Nanostructured poly(ethylene oxide)-like dendron-block-linear poly(ethylene-alt-propylene) copolymers: design, synthesis, and thermal and assembling properties

Abstract

A series of self-assembling dendron-coil block copolymers with well-defined molecular structures were prepared via a divergent method. The hydrophilic and hydrophobic blocks are composed of PEO-like dendrons and poly(ethylene-alt-propylene) (PEP) linear coils, respectively. A dendron-coil, 2G-2400 (2G and 2400 indicate the dendron generation and PEP molecular weight, respectively), showed a melting transition of the peripheral PEO chains. In contrast, the dendron-coils containing the 3^rd generation dendron did not show any melting transition because of the plasticization effect of the tri(ethylene oxide) branches. Due to the strong immiscibility between the PEO-like dendron and PEP blocks, diverse microphase-separated morphologies were observed. 2G-2400 displayed a lamellar mesophase with an interdigitated dendron packing structure. For the larger 3^rd generation dendron series, A15 micellar cubic (Pm3n space group), hexagonal columnar, and lamellar morphologies were revealed depending on the PEP coil length and/or temperature. For the columnar mesophases, the molecular wedge angles (α) were calculated to be 46.8° and 34.6° for 3G-2400 and 3G-3400, respectively. This suggests that the longer PEP coil of 3G-3400 is conformationally more deformed in the columnar structure. As a consequence, the columnar phase was transformed into an interdigitated lamellar structure as the temperature increased. This order to order transition is a reversal of the well-known phase sequence of linear block copolymers. This unusual morphological behavior is mainly attributed to the unique molecular arrangement associated with non-conventional dendron-coil chain architecture.