## 7 Melt Electrospun Polymer Blends  
As discussed in melt processing section, addition of one or more polymers to another polymer is an efficient and economical way to improve properties of the targeted polymer. Modification of mechanical properties, glass transition and crystallization temperatures, increase of biodegradability and biocompatibility, decrease of the price and increase of the life time are the main reason that brings polymer blending into the attractive category [164; 165; 166]. Some polymer blends are miscible and result in homogeneous structures after mixing, nonetheless, majority of blends are immiscible and the multicomponent materials complicate the system and melt processing due to different chemical/physical structure and viscoelastic behaviour [167]. Since the elongational flow of polymer melt plays an important role in the melt electrospinning process, properties and behaviour of polymer blends needs to be rationalized. Even if a lot of studies on polymer blends have been carried out with solution electrospinning, there are only a few that addresses on melt electrospinningning of blends. In this scope, information about the properties of polymer melt during the melt electrospinning is scarce, so this could be an open area for research studies. As discussed before, one polymer used widely in tissue engineering and scaffold construction is PCL. This polymer is attractive due to applications in bone, cartilage and nerve regeneration among other tissues in the body and showed slow degradation rate [168]. Although solution electrospun was successfully applied for preparation of PCL fibers, the addition of an amphiphilic polymer can extend the functionality of the material [167]. In addition, hydrophobic drugs can be added into PCL that are released as the material degrades. In this case, the presence of an amphiphilic material, such as poly (ethylene glycol)-block-poly(\(\varepsilon\)-caprolactone) (PEG-\(b\)-PCL), allows the delivery of hydrophilic compounds from the fiber and influences the degradation kinetics and hydrophilicity of PCL [169]. Additionally, using a low-molecular-weight PEG-\(b\)-PCL copolymer is expected to decrease the average fiber diameter by increasing the jet draw ratio, due to the introduction of polar PEG segments, as well as to reduce the viscosity of the polymer melt. From a viscoelastic perspective, mixing a high-molecular weight polymer (i.e., PCL) with a low-molecular weight polymer (i.e., PEG-\(b\)-PCL) complicates the blend system regarding the difference in physics of molecules as well as on chemistry. Pure PEG-\(b\)-PCL copolymer melt electrospinning did not result in consistent, continuous and uniform fibers due to its low molecular weight, insufficient macromolecular entanglements and Rayleigh instabilities. By contrast blends of PCL and PEG-\(b\)-PCL, for some parameter combinations and certain weight ratios of the two components, were able to produce continuous fibers significantly thinner (i.e., with an average diameter close to 2 \(\upmu\)m) compared to pure PCL [167]. Figure 9: (**a**) 3D-model diagram of hand-held melt-electrospinning apparatus. (**b**) Schematic diagram of hand-held melt-electrospinning apparatus. (**c**) Melt electrospinning schematic by hand-held melt-electrospinning apparatus. Copyright 2020 Elsevier [160]. PEG is also considered as an effective plasticizer for PLA-based systems. Thus, PLA/PEG blends with improved cell affinity, biodegradability, as well as physical and mechanical properties, have been found good candidates for both industrial and biomedical applications [7; 142]. The effects of spinning parameters were investigated by [PERSON] and [PERSON] [170; 171; 172]. Increasing the PEG content, temperature, and voltage decreased the diameter of fibers since it influenced on the viscosity, the quenching, and the stretching of polymer jets.