Structure and Crystallization Behavior of Poly(ethylene oxide) (PEO) Chains in Core-Shell Brush Copolymers with Poly(propylene oxide)-block-poly(ethylene oxide) Side Chains

Core-shell brush copolymers featuring a poly(p-hydroxystyrene) (PHOS) backbone and PPO-b-PEO (PPO and PEO stand for poly(propylene oxide) and poly(ethylene oxide)) side chains with different molecular compositions and exhibiting two inverse molecular architectures in regard to the side chains were investigated. Differential scanning calorimetry (DSC) and temperature-resolved wide- and small-angle X-ray scattering (WAXS/SAXS) were used to characterize the thermal and structural behavior. For the sample with the crystallizable PEO block linked directly to the backbone and a high PEO fraction (84.9 wt %), our results reveal a PEO crystallization/melting behavior similar to the one of bulk PEO. Surprisingly, the crystalline order, as determined by WAXS, persists up to 30 K above the melting point determined by DSC (T_m = 54 °C). For the samples where the PPO block is directly linked to PHOS backbone and the PEO chains are dangling, our results indicate that the side arm architecture has remarkable effects on the thermal and structural behavior. With decreasing PEO fraction in the side arms, the calorimetric crystallization temperature, T_c, and the melting point, T_m, of the PEO domains are strongly suppressed, reaching values as low as -45 °C and -8 °C, respectively. Furthermore, PEO crystallizes in an asymmetric lamellar phase with a distorted PEO crystalline phase. Above T_m the morphology changes from microphase-separated symmetric lamellae to hexagonally perforated lamellae with PEO domains immersed within a PHOS/PPO matrix with decreasing PEO fraction. Our results suggest that this specific brush copolymer architecture allows for tuning the ability of PEO blocks to crystallize.