Mechanical and Thermal Properties of Mixed-Tacticity Polyhydroxybutyrates and Their Association with Iso- and Atactic Chain Segment Length Distributions

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.