Vertically oriented nanoporous cylinders, demonstrating an unprecedented alignment persistence, were produced within freestanding poly(1,1-dimethyl silacyclobutane)-block-polystyrene-block-poly(2-vinyl pyridine) (PDMSB-b-PS-b-P2VP) layers (~15 μm thick) blended with short PS-b-P2VP chains by combining the non-solvent induced phase separation (NIPS) process with a solvent vapor annealing (SVA) treatment. Here, the NIPS step allowed for the formation of an asymmetric and porous PDMSB-b-PS-b-P2VP film having a top surface exhibiting poorly-defined nanopores while the subsequent SVA treatment enabled to produce a symmetric layer that possesses highly-ordered cylindrical nanodomains arranged into a 27 nm period square array. As the unblended NIPS/SVA-made PDMSB-b-PS-b-P2VP monoliths exhibited a mixed orientation of parallel and perpendicular cylinders, a blending strategy was used to achieve tetragonally-packed PDMSB and P2VP nanodomains having an exceptional vertical alignment persistence. Such solvent-annealed (3 h, CHCl3) PDMSB-b-PS-b-P2VP monoliths blended with 20 wt% of PS-b-P2VP chains showed a water permeance close to the value measured through their parent NIPS-made terpolymer films having poorly-ordered nanopores.