Dewetting of thin polymer films

We study the dewetting of thin polymer films deposited on slippery substrate. Recent experiments on these systems have revealed many unexpected features. We develop here a model that takes into account the rheological properties of polymer melts, focussing on two dewetting geometries (the receding of a straight edge, and the opening of a hole). We show that the friction law associated with the slippage between the film and the substrate has a direct influence on the dewetting dynamic. In addition, we demonstrate that residual stresses, which can be stored in the films due to their viscoelasticity, are a source of destabilization for polymer films, and accelerate the dewetting process.     

Reduced viscosity in thin polymer films

The dewetting of thin polystyrene films (20-500 nm) on a liquid substrate is studied at time scales that are long compared to the reptation time. It is shown that the kinetics correspond to those of purely viscous flow and that the viscosity measured by this technique is, for the thickest films, consistent with bulk measurements. Films on the order of the coil size are then studied. The effective viscosity of these films displays a large decrease when the film thickness h is below several radius of gyration, R(g). This viscosity reduction is found to depend only on the ratio h/R(g).     

Molecular mobility in polymer thin films

Fluorescence recovery after photobleaching was used to measure in-plane dye-probe diffusion coefficients, D, in thin films of monodisperse polystyrene supported on fused quartz substrates. The substrates were prepared with a high density of surface hydroxyl groups which interact favorably with repeat units of the polymer. The effects of temperature and film thickness were investigated, at temperatures above the bulk glass transition of the polymer, T(g), and in the range of film thicknesses from 1-10(2) times the radius of gyration (R(g)) of individual polymer molecules. As the film thickness decreases towards R(g) the value of D increases above the bulk values, with significant effects first appearing in films approximately 20R(g). In the thinnest films studied, about 4R(g), the values of D lie as much as two orders over bulk values. At the same time, the temperature dependence of D becomes much weaker than in bulk. Analysis by free volume theory indicates that apparent values of both T(g) and the thermal expansion coefficient for liquid state, alpha(L), decrease as the film thickness decreases. The possible effects of surface segregation of the dye probe are discussed.     

Glass transitions in thin polymer films

Freely standing polystyrene films show an anomalous drop of the glass temperature when the molecular weight is high and the thickness smaller than the coil size R₀. We present here a tentative explanation for these features, where two types of motions compete: a) standard motions, controlled by the free volume, and independent of chain length, b) collective motions along the chain, which require a weaker free volume (except for the end groups). For bulk systems, the standard motion always wins because of the end group hindrance. But for films thinner than the coil size, the dominant process may be the collective motion of a “loop” which does not involve the chain ends. What matters then is not the overall polymerisation index (N), but the length g of a typical loop starting from the surface (which is a more fluid region) and reaching deep into the film. With these ingredients, some surprising aspects of may possibly be understood. Received 7 December 1999     

Acoustic instabilities in thin polymer films

The spatial confinement of a fluctuation spectrum leads to forces at the confining boundaries. While electromagnetic (EM) fluctuations lead to the well-known dispersion forces, the acoustic analogy has widely been neglected. We show that the strength of the forces resulting from confined acoustic modes may be of the same order of magnitude as van der Waals forces. Additionally, the predicted scaling behavior is identical to the non-retarded case of the EM fluctuations. Our results suggest that dewetting experiments using polymer films are strongly influenced by the acoustic dispersion forces. Received 5 March 2002 and Received in final form 21 May 2002     

Porphyrin-sensitized laser swelling and ablation of polymer films

Laser-induced morphological changes of poly(methyl methacrylate), poly(N-vinylcarbazole), and gelatin films doped with porphyrins have been studied by etch depth measurement and scanning electron microscopy. An irreversible swelling of the irradiated surface was observed for all films in the case of low laser fluence. The swelling was replaced by ablation when the fluence was increased. The etch depth depends on the irradiation fluence and the dye concentration in the polymer. The observation of the irradiated surfaces suggests that the thermal effect is predominant both for swelling and ablation. The surface temperature at which swelling or ablation is initiated was estimated, assuming that these morphological changes take place at a certain temperature for any dye concentration in each polymer film.     

Electrical breakdown of thin polymer films

Data are presented on the dielectric strength of thin polymer films. The conclusion is drawn that the electron avalanche concept is inapplicable to the breakdown of thin films. It is proposed to consider electrical breakdown as a consequence of an abrupt local field enhancement induced by evolution of the space charge injected into the polymer from electrodes. It is shown that the lifetime of polymer films depends exponentially on electric field strength.     

Chain entanglement in thin freestanding polymer films

When a thin glassy film is strained uniaxially, a shear deformation zone (SDZ) can be observed. The ratio of the thickness of the SDZ to that of the undeformed film is related to the maximum extension ratio, lambda, which depends on the entanglement molecular weight, M(e). We have measured lambda as a function of film thickness in strained freestanding films of polystyrene as a probe of M(e) in confinement. It is found that thin films stretch further than thick films before failure, consistent with the interpretation that polymers in thin films are less entangled than bulk polymers, thus the effective value of M(e) in thin films is significantly larger than that of the bulk. Our results are well described by a conceptually simple model based on the probability of finding intermolecular entanglements near an interface.     

Electron-beam deposition of vanadium dioxide thin films

Developing a reliable and efficient fabrication method for phase-transition thin-film technology is critical for electronic and photonic applications. We demonstrate a novel method for fabricating polycrystalline, switchable vanadium dioxide thin films on glass and silicon substrates and show that the optical switching contrast is not strongly affected by post-processing annealing times. The method relies on electron-beam evaporation of a nominally stoichiometric powder, followed by fast annealing. As a result of the short annealing procedure we demonstrate that films deposited on silicon substrates appear to be smoother, in comparison to pulsed laser deposition and sputtering. However, optical performance of e-beam evaporated film on silicon is affected by annealing time, in contrast to glass.     

Deodorisation effect of diamond-like carbon/titanium dioxide multilayer thin films deposited onto polypropylene

Many types of plastic containers have been used for the storage of food. In the present study, diamond-like carbon (DLC)/titanium oxide (TiO₂) multilayer thin films were deposited on polypropylene (PP) to prevent flavour retention and to remove flavour in plastic containers.For the flavour removal test, two types of multilayer films were prepared, DLC/TiO₂ films and DLC/TiO₂/DLC films. The residual gas concentration of acetaldehyde, ethylene, and turmeric compounds in bottle including the DLC/TiO₂-coated and the DLC/TiO₂/DLC-coated PP plates were measured after UV radiation, and the amount of adsorbed compounds to the plates was determined. The percentages of residual gas for acetaldehyde, ethylene, and turmeric with the DLC/TiO₂ coated plates were 0.8%, 65.2% and 75.0% after 40 h of UV radiation, respectively. For the DLC/TiO₂/DLC film, the percentages of residual gas for acetaldehyde, ethylene and turmeric decreased to 34.9%, 76.0% and 85.3% after 40 h of UV radiation, respectively. The DLC/TiO₂/DLC film had a photocatalytic effect even though the TiO₂ film was covered with the DLC film.     

Temperature accelerated dielectric breakdown of PECVD low-k carbon doped silicon dioxide dielectric thin films

The dielectric breakdown strength of carbon doped silicon dioxide thin films with thickness d from 32 nm to 153 nm is determined at 25 °C, 50 °C, 100 °C, 150 °C and 200 °C, using I–V measurements with metal-insulator-semiconductor (MIS) structures. It is found that the dielectric breakdown strength, E_B, decreases with increasing temperature for a given film thickness. In addition, a film thickness dependence of breakdown is also observed, which is argued to show an inverse relation to thickness d in the form of E_B∝(d-d_c)^-n. The exponential parameter n and critical thickness limit d_c also exhibit temperature dependent behavior, suggesting a temperature accelerated electron trapping process. The activation energy for the temperature acceleration was shown to be thickness dependent, indicating a thickness dependent conduction mechanism. It is thereafter demonstrated that for relatively thick films (thickness >50 nm), the conduction mechanism is Schottky emission. For relatively thin films (thickness <50 nm), the Schottky conduction mechanism was obeyed at low field region while FN tunnelling was observed as a prevail one in the high field region. PACS 73.40.Qv     

Gradients of topographical structure in thin polymer films

We report the fabrication of centimeter-long gradients of topography. Structured polymer thin films were made by spin-coating an immiscible poly(methyl methacrylate)/poly(2-vinylpyridine) (PMMA/P2VP) blend from a common solvent on a substrate presenting a surface-energy gradient. Due to the interplay between polymer-wetting of the substrate and polymer phase-separation, different morphologies were observed along the sample. Atomic force microscopy (AFM) revealed a gradual transition from a flat to a structured morphology as the surface energy of the substrate was reduced. The transition occurred for a well-defined range of surface energies. A selective solvent was used to remove one of the polymer phases, increasing the topographical contrast and providing insight into the different stages of the transition. Image analysis allowed a quantitative characterization of the structure gradient.     

Charge transfer in thin polymer films of polyarylenephthalides

Transitions from an insulating state to a high-conductivity state are observed in thin polymer films of polyarylenephthalides. The temperature dependences of the conductivity for thin films of wide-band-gap polymer insulators are determined, the relaxation of excess charge is investigated, and the mechanisms of charge transfer in the temperature range preceding the transition to the high-conductivity state are revealed. It is shown that hopping transport over trap states in the band gap is the main mechanism of charge transfer in polyarylenephthalide film.     

Photothermal deflection spectroscopy of polymer thin films

Photothermal Deflection Spectroscopy (PDS) is known to be one of the most sensitive techniques for measuring the absorption of weakly absorbing materials. We have applied PDS for measuring the optical absorption of a few polymer thin-film samples over the wavelength region from 0.4–2.0 m. The results are useful for optical evaluation of these polymers.     

Spinodal-like dewetting of thermodynamically-stable thin polymer films

Energetic considerations indicate that long-range Van der Waals forces stabilize thin polystyrene (PS) films against height fluctuations on silicon substrates. Nevertheless, we report here on the amplification of capillary waves of specific wavelengths for 15 nm thick PS films on silicon, ultimately leading to dewetting in a spinodal-like process. However, the temporal dependence of the wavelength of the growing instability does not agree with the classical spinodal dewetting mechanism. Therefore, this phenomenon is ascribed to the existence of structural forces resulting either from the restructuring of the films or from density variations within the films during annealing, in accordance with recent theoretical treatments. The process is shown not to be limited to polystyrene films, which indicates the generality of our findings.Received: 1 August 2003PACS: 68.15. + e Liquid thin films - 47.20.-k Hydrodynamic stability - 47.20.Ma Interfacial instability - 68.08.-p Liquid-solid interfaces     

Supramolecular structure of electroactive polymer thin films

This paper presents the results of an experimental investigation of the supramolecular structure of polydiphenylenephthalide thin films that exhibit effects of resistive switching. The supramolecular structure of the polymer has been investigated using small-angle neutron scattering in conjunction with atomic force microscopy. It has been found that the internal structure of polymer films consists of structural elements in the form of spheroids. The sizes of the structural elements, which were obtained from the neutron scattering data and analysis of the atomic force microscopy images, correlate well with each other. A model of the formation of polymer layers has been proposed. The observed structural elements in polymer films are formed due to the association of macromolecules in the initial polymer solution.     

Phase-sensitive acoustic microscopy of polymer thin films

The three-dimensional images obtained by scanning acoustic microscopy with vector contrast (PSAM), contain significant qualitative and quantitative information that is not easily obtainable by other methods. We employ this technique to examine homopolymer and polymer blend thin films. The complex V(z) functions derived from the images, and the results obtained by image processing and meticulous analysis are employed to render the morphology, composition and micro-mechanical properties of the polymer films. In addition, ways by which the information inherent in the phase images can be extracted are examined. This is highly desirable, as the phase images contain very useful additional information.     

Evolution of stress in evaporated silicon dioxide thin films

The evolution of stress in evaporated SiO2, used as optical coatings, is investigated experimentally through in situ stress measurement. A typical evolution pattern consisting of five subprocedures （thin film deposition, stopping deposition, cooling, venting the vacuum chamber, and exposing coated optics to the atmosphere） is put forward. Further investigations into the subprocedures reveal their features. During the deposition stage, the stresses are usually compressive and reach a stable state when the deposited film is thicker than 100 nm. An increment of compressive stress value is observed with the decrease of residual gas pressure or deposition rate. A very low stress of-20 MPa is formed in SiO2 films deposited at 3×10^-2 Pa. After deposition, the stress increases slightly in the compressive direction and is subject to the stabilization in subsequent tens of minutes. In the process of venting and exposure, the compressive component increases rapidly with the admission of room air and then reaches saturation, followed by a logarithmic decrement of the compressive state in the succeeding hours. An initial discussion of these behaviors is given.