Crystal lamella stack

CRYSTALS GROWN FROM THE MELT AND THE CRYSTAL LAMELLA STACK... 

The existence of crystal lamellae in melt-crystallised polyethylene was independently shown by Fischer [28] and Kobayashi [39]. They observed stacks of almost parallel crystal lamellae with amorphous material sandwiched between adjacent crystals. At the time, another structure was well known, the spherulite (from Greek meaning small sphere ). Spherulites are readily observed by polarised light microscopy and they were first recognised for polymers in the study of Bunn and Alcock [40] on branched polyethylene. They found that the polyethylene spherulites had a lower refractive index along the spherulite radius than along the tangential direction. Polyethylene also shows other superstructures, e.g. structures which lack the full spherical symmetry referred to as axialites, a term coined by Basset et al. [41]. 

Sample PTEOX-83P, on the other hand, shows a ring-like WAXS pattern (Figure 6C). The reason for the absence of discrete scattering may possibly be attributed to the Isotropic structure of lamella stacking. The amorphous halos in all of the PTEOX samples may be due to the amorphous regions between the lamellae of folded chain crystals. 

The morphology of crystalline isotactic polystyrene, i-PS, has been investigated by others, and they have concluded that i-PS normally crystallizes as stacks of folded chain lamellae which are arranged in volume filling spherulites. The melting point of lamellar polymer crystals depends on the lamella thickness, L, as follows (28 )... 

Figure 5. Cold drawing process of a semicrystalline material. The dispersed crystal lamellae (a) under stress are arranged in a tilted stack formation (b).

This relations are only correct, if all amorphous material is solely located between the crystal lamellae within the stacks. In Fig. 9 lc and la at the end of crystallization are plotted as a function of the crystallization temperature Tc. Up to Tc = 235 °C la is almost temperature independent, while lc increases steadily. This increase is in a good agreement with the theoretical predictions. Like the initial crystal thickness in PE also the thickness of the lamellae in PET is defined by the size of the critical nucleus. But in PET these initial lamellae do not grow further. Instead of this, the decrease of the... 

It was shown that amorphous polymers certainly reside between crystal lamellae as a single phase mixture in poly(8-caprolactone) (PCL)/poly(vinyl chloride) (PVC) blends [Stein, 1978]. When the content of the amorphous polymer is high, its excess is excluded from interlamellar region to the outside of lamellar stacks in poly(etheretherketone)/ poly(etherimide) blends [Lee et ah, 1997]. 

Fig. 3.12 A stack of crystal lamellae, shown schematically. The separation determined by X-ray scattering is the repeat distance shown.

The amorphous component of the crystalline polymer solid contains beyond the amorphous component of single crystals, i.e. crystal defects (linear vacancies, kinks, and interstitials), chain folds and free chain ends, as new elements the rejected non-crystallisable impurities and tie molecules. The former concentrate on the outer boundaries of lamella stacks and spherulites, the latter in the amorphous layers separating the lamellae of the same stack. With the exception of impurities all other components of the amorphous phase are intimately connected with the crystals and cannot be physically separated from them or moved independently of them. 

Figure 1.10. Surface replica of molded polyoxymethylene fractured at liquid nitrogen temperatures. While the lamellae to the lower left are oriented at an angle to the fracture surface, the lamellae elsewhere are nearly parallel to the fracture surface. The important point is that lamellae (resembling those in single crystals) are stacked up like cards or dishes in the bulk state. (Geil, 1963.)...

Crystals Grown from the Melt and Lamellae Stacks... 

Blends of linear and branched polyethylene normally crystallize in two stages. The components crystallize separately provided that they are of similar molar mass. Linear polyethylene will crystallizes at the highest temperatures, forming regular shaped crystal lamellae. Branched polymers crystallize at lower temperatures in finer, S-shaped lamellae located between the stacks of the dominant lamellae. Although linear and branched polyethylenes are chemically very similar they can phase separate in the molten state. A characteristic of phase separated behaviour is the observation of a dominant lamella structure (Figure 6.14). ... 

When a PP is extruded and taken-up at a high draw ratio and crystallized under a high stress and annealed, a lamella-stacked structure is formed. In this structure, the lamellae are oriented perpendicular to the extrusion direction and connected by the tie molecules. When such an extrudate is stretched in the extrusion direction, the lamellae open elastically with the tie molecules working as fixing points. Therefore, after stress removal, the initial shape and structure are restored. Since such a part shows a high elastic recovery after a deformation and the elastic modulus is nearly the same as that of the usually processed article, it is called a Tiard-elastic item. When the hard-elastic film or fiber is drawn beyond the yielding point, plastic deformation occurs, leading to void formation, and a microporous film or fiber can be obtained. 

FIGURE 4 Random stacks of liquid and crystal lamellae for a sample of PVDF grown from a melt (upper) and after drawing the film several times its original length (bottom). (From Ref. 43.)... 

Fig. 24. Schematic representation of the possible deformation processes of a stack of crystal lamellae (a) the initial state, (b) interlamellar shear, (c) interlamellar separation, (d) intralamellar block shear, (e) intralamellar fine shear (not shown bending and rotation of lamellae), and (f) cavitation within the amorphous regions.

Pig. 1. Schematic illustration of random stacks of amorphous and crystal lamellae in the PVDF polymer (a) the morphology after the film is melt cast (b) after orientation of the film by mechanically stretching several times its original length and (c) after depositing metal electrodes and poling through the film thickness. 

Melt-grown crystals have the same shape as the solution-grown single crystals in most respects. They are lamella-shaped with a thickness-to-width ratio of 0.01-0.001. A typical feature of melt-crystallized samples, shown in Fig. 7.27, is the crystal stack. The lamellae of the stack are almost parallel and the amorphous component is located in the space between the crystal lamellae. The number of lamellae in a stack varies considerably between different samples. Linear polyethylenes of low or intermediate molar mass form stacks with a great many lamellae, whereas branched and/or high molar mass samples exhibit stacks containing only a few crystal lamellae (Fig. 7.27). 

Figure 7.31 Simple model of lamellar stack showing two adjacent crystal lamellae and the two types of chain present random tie chain and tight, (regular) fold.

Linear polyethylene displays relatively straight and long crystal lamellae. The amorphous interlayers are generally very thin. Occasional roof-ridged lamellae are found. The number of lamellae per stack is high. Branched polyethylene... 

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