Tunable and Degradable Dynamic Thermosets from Compatibilized Polyhydroxyalkanoate Blends

Polyhydroxyalkanoates
(PHAs) are versatile, biobased polyesters
that are often targeted for use as degradable thermoplastic replacements
for polyolefins. Given the substantial chemical diversity of PHA,
their potential as cross-linked polymers could also enable similar
platforms for reversible, degradable thermosets. In this work, we
genetically engineered Pseudomonas putida KT2440 to synthesize poly(3-hydroxybutyrate-co-3-hydroxyundecenoate)
(PHBU), which contains both 3-hydroxybutyrate and unsaturated 3-hydroxyundecenoate
components. To reduce the brittleness of this polymer, we physically
blended PHBU with the soft copolymer poly(3-hydroxydecanonate-co-3-hydroxyundecenoate) in mass ratios of 1:3, 1:1, and
3:1. Upon observing varying degrees of immiscibility by scanning electron
microscopy, we installed dynamic boronic ester cross-links via thiol–ene
click chemistry, which resulted in compatibilized dynamic thermoset
blends ranging in hard, medium, and soft rubber or elastomer thermomechanical
profiles. These dynamic thermoset blends were subjected to controlled
biological degradation experiments in freshwater conditions, achieving
timely mass loss despite the cross-linked architectures. Overall,
this work highlights a two-component platform for the production of
degradable and reprocessable dynamic thermoset blends suitable for
several classes of cross-linked polymer technologies from tailored,
biological PHA copolymers.