Pure polyfvinyl chloride

Polyfvinyl chloride) (PVC) is produced by mass, suspension, and emulsion processes. Mass polymerization is an exatiiple of a heterogeneous bulk system. PVC is virtually insoluble in vinyl chloride because the polymer is about 35% more dense than the monomer under normal polymerization conditions. Vinyl chloride, however, is quite soluble in polymer. The two phases in PVC polymerizations are pure monomer and monomer-swollen polymer. Polymerization proceeds in both phases, but it is very much faster in the polymer-rich phase because the mobility of macro radicals and mutual termination reactions are. severely restricted (cf. Section 6.13.2). 

Plasticizers—small organic molecules that act as lubricants between chains—eire usually added to thermoplastics to keep them from becoming brittle at room temperature. A good example is polyfvinyl chloride), which is brittle when pure but becomes supple and pliable when a plasticizer is added. In fact, the drip bags used in hospitals to deliver intravenous solutions are made of polyfvinyl chloride). Dialkyl phthalates such as di(2-ethyl-hexyl) phthalate are commonly used for this purpose, although questions about their safety have recently been raised. 

Molecular weights are not often measured directly for control of production of polymers because other product properties are more convenient experimentally or are thought to be more directly related to various end uses. Solution and melt viscosities are examples of the latter properties. Polyfvinyl chloride) (PVC) production is controlled according to the viscosity of a solution of arbitrary concentration relative to that of the pure solvent. Polyolefin polymers are made to specific values of a melt flow parameter called melt index, whereas rubber is characterized by its Mooney viscosity, which is a different measure related more or less to melt viscosity. These parameters are obviously of some practical utility, or they would not be used so extensively. They are unfortunately specific to particular polymers and are of little or no use in bringing experience with one polymer to bear on problems associated with another. 

Swelling of rubbers and polymers was mentioned in Section 2.6 in connection with cohesive energy density. Solvents may extract portions of polymer systems, for example, the plasticizer from a polyfvinyl chloride)-based system. Swelling at a surface may set up a stress that is relieved by crazing. Craze lines consist of oriented polymers. They become evident by their opacity in glassy polymers stressed by a purely mechanical couple or by surface absorption of a liquid (see Section 10.7). 

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