Some Individual Polymers

Gas-phase polymerization for ethylene was commercialized in 1968. Elimination of solvent recovery and catalyst removal as process steps gave significant economics. The process was described in Section 5.3 as an example of solution polymerization with insoluble initiator and polymer. It is used to produce HDPE, linear polymers with short branches by the copolymerization of ethylene with other branches of a-olefins (linear LDPE [LLDPE]), and polypropylene. 

The metallocene single-site catalysts (see Section 4.7) have been used in gas-phase and slurry reactors of various descriptions. The fact that metallocenes have been adapted to existing production lines has sped the introduction of the new poly-ethylenes into commercial markets. Other monomers that can be polymerized with metallocene catalysts include propylene, other a-olefins, and styrene. 

Another rubbery material, chlorinated polyethylene, can be made directly from branched polyethylene by postchlorination. A major use of this material is as a polymeric plasticizer for poly(vinyl chloride) (PVC). 

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In this chapter we have focused attention on various aspects of individual polymer molecules. In the next three chapters we shall examine some properties of assemblies of polymer molecules. Our interest in these chapters will be mostly directed toward samples of pure polymer assemblies of high and low molecular weight molecules-polymer solutions—will be discussed in Part III of this book. 

Tables 2—5 Hst some typical properties or ranges of properties for the more common film and sheet products. Although these values are good for comparative purposes, actual performance tests are best to determine suitabiHty for use. Properties of multiple-layer films or sheets in laminar stmctures cannot always be predicted from values for the individual polymer layers. Use conditions of stress, temperature, humidity, and light exposure all strongly influence performance. Film and sheet manufacturers can recommend product combinations or variations that may provide significant performance advantages to the user.

Copolymerisation is the process in which a mixture of two or more monomers gets polymerised to yield a product. The product obtained is known as a copolymer. A copolymer product contains some units of each type of monomer and is different from a physical mixture of individual polymer molecules formed by different monomers. It is not always possible to make a copolymer with any two or more monomers. When two monomers A and B are copolymerised the rate of polymerisation is determined by concentration of monomers. Four different propagation reaction can occur for copolymerisation of A and B. AA, AB, BB, BA". 

Multiple episodes of PTFE-induced inhalation fever over an 18-month period were associated with marked progression of chronic obstructive pulmonary disease in a carding machine operator. Permanent airway damage may occur in some individuals after repeated instances of polymer fume fever. 

Due to the fact that industrial composites are made up of combinations of metals, polymers, and ceramics, the kinetic processes involved in the formation, transformation, and degradation of composites are often the same as those of the individual components. Most of the processes we have described to this point have involved condensed phases—liquids or solids—but there are two gas-phase processes, widely utilized for composite formation, that require some individualized attention. Chemical vapor deposition (CVD) and chemical vapor infiltration (CVI) involve the reaction of gas phase species with a solid substrate to form a heterogeneous, solid-phase composite. Because this discussion must necessarily involve some of the concepts of transport phenomena, namely diffusion, you may wish to refresh your memory from your transport course, or refer to the specific topics in Chapter 4 as they come up in the course of this description. 

In reality, the morphology of a polycrystalline thermoplastic consists of spherulites which holds for common PP, PE, PA 6, PA 6,6 and PEEK crystalHzed under common conditions. Some semicrystalhne polymers as weU as the above mentioned moderately filled ones may exhibit lameUar crystahine morphology without any spherulitic order. As a result of random orientation of individual crystallites in spheruhtes and the manner of their connectivity, the elastic modulus of about 10 GPa has been extrapolated for a hypothetical ideal polycrystalline PE containing no amorphous phase from the dependence of the elastic modulus of PE on the degree of crystallinity. The presence of an amorphous phase which reduces the content of the crystalline phase results in a further reduction of the overaU elastic modulus of the semicrystalhne polymers compared to ideal mono crystals. 

In addition to their local irritant effects on the respiratory tract, the diisocyanates also have a propensity to induce an immimological response in some individuals, which is characterized by an asthma-like respiratory reaction, and will induce the formation of antibodies to both the monomeric and polymer forms of HDl. A few studies have examined the immimological effects of HDl toxicity in humans, with some data available from laboratory animal studies as well. 

In earlier Sections, the individual polymers that make up the constituent parts of the primary cell-wall have been discussed, and partial structures proposed for some of these polymers are shown in Figs. 1-4. A model of the primary cell-wall of suspension-cultured sycamore-cells, illustrating proposed interconnections between these constituent polymers within the intact wall, has been constructed by Albersheim and his associates.5,10 57,64,65 The model, not designed to be spatially or quantitatively accurate, is depicted in Fig. 6. 

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