Poly-3-hydroxybutyrate, a Crystal-Mobile Biodegradable\nPolyester

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