THE EFFECT OF END-GROUPS ON THERMODYNAMICS OF IMMISCIBLE POLYMER BLENDS .1. INTERFACIAL-TENSION

Interfacial tension is measured for immiscible blends of poly(butadiene) (PBD) and poly-(dimethylsiloxane) (PDMS) as a function of molecular weight and end group type of the siloxane. Blends containing amine-terminated PDMS (PDMS-NH2) show reduction in interfacial tension by as much as 30% compared to blends containing methyl-terminated PDMS (PDMS-CH3). Interfacial tensions for blends of PBD against mixtures of PDMS-CH3 and PDMS-NH2 are linearly dependent on the amine content, indicating an absence of any significant surfactant effect for the lower interfacial energy aminated component. The interfacial tension reduction observed for the amine-terminated systems is therefore attributed to changes in the bulk interactions resulting from end group substitution. These changes are qualitatively explained through binary interaction theory by considering the PDMS as a copolymer of chain backbone and end group units. Interfacial tensions for blends of fixed molecular weight PBD against high molecular weight PDMS (both methyl and amine terminated) are found to follow the relationship: gamma = gamma0 - k/M(n), where M(n) is the number-average molecular weight of the PDMS and gamma0 is a parameter related to the interfacial tension for infinite molecular weight PDMS, which is independent of end group type, while k is a parameter that is found to depend strongly on the end group type, but is a constant for a particular end group. The observed experimental scaling behavior compares favorably with quantitative theoretical predictions proposed by Anastasiadis, Gancarz, and Koberstein, and by Broseta, Fredrickson, Helfand, and Leibler.