Polyethylene kink defects

In addition, conformational disorder in polymer crystals may give rise to point and line defects which are tolerated in the crystal lattice at a low cost of free energy as kinks [104,105], jogs [106,107] and dislocations [108,109]. Such crystallographic defects arise whenever portions of chain adopt conformations different from the conformation assiuned by the chains in the crystal state [99], and have been widely discussed in the literature, in the case of polyethylene [108,109] and some aliphatic polyamides [99,106]. Point and... 

The agreement with experiment shown in Fig. 14 is fascinating, and with the vast increase in computer power since 1986, it would be valuable to follow up the approach pioneered by Termonia and Smith for models which included the possible defects in the structure. In HMPE fibres, it seems right to attribute creep to the movement of whole molecules past one another, which eventually leads to separation. However, the most likely mechanism would be the movement of defects such as those described by Reneker and Mazur (1983). A kink in a polyethylene chain due to an extra -GHj- group could move like a ripple in a carpet. 

In general, however, identification of the crystal cell is only part of the problem of characterizing the structure of crystalline polymers. Crystals are never perfect and the units cells do not infinitely duplicate through space even when they are grown very carefully from dilute solution using low molecular mass materials. As with the organic crystals considered in Chapter 3, a variety of defects can be observed and are associated with chain ends, kinks in the chain and jogs (defects where the chains do not lie exactly parallel). The presence of molecular (point) defects in polymer crystals is indicated by an expansion of the unit cell as has been shown by comparison of branched and linear chain polyethylene. The c parameter remains constant, but the a and b directions are expanded for the branched polymer crystals. Both methyl and... 

Figure 7.8 Kink (2gl type) ... GTG. .. in polyethylene the stems on each side of the defect are parallel but shifted by one lattice unit.

Figure 1. Representative conformation defects in polyethylene chains, (a) Pech-hold kink, (b) Reneker twist, and (c) smooth twist.

Figure 4. Convergence of the relaxation method. The energy of a crystalline array of polyethylene chains with the central one containing the defect indicated (relative to a nondefective array), see Figure 1. The Pechhold kink is in an array of two free coordirwtion shells (6 +12 = 18 chains), each chain is C,fl SO The other two defects are in an array of one free coordination shell plus a rigid second coordination shell, each chain is CtgHss-...

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