In most cases, polymers are not miscible together and when they are mixed, we observe a dispersion of the minor phase (dispersed phase) in the major one (matrix). This phenomena offer many possibilities of modification, properties adjustment and surface modification needed to serve customers needs. However, when the cohesion between the two phases is low - it's true in most cases - mechanical performances of polymer blends are generally low.
This lack of compatibility explain also in some cases why adhesion between polymers is low particularly with polyolefins (no polar groups) and when there is no specific interaction. Preparation of multi-layer products (films, bottles…) or "multi-material" parts can become difficult or even impossible. For plastics reinforcement by fillers or fibres, interfacial adhesion with the polymers is key to get good performances.
For these reasons, formulations containing different polymers or fillers and multi-component finished product requires good interfacial cohesion.