We study the influence of supercritical carbon dioxide (scCO(2)) on the phase behavior of a cylinder-forming polystyrene-block-polybutadiene-b-polystyrene triblock copolymer thin film. Solvent annealing with scCO(2) can produce patterns with long-range order but these structures become unstable for thin films with small thicknesses. These results are in good agreement with self-consistent mean field calculations, which indicate that a drying transition occurs for thicknesses below the radius of gyration of the molecule. After decompression and solvent extraction, the initially swollen polymer nanostructure suffers a strong reduction in the average domain spacing, which has a deleterious effect on the degree of order in the resulting pattern. Both, experiments and Cahn-Hilliard simulations suggest that during decompression the pattern suffers an order-order instability where the collapse of the lattice constant leads to uncommon patterns with long-range orientational order but structural distortions at small-length scales.