Synthesis and characterization of PEO–PCL–PEO triblock copolymers: Effects of the PCL chain length on the physical property of W1/O/W2 multiple emulsions

A series of poly(ethylene glycol)-block-poly(ɛ-caprolactone)-block-poly(ethylene glycol) (PEO–PCL–PEO) triblock copolymers were prepared and then used for the investigation of the effects of the ratio of ɛ-caprolactone to poly(ethylene glycol) (i.e., [CL]/[EO]) on the physical properties of water-in-oil-in-water (W1/O/W2) multiple emulsions containing a model reagent, ascorbic acid-2-glucoside (AA2G). In the synthesis, the [CL]/[EO] was varied from 0.11 to 0.31. The molecular weights and compositions of PEO–PCL–PEO were determined by GPC and 1H NMR analyses. Thermal behavior and crystal formation were studied by DSC, XRD, FT-IR, and polarized optical microscopy (POM). Aggregate behavior of PEO–PCL–PEO was confirmed by DLS, UV, and 1H NMR. Morphology and relative stiffness of the W1/O/W2 multiple emulsions in the presence of PEO–PCL–PEO were studied by confocal laser scanning microscopy (CLSM) and rheometer. Variation in the [CL]/[EO] significantly affects the crystalline temperature and spherulite morphology of PEO–PCL–PEO. As the [CL]/[EO] increases, the CMCs of PEO–PCL–PEO decreases and the slope of aggregate size reduction against the copolymer concentration becomes steeper except for the lowest [CL]/[EO] value of PEO–PCL–PEO (i.e., P-222). P-222 significantly increases the viscosity of continuous (W2) phase, which implies the copolymer would exist in the W2 phase. On the other hand, the triblock copolymers with relatively high [CL]/[EO] ratios mainly contribute to the size reduction of multiple emulsions and the formation of a firm wall structure. The particle size of the multiple emulsion decreases and the elastic modulus increased as [CL]/[EO] increases, confirmed by microscopic and rheometric analyses.