Rapid strain-induced crystallization

Rapid strain-induced crystallization has been associated with the outstanding mechanical properties characteristic of crosslinked NR compared to its synthetic analogue, i.e. crosslinked IR. This may be due in part to a reinforcing effect of strain-induced crystals on the properties of NR as a filler or physical crosslinking junctions. In fact, tensile and tear strengths of crosslinked NR are practically higher than those of crosslinked IR, at the high-speed limit of the tear test." Therefore, it is important to control the rate of crystallization, in order to control the mechanical properties of not only NR itself, but also the NR blends. 

Limiting our analysis to the effect of the base polymer on rubber failure, the polymer structure could be tailored as a dissipative one by increasing Tg up to the maximum value acceptable for an elastomer or toward a strain crystallizable structure by increasing the microstructure regularity up to a point in which a rapid strain induced crystallization is achieved. 

The main conclusions of the strain induced crystallization behavior of high trans polybutadiene based rubber and natural rubber are (1) the rate of crystallization is extremely rapid compared to that of NR (2) the amount of strain induced crystallization is small compared to that of NR, especially at room temperature and (3) for the high trans SBR s relative to NR, crystallization is more sensitive to temperature at low extension ratios, and crystallization is less sensitive to strain. 

Andrews et al., 1971). The latter, secondary crystallization, can be quite slow (Mitchell and Meier, 1968), and its rate is unaffected by strain (Andrews et al 1971). However, the row nucleation rate is greatly enhanced by orientation, effecting rapid initial crystallization (Krigbaum and Roe, 1964 Ziabicki and Jarecki, 1978 Elyashevich, 1982 Roland and Sonnenschein, 1991). The time for strain-induced crystallization of NR is less than 60 ms at RT (Mitchell and Meier, 1968), so that synchrotron X-ray sources (Toki et al., 2003,2011) are required for on-the-fly measurements. 

The network theory gives a good representation of the experimental results only for moderate strains (e < 1.5). For larger extensions, the theory overpredicts the stress because the conformational states are no longer adequately represented by the Gaussian distribution. At still higher extensions (e > 6), the observed stress rapidly rises because of development of strain-induced crystallization [see Fig. 8 (here A, = strain e)]. 

Finite chain extensibility is the major reason for strain hardening at high elongations (Fig. 7.8). Another source of hardening in some networks is stress-induced crystallization. For example, vulcanized natural rubber (cw-polyisoprene) does not crystallize in the unstretched state at room temperature, but crystallizes rapidly when stretched by a factor of 3 or more. The extent of crystallization increases as the network is stretched more. The amorphous state is fully recovered when the stress is removed. Since the crystals invariably have larger modulus than the surrounding... 

Kundler, L Finkelmann, H., Strain-Induced Director Reorientation in Nematic Liquid Single Crystal Elastomers. Makromol. Rapid Commun. 1995,16, 679-686. 

Klimov, E. Hoffmann, G. G. Gumenny, A. Seisler, H. W., Low-Temperature FT-NIR Spectroscopy of Strain-Induced Orientation and Crystallization in a Poly(dimethylsiloxane) Network. Macromol. Rapid Commun. 2005,26, 1093-1098. 

The process uses crystallizable polymers, of which the most important in PET. The first step is to injection mould (hence the name) a parison, or preform as it is more usually termed here. The preform is closed at the bottom and is considerably shorter and thicker than the final bottle. It is rapidly cooled (quenched) by using chilled water to cool the injection mould and this ensures that it is in its amorphous condition, i.e. no crystalline structure. Next it is reheated with infra-red elements to above its Tg, about 90-100 C for PET and enters the bottle mould and the mould is closed. The blow pin enters and pushes the soft preform downwards almost simultaneously the blow occurs, compressed air blowing the material outwards. The result is biaxial orientation - downwards from the movement of the blow pin, outwards from the action of the expanding air. The orientation induces crystallization, but in the form of lamellar crystals rather than spherulitic ones. This type of crystallization is strain-induced, and is characteristic of synthetic fibres and film, e.g. Melinex. It gives a transparent product with enhanced physical properties, both important for bottling carbonated drinks. The alternative name for the process is the stretch-blow process. Its main feature as a process is the control of the crystallinity of the polymer at its different stages. 

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