Polymers stress-induced crystallisation

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)... 

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. 

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. 

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. 

The infrared and mechanical data all indicate that in the case of DBDI based PU, the conformational mobility of the two benzene rings around the ethylene bridge induces a specific stress - strain and stress relaxation behaviour in PU, both in tensile and compression experiments. Experimental DSC, IR diehroism, and X-ray diffraction techniques were employed to study the influence of the compatibility of PU microphases (hard and soft blocks) on the PU physical/ mechanical properties, by the study of the structural modifications which appear during mechanical stresses i.e. phase separation, crystallization and orientation phenomena. The morphological study on such polymers have proved a high tendency of crystallisation and hard segment blocks phase separations [2,4]. ... 

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