# 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
#### 3.3.3 Morphology  
The electrospray process makes it possible to obtain various morphologies depending on the properties of the solutions used and the settings of the equipment. Morphological control is an important criterion to consider given their specific properties. These morphologies, whether porous, hollow, pleated, cup-shaped, semi-spherical, oval, or polygonal, are mainly related to the mechanisms of solidification of droplets, and in particular to the phenomena governing the change of state or the evaporation of solvents used according to environmental conditions. The transition from a liquid phase to a solid polymer phase is related to the vitrification of a rubber phase to an amorphous or glassy state of the polymer when the solvent evaporates. [PERSON] et al. identified that particle morphology varied according to the dynamic regime of the fluid at which polymer vitrification occurs, and for reduced polymer concentrations, i.e., (i) incomplete jet failure, (ii) complete jet failure without Coulomb instabilities, (iii) Coulomb instabilities without progeny droplets emission, and (iv) Coulomb instability of main droplets with progeny droplets emission (coulomb fission) [151]. The first regime leads to the production of main particles surrounded by thin nanofibers, which have a secondary bead, the second to globular particles, the third to particles with one or two different filaments, or having elongated shapes, and the last to particle residues from droplets of progeny from Coulomb droplet fissions. The molecular weight of the polymer used and its concentration favors a particular structure. Thus, low concentrations and/or low molecular weight of the polymer lead to the first morphology. The increase in molecular weights and concentrations allows globular particles to be obtained first, then in a second phase at pearled structures. A relatively high concentration of polymer correlated with higher molecular weights allows maintaining a sufficient macromolecular chain entanglement rate to lead to a spherical morphology by promoting its precipitation during the evaporation of the solvent. The characteristics of the solvent, such as its saturation vapor pressure or evaporation rate, also influence morphologies. Thus, the use of a low-volatility solvent produces spherical particles with a smooth surface, but which tend to deform when they reach the collection in semi-spherical particles. On the contrary, a solution containing a more volatile solvent leads to the creation of hollow, porous and irregular particles, linked to an excessive evaporation rate and the precipitation of macromolecular chains [152]. Also, the number and size of pores on the surface of PCL microspheres also decrease with the increase in PCL concentration from 2 to 4% by weight. This porosity can be controlled by adding a non-solvent to the initial solution [153]. Due to differences in solubility and compatibility between two solvents and the polymer, the polymer tends to diffuse from the low-affinity solvent to the one where it is most soluble, leading to phase separation. Thus, concerning differences in evaporation rate, the polymer-rich phase allows the formation of matrix particles, and the poor one to more porous structures. This non-solvent can also be used in a bath to collect particles and leads to porous and non-homogeneous structures [49; 152]. Due to the concentration gradient between the droplets and the collection medium, the solvent present in the electrosprayed droplets tends to diffuse into the bath, causing phase separation and solidification of the particles. Also, with the extraction of the solvent and the solidification of the droplets in the bath, part of the non-solvent is also encapsulated in a polymer matrix. After drying, the non-solvent evaporates to form pores.