Polyacrylonitrile chain

Acrylonitrile monomer when masticated in the presence of polymer leads to the formation of pseudocrosslinked block copolymers by mechanical scission of soluble block copolymers. The aggregation of the polyacrylonitrile chains of the block copolymer fraction results in the formation of swollen gels when the polymerization products are extracted with solvents from the initial polymer (78-80). 

FIGURE 8-10 Schematic diagram depicting dipole/dipole interactions in segments of polyacrylonitrile chains. 

POLYfMETHYL METHACRYLATEJ CHAINS POLYACRYLONITRILE CHAINS POLYACRYLONITRILE KNOTS... 

Use Intermediate in preparing grafted fibers when combined with polyacrylonitrile, there is an average substitution of one polyacrylonitrile chain for every 100 to 300 anhydroglucose units of the cellulose. 

Hydroxyl-Containing Graft Copolymers and Polyurea Dispersions. Graft copolymers of polyethers containing polyacrylonitrile chains have become available commercially, primarily for the manufacture of flexible foams, although they have also been evaluated in other applications (106-108). 

Critchfield et al. (109) have also reported the use of acrylonitrile-styrene graft polyols in urethane polymers. The presence of styrene prevents the tendency toward yellowing of the acrylonitrile graft copolymers, which is due to cyclization of the polyacrylonitrile chains at elevated temperatures. Similar types of graft polyols have been reported elsewhere (110). 

FIGURE 12.10 Model of assumed rigid, irregular helical conformation of the polyacrylonitrile chain as it would exist in the solid-state polymer. (From Hinrici-Olive, G. and Olive, S., Adv. Polym. Sci. 32, 128, 1979.)... 

Very recently, the same group has reported lyotropic LCs of polyacrylonitrile-grafted GOs [119]. Polyacrylonitrile chains were covalently and uniformly grafted onto GO surfaces via a simple free radical polymerization process. These functionalized sheets were well-dispersed in polar organic solvents such as dimeth-ylformamide and dimethyl sulphoxide forming nematic and lamellar LCs upon increasing concentrations. Continuous nacre-mimetic fibers have been assembled from these liquid crystalline phases. 

Secondary bonds are considerably weaker than the primary covalent bonds. When a linear or branched polymer is heated, the dissociation energies of the secondary bonds are exceeded long before the primary covalent bonds are broken, freeing up the individual chains to flow under stress. When the material is cooled, the secondary bonds reform. Thus, linear and branched polymers are generally thermoplastic. On the other hand, cross-links contain primary covalent bonds like those that bond the atoms in the main chains. When a cross-linked polymer is heated sufficiently, these primary covalent bonds fail randomly, and the material degrades. Therefore, cross-linked polymers are thermosets. There are a few exceptions such as cellulose and polyacrylonitrile. Though linear, these polymers are not thermoplastic because the extensive secondary bonds make up for in quantity what they lack in quahty. 

Hemicya.nines, These half-cyanine dyes may be represented by stmcture (48). They may be considered as cyanines in which a benzene ring has been inserted into the conjugated chain. Hemicyanines provide some bright fluorescent red dyes for polyacrylonitrile. 

Hydrophobic fibers are difficult to dye with ionic (hydrophilic) dyes. The dyes prefer to remain in the dyebath where they have a lower chemical potential. Therefore nonionic, hydrophobic dyes are used for these fibers. The exceptions to the rule are polyamide and modified polyacrylonitriles and modified polyester where the presence of a limited number of ionic groups in the polymer, or at the end of polymer chains, makes these fibers capable of being dyed by water-soluble dyes. 

This combination of monomers is unique in that the two are very different chemically, and in thek character in a polymer. Polybutadiene homopolymer has a low glass-transition temperature, remaining mbbery as low as —85° C, and is a very nonpolar substance with Htde resistance to hydrocarbon fluids such as oil or gasoline. Polyacrylonitrile, on the other hand, has a glass temperature of about 110°C, and is very polar and resistant to hydrocarbon fluids (see Acrylonitrile polymers). As a result, copolymerization of the two monomers at different ratios provides a wide choice of combinations of properties. In addition to providing the mbbery nature to the copolymer, butadiene also provides residual unsaturation, both in the main chain in the case of 1,4, or in a side chain in the case of 1,2 polymerization. This residual unsaturation is useful as a cure site for vulcanization by sulfur or by peroxides, but is also a weak point for chemical attack, such as oxidation, especially at elevated temperatures. As a result, all commercial NBR products contain small amounts ( 0.5-2.5%) of antioxidant to protect the polymer during its manufacture, storage, and use. 

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. 

In other instances the reactions appear to occur in sequence down the chain, for example in the depolymerisation reaction of polyformaldehyde (polyacetal) and polymethyl methacrylate which are referred to as zippering or sometimes unzippering reactions. In other cases cyclisation reactions can occur such as on heating polyacrylonitrile ... 

Though the increase of the concentration of Ce4+ ions also increases the yield of copolymer, it facilitates at the same time the decrease of the length of polyacrylonitrile blocks due to cerium ions participating in the disruption of the growing chain. 

This is a free-radical polymerization with short chain lives. The first molecules formed contain nearly 58 mol% styrene when there is only 50% styrene in the monomer mixture. The relative enrichment of styrene in the polymer depletes the concentration in the monomer mixture, and both the polymer and monomer concentrations drift lower as polymerization proceeds. If the reaction went to completion, the last 5% or so of the polymer would be substantially pure polyacrylonitrile. 

Chain growth polymerization. Important polymers manufactured hy chain growth are polyethylene, polystyrene, polyacrylonitrile, and polymethacrylates. 

An atactic polymer can crystallize, for instance the case of polyacrylonitrile, whose crystallizability has been explained on the basis of local conformations, which produce extended and straight chains.140... 

Polyacrylonitrile is an excellent textile fibre but is difficult to dye. However, by its copolymerisation or by grafting on a second polymer, it is possible to maintain the desirable properties of the fibre, yet produce a textile which can be processed in the usual way. Among the various factors that govern the copolymerisation process, the concentration and reactivity of the monomer are quite important. At any given time, the chain may grow in four different ways as under. Here A and B are the radicals which are involved in propagating steps, whereas A and B are the respective monomers. 

Grafting through Chain-transfer Fist the base polymer (for example Polyacrylonitrile) is dissolved in a monomer such as styrene and then an initiator is added. The two possible ways of polymerisation are ... 

It is also possible to prepare polymers with functional groups by partial saponification of polyvinyl esters to give OH groups or polyacrylic esters or Polyacrylonitrile to give COOH groups. Polymer with such functional groups at the chain ends are prepared... 

These results conclusively demonstrate that precipitation of polyacrylonitrile as a fine powder and occlusion of growing chains resulting in post-polymerization do not bring about autoacceleration if a highly polar solvent is present in the system. 

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