The ability to force immiscible polymers into specific, targeted structures would enable the generation of blends with tailored performance by exploiting the intimate relationship between structure and blend properties. Here we present a strategy for the forced assembly of immiscible polymers into targeted structures via development of a planar polymer micro-mixer (PPMM). The PPMM drives streams of molten polymer through mixing chambers, which are fabricated from metal shims that contain flow channels. By stacking the shims, complex 3D mixing flows can be generated. The advantages of this mixing technology include sample sizes significantly less than traditional micro-mixers (<100 mg), simple reconfiguration of the flow geometry, and optical access to the flow. Most significantly, it offers a path towards targeted blend structures rather than the more typical domain/matrix or random co-continuous ones. We observe the creation of multi-layers and coaxial cylinders in the first five mixing units, beyond that that interfacial tension and non-ideal flow tends to force the creation of mixed domain/matrix structures. The PPMM, along with the recently developed multi-sample micro-slit rheometer, is expected to be a key component of the "polymer processing lab-on-a-chip". Published by Elsevier Ltd.