Sophorolipid-induced dimpling and increased porosity in solvent-cast short-chain polyhydroxyalkanoate films: Impact on thermomechanical properties


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Sophorolipids (SL; microbial glycolipids) were used as additives in solvent-cast short-chain polyhydroxyalkanoate (sc-PHA) films to enhance surface roughness and porosity. Poly-3-hydroxybutyrate (PHB), poly-(6%)-3-hydroxybutyrate-co-(94%)-3-hydroxyvalerate (PHB/V), and poly-(90%)-3-hydroxybutyrate-co-(10%)-3-hydroxyhexanoate (PHB/HHx) films were evaluated with up to 43 wt % of SL. Sophorolipid addition caused surface dimples with maximum diameters of 131.8 µm (PHB), 25.2 µm (PHB/V), and 102.8 µm (PHB/HHx). A rise in the size and number of pores in the polymer matrix also occurred in PHB and PHB/V films. Surface roughness and film porosity were visualized by scanning electron microscopy and quantitated using confocal microscopy by correlating the surface area (A′) to the scanned area (A) of the films. The phenotypic alterations of the films caused a gradual decline in tensile strength and modulus and increased the elongation to break. Reductions in the enthalpies of fusion (ΔHf) in both the PHB (41% reduction) and PHB/HHx (36% reduction) films implied diminished crystallinity as SL concentrations increased. Over the same SL concentrations the Tan δ maxima shifted from 4 to 30°C and from 2 to 20°C in these respective films. These results provide a novel means by which sc-PHA properties can be controlled for new/improved applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40609.