Arman Ashraf

Morphologies of diblock copolymer/homopolymer blends near the order-disorder transition

Abstract The morphologies of three low-molecular-weight poly(styrene-b-isoprene) diblock copolymers and their blends with homopolystyrene (hPS) at 66 vol% styrene have been investigated. Addition of hPS to one copolymer produces lamellar-catenoid or double-diamond morphologies, depending on hPS chain length. In another blend, microstructural disorder arises.

Microphase-separated poly(styrene-b-isoprene)n multiblock copolymers with constant block lengths

Abstract Linear multiblock copolymers, like their diblock analogues, are capable of ordering into periodic microstructures when the blocks are sufficiently incompatible. In this work, a series of four linear poly(styrene-b-isoprene)n (SI)n (1 ≤ n ≤ 4) multiblock copolymers with nearly equal block lengths has been synthesized via living sequential anionic polymerization. All of the copolymers are microphase-separated, as discerned by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS), and exhibit lamellar morphologies in which the microdomain periodicity decreases with n. This behaviour suggests that the middle blocks contract the microdomains along the lamellar normal. Microstructural characteristics are compared with predictions from formalisms proposed for linear multiblock copolymers and, along with conformational considerations, are used to interpret the thermal and tensile properties of the copolymers.

Study of blends with narrow molecular weight distribution: hydrogen and deuterium labeled polystyrene and poly(vinyl methyl ether)

Abstract Polyvinyl methyl ether) PVME with polydispersity of less than 1.1 has been synthesized. This material has been used to study the effects of molecular weight on the phase diagram of blends of PVME with polystyrene (PS). The role of specific interactions in the phase behavior of the PS/PVME system also was studied using specifically deuterated PVME. Dynamic infrared linear dichroism (DIRLD), has indicated that two different chemical environments for the methoxyl groups of PVME may exist. One type of methoxyl interacts with the PS phenyl group such that their reorientational motions under oscillatory strain are synchronized, while another substantial population of methoxyl groups shows no such synchronization.

Bicomponent Block Copolymers Derived from One or More Random Copolymers as an Alternative Route to Controllable Phase Behavior

Block copolymers have been extensively studied due to their ability to spontaneously self-organize into a wide variety of morphologies that are valuable in energy-, medical-, and conservation-related (nano)technologies. While the phase behavior of bicomponent diblock and triblock copolymers is conventionally governed by temperature and individual block masses, it is demonstrated here that their phase behavior can alternatively be controlled through the use of blocks with random monomer sequencing. Block random copolymers (BRCs), i.e., diblock copolymers wherein one or both blocks are a random copolymer comprised of A and B repeat units, have been synthesized, and their phase behavior, expressed in terms of the order-disorder transition (ODT), has been investigated. The results establish that, depending on the block composition contrast and molecular weight, BRCs can microphase-separate. We also report that large variation in incompatibility can be generated at relatively constant molecular weight and temperature with these new soft materials. This sequence-controlled synthetic strategy is extended to thermoplastic elastomeric triblock copolymers differing in chemistry and possessing a random-copolymer midblock.

Synthesis and Morphological Studies of (AB)n Multiblock Copolymers

Abstract While numerous efforts have been put forth to correlate the morphology of microphase-separated diblock and triblock copolymers with molecular characteristics such as composition and chain length, far fewer studies have been devoted to elucidating the role of molecular architecture on microstructure in more complex linear copolymers. This is particularly surprising since many segmented copolymers, for example, have found their way into commercial applications. In this work, we present some of our recent work to bridge the gap between well-defined di/triblock copolymers and segmented copolymers. Here, a novel set of well-defined linear (AB) n multiblock copolymers possessing equal compositions and chain lengths, but different numbers of AB block pairs (n= 1,2,3,4), have been synthesized. Their microphase-separated morphologies have been characterized using both transmission electron microscopy (TEM) and small-angle x-ray scattering (SAXS). A recently-developed thermodynamic model based on confined-chain statistics is presented, and predictions are found to compare well with experimental data.

Effect of Systematic Hydrogenation on the Phase Behavior and Nanostructural Dimensions of Block Copolymers

Unsaturated polydienes are frequently hydrogenated to yield polyolefins that are more chemically stable. Here, the effects of partial hydrogenation on the phase behavior and nanostructure of polyisoprene-containing block copolymers are investigated. To ensure access to the order-disorder transition temperature (TODT) over a wide temperature range, we examine copolymers with at least one random block. Dynamic rheological and scattering measurements indicate that TODT increases linearly with increasing hydrogenation. Small-angle scattering reveals that the temperature-dependence of the Flory-Huggins parameter changes and the microdomain period increases, while the interfacial thickness decreases. The influence of hydrogenation becomes less pronounced in more constrained multiblock copolymers.

Macromolecular self-assembly in dilute sequence-controlled block copolymer/homopolymer blends

Abstract Conventional block copolymers consist of two long contiguous monomer sequences (‘blocks’) that can, in the same fashion as low-molar-mass surfactants, self-assemble into various microstructural elements (e.g., micelles at low copolymer concentrations) to minimize repulsive contacts in the presence of a parent homopolymer. In this work, we explore the existence of segment-specific interactions, as well as the possibility of tailoring these blend morphologies (and producing altogether new ones), with novel sequence-controlled block copolymers. These copolymers are comprised of at least one block that is a random segment composed of both constituent monomer species. Transmission electron microscopy is employed here to examine the bilayered membranes and channel structures that form in two different series of such copolymers in dilute copolymer/homopolymer blends.