Figure 3: Distribution of relaxation times and interfacial conformations.

The impact of adsorption on the deviations from bulk behaviour can be rationalized in terms the dimensionless parameter t*, corresponding to the ratio between the annealing and the adsorption time. The molecular picture corresponding to the perturbation in the dynamics on adsorption is given in (a–c). Up to t*∼1, adsorption proceeds via gradual filling of the available pinning sites at the interface, which provokes a decrease of the free volume in between molecules and causes the reduction in the population of faster modes. In the latest stages of adsorption, t*>1, the screening of previously adsorbed chains permits adsorption only on reorganization and pinning at fewer and fewer surface sites; the corresponding lower train fraction (percentage of sequences of consecutive monomers in direct contact with the surface) yields a reduction of the slower modes. Experimental validity of this molecular picture was achieved by analysing the distribution of relaxation times of the dielectric spectra of a 44-nm-thick film of PS97 during annealing at 423 K, at different adsorption conditions, t*=0.04 (d), t*=0.7 (e), t*=4 (f). Compared with short annealing times (t*≪1, d), adsorption induces a pronounced cutoff of the faster relaxation modes, after moderate annealing times (t*∼1, e). Further annealing (t*>1, f) induces a reduction in the polymer segments relaxing at slower characteristic times.