# 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
#### 3.3.2 Particle Size and Size Distribution  
Size distribution of particles increases with increasing polymer concentration and/or polymer molecular weight, related to an increase in the solution viscosity, and a decrease in conductivity. It was also expected that at low polymer concentration, the changes in concentration have more variation in viscosity compared with conductivity. At high concentration, the viscoelastic forces in the droplets are opposed to those of Coulomb repulsion to prevent droplet break-up, resulting in the formation of bigger particles. Thus, the increase in PCL concentration from 0.5 to 6 (wt.%) induces an increase in particle size according to [PERSON] et al. (Table 4) [147]. When PCL concentration increases further, to 9.6%, PCL particle size decreases as observed by [PERSON] et al. (Figure 6) [95]. In their work, [PERSON] et al. studied the effect of the variation in the molecular weight of PCL (from 10,000 to 45,000 g/mol) at various concentrations in toluene [126]. They found that changes in formulation parameters leading to an increase in the viscosity of the solution also lead to an increase in size. Some researchers have also reported that working with very low polymer concentrations, weak viscosity makes the process unstable and therefore particle size increase. Besides, at higher concentration, the observed decrease in conductivity leads to an unstable cone jet and therefore an increase in size particle distribution. Besides, the increase in viscosity also results in a decrease in jet diameter [148], which can be correlated with a lower mean droplet diameter during the rupture process according to [PERSON] et al. and [PERSON] et al. [66; 69; 74; 76]. Figure 6: Scanning electron micrographs of dried PCL microparticles. PCL concentration in chloroform is: (a) 5%, (b) 7.4%, (c) 8.7%, (d) 9.6% (a/v). Electrospraying conditions are 26 G for needle gauge, 20–25 cm for tip-to-collector distance, 0.5 mL/h for flow rate, and 10 kV for voltage. The molecular weight of PCL on average is 130 kg/mol with a polydispersity index of 1.45. Scale bar is 10 \(\upmu\)m (reprinted with permission from [95]). The surface tension of the electrospray solution influences the breakage of the charged droplets and thus the final particle size. Indeed, the particle size increases with decreasing surface tension [92]. According to [PERSON] et al., the use of higher concentrations of PCL produces larger particles due to increased surface tension [128]. However, during the electrospraying process, due to the viscosity changes and evaporation of the solvent, it is difficult to determine the value of the variation of the surface tension at equilibrium at the time scale of the phenomenon. Therefore, the relationship between particle size and surface tension is not always correlated or interpreted as such. During the solidification of the droplets, the size is influenced by the polymer-polymer interactions that take place during the evaporation of the solvent to obtain solid particles. Thus, a high vapor saturation pressure value of the solvent induces its evaporation when passing from the nozzle to the collector. For low values, the evaporation rate is low, and the residual solvent limits the condensation of the macromolecular chains and causes the particles to aggregate on the collector. Besides, the ability of the solvent to evaporate during the process is also related to the concentration of the solutions. An increase in the latter due to a higher rate of entanglement allows complete evaporation during flight time and promotes the aggregation of macromolecular chains. The use of the electrospraying process allows obtaining monomodal and narrow particle size distribution, which promotes a controlled release behavior of the drug. Its release rate, as well the shell erosion phenomena, is enhanced for particles with a low mean diameter and a narrow size distribution. These kinds of particles are mainly used in biomedical engineering and medical treatment.