Discovery of the β-form crystal structure in electrospun nanofibers of bio-based poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] and its implication on properties
Date
2017
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
Bacterially produced poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate]
(PHBHx) is a new type of bioplastic which not only inherits the excellent
biodegradability and biocompatibility of its parent homopolymer,
polyhydroxybutyrate (PHB), but also overcomes PHB’s brittleness and stiffness with
the incorporation of 3-hydroxyhexanoate (Hx) comonomer units with medium-chainlength
(mcl) side chains. The tough and ductile PHBHx, with a much lower
crystallinity and melting temperature, is well-suited for many practical applications.
Efforts have been made to broaden the application range of PHBHx by introducing the
beta-form crystalline structure, where the molecular chains adopt a planar zig-zag
conformation. However, it is extremely difficult to produce this beta-form in PHBHx
due to its much lower crystallinity and much more flexible molecular chains. In this
study, we report an approach using the technique of electrospinning. ☐ The strain-induced metastable β-form crystalline structure was successfully
introduced in PHBHx by collecting the macroscopically aligned electrospun PHBHx
nanofibers across the air gap on a piece of aluminum foil and on the tapered edge of a
high-speed rotary disk. The presence of the β-form crystal structure in electrospun
fiber mats was confirmed by wide-angle X-ray diffraction (WAXD) and Fourier
transform infrared spectroscopy (FTIR), with molecular orientation of the polymer
chains along the fiber axis revealed by polarized FTIR. ☐ Selected area electron diffraction (SAED) and AFM-IR were utilized to
investigate the morphological and structural details of individual PHBHx nanofibers.
The results demonstrated a coexistence of the thermodynamically stable α-form
crystalline structure, where molecular chains adopt a left-handed 21 helical
conformation, and the β-form in single fibers. The molecular orientation level and the
relative amounts of the two crystalline polymorphs were found to be highly dependent
on fiber collection methods and fiber diameter. Moreover, the α and β-form were
revealed to be spatially distributed as a core-shell structure consisting of an α-formrich
core and a β-form-rich shell, with the thickness of the shell remaining constant
despite the variation of fiber diameter. According to these observations, a possible
mechanism for the generation of the β-form was proposed. ☐ The effects of electrospinning parameters on the formation of the beta-form
were systematically investigated. The results indicated that more β-crystals can be
produced when 1) a higher fiber take-up is used, so that the polymer chains are further
stretched before fiber solidification; 2) an optimal solution concentration is chosen, so
that a balance between polymer chain deformation and relaxation is maintained
throughout the whole electrospinning process; and 3) a more volatile solvent is used,
so that more planar zig-zag chains are kinetically frozen in the fibers without being
converted to the helical conformation as the fibers solidify. These experimental results
indicate that the β-content in PHBHx nanofibers can be easily regulated by modifying
the electrospinning conditions. ☐ Finally, the influence of the presence of the β-form on the piezoelectric
response of the electrospun PHBHx nanofibers was studied. It was observed that the
fibers containing the β-form exhibited an obvious piezoelectric response to the applied
pressure, possibly due to the planar zig-zag conformation of the chains which gives
rise to a significant dipole moment change when subjected to mechanical deformation.
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In addition, the sensitivity of the piezoelectric PHBHx nanofibers to mechanical
pressure was measured to be 7.46 mV/kPa. These preliminary investigations indicate
that the piezoelectric performance of PHBHx can be largely improved by increasing
the concentration of the piezoelectric-active β-form crystalline structure. The
piezoelectric PHBHx distinguishes itself from all the other piezoelectric polymers
with its excellent biodegradability and biocompatibility, environmental-friendliness
and most importantly, low manufacturing cost. It is a promising piezoelectric polymer
which can be applied in advanced areas including portable/foldable electronic devices,
artificial electronic skins and implantable sensors.