The chemical nature of polymers

In this book the term polymer is used to mean a particular class of macromolecules consisting, at least to a first approximation, of a set of regularly repeated chemical units of the same type, or possibly of a very limited number of different types (usually only two), joined end to end, or sometimes in more complicated ways, to form a chain molecule. If there is only one type of chemical unit the corresponding polymer is a homopolymer, if there is more than one type it is a copolymer. This section deals briefly with some of the main types of chemical structural repeat units present in the more widely used synthetic polymers and with the polymerisation methods used to produce them. Further details of the structures of individual polymers will be given in later sections of the book. 

It should be noted that the term monomer or monomer unit is often used to mean either the chemical repeat unit or the small molecule which polymerises to give the polymer. These are not always the same in atomic composition, as will be clear from what follows, and the chemical bonding must of course be different even when they are. 

The simplest polymers are chain-like molecules of the type 

The phrase typical length of chain was used above because, unlike those of other chemical compounds, the molecules of polymers are not all identical. There is a distribution of relative molecular masses Mf) often called molecular weights) and the corresponding molar masses, M. This topic is considered further in section 3.2. The value of Mr for the chain considered in the previous paragraph would be 280 000, corresponding to M = 280 000 g mol. Commercial polymers often have average values of M between about 100000 and 1000000 g moP, although lower values are not infrequent. 

The flexibility of polyethylene chains is due to the fact that the covalent bonds linking the units together, the so-called backbone bonds, are non-collinear single bonds, each of which makes an angle of about 112° with the next, and that very little energy is required to rotate one part of the molecule with respect to another around one or more of these bonds. The chains of other polymers may be much less flexible, because the backbone bonds need not be single and may be collinear. A simple example is poly-paraphenylene,  

---

Naturally, the effect of the chemical nature of polymers on the flammability may be established only in tests under identical conditions. At present, many procedures are available for estimating material flammability, each reflecting one or another aspect of the complex combustion process. 

Reinforcement is not specific to any one polymer, filler or vulcanization system. For example, a highly reinforcing carbon black will reinforce all rubbers to a similar (but not necessarily identical) degree, which will be nearly independent of the vulcanization reaction, provided that the black does not interfere with the latter. This is not to say that the chemical nature of polymer, filler and cross-linking reaction are not important variables in reinforcement, but the specific effects they produce are differences in degree and not in kind. 

The binding interactions of polymers to hair The chemical nature of polymers used in hair products In situ polymerization reaction mechanisms Rheological or flow properties of polymer solutions Film formation and adhesional properties of polymers... 

It must not, however, be thought that the development of new polymers has come to an end. This is by no means the case. Polymer chemists continue to develop both new polymers and new polymerisation processes for older polymers. This leads not only to the introduction of polymers for special uses, which are often expensive, but also to the production of polymers speeially constructed to test theoretical understanding of how specific features of structure affect physical properties. Totally novel types of polymer are also synthesised with a view to investigating whether they might have useful properties. These developments are considered further in section 1.3.4, and the following section describes the chemical nature of polymers in more detail than has so far been considered. 

Effects of the Chemical Nature of Polymers/Polymeric Additives/ Surface Roughness... 

The use of flame retardants came about because of concern over the flammabiUty of synthetic polymers (plastics). A simple method of assessing the potential contribution of polymers to a fire is to examine the heats of combustion, which for common polymers vary by only about a factor of two (1). Heats of combustion correlate with the chemical nature of a polymer whether the polymer is synthetic or natural. Concern over flammabiUty should arise via a proper risk assessment which takes into account not only the flammabiUty of the material, but also the environment in which it is used. 

Reverse osmosis membrane separations are governed by the properties of the membrane used in the process. These properties depend on the chemical nature of the membrane material, which is almost always a polymer, as well as its physical stmcture. Properties for the ideal RO membrane include low cost, resistance to chemical and microbial attack, mechanical and stmctural stabiHty over long operating periods and wide temperature ranges, and the desired separation characteristics for each particular system. However, few membranes satisfy all these criteria and so compromises must be made to select the best RO membrane available for each appHcation. Excellent discussions of RO membrane materials, preparation methods, and stmctures are available (8,13,16-21). 

Manufactured fibers produced from natural organic polymers are either regenerated or derivative. A regenerated fiber is one which is formed when a natural polymer or its chemical derivative is dissolved and extmded as a continuous filament, and the chemical nature of the natural polymer is either retained or regenerated after the fiber-formation process. A derivative fiber is one which is formed when a chemical derivative of the natural polymer is prepared, dissolved, and extmded as a continuous filament, and the chemical nature of the derivative is retained after the fiber-formation process. 

Plasticization, whether internal (by copolymerization) or external (with additives), is also extremely important for proper performance at the time of apphcation. The ease of coalescence and the wetting characteristics of the polymer emulsion particles are related to their softness and the chemical nature of the plasticizer. 

An important characterization parameter for ceUulose ethers, in addition to the chemical nature of the substituent, is the extent of substitution. As the Haworth representation of the ceUulose polymer shows, it is a linear, unbranched polysaccharide composed of glucopyranose (anhydroglucose) monosaccharide units linked through thek 1,4 positions by the P anomeric configuration. 

The chemical nature of amber is complex and not fully elucidated. It is believed not to be a high polymer, the resinous state being accounted for by the complexity of materials present. The empirical formula is CioHigO and true amber yields on distillation 3-8% of succinic acid. 

The hydrophilic surface characteristics and the chemical nature of the polymer backbone in Toyopearl HW resins are the same as for packings in TSK-GEL PW HPLC columns. Consequently, Toyopearl HW packings are ideal scaleup resins for analytical separation methods developed with TSK-GEL HPLC columns. Eigure 4.44 shows a protein mixture first analyzed on TSK-GEL G3000 SWxl and TSK-GEL G3000 PWxl columns, then purified with the same mobile-phase conditions in a preparative Toyopearl HW-55 column. The elution profile and resolution remained similar from the analytical separation on the TSK-GEL G3000 PWxl column to the process-scale Toyopearl column. Scaleup from TSK-GEL PW columns can be direct and more predictable with Toyopearl HW resins. 

Hence, the transition of a polymer system into the oriented state is a result of the competition of two fundamental properties of a polymer molecule (1) its inherent anisotropy which is the main reason for the ability of polymer systems to form an oriented phase and (2) its flexibility which favours coiling of a long molecule. The result of this competition is determined by the chemical nature of the molecule however, kinetic hindrance can prevent the transition into the oriented state. 

Upon cooling, molten and rubberlike polymers pass the glass transition and solidify as glassy materials. The temperature TB of the glass transition depends on the chemical nature of the polymer as well as on the number of crosslinks between the molecular chains. Two different test methods were used for the determination of the glass transition range ... 

---