Studies of hydroxybutyrate oligomers crystallization behaviour

Abstract

Monodisperse long chain oligomers such as n-alkanes provide excellent model systems for fundamental studies of polymer crystallization and annealing. Previous studies revealed important results, including a preference for discrete crystal thicknesses corresponding to integer folded chain forms, minima in growth and nucleation rates as the system changes from one chain conformation to another, and clear unfolding transitions during melting. The work presented here extends previous studies to oligomers of hydroxybutyrate (OHB), which serve as models for the polymer poly(3-R-hydroxybutyrate) (PHB). A range of exact length hydroxybutyrate oligomers have been synthesized and their crystallization behaviour and morphology studied using optical and electron microscopy, together with small and wide angle X-ray scattering, including dynamic measurements at the ESRF Grenoble. The oligomers form crystals from dilute solution and from the melt, exhibiting similar overall morphologies and structure to PHB. Growth rate data for HB 24-mer and 32- mer spherulites grown from the melt and crystallization rate data from solution reveal discontinuities in the rate gradient which can be linked to changes in chain conformation. These features could arise from a ‘self-poisoning’ effect previously postulated for the growth minima in long n-alkanes. Crystals grown at the lower temperatures contain folded chains, which transform during heating through a process of partial melting/dissolution and re-crystallization to form extended chain crystals. These unfolding transitions were accompanied by changes in crystallinity and lattice parameter. Crystals grown at higher temperatures contain extended chains that do not rearrange further. Preferred crystal thicknesses are those which result in relatively high proportions of chain ends in the surface. For the 24-mer, they correspond to the extended chain length (E), and to E/2, 2/3E, 3/4E and 5/6E. This wide range of thicknesses is in contrast to results from long n-alkanes, possibly due to hydrogen bonding between chain ends, which effectively links chains together into longer units. The current work reveals a great deal about the way in which HB oligomer chains fold and how they re-arrange themselves from one folded form to another which, combined with previous results on PHB, will contribute towards a more complete view of the whole polymer crystallization process