Polyacrylonitrile branching

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

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.)...

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. 

Hydrolysis of Bamboo-Polyacrylonitrile Copolymer. In order to isolate the polymer branches of the graft copolymers and to check the accuracy of the grafting percentages... 

Still another type of a copolymer is one that possesses backbones composed of one individual polymer and the branches from another one. It is called a graft copolymer, because many such materials were formed by grafting the branch polymers to the polymer backbone. This, however, is not always the case and many graft copolymers were formed by polymerizing the branch copolymer from a different polymer backbone. (The subject of block and graft copolymers is discussed in Chap. 9) A graft copolymer of polyacrylonitrile on polyethylene can serve as an example ... 

There are numerous examples of random coil polymers forming networks (gels) not involving covalent bonds, e.g., poly(vinyl chloride) in di-2-ethylhexyl phthalate, polyacrylonitrile in dimethylformamide, poly-acrylglycinamide in water, celluloseacetate in benzyl alcohol, and polyethylene in xylene. In some systems the network branch points are clearly a result of specific interchain aggregation as, for example,... 

A third variety is the graft copolymer. In this case, branches of one monomer are grown on a main stem of a previously formed polymer molecule. For example, polyethylene can be irradiated in air with gamma rays or accelerated electrons, which leave peroxides or free radicals trapped on the polymer backbone. Exposed to a reactive monomer such as acrylonitrile (CH2=CHCN), polymerization is initiated at the free-radical sites, and branches of polyacrylonitrile grow on the polyethylene crystal stem [57]. 

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