# Influence of Solvent Selection in the Electrospraying Process of Polycaprolactone
## 3 Selection of Solvent Systems Used in PCL Electrospraying
### Solution Properties for Tailoring Characteristics of Electrosprayed PCL Particles
#### 3.3.1 Droplets or Fibers  
Electrohydrodynamic methods are divided into two categories having the same processes and mechanisms but differ in the obtained morphologies (spherical and fibrous materials), i.e., electrospinninging and electrospraying processes. The generation of materials with different shape mainly depends on the fission process of the charged droplet, and more especially on the strength of the intermolecular interactions between macromolecular polymeric chains. Fiber shape is obtained with strong interactions since they stabilize the charged jet and match the Coulomb repulsions of the charged droplets. The polymeric solution is further stretched, and the solvent evaporation leads to the formation of ultrathin fibers [145]. The formation of tiny particles is promoted by the presence of low interactions and strong Coulomb repulsions. Thus, the extent of intermolecular entanglement among PCL chains in working solution is one of the main factors governing the resulted shape [95]. Limited polymeric chain entanglements in the system represent a key parameter to achieve particle morphology by electrospraying according to [PERSON] et al. [46]. The degree of entanglement per chain in the electrosprayed solution (\(\left(n_{e}\right)_{sol}\)) is obtained from Equation (11). \[\left(n_{e}\right)_{sol}=\frac{\varphi M_{w}}{M_{e}} \tag{11}\]  
where \(\varphi\) is the polymer volume fraction in the electrosprayed solution, \(M_{w}\) and \(M_{e}\) are the average molecular weight of polymer and entanglement molecular weight, respectively. The number of entanglement per chain, (\(\left(n_{e}\right)_{sol}\)), and the critical chain overlap concentration, \(C_{ov}\), corresponding to the crossover concentration between the dilute and the semi-dilute concentration regimes or the concentration inside the radius of gyration of every single macromolecular chain [45], influence the obtained morphology of the final material. Thus, for (\(\left(n_{e}\right)_{sol}\)) \(\sim\) 2.5 (1 entanglement per chain), or a polymer concentration less than 3\(C_{ov}\), the process leads to the formation of spherical particles. The increase of (\(\left(n_{e}\right)_{sol}\)) up to 3.5 or the use of a polymer concentration >3\(C_{ov}\) allow having the entangled regime and therefore obtained in the first step a beaded-fiber structure, which evolves in fiber shape morphology. The entanglement is also related to the molecular weight of the polymer used [98] (Figure 5). Thus, the use of PCL with a molecular weight between 10,000 and 45,000 g/mol and a concentration of 9 to 30% by weight leads to the formation of spherical particles. At higher molecular weights, fibrous morphology is preferred, and when it reaches 80,000 g/mol, even working at a relative low concentration, from 0.5% to 10% by weight, ultrafine fibers are also obtained. Besides, the degree of entanglement of the polymer chains of the electrosprayed solution is also related to the nature of the solvents used. Thus, intermolecular tangles are of two kinds, i.e., (i) interactions between solvents and molecular solutes, (ii) intermolecular tangles between macromolecular chains. The calculation of the parameter (D) is used to guide the approach, since a low value illustrates a good solubility of the polymer in the solvent and a low degree of entanglement  
Figure 5: The effects of polymer molecular weight and solution concentration on the results of PCL electrospraying (**a**) smooth fibers from PCL with 80,000 g/mol, 10 wt.%; (**b**) beaded fibers from PCL with 80,000 g/mol, 0.5 wt.%; (**c**) particles from PCL with 45,000 g/mol, 9 wt.%; (**d**) particles from PCL with 45,000 g/mol, 11 wt.%; (**e**) particles with fibrils from PCL with 45,000 g/mol, 30 wt.%; (**f**) particles from PCL with 10,000 g/mol, 30 wt.% (reprinted with permission from Taylor & Francis Ltd., [[http://www.tandfonline.com](http://www.tandfonline.com)]([http://www.tandfonline.com](http://www.tandfonline.com)) [98]).