Classically, stress and strain rate in linear viscoelastic materials are related by a constitutive relationship involving the viscoelastic modulus G(t).The same constitutive law, within Linear Response Theory, relates currents of conserved quantities and gradients of existing conjugate variables, and Polish it involves the autocorrelation functions of the currents in equilibrium.We explore the consequences of the latter relationship in the case of a mesoscale model of a block copolymer, and derive the resulting relationship between viscous friction and order parameter diffusion that would result in a lamellar phase.We also explicitly consider in our derivation the fact that the dissipative part of the stress tensor must be consistent with the uniaxial symmetry of the phase.We then obtain a relationship between the stress and order parameter autocorrelation functions that can be interpreted as an extended constitutive law, one that offers a way to determine them from sword stand microscopic experiment or numerical simulation.