1983 Volume 1983 Issue 9 Pages 1254-1259
Rheological properties of hydrogels of poly (vinyl alcohol) (PVA) (frozen at -10°C and -20°C, and melted at 15°C every 24 h) were examined by the measurement of longitudinal vibration. The freezing time was chosen as 24, 48, 72 and 96 h. The degree of polymerization (DP) and the degree of saponification (DS) of the PVA were 500, 1000, 1700, 2000, 2400 and 98.5±0.5, 99.6±0.3 mol%, respectively. Measurements were done for the PVA solutions of 5, 7, 10, 15, and 20 wt%. The dynamic Young's modulus, E′, and the mechanical loss tangent, tan 8, were obtained at 2.5 Hz by longitudinal vibrations of a cylindrical gel. The E′ increased both increasing freezing time and with decreasing freezing temperature. The E′ increased steeply with increasing DP up to about 1000 and became almost constant in the range of 1700 to 2400. The effect of DS on the E′ was remarkable: the E′ decreased drastically with the decrease of DS of only about 1 mol%. The effects of the concentration and melting temperature on the E′ of PVA gels were enhanced by the increase in the number of steps in the temperature cycle. This phenomenon was explained through the formation of microcrystalline structure within the gels.
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