Scaling laws and internal structure for characterizing electrospun poly[(R)-3-hydroxybutyrate] fibers

Using chloroform/dimethylformamide (CF/DMF) co-solvent, electrospinning of poly[(R)-3-hydroxy-butyrate] (PHB) solutions was carried out at ambient temperature. The effects of the applied voltage (V), flow-rate (Q), and solution viscoelastic properties on the Taylor cone, electrified jet, and fiber morphology were investigated. In addition, the electric field developed by the needle-plate electrode configuration was calculated using a finite element analysis to reveal the tip-to-collector (H) effect. Among the processing parameters (V, Q and H), it was found that Q played a key role in determining the jet diameter (d(j)) and electrospun fiber diameter (d(f)), and scaling laws existed between them, i.e., d(j)-Q(0.61) and d(f)-Q(0.33). The diameter reduction ratios of D-o/d(j) (D-o is the needle diameter) and d(j)/d(f) were measured as 50-120 and 5-10, respectively; it suggested that major jet stretching took place in the straight electrified jet region, and further chain orientation could be gained by the subsequent process of jet whipping. By changing PHB concentrations from 5 to 15 wt%, the solution viscosity (eta(o)) was increased from 100 to 4900 cP, whereas the surface tension and solution conductivity remained unchanged: it provided a good model solution to exclusively reveal the eta(o) effect on the electrospinning process. Our results showed that the eta(o)-dependence of d(j) and d(f) also followed simple scaling laws: dj-eta(0.06)(o), and df-eta(0.39)(o), with a prefactor depending on the processing variables, mainly the flow-rate. Regardless of the PHB concentrations used, the obtained PHB fibers showed a similar crystallinity fraction of ca. 0.63 and possession of major a-crystals together with a small amount of beta-crystals with zigzag chain conformation. (C) 2008 Elsevier Ltd. All rights reserved.