In this study, a facile solvent vapor annealing (SVA) method is utilized to inscribe hierarchical secondary nanostructures onto electrospun poly(ε‐caprolactone)(PCL)/poly(ethylene oxide) (PEO) blend fibers. By carefully understanding the phase separation and crystallization behavior of PCL/PEO blends during the electrospinning process, one can tune the spatial distribution of the PCL phase, the growth of the PCL crystalline regions, and therefore the amount and even the sensitivity of free amorphous PCL chains in response to acetone vapor. Here, the PEO domains serve as mini‐dividers to restrict the growth of the semicrystalline PCL phase. During acetone vapor annealing, the PEO phase remains largely unchanged while swollen‐free amorphous PCL chains are deposited on pre‐existing PCL or even PEO crystalline lamellae, giving rise to hierarchical structures of high regularity. The morphologies of PCL/PEO hierarchical structures reported in this study are of striking uniformity, further demonstrating the reliability of the facile SVA method, not only for a few layers of thin fiber mats but also for thicker fiber mats.

Polymer‐based electrospun fibers have been intensively studied as antimicrobial membranes, drug carriers, and energetic materials. Inorganic fillers or small molecules have been routinely added into polymer matrices in order to enhance product functions. However, the electrospinning process is kinetically controlled and solvent rapidly evaporates due to the large surface‐to‐volume ratio of spinning liquid jet. When electrospinning a multicomponent system, complex phase behavior may occur and give rise to interesting internal structures of resulting products. Such kinetically driven phenomena deserve more attention for optimizing product performance. Here, electrospun poly(ε‐caprolactone)(PCL)/aminopropyl‐heptaisobutyl‐polyhedral oligomeric silsesquioxane (AMPOSS) fibers with AMPOSS content up to 30 wt% are studied as a model system to understand the impact of kinetically controlled phase separation on the fibers' internal structure, properties, and thermal stability. With sufficient AMPOSS loading, the hybrid fibers are found to have an AMPOSS‐shell/PCL‐core structure. The thermal stability of the as‐spun PCL/AMPOSS fibers is therefore greatly enhanced.

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