Interfacial Tension

Introduction

Interfacial tension is just another aspect of thermodynamic interaction between the components of a system. In case of polymer/solvent systems typical measurements range from 10-4 - 0.1 mN/m, for polymer/polymer-systems this interval is typically 1-20 mN/m.

The main efforts of our group in this area are:

To study the interrelation of the interfacial tension for polymer/solvent and polymer/polymer systems with other thermodynamic properties, like the Flory-Huggins interaction parameters LitLink.

Fig. 1. The correlation between the Flory-Huggins interaction parameter (fixing the concentration dependence of the Gibbs energy of mixing) and the interfacial tension is performed on the basis of the reduced "hump energy" indicated in this scheme as the green area.

Fig. 2. Interdependence of the interfacial tension σ and the reduced hump energy ε.

Fig. 3 shows the difference between homopolymer and copolymer solutions concerning the interfacial tension: s of copolymer solutions is lower by a given t LitLink.

Fig. 3. Correlation between interfacial tension σ of copolymer containing solutions and reduced temperature τ with τ = (Tc-T)/Tc. For comparison such a dependence is also shown for a typical homopolymer solution.

To analyze the effects of additives (like block-copolymers) on the interfacial tension of polymer blends (influences of the chemical nature and molecular architecture) LitLink.

Fig. 4. Influence of block-copolymer additives consisting of dimethylsiloxane and ethylene oxide on the interfacial tension of poly(dimethylsiloxane)/poly(ethylene oxide) blends at 100 °C; xAdd is the base mole fraction of the additive in the poly(ethylene oxide) phase.

At the moment we examine, if there is any influence on interfacial tension when the additive is given to different phases.

Fig. 5. Temperature dependence of the interfacial tension for the indicated binary polymer blends and ternary mixtures for which the composition of the additive in the PDMS phase is given in wt.-%.

Adding an second polymer to a polymer solution can also lead to the opposite effect: an increase of interfacial tension LitLink.

Fig. 6. Interfacial tension of solutions of PS 96 in CH as a function of the concentration of PIB 87 for 18.3°C; also shown are data points for two other high molecular weight additives indicated in the graph.

Kinetic sudies

To study kinetic aspects of phase behavior with temperature jump experiments in a spinning drop apparatus. It is investigated how phase separated polymer solutions react on a rise in temperature LitLink.

Fig. 7. Schematic phase diagram demonstrating how the temperature jump experiments have been performed in a spinning drop apparatus. The critical value of the volume fractions φ2 of the polymer is indicated by the open circle; the full circles represent the composition of the phases coexisting at the starting temperature T1. The over-all composition of the two-phase system contained in the rotating tube (full square) is shown for a typical case in which the density of the less concentrated polymer solution is lower than that of the higher. At T1 this point is situated inside the two phase region, whereas it is located within the homogeneous regime at T2.

 

Fig. 8. Time dependence of the apparent interfacial tension σ* (obtained from the dimensions of the droplet at different times by means of the equation used to calculate the interfacial tension σ) reduced to σ(T1), and of the volume V of the droplet, reduced to ist volume at T1, after a temperature jump from 20.7°C to 21.0°C. For the present system MeCH/PS 17.5 the droplet consists of the sol phase.