Pages 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 9259 result(s) returned

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
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

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
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

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
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

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
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

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): The Open University

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
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

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
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

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
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

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
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

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
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

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
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

4 Impersonation and imagination

Activity 5

Listen to Track 3, where Jackie Kay, Paul Muldoon and Jean ‘Binta’ Breeze talk about the importance of autobiography to their poems as well as the importance of using the imagination to harness other people'
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

2.4 The First Consul

Click to see plate 11 Antoine-Jean Gros, Bonaparte as First Consul, 1802, oil on canvas, 205 x 127 cm, Musée Nationale de la Légion d’Honneur, Paris. Photo: Bridgeman Art Library

Exercise

Look at Gros's Bo
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

2.3.1 The model

Figure 12 shows some of the influences which bear on an organisation. These influences, of course, are felt not by ‘an organisation’ but by people within the organisation. It is sensible, therefore, to talk about the influences on the management or on the manager w
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

1.3 ‘Going public’

For many companies a point may be reached, particularly if the company has grown significantly in size and has aspirations for further expansion, to seek equity finance through an initial public offering of shares (IPO).

SAQ 4

Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

Introduction

This course introduces you to the contested area of educational inclusion. You will look at differing perspectives on inclusion, in particular the way that medical and social models have influenced and shaped current thinking. You will also think about barriers to inclusion and the difference between integration and inclusion. In addition, you will consider some of the key documents, such as the Salamanca Statement, that underpin current thinking in this area.

This OpenLearn course prov
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

Introduction

The course focuses on the knowledge, learning and thinking of children aged between 3 and 8 years old. It has been written for an audience of practitioners working in the full range of early years care and education settings: you may be a teaching assistant in an early years class, a nursery nurse, a playgroup worker or leader, or a childminder; you may work voluntarily in an early years setting. But whatever the context in which you are working, we expect you to be working there regularly, f
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

Introduction

What value does art have in the school curriculum? This unit, primarily aimed at colleagues teaching art in schools, explores the justification for including art in the school curriculum together with some of the current criticisms commonly heard.

Find out more about studying with The Open University by visiting our online prospectus.<
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

Introduction

Participating in the democratic processes is seen as being a fundamental aspect of citizenship. All pupils need a broad knowledge and understanding of the rights, responsibilities and duties of citizens, as well as an understanding of forms of government. Notions of citizenship have been forged alongside the expansion of the right to vote and the development of our ideas about democracy. In this unit we explore different interpretations of democracy and strategies for involving pupils in con
Author(s): The Open University

License information
Related content

Copyright © 2016 The Open University

Postgraduate study skills in science, technology or mathematics
Are you about to undertake a PhD in science, technology or mathematics? If so this free course, Postgraduate study skills in science, technology or mathematics, will help you to examine your work processes. You will consider and develop the nature of postgraduate work and look at the planning of work needed at doctoral level. First published on We
Author(s): Creator not set

License information
Related content

Except for third party materials and otherwise stated (see http://www.open.ac.uk/conditions terms and conditions), this content is made available under a http://creativecommons.org/licenses/by-nc-sa/2

Pages 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463