A polymer composition including a functional material, the composition containing at least a block polymer encapsulating a material of a predetermined function and a solvent, wherein a property of the polymer in the composition is changed in response to a stimulus, whereby the block polymer encapsulating the material agglomerates together, or an ink composition usable as an inkjet ink containing at least a block polymer encapsulating a coloring material and a solvent, wherein a property of the polymer in the composition is changed in response to a stimulus, whereby the block polymer encapsulating the coloring material agglomerates together. The change of the property of the block polymer in response to a stimulus is a change from the lyophilic nature to the lyophobic nature, or the lyophilic nature to the lyophobic nature.