Mechanical properties of two polymers in a blend or alloy would depend upon their compatibility or the level of interfacial forces. High adhesion can only be obtained if the interface can sustain sufficient stress to induce dissipative forms of deformation such as flow, yield or crazing in the polymer. Under most circumstances, such deformation modes can only be obtained when the interface is coupled with a sufficient density of covalent bonds, together with perhaps some toughening effects due to surface roughness.

Most thermoplastics gain their strength from entanglement between the chains. The polymer chains form an entangled network that cannot pull apart when the material is in the glassy or semi-crystalline state. Instead, under stress, the whole network deforms and then strain hardens as the chains become stretched. Fracture requires scission of these covalently bonded chains. To form strong polymer-polymer adhesion it is necessary for the network to be continuous across the interface. This continuity can be formed by:
* Inter diffusion of chains if the two polymers are sufficiently miscible
* Use of coupling chains placed at the interface
* Chemical reaction to form coupling chains at the interface

Di block copolymers are used commercially for the coupling between phases within polymer blends.
The form of coupling has been found to depend on the molecular weight of the coupling chains. Short chains can pull out of the bulk material at a force that increases with the length of the pulled-out section. As the length of the chains increases to somewhere between one and four times, the length required to from an entanglement in the melt, the force required for pullout, becomes greater than the force to break chains, so they fail by scission. The extent of adhesion is strongly affected by the molecular failure mechanism as tough interfaces are normally only obtained when the failure is by scission. Scission failure alone does not ensure a tough interface.

A common technique to couple two bulk polymers is to introduce into one or both of the materials a small percentage of chemically modified chains that can react with the other polymer to form coupling chains at the interface. A classic example of this technique is the introduction into Polypropylene of some Maleic anhydride grafted Polypropylene chains to induce coupling with a Polyamide such as Nylon 6. The maleic anhydride functionality can react with hydroxyl end groups on the Polyamide chain to form a graft or block copolymer at the interface. The molecular mechanism of interfacial failure can again be either pullout or scission of these coupling chains. Pullout at low force can occur if either one of the blocks in the copolymer formed at the interface is rather short or if there are too many coupling chains at the interface.
In the Polyamide-Polypropylene example, where typically there are very few grafts per Polypropylene chain, pullout is only expected if the grafted Polypropylene chain is rather short which can easily happen as the grafting process tends to cause scission. The Polyamide chain is typical of bulk material and so has a molecular weight well above the entanglement molecular weight to give the material a useful cohesive toughness.
In other systems, when multiple grafts are possible on a single chain, then the coupling chains can become so densely packed at the interface that they exclude other chains and so cannot entangle well with the bulk material, causing pullout failure. This situation has been observed when the chains themselves were long enough to entangle.

Coupling by chain inter diffusion can occur if the two polymeric materials are miscible in each other, or at least sufficiently miscible to form a broad interface. Welding is the most common form of inter diffusion but chain inter diffusion is also important in solvent bonding.