The domain structure and mobility of semi-crystalline poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate): A solid-state NMR study

Multi-scale instrumental analyses of plasticized polyhydroxyalkanoates (PHA) blended with polycaprolactone (PCL) and the effects of crosslinkers and graft copolymers

Details of the mechanism underlying the tensile properties of plasticized polyhydroxyalkanoates (PHA) including poly(butylene succinate) (PBS) were investigated by blending with poly(ε-caprolactone) (PCL) as well as the addition of compatibilizers. Multi-scale instrumental analyses employed micro-focus X-ray CT to provide micro-scale morphology information on the order of ten microns while solid-state NMR spectral and relaxation time analyses contributed knowledge of the environment and molecular mobility of each constituent at the molecular to nano-scale. The blend of plasticized PHA with 50% PCL adopted a sea-island morphology to improve elongation at break in a quasi-static tensile test, which was dominated by the tensile properties of the added PCL. However, impact tensile properties were less improved by PCL addition, because its molecular mobility was suppressed by blending. Meanwhile, peroxy crosslinkers changed the sea-island morphology to homogenous in X-ray CT observations. Although the homogenous morphology sharply lowered the elongation at break in a quasi-static tensile test, the homogenous morphology improved impact tensile properties. Furthermore, graft polymers having acrylonitrile–styrene side-chains did not change the sea-island morphology but increased the molecular mobility of PBS in the plasticized PHA. This weak interaction between the plasticized PHA and PCL improved tensile properties in both quasi-static and impact tensile tests.

Multi-scale instrumental analyses showed that the crosslinker changed the morphology to homogenous while the graft polymer increased molecular mobility.

Solid-State Nuclear Magnetic Resonance (NMR) and Nuclear Magnetic Relaxation Time Analyses of Molecular Mobility and Compatibility of Plasticized Polyhydroxyalkanoates (PHA) Copolymers

The molecular mobility and compatibility of plasticized polyhydroxyalkanoates (PHA) were investigated, focusing on changes due to copolymerization using either flexible poly (butylene succinate) (PBS) or rigid poly(lactic acid) (PLA) units. For the case of a poly(3-hydroxybutyrate) (PHB) unit in plasticized PHA, copolymerization of either PBS or PLA decreased ¹H and 13C spin-lattice relaxation times in the laboratory frame (T₁H and T₁C) in the same manner, while PBS produced a lower ¹H spin-lattice relaxation time in the rotating frame (T₁ρH) than PLA. Both the signals of ¹H MAS (magic-angle spinning) and 13C PST (pulse saturation transfer) MAS nuclear magnetic resonance (NMR) spectra were sharpened and increased by copolymerization with PBS. A variable temperature relaxation time analysis showed that the decrease of T₁H values was dominated by the ¹H spin diffusion via the interface between PHB and the added polyester because of the good compatibility. Meanwhile, the decrease of T₁C values was dominated by increasingly rapid molecular motions of PHB because of the lowered crystallinity due to the plasticization. Slow molecular motions (kHz order) were enhanced more by the addition of PBS than PLA, although rapid molecular motions (MHz order) were enhanced by either polyester. Several NMR parameters were beneficial for analyzing the manufacturing process as the indexes of polymer compatibility and molecular motions.

Enhanced polyhydroxyalkanoate (PHA) production from the organic fraction of municipal solid waste by using mixed microbial culture

BACKGROUND

In Europe, almost 87.6 million tonnes of food waste are produced. Despite the high biological value of food waste, traditional management solutions do not consider it as a precious resource. Many studies have reported the use of food waste for the production of high added value molecules. Polyhydroxyalkanoates (PHAs) represent a class of interesting bio-polyesters accumulated by different bacterial cells, and has been proposed for production from the organic fraction of municipal solid waste (OFMSW). Nevertheless, until now, no attention has been paid to the entire biological process leading to the transformation of food waste to organic acids (OA) and then to PHA, getting high PHA yield per food waste unit. In particular, the acid-generating process needs to be optimized, maximizing OA production from OFMSW. To do so, a pilot-scale Anaerobic Percolation Biocell Reactor (100 L in volume) was used to produce an OA-rich percolate from OFMSW which was used subsequently to produce PHA.

RESULTS

The optimized acidogenic process resulted in an OA production of 151 g kg^-1 from fresh OFMSW. The subsequent optimization of PHA production from OA gave a PHA production, on average, of 223 ± 28 g kg^-1 total OA fed. Total mass balance indicated, for the best case studied, a PHA production per OFMSW weight unit of 33.22 ± 4.2 g kg^-1 from fresh OFMSW, corresponding to 114.4 ± 14.5 g kg^-1 of total solids from OFMSW. PHA composition revealed a hydroxybutyrate/hydroxyvalerate (%) ratio of 53/47 and Mw of 8∙10⁵ kDa with a low polydispersity index, i.e. 1.4.

CONCLUSIONS

This work showed how by optimizing acidic fermentation it could be possible to get a large amount of OA from OFMSW to be then transformed into PHA. This step is important as it greatly affects the total final PHA yield. Data obtained in this work can be useful as the starting point for considering the economic feasibility of PHA production from OFMSW by using mixed culture.

Solid-state NMR analyses reveal the structure dependence of the molecular dynamics for ω-amino acids

The molecular dynamics of metabolites is structure dependent and vitally important for the interactive functions in their potential applications as natural materials. To understand the relationship between molecular structure and dynamics, the molecular motions of four structurally related ω-amino acids (β-alanine, γ-aminobutyric acid, 5-aminovaleric acid, and 6-aminocaproic acid) were investigated by measuring their proton spin-lattice relaxation times (T(1), T(1ρ)) as a function of temperature (180-440 K). (13)C CPMAS NMR and DSC analyses were performed to obtain complementary information. All of these ω-amino acids showed no phase transition in the temperature range studied but had outstandingly long proton T(1) at 300 MHz and even at 20 MHz for the deuterated forms. The molecular dynamics of all these ω-amino acids were dominated by the reorientation motions of amino groups and backbone motions except in β-alanine. The activation energies for amino group reorientations were positively correlated with the strength of hydrogen bonds involving these groups in the crystals and the carbon-chain lengths, whereas such energies for the backbone motions were inversely correlated with the carbon-chain lengths. These findings provided essential information for the molecular dynamics of ω-amino acids and demonstrated the combined solid-state NMR methods as a useful approach for understanding the structural dependence of molecular dynamics.

Comprehensive solid-state NMR analysis reveals the effects of N-methylation on the molecular dynamics of glycine

Molecular dynamics of metabolites are important for their interactions and functions. To understand the structural dependence of molecular dynamics for N-methylated glycines, we comprehensively measured the (13)C and (1)H spin-lattice relaxation times for sarcosine, N,N-dimethylglycine, betaine, and betaine hydrochloride over a temperature range of 178-460 K. We found that the reorientations of methyl groups were observed for all these molecules, whereas reorientations of whole trimethylamine groups were detected in betaines. While similar rotational properties were observed for methyl groups in N,N-dimethylglycine and those in betaine, three methyl groups in betaine hydrochloride had different motional properties (E(a) ≈ 20.5 kJ/mol, τ(0) ≈ 1.85 × 10(-13) s; E(a) ≈ 13.9 kJ/mol, τ(0) ≈ 2.1 × 10(-12) s; E(a) ≈ 15.8 kJ/mol, τ(0) ≈ 1.1 × 10(-12) s). N,N-Dimethylglycine showed a phase transition at 348.5 K with changed relaxation behavior for methyl groups showing distinct E(a) and τ(0) values. The DIPSHIFT experiments showed that CH(3) and CH(2) moieties in these molecules had dipolar-dephasing curves similar to these moieties in alanine and glycine. The activation energies for CH(3) rotations positively correlated with the number of substituted methyl groups. These findings provided useful information for the structural dependence of molecular dynamics for N-methylated glycines and demonstrated solid-state NMR as a useful tool for probing the structure-dynamics relationships.

Kinetics of solid-state NMR cross-polarization from protons to carbon-13 in surgical sutures

Commercial Dexon surgical sutures, made of polyglycolide (PGA), were examined using (13)C CP/MAS NMR. The study shows that detailed analysis of the cross-polarization (CP) process is useful in the peak assignments and in the assessment of molecular mobility in the polymer domains. Crystallinity of PGA in the sutures was estimated at ca. 55%.

Synthesis, Cocrystallization, and Enzymatic Degradation of Novel Poly(butylene-co-propylene succinate) Copolymers

A series of poly(butylene-co-propylene succinate) (PBPSu) random copolyesters were synthesized and characterized using 1HNMR spectroscopy and viscometry. Tensile properties decreased with increasing propylene succinate (PSu) content. Cocrystallization and multiple melting behaviors were investigated. The copolymers showed a eutectic behavior. The minimum melting point corresponded to 75 mol % PSu. Wide-angle X-ray diffraction (WAXD) patterns showed that the copolymers with up to 60 mol % PSu units formed poly(butylene succinate) crystals. The interplanar spacings slightly differentiated. The copolymer with 11.5 mol % poly(propylene succinate) (PPSu) units formed PPSu crystals. The results indicated isodimorphic cocrystallization. The cocrystallization was thermodynamically analyzed using the Wendling-Suter model. The defect free energy decreased for copolymers with high PPSu content. The banded spherulites of the copolyesters were studied, and growth rates were analyzed using the Lauritzen-Hoffman theory. Enzymatic hydrolysis study, using Rhizopus delemar and Pseudomonas cepacia lipases, showed that degradation was faster for copolymers with high PSu content, compared even to the fast-degrading PPSu.