Acetate, polyvinyl long-chain branching

The method outlined above for characterizing branched polymers will hereafter be referred to as the molecular weight and branching distribution (MWBD) method. In the following sections, its application to the long chain branching in polyvinyl acetate and high pressure low density polyethylene will be demonstrated. 

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

As already mentioned, multibranched polyvinyl acetate is produced by the chain transfer to acetyl groups of dead polyvinyl aeetate, but all these branches are cut off by the deacetylation. If chain transfer occurs at a carbon atom on the main chain, it is certain that the branch connected directly to the main chain carbon atom will not be broken to result in the long branches. There are some papers that insist on the chain transfer to the main chain carbon atoms however, there is no experimental evidence to show the existence of a main chain branch [58-60]. The main chain transfer probably occurs, but its rate is much lower than the rate of transfer to the acetyl group. 

With loose structures of linear molecules the exponent for instance to cellulose nitrate in acetone, precipitated by water, to Polyvinyl acetate in toluene, precipitated by a methanol-water mixture-, and to the methyles-ters of polymethacrylic acid in benzene, precipitated by cyclohexane With compact spherical particles we must expect n-values in the neighbourhood of 2/3, since in this case only the outer surface of the particles is subject to the action of the medium. Examples can be found in some proteins. Finally, if the long-chain molecule shows a pronounced ramification or if the randomly kinked structure is comparatively close-packed, n may assume values between 0.7 and 1. This is shown for instance, by branched polystryrene h by acetyl-starch and by glycogene An accurate check on the value of n, however, is usually impossible. Husemann s experiments with glycogene, for instance, can be equally well described by n = 2/3 as by n — J (Fig. 5). This is due to the fact that the value of P in this method is not unlimited. In practice the upper limit of the molecular weight lies in the neighbourhood of 5.10, the lower limit lies in the further to be noted that the constant b in equation... 

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