The organisation and mechanical effectiveness of a series of polystyrene-block-hydrogenated polybutadiene (PS-b-hPB) diblock copolymers (where the hPB block is semicrystalline) when placed as thin layers between the immiscible polymers, polyethylene (PE) and polystyrene (PS), has been explored using neutron reflectivity, transmission electron microscopy and mechanical peel tests. For thin copolymer layers (expected to leave the interface unsaturated) the trend in variation of interfacial strength with copolymer molecular weight was found to be similar to that previously reported for amorphous copolymers. However, for thick layers (oversaturating the interface) the interfacial strength became in one case too large to separate the layers in the peel test. This may suggest a different mechanism for increasing interfacial strength when at least one of the copolymer blocks is crystallisable.