Giant screw dislocations

Fig. 4. Melt-crystallized polyethylene lamellae (a) linear polymer crystallized at 130°C as a planar crystal in which successive layers spiraling around the central (etched-out) giant screw dislocation are not in contact (b) ridging along 6 in a 17,000 mass fraction of linear poljrmer crystallized at 129°C (c) an ethyl-branched copolymer crystallized at 123°C showing a central S-profile, asymmetrically placed screw dislocations and new layers diverging therefrom. From Ref. 66.

Figure 2.24 Hierarchy of structures observed when a polymer crystallizes. Molecules adopt an extended conformation, or fold. The crystallized molecules are organized into lamellae. Crystalline layers and amorphous layers form a lamellar stack. Radial growth of lamellae leads to spherulitic supermolecular structures. The spokes of the spherulite result from defects in lamellar organization, such as giant screw dislocations...

Keith H D and Chen W Y (2002) On the origins of giant screw dislocations in polymer lamellae. Polymer 43 6263-6272. 

Figure 1.11 AFM image of a larger PCL structure grown at 57 °C from a molten film. Spiral growths from giant screw dislocations are abundant in views that are intermediate between flat-on and edge-on. Beckmans and Vancso [49]. Reproduced with permission of Elsevier.

Figure 3.24 Giant screw dislocation characterized by Burgers vector of length b equal to the crystal thickness 4. The dislocation is a linear defect parallel to S that marks the boundary between slipped and unslipped portions of the crystal. Two steps of height 4 are formed that are the loci for further crystallization. In this example the dislocation is left handed (advancing with counterclockwise rotation).

Figure 3.25 Polygonal spiral growth from the giant screw dislocation in Fignre 3.24. Arrows on the side snrface indicate the initial advance directions of the two steps. Each layer of the spiral has a thickness b = 4- Other dimensions are the terrace width y and the size of the smallest surface nucleus...

Figure 3.27 Transmission electron micrograph of the edge of a polyethylene dendrite grown at 67°C from 0.01% xylene solution. Notice the channels or shts leading inward from reentrant comers. At two of these (A and arrow) giant screw dislocations have formed. Three other rather irregular growth spirals are evident. From Keith and Chen [43] with permission from Elsevier.

Figure 3.29 Two intersecting crystals of polyethylene grown at 80°C from 0.05% xylene solution. The top crystal has the longer a-axis pointing to about 10 o clock, while the bottom crystal has the fe-axis oriented at 4 o clock. Two oppositely directed steps of thickness k are evident on a line near 11 o clock. Two coincident giant screw dislocations, each left handed, have been created at common the tip of the notches. Transmission electron micrograph of unspecified magnification by Hirai [45] with permission from John Wiley Sons, Inc.

Figure 3.30 Double growth spiral formed from two coincident giant screw dislocations. Isotactic polystyrene grown at 19(TC in a 10% atactic polystyrene solution. Growth faces are [1120). The small lonzenge-shaped crystal in the insert, which formed in the same preparation, can be ignored. From Keith et al. [46] with permission from John Wiley Sons, Inc.

In at least one situation, the genesis of giant screw dislocations can be attributed to internal stresses associated with nonplanar crystals. Reference was made in Section 3.1 to the chair form of polyethylene crystals... 

Figure 3.31 Sketch of interlocking crystals growing in dilute solution. Should the lamellae rotate about x-x to bring the basal surfaces of crystals A and B together, two giant screw dislocations with four steps are formed. These giant screws will lead to double growth spirals as in Figure 3.30. From Lotz et al. [48] with permission from John Wiley Sons, Inc.

Figure 3.69 Sketch of an undislocated lamellar ribbon (left), a lamella with one left-handed topological screw dislocation of strength b = Ic (center) and a lamella with four left-handed topological giant screw dislocations of the same strength (right). As illustrated, the dislocations have generated a left-hand twist, but no spiral growth. From Schultz and Kinloch [107] with permission from Elsevier.

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