Glycolipid biomaterials: solid-state properties of a poly(sophorolipid)

Structural complexity inherited from a microbial synthesis of glycolipids was translated into unique poly(sophorolipid) biomaterials. ROMP polymerization of natural diacetylated lactonic sophorolipids gave a high molecular weight polymer with asymmetric bola-amphiphilic repeating units. The poly(sophorolipid) chain alternates C18 oleic-like aliphatic segments (90% cis-configured double bonds) with bulky di-acetylated disaccharide moieties. The solid-state properties were investigated by means of TGA, DSC, TMDSC and variable temperature X-ray diffraction. The poly(sophorolipid) is a solid at room temperature that undergoes the glass transition at 61°C and melts at 123°C. The crystal phase is associated with ordered packing of the aliphatic chain segments. The semi-crystalline poly(sophorolipid) also displays a long range order (d = 2.44 nm) involving sophorose groups, that is found to persist after crystal phase melting (in high-T diffractograms) with a slightly shortened distance (2.27 nm). Upon annealing at 80 °C the poly(sophorolipid) re-crystallizes and concomitantly the disaccharide units space out again at 2.44 nm. An exothermal phenomenon that immediately follows melting and is revealed by TMDSC might be associated with the observed adjustment of sophorose units spacing in the melt. The peculiar structural organization of this novel biomaterial is discussed.