In this article, we use cooling-rate dependent Tg measurements (CR-Tg) to indirectly probe the relaxation dynamics of supported polystyrene thin films of various molecular weights, all chosen to be above the entanglement molecular weight. We show that the dynamics in these films deviate from bulk dynamics below a temperature T(*) = Tg + 6 K = 380  K ± 1  K. We show that T(*) for films of all thicknesses and molecular weights is the same as the temperature at which the free surface dynamics deviate from the bulk dynamics. The apparent activation barrier of the glass transition in thin films decreases towards that of the free surface as the film thickness decreases. This provides strong evidence that thin film dynamics are facilitated by the enhanced mobility at the free surface. The observation of T(*) can help resolve some seemingly contradictory data by suggesting that studies performed at higher temperatures (T > T(*)), or which probe shorter relaxation times (τ < τ(*) ∼ 1 s) would not observe properties that deviate from bulk values. We also demonstrate that the relaxation dynamics of supported entangled polystyrene films slow down slightly as the molecular weight of polystyrene increases. An eight nanometer film of Mw =2240 kg/mol polystyrene shows a Tg reduction of 27 K at a cooling rate of 1 K/min, while a film of the same thickness made of Mw =45.8 kg/mol polystyrene has a 36 K reduction of Tg compared to the bulk film at the same cooling rate. We hypothesize this is either due to the density of a dynamically "dead" layer near the substrate increasing with molecular weight, or partial anchoring of long chains, which could hinder segmental diffusion near the free surface.

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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 T 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 T (CR-T) 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 T's (<16 nm). T increases with decreasing thickness in a similar manner to what has been observed in previous studies on P2VP, and T 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).

We use cooling-rate-dependent T g measurements (CR-T g) to indirectly probe the relaxation dynamics and fragility of thin films of polystyrene (PS)/poly­(2,6-dimethyl-1,4-phenylene oxide) (PPO) blends. Thin films of this miscible blend have a single glass transition temperature (T g) that can be systematically shifted over 100 K simply by varying the composition of the blend. This study shows that the T g of these blends decreases below the bulk T g as the film thickness is decreased. Additionally, the degree of change in T g strongly depends on the cooling rate of the experiment. We show that the T g of 16 nm films of a 50% PS blend is 15 K lower than that of bulk at a cooling rate of 1 K/min but decreases only 4 K at a cooling rate of 120 K/min. By analyzing the cooling rate dependence of the T g for various thicknesses of the 50% PS blend, we demonstrate that the fragility of these blends decreases with film thickness. This behavior is similar to what is observed in ultrathin films of polystyrene, which suggests that the deviations from bulk dynamics in PS/PPO blends are due to enhanced mobility near the free surface. Similar to pure PS, if extrapolated to higher temperatures, the dynamics of thin films intersect the bulk dynamics at a temperature a few degrees above bulk T g. The presence of this temperature (T*) can help explain why some experiments fail to see T g depression in thin films of these blends. Lastly, we show that while the fragility of the bulk blend changes due to differences in the fragility of the homopolymers, ultrathin films (h = 16 nm) have identical average dynamics and fragilities regardless of the blend composition at the same temperature relative to Tg. This result implies that enhanced mobility near the free surface affects the dynamics of these blend thin films similarly.

Physical vapor deposition (PVD) is widely used in manufacturing ultra-thin layers of amorphous organic solids. Here, we demonstrate that these films exhibit a sharp transition from glassy solid to liquid-like behavior with thickness below 30 nm. This liquid-like behavior persists even at temperatures well below the glass transition temperature, Tg. The enhanced dynamics in these films can produce large scale morphological features during PVD and lead to a dewetting instability in films held at temperatures as low as Tg-35 K. We measure the effective viscosity of organic glass films by monitoring the dewetting kinetics. These measurements combined with cooling rate-dependent Tg measurements show that the apparent activation barrier for rearrangement decreases sharply in films thinner than 30 nm. These observations suggest long-range facilitation of dynamics induced by the free surface, with dramatic effects on the properties of nano-scale amorphous materials.Nanometer-sized thin films of small organic molecules are widely used in applications ranging from organic photovoltaics[1] and light emitting diodes [2,3], to protective coatings [4] and high resolution nano-imprint lithography [5]. It is advantageous to use amorphous films because, compared to crystals, they do not have grain boundaries to hinder charge transport, generate cracks and defects, or disrupt the writing processes. Physical vapor deposition (PVD), the common method used to manufacture these films, is usually performed at substrate temperatures below T g to produce films in the glassy state. However, if the properties at nanoscale deviate significantly from bulk properties, the resulting films can have reduced kinetic and thermal stability. Recent experiments suggest that diffusion at the free surface of organic glasses can be several orders of magnitude faster [6,7], with weaker temperature dependence compared to bulk diffusion. Enhanced, weakly temperature-dependent dynamics on the surface of polymeric glasses [8,9] have been shown to significantly affect the properties of ultra-thin polymer films [9][10][11][12][13][14][15][16][17]. In polymeric systems, the molecular weight of the polymer [14], and the temperature range of the measurement [8,9,14] seem to also affect the observed properties, resulting in ambiguity in the relationship between enhanced dynamics at the free surface and properties of ultra-thin glass films. As such, these results can not be extrapolated to molecular and atomic glass systems.

We report the synthesis and characterization of an analogous series of small organic molecules derived from a well-known glass former, 1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene (α,α,β-TNB). Synthesized molecules include α,α,β-TNB, 3,5-di(naphthalen-1-yl)-1-phenylbenzene (α,α-P), 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene (α,α-A), 9,9'-(5-(naphthalen-2-yl)-1,3-phenylene)dianthracene (β-AA) and 3,3',5,5'-tetra(naphthalen-1-yl)-1,1'-biphenyl (α,α,α,α-TNBP). The design of molecules was based on increasing molecular weight with varied π-π interactions in one or more substituents. The synthesis is based on Suzuki cross-coupling of 1-bromo-3-chloro-5-iodobenzene with arylboronic acids, which allows attachment of various substituents to tailor the chemical structure. The bulk compounds were characterized using NMR spectroscopy and differential scanning calorimetry (DSC). Thin films of these compounds were produced using physical vapor deposition and were subsequently annealed above the glass transition temperatures (Tg). For each molecular glass, cooling rate-dependent glass transition temperature measurements (CR-Tg) were performed using ellipsometry as a high-throughput method to characterize thin film properties. CR-Tg allows rapid characterization of glassy properties, such as Tg, apparent thermal expansion coefficients, apparent activation energy at Tg and fragility. DSC measurements confirmed the general trend that increasing molecular weight leads to increasing melting point (Tm) and Tg. Furthermore, CR-Tg provided evidence that the introduction of stronger π-interacting substituents in the chosen set of structural analogues increases fragility and decreases the ability to form glasses, such that β-AA has the largest fragility and highest tendency to crystallize among all the compounds. These strong interactions also significantly elevate Tg and promote more harmonic intermolecular potentials, as observed by decreasing value of the apparent thermal expansion coefficient.

A series of organically templated vanadium tellurites has been prepared under mild hydrothermal conditions. Single crystals were grown from mixtures of NaVO3, Na2TeO3, and either 1,4-diaminobutane, 1,6-diaminohexane, or N,N,N′,N′-tetramethylethylenediamine in H2O. Each compound contains similar [V2Te2O10] n 2n– layers. The layer metrics of [1,4-diaminobutaneH2][V2Te2O10], [1,6-diaminohexaneH2][V2Te2O10], [N,N,N′,N′-tetramethylethylenediamineH2][V2Te2O10], and [piperazineH2][V2Te2O10] reflect the steric bulk of the respective amines. Topotactic conversions between compounds through amine exchange are possible in reactions in which an increase in the strength of the amine–[V2Te2O10] n 2n– hydrogen-bonding network is observed.

This report aims to fully describe the experimental technique of using ellipsometry for cooling rate dependent Tg (CR-Tg) experiments. These measurements are simple high-throughput characterization experiments, which can determine the glass transition temperature (Tg), average dynamics, fragility and the expansion coefficient of the super-cooled liquid and glassy states for a variety of glassy materials. This technique allows for these parameters to be measured in a single experiment, while other methods must combine a variety of different techniques to investigate all of these properties. Measurements of dynamics close to Tg are particularly challenging. The advantage of cooling rate dependent Tg measurements over other methods which directly probe bulk and surface relaxation dynamics is that they are relatively quick and simple experiments, which do not utilize fluorophores or other complicated experimental techniques. Furthermore, this technique probes the average dynamics of technologically relevant thin films in temperature and relaxation time (τα) regimes relevant to the glass transition (τα > 100 sec). The limitation to using ellipsometry for cooling rate dependent Tg experiments is that it cannot probe relaxation times relevant to measurements of viscosity (τα << 1 sec). Other cooling rate dependent Tg measurement techniques, however, can extend the CR-Tg method to faster relaxation times. Furthermore, this technique can be used for any glassy system so long as the integrity of the film remains throughout the experiment.