We study the range of orientational order of a single layer of cylindrical block copolymer microdomains annealed on several types of substrate. The orientational persistence length or nematic correlation length (xi) is evaluated using recently developed imaging and analysis methods to measure the grain size of the block copolymer microdomains. We show that the substrate can lower xi for block copolymers with a majority component that interacts strongly with the substrate, but this can be mitigated by attaching a buffer layer of polystyrene brushes to the substrate. In addition, we show that, for a block copolymer where the block that strongly interacts with the substrate is the minority component, the microdomain correlation length does not increase when substrates are treated with this buffer layer. We suggest that in this case the brushes do not increase xi not only because of the lower volume fraction of the strongly interacting component but also because there are block copolymer wetting layers at the free and substrate interfaces that decouple the microdomains from the substrate in a similar manner as the polystyrene brushes.