Elastomeric vitrimers from designer polyhydroxyalkanoates with recyclability and biodegradability

Abstract

Cross-linked elastomers are stretchable materials that typically are not recyclable or biodegradable. Medium-chain-length polyhydroxyalkanoates (mcl-PHAs) are soft and ductile, making these bio-based polymers good candidates for biodegradable elastomers. Elasticity is commonly imparted by a cross-linked network structure, and covalent adaptable networks have emerged as a solution to prepare recyclable thermosets via triggered rearrangement of dynamic covalent bonds. Here, we develop biodegradable and recyclable elastomers by chemically installing the covalent adaptable network within biologically produced mcl-PHAs. Specifically, an engineered strain of Pseudomonas putida was used to produce mcl-PHAs containing pendent terminal alkenes as chemical handles for postfunctionalization. Thiol-ene chemistry was used to incorporate boronic ester (BE) cross-links, resulting in PHA-based vitrimers. mcl-PHAs cross-linked with BE at low density (<6 mole %) affords a soft, elastomeric material that demonstrates thermal reprocessability, biodegradability, and denetworking at end of life. The mechanical properties show potential for applications including adhesives and soft, biodegradable robotics and electronics.

Fig. 1.. PHDU vitrimer life cycle.

Production of PHDU in engineered P. putida from fatty acid substrates, postfunctionalization into BE–cross-linked vitrimers (PHDU-BE), and multiple end-of-life scenarios. RT, room temperature; DMPA, 2,2-Dimethoxy-2-phenylacetophenone; THF, tetrahydrofuran.

Fig. 2.. Metabolic engineering of P. putida for PHDU biosynthesis and PHDU production experiments.

(A) mcl-PHA biosynthesis from fatty acids through the weakened β-oxidation pathway. (B) Quantification of the HAMEs in the purified materials from three PHDU-6 production batches with different surfactant (Brij-35) concentrations (batch 1 = 0.05 g/liter; batch 2 = 0 g/liter; batch 3 = 0.10 g/liter) using GC-MS. HAME-6 was only detected in the sample from batch 1 in trace amounts of 0.4%. (C) Maximum PHDU-6 and PHDU-22 titers and rates achieved in 3-liter bioreactors as well as total sodium decanoate (C₁₀), 10-undecenoic acid (usC11), and glucose fed in each case.

Fig. 3.. Characterization of linear PHDU-6.

(A) PHDU-6 crystallization behavior during DSC experiments with various cooling and heating rates. Two T_m values correspond to phase 1 and 2 crystal structures. The larger peak for each rate was integrated for ΔH_f values, which are 21.9, 23.1, and 12.5 J g⁻¹ for 1, 5, and 10°C min⁻¹ rates, respectively. (B) DSC scan (10°C min⁻¹) of a solvent-cast thin film of PHDU-6, showing the highest crystallinity and T_m of all samples. (C) Stress-strain curves for compression-molded PHDU-sat (n = 3) with inset photograph of the colorless and transparent dog-bone specimen (ASTM D638 type V).

Fig. 4.. Structural analysis of swollen PHDU-BE networks by HR-MAS and ssNMR.

(A) Structure of cross-linked PHDU, with color-coded assignments indicated. (B to E) 1D ¹H and 2D ¹H-¹³C HSQC HR-MAS spectra for PHDU-BE-6 (blue, top) or PHDU-BE-22 (purple, bottom) materials. Expanded regions (D) (6%) and (E) (22%) highlight select linker signals. (F) ¹³C ssNMR data (50 MHz and 10 kHz MAS) with three spectra reported for both materials: DP (direct polarization), all carbons visible regardless of mobility; INEPT (insensitive nuclei enhanced by polarization transfer), only mobile protonated carbons observed; and CP (cross-polarization), only rigid carbons observed.

Fig. 5.. Crystallization behavior and morphology of PHDU-BE-6.

(A and B) Aging study of PHDU-BE-6 (8 mm-wide disk, compression molded and stored in a glass vial). (C) WAXS of the linear (orange) and cross-linked (blue) PHDU-6. Linear PHDU shows a peak at q = 0.274 Å⁻¹ and a broad group of peaks centered at q = 1.4 Å⁻¹. Cross-linked PHDU depicts a broader peak at q = 0.268 Å⁻¹ and a broad group of peaks centered at q = 1.4 Å⁻¹. (D) Schematic of the hypothetical crystal structures of linear (top) and cross-linked (bottom) PHDU. a.u., arbitrary units.

Fig. 6.. Mechanical, viscoelastic, and rheological properties of PHDU-BE-6.

(A) Overlay of tensile stress-strain curves for PHDU-BE-6 processed four times (n = 3). (B) Tensile data showing averaged values and SDs for ɛ_B, σ_B, and E. (C) Elastic hysteresis test of amorphous PHDU-BE-6 over 10 cycles of tensile strain from 0 to 100%. (D) DMA showing storage modulus as a function of temperature for PHDU-6 (linear, orange) and PHDU-BE-6 (blue). (E) Arrhenius relationship or activation energy plot for network relaxation as a function of temperature for PHDU-BE-6. (F) Flow creep-recovery profile for PHDU-BE-6 (blue) and PHDU-BE-22 (purple) at varied temperatures (60°, 100°, 125°C, 1 kPa, 10-min creep, 5-min recovery).

Fig. 7.. Freshwater biodegradation of PHDU-BE-6 and PHDU-linear.

Results over an 89-day period of aerobic freshwater degradation (ISO 14851) for linear PHDU-6 (orange) and PHDU-BE-6 (blue) against an MCC (green).

Fig. 8.. PHDU-BE network characteristics.

Cartoon representation of a PHDU-BE network (middle) with chain entanglements, closed and open BE linkages, and crystallites (left); (right) the interplay among moisture, cross-link density, crystallization, and mechanical properties.