Moiré patterns in the electron image of solution-grown polymer crystals

Relative tilt between two similar thin crystals may result in the formation of a moiré pattern in the electron image. Patterns hitherto interpreted as being parallel moirés or rotation moiré obtained from overlapping solution-grown polymer crystals are reconsidered, and it is shown that in some cases a tilt model is more acceptable. The general features of tilt moiré patterns predicted for crystals of this kind are discussed and compared with micrographs appearing in the literature. Methods of distinguishing tilt moiré patterns are indicated, though it is admitted that such experiments would be difficult to complete in the time limited by destruction of the crystal structure under the action of the electron beam. Extinction contours may also sometimes be confused with long-period tilt moiré patterns, and again careful experimentation is required for accurate identification. The model of the structure of solution-grown polyethylene crystals inferred from a tilt moiré interpretation of many of the electron image fringe patterns found in the literature is consistent with that derived from other sources.