Synthesis and dilute solution properties of styrene-isoprene diblock copolymers with mesogenic-zwitterionic end groups

Diblock copolymers of styrene and isoprene with an ionic-liquid crystalline end group were synthesized by anionic polymerization high-vacuum techniques. Using (3-dimethylamino)propyllithium as initiator, the dimethylamino group was introduced at either chain end. This group was converted to a zwitterionic-liquid crystalline end group by reaction with 2-{6-[4-(4-methoxyphenylazo)phenoxy]-hexyloxy}- 2-oxo-1,3,2λ⁵-phospholane. The dilute solution properties of the diblock copolymers, containing approximately 30 wt % PS, were studied in CCl₄, a nonpolar equally good solvent for both the polymeric blocks, by means of static and dynamic light scattering and viscometry. Static light scattering experiments indicate that dimers are formed in most cases, except for copolymers with M_w≤12 000. The estimated aggregation numbers were found to be lower than in the case of sulfozwitterionic end-capped copolymers. It seems that the lower polarity of the nitrogen-phosphorus zwitterion and the increased steric hindrance of the mesogenic group attached to the zwitterionic group are mainly responsible for this behavior. Because of the presence of a liquid crystalline group and the polymeric tail directly bonded to the zwitterion, the formation of elongated aggregate structures is difficult in the studied systems as was observed in sulfozwitterion end-functionalized diblocks. Excluded-volume effects play also some role in the dependence of aggregation number on the molecular weight of the polymeric tails. The results from dynamic light scattering and viscometry agree well with the static light scattering picture if the star model is used to derive aggregation numbers from data on hydrodynamic radii of the end-functionalized copolymers.