A new model for the fold surfaces of polyethylene single crystals

The long‐standing debate concerning the perfection of single crystals of polyethylene has centred on the problem of formulating an acceptable model for the fold‐containing surfaces of the crystals. The literature offers very strong experimental evidence for the presence of a distinctive surface layer of significant volume characterized by the resemblance between some of its properties and those expected for amorphous polymer, notably the density as estimated by extrapolation from molten polymer. This surface layer has indeed come to be called amorphous, and its presence is apparently incompatible with other equally convincing evidence for high regularity at the surface. The new model proposed here is based on the supposition that it is neither necessary nor correct to endow the density‐deficient surface layer with all the properties of truly amorphous material; on the contrary, the surface layer must certainly have a low density but it may also possess considerable structural regularity including re‐entrant folds. The new model appears to be compatible with all the evidence including the experimental results presented in the paper. These concern the effect of crystallization temperature and solvent on density.