Would You Weigh Less in an Elevator?
What happens when you jump in a moving elevator? Do you weigh more when you're going up and less when you're going down? Carol
Hedden explores the relationship between gravity, weight, and relative
motion, using a moving elevator to explain the fascinating physics. (03:35)
5.6.3 Conformation and crystallinity If there are key connections between the chain configuration and crystallisation, you might also expect some more subtle effects from rotation about chain bonds. After all, polymer chains must be able to twist into the regular conformation demanded for crystal structures (Figure 57(a)). And what influence will rotation have on
5.6.2 Structure and crystallinity In addition to the structural constraints mentioned in Section 2, where tacticity and geometrical isomerism control whether or not a polymer chain can crystallise, molecular mass and copolymerization are other important variables which can influence crystallising properties. A related effect is plasticization where a low molecular mass mat
5.6.1 Morphology of polymer crystallites The fundamental unit of structure formed by crystalline polymers which is accessible using the optical microscope is the spherulite. Isolated spherulites are formed easily at relatively slow spherulite growth rates such as those exhibited by polypropylene and isotactic polystyrene. Unlike aramid fibres where the degree of crystallisation is close to 100 per cent (Author(s):
5.6 Crystallisation of polymers The major benefits of crystallisation of chain molecules to end users are: since Tm > Tg, the maximum service temperatures are higher than with amorphous polymers; above Tg
, the modulus of a crystalline polymer is higher than that of an amorphous polymer owing to reinforcement and physical crosslinking of the amorphous matrix by the crystallites. Both effects are
5.5.2 Molecular orientation As polymers are processed and shaped by flowing into moulds the shear stress fields induce preferred orientations in the molecules. The hydrostatic components of the stress field cause packing. These orientation and packing effects will relax with time if the temperatures are high enough, but the moulding cycle is frequently such that they are ‘frozen-in’ by cooling or perhaps fixed into the structure because the material has been crosslinked. The consequent moulded-in or residual stresse
5.5.1 Non-uniform mixtures Moulded rubbers and plastics are compounds of a polymer matrix and a variety of additives. The mixing history of the material before and during the moulding process can have a critical influence upon the final product properties. If mixing is done badly then the microstructure of the moulding can be non-uniform. Lack of uniformity can cause variations of strength and other physical properties within the moulding. The degree of dispersion or distribution of relatively minor quantities of addit
5.5 Orientation in polymers Viscoelasticity, like thermodynamics, is concerned with the correlation of controllable variables and bulk, macroscopic phenomena. But one unique feature of polymeric materials is that the molecular unit, the polymer chain, can be highly anisotropic, i.e. the chain can be fully extended, or curled up in an amorphous equilibrium state without any net orientation. In fact, unoriented polymer is rarely encountered in manufactured products because of the different ways it is processed to shape. B
5.3.1 Time-temperature superposition For amorphous polymers above their Tgs, there is a convenient approximation which makes experiments easier. It is known as time-temperature superposition, and it relates time to temperature for viscoelastic materials. A sequence of measurements of ER
(t) is performed at different temperatures at a fixed initial strain. The time scale might be limited between several seconds and say 100 hours. The curves obtained on uncrosslinked polyisobutylen
5.3 Viscoelasticity and master curves An immediate consequence of the viscoelasticity of polymers is that their deformations under stress are time dependent. If the imposed mechanical stress is held constant then the resultant strain will increase with time, i.e. the polymer creeps. If a constant deformation is imposed then the induced stress will relax with time (stress relaxation). Author(s):
5.2.2 Viscous behaviour Viscous flow is not recoverable. When the stress is removed from a viscous fluid the strain remains. Hence the work energy is not returned to the forcing agency and has to be otherwise dissipated. Figure 45 illustrates this schematically by showing the strain response in such a viscous material when a simple stress history has
5.2.1 Elastic and viscoelastic behaviour When an elastic (not elastomeric, or long range elastic) material is stressed, there is an immediate and corresponding strain response. Figure 43 illustrates this by showing schematically the strain response to a particular stress history. Note that when the stress is removed the strain also returns to zero. So in a per
5.2 Viscoelasticity of polymers The simplest models for the deformation behaviour of an ideal material are those of Hookean linear elasticity in the solid state, and Newtonian linear viscosity in the liquid state. The end point of elastic deformation is either fracture or plastic flow, with the latter taking place at a constant yield stress in the ideal case. Whilst the behaviour of many real materials does approximate to these idealised models, that of polymers deviates markedly from them. In particular, their solid state
5.1 The behaviour of polymers The manufacture of polymer products is controlled by two often conflicting demands: the quality of the finished article in terms of its response to its environment and the ease or difficulty of processing it to shape. Both factors are controlled by what is termed viscoelasticity, namely, the behaviour of the polymer in response to applied stress or strain, and temperature. It is important to appreciate the duality in terms of the elastic and viscous responses of polymer solids and poly
4.6.2 Material costs in manufacturing For high added-value products like boats and cars, material costs form a relatively small proportion of total costs. For directly manufactured products, however, which are sold without much assembly or finishing, material costs do form a relatively large proportion of the total production cost. This applies particularly to polymeric containers for foods and drinks but not, for example, to containers for more sophisticated products like electronic or electrical goods. What is much more importa
4.6.1 Prices of polymers Prices of bulk and speciality polymers (Table 9) broadly reflect the degree of chemical processing and treatment needed to make them. Thus the polyolefins, which are directly polymerized from cracker streams, are generally the cheapest followed by vinyl derivatives of ethylene like PS and PVC. Derived polymers which require mo
2.7 Legitimating the regime The failure of Ingres's painting is revealing of the problems of political legitimation faced by the regime. If it was difficult to justify the authority of a ruler who had seized power, it was even harder to justify a monarchy based on usurpation (the authority Napoleon had usurped being either that of the Bourbon dynasty from a royalist point of view or that of the people from a republican one). Ingres's image of timeless, otherworldly majesty can thus be seen as compensating, or rather try
2.2 Input-output diagrams An input-output diagram shows the inputs to a system or to an operation and the outputs from it.
Acknowledgements The content acknowledged below is Proprietary (see terms and conditions) and is used under licence. All other materials included in this unit are derived from content originated at the Open University.
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5.3 Summary This section made an interesting contrast between simple data that generates large and complex structures that require large and complex programs to handle them, and complex data which a complex but easy to use program helps a non-expert handle in some interesting, creative, flexible ways. The case study on DNA illustrated how simple data (consisting of only four elements) can be combined into very large and complex structures (genes and chromosomes). You learned how such large and comp
