Polymer phase separation, pattern formation

Pattern Formation by Polymer Phase Separation. Like the SAM technique, polymer phase separation plays an important role in fabrication of microstructures, as it provides the opportunity for nanoscale patterning which otherwise is difficult by lithographic techniques 278, 279). The conditions necessary for microphase separation in immiscible polymer mixtures depend on their molecular architectures, nature of monomers, compositions, and molecular weights 280). Briefly, for linear homopolymer mixtures of A and B, the free energy of mixing per unit volume is given by 280, 281) ... 

Gutmann, J.S., Muller-Buschbaum, P., Stamm, M. Complex pattern formation by phase separation of polymer blends in thin films. Faraday Discuss. 112, 285-297 (1999)... 

Direct self-assembly (DSA) pays attention to pattern formation in materials smaller than 20 nm in size. DSA does not require special equipment for patterning, which is different from immersion lithography and nanoimprint. Pattern formation is performed by microphase separation in the DSA process. DSA patterning is quite different from conventional patterning. Diblock polymers composed of hydrophilic and hydrophobic polymer units are applied for DSA materials. Diblock polymers of the A-B type, where the A block is reciprocal to the B-block, accelerate to concentrate in the same block. A diblock polymer produces self-assembly and microphase separation by formation of a sphere, cylinder, and lamellar, which are controlled by product P of the % parameter (Flory interaction parameter) and degree of polymerization N siP = Z ab)- Classical structures of phases are shown in Figure 3.20. 

Fig. 4.27 Sketch of the demixing process observed in a sample containing a colloid polymer mixture at high polymer concentrations at q<03. The corresponding light scattering patterns are indicated as well a gel formation birth , b gel lifetime life , c gel collapse death , d macroscopic phase separation. Redrawn from Verhaegh et al. [107]...

The mechanism of pattern formation in photo-crosslinked polymer blends is not completely understood. Besides the competition between phase separation and photo-crosslinking reactions, elastic stress in the sample due to network formation, as well as an autocatalytic feedback arising from concentration fluctuations amplified by the crosslinking reaction, may play an important role in these ordering processes. 

When polymers undergo phase separation in thin films, the kinetic and thermodynamic effects are expected to be pronounced. The phase separation process can be controlled to effect desired morphologies. Under suitable conditions a film deposition process can lead to pattern replication. Demixing of polymer blends can lead to structure formation. The phase separation process can be characterized by the binodal and spinodal curves. UCST is the upper critical solution temperature, which is the temperature above which the blend constituents are completely miscible in each other in all proportions. LUST behavior is not found as often in systems other than among polymers. LUST is the lower critical solution temperature. This is the... 

Different phase separated morphologies can be found in different polymer solvent systems. The pattern formation consists of several stages. In the initial stage, phase separation results in a layered morphology of the two solvent swollen phases. As more solvent evaporates, this double layer is destabilized in two ways (1) capillary instability of the interface, and (2) surface instability. Each of the mechanisms results in different morphological length scales. Core shell spherical domains in phase-separated ternary systems have also been found. The shell thickness can be a few nanometers. 

Phase-separation phenomena are commonly observed in various kinds of condensed matter, including metals, semiconductors, simple liquids, and complex fluids such as polymers, surfactants, colloids, and biological materials. The study of these processes of pattern evolution is very important for both engineering applications and basic understanding of nonequilibrium dynamics of pattern formation [1]. Phase separation in each material group of... 

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