Dewetting Experiments

Some comparative dewetting experiments were conducted on a fluoropolymer. Teflon PFA (du Pont de Nemours and Co.) representing a relatively rigid substrate (p, = 250 MPa) with similar surface characteristics (surface free energy ys = 20 mJ m ). 

Dewetting Experiments Using a Visco-Elastic Fluid Polystyrene Films Slightly Above... 

We will present a few examples, which demonstrate that the balance of forces enables us to extract in a highly quantitative manner detailed information on molecular and interfacial properties of thin polymer films from relatively simple dewetting experiments. 

As in dewetting experiments with simple (Newtonian) fluids, one can also measure the width w of the rim. To do this, the exponential decay is truncated when the height of the rim (measured from the substrate level) has decayed to a value about 1.1 ho. Then, according to theory [42,44,46,47], w = v/2 Aq ln(10f/T), with T being a characteristic relaxation time of the polymer. As can be seen in... 

Fig. 17 Results of a typical dewetting experiment for high molecular weight PS film. A 55-nm thick film = 4,840 kg/mol) was dewetted at 130°C on a silicon wafer coated with an adsrahed PDMS layer (M =139 kg/mol). The growth of a nucleated hole is compared to dewetting liom a straight edge, established by breaking the silicon wafer into two parts...

We want to emphasize that a logarithmic time dependence of d and w, and the corresponding decrease of V, are not expected for a Newtonian liquid [42]. Moreover, our results cover times shorter than the longest relaxation time in equilibrated bulk samples (i.e., the reptation time). Thus, the visco-elastic properties of PS certainly affect our dewetting experiments. Thus, a detailed theoretical model has been developed that takes into account residual stresses, interfacial friction (i.e., slippage), and visco-elasticity [42,44,46],... 

Fig. 1.24. Dewetting experiment (a) small droplet deposited (b) large droplet deposited (c) spreading of the large droplet, using a pipette, to make a metastable film, fixed around its outer edge to the wetting ring (d) nucleation and growth of a dry zone...

C. Neto, K. Jacobs, R. Seemann, R. Blossey, J. Becker, and G. Griin, Satellite hole formation during dewetting experiment and simulation, J. Phys. Condens. Matter, 15,3355-3366 [2003). 

Dewetting experiments by Reiter [59-61] and subsequently by many others [62-66, 68-75,92-128,137-164] revealed the morphological evolution, self-organization, and pattern formation in thin polymer films. In most experiments the onset of instability is with the form of an undulation of the film surface (Figure 11.1a) [62] or the appearance of fairly equally sized but random collection of holes (Figure 11.1b) [59,60]. The mean... 

The discussion up to this point may give the impression that polymer thin Aims dewetting experiments are rather simple, as they involve a few easy steps like Aim preparation, heating/solvent vapor exposure and Anally, characterization of the evolved structures with an optical or atomic force microscope. However, extracting the exact information about the dewetting mechanism from a particular experiment... 

Particularly relevant for this chapter are the results obtained by Hu et al. on the confined crystallization of nanodroplets [147] and spherical MDs within block copolymers [148]. The simulated nanodroplets, which could have resulted from a dewetting experiment such as those already explained above, resembled a pancake. Upon decreasing the size of the droplet, both nucleation and crystallization rate became slower. At the interfaces, there was a strong preference for edge-on crystal orientation in concordance with recent experimental observations [81, 96]. Nevertheless, the simulation results indicated that the contribution of interfacial or surface nucleation rate to the overall nucleation rate was equally important to the nucleation of randomly oriented nuclei, which could homogeneously nucleate inside the pancake volume. Therefore, Hu et al. [147] concluded that the nucleation rate of the nanodroplets depends on both the thickness and the volume of the droplets [147]. 

In Fig. 1.3, we define the central parameters, which have to be measured systematically in a dewetting experiment. The changes of the dewetted distance (this is either the radius R(t) of a hole, when dewetting is initiated by a point, or the distance L(t) to an initiating straight contact line) are controlled by the balance of acting forces, for example related to frictional properties at the film-substrate interface. The parameters W(t) and H(t), which characterize the rim, reflect information on the rheological properties of the fluid film. 

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