  We study the phase transitions of a random copolymer chain with quenched
disorder. We apply a replica variational approach based on a Gaussian trial
Hamiltonian in terms of the correlation functions of monomer Fourier
coordinates. This allows us to study collapse, phase separation and freezing
transitions within the same mean field theory. The effective free energy of the
system is derived analytically and analysed numerically. Such quantities as the
radius of gyration or the average value of the overlap between different
replicas are treated as observables and evaluated by introducing appropriate
external fields to the Hamiltonian. We obtain the phase diagram and show that
this system exhibits a scale dependent freezing transition. The correlations
between replicas appear at different length scales as the temperature
decreases. This indicates the existence of the topological frustration.
