Molecular weight distribution curves determined by electron microscopy exhibit a deviation when approaching a molecular weight of 10'. Picturing of single molecules therefore was improved by the introduction of shadowing by electron beam. Techniques of preparation (by freeze-drying) and shadowing are described in detail. The scanning size of the electron beam shadowing is in between 15 and 20 A and is given by the structure of the supporting film. By double shadowing from different sides it is demonstrated, that there is no "decoration effect" in electron beam shadowing. The lower limit of an exact determination of the dimensions is a molecular weight of ca. lo5. It is shown with latex particles that there is a change of shape with time of the particles after sedimentation on the support which is dependent on the kind of polymer and of the conditions. Their volume though remains constant within the limits of error.

A mathematical treatment is described, by means of which the effect of the measuring errors on the number molecular weight distribution of polymers, as obtained with an electron microscope (EM), can be eliminated. It is shown, that the difference in the nonuniformities of the measured and the corrected distribution would be negligible only in the case of samples with high average degrees of polymerization and sufficiently broad distributions. Otherwise differences of 10% or more can be found, depending on the size of the actual measuring error. This is shown by means of four practical examples. For a practical application the measuring error may not exceed some critical value, which, though always lies above the mean error of the EM-method.

A method is described for the determination of the molecular weight distribution (MWD) of various polymers from electron microscopy pictures of their molecules. It is shown, that the number of molecules needed to construct the distribution is between 200 and 300. Furthermore, with macroscopic density of the polymer and the volume determined by electron microscopy, the correct mass of a single molecule is obtained. The method is demonstrated for samples of polystyrene and poly(methy1 methacrylate) and the results are compared with those of GPC-measurements. I . EinleitungIn unserer vorangehenden Mitteilung" wurde beschrieben, wie die einzelnen Molekule eines makromolekularen Stoffes aus einer sehr verdunnten Losung durch Gefriertrocknung auf einer Unterlage abgeschieden und durch Schragbedampfung im Elektronenmikroskop (EM) sichtbar gemacht werden konnen. In dieser Mitteilung wird gezeigt, daB man aus dem gemessenen Durchmesser und aus der gemessenen Schattenlange das Volumen der einzelnen Teilchen berechnen kann. Daraus ergibt sich unter Annahme einer bestimmten Dichte -wir setzen dafur die makroskopische Dichte des gesamten Praparates ein *I 4. Mitteilung: cf.".

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