  Interfaces between lamellar and disordered phases, and grain boundaries
within lamellar phases, are investigated employing a simple Landau free energy
functional. The former are examined using analytic, approximate methods in the
weak segregation limit, leading to density profiles which can extend over many
wavelengths of the lamellar phase. The latter are studied numerically and
exactly. We find a change from smooth chevron configurations typical of small
tilt angles to distorted omega configurations at large tilt angles in agreement
with experiment.
