Bacterially produced Poly [(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate) (PHBHx) is a new type of bioplastic with excellent biocompatibility, biodegradability and processing properties. Electrospinning was used as the processing technique to fabricate PHBHx nanofibers. For the first time, the strain-induced metastable β-form crystal structure, consisting of planar zig-zag chains, was discovered in the macroscopically aligned PHBHx nanofibers collected with a high-speed rotary disk. In addition, this β-form was confirmed to coexist with the thermodynamically stable α-form with 21 helical chains. Because the α and β-form crystal structures have significantly different chemical and physical properties, it would be intriguing to investigate the spatial distribution of the two crystalline polymorphs in the nanofibers, providing insights into the internal structure of the fibers, with the goal of precise control of the macroscopic properties of the material. The combination of atomic force microscopy (AFM) and infrared (IR) spectroscopy has been shown with the ability of providing topographic information as well as chemical and structural information at a spatial resolution of 50-100 nm, well below the diffraction limit in the infrared. Using AFM-IR, we explored the correlation between molecular orientation, crystalline structure and processing in electrospun PHBHx nanofibers at the single fiber scale. The ability to obtain IR spectra at high spatial resolutions allowed us to probe the crystalline populations as a function of nanofiber diameter and as a function of spatial location within the fiber. In addition, by utilizing the spectroscopic imaging capability of the AFM-IR instrument, we created a distribution map of the α and β-form crystal structure in a single PHBHx nanofiber, which revealed the existence of a core-shell structure consisting of a α-form-rich core and a β-form-rich shell. Besides, the thickness of the shell was observed to remain consistent with variation in fiber diameter. According to the experimental results, a new generation mechanism of the β-crystalline form was proposed which is significantly different from those previously reported. The morphological and structural details of individual electrospun PHBHx nanofibers provide us a deeper understanding of the structure/property/processing relationships at the single fiber scale, facilitating the fine-tuning of nanofiber properties to meet the application needs.

AFM-IR studies of individual electrospun nanofibers of bio-based poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBHx)

255th National Meeting (2018)

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