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 angstrom. 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 angstrom (for M(w)
= 2200) and 224 angstrom (for M(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%
