Effect of nanofiller incorporation on thermomechanical and toughness of poly (vinyl alcohol)-based electrospun nanofibrous bionanocomposites

Highlights

• Nanohydroxy apatite and cellulose nanofibers were incorporated into electrospun PVA nanofibers.

• Both nanofillers significantly increased the stiffness of the fibrous nanocomposites.

• Neat electrospun PVA mats showed ductile fracture whereas by nanofiller incorporation, fracture altered to a brittle mode.

Abstract

The current work studies the electrospun poly (vinyl alcohol) (PVA) nanofibers and its nanocomposites including nanohydroxy apatite (nHAp) and nHAp/cellulose nanofibers (CNFs), emphasizing the impact of nanofillers on the toughness of nanofibers. PVA nanofibers were incorporated with 10 wt% of nHAp and then various amounts of CNF were added to subsequent PVA/nHAp fibrous nanocomposites. The morphology of nonwoven mats was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). While neat PVA nanofibers were smooth and uniform in thickness, the nanofiller loading resulted in thinner fibers with less uniformity. Furthermore, the thermal properties of the nonwoven network of fibers were characterized employing thermogravimetric analysis (TGA). Although the maximum loss mass temperature of PVA was partially reduced upon addition of nanofillers, the onset of decomposition was not altered. The mechanical characterizations were performed using static tensile and dynamic mechanical analysis (DMA). Compared to neat PVA mats, the tensile test of nanocomposites mats demonstrated the significant increase in Young’s modulus; however, strain at break was dramatically reduced. In addition, the fracture work was assessed from the area under the stress-strain curve, which showed brittleness of fibrous nanocomposites due to the nanofiller incorporation. Field emission SEM (FE-SEM) was employed to scan the fracture surface of stretched fibers. The increase in modulus of electrospun mats was also shown by DMA in frequency mode. In parallel, both tensile test and DMA confirmed the change in fracture of PVA fibers from a tough to brittle mode, due to the nanofiller addition.

Introduction

A vast majority of current studies have been concentrated on the fabrication of porous structures via electrospinning [1]. This technique produces nonwoven fibrous mats which gained overwhelming supports from medicine, membrane, sensor [2], [3], etc. fields. In competition with its counterparts, e.g., gas foaming [4], [5], [6], phase separation [7], and particulate leaching [8], electrospinning is simpler, industrial scalable, cheaper and one step approach with possible control on fiber morphology [9], [10], [11].

A broad range of synthetic and naturally-occurring polymers have been electrospun for various applications. Meanwhile, biomedicine has mostly focused on synthetic biopolymers such as polycaprolactone (PCL), PVA, and so on [12], [13], [14] due to their higher strength, abundant production, ease of processing, etc. when compared with natural biopolymers. Among the mentioned polymers, since PVA is a water-soluble polymer, it can be a superior option for those who take into account environmental issues. Other merits such as hydrophilicity, nontoxicity, biodegradability, and biocompatibility are accounted for this semi-crystalline biopolymer [15], [16]. Although neat biopolymers have their own particular advantages, they are not able to meet all requirements of the intended application. For example, bone tissue engineering requires porous scaffolds with mechanical properties comparable to body bone and also osteoconduction and osteoinduction capabilities [17]. To reach these properties, inserting nanomaterials with attractive characteristics within the scaffold can be a solution. In general, nanomaterials have been extensively used to obtain reinforced polymeric nanocomposites. Specifically, in recent years, the incorporation of nano-scale materials, in the form of particles, fibers, and whiskers, into the electrospun fibers has paved the path for developing either fibrous nanocomposites or bionanocomposites when matrix and/or fillers are biomaterials [18], [19], [20].

Tissue engineering is one of the attractive areas for the use of electrospun bionanocomposite scaffolds, as a mimic of native tissues in which cells can grow, proliferate, differentiate, and finally regenerate a damaged tissue [21]. In the case of bone tissue engineering, nano-hydroxyapatite is an interesting nanoparticle, since it resembles the natural bone tissue [22]. In addition, in recent years, cellulose nanofibers have been accepted as reinforcements for various polymeric systems due to their unique properties including renewable sources, high aspect ratio, large Young’s modulus and recent widespread availability [22]. According to its proved biocompatibility and low cytotoxicity, CNFs have also recently gathered attention in biomedical applications such as drug delivery, tissue engineering, and bioimaging [23]. To date, CNF has been also employed as reinforcement in biocomposite materials such as electrospun scaffolds [24], [25], [26]. Considering the worthy characteristics of PVA, nHAp, and CNFs, together with the valuable properties of electrospun fibers, a ternary fibrous bionanocomposite scaffold based on these components could be a promising candidate for bone tissue engineering as demonstrated in previous reports [18], [24].

For many years, fractures of polymeric composites/nanocomposites have been studied by researches [27], [28], [29], [30], [31]. To go beyond the typical fracture analysis of reinforced polymeric matrices, the current study explores the toughness of the electrospun nanofibers in the presence of the nanofillers under the tensile force. Here, we proceed with the novel characterization of electrospun nanofibers, where the fracture surface of broken fibers was scanned by FE-SEM to investigate the impact of nanofillers on the toughness. Additionally, TEM, TGA, and DMA were applied to characterize more detailed the electrospun bionanocomposites.