*2.2.2 Tg*

The Tg value may impact the kinetics of drug release from delivery systems. For instance, PLGA micro/nanoparticles were prepared via the emulsion solvent evaporation method [6, 7, 21–23]. The residual solvent was removed via evaporation to harden the nanoparticles. Solvent evaporation promotes the transition to the glassy state from the rubbery state, as observed by hardening. As a result, the removal of solvents is comparable to the glass transition caused by reducing the temperature below Tg. Additionally, drug loading in micro/nanoparticles can decrease Tg through plasticizing effects. Increased drug loading leads to drug dispersion throughout the polymeric matrix, which increases the distance between polymeric chains. This causes the free volume to increase and reduces the Tg of the polymer [23]. The formation of glassy PLGA affects delivery systems by, for example, lowering the elastic modulus and transfer rates of water and the drug molecules and slowing the degradation and drug release rates. In comparison, the formation of the rubbery state facilitates higher mass transfer rates, and a higher elastic modulus leads to faster degradation and drug release rates.
