Synthesis of soy protein–lignin nanofibers by solution electrospinning Carlos Salas a,b,⇑ , Mariko Ago c , Lucian A. Lucia a,d , Orlando J. Rojas a,e,⇑ a North Carolina State University, Department of Forest Biomaterials, Raleigh, NC 27695, USA b Departamento de Ingenieria Quimica, Universidad de Los Andes, Merida 5101, Venezuela c Tokushima Bunri University, Faculty of Science and Engineering, Sanuki, Kagawa, Japan d Qilu University of Technology, Key Laboratory of Pulp & Paper Science and Technology, Jinan 250353, PR China e Aalto University, School of Chemical Technology, Department of Forest Products Technology, FI-00076 Aalto, Espoo, Finland article info Article history: Received 3 September 2014 Received in revised form 21 September 2014 Accepted 23 September 2014 Available online xxxx Keywords: Soy protein Composite fibers Lignin Electrospinning abstract Nanofibers were produced by electrospinning aqueous alkaline solutions containing different mass ratios of soy protein and lignin in the presence of poly(ethylene glycol) coadjutant, all of which presented shear thinning behavior. SEM revealed that the addition of polyethylene oxide as a coadjutant indeed facilitated the formation of defect-free fibers whose diameter increased with lignin concentration, in the range between  124 and  400 nm. Favorable interactions between lignin and soy protein were identified from data provided by differential scanning calorimetry. In addition, an increased hydrogen bonding and the loss of secondary structure of the proteins as the lignin concentration increased were observed from the disappearance of amide II (1500 cm 1 ) and III (1400–1200 cm 1 ) bands and a red shift of amide I band in the FT-IR spectrum. The unfolding of the protein contributed to a better interaction with lignin macromolecules, which further improved the electrospinning process. It is concluded that mix- tures of lignin and soy proteins, two major renewable resources with interesting chemical features, are suitable for the development of composite sub-micron fibers. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Interest in the development of environmentally friendly materi- als has sparkled the use of renewable precursor polymers and green manufacturing techniques. In the area of fiber formation, the time- renowned electrospinning technique allows production of continu- ous filaments from the nanometer to micrometer scale [1–6]. During the process, a polymer solution is pumped through a nozzle at the same time that a high voltage is applied between the nozzle and a metallic collector. The voltage induces electrostatic repulsion that stretches the liquid droplet at the tip of the nozzle forming a conical shape, the so-called Taylor cone. When the electrostatic repulsion overcomes the surface tension of the solution, the liquid is expelled from the nozzle as a jet that forms the fibers. The high surface area of fibers produced by electrospinning makes them ideally suitable for a wide variety of applications including tissue engineering, cosmetics, wound dressing, filtration media, catalysis, nanosensors and mili- tary protective clothing [2,3,6]. The use of natural polymers alone or in combination with synthetic polymers to produce nanofibers by electrospinning has gained increased attention over the last dec- ade; some of the polymers studied include cellulose acetate [7–9], collagen [10], hyaluronic acid [11], chitosan [12,13], polylactic acid (PLA), silk proteins [5,6], poly-e-caprolactone [14], cellulose [15– 17], soy proteins [18,19], and lignin [20–22]. Lignin, the most abundant non-cellulosic polymer on earth, [23] is a complex aromatic (phenolic) macromolecule present in the cell wall of plants; it is composed of three main phenylpropanoid monomers or precursors: coniferyl, sinapyl and p-coumaryl alco- hol [24]. Uses of lignin different than energy co-generation include those that take advantage of its chemical features as well as its physical and thermal properties when combined with other (bio)polymers; for example, in composite materials. Lignin has been studied for a variety of applications, from dispersants [23], and adhesives to the production of composite materials (phenolic resins) and carbon fibers [23,25–28]. The production of lignin fibers (micro and nanofibers) by the electrospinning technique has been reported recently [20– 22,25,26,29]. The addition of polyethylene oxide [20] or polyvinyl alcohol [21,22] coadjutants has been noted to improve intermolec- ular interactions, which leads to uniform fibers. In contrast with lignin, soybean proteins display a thermo- plastic behavior and are among the most investigated renewable materials for non-food applications, including wood adhesives, http://dx.doi.org/10.1016/j.reactfunctpolym.2014.09.022 1381-5148/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding authors at: North Carolina State University, Department of Forest Biomaterials, Raleigh, NC 27695, USA (C. Salas), Forest Products Technology, Aalto University, FI-00076 Aalto, Finland (O.J. Rojas). E-mail addresses: clsalasa@ncsu.edu (C. Salas), orlando.rojas@aalto.fi (O.J. Rojas). Reactive & Functional Polymers xxx (2014) xxx–xxx Contents lists available at ScienceDirect Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react Please cite this article in press as: C. Salas et al., React. Funct. Polym. (2014), http://dx.doi.org/10.1016/j.reactfunctpolym.2014.09.022