Prevalence of patterned microstructures in interpenetrating polymer networks is well documented experimentally. Thermodynamic theories explaining this feature nonetheless have not been forthcoming. These enmeshed structures carry elastic energy and elevate in the thermodynamic limit the free energy of mixing. Yet why is the deviation from a supposed macroscopic phase separation? This confounding question needs to be answered; and in this paper a thermodynamic explanation is offered. Crosslinking is shown to reduce the effective driving force for macrophase separation. Long-range free energy of deformation gives rise to sinks in the phase evolution, suppressing long-wavelength (hydrodynamic mode) composition fluctuations which usually implode during spinodal decompositions, prodding macrophase separations. Consequently, nonequilibrium metastable structures with small phase domains would evolve. This study highlights lineaments of pattern in physical systems: a competition between short-and long-range forces or a time evolution of nonconserved order parameters or both. A new free energy of mixing is formulated to elucidate the structural novelties.