Glass Transition In Thin Films Research Articles

Investigations of the adsorbed layer of polysulfone: Influence of the thickness of the adsorbed layer on the glass transition of thin films.

This work studies the influence of the adsorbed layer on the glass transition of thin films of polysulfone. Therefore, the growth kinetics of the irreversibly adsorbed layer of polysulfone on silicon substrates was first investigated using the solvent leaching approach, and the thickness of the remaining layer was measured with atomic force microscopy. Annealing conditions before leaching were varied in temperature and time (0-336h). The growth kinetics showed three distinct regions: a pre-growth step where it was assumed that phenyl rings align parallel to the substrate at the shortest annealing times, a linear growth region, and a crossover from linear to logarithmic growth observed at higher temperatures for the longest annealing times. No signs of desorption were observed, pointing to the formation of a strongly adsorbed layer. Second, the glass transition of thin polysulfone films was studied in dependence on the film thickness using spectroscopic ellipsometry. Three annealing conditions were compared: two with only a tightly bound layer formed in the linear growth regime and one with both tightly bound and loosely adsorbed layers formed in the logarithmic growth regime. The onset thickness and increase in the glass transition temperature increases with annealing time and temperature. These differences were attributed to the distinct conformations of the formed adsorbed layers.

Decoupling of Dynamic and Thermal Glass Transition in Thin Films of a PVME/PS Blend.

The discussions on the nanoconfinement effect on the glass transition and glassy dynamics phenomena have yielded many open questions. Here, the thickness dependence of the thermal glass transition temperature Tgtherm of thin films of a PVME/PS blend is investigated by ellipsometry. Its thickness dependence was compared to that of the dynamic glass transition (measured by specific heat spectroscopy) and the deduced Vogel temperature (T0). While Tgtherm and T0 showed a monotonous increase, with decreasing film thickness, the dynamic glass transition temperature (Tgdyn) measured at a finite frequency showed a nonmonotonous dependence that peaks at 30 nm. This was discussed by assuming different cooperativity length scales at these temperatures, which have different sensitivities to composition and thickness. This nonmonotonous thickness dependence of Tgdyn disappears for frequencies characteristic for T0. Further analysis of the fragility parameter showed a change in the glassy dynamics from strong to fragile, with decreasing film thickness.

Facilitation of interfacial dynamics in entangled polymer films.

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.

Molecular Mobility and Glass Transition of Thin Films of Poly(bisphenol A carbonate)

Glass transition behavior of thin poly(bisphenol A carbonate) (PBAC) films capped between two aluminum (Al) layers is investigated by means of dielectric expansion dilatometry and dielectric relaxation spectroscopy accompanied by contact angle measurements. The thermal glass transition temperature is more or less independent of the film thickness down to 20 nm. For thickness below 20 nm, an increase of Tg is observed. Meanwhile, an increase of the relaxation time at a fixed temperature is observed for the film with the thickness of 19 nm on the basis of a careful analysis of the temperature dependence of the relaxation rates. A more detailed analysis of the relaxation map reveals that the Vogel temperature increases and the fragility decreases systematically with decreasing film thickness. These properties are discussed in terms of the formation of a boundary layer with PBAC segments adsorbed onto the Al electrode due to the strong interaction between the Al and PBAC layers (2.51 mJ/m2), which results in ...

Effect of nanoparticle dispersion on glass transition in thin films of polymer nanocomposites

We present spectroscopic ellipsometry measurements on thin films of polymer nanocomposites consisting of gold nanoparticles embedded in poly(styrene). The temperature dependence of thickness variation is used to estimate the glass transition temperature, T(g). In these thin films we find a significant dependence of T(g) on the nature of dispersion of the embedded nanoparticles. Our work thus highlights the crucial role played by the particle polymer interface morphology in determining the glass transition in particular and thermo-mechanical properties of such nanocomposite films.

Calorimetric study of blend miscibility of polymers confined in ultra-thin films

Miscibility in blends of polystyrene and poly(phenylene oxide) (PS/PPO) confined in thin films (down to 6 nm) was investigated using a recently developed sensitive differential alternating current (AC) chip calorimeter. Comparison of composition dependence of glass transition in thin films with common models should provide information on miscibility. This study focuses on the blend system polystyrene and poly(phenylene oxide) (PS/PPO) because it is thought as a miscible model system in the whole composition range. Furthermore, its local dynamic heterogeneity is already identified by dynamic mechanic thermal analysis (DMTA) and solid state NMR techniques. For this blend, we find that even for the thinnest films (6 nm, corresponding to about half of PPO’s radius of gyration R g) only one glass transition is observed. The composition dependence of T g is well described by the Fox, Couchman or Gordon-Taylor mixing law that are used for the miscible bulk blends. Although there is a contradicting result on whether T g decreases with decreasing film thickness between our calorimetric measurements and Kim’s elipsometric measurements on the same blend (Kim et al. Macromolecules 2002, 35, 311–313), the conclusion that the good miscibility between PS and PPO remains in ultrathin films holds for both studies. Finally, we show that our chip calorimeter is also sensitive enough to study the inter-layer diffusion in ultrathin films. PS chain in a thin PS/PPO double layer that is prepared by spin coating PPO and PS thin film in tandem will gradually diffuse into the PPO layer when heated above T g of PS, forming a PSxPPO100−x blend. However, above the PSxPPO100−x blend, there exists an intractable pure PS like layer (∼30 nm in our case) that does not diffuse into the blend beneath even staying at its liquid state over 10 hours.

Impact of low-molecular mass components (oligomers) on the glass transition in thin films of poly(methyl methacrylate)

Commercial polydisperse atactic poly(methyl methacrylate) (PMMA) exhibits a decreased glass transition temperature (Tg) when the film thickness is less than ∼60 nm, whereas more model atactic PMMA shows an increased Tg in thin films supported on clean silicon wafers. NMR indicates no difference in tacticity, so the divergent thin film behavior appears related to the relative distribution of molecular mass. Extraction of some low molecular weight PMMA components from the commercial sample results in a significant modification of the thin film Tg compared with the initial PMMA fraction. The extracted sample exhibits initially a slight decrease in Tg as the film thickness is reduced below ∼60 nm, but then Tg appears to increase for films thinner than 20 nm. These results illustrate the sensitivity of polymer thin film properties to low-molecular mass components and could explain some of the contradictory reports on the Tg of polymer thin films that exist in the literature. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010

Size Effects and Extraordinary Stability of Ultrathin Vapor Deposited Glassy Films of Toluene

We report the first experimental evidence of size effects in the glass transition of thin films of an organic molecule grown from the vapor phase. In as-deposited films grown at 90 K (0.80Tg), both the fictive temperature, Tf, and the onset of the glass transition, Ton, decrease with thickness. The thinnest layers (∼4 nm) exhibit the highest thermodynamic and lowest kinetic stability. Films refrozen at 2000 K/s after being heated to the liquid state during a previous scan demonstrate no size effects. The width of the glass transition for both as-deposited and refrozen films is independent of the film thickness down to 4 nm. Our heat capacity data suggest that ultrathin vapor-deposited glasses transform into liquid by a faster dynamic influenced by the outer film surface.

Influence of substrate chemistry and the network homogeneity of ultra‐thin cross‐linked polymer films cured at their surface on the main relaxations temperatures probing by microthermal analysis

AbstractThin 200‐nm epoxy–amine mixtures were cured on silicon wafers with different surface chemistry to quantify the effect of the chemistry on the glass transition temperature evolution in ultra‐thin thermosetting films. Two surface treatments were investigated: the first one only consisted in the activation of the silanols groups at the silicon surface, whereas the second one consisted in the grafting 3‐aminopropyltrimethoxysilane (APTMS) monolayer on the silicon wafers. The epoxy films were deposited on these chemistry modified wafers by spin coating a toluene solution of DGEBA–amine mixture at stoichiometric ratio. The same cure processing was used for both samples. Thin films were analysed not only using microthermal and thermomechanical analysis to determine the relaxation transitions temperatures of these films but also using FTIR in infrared reflection absorption spectroscopy mode to determine the curing rate of these networks. It was found that all these thin films showed two different glass transitions, the first one at 96 °C and was independent of the surface treatments, whereas the second one increasing from 142 °C for the oxidised wafers surface to 167 °C for the aminosilane grafted on the silicon wafer. The substrate chemistry extent on the film network structure, the interfacial bonds and interactions are discussed. This work also illustrates the interest in using microthermal analysis to obtain relevant temperature glass transition of thin film at sub‐micrometre scale, strongly dependant of local structure and chemistry composition. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Evidence of glass transition in thin films of maleic anhydride derivatives: Effect of the surfactant coadsorption

The glass transition temperature of poly (maleic anhydride-alt-1-octadecen) and poly (styrene-co-maleic anhydride) cumene-terminated thin films has been measured by mechanical relaxation of Langmuir films of these polymers. The dynamical properties show glass-like features (non-Arrhenius relaxation times and non-Debye mechanical response) interpreted by the coupling model. The glass transition temperature values determined by a mechanical relaxation experiment (step-compression) agree very well with those obtained by surface potential measurements. It is found that the glass transition temperature values in thin films decrease by about 100 K as compared with those corresponding to the bulk polymers. The coadsorption of the water-insoluble surfactant DODAB decreases the glass transition temperature.

Metastable States of Glassy Dynamics, Possibly Mimicking Confinement‐Effects in Thin Polymer Films

AbstractThe impact of the preparation and thermal history on the dynamic glass transition in thin films of poly(methyl methacrylate) and poly(vinyl acetate) is systematically investigated by means of broadband dielectric spectroscopy. Metastable states of glassy dynamics are detected, exhibiting shifts in the mean‐relaxation time from the bulk values. These shifts ‐ possibly mimicking confinement‐effects in thin polymer films ‐ become weaker (or even disappear) after an extensive annealing.magnified image

Differential AC-chip calorimeter for glass transition measurements in ultra thin polymeric films

A differential AC-chip calorimeter capable to measure the glass transition in nanometer thin films is described. Due to the differential setup pJ/K sensitivity is achieved. Heat capacity can be measured for sample masses below one nanogram even above room temperature as needed for the study of the glass transition in nanometer thin polymeric films. The calorimeter allows for the frequency dependent measurement of complex heat capacity in the frequency range from 1 Hz to 1 kHz. The glass transition in thin films of polystyrene (PS) (100–4 nm) and polymethylmethacrylate (PMMA) (400–10 nm) was determined at well defined experimental time scales. No thickness dependency of the glass transition temperature was observed within the error limits (±3 K) - neither at constant frequency nor for the traces in the activation diagrams (1 Hz–1 kHz).

Discrepancies in the characterization of the glass transition in thin films of hyperbranched polyesters

AbstractThe dynamic glass transition and the dilatometric glass transition temperature are simultaneously characterized in thin films of hyperbranched aromatic polyesters by broadband dielectric spectroscopy and capacitive scanning dilatometry. A diverging thickness dependence is detected: while the temperature position of the alpha relaxation peak Tα decreases by ∼30 K, the dilatometric Tg increases by ∼10 K with decreasing film thickness. This emphasizes the subtle character of the glass transition phenomenon—as manifested in the molecular dynamics and in the (structural) thermal expansion—and proves that, in contrast to the bulk, different experimental techniques do not necessarily deliver similar results in confinement. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3006–3010, 2006

Modelling irradiation induced glass transition in thin films

By the interface migration-charge transfer (IMCT) atomistic model the nucleation of amorphous or crystalline phases is modelled in binary compound films, both metallic and non-metallic, where dense collision cascades form upon irradiation. Non-equilibrium compositional and electronic density profiles develop at the interface between each cascade and the surrounding crystalline matrix due to interface enrichment of one of the film constituents. By local charge transfer reactions (CTR), system relaxation is mimicked via formation of dimers of an effective compound. We calculate the energy cost to produce one such dimer, the difference of formation enthalpy between each effective compound and the corresponding initial compound and the local strain associated to a CTR for representative sets of metallic and non-metallic ion bombarded compounds. Qualitative differences are found in the parameter trends between the two groups of compounds amorphized and respectively, crystalline under ion beam irradiation.

Temperature Derivative Fluorescence Spectroscopy as a Tool to Study Dynamical Changes in Protein Crystals

Motions through the energy landscape of proteins lead to biological function. At temperatures below a dynamical transition (150–250 K), some of these motions are arrested and the activity of some proteins ceases. Here, we introduce the technique of temperature-derivative fluorescence microspectrophotometry to investigate the dynamical behavior of single protein crystals. The observation of glass transitions in thin films of water/glycerol mixtures allowed us to demonstrate the potential of the technique. Then, protein crystals were investigated, after soaking the samples in a small amount of fluorescein. If the fluorophore resides within the crystal channels, temperature-dependent changes in solvent dynamics can be monitored. Alternatively, if the fluorophore binds to the protein, local dynamical transitions within the biomolecule can be probed directly. A clear dynamical transition was observed at 175 K in the active site of crystalline human butyrylcholinesterase. The results suggest that the dynamics of crystalline proteins is strongly dependent on solvent composition and confinement in the crystal channels. Beyond applications in the field of kinetic crystallography, the highly sensitive temperature-derivative fluorescence microspectrophotometry technique opens the way to many studies on the dynamics of biological nanosamples.

Ideal glass transitions in thin films: An energy landscape perspective

We introduce a mean-field model for the potential energy landscape of a thin fluid film confined between parallel substrates. The model predicts how the number of accessible basins on the energy landscape and, consequently, the film’s ideal glass transition temperature depend on bulk pressure, film thickness, and the strength of the fluid–fluid and fluid–substrate interactions. The predictions are in qualitative agreement with the experimental trends for the kinetic glass transition temperature of thin films, suggesting the utility of landscape-based approaches for studying the behavior of confined fluids.

Glass transition of thin films of poly(2-vinyl pyridine) and poly(methyl methacrylate): nanocalorimetry measurements

Glass transitions were observed in thin films of poly(2-vinyl pyridine) (P2VP) and poly(methyl methacrylate) (PMMA) using a scanning nanocalorimetry technique which has both high sensitivity (10 −9 J/K) and high scan rates (10 4–10 5 K/s). Samples were deposited by the spin-cast method. The thickness of samples was 100–400 nm. Glass transition temperature, obtained by nanocalorimetry, is shifted toward higher temperatures by 10–20 K and activation enthalpy of glass transition is shifted to lower values by factor of 2–4. The glass transition characteristics of both polymers are discussed in terms of the standard Tool–Narayanaswamy–Moynihan (TNM) multi-parameter model.

The glass transition of thin film polymer/polymer blends: Interfacial interactions and confinement

We examined the influence of film thickness and composition on the effective Tg of compatible thin film mixtures of polystyrene (PS) and tetramethylbisphenol-A polycarbonate (TMPC) on SiOx/Si substrates using spectroscopic ellipsometry. Our measurements reveal that while the Tg of TMPC films increased with decreasing film thickness, h, the effective Tg of thin film mixtures of PS and TMPC decreased with decreasing film thickness. In these mixtures, Tg was independent of film thickness at large h. We also found that while the Tg of bulk mixtures of TMPC/PS exhibited large negative deviations from additivity with composition, such deviations were negligible in the thin film mixtures. The thickness dependence of Tg is compared with theory.

Relaxation of polymer thin films in isothermal temperature‐jump measurements

The dynamic behavior of thin polymer films is of interest in the fabrication of microelectronics and optoelectronics and in the coatings industry. It is known that polymer relaxation is affected by film thickness and the particular substrate/polymer pair. We previously used a spectroscopic ellipsometer to investigate the glass transition in thin films. In addition to information on the modification of thermal transitions such as the glass-transition temperature, the speed of data acquisition in an automated, spectroscopic ellipsometer, operated at a single wavelength of 780 nm, allows for the direct observation of the isothermal dimensions of a thin polymer film as a function of time after a rapid temperature change. In this article, we discuss recent results from the observation of the time dependence of film-normal thickness and normalized, in-plane, lateral dimension as well as simple fits to this relaxation behavior in terms of a normalized viscosity and relaxation time. The results support a highly asymmetric initial thermal expansion normal to the film followed by close to isotropic relaxation and anisotropic “flow” (the flow in response to the vanishingly small shears of thermal expansion). These features may clarify issues involving the observation of chain confinement in thin polymer films in terms of potential differences between equilibrium and dynamic measurements. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2929–2936, 2000

Relaxation of polymer thin films in isothermal temperature-jump measurements

The dynamic behavior of thin polymer films is of interest in the fabrication of microelectronics and optoelectronics and in the coatings industry. It is known that polymer relaxation is affected by film thickness and the particular substrate/polymer pair. We previously used a spectroscopic ellipsometer to investigate the glass transition in thin films. In addition to information on the modification of thermal transitions such as the glass-transition temperature, the speed of data acquisition in an automated, spectroscopic ellipsometer, operated at a single wavelength of 780 nm, allows for the direct observation of the isothermal dimensions of a thin polymer film as a function of time after a rapid temperature change. In this article, we discuss recent results from the observation of the time dependence of film-normal thickness and normalized, in-plane, lateral dimension as well as simple fits to this relaxation behavior in terms of a normalized viscosity and relaxation time. The results support a highly asymmetric initial thermal expansion normal to the film followed by close to isotropic relaxation and anisotropic “flow” (the flow in response to the vanishingly small shears of thermal expansion). These features may clarify issues involving the observation of chain confinement in thin polymer films in terms of potential differences between equilibrium and dynamic measurements. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2929–2936, 2000