CRYSTALLIZATION BEHAVIOR OF PREDOMINANTLY SYNDIOTACTIC POLY(BETA-HYDROXYBUTYRATE)

Predominantly syndiotactic poly(beta-hydroxybutyrate), syn-PHB, of variable syndioregularity (syndyad fractions 0.59, 0.62, 0.64, and 0.71) and molecular weight was prepared by the dibutyltin dimethoxide catalyzed ring opening of racemic beta-butyrolactone (BL). The crystallization behavior of the syn-PHB polymers was investigated by DSC and X-ray diffraction analyses. DSC of films after melting and annealing showed at least one, and often two distinct melting transitions occuring over a broad (often similar to 40 degrees C) temperature range. These results indicate that syn-PHB chain segments of variable syndioregularity form crystalline regions with very different thermodynamic stabilities. Maximum degrees of crystallinity for melt annealed 0.64- and 0.71-syn-PHB was observed at an annealing temperature (T-c) of 30 degrees C. At T-c values at 45 degrees C and higher, crystallization :of relatively lower syndioregular chain segments was apparently excluded to variable degrees dependent on T-c and sample syndiotactic dyad content. After crystallization of syn-PHB samples at elevated temperatures, ambient temperature annealing resulted in an observed lower temperature melting transition at similar to 50 degrees C. This result showed little to no dependence on syn-PHB syndioregularity and T-c. Both solution precipitated 0.62-syn-PHB and 0.71-syn-PHB have WAXS patterns with poorly resolved crystalline reflections superimposed on amorphous haloes indicating low levels of crystallinity (17% and 25%, respectively) and poorly formed crystals. Isothermal crystallization monitored by DSC showed that the syn- and natural origin PHB showed fastest crystallization rates at temperatures between similar to 50 degrees C and 70 degrees C and 60 degrees C and 90 degrees C, respectively. From the dependence of the higher melting transition on T-c it was determined that the equilibrium melting temperatures for 0.62-syn-PHB (M(n) = 83,700 g/mol) and a 0.64-syn-PHB (M(n) = 11,900 g/mol) were similar to 157 and 154 degrees C, respectively. An Avrami analysis of syn-PHB yielded results similar to that found for natural origin PHB indicating that crystal growth occurs by a two-dimensional mechanism.