Orientation-induced crystallization

The increase in the equilibrium melting point is thus due to the increase in free energy of the melt, the latter being ideally entropy-driven  

Chapter 3). The ef ective degree of supercooling at a given crystallization temperature (TJ increases due to orientation  

The overall crystallization can also be compactly described by the Avrami equation, which for polymers takes the form  

The kinetics of orientation-induced crystallization has been treated by Janeschitz-Kriegl (1992), Eder, Janeschitz-Kriegl and Liedauer (1992) and Haudin and Billon (1992), essentially using the Avrami equation. 

Crystallization involves the short-distance diffusion of crystallizable units and the passage of a free energy barrier of enthalpic or entropic origin. The overall crystallization rate w ) can be described according to the equation  

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Biaxial orientation effects are important in the manufacture of films and sheet. Biaxially stretched polypropylene, poly(ethyleneterephthalate) (e.g., Melinex) and poly(vinylidene chloride) (Saran) produced by flat-film extrusion and tentering are strong films of high clarity. In biaxial orientation, molecules are randomly oriented in two dimensions just as fibers would be in a random mat the orientation-induced crystallization produces structures which do not interfere with the light waves. With polyethylene, biaxial orientation often can be achieved in blown-film extrusion. 

Jinan, C. et al. (1989). Nonisothermal orientation-induced crystallization in melt spinning of polypropylene. J. Appl.Polym. Sci, Vol. 37, Issue 9, pp. 2683-2697. 

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. 

Well known examples of orientation induced crystallization are the stretching of rubber, fiber orientation in polyamides and polyesters, and the biaxial stretching of polycarbonate filuK. An example of thermal effects during induct crystallization in rubber is shown in Fig. 45, where the heat flow (Q) for successive steps of elongation and retraction is plotted as a function of time. 

Hirahata H, Seifert S, Zachmann H G and Yabuki K (1996) On-line measurements of orientation induced crystallization of PET during high speed spinning. Polymer 37. BIZI- 5138. 

At one time the pronounced upward sweep in the tensile stress-strain curve of a rubber vulcanizate was considered to be due to orientation-induced crystallization. It has however since been observed that the stress-strain curve for natural rubber does not greatly change in shape at temperatures as high as 100 C when such orientation-induced crystallinity would be very much reduced. In addition... 

A unique aspect of high polymer solid/liquid equilibria is the orientation-induced crystallization at temperatures above T ... 

This chapter presents first some fundamental aspects of nucleation, and second the general Avrami equation, which is frequently used to describe overall crystallization. The growth theories of Lauritzen and Hoffman and Sadler and Gilmer are discussed in sections 8.4.2 and 8.4.3. Molecular fractionation and orientation-induced crystallization are dealt with in sections 8.5 and 8.6. 

Somani RH, Yang L, Sics I, Hsiao BS, Pogodina NV, Winter HH, Agarwal P, Fruitwala H, Tsou A. Orientation-induced crystallization in isotactic polypropylene melt by shear deformation. Macromol Symp 2002 185 105-117. 

Larin B, Avila-Orta CA, Somani RH, Hsiao BS, Marom G (2008) Combined effect of shear and fibrous fillers on orientation-induced crystallization in discontinuous aramid fiber/isotactic polypropylene composites. Polymer 49(l) 295-302... 

Heuvel, H. M., and R. Huisman, Effect of Winding Speed on the Physical Structure of As-Spun Poly(ethylene terephthalate) Fibers, Including Orientational-Induced Crystallization, J. Appl. Polym. ScL, 22, 2229-2243, 1978. 

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