We describe results from experiments in which spontaneous embedment of 20 nm silica particles was used to probe under-layer effects on the free surface dynamics and the glass transition temperature (T g ) of polystyrene. Both 13 and 20 nm thick polystyrene (PS) films were prepared and placed on different under-layer substrates, which in turn were supported on a silicon wafer (Si) substrate. The under-layer substrates used were PS, poly(2-vinylpyridine) (P2VP), and poly(methyl methacrylate) (PMMA) with thicknesses ranging from 13 to 350 nm. The particle height change during the embedment was monitored using an atomic force microscope. For the PS films supported on the PS and P2VP under-layers, experimental temperatures varied from T g − 10 K to T g + 5 K. In the case of the PMMA under-layer, experimental temperatures varied from T g − 10 K to T g + 10 K. The Hutcheson and McKenna model [Phys. Rev. Lett. 2005, 94 (7)] was applied to the particle embedment depth to obtain the surface rheological temperatures [Eur. Phys. J. E 2007, 22 (4), 281−286] and the T g . It is found that the dynamics of the top-layer PS films were faster than the bulk material below the macroscopic T g and slower above it for all under-layers considered. The T g of both the 20 and 13 nm top-layer PS films were found to be essentially independent of under-layer thickness and reduced by less than 7 °C. Upon replacing PS under-layers with the same thickness of P2VP and PMMA as the under-layers, the T g of the 20 nm PS top-layer films changed by less than 5 K.

Vacuum pyrolysis deposition (VPD) has been used to create an ultrastable polymer glass having a fictive temperature T f of as much as 57 K below the nominal glass transition temperature of the thermally rejuvenated polymer. Amorphous fluoropolymer films 300 to 700 nm thick were created by VPD followed by characterization of the thermal response using rapid-scanning chip calorimetry. The deposition was performed for substrates held at temperatures from 30.0 °C (303.2 K) to 116.7 °C (389.9 K) corresponding to approximately 0.75 to 0.97 times the limiting fictive temperature T f ′ ≈ T g of the same material determined by cooling then heating at 600 K/s. Consistent with literature observations for small molecules that are vapor deposited in similar conditions relative to the material T g, large enthalpy overshoots are observed, typical of both highly aged and ultrastable glasses. The 57 K reduction in T f for the VPD polymers is greater than prior reports for physical vapor deposition of small molecules to form ultrastable glasses as well as greater than the T f reductions seen in ambers from 20 million to over 200 million years of age. The potential of using such materials to investigate systems extremely deep into the glassy condition is discussed.

Measurements on nanogram samples of ultrastable polymer Teflon films challenge paradigms of the glass transition event.

Prior bubble inflation studies have shown strong evidence of rubbery-like stiffening behaviors for several organic and inorganic thin films. The possible origin is still unclear, but recently Ngai et al. [J. Polym. Sci., Part B: Polym. Phys.201351214224] and Li and McKenna [Macromolecules20154863296336] proposed that the separation of molecular motions and dynamic fragility are closely related to the observed rubber-like stiffening behavior. In the present work, we report observations from bubble inflation measurements on nanometric thin polyisobutylene (PIB) films that test the correlations suggested by the Ngai coupling model (stiffening is related to the shape parameter describing the α-relaxation) and Li and McKenna (the rubbery stiffening correlates with the dynamic fragility). Mechanical properties and surface tension of nanometric thin PIB films were investigated through strain–stress measurements for film thicknesses ranging from 13 to 126 nm. The tests were performed at room temperature far above the glass transition temperature of PIB. In addition to stiffness and surface tension, rupture strengths were also measured. We find that the stiffness increases with decreasing film thickness and that the surface tension remains constant, independent of the film thickness. The rupture stress is found to increase with decreasing film thickness, whereas the rupture strain decreases with decreasing film thickness. Similar to the prior bubble inflation measurements in polymeric thin films, the thickness dependence of the stiffening followed a power law behavior with film thickness. The observed stiffening behavior agrees with the suggestion from Ngai et al. that the rubbery stiffening should correlate with the separation of the α- and Rouse-mode relaxations. However, unlike prior results for ultrathin polymer films, the stiffening behavior of PIB did not follow the linear relationship with dynamic fragility that was proposed by Li and McKenna.

Here, we report results from an investigation of nano-scale size or confinement effects on the glass transition and viscoelastic properties of physical vapor deposited selenium films. The viscoelastic response of freely standing Se films was determined using a biaxial membrane inflation or bubble inflation method [P. A. O'Connell and G. B. McKenna, Science 307, 1760-1763 (2005)] on films having thicknesses from 60 to 267 nm and over temperatures ranging from Tg, macroscopic - 15 °C to Tg, macroscopic + 21 °C. Time-temperature superposition and time-thickness superposition were found to hold for the films in the segmental dispersion. The responses are compared with macroscopic creep and recoverable creep compliance data for selenium [K. M. Bernatz et al., J. Non-Cryst. Solids 307, 790-801 (2002)]. The time-temperature shift factors for the thin films show weaker temperature dependence than seen in the macroscopic behavior, being near to Arrhenius-like in their temperature dependence. Furthermore, the Se films exhibit a "rubbery-like" stiffening that increases as film thickness decreases similar to prior observations [P. A. O'Connell et al., Macromolecules 45(5), 2453-2459 (2012)] for organic polymers. In spite of the differences from the macroscopic behavior in the temperature dependence of the viscoelastic response, virtually no change in Tg as determined from the thickness dependence of the retardation time defining Tg was observed in the bubble inflation creep experiments to thicknesses as small as 60 nm. We also find that the observed rubbery stiffening is consistent with the postulate of K. L. Ngai et al. [J. Polym. Sci., Part B: Polym. Phys. 51(3), 214-224 (2013)] that it should correlate with the change of the macroscopic segmental relaxation.