  The mesoscale rheology of a lamellar phase of a block copolymer is modeled as
a structured fluid of uniaxial symmetry. The model predicts a viscoelastic
response that depends on the angle between the the local lamellar planes and
velocity gradients. We focus on the stability under oscillatory shear of a two
layer configuration comprising a parallel and a perpendicularly oriented
domain, so that the two layers have a different viscoelastic modulus
$G^{*}(\omega)$. A long wave, low Reynolds number expansion is introduced to
analytically obtain the region of stability. When the response of the two
layers is purely viscous, we recover earlier results according to which the
interface is unstable for non zero Reynolds number flows when the thinner layer
is more viscous. On the other hand, when viscoelasticity is included, we find
that the interface can become unstable even for zero Reynolds number. The
interfacial instability is argued to dynamically favor perpendicular relative
to parallel orientation, and hence we suggest that the perpendicular
orientation would be selected in a multi domain configuration in the range of
frequency $\omega$ in which viscoelastic contrast among orientations is
appreciable.
