Mesophase lamellae

Thus, the morjAology of the mesomorj uc fledble PDES resembles in many respects the morphology of crystalline polymers. However, in ccmtrast to the usual crystalline lamellae the mesophase lamellae have considerably larger dimensions, their thickness being 1.5 to 2 p, they consist of some layers with extended-chain macromolecules. 

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... 

Figure 1. Schematic representation of two lyotropic mesophases. The lamellar phase (left) is a periodical stacking along one dimension of soap and water lamellae. In the hexagonal phase (right), the soap cylinders are organized in a two-dimensional array.

Discussion. We can now propose a coarse description of the paraffinic medium in a lamellar lyotropic mesophase (potassium laurate-water). Fast translational diffusion, with D 10"6 at 90 °C, occurs while the chain conformation changes. The characteristic times of the chain deformations are distributed up to 3.10"6 sec at 90 °C. Presence of the soap-water interface and of neighboring molecules limits the number of conformations accessible to the chains. These findings confirm the concept of the paraffinic medium as an anisotropic liquid. One must also compare the frequencies of the slowest deformation mode (106 Hz) and of the local diffusive jump (109 Hz). When one molecule wants to slip by the side of another, the way has to be free. If the swinging motions of the molecules, or their slowest deformation modes, were uncorrelated, the molecules would have to wait about 10"6 sec between two diffusive jumps. The rapid diffusion could then be understood if the slow motions were collective motions in the lamellae. In this respect, the slow motions could depend on the macroscopic structure (lamellar or cylindrical, for example)... 

The homopolymer showed an enantiotropic nematic mesophase, whereas the diblock copolymer generated microphase-separated lamellae, in which the SCLCP block possessed a nematic-isotropization transition similar to the homopolymer (Table 17). Upon heating, the nematic microphase decreased continuously in the nematic phase from 38.5 nm to 27 nm and showed a constant value of about 26 nm after the nematic-isotropization transition. Therefore, materials in which these block copolymers are macroscopically aligned are expected to show reversible contraction in one dimension, making this polymer system an interesting candidates for an artificial muscle or actuator. 

This chapter deals almost exclusively with neat, or pure, diblock copolymer melts. Polymer blends are discussed in Chapter 9, micellar solutions in Chapter 12, and stabilized suspensions in Chapter 6. In the following, Section 13.2 briefly reviews the thermodynamics of block copolymers, and Section 13.3 describes the rheological properties and flow alignment of lamellae, cylinders, and sphere-forming mesophases of block copolymers. More thorough reviews of the thermodynamics and dynamics of block copolymers in the liquid state have been written by Bates and Fredrickson (1990 Fredrickson and Bates 1996). The processing of block copolymers and mechanical properties of the solid-state structures formed by them are covered in Folkes (1985). Biological applications are discussed in Alexandridis (1996). 

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. 

Figure 3. Lamellar packing model for polypeptide containing rod-coil diblock copolymers (A) Model for polybuta-diene-b-PBLG diblock copolymer in which the PBLG chains fold in the lamellar layer and adopt hexagonal packing. This corresponds to a SmB mesophase [9]. (B) Model for amphiphilic polypeptide diblock copolymer. Note that the rods are tilted relative the lamella layer normal while maintaining hexagonal close packing with a constant domain D independent of tilt angle.

These authors have used the technique of lamellar decoration [76] which enables detailed assessment of cheu-acteristic mesophase defects and texture on a much finer scale than previously possible with conventional electron-microscopy preparations. The defects and texture existing in the polymer melt state are first retained by thermal quenching of the polymer fluid to room temperature. The glassy LCP film is then annealed above its glass transition, but below the melting point. Crystalline lamellae grow perpendicular to the local chain axis and effectively decorate the molecular director... 

The relative decrease in enthalpy should represent the extent of disorder at the interface of the two blocks, and has actually been directly correlated to the thickness of the interphase. That is, the AH of isotropization of lamellar PS-PChEMA (Scheme 26) containing pchEMA=0.56 is 81.4% of that of the homopolymer, indicating that the interphase should be approximately 18.6% of the liquid crystalline lamellae (12.5 nm). The calculated value of 2.3 nm corresponds very well to that measured by TEM following preferential staining of the interphase [207]. Nevertheless, variations in the extent of disorder at the interface as measured by decreased enthalpies of transitions may be due to variations in sample preparation and thermal history. For example, Gronski et al. s H-NMR experiments on deuterated PS-PBAz (Scheme 21) indicate that the disordered interphase present in powder samples is eliminated when the samples are oriented by shear for extensive time in the nematic mesophase [203]. 

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