Surface structure of polyethylene crystals as revealed by surface decoration. III. Origin of mobilizable surfaces

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Abstract
It was shown in Part II of the series that surfaces of polyethylene single crystals could be divided into mobilizable and nonmobilizable categories by means of a test involving surface decoration and swelling. It has been established that the factor determining this behavior is the molecular weight. Material of low molecular weight (fractions up to 8000 mol wt) do not mobilize, beyond this molecular weight the ability to mobilize increases up to a maximum at around 20,000 mol wt beyond which there is a noticeable decrease in the effect. The ability to mobilize is removed by a slight amount of oxidation that does not affect the molecular weight but removes the terminal vinyl groups. All these results, together with evidence from other related work (on crystals with dislocation networks) on selective chain cutting, lead to the conclusion that only a very small amount of material situated at the outermost surface is involved which consists essentially of long cilia and, only to a lesser extent, of large loose loops. In conclusion, the decoration technique provides a very sensitive method for identification of a very minor yet most distinctive constituent of the fold surface.