The crystal lamella

It was known from early X-ray diffraction work that polymers never crystallize to 100%. The prevailing view of polymer crystals was that they were fringed micelles (Fig. 7.10). The modern view of chain folding was first introduced by Storks (1938). Storks concluded that the chains of semicrystalline trans-(polyisoprene) had to fold back and forth. This proposal went by largely unnoticed by the scientific community. Three papers were independently published by Keller (1957), Till (1957) and Fischer (1957) reporting that single crystals were 10 nm thick 

Andrew Keller entered the H. H. Wills Physics Laboratory in 1955 and was stunned by what he saw  

Most of the examples presented here are taken from the extensive work carried out on polyethylene. They highlight general principles valid for any crystalline polymer. Some of the features are, however, special and unique to the particular polymer, and these cases will be discussed separately. 

Single crystals of linear polyethylene prepared from dilute solutions in xylene and similar solvents provided the evidence in favour of chain folding. The large surfaces which contain the chain folds are commonly referred to as the fold surfaces. The single crystals shown in Fig. 7.11 exhibit planar, lateral 

The hollow pyramid shape typical of a single crystal of polyethylene indicates that the chain axis is not parallel with the normal of the lamella. The chain axis is generally at an angle, about 30°, with respect to the lamella normal. The reason for the chain tilt is essentially that a certain type of regular chain fold requires a small vertical displacement of the linear chain in the adjacent position (Fig. 7.14). 

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In semicrystalline polymers such as polyethylene, yielding involves significant disruption of the crystal structure. Slip occurs between the crystal lamellae, which slide by each other, and within the individual lamellae by a process comparable to glide in metallic crystals. The slip within the individual lamellae is the dominant process, and leads to molecular orientation, since the slip direction within the crystal is along the axis of the polymer molecule. As plastic flow continues, the slip direction rotates toward the tensile axis. Ultimately, the slip direction (molecular axis) coincides with the tensile axis, and the polymer is then oriented and resists further flow. The two slip processes continue to occur during plastic flow, but the lamellae and spherullites increasingly lose their identity and a new fibrillar structure is formed (see Figure 5.69). 

Abstract The morphology of polyethylene has been an important theme in polymer science for more than 50 years. This review provides an historical background and presents the important findings on five specialised topics the crystal thickness, the nature of the fold surface, the lateral habit of the crystals, how the spherulite develops from the crystal lamellae, and multi-component crystallisation and segregation of low molar mass and branched species. 

It is now generally accepted that the very low sliding friction of molybdenum disulphide is due to the very low shear strength parallel to the basal plane of the crystal lamellae, compared with the high strength or hardness perpendicular to the basal plane. The low shear strength is caused by the wide separation distance... 

Fig. 42. Schematic representation of the flattening of the crystal lamellae during crystallization...

The results show that above 240 °C the recrystallization occurs after complete melting of the crystal lamellae as in the case of polyethylene with rapid heating. In addition, some lamelae melt and do not recrystallize at the high temperature they crystallize however after cooling down to 120 °C with the same thickness as they had before heating. As long as the material is heated up only to temperatures below... 

Orientation of semicrystalline polymers below the melting point is often referred to as "cold drawing." Although some stress crystallization does occur, the process primarily involves the transformation of existing crystalline structures. A widely accepted model of the deformation mechanism is that provided by Peterlin (Figure 5) (41). Prior to necking, the crystal lamellae which... 

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... 

Fig. 20. Thickness of the crystal lamellae lc and of the amorphous regions 1, in PET at the end of annealing as a function of annealing temperature Ta. The samples were previously crystallized at 120 °C...

The complicated morphology of crystalline polymer solids and the coexistence of crystalline and amorphous phases make the stress and strain fields extremely nonhomogeneous and anisotropic. The actual local strain in the amorphous component is usually greater and that in the crystalline component is smaller than the macroscopic strain. In the composite structure, the crystal lamellae and taut tie molecules act as force transmitters, and the amorphous layers are the main contributors to the strain. Hence in a very rough approximation, the Lennard-Jones or Morse type force field between adjacent macro-molecular chain sections (6, 7) describes fairly well the initial reversible stress-strain relation of a spherulitic polymer solid almost up to the yield point, i.e. up to a true strain of about 10%. 

A miscible blend of amorphous and crystalline polymers usually means a single phase in the melt and a neat crystalline phase with a mixed amorphous region in the sohd. Because of chain folding during crystallization, the crystal lamellae are formed. Their radical growth usually lead to the formation of spheniUtes [Nadkami and Jog, 1991]. 

HDPE has an enormous range of ESC resistance. Figure 10.21 shows creep rupture data for blow moulding and pipe grades under extreme conditions, of 80 °C in 2% Arkopal 110 surfactant solution, which allow the reasonably rapid selection of promising candidate materials. The samples have a 10 X 10 mm cross section, with a circumferential, 1.6 mm deep, razor blade notch. The samples with the longest lives have more tie links between the crystal lamellae. 

That applies even to single crystals grown from dilute solution. The amorphous component is partially distributed all over the crystal in form of crystal defects, i.e. vacancies and kinks, and to a larger extent on the surfaces of the crystal lamellae and mosaic blocks. 

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