Structure mesophases

Several works have been published about the phase behavior of polybibenzoates, showing the ability of the biphenyl group to produce mesophase structures. Different spacers have been used, and the results show that the structure of the spacer influences very much the transition temperatures and the nature and stability of the mesophases, as well as the ability to generate three-dimensional crystals. 

The mesophase structure that coexists with the Laphase above 75 wt % lipid is unknown. 

The study of the microphase morphology in several types of crystallizable triblock copolymers has shown that when the triblock copolymers have compositions from which the crystallizable component is able to build lamellae, there is a tremendous influence on the mesophase structure. The MDs do... 

In addition to the crystal forms, X-ray scattering studies indicate that when unoriented PEN fiber was drawn at 120 °C ( 7 g), a mesophase is generated. In this form, the molecular chains are in registry with each other in the meridional direction but not fully crystallized in the equatorial direction. This conclusion was based on the presence of additional meridional peaks not accounted for by the crystal structure obtained by X-ray scattering. The mesophase is a intermediate phase and its existence is strongly dependent upon the processing conditions consequently, it could have implications with respect to the properties of commercially produced fibers and films, since it appears to be stable and not easily converted to the crystalline form, even at elevated temperature [25, 26], The mesophase structures of PET, PEN and poly(ethylene naphthalate bibenzoate) were compared by Carr et al. [27], The phase behavior of PEN and PEN blends with other polymers has also been studied [28-32],... 

Note 1 One- or two-dimensional order leads to mesophase structure. 

FIGURE 4.11 Different mesophasic structures where the mesogenic unit is designated by a rounded square. 

For crystalline polymer systems, transition from the crystalline structure to a mesophase structure occurs, whereas for amorphous polymer systems, the mesophase occurs after the Tg has occurred. Some polymer LC systems form several mesophases. Mesophases can be detected using DSC, x-ray diffraction, and polarizing microscopy. 

An artificial neural network (ANN) model was developed to predict the structure of the mesoporous materials based on the composition of their synthesis mixtures. The predictive ability of the networks was tested through comparison of the mesophase structures predicted by the model and those actually determined by XRD. Among the various ANN models available, three-layer feed-forward neural networks with one hidden layer are known to be universal approximators [11, 12]. The neural network retained in this work is described by the following set of equations that correlate the network output S (currently, the structure of the material) to the input variables U, which represent here the normalized composition of the synthesis mixture ... 

RF-substituted benzoic acids (compounds 44—49 in Fig. 16) are somewhat distinct from the other taper shaped amphiphiles as they tend to form discrete cyclic hydrogen bonded dimers, which provide some rigidity to the core region [126], This gives rise to some interesting effects on mesophase structure. For example, the single... 

Fig. 15 Binary phase diagrams showing the change of the mesophase structure depending on the ratio of single-chain to three-chain compounds (a) amphiphiles 41/43 [139] and (b) pentaery-thritol tertabenzoates 35/37 RP = (CH2)4C6F13 for all compounds (T/°C) [138], Reproduced with permission (a) [139] copyright 2002, American Chemical Society (ACS) (b) [138] copyright 2000, Wiley-VCH...

Mitchell, D.J., Tiddy, G.J.T., Warring, L., Bostock, T. and Me Donald, M.P. (1983) Phase behavior of polyoxyethylene surfactants with water, mesophase structures and partial miscibility (cloud points). /. Chem. Soc. Faraday Trans. I, 79(4), 975-1000. 

It is well known that some auxiliary organic components such as mesitylene can expand the mesopores of titania [6] and silica [7]. Here, DDA, TritonX-100, triethanolamine and ethanol were chosen to control the mesophase structure of the Ti-Zr samples. Considering the fast condensation of Ti- or Zr- components, these organic components may help to slow down the condensation and improve the thermal stability. 

At higher temperatures and water concentrations, the system may shift into the cubic mesophase structure (see Figure 15). The water is present as spheres totally surrounded by monoglyceride. This phase has a high viscosity and is sometimes called viscous isotropic in the hterature the two terms refer to the same structure. In the presence of more water than can be accommodated in the internal spherical phase, one obtains a mixture of lumps of this cubic structure dispersed in excess water. With a saturated monoglyceride such as GMS, the lamellar structure is the main mesophase found under practical conditions, while with unsaturated monoglycerides this cubic phase is the predominant one at lower temperatures. At lower water concentrations, the spherical water micelles are farther apart, so the viscosity of the mixture becomes lower, approaching that of melted pure surfactant. This is the fluid isotropic mesophase, sometimes referred to as the L2 phase. 

Ferrocene derivatives 15 exhibited remarkable liquid crystal properties (Fig. 9-13). Indeed, they all gave rise to enantiotropic mesophases. Structures with n = 1 to 11 showed nematic phases. From n = 12 a smectic C phase formed. The latter was monotropic only for 15 (n = 12). The smectic C domain increased from n = 13 to n = 16, and, inversely, the nematic range narrowed. The last member of this series (n = 18) presented one smectic C phase between 159 °C and 179 °C. A nematic to smectic C transition and a focal-conic texture of a smectic C phase are presented in Figs. 9-14 and 9-15, respectively. 

Polymers with poly(ethylene oxide) spacers were shown to have transition temperatures similar to those for polymers with polymethylene spacers of the same length, but the mesophases formed could be quite different, which again emphasizes the importance of the spacer in determining mesophase morphology. Furthermore, polymers with polysiloxane spacers and mesogenic units, identical to those present in polymers with polymethylene spacers having a comparable number of bonds, showed different mesophase structures in addition to much lower transition temperatures. 

The chiral side chain polymers derived from asymmetric esters of terephthalic acid and hydroquinone can form (in a broad temperature range, including ambient temperature) an unusual mesophase (the isotropic smectic phase, IsoSm ) characterized by high transparency and optical isotropy within the visible wavelength range, combined with a hidden layered smectic ordering and some elements of helical superstructure at shorter dimensions of 10 to 250 nm. The short-pitch TGB A model seems to be the most adequate for the mesophase structure. 

Ageing of this sample at higher temperature (105 C) led to anisotropy in its ESR spectrum with an amplitude of anisotropy of 0,9 mT [12]. The average g-value of the anisotropy spectrum is equal to that of isotropic spectrum. Anisotropy observation under conditions of intermolecular exchange of unpaired electrons is possible only in the case of mutual orientation of the main molecular axes of paramagnetic domains located in high ordered mesophase structures [13] This leads to a preferential orientation of PMS with respect to the magnetic fields. 

Mesophase structures self-assembled from surfactants (Figure 8.35) provide another class of useful and versatile templates for generating ID nanostructures in relatively large quantities. It is well known that at critical micellar concentration (CMC) surfactant molecules spontaneously organize into rod-shaped micelles [315c]. These anisotropic structures can be used immediately as soft templates to promote the formation of nanorods when coupled with appropriate chemical or electrochemical reaction. The surfactant needs to be selectively removed to collect the nanorods/nanowires as a relatively pure sample. Based on this principle, nanowires of CuS, CuSe, CdS, CdSe, ZnS and ZnSe have been grown selectively by using surfactants such as Na-AOT or Triton X of known concentrations [238, 246]. 

Cholesteryl para-substituted benzoates give mesophases with transition temperatures and thermodynamic parameters which depend upon the para-substituent. " Crystal and mesophase structures of cholesteryl myristate appear to show some similarities in molecular packing. " X-Ray studies show that cholesteryl 17-bromoheptadecanoate crystals contain alternating regions with cholesterol and hydrocarbon-chain packing. ""... 

Structure. The traditional methods for identifying and characterizing the texture of LC phases are light microscopy and x-ray diffraction (521. Electron microscopy has also been used when it is possible to freeze the mesophase structure in the solid state (53). The chapter by Viney reviews the optical microscopy technique as applied to LCPs. 

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