This study determined the phase and crystallization behaviors of blends composed of asymmetric polystyrene-block-poly(ethylene oxide) (PS-PEO) and symmetric polystyrene-block-poly(methyl methacrylate) (PS-PMMA). The PS blocks in the various binary block copolymers exhibited nearly identical molecular weights, whereas the molecular weight ratios of PEO and PMMA varied. The compatibility of the PEO and PMMA chains aided the binary block copolymers in co-ordering in a lamellar microdomain morphology, with the PEO and PMMA blocks sharing a common microdomain. Adding short tethered PMMA chains to long tethered PEO chains led to a decrease in the common microdomain spacing and an increase in the grafting density. These behaviors increased PEO chain stretching, causing macrophase separation. The mismatch in PEO and PMMA block lengths divided the common PEO/PMMA microdomain into two sections: the coexisting PEO/PMMA section close to the microdomain interface and the neat PEO section far away from it. The high-glass-transition-temperature PMMA reduced PEO chain mobility, inhibiting PEO crystallization in the coexisting PEO/PMMA section but not in the neat PEO section. When the block length ratio of PEO to PMMA decreased, the neat PEO section narrowed. The increase in the extent of PEO confinement led to a reduction in PEO crystallizability.