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As the first step in a series of works on the mechanical behavior of a concentrated hydrogel of agar-agar, the experimental determination of its elastic behavior in compression was made. An apparatus which could record a compression stress-strain curve automatically was devised and used to obtain the results reported in this paper. Influences of tem-perature, concentration, and gelation time upon the modulus of elasticity (Young’s) were considered and plotted in Figures 4, 5 and 6.

As the third report of the series of papers concerning the viscoelastic behavior of concentrated hydrogels of agar-agar, the results of an experimental investigation of the relaxation of stress in an agar-agar gel of a concentration 4 g./100 cc. have been presented, which are summarized as follows:(1) The general type of the relaxation curves obtained may be seen in Figure 2, in which particular attention must be drawn to the behavior that the plotted curves fall across the zero stress base after running certain finite intervals of time. An adequate interpretation of this has been presented in the last section of the text.(2) As was illustrated in the second paper of this series, an agar-agar gel block decreases spontaneously and gradually its volume under the experimental conditions adopted in this series of experiments. Taking into account he spontaneous volume contraction, a system of equations for representing the viscoelastic deformation of an agar-agar gel column subject to compressive load has been presented, in which as a mechanical model of the material two Maxwell relaxation elements connected in parallel have been assumed.(3) All the data of relaxation obtained have been considered in the light of this set of equations. In this way, the numerical results listed in Table 1 have been obtained.(4) The activation energy for relaxation has been calculated for each individual mechanical element by the use of the Arrhenius equation. It has been found that the activation energies so calculated are constant below about 40°C. for both elements, amounting to nearly equal values, the average of which is estimated at about 9 kcal./mole. Above 40°C., for both relaxation elements, deviations from the Arrhenius equation appeared, which are gradually marked with increase in temperature. We have ascribed these deviations to the decrease in the frequency factor in the Arrhenius equation.(5) Finally, the set of equations presented above has been solved for the case of compression creep under constant load, resulting in a creep equation coinciding exactly in form with the empirical creep equation derived in the previous paper II of this series. In addition to this formal agreement of the present theoretical and the previous empirical creep equations, quantitative agreement of them has-been demonstrated by showing that the viscosity values from the previous creep test can be well reproduced by the sum of the individual viscosity values obtained from the present relaxation test.

In this report, we have presented the results of an experimental investigation carried out of the creep under constant compressive load for concentrated hydrogels of agar-agar.The apparatus which was designed in a previous paper I of this series for the measurement of the compression stress-strain curve was modified to suit the measurement of the creep under constant compressive load. The shape of the creep curves obtained was found to be well fitted by Eq. (1). The influences of temperature, concentration and applied stress on the creep rate in the viscous flow region were investigated separately. One of the interesting facts found was a clear evidence showing that even at zero stress condition there occurred a definite creep process which was closely followed by a straight line within the interval of time of observation employed. It seemed likely that this phenomenon should be associated primarily with the molecular structural feature of the agar-agar gel network which leads the gel to syneresis.It was found for all the data obtained that the relation between λ and Sfollowed Eq.(2) within the limits of experimental error, where λ is the creep rate in the viscous flow region and Sis the magnitude of stress applied. This equation involves two parameters λ0and η, where λ0stands for the value of λ in the absence of external stress, and η is the Newtonian viscosity corresponding to the region in which a linear viscous flow appears. The influences of various environmental parameters on these two quantities were investigated, obtaining the results as shown in Figures 3, 4 and 5.