When two immiscible polymers are brought into intimate contact, highly localized mixing of polymer chains creates an "interphase" region. Materials that are entirely interphase are fabricated by forced-assembly using layer multiplying coextrusion to form assemblies of thousands of nanolayers. Analysis of the interphase materials with conventional methods of polymer analysis confirms certain theoretical predictions for the first time. The unique properties of the new interphase materials result from lower tree volume than the constituents.