Polymers often show an inhomogeneous distribution of their mechanical properties. In adhesive layers these inhomogeneities result from the formation of interphases close to the substrates. In addition, chemical aging processes are driven by diffusive processes entering the specimen from the boundaries and changing the local properties of the polymer. In the present contribution a model is presented that takes into account the local effects of the interphase's formation or of the aging processes, respectively. The model is based on a phenomenological continuum mechanical approach, i.e., the effects are described by the introduction of an additional field in combination with an additional balance equation and corresponding boundary conditions. In the first part of this paper the formation of interphases is investigated. Comparing the model with experimental results allows for the identification of the model parameters. In the second part the model is enhanced to describe effects due to chemical aging. Some illustrative examples show the general behaviour of the model and motivate a series of experiments allowing for the required parameter identification.