Interfacial tensions from drop retraction versus pendant drop data and polydispersity effects.

Interfacial tensions sigma were measured by means of both methods for the following polymer pair: polyisobutylene (PIB 3) plus poly(dimethylsiloxane) (PDMS 152) and poly(dimethyl-co-methylphenylsiloxane) (CoP26*) plus PDMS 48. The numbers after the abbreviation state the molar masses in kilograms; the homopolymers exhibit polydispersities on the order of 2. The reliability of the method of drop retraction is backed up by systematic measurements, which demonstrate that it is possible to study the time evolution of sigma. Because of the free choice of the phases (drop or matrix) and the possibility to vary the overall composition of the system in a wide range, drop retraction yields more information than the pendant drop method. For the present systems both types of experiments yield identical results for the droplets of higher density. Experiments with the inverse blends and at higher volume fractions of the disperse phase demonstrate that the polydispersity of the components plays an important role. In the case of the system PIB 3/PDMS 152 the steady-state interfacial tension at 25 degrees C is 2.25 mN m(-1) if the drop consists of PDMS, but only 1.3 mN m(-1) if it consists of PIB. Furthermore, the time-independent sigma values are attained much more rapidly in the latter case.