DIFFUSION AT POLYMER POLYMER INTERFACES PROBED BY RHEOLOGICAL TOOLS/

Diffusion at the polymer/polymer interface was probed by small-amplitu de oscillatory shear measurements carried out on polystyrene (PS)/poly styrene (PS) sandwich-like assembly as a function of the time of weldi ng in the molten state. It was found that the dynamic complex shear mo dulus G(t) at a fixed frequency increases with the time of contact in two time regimes. First G(t) increases proportionally to t(1/2) and then a second regime takes place where G(t) increases proportionally to t(1/4) At longer times, G(t) tends asymptotically toward G* of pur e polystyrene. The results were interpreted in terms of reptation theo ry and the time of transition between the two scaling law regimes was found to be in agreement with the time needed for the transition from the Rouse mode to the reptation mode. Special attention was given to t he initial state of the polymer surfaces before contact by performing experiments on (i) freshly prepared surfaces, (ii) presheared samples, (iii) fractured samples, and (iv) corona-treated samples. The results showed that the diffusion mechanism is strongly dependent on the init ial chain-end distribution at the surface before contact. Diffusion at surfaces with an excess of chain ends proceeds in a Rouse-like mode, whereas for surfaces without excess of chain ends, the diffusion proce eds in a reptational-like dynamics, as was predicted by de Gennes and by Prager and Tirrell.