Chain structure disentanglement

Stereo-regular polymers usually for crystals of folded-chain structure and they can be extended to give highly oriented fibers. In this case, the period of the folded-chain crystal seems to be close to the diameter of the B-chain coil or an order of rig ll (i.e., 16nm). This fact suggests that crystals are formed by disentanglement of the gel with pseudo-cross-links of the size of 16 or B [27d, p. 186]. 

From a thermodynamic point of view and at equilibrium, the phenomenon of crystallization is the opposite to that of melting, but the macromolecular nature of polymers makes it more complex than for low-molar-mass species. Indeed, the kinetics plays a key role in the case of polymers, with their crystallization being conditioned by the need of chains to disentangle before regular arrangement for instance, certain polymers of regular molecular structure and hence potentially suitable for crystallization cannot crystallize at all due to their inability to disentangle. 

For these reasons, PMMA and its maleimide and glutarimide copolymers represent very suitable materials for investigating the effect of the chemical structure and of the solid state molecular motions on the plastic deformation, the occurrence of the various micro-mechanisms of deformation (chain scission crazes, shear deformation zones, chain disentanglement crazes), as well as the fracture behaviour. 

An additional elegant route to obtain disentangled UHMW-PE, and thus to control the interphase, is by direct polymerization in the reactor. In order to make UHMW-PE, a relatively low polymerization temperature is needed and a situation is easily encountered where the polymerization temperature is lower than the crystallization temperature of UHMW-PE in the surrounding medium in which the catalyst is suspended. In this situation, the growing chains on the catalyst surface tend to crystallize during the polymerization process. These UHMW-PE reactor powders, often referred to as nascent or virgin UHMW-PE, can be remarkably ductile. It was shown by Smith et al. [26] that reactor powders, in the same manner as solution-cast UHMW-PE, could be drawn easily into high-modulus structures. 

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