The diffusion theory attributes the adhesion of polymeric materials to the
inter-penetration of chains at the interface.
The major driving force for polymer autohesion and heterohesion is due to
mutual diffusion of polymer molecules across the interface . This theory requires that both the adhesive and adherend are
polymers, which are capable of movement and are mutually compatible and
miscible. To describe the self-diffusion phenomenon of polymers, several
theories have been proposed: entanglement coupling , cooperativity , and reptation . The reptation model  has been applied to study tack, green strength, healing, and
welding of polymers.
Parameters affecting the diffusion process are:
contact time, temperature, molecular weight of polymers and physical form
(liquid, solid). Polarity generally increases adhesion.
Some evidence may has demonstrated that the interdiffusion phenomenon exists in
mobile and compatible polymers and may promote the intrinsic adhesion. The
diffusion theory, however, has found limited application where the polymer and
adherend are not soluble or the chain movement of the polymer is constrained by
its highly crosslinked, crystalline structure, or when it is below its glass