Glassy polymer nanofibers with spatially confined poly(ethylene oxide) (PEO) were fabricated by coaxial electrospinning of PEO with polyacrylonitrile (PAN) or polystyrene. The effect of melt-annealing on the crystallization behavior of the confined PEOs was studied using differential scanning calorimetry. It is found that the crystallization behavior of the confined PEOs varies with annealing temperature (T-a), annealing time (t(a)), and molecular weight of PEO. Notably, it is observed that the crystallization temperature (T-c) and melting temperature (T-m) of PEO increase with prolongation of t(a), for PEO600K/PAN and PEO2K/PAN coaxial electrospun fibers. This phenomenon can be interpreted by the annealing-induced demixing at the core-sheath interface. After the coaxial electrospinning, the core and sheath of the PEO/PAN coaxial fibers are partially compatible due to the miscible solvents used for the core and sheath polymers. Upon annealing, demixing occurs at the core-sheath interface, leading to improved crystallizability of PEO. (C) 2017 Wiley Periodicals, Inc.