Dewetting Of Thin Polystyrene Films Research Articles

Controlling Lengthscales in Water-Solvent Induced Self-Organized Dewetting of Thin Polystyrene Films by Modulating the Surface Properties of the Substrate

We report here a facile method of modulating the lengthscales of self-organized dewetting of polystyrene thin films by varying the extent of silane grafting on the substrate. The self-assembled monolayers (SAM) of octadecyltrichlorosilane (OTS) were grafted on glass substrates by exposing them to different doses of OTS vapors in a controlled, low pressure environment. Measurement of water contact angles on the modified glass surfaces showed that the contact angles could be continuously increased from 40° to 70° by increasing the time of exposure to the OTS vapors from a few minutes to several hours. Polystyrene (PS) thin films were spin coated on the modified glass substrates and water-solvent mixture induced dewetting of the PS thin films was carried out, where the dewetting kinetics depended on the spreadability of water on the surface. We observed a reduction in the length scales of the dewetting, i.e. the wavelength of instability (λ) and droplet size (d), by increasing the extent of OTS exposure. The λ for a 50 nm thick PS film decreased from 12 to 4 µm while the droplet diameter (d) also decreased, from 2.5 to 1.6 µm.

Patterned chemisorption of proteins by thin polymer film dewetting

The covalent attachment of proteins and their patterning on surfaces are important in the bio-medical industry, for use in applications such as bio-sensing and biocompatible medical devices. In this work, proteins are chemisorbed by reaction of their amine groups to glutaraldehyde- and carboxylic acid-functionalised surfaces, within patterns obtained by the dewetting of thin polystyrene films. The proof of principle is shown that consecutive attachment of proteins within the same pattern is possible using this method.

Relaxation of Residual Stress and Reentanglement of Polymers in Spin-Coated Films

Performing detailed studies of viscoelastic dewetting of thin polystyrene films on solid substrates, we demonstrate the existence of residual stress due to strongly out of equilibrium chain conformations and a reduced entanglement density resulting from film preparation by spin coating. The ratio of stress over elastic modulus was found to increase strongly with decreasing film thickness and increasing chain length. Full equilibration of chain conformations required long times comparable to bulk reptation times. However, for chains longer than about 3000 monomers, the residual stress relaxed faster, at a rate independent of chain length.

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).

Influence of Molecular Architecture on the Dewetting of Thin Polystyrene Films

The control of dewetting for thin polymer films is a technical challenge and of significant academic interest. We have used polystyrene nanoparticles to inhibit dewetting of high molecular weight, linear polystyrene, demonstrating that molecular architecture has a unique effect on surface properties. Neutron reflectivity measurements were used to demonstrate that the nanoparticles were uniformly distributed in the thin (ca. 40 nm) film prior to high temperature annealing, yet after annealing, they were found to separate to the solid substrate, a silanized silicon wafer. Dewetting was eliminated when the nanoparticles separated to form a monolayer or above while below this surface coverage the dewetting dynamics was severely retarded. Blending linear polystyrene of similar molecular weight to the polystyrene nanoparticle with the high molecular weight polystyrene did not eliminate dewetting.

Dewetting of thin polystyrene films absorbed on epoxy coated substrates

Various characteristics of dewetting of thin polystyrene (PS) films absorbed on highly cross-linked epoxy-coated and silicon oxide covered substrates are studied as a function of PS film thickness (20<h<1300 A) by optical microscopy, atomic force microscopy, and x-ray and neutron reflectivity. For a silicon oxide covered substrate, the nucleation of holes and growth (NG) mechanism is observed for h>h(c1) whereas the spinodal dewetting (SD) occurs through the growth of surface undulations for h<h(c1), where h(c1) is approximately 4R(g). For an epoxy-coated substrate, the NG mechanism is observed for h>h(c2) while the SD mechanism is observed for h<h(c2), where h(c2) is approximately 6R(g). We demonstrate that the highly cross-linked epoxy-coated silicon substrate leads to retardation of the PS film dewetting in comparison to the silicon oxide covered silicon substrate. Moreover, we confirm that the epoxy-coated substrate leads to a significant decrease in the fraction of dewetted area at the apparent equilibrium stage of dewetting due to the anchoring effect of PS molecules caused from the cross-linked networks of the epoxy layer. In contrast the retardation effect of the epoxy-coated substrate on the rate of dewetting is more remarkable for relatively thinner PS films (h< approximately 800 A) than thicker films ( approximately 800<h<1300 A) since the short-range intermolecular interactions are dominant for relatively thin PS films. Thus the highly cross-linked epoxy-coated substrate has a large influence on the kinetics, morphology, and mechanism of dewetting of thin PS films.

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.

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.

Correlated dewetting patterns in thin polystyrene films

We describe preliminary results of experiments and simulationsconcerned with the dewetting of thin polystyrene films (thickness< 7nm) on top of silicon oxide wafers. In the experiments we scratched an initiallyflat film with an atomic force microscopy (AFM) tip, producing dry channels inthe film. Dewetting of the films was imaged in situ using AFM and acorrelated pattern of holes (‘satellite holes’) was observed along the rimsbordering the channels. The development of this complex film ruptureprocess was simulated and the results of experiments and simulations are ingood agreement. On the basis of these results, we attempt to explain theappearance of satellite holes and their positions relative to pre-existing holes.

Influence of Dendrimer Additives on the Dewetting of Thin Polystyrene Films

A small mass concentration of poly(benzyl ether) dendrimer added to a low molecular mass polystyrene is found to inhibit the dewetting of a thin (≈50 nm) polystyrene film from an acid-etched silicon substrate. The inhibition effect is found to depend on generation number where the lowest generation (G = 3) tested was the most effective. Our findings are qualitatively similar to previous observations by Barnes et al. [Macromolecules 2000, 33, 4177−4185.] where the addition of fullerenes (“buckyballs”, C60) similarly inhibited the dewetting of thin polystyrene and polybutadiene films. Thus, dewetting inhibition by nanoparticles appears to be a general effect, although certain conditions apparently need to be met for its occurrence. Specifically, a general tendency for the particles to segregate to the solid substrate seems to be required and the interaction between the particles and polymer must not be too unfavorable. The phase boundaries of the dendrimer−polymer mixtures depend on the generation, the high...