Polyacrylonitrile

Polyacrylonitrile (PAN) has been used in the preparation of UP membranes for a long time [82, 83] due to its superior resistance to hydrolysis and oxidation. PAN is highly crystalline and relatively hydrophilic and is usually copolymerized with more hydrophilic monomers to improve processability and to make it less brittle. Hollow fibers can be prepared from PAN dissolved in nitric acid [84]. Preparation of PAN membranes by phase inversion from solutions in DMAC, DMF or NMP is also possible. An example is shown in Fig. 4.4. A Sumitomo patent [85] discloses the preparation of membranes from copolymers containing 89% acrylonitrile and 11% ethyl acrylate dissolved in DMF and formamide and coagulated in water. A microporous membrane is obtained. In order to make the membranes suitable for reverse osmosis, they were submitted to a plasma treatment in the presence of 4-vinyl pyridine. 

Polyacrylonitrile can be prepared by adopting acrylamide and acrylic acid as raw materials. The synthetic reaction can be given as follows  

Flotation results show that the flocculation of polyacrylonitrile decreases obviously with introducing small amounts of -COOH groups. When the introduced quantity of -COOH groups is 1/3, however, the flocculation of polyacrylonitrile increases. When the introduced quantity of -COOH groups is 2/3, polyacrylonitrile appears an anionic flocculant which is similar to Aerofloc 550. 

Polyacrylonitrile (PAN) is formed by the peroxide-initiated free-radical polymerization of acrylonitrile (CH2=CH—CN). The major application of PAN is as the fiber Orion. When copolymerized with butadiene, it forms Buna N or nitrile rubber, which is resistant to hydrocarbons and oils. As a copolymer with styrene (SAN), it is a transparent plastic with very good impact strength used for machine components and for molding crockery. As a terpolymer of acrylonitrile-butadiene-styrene (ABS), the plastic is known for its toughness and good strength and finds applications in water lines and drains. 

Carbon fibers have also been made from the pyrolysis of viscose (cellulose), rayon, and jute and fi om pitch. Though these methods produce slightly lower strength carbon fibers as compared to PAN, the lower cost ( to 5) makes them excellent reinforcement materials for noncritical items such as golf clubs, tennis rackets, skis, and related sports goods. 

Heating polyacrylonitrile up to 200 °C induces no noticeable changes in its chemical 

Accumulation of these polyconjugated structures during pyrolysis of the polymer under vacuum is characterised by a symmetrical electron paramagnetic resonance (EPR) signal of width 2.3 mT. 

It should be noted that the decomposition of polyacrylonitrile at temperatures up to 800 °C induces self-stabilisation, i.e., the quantity and the rate of elimination of volatile products gradually decrease. This is most probably associated with the production of the highly thermally resistant polymer with cyclised nitrile groups. 

Several thermal stability studies have been conducted on polyacrylonitrile [30] and its copolymers [31-33]. 

Petit and Neel [33] carried out thermal stability measurement on copolymers of cis and trans penta-1,3 diene and acrylonitrile. 

Heating polyacrylonitrile up to 200 °C induces no noticeable changes in its chemical composition. However, the polymer becomes firstly yellow, then red-brown and finally, blue-black. According to IR spectroscopic data the colouration of the polymer is associated with the cyclisation of nitrile groups  

Two investigations have been made on polyacrylonitrile in fibre form. The first reports on rates of formation of degradation products and the second on exothermic reactions and discolouration. The latter paper also reports the effect of copolymerization of acrylonitrile with 2-vinylpyridine upon these reactions. 

Decomposition products from a terpolymer of acrylonitrile, vinylidene chloride, and glycidyl methacrylate have been analysed. The elimination of hydrogen cyanide and hydrogen chloride were continuously monitored.  

The degradation of poly(a-chloroacrylonitrile) has also been studied. If the polymer were first partially dehydrochlorinated by treatment with triethylamine then cyclization of cyanide groups was inhibited.  

The most important commercial processes for polyacrylonitrile (XLIII) are solution and suspension polymerizations. Almost all the products containing acrylonitrile are copolymers. Styrene-acrylonitrile (SAN) copolymers are useful as plastics (Sec. 6-8a). 

Acrylic and modacrylic fibers have a wool-like appearance and feel, and excellent resistance to heat, ultraviolet radiation, and chemicals [Bajaj and Kumari, 1987]. These fibers have replaced wool in many applications, such as socks, pullovers, sweaters, and craft yams. Other applications include tenting, awning fabric, and sandbags for rivershore stabilization. The use of acrylic and modacrylic fibers in carpets is low since these materials do not hold up well to recycling through hot-humid conditions. This also prevents its use in the easy-care garment market. 

If a blend of poly(methyl methacrylate) and polyacrylonitrile is thermally degraded, the behaviour of the poly (methyl methacrylate) is profoundly affected, but that of the polyacrylonitrile is unaltered. The methacrylate units react with ammonia arising from the polyacrylonitrile, and the amide-ester copolymer so formed undergoes complex degradation reactions, rather than the depolymerization to monomer which takes place with unchanged poly(methyl methacrylate). 

The degradation of polyacrylonitrile deuteriated in the alpha position has been followed by Fourier transform i.r. spectroscopy. The results are consistent with imine-enamine tautomerism followed by oxidation to give pyridone structures. A variety of thermo-analytical techniques has been employed to study thermal and thermo-oxidative reactions and it is concluded from these that a carboxylate moiety is responsible for initiating cyclization in an oxidizing atmosphere. The role of oxygen is said to be three-fold, creation of the carboxylate initiator sites, dehydrogenation, and crosslinking. 

Copolymers that have been investigated include acrylonitrfle-cr-methyl-styrene, acrylonitrile-methyl acrylate, acrylonitrile-methyl methacrylate, acrylonitrile-vinyl acetate, acrylonitrile-vinyl bromide, acrylonitrile-acrylic acid, and acrylonitrile-lV-vinylpyrrolidone. As might be expected the course of the thermal decomposition is significantly affected by the presence of a comonomer some accelerate the cyclization of acrylonitrile sequences, whilst others inhibit the process. 

Polymethacrylonitrile and poly(a-chloroacrylonitrile) have also been studied. In the latter polymer there is competition between dehydrochlorination reactions and the cyclization of cyano groups. 

The photodegradation and photo-oxidative degradation of polyacrylonitrile [41, 302, 1200, 1951, 2313] and its copolymers such as poly(acrylonitrile-co-isopropyl ketone) [41], poly(acrylonitrile-co-methyl methacrylate) [820], poly(acrylonitrile-co-2 naphthyl methacrylate [338], poly(acrylonitrile-co-butadiene) [24,2001], poly(acrylonitrile-co-styrene) [232, 747], poly(acrylo-nitrile-co-p-styrene) [232] and poly(a-chloroacrylonitrile-co-methyl methacrylate) [820] have been studied in detail. 

The UV irradiation of polyacrylonitrile (3.48) causes chain scission and crosslinking, which occur by the following mechanism with the formation of hydrogen, methane, acrylonitrile and hydrogen cyanide [302,1200,2058]  

Photo-oxidation of polyacrylonitrile, especially at elevated temperatures, does not result in its degradation. The jS-ketonitrile groups [3.49) formed give further cyclic (ladder) structures [41, 302]  

Photolysis of polyacrylonitrile in ethylene carbonate ( 2115)200 and propylene carbonate (C3H70)2C0 solution causes random chain scission [1077, 1089). 

Photodegradation of poly(a-chloroacrylonitrile) (3.50) occurs via C—Cl scission to give a chlorine radical followed by its reaction with an adjacent hydrogen atom to give HCl and a double bond [820]  

In the exhaust method, the fibre material is introduced into a bath containing cationic brightening agent and 3-5% formic acid (85%) to maintain the pH 3 to 4 at 

The one bath two stage bleaching and brightening of acrylic fibres is quite popular. In this procedure, the material is boiled with chlorite, using oxalic acid as an accelerator for about 30 min, the bath is cooled to 80 C and excess chlorine is eliminated with sodium hydrosulphite. In the second phase, the optical brightening agent is added and the bath is again heated to the boil in 30 min and the material is treated at this temperature for further 30 min. The material is then worked up in the usual manner. 

Non-ionic brightening agent can also be taken up by the fibre from suspensions. The fibre material is treated with a solution containing disperse type brighteners and 2% formic acid (85%) (pH 3 to 4) at 100°C for 30 to 40 min. Exhaustion can be accelerated by raising the temperature of the bath to 110 C. 

Polyalkylene sulfides (thioplastics) have a relatively high density (1.3-1.6 g/cm ) and usually have a strong odor of hydrogen sulfide or mercap-tans (like rotten eggs). The odor is especially strong on heating, and in this way they can be qualitatively identified. 

The large group of plastics without heteroatoms can only be incompletely identified with this separation procedure. Place the sample in water. If it dissolves slowly, then it may be polyvinyl alcohol. (For specific identification, see Section 6.2.6.) If the plastic is insoluble in water, then check first for formaldehyde (Section 6.1.4). The only positive reaction in this group is given by phenol formaldehyde resins and polyoxymethylene (polyformaldehydes). 

test for phenols (see Section 6.1.3). They may result from phenol and cresol formaldehyde resins and also from epoxy resins or polycarbonates based on bisphenol A. 

A further test for acetate (Section 6.2.5) makes it possible to identify polymers containing vinyl acetate as well as cellulose acetate or cellulose acetate butyrate (Section 6.2.16). 

These tests, however, do not identify certain chemically very inert plastics such as polyethylene, polypropylene, polyisobutylene, polystyrene, polymethyl methacrylate, polyacrylates, polyethylene terephthalate, natural rubber, butadiene rubber, polyisoprene, and silicones. Their identification requires specific individual reactions, described in Chapter 6. 

Exchange resins of the type Amberlite CG 50-III for TLC (see Table 10, p. 46), containing carboxyl groups, have likewise been used for separation of flavone compounds. Isopropanol-water (40 + 6) was used for development [222, 223]. Good separations are accomplished and the sequence of substances on the chromatogram differs from that on perlon or sihca gel layers. The hi /-values found increased in the order  

This polymer has good chemical and oil and grease resistance. Its only application in which it might come into contact with food is as a filter cloth in food manufacturing 

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Highly colored, they have been used to dye cellulose acetate (552) and acrylic fibers (553). Cationic dyes prepared from 2-azothiazoles by simple alkylation on the ring nitrogen (552) have been used increasingly with the introduction of polyacrylonitrile fibers with basic sites that can be colored with such dyes (554). 

The principal monomer of nitrile resins is acrylonitrile (see Polyacrylonitrile ), which constitutes about 70% by weight of the polymer and provides the polymer with good gas barrier and chemical resistance properties. The remainder of the polymer is 20 to 30% methylacrylate (or styrene), with 0 to 10% butadiene to serve as an impact-modifying termonomer. 

Fig. 4.25 Adsorption isotherms showing low-pressure hysteresis, (a) Carbon tetrachloride at 20°C on unactivated polyacrylonitrile carbon Curves A and B are the desorption branches of the isotherms of the sample after heat treatment at 900°C and 2700°C respectively Curve C is the common adsorption branch (b) water at 22°C on stannic oxide gel heated to SOO C (c) krypton at 77-4 K on exfoliated graphite (d) ethyl chloride at 6°C on porous glass. (Redrawn from the diagrams in the original papers, with omission of experimental points.)...

The way in which these factors operate to produce Type III isotherms is best appreciated by reference to actual examples. Perhaps the most straightforward case is given by organic high polymers (e.g. polytetra-fluoroethylene, polyethylene, polymethylmethacrylate or polyacrylonitrile) which give rise to well defined Type III isotherms with water or with alkanes, in consequence of the weak dispersion interactions (Fig. S.2). In some cases the isotherms have been measured at several temperatures so that (f could be calculated in Fig. 5.2(c) the value is initially somewhat below the molar enthalpy of condensation and rises to qi as adsorption proceeds. In Fig. 5.2(d) the higher initial values of q" are ascribed to surface heterogeneity. 

The low-temperature (remember that this is a relative term Tj = 317°C for polyacrylonitrile) behavior of linear polymers may conveniently be divided into three regimes ... 

Combination and disproportionation are competitive processes and do not occur to the same extent for all polymers. For example, at 60°C termination is virtually 100% by combination for polyacrylonitrile and 100% by disproportionation for poly (vinyl acetate). For polystyrene and poly (methyl methacrylate), both reactions contribute to termination, although each in different proportions. Each of the rate constants for termination individually follows the Arrhenius equation, so the relative amounts of termination by the two modes is given by... 

Poly(vinylidene fluoride), tangential flow. Regenerated cellulose hoUow fiber. Polyacrylonitrile hoUow fiber. 50-nmhead, 150-nm tail. 

For nosetip materials 3-directional-reinforced (3D) carbon preforms are formed using small cell sizes for uniform ablation and small pore size. Figure 5 shows typical unit cell dimensions for two of the most common 3D nosetip materials. Carbon-carbon woven preforms have been made with a variety of cell dimensions for different appHcations (27—33). Fibers common to these composites include rayon, polyacrylonitrile, and pitch precursor carbon fibers. Strength of these fibers ranges from 1 to 5 GPa (145,000—725,000 psi) and modulus ranges from 300 to 800 GPa. 

Resin and Polymer Solvent. Dimethylacetamide is an exceUent solvent for synthetic and natural resins. It readily dissolves vinyl polymers, acrylates, ceUulose derivatives, styrene polymers, and linear polyesters. Because of its high polarity, DMAC has been found particularly useful as a solvent for polyacrylonitrile, its copolymers, and interpolymers. Copolymers containing at least 85% acrylonitrile dissolve ia DMAC to form solutions suitable for the production of films and yams (9). DMAC is reportedly an exceUent solvent for the copolymers of acrylonitrile and vinyl formate (10), vinylpyridine (11), or aUyl glycidyl ether (12). 

Reference methods for criteria (19) and hazardous (20) poUutants estabHshed by the US EPA include sulfur dioxide [7446-09-5] by the West-Gaeke method carbon monoxide [630-08-0] by nondispersive infrared analysis ozone [10028-15-6] and nitrogen dioxide [10102-44-0] by chemiluminescence (qv) and hydrocarbons by gas chromatography coupled with flame-ionization detection. Gas chromatography coupled with a suitable detector can also be used to measure ambient concentrations of vinyl chloride monomer [75-01-4], halogenated hydrocarbons and aromatics, and polyacrylonitrile [25014-41-9] (21-22) (see Chromatography Trace and residue analysis). 

The first reported synthesis of acrylonitrile [107-13-1] (qv) and polyacrylonitrile [25014-41-9] (PAN) was in 1894. The polymer received Htde attention for a number of years, until shortly before World War II, because there were no known solvents and the polymer decomposes before reaching its melting point. The first breakthrough in developing solvents for PAN occurred at I. G. Farbenindustrie where fibers made from the polymer were dissolved in aqueous solutions of quaternary ammonium compounds, such as ben2ylpyridinium chloride, or of metal salts, such as lithium bromide, sodium thiocyanate, and aluminum perchlorate. Early interest in acrylonitrile polymers (qv), however, was based primarily on its use in synthetic mbber (see Elastomers, synthetic). 

PyraZolines. l,3-Diphenyl-2-pyia2olines (7) (Table 2) aie obtainable from appiopiiately substituted phenyUiydiazines by the Knoii reaction with either P-chloro- or P-dimethylaminopropiophenones (30,31). They are employed for brightening synthetic fibers such as polyamides, cellulose acetates, and polyacrylonitriles. 

A further development in the coumarin series is the use of derivatives of 3-phenyl-7-aminocoumarin ((13) where R, R = Cl or substituted amines) as building blocks for a series of light-stable brighteners for various plastics and synthetic fibers, and, as the quatemi2ed compounds, for brightening polyacrylonitrile (62). 

The most common chemical bleaching procedures are hypochlorite bleach for cotton hydrogen peroxide bleach for wool and cotton sodium chlorite bleach for cotton, polyamide, polyester, and polyacrylonitrile and reductive bleaching with dithionite for wool and polyamide. 

In the case of solvent spinning, ie, secondary acetate, polyacrylonitrile, and poly(vinyl chloride), the FWA is added to the polymer solution. An exception is gel-whitening of polyacrylonitrile, where the wet tow is treated after spinning in a washbath containing FWA. 

Other Films. Although commercially less important than polyethylenes and polypropylenes, a number of other plastic films are in commercial use or development for special appHcations, including ethylene—vinyl acetate, ionomer, and polyacrylonitrile [25014-41-9]. 

Polyacrylonitrile (PAN) films have outstanding oxygen and CO2 barrier properties, but only modest water-vapor barrier properties. They are for processed-meat and fresh pasta packaging laminations where an oxygen barrier is required for vacuum or gas flush packaging. 

Worldwide demand for DMF in acryhc fiber production has held up better than in the United States. The high solubiUty of polyacrylonitrile in DMF, coupled with DMF s high water miscibility, makes it an attractive solvent for this appHcation. Its principal competition in this area comes from DMAC. 

The white cell adsorption filter layer is typically of a nonwoven fiber design. The biomaterials of the fiber media are surface modified to obtain an optimal avidity and selectivity for the different blood cells. Materials used include polyesters, eg, poly(ethylene terephthalate) and poly(butylene terephthalate), cellulose acetate, methacrylate, polyamides, and polyacrylonitrile. Filter materials are not cell specific and do not provide for specific filtration of lymphocytes out of the blood product rather than all leukocytes. 

Tetraethylene glycol may be used direcdy as a plasticizer or modified by esterification with fatty acids to produce plasticizers (qv). Tetraethylene glycol is used directly to plasticize separation membranes, such as siHcone mbber, poly(vinyl acetate), and ceUulose triacetate. Ceramic materials utilize tetraethylene glycol as plasticizing agents in resistant refractory plastics and molded ceramics. It is also employed to improve the physical properties of cyanoacrylate and polyacrylonitrile adhesives, and is chemically modified to form polyisocyanate, polymethacrylate, and to contain siHcone compounds used for adhesives. 

Process. Any standard precursor material can be used, but the preferred material is wet spun Courtaulds special acrylic fiber (SAF), oxidized by RK Carbon Fibers Co. to form 6K Panox B oxidized polyacrylonitrile (PAN) fiber (OPF). This OPF is treated ia a nitrogen atmosphere at 450—750°C, preferably 525—595°C, to give fibers having between 69—70% C, 19% N density less than 2.5 g/mL and a specific resistivity under 10 ° ohm-cm. If crimp is desired, the fibers are first knit iato a sock before heat treating and then de-knit. Controlled carbonization of precursor filaments results ia a linear Dow fiber (LDF), whereas controlled carbonization of knit precursor fibers results ia a curly carbonaceous fiber (EDF). At higher carbonizing temperatures of 1000—1400°C the fibers become electrically conductive (22). 

The (A/-alkylated) lactam of 8-aminonaphthalenecarboxylic acid (47) also is a valuable dye iatemiediate, eg, for cyclometbine-type dyes used for dyeiag polyacrylonitrile fibers and other synthetics. 1,8-Naphtholactams are prepared in high yield and purity by the reaction of naphtholactones with RNH2 (R = H, Cl—4 alkyl, cycloalkyl, or optionally substituted aryl) in aqueous medium, usually in the presence of bisulfite at 150°C over a period of 15 h (143). 

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