Dewetted structure

Applications of Dewetted Structures in Oi anic Photonics and Electronics... 

Applications of Dewetted Structures in Organic Photonics and Electronics 197... 

The values of Go and y are known and for the elastomer of Young s modulus of 2.1 MPa, [/o = 8 X 10 mm-s [12]. We can then evaluate 8 at ca. 20 mn. This value is perhaps a little high but of the same order of magnitude as earlier estimated [6]. Thus, despite some necessary approximations and simplifying hypotheses, we arrive at a semiquantitative explanation of the relationship between dewetting and therefore, presumably, wetting speed and the molecular structure of the elastomeric substrate. 

The effect of AB diblock size relative to the homopolymers on the compati-bilization of A/B homopolymer blends was examined using numerical self-consistent field theory (in two dimensions) by Israels et al. (1995). They found that the interfacial tension between homopolymers can only be reduced to zero if the blocks in the diblock are longer than the corresponding homopolymer. Short diblocks were observed to form multilamellar structures in the blend, whereas a microemulsion was formed when relatively long copolymers were added to the homopolymer mixture. These observations were compared to experiments on blends of PS/PMMA and symmetric PS-PMMA diblocks reported in the same paper. AFM was used to measure the contact angle of dewetted PS droplets on PMMA, and the reduction in the interfacial tension caused by addition of PS-PMMA diblocks was thereby determined. The experiments revealed that the interfacial tension was reduced to a very small value by addition of long diblocks, due to emulsification of the homopolymer by the diblock, in agreement with the theoretical expectation (Israels et al. 1995). 

Fig. 30. Different types of the momentary morphologies which are typically observed during wetting (a-c) and dewetting (d-f) events on a solid flat substrate, a droplet, b spherical cap with a precursor film.c thin film (eventually with a multilayer structure), d thin liquid film, e ruptured film with rims at the dewetting front,f droplets...

Modern coating technologies require increasingly thinner polymer films. This requirement is opposed by the surface pressure and the chain elasticity. Below a certain equilibrium thickness, the film is either metastable or even unstable and tends to break into droplets regardless of the chemical structure of the substrate [321, 322]. Anomalous wetting behaviour was observed for amphiphilic polymer films whose stability is controlled by the orientation of the surface active moieties [323,324]. All these phenomena belong to the dewetting problem. 

The bilayer morphology of thin asymmetric films of may be unstable. A regularly corrugated surface structure of the films was ascribed to spinodal transition into a laterally phase separated structure, where the surface morphology depended on the polymer incompatibility and the interfacial interactions [347, 348]. Recently, the phase separation and dewetting of thin films of a weakly incompatible blend of deuterated PS and poly(p-methylstyrene) have been monitored by SFM [349, 350]. Starting from a bilayer structure, after 454 h at T= 154 °C the film came to the final dewetting state where mesoscopic drops of... 

Recently, chemically patterned surfaces have attracted a lot of attention due to their potential use as templates for lateral ordering of polymer films. On a micrometer scale, liquid dewets such surfaces and segregates on surface areas which exhibit preferential interaction with the liquid [360,361 ]. A few successful attempts have been made to transfer a lateral variation in surface energy into a composition pattern in the polymer film [16,362,363]. Figure 39a shows a laterally patterned structure which consist of periodic stripes of alternating PVP and PS domains. One of the domains, e.g. PVP, could be removed by dissolution in a... 

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