Orientation frozen

As previously stated, molecular orientation occurs during melt processing of polymers. On removal of the deforming stresses the molecules start to coil up again but the process may not go to equilibrium before the polymer cools to below its Tg. This leads to residual orientation (frozen-in strain) and corresponding frozen-in stresses. 

A special category is formed by oriented polymers, which have considerably higher stiffnesses. The most obvious example is the textile fibre the orientation, frozen-in in a crystalline structure, raises E by a factor of 3 to 5. Extremely high orientations, as met in liquid-crystal polymers (LCP s) result in even higher E-values, namely 60 to 120 GPa ... 

Among the first molecules for which triplet state EPR signals were detected in randomly oriented frozen solutions were indole 3) and tryptophan The aromatic amino acids are well suited to triplet EPR studies by virtue of their long-lived phosphorescent states which allow large steady-state triplet populations to be achieved with continuous optical pumping. The triplet states of the aromatic amino acids and... 

To make shrink film, a polymer film can be oriented at an elevated temperature and the orientation frozen by rapid cooling. When the film is subsequently heated, the molecular memory of the polymer causes it to attempt to return to its original dimensions. Lightly cross-linked materials are often used to increase the tendency to shrink. In that case, electron beam irradiation of the plastic film produces free radicals, which then react to produce cross-links between adjacent molecules. The presence of these cross-links means the material will no longer become liquid and flow at its normal melting temperature. That, in turn, allows the shrink film to be exposed to high temperatures, at or above its former melt temperature, without flow, so these elevated temperatures can be used to promote shrinking. 

There are many methods to increase the amount of orientation frozen in during the blown film process. Systems have been developed to approach... 

Anisotropic behaviour is also exhibited in optical properties and orientation effects can be observed and to some extent measured by birefringence methods. In such oriented materials the molecules are in effect frozen in an unstable state and they will normally endeavour to take up a more coiled conformation due to rotation about the single bonds. If an oriented sample is heated up the molecules will start to coil as soon as they possess sufficient energy and the mass will often distort. Because of this oriented materials usually have a lower heat distortion temperature than non-oriented polymers. 

The dielectric constant of unsymmetrical molecules containing dipoles (polar molecules) will be dependent on the internal viscosity of the dielectric. If very hard frozen ethyl alcohol is used as the dielectric the dielectric constant is approximately 3 at the melting point, when the molecules are free to orient themselves, the dielectric constant is about 55. Further heating reduces the ratio by increasing the energy of molecular motions which tend to disorient the molecules but at room temperature the dielectric constant is still as high as 35. 

Cooling rates can affect product properties in a number of ways. If the polymer melt is sheared into shape the molecules will be oriented. On release of shearing stresses the molecules will tend to re-coil or relax, a process which becomes slower as the temperature is reduced towards the Tg. If the mass solidifies before relaxation is complete (and this is commonly the case) frozen-in orientation will occur and the polymeric mass will be anisotropic with respect to mechanical properties. Sometimes such built-in orientation is deliberately introduced, such as... 

The built-in hinge, particularly successful with polypropylene in which molecules are frozen-in oriented at right angles to the axis of the hinge. 

In general it may be said that the amount of frozen-in orientation will depend on ... 

As with thermoplastics melt processes, the setting is achieved by cooling. It will be appreciated that such cooling is carried out while the polymer is under stress so that there is considerable frozen-in orientation. This can be maintained throughout the life of the article. It is possible with the higher molecular weight materials to heat shapes made from blanks many years previously and see them return to the original shape of the blank. 

A characteristic feature of thermoplastics shaped by melt processing operations is that on cooling after shaping many molecules become frozen in an oriented conformation. Such a conformation is unnatural to the polymer molecule, which continually strives to take up a randomly coiled state. If the molecules were unfrozen a stress would be required to maintain their oriented conformation. Another way of looking at this is to consider that there is a frozen-in stress corresponding to a frozen-in strain due to molecular orientation. 

The reason for the activity of the above named classes of liquids is not fully understood but it has been noted that the most active liquids are those which reduce the molecular cohesion to the greatest extent. It is also noticed that the effect is far more serious where biaxial stresses are involved (a condition which invariably causes a greater tendency to brittleness). Such stresses may be frozen in as a result of molecular orientation during processing or may be due to distortion during use. 

The rather rigid molecules and high setting temperatures are conducive to molecules freezing in an oriented position with consequent high frozen-in stresses. 

Second-order stress is difficult to observe and much less extensively studied. The causes of internal stress are still a matter for investigation. There are broad generalisations, e.g. frozen-in excess surface energy and a combination of edge dislocations of similar orientation , and more detailed mechanisms advanced to explain specific examples. 

Sheet forming processes, such as vacuum forming, do have effects on the product. The designer should be aware that these will affect the performance of one s product and one should learn how to modify the design to minimize any deleterious effects. Probably the most serious problem encountered in formed film or sheet products results from the fact that the materials are made from film or sheet at temperatures well below the melt softening point of the plastic, usually near the heat distortion temperature for the material. Forming under these condition when the draw down ratio is exceeded for a specific plastic can result in over stretched orientation of the material, the production of frozen-in stresses, poor product reproducibil-... 

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