assemblable polymer the primer layer may be provided with gaps not illustrated in the figures to permit one of the block types of the self assemblable layer to come into direct contact with the substrate 21 below the primer layer 22 this may be useful for anchoring or aligning a domain of a particular block type of the self assemblable polymer layer 23 to the substrate 21 the a block of the copolymer is of monomers comprising azobenzene moieties which have a low energy trans and a high energy cis isomeric state the trans isomer may be converted to the cis isomer by irradiation with an appropriate first wavelength of radiation and converted back to the trans isomer by illumination with a different second wavelength of radiation in fig3 b the polymer layer 23 is subjected to radiation 24 such that the monomers of the a blocks are converted into the cis form the monomers of b are such that the monomers of a when in the cis form have a greater chemical affinity for the monomers of b than when they are in the trans form and so the flory huggins parameter for the self assemblable polymer is greatly reduced when the a monomers are in the cis arrangement second molecular arrangement rather than in the trans arrangement first molecular arrangement with the a monomers in the cis arrangement the self assemblable polymer is heated to a temperature above its glass transition temperature tg but below its order disorder transition temperature to d so that the polymer layer 23 may self assemble into domains of type a 25 and type b 26 providing a periodic patterned layer this is shown in fig3 c the polymer layer 23 may be annealed.