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" psi (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 sigma-due to swelling. If t is larger than T(S) (the reptation time of the short chains), psi 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 psi approximately N(e)/ s (t) approximately t-1/2. The width of the mixed region e(t) increases like s 1/2 (t) at t < 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).