Semi-crystalline poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymers are biodegradable systems with potential as substrates for use in tissue regeneration. Previous studies have shown that severe embrittlement occurs on storage at room temperature restricting their application possibilities. Concepts such as secondary, advancing crystallisation causing changes in the amorphous/crystalline ratio have been mooted as the cause of the embrittlement. Using films prepared by extrusion and compression moulding procedures we have attempted to probe not only the pure amorphous and crystalline phases but also the interfacial region. Interpretation of dynamic mechanical and dielectric data highlights the changes in the nature of the interfacial region on processing. Moreover, the use of the Thermally Stimulated Discharge technique is a powerful probe for highlighting the morphological changes induced in multiphase systems by the processing step.