Glass Transition Of Ultrathin Films Research Articles

Kinetic arrest of front transformation to gain access to the bulk glass transition in ultrathin films of vapour-deposited glasses.

Physical vapour deposition has emerged as the technique to obtain glasses of unbeatable stability. However, vapour deposited glasses exhibit a different transformation mechanism to ordinary glasses produced from liquid. Vapour deposited glasses of different thermodynamic stability, from ultrastable to those similar to ordinary glasses, transform into the liquid state via front propagation starting at the most mobile surfaces/interfaces, at least for the first stages of the transformation, eventually dynamiting the high thermal stability achieved for some of these glasses. A previous study showed that it was possible to avoid this transformation front by capping the films with a higher Tg material. We show here fast calorimetry measurements on TPD and IMC vapour deposited glasses capped respectively with TCTA and TPD. This capped configuration is very effective in suppressing the heterogeneous transformation of the stable glasses into the supercooled liquid and shifts the devitrification temperature to much higher values, where the bulk homogeneous mechanism becomes active. This approach may be useful to further study the bulk glass transition in thin films.

Exploring the broadening and the existence of two glass transitions due to competing interfacial effects in thin, supported polymer films.

In this report, we use ellipsometry to characterize the glass transition in ultra-thin films of poly(2-vinyl pyridine) (P2VP) supported on a silicon substrate. P2VP is known to have attractive substrate interactions, which can increase the Tg of ultra-thin films compared to the bulk value. Here, we use an extended temperature range to show that the glass transition can be extremely broad, indicating that a large gradient of the dynamics exists through the film with slow dynamics near the substrate and enhanced dynamics at the free surface. To observe the effect of these two interfaces on the average thin film dynamics, cooling rate-dependent Tg (CR-Tg) measurements were used to indirectly probe the average relaxation times of the films. We demonstrate that ultra-thin films have lower fragility compared to bulk films, and, when cooled at slow cooling rates (<1 K/min), exhibit extreme broadening of the dynamics (<70 nm) and eventually complete decoupling between the free surface and substrate regions to produce films with two distinct Tg's (<16 nm). Tg,high increases with decreasing thickness in a similar manner to what has been observed in previous studies on P2VP, and Tg,low decreases with decreasing film thickness in a similar manner to what has been observed in polymer films with enhanced free surfaces and neutral substrate interactions. These observations indicate that the dynamics in thin films of P2VP can be strongly coupled over a length scale of ∼10-20 nm, resulting in two co-existing layers with two distinct glass transitions when the range of the dynamical gradients become too large to sustain (breadth of the transition > 50 K).

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

Despite the decade long controversial discussion on the effect of nanometer confinement on the glass transition temperature (Tg) 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 Tg in dependence on the film thickness, it was shown that the Tg of the whole film was strongly influenced by that nanometer thick surface layer, with a lower Tg. Since the observed thickness dependence of the dynamic Tg is similar to the thickness dependence of the Tg for thin films of homopolymers, it is concluded that also for homopolymer a highly mobile surface layer is relevant for the widely observed Tg depression.

Glass transition in ultrathin films of amorphous solid water

Nanocalorimetry at ultrafast heating rates is used to investigate the glass transition of nanometer thick films of metastable amorphous solid water grown by vapor deposition in an ultrahigh vacuum environment. Apparent heat capacity curves exhibit characteristic features depending on the deposition temperature. While films grown at T ≥ 155 K are completely crystallized, those deposited at 90 K show a relaxation exotherm prior to crystallization. Films grown between 135 and 140 K and subsequently cooled down to 90 K reveal a clear endothermic feature before crystallization, which is compatible with a glass-to-liquid transition. The onset temperature is located at 174 K at a heating rate of 2.4 × 10(4) K/s and is independent of film thickness in the range of 16-150 nm. Comparison of our data with other calorimetric measurements at various heating rates suggests that water is a strong glass former in the deeply supercooled state.

Confinement Effects on the Relaxation Time Distribution of the Dynamic Glass Transition in Ultrathin Polymer Films

The dynamic glass transition in ultrathin films of poly(vinyl acetate) (PVAc) is investigated by broadband dielectric spectroscopy. With decreasing film thickness changes in the relaxation time distribution are detected, leading, for thicknesses below ∼20 nm, to an increase in the average relaxation rate. This is caused by an asymmetric suppression of the relaxation modespredominantly on the low-frequency side.

Evidence for size-dependent mechanical properties from simulations of nanoscopic polymeric structures

Discontinuous molecular dynamics simulations of a model polymer have been conducted to investigate the glass transition of ultrathin films and the mechanical properties of nanoscopic structures. Continuum mechanics models have been applied to interpret simulation data and extract apparent Young’s Moduli. Consistent with experiments, the results of simulations indicate that the glass transition temperature of thin films can be higher or lower than that of the bulk, depending on the nature of polymer–substrate interactions. Simulations also indicate that the mechanical properties of nanoscopic structures can be considerably different from those of the bulk. An analysis of molecular strain distributions in nanostructures undergoing a deformation indicate that significant stress relaxation occurs at air–polymer interfaces. A comparison of these distributions to the results of continuum, finite-element calculations reveal pronounced differences between the continuum and molecular approaches.

Glass transition of ultra-thin films of modified PVC

Ultra-thin layers (h=50–150nm) of PVC, functionalised in solution by thiophenol (t-PVC) and 4-aminothiophenol (am-PVC), were prepared by spin-coating on silicon wafers. The glass transition temperature of these thin films was investigated by kinetic scan ellipsometry. The Tg of the am-PVC and t-PVC modified PVC is found to increase more strongly with the degree of modification in thin layer geometry than in the bulk. Also, Tg(h) of t-PVC exhibits a larger increase in the confined state as compared to am-PVC. This result is unexpected since no specific interactions are assumed between thiophenyl group and the surface indicating that other driving forces for the alteration of Tg(h) should be taken into account.In a second step, the supported PVC thin films are chemically cross-linked in solution by terephthaloyl chloride. The Tg(h) of the polymer layer increases with the cross-linking time more severely at a higher degree of modification but independently of the thickness of the layer indicating that the diffusion process is not altered in thin layers. Above Tg, the thermal expansion coefficient α of the thin films is not modified after cross-linking, whereas, it was found to decrease strongly between 4 and 8% of modification in the uncured state.

Glass transition of ultrathin films probed by X-ray reflectivity

X-ray reflectometry was used to measure the temperature dependence of the thickness of ultrathin films of two low-molecular-weight glass formers. In bulk samples, this type of measurements reveals the presence of a glass transition by a sudden break of slope. We have observed a broadening of the glass transition in films thinner than roughly 150 nm, indicating an inhomogeneous distribution of relaxation times in these films. Our results are explained by confinement effect and interpreted in the frame of a theoretical model based on the concept of collective dynamics over a cooperative length.