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

Abstract

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.

Determining paracrystallinity in mixed-tacticity polyhydroxybutyrates

“…As described in the Results, the red and green curves in Fig. 3 are not equal to those presented earlier (Haslbö ck et al, 2019). However, their qualitative progressions are good matches for the observed f bc and f pc .…”

“…In general, thermal factors decrease with increasing elastic parameters (Debye, 1913), allowing one to determine elastic constants e (Sasaki et al, 2013). In earlier work, we were able to trace Young's moduli E, as determined by tensile testing, with a two-phase model using the total amorphous and crystalline contents (Haslbö ck et al, 2019). We note that the progression of B corresponds qualitatively and inversely to the E determined for the crystalline phases, which decreased to lower f meso .…”

“…An application-oriented aim of the overall venture was to determine the progressions of structural, mechanical and thermal characteristics with f meso . Hence, in two prior studies, we determined the progression of the fraction of crystalline phase and the accompanying crystallite sizes with f meso (Haslbö ck et al, 2018) and some mechanical and thermal ISSN 1600-5767 properties of the compositions (Haslbö ck et al, 2019). The latter included glass transition temperatures, which remained constant at 278 K, and melting points, which decreased linearly from 440 K at f meso = 1.00 to 330 K at f meso = 0.59, below which no melting signals were detected.…”

“…Furthermore, we attempted to find a characteristic that would account for observed anomalies in the progressions of various properties, at least qualitatively (Haslbö ck et al, 2019). We used a random-number generator to simulate R and S monomer chain makeups and then fitted the iso-and atactic chain segment lengths with exponential decay functions.…”

“…This is a qualitative indicator of chain disturbance due to mismatching tacticities and peaks at the intersect of the two s, which is at f meso = 0.7 for randomly added monomers. It is there that instances of unexpected mechanical and thermal behaviour were detected (Haslbö ck et al, 2019).…”