Crystallization behavior of predominantly syndiotactic poly(β-hydroxybutyrate)

John E. Kemnitzer, Richard A. Gross, Stephen P. McCarthy, John Liggat, David J. Blundell, Mike Cox

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Predominantly syndiotactic poly(β-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 β-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 ∼ 40°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°C. At T c values at 45°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 ∼ 50°C. This result showed little to no dependence on syn-PHB syndio-regularity 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 ∼ 50°C and 70°C and 60°C and 90°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 ∼ 157 and 154°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.

Original languageEnglish
Pages (from-to)37-47
Number of pages11
JournalJournal of Environmental Polymer Degradation
Issue number1
Publication statusPublished - 1 Jan 1995


  • crystallization
  • Poly(3-hydroxybutyrate)
  • ring-opening polymerization
  • syndioregularity
  • syndiotactic
  • thermal properties
  • X-ray diffraction

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