Stress-induced crystallisation

This thermodynamic behaviour is consistent with stress-induced crystallisation of the rubber molecules on extension. Such crystallisation would account for the decrease in entropy, as the disorder of the randomly coiled molecules gave way to well-ordered crystalline regions within the specimen. X-Ray diffraction has confirmed that crystallisation does indeed take place, and that the crystallites formed have one axis in the direction of elongation of the rubber. Stressed natural rubbers do not crystallise completely, but instead consist of these crystallites embedded in a matrix of essentially amorphous rubber. Typical dimensions of crystallites in stressed rubber are of the order of 10 to 100 nm, and since the molecules of such materials are typically some 2000 nm in length, they must pass through several alternate crystalline and amorphous regions. 

In practice, many fabrication processes take place under non-isothermal, non-quiescent and high-pressure conditions. Mechanical deformation and pressure can enhance the crystallisation as well as the crystal morphology, by aligning the polymer chains. This leads to pressure-induced crystallisation and to flow-induced or stress-induced crystallisation, which in fact is the basis for fibre melt-spinning (see Sect. 19.4.1)... 

Stress-induced crystallisation is accomplished by spinning processes, followed by drawing of the spun elements, just by drawing of the spun filaments or by spinning with high-speed winding,... 

In semi-crystalline polymers this rearrangement is so drastic that it may be called stress-induced crystallisation or recrystallisation. 

PLA is a polymer that may not be well suited to injection moulding. Its rate of crystallisation is too slow to allow cycle times typical of those for commodity thermoplastics such as polystyrene. Stress induced crystallisation that can enhance PLA crystallisation is better suited to processes such as fibre spinning or biaxial orientation of film. 

Those elastomers that exhibit low modulus plus high strength in the unfilled gum state because they undergo stress-induced crystallisation. 

It is shown that the role of zinc stearate in vulcanisation formulations is to promote the reaction of accelerator terminated polysulphidic pendant groups with neighbouring polymer chains, thereby resulting in higher crosslink densities. Cyclisation reactions may also be reduced, thus increasing stress induced crystallisation under load and thereby further enhancing the physical properties of the vulcanisate. 6 refs. 

PET displays a rich variety of non-linear phenomena in its constitutive behaviour, and the endeavour of finding means to describe them mathematically is formidable. This is because beyond the common constitutive response exhibited by other amorphous polymers (e.g. polystyrene and polymethy methacryalate) such as yielding/stress softening and entanglement slippage, the behaviom of PET is further complicated by the evolution of a stress induced crystallisation and crystallisation enhanced stress relaxation process, where the details of the physical processes involved are still a topic of dispute. 

One effect of this strain-induced crystallisation is that there is a characteristic upswing in the plot of stress against strain for natural rubbers, as illustrated in Figure 7.11. 

Density is also found to increase in this region, thus providing additional evidence of crystallisation. Certain synthetic elastomers do not undergo this strain-induced crystallisation. Styrene-butadiene, for example, is a random copolymer and hence lacks the molecular regularity necessary to form crystallites on extension. For this material, the stress-strain curve has a different appearance, as seen in Figure 7.12. 

Crystallisation of polymers depends on the possibilities of nucleation and growth. The structural regularity of the polymer has a profound influence on both. Interesting correlations were found for estimating the rate of spherulitic crystallisation. Besides this normal mode of bulk crystallisation, other modes are frequently observed induced crystallisation by pressure and stress and extended chain crystallisation. The latter mode occurs under special conditions for flexible chain polymers, but is the normal mode for rigid chain polymers. All modes of crystallisation are correlated with the structure of the polymer chain and with the two main transition temperatures, Tg and Tm. 

INDUCED CRYSTALLISATION OF FLEXIBLE POLYMERIC MOLECULES BY PRESSURE AND STRESS... 

The stress-strain curve for unfilled NR exhibits a large increase in stress at higher deformations. NR displays, due to its uniform microstructure, a very unique important characteristic, that is, the ability to crystallise under strain, a phenomenon known as strain-induced crystallization. This phenomenon is responsible for the large and abrupt increase in the reduced stress observed at higher deformation corresponding, in fact, to a self-toughening of the elastomer because the crystallites act as additional cross-links in the network. This process can be better visualized by using a Mooney-Rivlin representation, based on the so-called Mooney-Rivlin equation ... 

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