Critical concentration, liquid-crystalline composites

The new approach has been termed The Mesogenic Index (MI) and has successfully been applied to 23 copolymer systems in which the critical compositions for mesophase formation have been established by means of varying constituent monomer concentrations. It is also consistent in predicting liquid crystalline behaviour in several hundred main-chain polymer systems containing amide, ester, carbonate, ether and urethane groups. 

Fiber Spinning and Mechanical Properties Aside from providing the critical concentrations at which liquid-crystalline behavior appears. Table m also lists the processing concentrations of the candidate block copolymers. Since rigid molecules would aggregate together to form anisotropic domains as their concentration exceeds Cer the processing of molecular composites must be controlled within this limit. 

Two series of PBTA/PI block copolymers were synthesized in this study and solution processed into molecular composite fibers via dry-jet wet-spinning. The unique rheological properties of liquid-crystalline PBTA homopolymers and PBTA/PI block copolymers were studied with a cone-and-plate rheometer. For block copolymers, the critical concentration decreased with an increase in PBTA content. The flow curves of isotropic and anisotropic solutions could be described via the power-law model and Carreau model, respectively. Copolymer fibers possess tensile strength and modulus located between those of PBTA fibers and PI fibers. Moreover, the tensile strength and modulus of Col fibers increase with an increase in PBTA content. Besides, increasing the draw ratios would give rise to an increase in the mechanical properties of copolymer fibers... 

The concentrated regime is where n > 1/dL ox cp >ap. In this range the dynamic properties of the fibers can be severely affected by fiber-fiber interactions and can lead to solid-like behavior. It is interesting to note that most fiber composites of industrial interest typically have fiber concentrations of ( > 0.1 and fall within the concentrated regime. In addition to the three regimes defined above, molecular theories define a critical concentration in which molecules will preferentially align to form a nematic liquid crystalline phase, a phase intermediate to a purely crystalline phase and an isotropic liquid phase. However, it has yet to be proved that fiber suspensions will also go through this transition (Larson, 1999). 

A temperature-composition phase diagram for a surfactant solution is a characteristic phase diagrarr that delineates the conditions under which crystalline surfactant, monomers, or micelles will exist. On the phase diagram shown in Figure 12.5 (Smirnova, 1995), L represents the liquid phase, S the solid phase, and )(the surfactant mole fraction. The critical micellar temperature, CMT, is deLned as the line between the crystalline and micellar phases. Micelle formation occurs at temperatures greater than the CMT. The critical micellar concentration, CMC, line separates the micellar and... 

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