Measurement of the product of cross diffusion coefficients of ternary polymer solutions (cellulose acetate butyrate/styrene/methyl ethyl ketone) using Soret Forced Rayleigh Scattering method

K. Koba1 and Y. Nagasaka2

1School of Integrated Design Engineering, Keio University, Japan
2Department of System Design Engineering, Keio University, Japan

Keywords: polymer, diffusion phenomenna
property: cross diffusion coefficient
material: cellulose acetate butyrate

We have developed the measurement technique for mass diffusion coefficients in polymer solutions, namely Soret forced Rayleigh Scattering Method (SFRS). This method enables a noncontact and high-speed measurement using laser heating and Soret effect. The fringe patterned laser beams heat a sample to form a temperature distribution and Soret effect induces concentration distribution. Both the temperature and concentration distributions work as an optical phase grating and a probe beam is diffracted on these distributions. By analyzing the intensity variation of diffracted probe beam, the mutual diffusion coefficients can be determined.

In the present paper, we determined the product of cross diffusion coefficients in CAB/Styrene/MEK solutions (CAB: 10-30 wt%, Styrene: 5-85 wt%, MEK: 5-85 wt%) at 25 oC using SFRS. By solving the diffusion equations for ternary solutions with appropriate boundary conditions of SFRS, we determined two time constants of mass diffusion. These time constants relate with the fringe space of concentrations, two main diffusions, and the product of cross diffusion coefficients. We estimated two main diffusion coefficients with mutual diffusion coefficients of binary polymer solutions (CAB/MEK, CAB/Styrene, Styrene/MEK) and determined the product of cross diffusion coefficients. The results showed that the product of cross diffusion coefficients change its sign from positive to negative by the concentrations of Styrene and MEK.

In the next we have improved SFRS in order to be applied for the measurement of two cross diffusion coefficients of ternary solutions respectively. In our new method, the temperature distribution is formed by a moving interference fringe patterned laser beams, and concentration distributions induced by the Soret effect are also moving. Intensity of diffracted probe beam is changed by moving these distributions. The moving speeds of two components are different due to their different diffusion coefficients. We have attempted to obtain signal waveforms of component 1 and 2 which include the information on cross diffusion coefficients.

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