Thermodynamic Ultrastability of a Polymer Glass Confined at the Micrometer Length Scale

Abstract

We employ fast scanning calorimetry to assess the thermodynamic state attained after a given cooling rate and the molecular mobility of glassy poly(4-tert-butylstyrene) confined at the micrometer length scale. We show that, for such a large confinement length scale, thermodynamic states with a fictive temperature (T f) 80 K below the polymer glass transition temperature (T g) are attained, which allows to bypass the geological timescales required for bulk glasses. Access to such states is promoted by a fast mechanism of equilibration. Importantly, the tremendous T f decrease takes place while the molecular mobility remains bulklike, indicating marked decoupling between vitrification kinetics and molecular mobility.

Vitrification decoupling from α-relaxation in a metallic glass

“…The increase of  is evident at values of  larger than ∼0.1 s, which is equal to that for which the decoupling -relaxation/vitrification kinetics is observed. This finding is not typically observed in other glass-forming systems at these moderate time scales (2-5), unless the system is subjected to geometrical confinement, as widely reported for polymeric glasses (11,(22)(23)(24)48). Glasses subjected to extremely long annealing would also present a decoupling.…”

Direct observation of desorption of a melt of long polymer chains

“…1 b. We used thin films of poly(4 -t e r t -butylstyrene) (P t BS), a polymer whose glassy dynamics 24 , vitrification 25 , and adsorption kinetics 9 have been deeply characterized in the previous years. We prepared our samples by spin-coating, the most commonly used method to coat solid substrates with thin polymer layers.…”

“…In the inset, red stars indicate the values of τ eq , whereas the red line is obtained from the temperature dependence of the segmental mobility, taken from ref. 24 . Error bars account for the uncertainty in the determination of the time necessary to reach a constant Δ H des value in the data set shown in the main panel of the same figure.…”

“…Such value, too high for simple thermally activated processes, is common in the case of molecular rearrangements requiring cooperativity 37 . Hence, we tested 38 if the temperature dependence of τ e q could be associated with the segmental α -relaxation of P t BS 24 , that is, the cooperative relaxation mechanism responsible for density fluctuations, arising from changes in molecular conformations involving several structural units 39 . This analysis confirmed that adsorption has an activation energy comparable to that of segmental mobility, although the two processes are separated in time by almost 6 orders in magnitude, τ eq ≈ 10 5.7 τ α .…”

“…Error bars account for the uncertainty in the determination of the intensity of the overshoot. In the inset, red stars indicate the values of τ eq , whereas the red line is obtained from the temperature dependence of the segmental mobility, taken from ref 24. .…”