Network morphologies in monodisperse and polydisperse multiblock terpolymers.

Abstract

Multiply continuous network morphologies were previously identified in “monodisperse” (polydispersity index (PDI) < ~1.1 in all blocks) poly(isoprene-bstyrene- b-ethylene oxide) (ISO) triblock terpolymers. This work extends the investigation of multiply continuous network structures to two other classes of multiblock terpolymers: i) “monodisperse” OSISO pentablocks and ii) polydisperse ISO triblocks. The OSISO pentablocks are synthesized using a protected initiation strategy that required the development of the functional organolithium 3-triisopropylsilyloxy-1- propyllithium (TIPSOPrLi). TIPSOPrLi may be used to prepare α-hydroxypolystyrene with narrower molecular weight distributions (PDI ~ 1.1) than are attainable using the commercially available 3-tert-butyldimethylsilyloxy-1-propyllithium. A telechelic triblock terpolymer (HO-SIS-OH) with narrow molecular weight distributions in all blocks is prepared using TIPSOPrLi. A series of OSISO pentablocks is synthesized from this parent triblock, and a stable region of O70 (the orthorhombic Fddd network) is identified between two-domain lamellae (LAM2) and three-domain lamellae (LAM3) in OSISO materials. This sequence of morphologies was previously reported in ISO triblocks with comparable compositions. Mechanical tensile testing reveals that an OSISO sample with a lamellar mesostructure fractures in a brittle fashion at a strain of 0.06. An OSISO containing the O70 network, in contrast, has a strain at failure of 1.3, even though the crystallinity of the terminal blocks is above the brittle threshold established in other multiblock materials. This improved toughness is attributed to the combined effects of a triply continuous morphology and an intrinsically tough SIS core.

The ISO triblock studies probe the stability of network morphologies with respect to polydispersity in the polystyrene and poly(ethylene oxide) chains. Three series of ISO triblocks with polydisperse (PS PDI = 1.16, 1.31, 1.44) polystyrene blocks are prepared by anionic polymerization. While the network “window” in the PS PDI = 1.16 series is comparable in width and location to the window reported in the “monodisperse” ISO materials, it apparently shrinks for the higher PS PDI values. Only lamellar mesostructures are reported in the PS PDI = 1.31 materials, and network morphologies are identified over only a narrow range of compositions in the PS PDI = 1.44 samples. Polydispersity does not always destabilize network morphologies, however, as broadening the molecular weight distribution of the terminal poly(ethylene oxide) block drives a morphological transition from lamellae to the coreshell gyroid network. This result demonstrates that polydispersity can be used to tune block terpolymer phase behavior and stabilize technologically useful network mesostructures. Self-consistent field theory calculations augment the experimental analysis and offer insight into the physics underlying the polydispersity-driven morphological changes.