Direct observations of the growth of spherulites of poly(hydroxybutyrate-co-valerate) using atomic force microscopy

Abstract

Atomic force microscopy (AFM) has been used to observe, in real time, the growth of two-dimensional poly(hydroxybutyrate-co-valerate) (PHB/V) ‘spherulites’ in thin films. The AFM permits us to image the growth over a wide range of magnifications, from the macroscopic spherulitic growth down to observations of growth of individual lamellae. The lamellar growth images are obtained using a special, high resolution, phase-imaging technique. Low magnification images show, in common with optical microscope techniques, sharp circular growth fronts which move at a constant growth rate. At higher magnifications the rough nature of the growth front on a lamellar scale is clearly revealed with dominant lamellae leading the growth. The most remarkable observation is that these dominant lamellae do not grow at a fixed, constant rate, as predicted by most growth theories, but rather they initially spurt forwards at a rate substantially faster than the macroscopic growth rate, and then slow down or stop. A new theory, in which the spherulite growth rate is controlled not by the growth rate of the individual lamellae, but rather by the rate at which new lamellae nucleate on existing, dormant lamellae, is suggested. It is believed that these observations, although only made on one system, may be more widely applicable.