Complex Thermal and Bulk Assembling Properties of Dendritic-Linear-Dendritic Triblock Copolymers Depending on the Length of the Middle Block

Abstract

We report the synthesis, as well as the complex thermal and assembling properties, of a series of three triblock copolymers 1-3, each of which is based on aliphatic polyether third-generation dendron ends that contain tetradecyl peripheries and a hydrophilic linear poly(ethylene oxide) (PEO) middle block. Because of the large cross section of the third-gene ration dendron ends and the amorphous PEO-like dendritic core, the crystallinity of the central PEO coil in each block copolymer is significantly reduced. In particular, copolymers 1 and 2 with relatively short PEO coils (M-n = 1000 and 2000 g/mol) revealed no PEO melting. Also, the crystallinity of the peripheral tetradecyl chains was investigated, depending on the PEO morphology. Copolymers 1 and 2, both with amorphous PEO, coils, had similar degrees of the tetradecyl crystallinity, 33% and 34%, respectively, while copolymer 3, with a crystalline PEO coil (M-n = 4000 g/mol), showed a somewhat higher value of 41%. This is probably due to the hard confinement effect caused by the crystalline PEOs during the tetradecyl crystallization of copolymer 3. In addition to surveying the thermal behavior of the copolymers, their self-assembling behavior was surveyed in the crystalline and liquid crystalline (mesophase) states by SAXS and TEM techniques. The copolymers showed a complex bulk morphological behavior as a function of the PEO coil length. In the crystalline state, copolymers 1 and 2 exhibited columnar assemblies, but they showed different column packing (i.e., oblique and rectangular symmetries for 1 and 2, respectively). Meanwhile, copolymer 3 formed lamellar structures in both of its crystalline states. In the molten states, all copolymers showed microphase-separated microstructures, as indicated by the SAXS and TEM results. Copolymer 1 exhibited a cylindrical mesophase without a long-range order, while copolymers 2 and 3 showed hexagonal olumnar and disordered cylindrical mesophases as temperature increased.