Viscoelastic dynamics of polymer thin films and surfaces

The strain relaxation behavior in a viscoelastic material, such as a polymer melt, may be strongly affected by the proximity of a free surface or mobile interface. In this paper, the viscoelastic surface modes of the material are discussed with respect to their possible influence on the freezing temperature and dewetting morphology of thin polymer films. In particular, the mode spectrum is connected with mode coupling theory assuming memory effects in the melt. Based on the idea that the polymer freezes due to these memory effects, surface melting is predicted. As a consequence, the substantial shift of the glass transition temperature of thin polymer films with respect to the bulk is naturally explanied. The experimental findings of several independent groups can be accounted for quantitatively, with the elastic modulus at the glass transition temperature as the only fitting parameter. Finally, a simple model is put forward which accounts for the occurrence of certain generic dewetting morphologies in thin liquid polymer films. It demonstrates that by taking into account the viscoelastic properties of the film, a morphological phase diagram may be derived which describes the observed structures of dewetting fronts. It is demonstrated that dewetting morphologies may also serve to determine nanoscale rheological properties of liquids.Received: 1 January 2003, Published online: 14 October 2003PACS: 47.50. + d Non-Newtonian fluid flows - 68.47.Mn Polymer surfaces - 68.60.Dv Thermal stability; thermal effects     

Influence of the addition of silsesquioxane on the dewetting behavior of polystyrene thin film

A strategy to suppress the dewetting of polystyrene (PS) thin films by the addition of octacyclopentylsilsesquioxane (cPOSS) as a nanofiller was proposed. PS thin films with cPOSS were prepared by spin-coating. The bulk glass transition temperature of PS was not changed with an addition of the nanofiller up to 10 wt%. On the other hand, the addition of cPOSS to the PS thin films led to a great inhibition of dewetting. After annealing for 3 h at 373 K, no appreciable dewetting was observed by optical microscopy in the PS film with 15 wt% cPOSS, in contrast, the PS film without cPOSS was completely dewetted. Holes formed on the PS films with 10 wt% cPOSS. However, in that case, the growth of the holes stopped before reaching the final stage of the dewetting. This suggests that the increase of the cPOSS concentration per unit area at the rim of the holes prevents further growth of the holes, and this inhibition effect can be attributed to the interaction between cPOSS and the substrate accompanying modification of the PS-substrate interface.     

Suppress polystyrene thin film dewetting by modifying substrate surface with aminopropyltriethoxysilane

Aminofunctional organosilanes, such as 3-aminopropyltriethoxysilane (APTES), have been widely utilized as adhesion promoters, and have also been found to have the ability to prevent dewetting of polymer thin films from substrates. The APTES molecule contains an active terminal amino group that can hydrogen bond with the multiple hydroxylated head groups in itself and hydroxyl groups on a substrate, thus forming cyclic structures and a complex loose network at ambient conditions. Upon heating, the hydrogen bond can be broken, allowing more silanol groups to condense with each other and form siloxane linkages, hence tightening the three-dimensional network. When a polymer thin film is in contact with the APTES layer during the thermal process, the polymer chains can diffuse/penetrate into the APTES network while the network is being tightened by the additional crosslinking. The penetrated and subsequently anchored chains could help to stabilize the thin film on the substrate. This hypothesis was verified by dewetting studies of thin films of polystyrene (PS), having molecular weights above and below the entanglement molecular weight (M_e), from APTES and control surfaces when the systems were subjected to various treatments. Dewetting suppression was observed for PS/APTES that was thermally treated at ∼80 °C or 120 °C prior to the annealing of the thin film at higher temperatures. Much stronger suppression was noticed for PS having a molecular weight higher than M_e. When PS thin films were deposited onto a precured APTES network, no dewetting suppression was observed.     

Role of the external pressure on the dewetting of soft interfaces

We consider dewetting at soft interfaces, e.g., a rubber ball squeezed in a fluid against a flat hard substrate. We show that the applied squeezing pressure, which was not taken into account in earlier studies of the dewetting transition, may have a big effect on the squeeze-out dynamics.Received: 10 August 2003, Published online: 3 September 2003PACS: 68.15. + e Liquid thin films - 81.40.Pq Friction, lubrication, and wear     

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     

The timescale of spinodal dewetting at a polymer/polymer interface

We investigate the dynamics of spinodal dewetting in liquid-liquid polymer systems. Dewetting of poly(methyl-methacrylate) (PMMA) thin films on polystyrene (PS) “substrates” is followed in situ using neutron reflectivity. By following the development of roughness at the PS/PMMA interface and the PMMA surface we extract characteristic growth times for the dewetting process. These characteristic growth times are measured as a function of the molecular weight of the two polymers. By also carrying out experiments in the regime where the dynamics are independent of the PS molecular weight, we are able to use dewetting to probe the scaling of the PMMA thin film viscosity with temperature and molecular weight. We find that this scaling reflects bulk behaviour. However, absolute values are low compared to bulk viscosities, which we suggest may be due in part to slippage at the polymer/polymer interface. Received 25 June 2001 and Received in final form 5 December 2001     

Modelling thin-film dewetting on structured substrates and templates: Bifurcation analysis and numerical simulations

We study the dewetting process of a thin liquid film on a chemically patterned solid substrate (template) by means of a thin-film evolution equation incorporating a space-dependent disjoining pressure. Dewetting of a thin film on a homogeneous substrate leads to fluid patterns with a typical length scale, that increases monotonously in time (coarsening). Conditions are identified for the amplitude and periodicity of the heterogeneity that allow to transfer the template pattern onto the liquid structure ("pinning") emerging from the dewetting process. A bifurcation and stability analysis of the possible liquid ridge solutions on a periodically striped substrate reveal parameter ranges where pinning or coarsening ultimately prevail. We obtain an extended parameter range of multistability of the pinning and coarsening morphologies. In this regime, the selected pattern depends sensitively on the initial conditions and potential finite perturbations (noise) in the system as we illustrate with numerical integrations in time. Finally, we discuss the instability to transversal modes leading to a decay of the ridges into rows of drops and show that it may diminish the size of the parameter range where the pinning of the thin film to the template is successful.Received: 29 January 2003, Published online: 15 July 2003PACS: 68.15.+e Liquid thin films - 81.16.Rf Nanoscale pattern formation - 47.20.Ky Nonlinearity (including bifurcation theory)     

Structural relaxation of spin-cast glassy polymer thin films as a possible factor in dewetting

Reiter [1] has recently reported a situation in which the dewetting of quasi-solid films is linked to plastic deformation - rather than viscous flow - resulting from capillary forces. Herein we propose that, in thin films of some glassy polymers - especially poly(methyl methacrylate) (PMMA) - prepared by spin-casting from solvent, structural relaxation might impart sufficient stress to cause plastic deformation. We find that PMMA films decrease in thickness by several percent, which is sufficient to create significant stress in those cases in which the film is attached to a rigid substrate. The floating technique, which can take tens of minutes, might allow most of the structural relaxation to occur prior to dewetting experiments.Received: 1 August 2003PACS: 65.40.De Thermal expansion; thermomechanical effects - 82.60.Lf Thermodynamics of solutions - 61.41. + e Polymers, elastomers, and plasticsM. Sferrazza: Current address: Département de Physique, Université Libre de Bruxelles, Boulevard du Triomphe, CP223, 1050 Bruxelles, Belgium     

Growth and morphology of thin films of aromatic molecules on metals: the case of perylene

The morphology and growth of perylene films on copper and gold surfaces have been characterized by XPS, AFM, SEM and polarization microscopy. Deposition at cryogenic temperatures leads to amorphous but homogeneous films whereas growth a room temperature results in a formation of disjointed crystalline islands. A similar morphology was observed after thawing the amorphous films which were grown before at low temperature and hence demonstrates a pronounced dewetting. Furthermore, it was found that the geometry of the resulting islands depends on the actual substrate surface which is attributed to the formation of seed layers and their influence on the subsequent film growth. The presently described dewetting and island formation appears to be a quite general phenomenon of organic film growth which needs to be considered in the interpretation of spectroscopic data and STM measurements for organic thin films. PACS 68.37.-d; 68.37.Ps; 68.55.Jk     

Stability of thin polymer films on a corrugated substrate

We study the wetting behaviour of thin polystyrene (PS) films on regularly corrugated silicon substrates. Below a critical film thickness the PS films are unstable and dewet the substrates. The dewetting process leads to the formation of nanoscopic PS channels filling the grooves of the corrugated substrates. Films thicker than the critical thickness appear stable and follow the underlying corrugation pattern. The critical thickness is found to scale with the radius of gyration of the unperturbed polymer chains. Received 6 April 2000 and Received in final form 24 August 2000     

Open questions and promising new fields in dewetting

This contribution summarizes the present understanding of dewetting focusing on three points that are either controversial or open. The first issue concerns the initial formation of holes, i.e. the film rupture. The second point concerns the unstable growth of holes, i.e. the transversal instability of the receding contact line. Finally, recent extensions towards dewetting on heterogeneous substrates are examined. In passing the long time evolution in dewetting and the coupling of dewetting with other effects are discussed.Received: 1 August 2003PACS: 68.15. + e Liquid thin films - 68.55.-a Thin film structure and morphology - 47.20.Ma Interfacial instability     

Dewetting near the glass transition: transition from a capillary force dominated to a dissipation dominated regime

Dynamics and corresponding morphology of dewetting of thin polystyrene films at temperatures close to the glass transition were investigated by measuring simultaneously dewetted distance and width of the rim. Comparing the opening of cylindrical holes with the retraction of a straight contact line revealed (i). a drastic influence of the geometry (planar or radial symmetry) on the dynamics at early stages, (ii). a new logarithmic dewetting regime, and (iii). transitions between four dewetting regimes clearly indicated by changes in the shape of the rim. The complete dewetting scenario can be understood as an initial dominance of capillary driving forces, which is progressively overtaken by dissipation related to the increasing size of the rim.     

Capillary instabilities by fluctuation induced forces

The spontaneous break-up of thin films is commonly attributed to the destabilizing effect of van der Waals forces. Dispersion forces can be considered in terms of the confinement of the electromagnetic fluctuation spectrum. The principle of confinement is more general than the usual argument of interacting dipole fluctuations. It includes also disjoining pressures that are caused by thermal fluctuations. In this context, we review recent publications on the dewetting of thin polymer films, and argue that the presence of an acoustic disjoining pressure is necessary to adequately describe some of these experimental results.Received: 1 August 2003PACS: 47.20.Ma Interfacial instability - 68.15. + e Liquid thin films - 43.25.Qp Radiation pressure     

Generic morphologies of viscoelastic dewetting fronts

A simple model is put forward which accounts for the occurrence of certain generic dewetting morphologies in thin liquid coatings. It demonstrates that, by taking into account the elastic properties of the coating, a morphological phase diagram may be derived which describes the observed structures of dewetting fronts. It is demonstrated that dewetting morphologies may also serve to determine nanoscale rheological properties of liquids.     

Dynamics and instability of solid-state dewetting

Spontaneous dewetting of solid thin films proceeds by edge retraction of film edges and/or by heterogeneous void growth. Classical 1D and 2D continuous models of the evolution of a dewetting film, based on surface diffusion mechanisms, predict that in the long-time limit dewetting obeys universal scaling laws. In this paper, we review 1D and 2D predictions and recent experimental results. For this purpose, using Si(001)/SiO₂ and Ge(001)/SiO₂ single-crystalline thin films in different geometries, we have been able to compare theoretical predictions to experimental results obtained by combining in situ LEEM and ex situ AFM measurements. For dewetting from film edges, experimental results partially differ from continuous models predictions. More precisely, because of the crystallographic anisotropy: (i) the facetted edges remain stable during dewetting (they simply recede at constant shape) while poorly or un-facetted edges are unstable (they recede by finger formation); (ii) rim formation, induced by mass-conservation condition, proceeds in a layer-by-layer mode and is limited by 2D nucleation properties on the top of the rim; (iii) the island generation mechanism differs from the mass shedding behaviour predicted by 1D models. For dewetting mechanisms involving void growth, different behaviours are reported and discussed. For thin Si(001)/SiO₂ films, the corners of the opening square-shaped voids lead to a local destabilisation of the growing voids. For thin Ge(001)/SiO₂ films, the side of the voids invariably turns instable and forms tip dendrites whose branch density depends on the temperature and the initial film thickness. Finally, ultra-thin films, more sensitive to local fluctuations, dewet in a fractal geometry.     

Stability of thin nematic films

We discuss the peculiarity of thin nematic films on solid substrates with a free surface, underlining the differences with what is usually seen in dewetting. We review the thermodynamic basis of the coupled phase/thickness separation that has previously been shown experimentally. We give new experimental evidences for the origin of the coupling force chosen in our previous theoretical model. This additional information contributes to the discussion raised by the article of Ziherl and Zumer in this issue [19].Received: 3 December 2003PACS: 68.15. + e Liquid thin films - 64.70.Md Transitions in liquid crystals - 61.30 Surface phenomena in liquid crystals including anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions and wetting transitions     

Influence of surface cleaning on dewetting of thin polystyrene films

Thin polystyrene (PS) films on top of silicon substrates are a frequently investigated model system in the framework of unstable films. However, with respect to stability the various experiments yielded contradictory results. Focussing on the influence of preparation conditions such as the surface cleaning solves these contradictions. By applying different surface cleans the PS film can be changed from a stable homogeneous one into a completely dewetted one. In addition to the type of clean applied, the time between cleaning the surface and spin-coating the polymeric layer on top turned out to be an important experimental parameter.Received: 1 August 2003PACS: 68.55.-a Thin film structure and morphology - 68.15. + e Liquid thin films - 68.47.Mn Polymer surfaces     

Visualization of nanosecond laser-induced dewetting,ablation and crystallization processes in thin silicon films

In the present work, nanosecond pulsed laser crystallization, dewetting and ablation of thin amorphous silicon films are investigated by time-resolved imaging. Laser pulses of 532 nm wavelength and 7 ns temporal width are irradiated on silicon film. Below the dewetting threshold, crystallization process happens after 400 ns laser irradiation in the spot central region. With the increasing of laser fluence, it is observed that the dewetting process does not conclude until 300 ns after the laser irradiation, forming droplet-like particles in the spot central region. At higher laser intensities, ablative material removal occurs in the spot center. Cylindrical rims are formed in the peripheral dewetting zone due to solidification of transported matter at about 500 ns following the laser pulse exposure.     

Low temperature N2-based passivation technique for porous silicon thin films

A technique is presented for the passivation of porous silicon (PS) thin films via nitrogen based annealing at the lowest temperature ever reported. Annealing freshly anodized PS thin films at temperatures as low as 520  ^°C under N₂ flow in a rapid thermal annealer produces films that show no change in refractive index when exposed to ambient conditions over 60 days. These films also exhibited chemical resistance by surviving a brief dip in both concentrated KOH and buffered HF. Unlike most other PS surface passivation methods, this technique causes negligible reduction in refractive index of the annealed PS thin films. Passivation only occurs when dangling bonds and mono-hydrides populate the PS surface, providing a path for thermal interactions with the N₂ gas.     

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.