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4.3 Chain growth polymerization

Chain growth polymerization is basically a three-stage process, involving initiation of active molecules, their propagation and termination of the active chain ends.


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3.3.1 Ethylene, propylene and butadiene

Nowadays ethylene is the most important building block for the chemical industry, particularly as a monomer in its own right, as a co-monomer with other vinyls, and as a source of vinyl monomers. It is the prime source for ethylene oxide, which is another major source of polymers, glycols and ethers. They can also be used to build up more complex C4 molecules and aromatics.

Some of the ways in which the ethylene molecule is modified to create other chemicals and polymers are
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2.7 Commercial polymers

The increasing control of polymer structure by fine-tuned catalysis of polymerization opened up an enormous area for commercial exploitation, and new polymers are still being produced in this way (such as the metallocene polymers). A revolution of equal magnitude has occurred with polymers containing functional groups, for example, the nylons, polyesters and polyurethanes, resulting in polymers ranging from quite simple structures like aramid fibre to relatively complex repeat units like thos
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2.6 Molecular mass distribution

Figure 28

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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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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4.4.2 Passive optical networks and Ethernet in the first mile

If FTTH uses a dedicated fibre link between each house and the telephone exchange then each house carries the cost of two sets of terminal equipment: one in the house and one in the exchange.

A significant saving can be made with the passive optical network (PON) configuration (Author(s): The Open University

2.4.1 Multimode distortion

With multimode fibre, the main cause of pulses spreading is the multiple paths that signals can traverse as they travel along the fibre. This phenomenon of multimode distortion is illustrated in Figure 5.


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Learning outcomes

By the end of this unit you should be able to:

  • discuss what ethics is and what constitutes an ethical issue;

  • identify and discuss ethical issues that arise in the media, in routine conversations and, in particular, in your own everyday professional practice;

  • discuss the role of emotions in ethical deliberations;

  • discuss how negotiation might resolve apparent ethical differences;

  • identify and discuss the ethical issues p
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6.5 Costs

  • 11. What are the financial costs involved (capital and recurring)?

  • 12. What additional non-financial resources will be needed?

  • 13. When are the all the various financial and non-financial resources needed?


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8.4.1 Open-loop control

Open-loop is the crudest way of controlling etch depth. It relies on ensuring that every aspect of the process that can affect the rate of progress of the etch is kept under tight control. This can add up to a sizeable list. Table 5 shows just some parameters that affect both wet and dry etching.

Whether
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7.3.3 Plasmas

More control can be achieved in vapour deposition if a plasma is generated. A plasma is simply a gas where a proportion of the molecules have been ionised. The ions remain in an uneasy equilibrium with the electrons they have released, prevented from recombining only because the electrons are hot and fast-moving, and so are difficult to trap.

Plasmas are widely used in materials processing, with pressure ranging from 10−3 mbar to 1 mbar and typically up to 1% of the molecul
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4.3 PZT

The above requirements lead us to a range of ceramic materials with crystalline structure. One such material is lead zirconate titanate (PZT), which is an oxide alloy of lead, zirconium and titanium. It is often used in a specific composition (sometimes with additives) in order to achieve a particular crystal structure and the desired piezoelectric response. Author(s): The Open University

3.6 Design considerations for AFM probes

AFMs have proved so useful in so many areas of science and engineering that they are now to be found in most universities and many manufacturing companies. The making of probes for these instruments is no longer a cottage industry, partly because of the sheer numbers involved, but also because of the need for consistency of performance from probe to probe. This requires precise shaping, both of the tip itself and of the cantilever on which it is mounted. The quality of image obtained from an
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3.5.3 Lateral force (friction) mode

If the AFM probe is put in contact with the surface and dragged sideways across it, the cantilever will be sensitive to variations in the lateral force between the probe tip and the sample. Figure 8 illustrates the principle.

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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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3.3 Description of the bridge

An outline plan of the bridge shows the main piers on which the bridge was laid (Figure 10). To allow shipping to pass up the Tay to Perth, a height of about 88 feet was required between the bridge girders and the high water mark in the middle of the firth. On the south bank, at Wormit, the land rose steeply t
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2.3 Railways in Britain

The railway age started with attempts to make a steam engine small enough to be fitted to a wagon for hauling coal at collieries, the wheels moving on a wooden or iron rail for guidance. Improvements to the drive mechanism led directly to the Locomotion designed by George Stephenson, and the opening of the first passenger and goods service for the 27 miles between Stockton and Darlington in County Durham. It was opened in 1825 and was quickly followed in 1830 by a line between Manchest
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2.2 Transportation disasters

Movement of people and goods was one of the main outcomes of the industrial revolution in Britain in the late-eighteenth century, starting with canals, which were displaced gradually by railways. Industrialisation came through innovation in manufacture, especially the development of mass-produced materials such as cast-iron. While the material had been known and used since the Elizabethan period, it could only be made in small quantities by smelting iron ore with charcoal.

The Darby fam
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2.8 Good times and bad

The music industry, like any other large industrial business, had good times and bad times. By 1924 the burgeoning of radio broadcasting in the United States caused a severe downturn in record and equipment sales, leading to amalgamations and bankruptcies of many of the record companies. Actually, radio broadcast studio technology proved of great importance to the record industry. The sensitive microphones and electronic amplifiers used in broadcast studios offered improved characteristics th
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