters ranging from 0.2 to 1 fim has been observed with photon correlation spectroscopy. The submicron gels undergo a sharp, but continuous volume phase transition in water in response to temperature change. The transition temperature depends on the degree of ionization of the gel and is approximately 34 "C for a nonionic gel and 38 "C for a gel with 4.5% ionizable groups. The volume change at transition increases with ionization of the gel. Larger size gels made by the same emulsion polymerization method are studied under the microscope for comparison. The light scattering and microscopic data show excellent agreement.

The photophysical properties of tetra-tert-butylphthalocyaninatosilicon (SiPc) covalently linked to one or two 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radicals (R1, R2) have been studied by fluorescence, transient absorption, and time-resolved electron paramagnetic resonance (TREPR) spectroscopies. It is found that the fluorescence quantum yields and lifetimes of R1 and R2 decrease compared with those of (dihydroxy)SiPc ((dihydroxy)SiPc = 6.8 ns, R1 = 4.7 ns and 42 ps, and R2 = 4.7 ns and <30 ps). Transient absorption measurements indicate that the lifetime of the excited triplet SiPc is markedly dependent on the number of linking TEMPO radicals ((dihydroxy)SiPc = 500 micros, R1 = 7.6 micros, and R2 = 3.7 micros). These short lifetimes of R1 and R2 in the excited states are explained as a result of the interaction with TEMPO changing the ISC between the singlet and triplet states to spin-allowed transitions. Quantitative TREPR investigations have been carried out for the radical-quartet pair mechanism of R1 and the photoinduced population transfer of R2. It is determined that the rise and decay times of these electron spin polarizations denote the spin-lattice relaxation time of the ground state and the lifetime of the excited multiplet state, respectively. This study contributes not only to an elucidation of radical-chromophore interactions but also to a novel approach for controlling magnetic properties by photoexcitation.

The present study examined the effect of applying a pulsed electromagnetic field (PEMF) on bone formation around a rough-surfaced dental implant. A dental implant was inserted into the femur of Japanese white rabbits bilaterally. A PEMF with a pulse width of 25 microseconds and a pulse frequency of 100 Hz was applied. PEMF stimulation was applied for 4 h or 8 h per day, at a magnetic intensity of 0.2 mT, 0.3 mT or 0.8 mT. The animals were sacrificed 1, 2 or 4 weeks after implantation. After staining the resin sections with 2% basic fuchsin and 0.1% methylene blue, newly formed bone around the implant on tissue sections was evaluated by computer image analysis. The bone contact ratios of the PEMF-treated femurs were significantly larger than those of the control groups. Both the bone contact ratio and bone area ratio of the 0.2 mT- and 0.3 mT-treated femurs were significantly larger than the respective value of the 0.8 mT-treated femurs (P < 0.001). No significant difference in bone contact ratio or bone area ratio was observed whether PEMF was applied for 4 h/day or 8 h/day. Although a significantly greater amount of bone had formed around the implant of the 2-week treated femurs than the 1-week treated femurs, no significant difference was observed between the 2-week and 4-week treated femurs. These results suggest that PEMF stimulation may be useful for promoting bone formation around rough-surfaced dental implants. It is important to select the proper magnetic intensity, duration per day, and length of treatment.

SYNOPSISDeformation of poly (vinyl alcohol) -poly (sodium acrylate) composite hydrogel ( PVA-PAA gel) under sinusoidally varying electric fields was studied in electrolyte solutions. The PVA-PAA gel was prepared by repeatedly freezing and thawing a mixed solution of PVA and polyacrylic acid. A cyclic bending-straightening motion of the PVA-PAA gel rods of about 1 mm in diameter have been observed in Na2C03 aqueous solutions under the fields. The PVA-PAA gel had a response time of less than several hundreds milliseconds. The bending has also been observed in organic solvents containing a n electrolyte when the organic solvent is electrolyzed. It was found that the motion of the gel under electric fields of less than 1 Hz occurred mainly through swelling due to the change of the osmotic pressure based upon the difference of the ion concentration. However, it has not been determined whether the motion at higher frequencies is caused by the osmotic effect. 0 1993