2.3.4 Stereoisomerism A final type of isomeric variation occurs as a result of the three-dimensional structure of some polymers. It is possible because a four-valent atom like carbon can exist in two different forms when the subsidiary groups or atoms attached to the carbon are all different. The carbon atom is then known as an asymmetric carbon atom. A very simple example of the phenomenon is the structure of a small molecule, lactic acid. As Figure 20 shows, it can exist in two forms which are mirr
2.3.3 Geometrical isomerism
A second type of isomerism occurs with diene monomers, and is present in both NR and butadiene rubbers (BR). It occurs because the single double bond in the final polymer can exist in two ways: a cis form and a
trans
form. The repeat unit shown in Table 3 for NR does less than justice to the two-dimensional structure of this material (Author(s):
2.3.2 Chain branching A germ of the idea is shown by the formulae for 2- and 3-methylpentane in Figure 16. A single methyl group (CH3—) can occur in two different positions along an essentially linear carbon-carbon chain. The methyl group is a very simple kind of branch along the chain, and it is easy to extend the idea to much larger molecules. Thus LDPE is a polymer based on a linear backbone chain with the repeat unit [CH<
2.3.1 Structural isomerism In the saturated hydrocarbons, whose structural formulae are shown in Figure 16, it is not possible to form distinct isomers with just three or less carbon atoms linked together. There is only one way in which one carbon and four hydrogen atoms can be linked together, the single compound being methane, CH4. A similar situation holds for ethane, C2H6 and propane, C3H8.
2.3 Chain configuration The structure of repeat units is fixed by the chemical bonds between adjacent atoms. The shape or shapes thus created is known as the configuration, and for chains will be the chain configuration. Like children's plastic building blocks, however, there can be many different configurations for a given set of atoms of a particular type. The different structures which have identical chemical formulae are known as isomers, and such isomers can have quite different properties. Isomer
2.2 Chain repeat units The repeat units of a range of polymers together with the monomer units from which they are derived are shown in Table 3. The simplest repeat unit is that for polyethylene, and consists of two carbon atoms linked to four hydrogen atoms. The difference between the monomer and the repeat unit is the loss of the double bond in the former to give the chain-linked repeating group. Thus the molecular masses of both monomer and unit are identical at 28. The molecular mass of the repeat unit is usual
2.1 Understanding the polymer state It was the pioneering scientific work of Hermann Staudinger in the early part of the twentieth century which led to an understanding of the polymer state at an atomic and molecular level. Until then, plastics and rubbers had been developed from naturally occurring substances or discovered during routine synthesis. His research laid the basis for all subsequent discoveries and their commercial development. In essence, he realised that polymers were large molecules built up by the repetition of
1.3 Product design and manufacture So what are the reasons for the continued growth in the use of polymers as shown in Figure 1? It cannot be raw material cost, since the source of synthetic polymers is crude oil or natural gas, prices of which have risen over the same period of time. The comparative prices of polymers are considerably greater than traditional materials like mild steel, so we must look elsewhere for their success. It is really necessar
1.2 Polymer types Traditionally, the industry has produced two main types of synthetic polymer – plastics and rubbers (Figure 3). The distinction is that plastics are, by and large, rigid materials at service temperatures while rubbers are flexible, low modulus materials which exhibit long-range elasticity. Plastics are further subdivided into thermoplastics and thermosets, the latter type being materials where the long chains are linked t
1 1The growth of poymers Polymers, or materials composed of long molecular chains, are now well-accepted for a wide variety of applications, both structural and non-structural, and for mass-manufactured as well as one-off speciality products. The growth in their use has continued in the last two decades or more, despite the effects of several recessions in industrial activity (Figure 1). In the same period the demand for traditional materials like
4.8 Diagrams for communication
Pictures speak louder than words. But how can you use diagrams to help you? This unit looks at how diagrams can be used to represent information and ideas about complex situations. You will learn how to read, draw and present diagrams to help illustrate how ideas or processes are connected.
4.7 Diagrams for planning and implementation
Pictures speak louder than words. But how can you use diagrams to help you? This unit looks at how diagrams can be used to represent information and ideas about complex situations. You will learn how to read, draw and present diagrams to help illustrate how ideas or processes are connected.
4.6 Diagrams for diagnosis
Pictures speak louder than words. But how can you use diagrams to help you? This unit looks at how diagrams can be used to represent information and ideas about complex situations. You will learn how to read, draw and present diagrams to help illustrate how ideas or processes are connected.
4.5 Diagrams for further analysis and quantitative model building
Pictures speak louder than words. But how can you use diagrams to help you? This unit looks at how diagrams can be used to represent information and ideas about complex situations. You will learn how to read, draw and present diagrams to help illustrate how ideas or processes are connected.
4.4 Diagrams for connectivity
Pictures speak louder than words. But how can you use diagrams to help you? This unit looks at how diagrams can be used to represent information and ideas about complex situations. You will learn how to read, draw and present diagrams to help illustrate how ideas or processes are connected.
4.3 Diagrams for understanding
Pictures speak louder than words. But how can you use diagrams to help you? This unit looks at how diagrams can be used to represent information and ideas about complex situations. You will learn how to read, draw and present diagrams to help illustrate how ideas or processes are connected.
4.2 Systems diagrams and diagrams helpful for systems work
Pictures speak louder than words. But how can you use diagrams to help you? This unit looks at how diagrams can be used to represent information and ideas about complex situations. You will learn how to read, draw and present diagrams to help illustrate how ideas or processes are connected.
4.1 Systems thinking and concept
Pictures speak louder than words. But how can you use diagrams to help you? This unit looks at how diagrams can be used to represent information and ideas about complex situations. You will learn how to read, draw and present diagrams to help illustrate how ideas or processes are connected.
3.5.2 Key points
Pictures speak louder than words. But how can you use diagrams to help you? This unit looks at how diagrams can be used to represent information and ideas about complex situations. You will learn how to read, draw and present diagrams to help illustrate how ideas or processes are connected.
3.5.1 Reading diagrams: questioning what they say
Pictures speak louder than words. But how can you use diagrams to help you? This unit looks at how diagrams can be used to represent information and ideas about complex situations. You will learn how to read, draw and present diagrams to help illustrate how ideas or processes are connected.
