Blends of poly(ethylene oxide) (PEO) and poly(methyl acrylate) (PMA) as well as the respective diblock copolymers PEO-b-PMA form a homogeneous melt and undergo. crystallization of PEO upon cooling. Although an identical PEO (M-n = 5000 g/mol) is used in blends and diblock copolymers, crystallization and melting behavior at comparable PMA contents differs strongly as revealed by temperature-resolved small-angle X-ray scattering (TR-SAXS) and differential scanning calorimetry (DSC) measurements. After isothermal crystallization, PEO lamellae in the blends thicken during heating from once-folded to extended chain crystals prior to melting as revealed by TR-SAKS. Contrarily, in PEO-b-PMA a thickening to extended chain lamellae is impossible when the PMA block exceeds an M of about 3000 g/mol. This behavior is caused by a balance between the tendency of the crystallizable block to form extended chain crystals, the tendency of the noncrystallizable chains to adopt a maximum in conformational entropy, and the space requirements of the these chains at the crystalline amorphous interface. Thus, chain-folded crystals of PEO are formed as a compromise when the noncrystallizable chains become sufficiently long and can be considered to be in thermodynamic equilibrium. Equilibrium melting temperatures for neat PEO, T-m(0), and for PEO in blends and diblock copolymers, T-m,b(0), are determined using both the Hoffman-Weeks and the Gibbs-Thomson approach. Values determined by the Hoffman-Weeks method are generally lower compared to values obtained by the Gibbs Thomson approach, which can be explained by inherent differences in extrapolation procedures. The maximum equilibrium melting point depression T-m(0) - T-m,b(0) is found to be 2 K in blends and 7 K in diblock copolymers. In the case of blends, melting point depression can be explained by Flory's entropy contribution of miscible polymer blends.