Poly-3-hydroxybutyrate, a Crystal-Mobile Biodegradable Polyester

Poly(3-hydroxybutyrate) (P3HB), a biodegradable thermoplastic with the potential to partially replace oil-based commodities, suffers from embrittlement over storage time. We here investigate possible causes by monitoring aging processes in the sample through changes in mechanical and thermal properties as well as the semicrystalline morphology (crystallinity, lamellar stack dimensions, and its inhomogeneity) over time, using advanced small-angle X-ray scattering analyses for the latter. We find that the slow rise in the shear modulus of the materials is accompanied by secondary crystallization, detected as lamellar thickening and an increase in the melting point. These morphology changes may be due to the chain mobility in the crystallites. This is a feature of “crystal-mobile” polymers that are known to be able to crystallize more fully than their crystal-fixed counterparts. We present the first evidence of the related crystalline αc relaxation in P3HB by dynamic-mechanical analysis (DMA) and investigate the process microscopically by advanced magic-angle spinning solid-state exchange NMR techniques, specifically centerband-only detection of exchange. We show that monomer jumps in the P3HB crystals are comparably slow (i.e., with an average correlation time τc of 0.1 s at 120 °C). Their apparent activation energy (80 to 90 kJ/mol) is found to be independent of the crystallization temperature and in good agreement with the DMA results.