Macrolattice formation in amorphous associating polymers

We report on high-resolution synchrotron x-ray scattering studies of melts of a series of linear polyisoprenes with highly polar sulfozwitterion groups at one end. Studies were carried out for six different molecular weights between 2200 and 28 000. For low molecular weights between 2200 and 4650, we observe a lattice with the symmetry of a triangular array of cylinders. The lattice ordering is strong; the nominal domain size exceeds 5000 Å. A structural phase transition to a cubic (bcc) lattice with long-range order is observed to occur for melts with molecular weights between 4650 and 14 000. The lattice spacing increases over the molecular range between 83 Å (for ${\mathit{M}}_{\mathit{w}}$=2200) and 224 Å (for ${\mathit{M}}_{\mathit{w}}$=28 000). In contrast to usual block copolymers, for these copolymers the long-range order sets in at extremely small volume fraction of ionic species, less than 1%.