Molecular weight polyacrylonitrile

Carbon Cha.in Backbone Polymers. These polymers may be represented by (4) and considered derivatives of polyethylene, where n is the degree of polymeriza tion and R is (an alkyl group or) a functional group hydrogen (polyethylene), methyl (polypropylene), carboxyl (poly(acryhc acid)), chlorine (poly(vinyl chloride)), phenyl (polystyrene) hydroxyl (poly(vinyl alcohol)), ester (poly(vinyl acetate)), nitrile (polyacrylonitrile), vinyl (polybutadiene), etc. The functional groups and the molecular weight of the polymers, control thek properties which vary in hydrophobicity, solubiUty characteristics, glass-transition temperature, and crystallinity. 

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

On the other hand, organic materials of relatively low molecular weight such as acetylene, benzene, ethylene and methane, can produce vapour-grown carbon materials by imperfect combustion or by exposing their vapour to a heated substrate in an electric furnace in the presence of a metal catalyst. The latter process generates VGCFs. Other precursors to VGCF include polyacrylonitrile (PAN), isotropic or mesophase pitch, rayon or nylon [8]. 

In addition to the established large volume products already mentioned, other plastic materials are known to be under study or have been introduced so recently that their markets have not been fully developed. It seems certain that products such as polyethylene terephthalate and polyacrylonitrile fibers will attain large volume production. A new type of resin that has appeared very recently is Shell Chemical Co. s Epon series (32), a group of polymers of various molecular weight ranges which are produced from phenol, acetone, and epichlorohydrin. 

A large number of papers has dealt with the question of molecular weight and molecular weight distribution in polyacrylonitrile. Recent discussions are those of Onyon 108), Krigbaum and Kotliar 90), Booth and Beason 33), Bamford, Jenkins, Johnston and White 19) and Kobayashi 87). The intrinsic viscosity vs. molecular weight relationship of Cleland and Stockmayer 41) is probably as well supported as any. Fortunately, much of the material in this review does not depend heavily on detailed knowledge of molecular weights or of their distributions. 

Kobayashi, H. Molecular weight dependence of intrinsic viscosity, diffusion constant, and second virial coefficient of polyacrylonitrile. J. Polymer Sci. 39, 369-388 (1959). 

The degradation of the cellulose fraction of the copolymer and subsequent recovery of the polyvinyl polymer have often been used to characterize the polymer. For example, cellulose may be acetylated and acid hydrolyzed to remove it from the copolymer. Then the recovered polymer can be dissolved, in solvent normally used for the polymer, and i the molecular weight of the polymer determined viscometrically (12, 42). As reported previously for polymers, such as polyacrylonitrile, a functional group on the polymer may be altered during the fractionating. These changes have been determined by infrared spectroscopy. For free-... 

Molecular Weight of Grafted PAN Chains. A homopolymer-free graft copolymer sample was hydrolyzed in 2 M HC1 for 48 h at room temperature to depolymerize the cellulose chains. The residue, mainly polyacrylonitrile (PAN) chains, was washed with water and... 

Figure 4. Molecular weight distribution for polyacrylonitrile branches of grafted...

O 2 Onyon, P. F. Molecular weights and intrinsic viscosities of solution polymerized polyacrylonitrile. J. Polymer Sci. 37, 315 (1954). 

Zilkha, Feit, and Frankel (106), in their study of the anionic polymerization of acrylonitrile and methacrylonitrile with quaternary ammonium hydroxides, found the molecular weight of polyacrylonitrile to be independent of monomer and catalyst concentration while that of polymethacrylonitrile was not. The infrared spectra of the polyacrylonitrile indicated terminal CH2= groups. They suggested that termination by chain transfer to monomer was the explanation. 

Polyacrylonitrile and polymethacrylonitrile make a fifth pair of this kind, but the viscosity relations for the latter are based on osmotic measurements for only four unfractionated polymers with molecular weights lying between 180,000 and 500,000 [Fuhrman and Mesrobian (115a)], so that only a crude estimate of a can be made for this polymer. Nevertheless, the effect of the methyl group is about the same as in the other, better established cases. 

Molecular weight characterization of modacrylic fibers is difficult because of the limited number of solvents available and inhomogeneties in composition between individual polymer chains that affect solution properties, particularly if the comonomers are ionic in character. Di-methylformamide and dimethylacetamide are suitable for measurement of molecular weight of polyacrylonitrile, but errors are introduced when copolymers are analyzed (126). Bortniak et al. (127) have analyzed modacrylic fibers quantitatively in microgram quantities by using pyrolysis gas chromatography. 

The oldest and probably the most widely used initiator is 4,4 -azobis(4-cyano-n-pentanol). Synthesized from 5-keto-n-pentanol as starting material (Strecker synthesis), this initiator was used in hydroxytelechelic polystyrene, polyacrylonitrile, and polymethacrylate syntheses, 4). Dienes were also polymerized with this initiator by a different method the molecular weights of liquid hydroxytelechelic polymers are between 2000 and 20000, their functionality is usually higher than two 15,16) (Table 1.1). The polymerization has been studied in dependence on reaction time, monomer and initiator concentrations, temperature, and solvent. The molecular weight increases with decreasing initiator concentration or increasing reaction time. The yield is a function of the nature of monomer and solvent. It decreases in the series chloroprene > butadiene > isoprene and dioxane > toluene. 

---