Direct imaging of the surfaces of poly(β) hydroxybutyrate and hydroxybutyrate oligomers by atomic force microscopy

Abstract

Attempts have been made to image the fold surface of a single crystal of polyhydroxybutyrate (PHB) using the relatively new technique of atomic force microscopy (AFM). To overcome the obscuring of the fold surface by loose loops of polymer and chain ends, two different approaches were used. We first studied the single crystals of an oligomer of 32 HB units, which is known to fold once very tightly within a crystal, using AFM. Secondly, studies were made of single crystals of PHB which have been chemically degraded with methylamine to etch away the “amorphous” layer of loosely folded material, in an attempt to expose the fold surface. The crystals of the 32-mer had a similar morphology to those of the polymer PHB. However, at high magnification, lines of ridges were observed which ran parallel to the crystallographic b axis with a spacing of 0.7 nm, similar to the dimensions of the unit cell (0.58, 1.32, and 0.60 nm). It was not possible to differentiate between chain ends and folds. The partially etched PHB crystals maintained enough integrity to permit imaging by AFM, although surface detail could not be resolved on a molecular scale.