Abstract: A supported or cast film of a polyhydroxyalkanoate polymer, such as a polyhydroxybutyrate/valerate polymer, can be prepared by applying a layer of molten polymer to a cool (typically 4.degree. to 20.degree. C.) surface not substantially above the glass transition temperature of the polymer, so as to form a solid, glassy film with a high density of nucleation sites; subsequently the temperature of the film so formed is raised, for example to 40.degree. C. or more, towards the optimum temperature for growth of the polymer's spherulites effectively separating the film formation/nucleation step from the crystallisation step (growth of spherulites around the nucleation sites). Smaller crystallites are formed, and the film has improved barrier properties.

Abstract: A supported or cast film of a polyhydroxyalkanoate polymer, such as a polyhydroxybutyrate/valerate polymer, can be prepared by applying a layer of molten polymer to a cool (typically 4.degree. to 20.degree. C.) surface not substantially above the glass transition temperature of the polymer, so as to form a solid, glassy film with a high density of nucleation sites; subsequently the temperature of the film so formed is raised, for example to 40.degree. C. or more, towards the optimum temperature for growth of the polymer's spherulites effectively separating the film formation/nucleation step from the crystallisation step (growth of spherulites around the nucleation sites). Smaller crystallites are formed, and the film has improved barrier properties.