Study on the condensed state physics of poly(ε-caprolactone) nano-aggregates in aqueous dispersions

Narrowly size distributed spherical, ellipsoid-like and lamellae stacked poly(ε-caprolactone) (PCL) nano-aggregates in aqueous dispersions with a diameter ranging from about (∼) 50 to 330 nm were prepared via nanoprecipitation method in the present study. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) were used to characterize the morphology and size of the PCL nano-aggregates. We investigated the melt behaviors of the original (without any thermal treatment after preparation) PCL nano-aggregates in aqueous dispersions by nano differential scanning calorimetry (nano-DSC). In particular, the condensed state of the original ∼50 nm PCL nanospheres was demonstrated to be amorphous as a result of exhibiting no melting peak in the first nano-DSC heating scan. Furthermore, the rubbery↔flow condensed state transition of the amorphous PCL nanospheres was explored by fluorescence measurements. Moreover, the confined crystallization of the ∼50 nm PCL nanospheres from rubbery state in aqueous dispersions was investigated via isothermal crystallization process. Enormous supercooling was observed during crystallization due to nanoconfinement effect. In addition, when the diameter of the original PCL aggregates was increased to more than 150 nm, PCL is in semi-crystalline state and the crystallinity increases with the diameter.