Liquid crystals Microphase separation

Several structure sizes caused by microphase separation occurring in the induction period as well as by crystallization were determined as a function of annealing time in order to determine how crystallization proceeds [9,18]. The characteristic wavelength A = 27r/Qm was obtained from the peak positions Qm of SAXS while the average size of the dense domains, probably having a liquid crystalline nematic structure as will be explained later, was estimated as follows. The dense domain size >i for the early stage of SD was calculated from the spatial density correlation function y(r) defined by Debye and Buche[50]... 

We are currently initiating three research projects that include (1) the synthesis of reflective liquid crystal/polymer composite films, (2) a study of microphase separation in hyperbranched block copolymers, and (3) the design and synthesis of polar organic thin films, which is the subject of this proposal. (47 words aim for 41 words)... 

In liquid crystals or LC-glasses one looks for orientational order and an absence of three-dimensional, long-range, positional order. In liquid crystals, large scale molecular motion is possible. In LC-glasses the molecules are fixed in position. The orientational order can be molecular or supermolecular. If the order rests with a supermolecular structure, as in soap micelles and certain microphase separated block copolymers, the molecular motion and geometry have only an indirect influence on the overall structure of the material. 

From macroscopic observations, it appears that in the DMDBTDMA-dodecane system the nature of the third phase (liquid, gel, or solid) (140) depends to a large extent on the extracted species. In some cases, microphase separations can be obtained, that is, the coexistence of a more crystalline phase with domains of diluted phase that do not separate upon centrifugation. In classical colloidal literature (141), this situation is described as a dispersion of tactoids in the form of small amounts of liquid crystals, giving macroscopically a gel. 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

A variety of experimental techniques have been used to investigate IL structures. Neutron diffraction, X-ray scattering, extended X-ray absorption fine structure (EXAFS) [97-105], and theoretical methods [64, 106-111] are probably the most powerful approaches to quantify IL structure and interactions. For example, X-ray diffraction and absorption studies along with computer simulations have shown that ILs are by no means molecular liquids [64, 89,94,112,113], Unlike a conventional solvent like hexane or chloroform, ILs contain a large number of internal interfaces and exhibit different degrees of order [89, 114], ILs can form liquid crystals, extended hydrogen-bonded networks, inclusion compounds, or microphase separated structures, where polar and non-polar regions are separated by a complex interface [86, 89, 114],... 

If pushed to an extreme the perfluorinated bridge concept can be used to obtain liquid crystals with no cyclic moieties in their mesogenic core structure [4-8]. Since the beginning of the 1980s it has been known that semi-fluorinated n-alkanes, so-called diblock compounds, F(CF2) (CH2) H, form smectic phases [49], because of microphase separation as a result of separate, layer-like aggregation of the hydrocarbon and fluorocarbon moieties. Nevertheless, if introduced into a nematic host mixture, even small quantities of these diblocks cause gelation of the mixture. Their solubility is also limited to a few percent by weight. 

Lyotropic liquid crystals are principally systems that are made up of amphiphiles and suitable solvents or liquids. In essence an amphiphilic molecule has a dichotomous structure which has two halves that have vastly different physical properties, in particular their ability to mix with various liquids. For example, a dichotomous material may be made up of a fluorinated part and a hydrocarbon part. In a fluorinated solvent environment the fluorinated part of the material will mix with the solvent whereas the hydrocarbon part will be rejected. This leads to microphase separation of the two systems, i.e., the hydrocarbon parts of the amphiphile stick together and the fluorinated parts and the fluorinated liquid stick together. The reverse is the case when mixing with a hydrocarbon solvent. When such systems have no bend or splay curvature, i.e., they have zero curvature, lamellar sheets can be formed. In the case of hydrocarbon/fluorocarbon systems, a mesophase is formed where there are sheets of fluorocarbon species separated from other such sheets by sheets of hydrocarbon. This phase is called the La phase. In the La phase the molecules are orientationally ordered but positionally disordered, and as a consequence the amphiphiles are arranged perpendicular to the lamellae. The La phase of lyotropics is therefore equivalent to the smectic A phase of thermotropic liquid crystals. 

Liquid crystal block copolymers are a recently explored group of polymers which combine microphase separation and liquid crystallinity. Yet, as early as 1963, Gratzer and Doty [37] reported the first block copolymers containing two polypeptide blocks in which one of the blocks, poly(y-benzyl-L-glutamate) (PBLG), was already well known to be liquid crystalline. In principle, all the LCP structures shown in Fig. 1 can be incorporated into block copolymer structures with a second flexible coil or LC block. Some of the possible LC-BCP structures are shown in Fig. 2. Due to the limited nature of this review, structures such as the grafted LC copolymers [38] and various multiblock LC copolymers [39] will not be covered. 

These materials represent the first observation of the SmC (zig-zag) and SmO (arrow head) structure in rod-coil diblock copolymers [41] in contrast to the homopolymer of poly( -hexyl isocyanate) which only form a nematic mesophase (both lyotropic [65] and thermotropic [66]). This confirms the idea by Halperin [60, 69] that rod-coil systems are a microscopic model for smectic liquid crystals in general. Although the SHIC rod-coil system has a relatively broad polydispersity, a smectic mesophase over a size scale of as much as 10 xm has been observed (Fig. 4B). This indicates that microphase separation plays a very important role in determining the self-assembly of the liquid crystalline process of these blocks. The existence of only a nematic phase in the rod homopolymer system is probably due to its broad polydispersity in contrast to the fact that a smectic meso-... 

Ferroelectric liquid crystals (FLC) are of great interest due to their fast electro-optical response which is about 1,000 times faster than conventional twisted nematic cells [131]. The geometry used is called a surface stabilized FLC cell which utilizes a very thin gap (=2 pm) to unwind the FLC supramolecular pitch (=1-2 pm) since the bulk FLC materials do not show macroscopic polarization. This very thin gap, however, leads to difficulties in manufacturing large panels and very poor shock resistance. Researchers have proposed the concept of microphase stabilized FLC [79,109, 130] using FLC-coil diblock copolymers for electro-optical applications as shown in Fig. 15. This concept takes advantage of ferroelectric liquid crystallinity and block copolymer microphase separation since the block... 

Hamley IW, CasteUetto V, Lu Z, Imrie C, Itoh T, A1 Hussein M. 2004. Interplay between smectic ordering and microphase separation in a series of side group liquid crystal block copolymers. Macromolecules 37 4798 4807. 

Liquid crystals represent an intermediate state of order (mesophase) between crystals and liquids. Crystals have a three-dimensional long-range order of both position and orientation (Fig. 5.1-la). Liquids, in contrast, do not show any long-range order (Fig. 5.1-lb). In plastic crystals (disordered crystals, Fig.5.1-lc), positional order is maintained, but orientational order is lost. In mesophases, imperfect long-range order is observed, and thus they are situated between crystals and liquids. The reasons for the formation of a mesophase can be the molecular shape or a microphase separation of amphiphilic compounds. 

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