Poly(styrene-b-paramethylstyrene) block copolymer samples have been prepared in a mechanical press, cooled under pressure below the glass transition temperature and investigated by small angle neutron scattering. We observe a microphase separation transition from an orderd to a disorderd state with ascending pressure. The observed effect is not expected solely due to pressure, but the situation in the press is more complicated : (i) Ascending pressure leads to an increase of the glass transition temperature which slows down relaxations and hinders the evolution of thermodynamic equilibrium, (ii) the uniaxial movement of the piston leads to an uniaxial shear field and samples get oriented. An adiabatic heating during the pressing procedure has been found to be of minor influence. The situation is discussed with respect to common sample preparation conditions and processing.