Tuning the properties of polyhydroxybutyrate films using acetic acid via solvent casting

Abstract

Biodegradable polyhydroxybutyrate (PHB) films were fabricated using acetic acid as an alternative to common solvents such as chloroform. The PHB films were prepared using a solvent casting process at temperatures ranging from 80 °C to 160 °C. The crystallinity, mechanical properties and surface morphology of the films cast at different temperatures were characterized and compared to PHB films cast using chloroform as a solvent. Results revealed that the properties of the PHB film varied considerably with solvent casting temperature. In general, samples processed with acetic acid at low temperatures had comparable mechanical properties to PHB cast using chloroform. This acetic acid based method is environmentally friendly, cost efficient and allows more flexible processing conditions and broader ranges of polymer properties than traditional methods.

Figure 1. Images of PHB films processed at different temperatures overlaid on the right side (separated by red lines) of a printed pattern, demonstrating the translucency of PHB films.

Results from a film processed using chloroform (CF) is shown for comparison. The film thickness of each sample was 40 ± 10 μm.

Figure 2

Optical transmittance vs. solvent casting temperature. The transmittance values were obtained at wavelengths of 600 nm, 500 nm, 400 nm and 300 nm. The inlay shows the images of transmitted laser beam after passing through the PHB films, arranged from lowest processing temperature (left) to highest processing temperature (right).

Figure 3. Stereomicroscope images of the PHB films solvent cast at different temperatures using acetic acid (AA) as a solvent.

False-colored images (in blue) show the undulations and macroscopic features on the surface, which indicate the presence of two distinct regions in the samples. A sample processed with chloroform (CF) as the solvent is shown for comparison.

Figure 4. Percent crystallinity with respect to the processing temperature.

The straight red line indicates the crystallinity of PHB prepared using chloroform at room temperature.

Figure 5. Combined XRD plot of PHB processed in acetic acid (AA) at different temperatures.

The pattern obtained from PHB prepared with chloroform (CF) is shown in red for comparison.

Figure 6. TGA plot of PHB films prepared under different conditions: solvent cast in chloroform (CF), solvent cast in acetic acid at 80 °C, and solvent cast in acetic acid at 160 °C.

The upper and lower horizontal lines correspond to 95% normalized mass and 5% normalized mass respectively, while the vertical lines indicate the thermal degradation onset temperature (T_i) and the complete degradation temperature (T_c) of each sample.

Figure 7. Combined plot of the DSC melting curves for PHB samples prepared at different temperatures in acetic acid (AA).

The as-received sample and sample solvent using chloroform (CF) are also shown for comparison. All samples were run at a scan rate of 20 °C /min (endothermic down).

Figure 8. Mechanical characterization of PHB films prepared in acetic acid (AA) at different casting temperatures.

(a) Strain to failure vs. casting temperature, (b) Elastic modulus vs. processing temperature, (c) Ultimate tensile stress vs. processing temperature, (d) Representative stress-strain curves. Average values and standard error are shown in (a–c) and are based on at least 4 measurements. Results from samples cast with chloroform (CF) are also shown for comparison.

Figure 9. AFM scans of PHB samples processed at different temperatures in acetic acid (AA) or chloroform (CF).

Scan area: 20 μm². The topography scale is ±120 nm for all acetic acid-processed samples, and ±1000 nm for the chloroform-processed sample.

Figure 10. RMS roughness of PHB films solvent cast with acetic acid with respect to the processing temperature.

Figure 11. Ensemble of different forms of PHB produced using acetic acid as a solvent.

(a) Porous PHB by rapid removal of solvent, (b) spray coated PHB layer on a glass substrate, (c) PHB thin films that have different optical transmittance. Films prepared at different acetic acid concentrations at 80 °C. Optical transmittance decreases from left to right, (d) Flexible PHB films prepared by solvent casting from acetic acid.

Figure 12. Schematic of PHB film preparation by solvent casting.

(a) The PHB is dissolved in acetic acid at 160 °C, (b) the solution is poured onto a glass slide held at the casting temperature, (c) samples are baked for 3 or 6 minutes (depending on the casting temperature) at which point films can be removed from the slide.