The subject invention pertains to heteromorphic polymer compositions characterized as comprising: (a) a homogeneous linear or substantially linear ethylene/.alpha.-olefin interpolymer backbone; and (b) a branch appending from the backbone, which branch comprises an ethylene homopolymer or ethylene/.alpha.-olefin interpolymer having a density which is at least 0.004 g/cm.sup.3 greater than that of the backbone. At least one of the backbone polymer or the branch polymer may be optionally functionalized to promote adhesion to polar surfaces. The heteromorphic polymer compositions of the invention exhibit enhanced upper service temperature. Also disclosed is a process for preparing the heteromorphic polymer compositions of the invention.