Characterization of solute-membrane interactions by TMA-measurements

Abstract

On the basis of relationships derived by Oplatka et al. [1] correlating the derivative of tensile stress with respect to temperature for a polymer strip at constant length, (∂σ/∂T)t, with the derivative of entropy with respect to strip length, (∂S/∂l)T, relationships are derived relating the derivation of strip length with respect to temperature under constant tensile stress, (∂l/∂T)σ, with the derivative of entropy with respect to tensile load, (∂S/∂σ)T. The developed relationships are applied to evaluate TMA (thermomechanical analysis) measurements performed with asymmetric and homogeneous cellulose acetate membranes where the membrane strips were kept in deionized water. Moreover, TMA measurements were performed with homogeneous CA membranes while maintaining the membrane strips in bathing solutions of different, but in each instance, constant electrolyte concentrationcs(e.g., LiCl, LiNO3, Li2SO4, CaCl2; 0 ≤cs≤ saturation). Under constant tensile stress, σ, as-cast asymmetric CA membranes, which are kept in deionized water, first elongate with increasing temperature at temperatures up to about 50°C. However, they shrink with a further increase of temperature in the temperature range 50°C up to about 90°C, whereas creeping starts at temperatures beyond 90°C, leading again to an elongation of the strip with a further increase of temperature. The negative apparent coefficient of thermal expansion, existing between 50° to 90°C, is consistent with shrinking phenomena observed with asymmetric CA membranes [2]. In addition, it correlates well with both an entropy decrease with increasing tensile stress at constant strip length and an entropy increase with decreasing strip length at constant tensile stress. Contraction of the strip with increasing temperature at constant tensile load might be due to increasing coil formation of polymer molecules rendered possible by disintegration of joints such as H-bonds between polymer chains. Depending on the electrolyte of the bathing solution and its concentration, homogeneous CA membranes exhibit positive and negative apparent linear coefficients of thermal expansion under constant tensile stress.