Self-assembly and chain-folding in hybrid coil-coil-cube triblock oligomers of polyethylene-b-poly(ethylene oxide)-b-polyhedral oligomeric silsesquioxane

Self-assembly and chain-folding in well-defined oligomeric polyethylene-Mocik-poly(ethylene oxide)-block-polyhedral oligomeric silsesquioxane (PE-b-PEO-b-POSS) triblock molecules were studied by small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), and transmission electron microscopy (TEM). The triblock oligomers were synthesized by attaching two kinds of functional POSS molecules, namely, isocyanatopropyldimethylsilylisobutyl-POSS (Ib-POSS) and isocyanatopropyldimethylsilylcyclopentyl-POSS (Cp-POSS), to a hydroxyl-terminated PE-b-PEO-OH diblock oligomer (denoted as E ₃₉EO ₂₃) via urethane reactions. In these triblock oligomers, both PE and POSS were crystalline, whereas PEO became amorphous due to tethering of its both ends to other two blocks. In the crystalline state, PE chains tilted 32° from the lamellar normal, and both Ib-POSS and Cp-POSS molecules stacked into four-layer (ABCA) lamellar crystals, having the same trigonal (R3̄m) symmetry as in pure POSS crystals. Because the cross-sectional area for a PE chain in the PE crystals (0.216 nm ²/chain) at the interface was much smaller than that for a POSS molecule in POSS crystals (1.136 nm ²/molecule), the self-assembly and PE chain-folding were substantially affected by the sequence of PE and POSS crystallization when crystallizing from the melt. For example, PE crystallization induced the POSS crystallization in the bulk E ₃₉EO ₂₃-Ib-POSS, and thus extended-chain PE crystals were observed. The grains of crystalline lamellae again were small with often highly curved lamellar crystals. This could also be attributed to the unbalanced interfacial areas for POSS and PE blocks (the interfacial area ratio being 2.6 for interdigitated PE crystals, i.e., two PE chains per POSS molecule). For the E ₃₉EO ₂₃-CP-POSS triblock oligomer, POSS molecules crystallized before PE crystallization, forming a well-defined lamellar structure. The preexisting POSS crystals confined the PE crystallization, and thus almost once-folded PE crystals were obtained. Because the interfacial area ratio between POSS and PE blocks decreased to 1.3, the crystalline lamellae were straight and their grains were relatively large. On the basis of this study, we conclude that confinement effect plays an important role on chain-folding of crystalline block oligomers, which in turn determines the unbalanced cross-sectional areas between chemically different blocks and finally dictates their final crystalline morphology.