Noreen Louise Thomas

This paper investigates the effectiveness of a viscosity ratio model to predict phase inversion in poly(lactic acid)/poly(caprolactone) (PLA/PCL) blends. From capillary rheometry measurements, the melt viscosities of PLA and PCL were found to be 1298 and 1758 Pas respectively, at the temperature and shear strain rate of the melt blending process. Using these values, phase inversion was predicted to occur at 54 wt% of PCL. This prediction was validated both by optical microscopy and also by scanning electron microscopy (SEM) in which one of the phases had been removed by etching. For mechanical properties, the co-continuous phase morphology gave good ductility with balanced modulus and yield stress. Contact angle measurements were used to evaluate the surface energies of PLA and PCL. Experimental results of 35.5 and 44.0 mNm⁻¹, respectively, agreed well with the values of 37.6 and 46.0 mNm⁻¹ calculated from the Parachor method. Biodegradation tests in compost and degradation tests in phosphate buffer solution revealed that PLA and PCL hydrolyze by different mechanisms. PLA undergoes bulk degradation and brittle failure occurs after an incubation time, whereas PCL undergoes surface degradation. Prediction of phase inversion allows control of the morphology and hence the biodegradation and hydrolytic degradation behavior. GRAPHICAL ABSTRACT

In this paper we review the literature on electrospinning of porous fibers of micron and nanoscale diameters. Selection of an appropriate polymer-solvent system can yield porous fibers due to phase separation, and different types of porosity are generated by different phase separation mechanisms. In comparison with porous polymeric fibers, production of porous inorganic fibers is much more complex: small molecules or atoms cannot be directly electrospun into continuous fibers and so polymer precursors are required and subsequently removed. Inorganic porous fibers can be generated based on a range of materials: carbon, silica and numerous metal oxides and mixed metal oxides. Some of the many applications of electrospun porous fibers are discussed: applications ranging from tissue engineering and drug delivery, to water treatment, sensors, photocatalysis and lithium-ion batteries.

In this study an integrated methodology was proposed for the selection of solvent systems to produce electrospinnable solutions that form defect‐free poly(lactic acid) (PLA) fibers with narrow diameter distributions. The solvent systems were chosen using a thermodynamic approach, combined with electrical and rheological property criteria. More specifically, the three step methodology includes (1) initial choice of solvent by solubility evaluation to meet thermodynamic criteria, (2) electrical properties, that is, conductivity and dielectric constant adjustment by using solvent mixtures to meet electrical property criteria, and (3) critical entanglement concentration (C_e) determination by viscosity measurements, supported by elastic and plastic moduli measurements, followed by concentration adjustment to meet rheological criteria. All three criteria need to be met to ensure defect‐free nanofiber morphology. The methodology was demonstrated using PLA solutions that were characterized in terms of thermodynamic properties, conductivity, surface tension, and viscosity measurements. These data were analyzed and related to the nanofiber morphology and diameter as determined from scanning electron microscopy (SEM). Measurements of the elastic (G′) and the plastic (G″) moduli of PLA solutions showed a sharp increase of G′ at the chain entanglement concentration. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2016,54, 1483–1498