Polyacrylonitrile structure

AN revealed polyacrylonitrile two-dimensional order, whereas copolymers containing less AN were amorphous. Thus the decrease in strength at very high AN contents was associated with polyacrylonitrile structure which developed because the styrene present was insufficient to disrupt the structure even though it was evenly distributed. 

Because of their unique blend of properties, composites reinforced with high performance carbon fibers find use in many structural applications. However, it is possible to produce carbon fibers with very different properties, depending on the precursor used and processing conditions employed. Commercially, continuous high performance carbon fibers currently are formed from two precursor fibers, polyacrylonitrile (PAN) and mesophase pitch. The PAN-based carbon fiber dominates the ultra-high strength, high temperature fiber market (and represents about 90% of the total carbon fiber production), while the mesophase pitch fibers can achieve stiffnesses and thermal conductivities unsurpassed by any other continuous fiber. This chapter compares the processes, structures, and properties of these two classes of fibers. 

Low density, carbon fiber-carbon binder composites are fabricated from a variety of carbon fibers, including fibers derived from rayon, polyacrylonitrile (PAN), isotropic pitch, and mesophase pitch. The manufacture, structure, and properties of carbon fibers have been thoroughly reviewed elsewhere [3] and. therefore, are... 

Figure 11.5. Model of structure of polyacrylonitrile-based carbon fibre (after Johnson 1994).

Acrylonitrile, CH,CHCN, is used in the synthesis of acrylic fibers (polyacrylonitriles), such as Orion. Write the Lewis structure of acrylonitrile and describe the hybrid orbitals on each carbon atom. What are the approximate values of the bond angles ... 

Polyacrylonitrile, known commercially as Orion, is made by polymerizing acrylonitrile (see Figure 13-3) Orion is used to make fibers for carpeting and clothing. Draw the Lewis structure of polyacrylonitrile, showing at least three repeat units. 

Acrylonitrile polymerizes in the same way as ethylene. Notice that this polymer has the same structure as polyethylene, except that a CN group is attached to every second carbon atom, so the structure is a reasonable one. A line structure of polyacrylonitrile eliminates the clutter caused by the H atoms. A ball-and-stick model of the same polymer segment is included for comparison. 

Membranes UF membranes consist primarily of polymeric structures (polyethersulfone, regenerated cellulose, polysulfone, polyamide, polyacrylonitrile, or various fluoropolymers) formed by immersion casting on a web or as a composite on a MF membrane. Hydrophobic polymers are surface-modified to render them hydrophilic and thereby reduce fouling, reduce product losses, and increase flux [Cabasso in Vltrafiltration Membranes and Applications, Cooper (ed.). Plenum Press, New York, 1980]. Some inorganic UF membranes (alumina, glass, zirconia) are available but only find use in corrosive applications due to their high cost. 

Two nitrogen-containing polymeric materials with extended aromatic ladder structures have been chosen for direct fluorination studies (Figure 14.9).57 Pyrolyzed polyacrylonitrile (3) and paracyanogen (4) [poly(pyrazinopryazine)] have been subjected to direct fluorination to produce perfluorinated analogues. 

Structural changes in the polymer, which will accompany the formation of small molecule products from the polymer, or may be produced by other reactions, can cause significant changes to the material properties. Development of colour, e.g. in polyacrylonitrile by ladder formation, and in poly(vinyl chloride) through conjugated unsaturation, is a common form of degradation. 

Itaconic Acid. Structurally an a-substituted methacrylic acid, itaconic acid constitutes a C5 building block with significant market opportunities. It is currently produced via fungal fermentation at about 10,000 t/a and mainly used as a specialty comonomer in acrylic or methacrylic resins, as incorporation of small amounts of itaconic acid into polyacrylonitrile significantly improve their dyeability. 

A completely different approach was taken by Koresh and Soffer (1980, 1986, 1987). Their preparation procedure involves a polymeric system like polyacrylonitrile (PAN) in a certain configuration (e.g. hollow fiber). The system is then pyrolyzed in an inert atmosphere and a dense membrane is obtained. An oxidation treatment is then necessary to create an open pore structure. Depending on the oxidation treatment typical molecules can be adsorbed and transported through the system. 

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. 

Cyclization is a key reaction in the production of carbon fibers from polyacrylonitrile (PAN) (acrylic fiber see Sec. 3-14d-2). The acrylic fiber used for this purpose usually contains no more than 0.5-5% comonomer (usually methyl acrylate or methacrylate or methacrylic acid). Highly drawn (oriented) fibers are subjected to successive thermal treatments—initially 200-300°C in air followed by 1200-2000°C in nitrogen [Riggs, 1985]. PAN undergoes cyclization via polymerization through the nitrile groups to form a ladder structure (XXVII). Further reaction results in aromatization to the polyquinizarine structure (XXVIII)... 

Chapter 5 shows that the application of hydrolytic enzymes is a powerful yet mild strategy to directly improve polymer surface properties (i.e. hydrophilicity) or activate materials for further processing. The surface hydrolysis of polyamides (PA), polyethyleneterphthalates (PET) and polyacrylonitriles (PAN) is discussed, as well as the mechanistic details on the enzymatic surface hydrolysis. The mechanistic data, combined with advances in structural and molecular biology, help to explain different activities of closely related enzymes on polymer surfaces. 

Martin and coworkers tried to prepare carbon tubes from the carbonization of polyacrylonitrile (PAN) in the channels of anodic oxide film (10). A commercially available film with a pore diameter of 260 nm was immersed in an aqueous acrylonitrile solution. After adding initiators, the polymerization was carried out at acidic conditions under N2 flow at 40°C. The PAN formed during the reaction was deposited both on the pore walls and on both sides of the film. Then the Film was taken from the polymerization bath, followed by polishing both faces of the film to remove the PAN deposited on the faces. The resultant PAN/alumina composite film was heat-treated at 250°C in air, and then it was heat-treated at 600°C under Ar flow for 30 min to carbonize the PAN. Finally, this sample was repeatedly rinsed in I M NaOH solution for the dissolution of the alumina film. The SEM observation of this sample indicated the formation of carbon tubes with about 50 xm long, which corresponds to the thickness of the template film. The inner structure of these tubes was not clear because TEM observation was not done. The authors claim that it is possible to control the wall thickness of the tubes with varying the polymerization period. 

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. 

Within polymer solids, volatile, reactive fragments are trapped and often rereact, forming rearranged structures. If the rearranged structures exhibit markedly better stability, excessive char results. Thus solid ldpe decomposes with little char, whereas polyacrylonitrile (PAN) gives excessive char because of the formation of thermally stable rearranged products. 

The property of mesophase that makes it suitable for carbon fiber and premium coke manufacture is that it forms ordered structures under stress which persist following carbonization. However, most carbon fiber production in the 1990s is based on polyacrylonitrile (PAN). 

Perhaps one of the best known syntheses of a heterocyclic polymer via the modification method is the generation of nitrogen-containing ladder polymers by pyrolysis of polyacrylonitrile) (77MI11109). The thermolysis is known to take place in discrete steps. The first step in the sequence, which can take place with explosive violence if the heating rate is not sufficiently slow, occurs at about 150 °C and can be detected by the onset of intense color formation. The product of this reaction (Scheme 101) is the cyclic tetrahydropyridine ladder structure (209). The next step, which is conducted in the presence of air at ca. 250 °C, involves the thermooxidation of polymer (209) to form what is best described as terpolymer (210) containing dihydropyridine, pyridone and pyridine units. 

---