Solvent-induced crystallization in electrospun poly(3-hydroxybutyrate) (PHB) was systematically investigated, along with the hierarchical structural changes that occur upon heating. The study employed temperature-dependent wide-angle and small-angle X-ray scattering (WAXS/SAXS), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR) to comprehensively analyze structural evolution across multiple length scales. Solvent molecules, like cyclopentanone, N,N-dimethylformamide, 1,4-dioxane, 1,3-dioxolane, and dimethyl carbonate, induce the formation of the β form in addition to the existing α form in electrospun PHB. Interactions between solvent molecules and stretched polymer chains promote the formation of the β form, as evidenced by the appearance of a reflection at 2θ ≈ 18.7° in the WAXS pattern. Thermal annealing of the solvent crystallized electrospun fibers at 75 °C enhances the β form, suggesting that the enhanced mobility of the oriented amorphous chains plays a critical role in the formation of the β form. DSC and temperature-dependent WAXS results revealed that the β form was stable up to 90 °C and melts completely at 125 ± 3 °C. At higher temperatures, the β form completely disappears, and the well-resolved α form peaks emerge, indicating the formation of a highly ordered α form. Additionally, systematic changes in lamellar structure parameters derived from temperature-dependent SAXS measurements confirm the structural evolution associated with lamellar thinning and lamellar fusion due to the premelting and recrystallization. FTIR spectroscopy further confirms the presence of the β form in solvent-crystallized samples, and temperature-dependent FTIR spectra corroborate the structural transitions observed by DSC, WAXS, and SAXS. The presence of the β form in solvent crystallized PHB fibers contributes to improved piezoelectric and mechanical performance compared to as-electrospun PHB fibers.
Thomas et al. (Mon,) studied this question.