In order to provide parameters that can be used to tailor the crystalline and supramolecular structures of pure polyhydroxybutyrate, we synthesized polymers with fractions of meso groups in the range 0.5−1. We confirmed the random polymerization of R and S enantiomers by the catalyst. From Xray diffractograms, the lattice parameters were determined; they remained constant for the observed range of fractions. We also traced the directional crystallite sizes over tacticity, which change significantly for one unit cell direction. The respective crystalline phase atom fractions were quantified by iteratively fitting amorphous phase diffraction patterns. We found that the crystalline contents of small-crystallite polyhydroxybutyrates have so far been underestimated. X-ray diffraction and transmission electron microscopical observations from polymers with meso group fractions of 0.5 are discussed. To facilitate the quantification of crystalline atom fractions, we refined two accessible infrared absorption spectroscopy-based indices. These indices, and the fundamental correlations between chemical and crystallite structuring reported herein, allow to tune structure-dependent properties, e.g., melting point and toughness, of mixedtacticity polyhydroxybutyrates over wide ranges.

We investigated the effect of including S enantiomers on the mechanical and thermal properties of predominantly (R)-β-polyhydroxybutyrate (PHB). From tensile testing, we determined resulting ratios of meso to racemo diads, for which elastic modulus, strength, and fracture strain combine to provide maximized fracture energies. We found that these coincide with an inversion of the respective elastic moduli of the amorphous and crystalline phases. From thermocalorimetric analyses, we determined the glass-transition temperatures and enthalpic relaxations, the heat capacities of the materials and their constituent phases, the directional crystallization rates and melting points, as well as the melting enthalpies for the α-PHB phase as functions of tacticity. We present a unifying characteristic, accounting for tacticity mismatches, based on the previously determined random polymerization action of the catalyst ethylzinc β-diketiminate and 4-methoxybenzyl alcohol. This characteristic provides a qualitative indication of the transition points in nonlinear correlations encountered between the ratios of meso to racemo diads and mixed-tacticity polyhydroxybutyrates’ fracture energies, amorphous and crystalline phase elastic moduli, melting enthalpies, and lattice vibrational frequencies.

Recently, the authors reported on the development of crystallinity in mixed-tacticity polyhydroxybutyrates. Comparable values reported in the literature vary depending on the manner of determination, the discrepancies being partially attributable to scattering from paracrystalline portions of the material. These portions can be qualified by peak profile fitting or quantified by allocation of scattered X-ray intensities. However, the latter requires a good quality of the former, which in turn must additionally account for peak broadening inherent in the measurement setup, and due to limited crystallite sizes and the possible presence of microstrain. Since broadening due to microstrain and paracrystalline order both scale with scattering vector, they are easily confounded. In this work, a method to directionally discern these two influences on the peak shape in a Rietveld refinement is presented. Allocating intensities to amorphous, bulk and paracrystalline portions with changing tactic disturbance provided internal validations of the obtained directional numbers. In addition, the correlation between obtained thermal factors and Young's moduli, determined in earlier work, is discussed.