fibers Article Humidity Controlled Mechanical Properties of Electrospun Polyvinylidene Fluoride (PVDF) Fibers Piotr K. Szewczyk , Daniel P. Ura and Urszula Stachewicz * International Centre of Electron Microscopy for Materials Science, Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Al. A. Mickiewicza 30, 30-059 Kraków, Poland; pszew@agh.edu.pl (P.K.S.); urad@agh.edu.pl (D.P.U.) * Correspondence: ustachew@agh.edu.pl Received: 28 September 2020; Accepted: 14 October 2020; Published: 16 October 2020   Abstract: Processing parameters in electrospinning allow us to control the properties of fibers on a molecular level and are able to tailor them for specific applications. In this study, we investigate how relative humidity (RH) affects the mechanical properties of electrospun polyvinylidene fluoride (PVDF). The mechanical properties of single fibers were carried out using a specialized tensile stage. The results from tensile tests were additionally correlated with high-resolution imaging showing the behavior of individual fibers under tensile stress. The mechanical characteristic is strongly dependent on the crystallinity, chain orientation, and fiber diameter of electrospun PVDF fibers. Our results show the importance of controlling RH during electrospinning as the mechanical properties are significantly affected. At low RH = 30% PVDF fibers are 400% stiffer than their counterparts prepared at high RH = 60%. Moreover, the vast differences in the strain at failure were observed, namely 310% compared to 75% for 60% and 30% RH, respectively. Our results prove that humidity is a crucial parameter in electrospinning able to control the mechanical properties of polymer fibers. Keywords: PVDF; mechanical properties; humidity; electrospinning; tensile testing 1. Introduction Many applications of electrospun polymer fibers depend greatly on their mechanical properties, which are governed by processing conditions in electrospinning [1–3]. With the right set of parameters, we are able to obtain a broad range of morphologies and physicochemical properties from the same raw material [2,4–6]. Thus far, electrospun fibers have been researched in a wide variety of applications such as cost-effective water harvesting systems [7,8], energy harvesters [9–11], tissue engineering scaffolds [12,13], filters [14–16], composite strengtheners [17–19], sensors [20], drug delivery systems [21,22] and many more. Characterization of mechanical properties of single polymer fibers is difficult due to often micro- and nanosized dimensions of the samples, making the handling and testing challenging [23]. There are many reports on fiber meshes that were measured using macroscale experimental methods [23–26]. Such an approach gives important information on how electrospun membranes behave under load together with a few proposed models of stretched electrospun samples [27]. Importantly, there are many parameters in electrospinning and all of them affect the properties of produced fibers [28,29]. One of them is relative humidity (RH) [30]. Humidity influences fiber diameters [31], crystallinity [32], internal structure [33], surface roughness [34], and mechanical properties of electrospun fibers [30,35,36]. Recent reports have shown that key properties such as stiffness, strain at break, or Young’s modulus can be controlled using just RH [30,35,36]. The importance of RH is often correlated with the phase separation processes that take place in a humid atmosphere. Vapor-induced phase separation (VIPS) is frequently proposed as the main mechanism of phase separation in the electrospinning process. In this Fibers 2020, 8, 0065; doi:10.3390/fib8100065 www.mdpi.com/journal/fibers