## 4 Melt Electrospinning
### Melt Electrospinning Conditions  
Defining the melt electrospinning process conditions mainly depends on the polymer system and the used technique as well as the desired mechanical properties. These properties can undergo a marked improvement compared to typical nonwoven fiber mats in which only the physical entanglement of the fibers contributes significantly to the mechanical properties. The most important parameters in a simple melt electrospinning process are the temperature, tip-to-collector distance, applied voltage, flow rate, and spinneret diameter [16]. Selecting the suitable tip to collector distance relies on the cooling diagram and the ability of stretching the polymer melt. Cooling and consequent solidification of the fibers take place as they stretch, therefore, short tip to collector distances may result in the deposition of slightly molten fibers and in crossover points caused by their fusion [37]. [PERSON] et al. [38] demonstrated that the final fiber diameter not only depends on the distance but also on the collector itself and the temperature. In this study, a KCl/ice water collector (cold collector) proved to produce thinner fibers compared to those obtained with regular tin foils. These thin fibers could have an improved molecular chain orientation and better resistance to the usual shrinkage caused by a reduction of the electric force [37; 39; 40]. Figure 2: Schematic of a typical melt spinning process. Copyright 2015 Elsevier [34]. Choosing an appropriate processing temperature firstly depends on the melting point of the polymer and then, on obtaining a suitable melt viscosity to stretch the jet, which, as discussed in previous sections, is influenced by temperature. Although it varies among the different techniques, the process temperature is typically chosen between 10 and 30 \({}^{\circ}\)C above the polymer melting point. A main drawback of this technique is that melting points are usually so high that polymers may suffer degradation. To avoid this problem, additives can be used so that viscosity is controlled and lower process temperatures can be achieved [41; 42; 43]. Influence of different applied voltage in both solution and melt electrospinning follow the same trends reported in a lot of studies [11]. Voltage may range between 7 kV and 50 kV, depending on the technique and material to be electrospun. Polarity (conductivity) and viscosity of the polymer melt are the key properties when it comes to optimize the voltage in order to obtain the desired fiber size and morphology, and get also a stable Taylor cone without sputtering the jets all over the collector and the insulating chamber [17; 44]. Another important parameter is the flow rate as it is directly proportional to the fiber diameter (i.e., polymer jets have less time to be stretched at higher flow rates and consequently larger diameters are derived) [45]. The flow rate is remarkably low (between 5 \(\mu\)L h\({}^{-1}\) and 9 mL h\({}^{-1}\)) compared to solution electrospinning due to elimination of the volatile solvent in the initial system. Nonetheless, the production capacity is much more high in the melt electrospinning process as consequence of working with multi jets and non volatile materials [46; 47; 48]. The diameter of spinneret holes has logically a great influence on the size of the fiber and may be a limitation to get ultrafine fibers. Since the flow of highly viscous melt polymers through small orifices is challenging and a sufficient flow is necessary to obtain the initial jet, spinneret holes are larger in melt electrospinning than those used in solution electrospinning [37]. Different designs of new spinnerets are now available. For example, non-cylindrical spinnerets could be an option to improve the melt electrospinning technique providing a multi-jet system that could overcome the large-scale barrier [46; 47]. On the other hand, in some techniques such as laser or wire melt electrospinningning, the spinneret is removed and the initial jets are provided and holed by the solid part of the polymer.