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ABSTRACT We investigate the formation of poly(L‐lactic acid) (PLLA) nanoparticles using a green solvent, propylene carbonate (PC), via a combination of nonsolvent‐induced phase separation (NIPS) and temperature‐induced phase separation (TIPS). Water (W) is used as the nonsolvent to induce liquid/liquid phase separation (LLPS) of the PLLA/PC solution at high temperature. Reducing the solution temperature can induce a second LLPS, followed by PLLA crystallization. Three types of polymer nanoparticles (PNPs), including PLLA single crystals, spherical Ouzo crystalsomes, and sheaf‐like crystals named cascade PNPs (CasPNPs), have been observed. The crystalline structure and thermal behavior of the PNPs have been investigated using Wide‐Angle X‐ray diffraction, Fourier Transform Infrared Spectroscopy, and Differential Scanning Calorimetry. A PNP morphology map of the PLLA/PC/W ternary system has been established. The complex PNP morphology is attributed to the two levels of cascade LLPS coupled with confined PLLA crystallization. Our results demonstrate that LLPS directs the formation of both PLLA spherical particles and curved single crystals, with improved crystallization kinetics. 

Abstract Nanoparticle (NP) assembly has been extensively studied, and a library of NP superstructures has been synthesized. These intricate structures show unique collective optical, electronic, and magnetic properties. In this work, we report a bottom‐up approach for fabricating spherical gold nanoparticle (AuNP) assemblies that mimic colloidosomes. Co‐crystallization of lipoic acid‐end‐functionalized poly(ethylene oxide) (PEO) and AuNPs in solution via a self‐seeding method led to the formation of hollow spherical NP assemblies named nanoparticle crystalsomes (NPCs). Due to the spherical shape, the translational symmetry of PEO crystals is broken in NPCs, which can be attributed to the competition between NP close packing and polymer crystallization. This was confirmed by tuning the NPC morphology via varying the self‐seeding temperature, crystallization temperature, and PEO molecular weight. We envisage that this strategy paves the way to attaining exquisite morphological control of NP assemblies with broken translational symmetry.