Pitch, cellulose mesophases

In addition to the rigidity, steric effects and flexibility of side groups seems to Influence the formation and properties of cellulosic mesophases they allow (or not) the existence of a mesophase before crystallization, influence the temperature for onset of a mesophase, and contribute to the value of the cholesteric pitch. 

Besides degree of substitution, nature of substituents, solvent, concentration, and temperature, other factors that change the polymer-solvent interactions can affect the pitch of lyotropic cellulosic mesophases. Doping inorganic salts " or small chiral molecules into the lyotropic mesophase changes the polymer-solvent interactions. As the results, the pitch of the chiral nematic mesophase changes accordingly. 

The surface properties of carbon fibers are intimately related to the internal structure of the fiber itself, which needs to be understood if the surface properties are to be modified for specific end applications. Carbon fibers have been made from a number of different precursors, including polyacrylonitrile (PAN), rayon (cellulose) and mesophase pitch. The majority of commercial carbon fibers currently produced are based on PAN, while those based on rayon and pitch are produced in very limited quantities for special applications. Therefore, the discussion of fiber surface treatments in this section is mostly related to PAN-based carbon fibers, unless otherwise specified. 

Aqueous suspensions of cellulose microcrystalhtes obtained by acid hydrolysis of native cellulose fibers can also produce a cholesteric mesophase [ 194]. Sulfuric acid, usually employed for the hydrolysis, sulfates the surface of the micro crystallites and therefore they are actually negatively charged. Dong et al. performed some basic studies on the ordered-phase formation in colloidal suspensions of such charged rod-like cellulose crystallites (from cotton filter paper) to evaluate the effects of addition of electrolytes [195,196]. One of their findings was a decrease in the chiral nematic pitch P of the anisotropic phase, with an increase in concentration of the trace electrolyte (KC1, NaCl, or HC1 of < 2.5 mM) added. They assumed that the electric double layer on... 

Cellulose based mesophases have displayed a somewhat steeper inverse dependence of pitch to concentration than that reported by Toriumi (29). A value of x = 3 has been reported for hydroxypropyl cellulose in water (31), for acetoxypropyl cellulose in acetone (32) and for hydroxypropyl cellulose in acetic acid and in methanol (28). Our data for cellulose in NH3/NH4SCN show a reasonably straight line when p l/3 was plotted vs. cellulose concentration (Figure 11), which is in accord with earlier reports (28,29,31,32). 

Lyotropic liquid crystalline cellulose derivatives exhibit unique optical properties because of their helicoidal supramolecular structure.The chiro-optical properties of the helicoidal structure can be described by a pitch p (or its inverse, the twist p ) p = 2o/fi, where is the reflection wavelength and h is the mean refractive index of a sheet, and the corresponding handedness of the twist right-handed helicoidal structure being assigned to a positive pitch p > 0) and left-handed helicoidal structures to a negative pitch p < The nematic mesophase can be... 

AEC) in chloroform, with the increase of degree of acetylation, the rigidity of the cellulose chain increases monotonically. However, the pitch of the mesophase first increases to infinity and then decreases with an inversion of the handedness of the chiral nematic phase, from left-handed to right-handed. 

The theory predicts that the handedness of cellulosic liquid crystalline solutions, designated by the sign of the pitch, depends not only on temperature (T) and on steric repulsion of the chain X), but also on an attractive interaction parameter, %, which depends on the nature of the solvent. The chiral forces are balanced when (x XkT) = 0. In this compensated condition, the pitch of the mesophase should become infinite, and the mesophase resembles a normal nematic phase. 

Figure 10.13(a) shows an SEM micrograph of a 4-methylphenyl urethane of cellulose (4-MPC) sample photopolymerized from an Af-vinyl pyrrohdinone solution (43.8 wt%). At polymer concentrations above 35 wt%, 4-MPC forms a cholesteric mesophase (Chapter 11). The SEM image shows period striations with repeat distance equal to 140 30 nm, a value that is comparable to that of half the pitch, P/2, of the cholesteric mesophase. 

Carbon fibers typically are produced by the high temperature treatment of stmc-turally stable precursor fibers that have been thermally, mechanically, and/or chemically pre-treated. Based on the precitrsor type, carbon fibers can be divided into four major classes, namely (i) polyacrylorritrile (PAN)-based, ( ) mesophase pitch-based, (Hi) cellulose-based, and (iv) vapor grown carbon fibers. 

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