Demixing of polymer blends

Apart from the use of block copolymers providing nanoscale features other methodologies alone or in combination can provide hierarchically stmctured patterns. For instance taking advantage of the template assisted stmcturation, demixing of polymer blend solutions on stmctured substrates can also be employed to create multiscale ordered surfaces. Boneberg et al. [202] employed this approach and blended two components, PVP and PS, already known to form ordered porous films during phase separation [203]. The authors used a micrometer patterned... 

Geldhauser, T., Walheim, S., Schimmel, T., Leideier, P., Boneberg, J. Influence of the relative humidity on the demixing of polymer blends on prepattemed substrates. Macromolecules 43, 1124-1128 (2009)... 

In applications in the semiconductor industry, polymer structures are required on length scales down to individual molecules. A bottom-up approach is better than a top-down approach in order to achieve this. A lateral resolution less than 100 nm can be created by surface instabilities and pattern formation in polymer films. Steiner [6] discussed demixing of polymer blends and pattern formation by capillary instabilities for nanostructure formation. 

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... 

Demixing of polymer blends consisting of macromolecules synthesized from the same mmiomers and differing practically only in their molecular architecture plays... 

One practical example of demixing that might be attributed to a difference in crystallizability is the incompatibility in blends of polymers with different stereochemical compositions. The stereochemical isomers contain both chemical and geometrical similarities, but differ in the tendency of close packing. In this case, both the mixing energy B and the additional mixing entropy due to structural asymmetry between two kinds of monomers are small. However, the stereochemical differences between two polymers will result in a difference in the value of Ep. Under this consideration, most experimental observations on the compatibility of polymer blends with different stereochemical compositions [89-99] are tractable. For more details, we refer the reader to Ref. [86]. 

Compared to bulk polymer mixtures, the interfacial behaviour of polymer blends is essentially different [341]. The demixing process in thin films is strongly affected by the thin film confinement and the interfacial interactions of the blend components with the confining phases (e.g., substrate and air). Even in the one-phase region of the phase diagram, preferential segregation of the components at one of the interfaces leads to a certain composition profile as a function of the distance from the free surfaces and the substrate plane [342,343]. In the... 

The main advantage of using a ternary blend (as opposed to the direct replication of Fig. 1.6, where the width of the polymer structures was directly imposed by the substrate pattern), is the relative independence of the structure parameters (width, aspect ratio) with respect to the substrate pattern. The width (and thereby the aspect ratio) of the PMMA rings in Fig. 1.7 is controlled by the relative amount of PMMA in the PS/PMMA/PVP blend. While the lateral periodicity of the polymer structures is determined by the substrate, the structure size is controllable by the relative amount of PMMA in the blend. Similar to the replication technique using two polymers, pattern replication by demixing of ternary blends should be expandable to other polymer system, with the main requirement that one of the components wets the interface of the other two. 

Kuhn R (1983) Characterization of polymer blends, block copolymers, and graft copolymers by fractionation procedures using demixing solvents. Polym Sci Technol 20 45-58. 

It is known that, in a polymer blend, thermodynamic incompatibility between polymers usually causes demixing of polymers. If the polymer is equilibrated in air, the polymer with the lowest surface energy (hydrophobic polymer) will concentrate at the air interface and reduce the system s interfacial tension as a consequence. The preferential adsorption of a polymer of lower surface tension at the surface was confirmed by a number of researchers for a miscible blend of two different polymers. Based on this concept, surface modifying macromolecules (SMMs) as surface-active additives were synthesized and blended into polymer solutions of polyethersulfone (PES). Depending on the hydro-phobic or hydrophilic nature of the SMM, the membrane surface becomes either more hydrophobic or more hydrophilic than the base polymeric material. ... 

Matsuzaka, K., Jinnai, H., Koga, T., and Hashimoto, T. (1997) Effect of oscillatory shear deformation on demixing processes of polymer blends. Atacromolecules, 30 (4), 1146-1152. 

During transportation of polymer solutions in production units and processing polymer blends the system is subjected to considerable shear rates. It is therefore very important to know the effect the shearing forces have on the phase behavior of a (co-)polymer system. Over the years there have been many discussions about the possible influence of flow/shear on the demixing behaviour of polymer blends. Very recently the importance of shear was clearly demonstrated by several groups, e.g. by Aelmans et al. [10]. 

Mixing of two amorphous polymers can produce either a homogeneous mixture at the molecular level or a heterogeneous phase-separated blend. Demixing of polymer chains produces two totally separated phases, and hence leads to macrophase separation in polymer blends. Some specific types of organized structures may be formed in block copolymers due to microphase separation of block chains within one block copolymer molecule. 

Phase separation can emerge spontaneously in thin films of polymer blend, and it is this thermodynamically driven phase-separation process that produces the patterning of thin polymer blend films. It can be induced by several parameters, each with critical values, including (i) the ratio between the components [10,22] (ii) the demixing temperature [12] and (iii) the chemical nature of the solvent. 

Then in section III, we analyze the liquid-liquid phase separation of polymer blends in solvents of various quality. Particular emphasis is put on the case of a common good solvent and on the discussion of the critical properties of demixing which are very unusual as the critical behavior is not of the mean-field type (except for very long chains and low incompatibility degrees) and is als very different from that of low molecular weight ternary mixtures. We also focus on well- demixed systems and consider the interfacial properties following the work of Broseta et al. ... 

Fig. 8 Theoretical liquid-liquid demixing curve (solid line) and the bulk melting temperature (dashed line) of a flexible-polymer blend with one component crystallizable and with athermal mixing. The chain lengths are uniform and are 128 units, the linear size of the cubic box is 64, and the occupation density is 0.9375 [86]...

The kinetics of polymer/polymer demixing are many orders of magnitude slower than those for polymer/solvent demixing. In a typical instrument used to study blend demixing a polymer film supported on a thin glass cover slip is placed in a ther-mostatted pressure cell (Fig. 7.18c). The sample is annealed for some hours well... 

It is well-known that one can reduce the droplet size of a demixed polymer blend by adding a well-chosen block-copolymer at given p and T, and achieve interesting improvements of the blend s mechanical properties. 

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