Ultrasonic Velocity and Absorption of Aqueous Solutions of Polyvinyl Alcohol

Abstract

An ultrasonic pulse technique was applied to the rheological study of aqueous solutions of polyvinyl alcohol in the ultrasonic frequency region.Two samples of polyvinyl alcohol were used. The average degree of polymerization of sample A and sample B was 1800 and about 2000, respectively. The content of residual acetyl groups was smaller than 0.7% by mole for sample A and was 12.2% by mole for sample B. The molecular weight distribution curve of sample A was obtained by the successive precipitation method proposed by Spencer.Ultrasonic velocity and absorption were measured over the temperature range of 3∼70°C for the solutions in the concentration range of 5∼15% by weight. The frequency of ultrasound was 1.42Mc/s throughout the experiment.The following results were obtained:In sample A, the temperature dependence of ultrasonic velocity has similar characteristics on the whole to that observed in pure water, and the ultrasonic velocity increases with the increase of concentration. In sample B which contains acetyl groups by 12.2% by mole, however, the behavior is fairly different. In the low temperature region no remarkable difference between A and B is found, but above 40°C it is clearly seen that sound velocity is smaller than that for the solutions of sample A which have the same concentration. The tendency of smaller sound velocity becomes more evident in high temperature region, and the velocity decreases with increase of polymer concentration in sharp contrast to the case of sample A. The velocity-temperature curve of solutions of sample B has a broader peak than in pure water, and the peak becomes lower, and the peak temperature shifts to lower temperature side. For example, in the case of the 10wt.% solution of sample B the peak temperature is about 50°C, whereas the value for pure water is 74°C.The average absorption coefficient in the temperature range of the present experiment increases with increase of concentration in both samples. The data for solutions of sample B are seen to be larger than those for solutions of sample A of the same concentration.The longitudinal elastic modulus M1 and the longitudinal loss modulus divided by angular frequency M2/ω are obtained from the data of sound velocity and absorption.