A DSC and XPS characterization of core–shell morphology of block copolymer nanoparticles

Self-assembly of amphiphilic block copolymer chains is known to produce core–shell nanoparticles, but imaging techniques have generally failed to provide clear evidence about the multiphase structure. We report herein the advantages and limitations of modulated temperature differential scanning calorimetry (MDSC) and X-ray photoelectron spectroscopy (XPS) for the morphology study of spherical poly(hydroxyethyl acrylate)-b-polystyrene diblock copolymer nanoparticles with an intensity-average diameter of 40 nm. Using lyophilized particles, MDSC is more informative than XPS since it allows the three morphological features of composite latex particles to be distinguished: polystyrene core, poly(hydroxyethyl acrylate) shell, and interface. In MDSC, phase separation is evidenced by two distinct increments of heat capacity (ΔC_p) in the glass transition regions of the two blocks. By measuring ΔC_p values, an interface weight fraction of 70% is measured that gradually decreases to 50% with annealing time (150 °C, 2 h), indicating a higher extent of phase separation.