From neutron reflectometry we have obtained the interfacial width (typically 2-8 nm) between incompatible polymers above the glass transition temperatures. The investigated polymer bilayers consist of a deuterated polystyrene top layer and various poly(alkylmethacrylates) as a bottom layer. We obtained interesting effects of the length of the alkyl group on the interfacial width. Using mean-field theory we calculate Flory-Huggins-Stavermann interaction parameters chi from the interfacial width alpha(1) data. These results are compared with investigations of the interfacial width between deuterated poly(methylmethacrylate) and respective polyalkylmethacrylates.