PAN, crystalline

Chemical reactions proceeding in the system under the conditions of IR pyrolysis result in the changes of PAN crystalline structure. X-ray phase analysis (XRPA) has shown that at T>200°C in the course of formation of ordered carbon structures crystalline and amorphous phases of initial PAN disappear, while another amorphous carbon phases appear [12] ... 

Pougef ef al. gave an exhaustive sfudy to PAn crystallinity in 1991 and this work remains the main reference point for more recent studies. Pouget... 

Influence of Water and "Secondary Dopants" on PAn crystallinity. An increase in crystallinity of PAn emeraldine salt has been reported by the incorporation of secondary plasticizing agents. Pron et al. have shown that... 

Degree of PAN-crystallinity as a function of crystallite thickness and polymer tacticity... 

The ammonium sulfate and sodium chloride are simultaneously dissolved, preferably ia a heel of ammonium chloride solution. The sodium chloride is typically ia excess of about 5%. The pasty mixture is kept hot and agitated vigorously. When the mixture is separated by vacuum filtration, the filter and all connections are heated to avoid cmst formation. The crystalline sodium sulfate is washed to remove essentially all of the ammonium chloride and the washings recycled to the process. The ammonium chloride filtrate is transferred to acid resistant crystallising pans, concentrated, and cooled to effect crystallisation. The crystalline NH Cl is washed with water to remove sulfate and dried to yield a product of high purity. No attempt is made to recover ammonium chloride remaining ia solution. The mother Hquor remaining after crystallisation is reused as a heel. 

Fibers produced from pitch precursors can be manufactured by heat treating isotropic pitch at 400 to 450°C in an inert environment to transform it into a hquid crystalline state. The pitch is then spun into fibers and allowed to thermoset at 300°C for short periods of time. The fibers are subsequendy carbonized and graphitized at temperatures similar to those used in the manufacture of PAN-based fibers. The isotropic pitch precursor has not proved attractive to industry. However, a process based on anisotropic mesophase pitch (30), in which commercial pitch is spun and polymerized to form the mesophase, which is then melt spun, stabilized in air at about 300°C, carbonized at 1300°C, and graphitized at 3000°C, produces ultrahigh modulus (UHM) carbon fibers. In this process tension is not requited in the stabilization and graphitization stages. 

A relatively new class of high-performance carbon fibers is melt-spun from mesophase pitch, a discotic nematic liquid crystalline material. This variety of carbon fibers is unique in that it can develop extended graphitic crystallinity during carbonization, in contrast to current carbon fibers produced from PAN. 

Tables VIII-XI show examples of pon variations of several buffers. With such tables, it is easy to adjust any desired pan value in mixed solvents at any selected temperature or in a given range of temperatures. We will see in Section III,E how these values are essential to investigate safely both crystal structure and productive enzyme-substrate complexes in the crystalline state.

Apart from nitrile-hydrolyzing enzymes, some esterases and cutinases have been used for surface hydrolysis of PAN [74], These enzyme were shown to specifically hydrolyse vinyl acetate moieties present as co-monomer in many commercial PAN materials, with no changes in crystallinity as determined by X-ray diffraction [74],... 

Recall from Section 1.4.5.1 that there are two primary types of carbon fibers polyacrylonitrile (PAN)-based and pitch-based. There are also different structural forms of these fibers, such as amorphous carbon and crystalline (graphite) fibers. Typically, PAN-based carbon fibers are 93-95% carbon, whereas graphite fibers are usually 99+%, although the terms carbon and graphite are often used interchangeably. We will not try to burden ourselves with too many distinctions here, since the point is to simply introduce the relative benefits of continuous-fiber composites over other types of composites, and not to investigate the minute differences between the various types of carbon-fiber-based composites. The interested reader is referred to the abundance of literature on carbon-fiber-reinforced composites to discern these differences. 

In a 3-I. round-bottom flask equipped with a mechanical stirrer are placed 1500 g. of 20 per cent ethylene chlorohydrin solution (Note 1) and 750 g. of water. The flask is set in an empty pan of suitable size to serve as a bath in case cooling becomes necessary. With the stirrer in operation, 493 g. of crystalline sodium sulfide containing nine molecules of water of crystallization is added to the chlorohydrin solution at a rate which will maintain the temperature at 30-3 5 °. This will require from forty to sixty minutes. After all the sodium sulfide has been added the solution is stirred for thirty minutes. 

---