Phase separation and dewetting

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... 

Ogawa, H., Kanaya, T., Nishida, K., Matsuba, G. Phase separation and dewetting in polystyrene/poly(vinyl methyl ether) blend thin films in a wide thickness range. Polymer 49(1), 254-262 (2008)... 

Mliller-Buschbaum, P., Gutmann, J.S., Stamm, M. Influence of blend composition on phase separation and dewetting of thin polymer blend films. Macromolecules 33(13), 4886-4895 (2000)... 

Typically, a thin film blend remains stable if both phase separation and dewetting can be avoided, hi some cases, even if both phase separation and dewetting are present, the films may remain stable when a layered structure is adopted [108]. This observation was evidenced by Newby and coworkers [109] using poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN) films. The film exhibited stabihty when a tri-layer structure was formed with a middle SAN-rich layer and a PMMA-rich surface wetting layer. However, when the mid-layer ruptured and form-phase domains of the film became unstable. 

Muller-Buschbamn P, O Neill SA, Affrossman S, Stamm M. Phase separation and dewetting of weakly incompatible polymer blend films. Macromolecules 1998 31 5003-9. 

Muller-Buschbaum P, Gutmann JS, Stamm M. Control of snrface morphology by the interplay between phase separation and dewetting. J Macromol Sci Phys 1999 B38 577-92. 

You J, Liao Y, Men Y, Shi T, An L, Li X. Composition effect on interplay between phase separation and dewetting in PMMA/SAN blend ultrathin films. Macromolecules 2011 44 5318-25. 

Muller-Buschbaum, P., Bauer, E., Wimnicke, O., Stamm, M. The control of thin film morphology by the interplay of dewetting, phase separation and microphase separation. J. Phys. Condensed Matter 17(9), S363-S386 (2005)... 

In summary, the methods bom in recent decades to pattern polymer films by means of an underlying templated substrate have grown in sophistication and flexibility. These techniques harness the natural tendencies of polymer Aims to dewet and phase separate, and through a simple means guide these natural processes. In all cases, the template and the desired stmcture must be matched to the properties of the polymer film and the processing conditions, but the final Alms can exhibit a variety of designs over many length scales. 

AFM measurements have greatly contributed to our understanding of the interplay between phase separation and surface dewetting in thin films of miscible polymer blends [144-149],... 

Kanemoto, R., Anas, A., Matsumoto, Y, Ueji, R., Itoh, T., Baha, Y, Nakanishi, S Ishikawa, M. and Biju, V. (2008) Relations Between dewetting of polymer thin films and phase-separation of encompassed quantum dots. J. Phys. Chem. C, 112, 8184-8191. 

Fig. 4a—d. Lamellar structures in thin films that are not considered further in detail in the present article a Thin film confined between inequivalent walls, where the lower one favors the B-rich domains and the upper one the A-rich domains. Then an arrangement where the interfaces run parallel to the walls requires that thickness D and wavelength X are related as D=(n+1/2)A, n=0,l, 2... b Thin film on a substrate that favors B-rich domains undergo at the order-disorder transition (ODT) of the block copolymer melt a phase separation into a fraction x of thickness nXh and a fraction 1-x of thickness (n+1) Xh, such that D=[xn+(l-x) (n+l)] K if the air also favors B-rich domains, c If the air favors A-rich domains instead, the phase separation happens in a fraction x of thickness (n-l/2)A and a fraction 1-x of thickness (n+ 1/2)X with n= 1,2,3... d If the block copolymer film undergoes dewetting at the substrate, droplets form with a step-pyramide like structure ( Tower of Babel [30]). 

Hansen, C.M., Surface dewetting and coatings performance, J. Paint TechnoL, 44(570), 57-60, 1972. Misev, T.A., Thermodynamic analysis of phase separation in self-stratifying coatings — solubility parameters approach, J. Coatings TechnoL, 63(795), 23-28, 1991. 

Fig. 6.6 Schemes of two possible mechanisms whereby surface chemistry templates can guide the phase separation in polymer blend films. In both cases, the substrate is predominately one stnface chemistry—light green areas—with a second surface chemistry covering a minority of the substrate—black areas. In (a)-(e), the film vertically phase separates as induced by the majority surface chemistry, and then the minority surface chemistry induces dewetting of the lower polymer. In (f)-(j), the minority surface chemistry nucleates phase separation at precise locations before spontaneous nucleation can occur [6, 14]...

Just as topographic features can be used to control dewetting in single component polymer films, such templates also can be used to control polymer blend and block copolymer phase segregation. A difficulty with BCPs, however, is generating a template with sufficiently small feature sizes to match the natural phase separation... 

PS, PMMA ChlOToform Surface analysis showed an enrichment of the surface by PMMA (phase separation). The behavior is related to differences in solubility and dewetting of PMMA-rich domains by PS-rich phases. XPS, AFM (Ton-That et al. 2001)... 

Dewetting (304-306) and phase separation phenomena are also amenable to in situ real-time SFM stndies. As shown by Jacobs and co-workers, the dewetting of PS on an oxidized silicon wafer can be followed in situ at elevated temperatures. These observations are nseful to develop a better imderstanding for film stability necessary in many modem applications of polymer ultrathin films in miniaturized electronic and microfluidic device applications. 

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