Unusual Deviations from Bulk Behavior in Ultrathin Films of Poly(tert-butylstyrene): Can Dead Layers Induce a Reduction of T_g?

Abstract

The temperature and thickness dependence of the thermal expansivity of ultrathin thin films of poly(tert-butylstyrene) capped between aluminum layers revealed an unusual and intriguing confinement scenario. Below 50 nm, both the glass transition temperature and the thermal expansion coefficients decreased. Such a mixed behavior implies an enhancement of the molecular mobility, without the presence of any free surface, but dead layers. On the basis of a careful analysis of averaged quantities measured by capacitive dilatometry, we deduced a profile of thermal expansivity that explains our experimental data and previous observations in polymer nanocomposites. The effect of density-conformation coupling in proximity of a nonattractive interface allows the coexistence of an immobilized fraction in direct contact with the metal and an excess of thermal expansivity, arising from the long-range effects of packing frustration penetrating inside the bulk-like core of the film.

Effects of surface modification and thermal annealing on the interfacial dynamics in core–shell nanocomposites based on silica and adsorbed PDMS

“…Similar results were reported recently for core-shell systems based on fumed silicas and PDMS, same as in the present work [17]. Other models proposed for explaining the effects of annealing in these and similar systems involve the diffusion of free volume holes at the interfaces between polymer and nanoparticles [74] or redistribution of interfacial free polymer volume [75], and, also, possible changes in interfacial polymer density [4].…”

Is the Reduction in Tracer Diffusivity under Nanoscopic Confinement Related to a Frustrated Segmental Mobility?

“…space where diffusing in proximity of the interface (densification). [11,24,27] This phenomenon results in a reduction of thermal expansivity, as experimentally verified, [23] and a less efficient random walk, which leads to a reduction of the tracer diffusivity. Similar ideas are in agreement with the experimental evidence that the diffusion of smaller probes like decacyclene and lophine might be less affected (or remain constant).…”

“…[19] The increase in effective T g is furthermore in disagreement with the larger drops observed for example by ellipsometry [20] and fluorescent spectroscopy. [21,22] However, we noticed a strong correlation between the thickness dependence of the T g determined via tracer diffusivity and the glass transition temperature provided by capacitive dilatometry, [23,24] a technique which senses the temperature dependence of the dielectric constant in absence of molecular mobility and thus the density fluctuations, coupled to changes in the electric capacitance. The constant ''dynamic'' T g measured for single layers of l-PS similarly disproves that the observed reduction in mass transport are related to a frustrated intrinsic mobility of the probe molecules in proximity of the metallic interface.…”

The lifetime of the deviations from bulk behaviour in polymers confined at the nanoscale

“…Because of the direct proportionality between the capacitance of a dielectric medium and the inverse of its thickness, T g could be determined by the sudden reduction in thermal expansivity (here proportional to − dC  /dT) at the transition from the liquid to the glassy state 8 . Dielectric spectra were acquired under a continuous nitrogen flow with an effective cooling rate of − 2 K min − 1 .…”