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4.5.2 Commercial copolymers

The main reason for copolymerizing different monomers is to adjust the physical properties of a given homopolymer to meet a specific demand. SBR elastomer, for example (Table 1), based on 24 wt% styrene monomer shows better mechanical properties and better resistance to degradation than polybutadiene alone. By increasing the s
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3.2.1 Thermal cracking

The bulk of the major monomer and intermediate, ethylene (C2H4), is still produced in the UK by steam cracking without the use of catalysts. Paraffinic feedstocks are best for optimising ethylene yields, and the severity of cracking is specified by the rate of disappearance of a marker compound, usually n-pentane. The severity of the reaction can then be defined as follows:

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2.6 Molecular mass distribution

Figure 28

2.5.4 Melting and structure

For those polymers which can crystallise, one would expect some relation between chain rotation and melting. Since all crystallisation demands that chains form an ordered conformation (e.g. the PE planar zig-zag) before they can pack together, the chance of this happening should be related to the ease of twisting into the required conformation. That there is a rough correlation between Tg and Tm can be judged from Author(s): The Open University

2.5.1 Homologous series

Another approach to the problem is to consider what happens to the properties of a related series of compounds of increasing chain length. The simplest precedent is the sequence of properties for the saturated paraffin hydrocarbons (normal alkanes). Such a series of compounds is known as an homologous series. The lowest members are gases like methane and ethane, but as the length of the chain increases the hydrocarbons become first liquids and then waxy solids. Both the melting points
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2.4 Chain conformations

The repeat units or chains shown in the previous figures are all static representations of real chains and they are therefore of limited use. The key idea we need to explore real chains, and their influence on properties is that of the conformation of a chain. A single conformation is just a single shape that a chain can adopt, so that for example, when a polyethylene chain is shown as a linear zig-zag, this is one possible conformation. Static formulae do not show an important aspect
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2.3.6 Copolymers

So far, the discussion has been confined to polymers with only a single type of repeat unit, but in reality, a large and growing number of commercial polymers are actually composed of different types of unit attached together by chemical covalent bonds. They are known as copolymers, and can comprise just two different units (binary copolymers) or three (ternary), and so on. It is one of the common strategies used by molecular engineers to manipulate the properties of polymers to gain j
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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
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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-di
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1.3.2 Human/product interaction

The balance of properties needed in a particular product varies enormously, depending on the exact duty that product will perform in service, the environment in which it will operate, and the way it will interact with the user or consumer. The last factor has assumed much greater influence in product design as competition between different manufacturers sharpens the perception of quality in users’ eyes. The study of human-product interactions is variously known as human factors or
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3.1 The steps to systems modelling

Systems modelling in practice usually involves six broad steps, within each of which there may be many subsidiary steps and some checking and revision. There is also likely to be iteration back to the earlier steps, as issues which call for changes in earlier decisions are uncovered.

Nevertheless, in my experience, the following six steps are likely to cover the basics.

  1. Identify the system of interest, in particular specify the system boundary a
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2.4 Models as part of systems work

Thinking systemically involves identifying systems relevant to some situation, and models are invariably used as part of this process. An example of this forms part of Checklands' Soft Systems Methodology (SSM) (Checkland, 1981). One aspect of this methodology concerns the formulation of a root definition of some system that is relevant to the situation of interest and the construction of a conceptual model of this system. The root definition is a concise, verbal description of what a
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3.10 Systems techniques

The two systems methodologies provide a framework for the application of problem solving, analysis and design techniques. These fall into three groups.

  • Diagramming: ranging from single systems maps to complex flow charts. Diagrams of one sort or another provide a method of analysis, design and communication.

  • Modelling: simulation is used extensively to analyse the dynamics of an existing system and to predict the behaviour of a propos
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2.3 Attenuation

Activity 2

At approximately what wavelength is the attenuation of optical fibre lowest? What, approximately, is the attenuation at that wavelength? What other wavelengths are used and why?


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2.1 Light sources and detectors

The basic building blocks of an optical-fibre link are the light source, the fibre and the detector (Figure 1).


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3.4 Managing across interfaces

Increasingly, operations management is seen as an interface discipline (Voss, 1995). Managing across interfaces, both internal and external to the organisation, is a particular challenge for managers and this is discussed further in this section.

Information and communications technology is an important means of linking across the various interfaces. Author(s): The Open University

8.3.1 Fluorine-based etching of silicon

Given the noxious chemistry needed to etch silicon with a liquid, it is perhaps surprising that a gas can do the job at all. However, both xenon fluoride (XeF2) and chlorine trifluoride (ClF3) gases have been used successfully for just this purpose. Each acts as a source of fluorine atoms, which are just barely bound together into molecules and are easily rearranged around silicon atoms with which they form strong bonds, turning them into inert SiF4 gas. These
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7.3.6 Laser ablation deposition

Another close cousin to sputter deposition is laser ablation deposition. Ion bombardment of the target is replaced by a focused pulse of light from an ultraviolet laser. Although each pulse may carry only 1 J of energy, this is delivered within 1 ns to a 1 mm spot on the target surface. This represents an astonishing power density and the target surface explodes into vapour that can be caught on the waiting wafer surface. It is vital to scan the focus point across the target surface or you wi
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2.5 Review

The materials from which this simple sensor is made have been carefully chosen. They have had to be compatible with one another during the manufacturing process – so that for example, etching the material in one layer did not affect another material laid down previously. It had to be possible to shape them into the desired form, though some compromises also had to be struck. For instance, the V-groove trench is not the ideal geometry for the pit behind the beam but it is very easily made in
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4.8 Assess and review again

If you've been following the stages of our problem-solving map, then the chances are you're ahead of me here (Figure 19). Yes, if it works, hurrah; if it doesn't then off we go again, all the way back to ‘possible solutions’ and selecting the best of the rest. Or maybe even going back to the be
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