Crystallisation and orientation

If a rubbery polymer of regular structure (e.g. natural rubber) is stretched, the chain segments will be aligned and crystallisation is induced by orientation. This crystallisation causes a pronounced stiffening in natural rubber on extension. The crystalline structures are metastable and on retraction of the sample they disappear. 

Biaxial orientation effects are of importance in the manufacture of films and sheet. Biaxially stretched poly(ethylene terephthalate) (e.g. Melinex), 

Much of the success of the poly(ethylene terephthalate) bottle has arisen from the control of the biaxial orientation that occurs during manufacture to give a product both strong and of low gas permeability. 

When a rubbery polymer, such as natural rubber, is stretched the molecules become aligned. This orientation leads to crystallisation. The effect of this so-called strain-induced crystallisation is to make the extended polymer stiffer than the unstrained polymer. Such crystallisation is not permanent but disappears when the sample is allowed to retract and regain its original dimensions. 

Similarly, oriented crystallisation can be induced by stretching sheets or films of polymers in two directions simultaneously. The resulting materials have biaxially oriented polymer crystals. Typical examples of such materials are biaxially stretched poly(ethylene terephthalate), poly(vinylidene chloride), and poly (propylene). Since the oriented crystals do not interfere with light waves, such films combine good strength with high clarity, which makes them attractive in a number of applications. 

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The interplay of orientation and crystallisation leads to a wide range of super-molecular structures or morphologies. Each different morphology represents to the user a different compromise in physical properties, so that characterisation and control of morphology becomes very important for the efficient application of polymeric materials. 

Fibres of polycarborane 7, wet-spun from trifluoromethanesulfonic acid, can be oriented and crystallised by drawing to five times their original length at ca. 300 °C, and the X-ray diffraction pattern from such a fibre is shown schematically in Figure 1. The two meridional arcs, with d-spacings of 5.3 and 4.6 A, appear to correspond to sixth-and seventh-order reflections from a d-spacing of ca. 32 A, approximately the distance... 

There are two principal forces that govern the abdity of a polymer to crystallise the interchain attractive forces, which are a function of the chain stmcture, and the countervailing kinetic energy of the chain segments, which is a function of the temperature. The fact that polymers consist of long-chain molecules also iatroduces a third parameter, ie, the imposition of a mechanical force, eg, stretching, which can also enhance interchain orientation and favor crystallisation. 

Structurally the difference between PEN and PET is in the double (naphthenic) ring of the former compared to the single (benzene) ring of the latter. This leads to a stiffer chain so that both and are higher for PEN than for PET (Tg is 124°C for PEN, 75°C for PET is 270-273°C for PEN and 256-265°C for PET). Although PEN crystallises at a slower rate than PET, crystallization is (as with PET) enhanced by biaxial orientation and the barrier properties are much superior to PET with up to a fivefold enhancement in some cases. (As with many crystalline polymers the maximum rate of crystallisation occurs at temperatures about midway between Tg and in the case of both PEN and PET). At the present time PEN is significantly more expensive than PET partly due to the economies of scale and partly due to the fact that the transesterification route used with PEN is inherently more expensive than the direct acid routes now used with PET. This has led to the availability of copolymers and of blends which have intermediate properties. 

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