Oriented crystallizable polymers

Introduction Fundamental Aspects of Orientational Processes in Crystallizable Polymers... 

Crystallizable polymers tend to form randomly oriented crystallites which are oriented when the polymer is stretched or cold drawn at temperatures below the Tm. Crystallization under pressure may result in a fibrillar structure or extended chain structure. 

A process that greatly improves blow molded product properties is that of injection stretch blow molding, which introduces biaxial orientation in crystallizable polymers. There are two variants of this process involving the molding of a preform as the first step. The thermomechanical paths of the two process variants are shown in Fig. 14.19. In the... 

Crystallization from an oriented melt may occur during extrusion and injection molding processes of crystallizable polymers. The crystalline microstructure which results is... 

Ellis RJ (1997) Molecular chaperones avoiding the crowd. Curr Biol 7 R531-R533 Flory PJ (1953) Principles of polymer chemistry. Cornell University Press, Ithaca Graham PD, McHugh AJ (1998) Kinetics of thermally induced phase separation in a crystallizable polymer solution. Macromolecules 31 2565-2568 Hu WB, Frenkel D (2005) Oriented primary crystal nucleation in lamellar diblock copolymer systems. Faraday Discuss 128 253-260... 

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. 

Vasanthan N, Manne NJ, Krishnama A. Effect of molecular orientation on the cold crystallization of amorphous-crystallizable polymers the case of poly(trimethylene terephthalate). I EC Res 2013 52 17920-6. 

Still higher degrees of chain orientation are found in the extended chain crystallites, row structures, and highly lamellar hard-elastic or springy forms of crystallizable polymers which recently have become of considerable interest. Follow-... 

The stretching of amorphous but crystallizable materials can greatly increase the rate of crystallization in some cases. Natural rubber and polyethylene terephthalate are examples. The stretching of the polymer initially causes the crystallites to grow so that the chains in the crystallites are oriented parallel to the applied stress. Thus the growth of the crystallites... 

Fig. 15. Hypothetical dependence of the linear thermal expansion coefficient 3 on the degree of orientation 7). 1 — crystallizable stretched rubber, 2, 3 — crystalline drawn polymers, 4 — p n...

There is no comprehensive theory for crystallization in block copolymers that can account for the configuration of the polymer chain, i.e. extent of chain folding, whether tilted or oriented parallel or perpendicular to the lamellar interface. The self-consistent field theory that has been applied in a restricted model seems to be the most promising approach, if it is as successful for crystallizable block copolymers as it has been for block copolymer melts. The structure of crystallizable block copolymers and the kinetics of crystallization are the subject of Chapter 5. 

An example of a relevant optical property is the birefringence of a deformed polymer network.256 This strain-induced birefringence can be used to characterize segmental orientation, and both Gaussian and non-Gaussian elasticity.92,296-302 Infrared dichroism has also been particularly helpful in this regard.82,303 In the case of the crystallizable polysiloxane elastomers, this orientation is of critical importance with regard to strain-induced crystallization, and the tremendous reinforcement it provides.82... 

With all these examples it should be remarked, that not all polymers which are, in principle, crystallizable, actually crystallize spontaneously. The rate of crystallization may be too low, such as with PPE, PC, PIB, BR and IR. In particular for the rubbers (the last three mentioned) it can be said that they crystallize when strongly strained, in other words, when the chains are highly oriented (see also Qu. 4.1 to 4.3). 

The phenomenon of strain hardening in polymers is a consequence of orientation of molecular chains in the stretch direction. If the necked material is a semicrystalline polymer, like polyethylene or a crystallizable polyester or nylon, the crystallite structure will change during yielding. Initial spherulitic or row nucleated structures will be disrupted by sliding of crystallites and lamellae, to yield morphologies like that shown in Fig. 11-7. 

The good orientation of the polymer chains produced is due more to the crystallizability of the polymer molecule than to the orientation of the monomer in the crystal, since in all the cases in which polymerization occurs below the melting point a certain mobility of the monomer molecule is observed. Since mobility reduces the probability of orientation occurring during polymerization, the orientation of the polymer chain may be due to the crystallizability of the polymer molecule. 

The enhanced recovery characteristics were thought to be associated with the synergism between the anisotropic SWCNTs and the crystallizable switching segments of the polymer. The extent of SWCNT orientation in the direction of deformation (tube axis parallel with applied stress) increased with SWCNT concentration. In conjunction, the polymer crystallite fraction increased with SWCNT concentration. 

---