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Abstract
The slow crystallization kinetics of poly(hydroxyalkanoates) (PHAs) preclude simple incorporation in existing industrial thermoplastic process steps. This work addresses improving the nucleation density and crystallization speed of poly(hydroxybutyrate-co-hydroxyhexanoate) (PHBH), containing 9 mol% 3-hydroxyhexanoate content, via organic nucleating additives. First, xanthine and a series of its methylated derivatives (caffeine, theophylline, and theobromine) are examined to establish structure-function relationships of nucleating additives. Xanthine and theobromine, both of which contain a non-substituted imide hydrogen structural motif, successfully nucleated PHBH. Caffeine and theophylline, both of which contain a methyl-substituted imide nitrogen, did not affect PHBH nucleation.
Second, a biobased and biologically degradable B-vitamin (nicotinic acid, a.k.a. niacin) is evaluated as a melt-soluble PHBH nucleating agent. Nicotinic acid exhibits a moderate nucleating effect despite not containing an imide structural motif due to the hydrogen bonding capacity of its carboxylic acid functional group. Nicotinic acid displayed an interesting concentration-dependent nucleation performance, wherein concentrations exceeding 2 w/w% led to reduced functionality because of the absence of active nucleating surfaces due to increased crystal growth within the polymer melt.
Third, a series of orotic acid esters was synthesized and first reported as melt soluble PHBH nucleating agents. The orotate esters display reduced (but nonetheless valuable) nucleation performance while exhibiting melt solubility which is not observed in the parent compound. Unlike nicotinic acid, orotate esters do not degrade PHBH molecular weight, making them powerful tools for advanced PHBH extrusion operations. The crystal structures of all four synthesized derivatives were fully elucidated using X-Ray Diffraction (XRD).
Finally, PHBH is formulated with an appropriate nucleating agent to produce filaments for fused deposition modeling (FDM) 3D printing applications. The filament is shown to be capable of producing high quality medical illustrations despite the challenges posed by the semicrystalline nature of PHBH. The filament is additionally validated as biologically degradable using respirometry, to show that the PHBH formulation completely mineralizes in short timescales – an important hallmark of biologically degradable, non-polluting plastics.