Crystallization and Melting of Poly(ethylene oxide) in Blends and Diblock Copolymers with Poly(methyl acrylate)

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-SAXS. Contrarily, in PEO-b-PMA a thickening to extended chain lamellae is impossible when the PMA block exceeds an M_n 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_m⁰, and for PEO in blends and diblock copolymers, T_m,b⁰, 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_m⁰ − T_m,b⁰ 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.