Hindered interdiffusion in asymmetric polymer‐polymer junctions

We discuss theoretically the diffuse interface formed when a long (L) polymer is put into contact with shorter chains (S) of the same material (all chains being entangled). At time t shorter than the reptation time T L of the long chains, the L chains behave like a gel swollen by the S chains. The “penetration factor” ψ (i.e. the volume fraction of S near the gel surface) is controled by a balance between the osmotic pressure of the swollen L chains, and the elastic tension ψ due to swelling. If t is larger than T S (the reptation time of the short chains), ψ is of order N e /N S (where N e is the number of monomers between entanglement points, and N S is the degree of polymerisation of the short chains). On the other hand, if t < T S , N S must be replaced by the average number s (t) of monomers of an S chain which have entered the L region, and ψ ∼ N e / s (t) ∼t −1/2 . The width of the mixed region e(t) increases like s 1/2 (t) at T S , and like (D S t) 1/2 (where D S is the reptation diffusion constant of the S chains) at t>T S .