Suzana Popovic

Pores and diffusion characteristics of porous gels

Abstract A simple porous gel synthesis method is proposed in this paper. We found that the sequential modification of conventional radical polymerization gives rise to pore formation in acrylamide gels and investigated the parameters that are responsible for controlling the number and the size of pores. Pore formation is dominated by the amount of polymerization initiator (ammonium persulfate), accelerator ( N , N , N ′, N ′-tetramethylethylenediamine) and polymerization temperature. The initiator works as a pore nucleus. The accelerator helps the pore growth. However, temperature control is the easiest and the most effective way to create a number of large pores. Especially if the pregel solution is heated to or above 80°C, the pore formation is effectively promoted resulting in an increase in the number of pores and their size. Increase in the volume fraction of macro pores was found to result in the increase in the diffusion coefficients of the solution flowing through gels.

Mechanical testing of hydrogels and PAN gel fibers

Uniaxial and biaxial mechanical tester was developed tp perform basic test for characterization of soft, low strength polymer gels. We have considered stress/strain relationship of acrylamide-based and polyacrylonitrile (PAN) fiber and sheet gels. Acrylamide-based gels were tested uniaxially, where Youngs modulus estimated varies between 10 and 25 kPa. Uniaxial and biaxial tension test were performed on PAN sheets, from which Youngs modulus was estimated at 6.65 Mpa, which is an extraordinarily large strength than the conventionally investigated acrylamide- based gels. Its length change in response to EtOH/H2O solution exchange proved to be not small as an actuator material, and it was observed that PAN exhibited a deformation in response to the electric field. Two types of PAN gel fibers were tested as a bundle of 50 fibers. True stress-strain data were obtained showing similar non-linear trend in both case. Statistical methods were used to assess fiber strength distribution.

Electroactive nonionic poly(vinyl alcohol) gel actuator

A fast (0.8sec), large (>10%) and reversible deformation of Poly(vinyl alcohol) gel (PVA) swollen with dimethyl sulfoxide (DMSO) upon electric field was realized, and the maximum observed strain reached quite high 27% (DP2100). St-PVA gels were found to exhibit more stable and reversible deformation than at-PVA gels. Furthermore, we have studied the macroscopic structure of PVA/DMSO gels and its influence on their strain exhibition. Four differently structured PVA/DMSO gels, monolayer, circular, triple-layered and porous, were prepared. Monolayer gel exhibited the highly reproducible strain behavior. The mechanism of electric actuation of PVA/DMSO gel is proposed. Then a design of gel mechanical switch is shown, which exhibited the fast response to the electric field with a large, stable and reversible stretching deformation.

Influence of the thermal treatment applied to PAN gel on its length change and generated force

PAN gel is known for its strong matrix as well as for its fast length change by the acid-base environmental solution exchange. Besides, PAN gel is a quite soft material like a real cell. Therefore its been regarded as a most promising material as an artificial muscle. However, its matrix strength declines extremely unfortunately in basic solution. Its matrix should be improved so as not decline, otherwise it cannot be an artificial muscle for practical use. We applied a high temperature thermal treatment and a subsequent hydrolysis to PAN gel prepared through the nearly conventional processing method, and we investigated the time dependence of its length change ratio and generated force through the acid-base solution exchange, and its durability. Although its length change and force generation performances were impaired to some extent, we found an improvement of its matrix robustness and durability.

Photo- and electroactive color changeable acrylamide gel actuator

Polyacrylamide hydrogels containing bis-[4- {dimethylamine}phenyl]{4-vinyl-phenyl}methyl leucohydroxide which is so called vinyl derivative of Malachite Green have been studied as color changeable gels. The response times of the color and the volume changes of the gel were measured under 6 and 2 different stimuli, respectively. We found a way to increase their color change speed upon applied electric current (E-current), and designed a gel actuator using Nafion film as a separator between two compartments and as a cation conductor. In addition acrylamide gel swollen with Na2SO4 solution was used as a medium for increasing electric conductivity. We varied the concentration of dvMG in the gel to control the degree of color change. Furthermore, we have studied the influence of gel thickness on the color change rate. In light of the results obtained, we have proposed one device consisting of this color changeable gel.

Phase transition behavior of an amphoteric polymer gel

In order to characterize the deformation behavior of polymer gels for actuators, spatial distribution of deformation of anionic, cationic and amphoteric gels under the electric field was measured. Amphoteric gel was found to be a promising material for inducing symmetric deformation, compared with anionic and cationic gels. It was also found that the deformation of electro-active gels was mainly attributed to interfacial phenomenon between gel-electrode. It can be concluded that by the use of as many amphoteric gel-electrode interfaces as possible will provide us with electro-active polymer gels that are fast responsive, largely deformable with symmetric deformation mode.