Unambiguous Evidence for a Highly Mobile Surface Layer in Ultrathin Polymer Films by Specific Heat Spectroscopy on Blends

Abstract

Despite the decade long controversial discussion on the effect of nanometer confinement on the glass transition temperature (T g ) of ultrathin polymer films, there is still no consistent picture. Here, the dynamic calorimetric glass transition of ultrathin films of a blend, which is miscible in the bulk, is directly investigated by specific heat spectroscopy. By a self-assembling process, a nanometer thick surface layer with a higher molecular mobility is formed at the polymer/air interface. By measuring the dynamic calorimetric T g in dependence on the film thickness, it was shown that the T g of the whole film was strongly influenced by that nanometer thick surface layer, with a lower T g . Since the observed thickness dependence of the dynamic T g is similar to the thickness dependence of the T g for thin films of homopolymers, it is concluded that also for homopolymer a highly mobile surface layer is relevant for the widely observed T g depression.

Complex nonequilibrium dynamics of stacked polystyrene films deep in the glassy state

“…15) and acceleration of equilibrium recovery in the physical aging regime, 16,17 appear at film thicknesses considerably larger than those where alterations of the rate of spontaneous fluctuations are observed. [18][19][20] Such decoupling has been observed measuring simultaneously the rate of spontaneous fluctuations and the thermal T g . [21][22][23][24] Additional indications can be provided by experiments where the same perturbation is applied in the linear and non-linear regimes, for instance in calorimetry where the perturbed magnitude is the entropy.…”

“…27,28 In any case, effects on the rate of spontaneous fluctuations in nanostructured polymers are limited to the very first layers close to the interface. [18][19][20] Conversely, deviations of the nonequilibrium dynamics can be visible for length scale of nanostructuring as large as microns, as shown in films 17,[29][30][31][32][33] and polymer nanocomposites. [34][35][36][37] A plausible explanation to the more pronounced effect of reducing film thickness on the nonequilibrium dynamics relies on purely geometric arguments, in particular on the role of "free" interfaces.…”

Fluid-like Surface Layer and Its Flow Characteristics in Glassy Nanotubes

“…For instance, in glassy polymers, extensive studies have shown the presence of a highly mobile surface layer on the order of tens of nanometers thick. [19][20][21][22][23][24][25][26] In the case of glassy (or amorphous) inorganic materials, whether or not a similar surface layer exists, however, is largely unexplored. A prior study has indicated such a possibility through imaging the motion of surface atoms in ultrathin SiO 2 films 27,28 , but its effect on the dynamic mechanical properties is not known.…”

“…Nanoscale glassy films are ubiquitous and critical components for a variety of electronic and energy devices such as optical coatings, − amorphous oxide transistors , and gate dielectrics, − nonvolatile solid state memory, , memristors, − solid-electrolyte interphase layers and anodes , in batteries, and amorphous Si solar cells. , The mechanical properties of such amorphous materials are practically important in system design and issues can arise when shrinking them down to nanoscale dimensions. For instance, in glassy polymers, extensive studies have shown the presence of a highly mobile surface layer on the order of tens of nanometers thick. − In the case of glassy (or amorphous) inorganic materials, whether or not a similar surface layer exists, however, is largely unexplored. A prior study has indicated such a possibility through imaging the motion of surface atoms in ultrathin SiO 2 films, , but its effect on the dynamic mechanical properties is not known.…”

Adsorption Kinetics of Polystyrene and Poly(9-anthracenyl methyl methacrylate) onto SiO2 Surface Measured by Chip Nano-Calorimetry

“…The procedure of “Guiselin’s experiment” provides the adsorbed layer by annealing and subsequent solvent washing [ 13 ]. Many experiments have shown that the residual adsorbed layer thickness depends on the annealing time at an elevated temperature [ 14 , 15 , 16 , 17 , 18 , 19 ]. Dielectric spectroscopy provided real-time measurements of adsorption kinetics in a buried interface with an aluminum substrate [ 20 ].…”