Micro-phase structure formation

The formation mechanism of micro-phase structures for block copolymers essentially roots in a delicate balance between entropic and enthalpic contributions to the Helmholtz energy. Several theories have... 

Many polymer blends or block polymer melts separate microscopically into complex meso-scale structures. It is a challenge to predict the multiscale structure of polymer systems including phase diagram, morphology evolution of micro-phase separation, density and composition profiles, and molecular conformations in the interfacial region between different phases. The formation mechanism of micro-phase structures for polymer blends or block copolymers essentially roots in a delicate balance between entropic and enthalpic contributions to the Helmholtz energy. Therefore, it is the key to establish a molecular thermodynamic model of the Helmholtz energy considered for those complex meso-scale structures. In this paper, we introduced a theoretical method based on a lattice model developed in this laboratory to study the multi-scale structure of polymer systems. First, a molecular thermodynamic model for uniform polymer system is presented. This model can... 

Furthermore the processes of physical gelation may be due to such a phase change across spinodal lines since micro-phase separation may induce the formation of a porous structure which could have high chemo-mechanical response. 

Rod—coil block copolymers have both rigid rod and block copolymer characteristics. The formation of liquid crystalline nematic phase is characteristic of rigid rod, and the formation of various nanosized structures is a block copolymer characteristic. A theory for the nematic ordering of rigid rods in a solution has been initiated by Onsager and Flory,28-29 and the fundamentals of liquid crystals have been reviewed in books.30 31 The theoretical study of coil-coil block copolymer was initiated by Meier,32 and the various geometries of microdomains and micro phase transitions are now fully understood. A phase diagram for a structurally symmetric coil—coil block copolymer has been theoretically predicted as a... 

There are basically two topics that need to be addressed regarding the effect of amphiphilic polymers on the physical behaviour of microemulsions. The first topic is related to phase behaviour and structure formation. Amphiphilic polymers can strongly influence phase behaviour because of their impact on the bending rigidity of the surfactant film. For both droplet micro emulsions and bicontinuous microemulsions such phenomena were studied. Especially in droplet microemulsions, amphiphilic polymers were used to interconnect microemulsion domains. This leads to ordering phenomena and can alter the phase behaviour. The second topic again is based on systems where microemulsion domains are connected via polymers. It covers dynamic phenomena with a focus on viscoelastic properties. Important in this area is the formation of transient or permanent networks. 

Microemulsions and surfactant-stabilized (macro) emulsions are distinctively different with respect to thermodynamic stability and, therefore, while most significant for both types of systems, the role of studies of phase behavior is different in the two cases. For emulsions we are con-eemed with two- or multi-phase regions in the phase diagrams, and for microemulsions with one-phase regions. Beeause of that micro emulsion studies are closely related to studies of other thermo-dynamically stable phases, notably liquid crystalline phases and micellar solutions. Structural models of microemulsions have to a considerable extent been advanced on the basis of our understanding of other stable phases the formation and stability of a micro-emulsion phase for a certain surfactant results from the comope-tition with alternative phases. The principal differences between micro emulsions and emulsions, together with the related nomenclature, is bound to lead to considerable confusion for example, the persistence in literature of emulsion-based structural pictures of microemulsions can be traced to the related names. However, the term microemulsions is kept for historical reasons. 

The properties of the linear material 7.27 and the network copolymer 7.28 have been studied by dynamic mechanical analysis, DSC, and transmission electron microscopy. Evidence was obtained for the formation of highly ordered micro-phase-separated superstructures in the solid state from the materials 7.27. The Cu(bipy)2 moieties appear to form ordered stacks, and this leads to thermoplastic elastomer properties. In contrast, the network structure of 7.28 prevents significant microphase separation [51-53]. By means of related approaches, dinuclear Cu helical complexes have also been used to create block copolymers by functioning as cores [54], and polymer networks have also been formed by using diiron(II) triple helicates as cores for the formation of copolymers with methyl methacrylate [55]. 

An alternative approach concerns the preparation of multilayer films in which each layer has different refractive indexes. Li et al. [107] fabricated multilayers with an increasing refractive index from the top to the bottom of the film. They prepared blends of BCP/homopolymers (PS-b-PMMA/PMMA). They used an octadecyltri-chlorosilane-modified glass substrate, and a mixture of solvents made the PMMA domains form a gradient distribution in the vertical direction of the entire micro-phase-separated film. This unique behavior allows, upon removal of PMMA, to obtain porous PS-based structures with variable porosity in the vertical direction (Fig. 6.17). Later, the authors investigated further the parameters [total degree of polymerization (AO, the volume fraction of the PMMA block (/pmma). the weight percentage of the homopolymer, and the solution concentration] that direct the structure formation and related them with the broadband antireflection properties [108]. 

Fig. 39. Micro-EDX analysis of Ndl23 crystals grown by the modified TSSG method in low-Po, atmosphere from contamination-firee Nd Oj crucibles with different post-growth heat treatments. In all the cases final oxygenation at 340°C in oxygen was applied. The picture demonstrates (a) tweed structure formation and (b) nanoscale composition fluctuations in crystals with the anomalous peak effect on a magnetization curve. Note that the composition profile for heavy atoms (Ba/Nd ratio) is similar to wave-like fluctuations typical for demixing behavior or a spinodal homophase decomposition rather than for a heterophase decomposition with the formation of a boundary between the crystal matrix and the precipitated phase (M. Nakamura et al. 1996c).

A copolymer-homopolymer mixture provides us with a variety of domain morphology since both macro- and micro-phase separations take place simultaneously. (Equi-hbrium property has been studied extensively by a mean-field theory [1].) We consider a mixture of A-B diblock copolymer and C homopolymer assuming a short-range repulsive interaction between A and B monomers and B and C monomers. One may expect a multiple domain structure in a sense that microphase separated domains are developed in a macrophase separated domain [2, 3]. It is also expected that formation of vesicles is also possible... 

The flow and heat transfer in heated micro-channels are accompanied by a number of thermohydrodynamic processes, such as liquid heating and vaporization, boiling, formation of two-phase mixtures with a very complicated inner structure, etc., which affect significantly the hydrodynamic and thermal characteristics of the cooling systems. 

In essence two types of carbonitride are formed in a Ti,Nb-hardened micro-alloyed steel. At high temperatures a predominantly TiN-rich carbonitride is formed. However, on cooling to lower temperatures a predominantly NbC-iich carbonitride also precipitates. Both caibonitrides are based on the NaCl structure and form part of a continuum usually described by a formula such as (TixNb. xXCzNi.2). This can be expanded to include elements such as V and Ta, so the formula becomes (TazTiyNb Vi.,. z)(CzNi.z). The formation of two types of carbonitride can be consisted due to phase separation and Fig. 10.54 shows a projected miscibility... 

Many reports are available where the cationic surfactant CTAB has been used to prepare gold nanoparticles [127-129]. Giustini et al. [130] have characterized the quaternary w/o micro emulsion of CTAB/n-pentanol/ n-hexane/water. Some salient features of CTAB/co-surfactant/alkane/water system are (1) formation of nearly spherical droplets in the L2 region (a liquid isotropic phase formed by disconnected aqueous domains dispersed in a continuous organic bulk) stabilized by a surfactant/co-surfactant interfacial film. (2) With an increase in water content, L2 is followed up to the water solubilization failure, without any transition to bicontinuous structure, and (3) at low Wo, the droplet radius is smaller than R° (spontaneous radius of curvature of the interfacial film) but when the droplet radius tends to become larger than R° (i.e., increasing Wo), the microemulsion phase separates into a Winsor II system. 

Block or graft copolymers in a selective solvent can form structures due to their amphiphilic nature. Above the critical micelle concentration (CMC), the free energy of the system is lower if the block copolymers associate into micelles rather than remain dispersed as single chains. Often the micelles are spherical, with a compact core of insoluble polymer chains surrounded by a corona of soluble chains (blocks) [56]. Addition of a solvent compatible with the insoluble blocks (chains) and immiscible with the continuous phase leads to the formation of swollen micelles or polymeric micro emulsion. The presence of insoluble polymer can be responsible for anomalous micelles. 

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