Study on Unconventional Morphologies by Linear A1B1A2B2 Tetrablock Copolymers

Abstract

Block copolymers have received great attention due to their various morphologies such as spheres, cylinders and lamellae which depend on the volume fraction (f), degree of polymerization (N), and the Flory-Huggins interaction parameter (χ). Though the phase behavior and morphology of linear AB diblock or ABA triblock copolymers have been extensively investigated, those of linear multiblock copolymers composed of more than four blocks have been rarely studied due to difficulties on polymerization. However, recent advance of synthetic skills and simulation techniques have provided opportunities for studying phase behavior of multiblock copolymers. Because the chain configurations of multiblock copolymers are quite different from that of diblock (or triblock) copolymers, multiblock copolymers show novel morphologies not attained by AB diblock or ABA triblock copolymers. In this thesis, I studied the phase behavior and morphology of linear A1B1A2B2 tetrablock copolymers. For this, I polymerized unimodal and well-defined polystyrene-block-polyisoprene-block-polystyrene-block-polyisoprene tetrablock copolymers (S1I1S2I2) with various volume fractions. Then, I investigated the unique morphology and phase behavior. In chapter 2, I investigated the phase behavior of S1I1S2I2 with various volume fractions of PI1 block (fPI1), while maintaining the symmetric volume fraction of total PS blocks and PI blocks (fPS1+fPS2: fPI1+fPI2 ≈ 1:1). S1I1S2I2 was polymerized by sequential anionic polymerization. I found that even at the same overall volume fraction of PS and PI blocks, diverse morphologies were observed as fPI1 was increased: lamellae → asymmetric lamellae → hexagonally packed PI-cylinders → double gyroid with PI-network domains → short-period lamellae. The domain spacing gradually decreased as fPI1 increased. Particularly, cylindrical and gyroid morphologies were observed depending on fPI1 at symmetric overall volume fraction and intermediate segregation. The experimentally observed morphologies were consistent with the predicted equilibrium phases by the self-consistent mean field theory (SCFT). In chapter 3, I studied the order to order transition (OOT) of the S1I1S2I2 with the symmetric volume fraction of total PS and PI blocks, but asymmetry of PI block (τPI = fPI1/(fPI1+fPI2) = 0.19). Very interestingly, this linear tetrablock copolymer showed cylindrical to lamellar microdomains transition upon heating with upper critical ordering transition (UCOT). Such behavior is exactly the opposite of commonly observed OOT for linear AB diblock or A1BA2 triblock copolymers, where lamellar microdomains have been transformed to cylindrical microdomains upon heating. In chapter 4, I found that S1I1S2I2 in a selective solvent showed the core-satellite micellar structures which consist of a large central core and small satellites surrounding the core. When a PS-selective solvent, for example, dimethylacetamide (DMAc) or diethylphthalate (DEP), was used, the PI blocks formed the inside core, while the PS blocks became a shell as long as the volume fraction of mid PI1 block (fPI1) is large enough (0.2) to form loop conformation resulting in merging into PI spherical cores consisting of the PI2 block. However, for smaller fPI1 (0.08), the short mid PI1 blocks could not merge into the PI2 core; thus separately small spherical micelles, which is referred to as the “satellite” micelles.