Folded-chain macroconformation

A path with a lower positive free-enthalpy barrier to crystallization than to the extended-chain crystals involves ufolded-chain macroconformation and leads to the chain-folding principle. Crystallization occurs first with shorter, chain-folded segments of the molecules, as shown schematically at B of Fig. 5.42 (A B). From... 

A fringed-micellar structure was proposed in 1930 for the sttuctures of colloids and gels [19]. By adding the possibility of chain-folded crystals, as illustrated in the center of Fig. 5.42, all three limiting macroconformations are combined. Semicrystalline polymers, thus, are a system consisting of folded-chain crystals, intercrystalline amorphous, and possibly extended-chain subsystems (see Fig. 2.80). The latter are expected particularly in fibers drawn to large extension. Most samples have a macroconformation somewhere within the triangle of Fig. 5.42. 

A sufficiently regular, flexible linear macromolecule, crystallized from the mobile random state, will always crystallize first in a chain-folded macroconformation. 

The melting of polymer crystals exhibits many instructive features of non-equilibrium behaviour. It has been known since the 1950s that the crystals of flexible-chain polymers, e.g. polyethylene (PE), are lamella-shaped with the chain axis almost parallel to the normal of the lamella. The lamellar thickness (LJ is of the order of 10 nm, corresponding to approximately 100 main chain atoms, which is considerably less than the total length of the typical polymer chain. This fact led to the postulate that the macroconformation of the chains must be folded. The... 

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