Head-Head Interactions in Zwitterionic Associating Polymers

Theoretical calculations are presented, both at the molecular mechanics empirical level and at the Hartree-Fock semiempirical level (MNDO), on molecules containing (ammonioalkyl)sulfonate zwitterions as models for semitelechelic associating polymers. In particular the interactions between the zwitterionic head groups and the evolution of the energetics and structures of the molecules are examined as a function of their aggregation. The clustered zwitterions adopt an extended (all-trans) conformation with their dipoles aligning in an antiparallel fashion in order to favor interchain electrostatic interactions. For large aggregation numbers in apolar solvents, associating polymers with zwitterionic head groups are predicted to form reverse micelles that possess either a tubelike or a disklike structure. Spherical structures are not energetically favorable. The excluded-volume interaction of the long hydrocarbon tails in the semitelechic polymers is expected to favor the tubelike structure.