Chain branching, polyethylene crystal structure

Structural Regularity. Structural regularity enhances the tendency for crystallization. Thus, linear polyethylene is difficult to obtain in any form other than a highly crystalline one. Low density, branched polyethylene is typically largely amorphous. The linear polyethylene chains are nonpolar, and the crystallization tendency is mainly based on the flexibility of the chains to achieve a regular, tightly packed conformation that takes advantage of the special restrictions inherent in dispersion forces. 

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... 

Because of its exceptional symmetry, the polyethylene molecule fits so readily into a crystalline lattice, in spite of its lack of molar cohesion, that its tendency is to crystallize into spherulite structures on relaxation rather than revert to a disordered state. However, the degree and type of chain branching that interrupts the geometrical regularity of the polyethylene molecular structure causes a local... 

Physical Properties. All polyethylene above 0.86 g/cm density is semicrystalline. The basic crystalline structure for most commercial LLDPE is chain-folded lamellae (Fig. 7). The body of the crystal consists of polymer backbone segments, and the surfaces are a collection of chain folds, loose cilia, and tie chains (chains incorporated into more than one crystal). When crystallized isothermally, it has been foimd that 95% of the lamellae in a given sample are within 5% of the same thickness (10). There is some debate over the mechanism of chain folding and of the subsequent fold loops. The most likely model includes adjacent reentry, loose adjacent reentry, and nonadjacent reentry. Short-chain branch length... 

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