Spherulite in poly-3-hydroxy butyrate (PHB)
DoITPoMS, University of Cambridge
Polymer melts often crystallise from heterogeneous nuclei to form ribbon-like lamellae, which have a folded chain molecular structure. They commonly radiate outwards from the nucleation point, to form spherical features called spherulites - which are often quite large. The characteristic Maltese cross pattern, seen when viewed between crossed polars, results from isoclinic fringes formed when one of the principal vibration directions is approximately parallel to the polariser. In some cases (including the Poly-3-hydroxy butyrate (PHB) viewed here), the lamella twist as they grow outwards, in phase with their neighbours. This gives rise to the observed ring pattern. When viewed with a full wave plate, alternate (fast and slow) rings are coloured blue and yellow.
4 minutes @ 210ºC, 5 minutes @ 160ºC, followed by quench to room temperature
Thin specimen on glass slide with cover slip
Cross-polarised light microscopy
Prof T W Clyne
Department of Materials Science and Metallurgy, University of Cambridge