Observation Of A Non-constant Mean Curvature Interface In An Abc Triblock Copolymer

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Morphological transitions in an I2S simple graft block copolymer: From folded sheets to folded lace to randomly oriented worms at equilibrium

A new equilibrium morphology consisting of randomly oriented wormlike micelles dispersed in a continuous matrix is observed in a neat, strongly segregated I2S simple graft block copolymer. The equilibrium nature of the worm phase is determined via a set of selective solvent casting and prolonged annealing experiments. Transmission electron microscopy (TEM) experiments on quenched samples allow a unique opportunity to directly observe the transition of a kinetically trapped, nonequilibrium folded-layer morphology, formed by casting the sample with a solvent selective for polyisoprene (PI), into the equilibrium, randomly oriented worm phase through an intermediate folded-lace morphology. The folded-lace intermediate is similar to the "mesh" structure previously observed by Hashimoto et al. in starblock/ homopolymer blends.1 The simple graft block copolymer, formed by grafting a single polystyrene (PS) chain onto the center of a polyisoprene backbone, introduces a 2:1 PI/PS arm number asymmetry in the microphase separated state. The 0.81 volume fraction of the PS graft is theoretically predicted2 to be the first volume fraction of graft large enough to force the two PI arms per molecule to the concave side of the PI/PS interface in the microphase separated state. This unique volume fraction, coupled with the novel graft architecture, seems to frustrate the system from choosing a lattice during the microphase separation process

I5S miktoarm star block copolymers: Packing constraints on morphology and discontinuous chevron tilt grain boundaries

A morphological study of three I5S six-arm miktoarm star block copolymers is presented. These miktoarm stars are comprised of five arms of polyisoprene (PI) and one arm of polystyrene (PS) joined together at a single junction point. The strong segregation limit theory for the morphological behavior of miktoarm stars predicts that these materials should form spherical morphologies, but only lamellar and cylindrical morphologies were observed by TEM and SAXS. These results are similar to previously reported discrepancies between experimentally observed morphological behaviors of miktoarm stars and the predictions of the theory. Previous work has attributed the discrepancies to the neglect of the effect of the multifunctional junction points on calculated free energies. The current results suggest that, in addition to this, geometrical packing constraints prevent the formation of morphologies such as spheres and cylinders in highly asymmetric miktoarm stars in which the minor volume fraction component would need to occupy the matrix phase. Finally, unusual broken chevron tilt grain boundary morphologies were observed in a lamellar I5S material. We attribute these new structures to the asymmetric energy penalties for interfacial bending which result from the molecular asymmetry of the miktoarm stars

Morphology of vergina star 16-arm block copolymers and scaling behavior of interfacial area with graft point functionality

The morphological behavior of three well-defined miktoarm star block copolymers having 16 arms/molecule was characterized using transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS) techniques. The molecules, called Vergina stars, have 8 arms of polyisoprene and 8 arms of polystyrene radiating from a single junction point. The samples, with polystyrene volume fractions of 0.37, 0.44, and 0.47 and total molecular weights ranging from 330 000 to 894 000, were all found to microphase separate into lamellar morphologies. In this respect all three samples, in agreement with theory, behave in the same way as linear diblock copolymers of the same relative volume fractions. Incorporating results from previous studies in the literature of miktoarm block copolymers containing trifunctional and tetrafunctional branch points, as well as the new Vergina star data, the scaling behavior of the area per junction versus junction functionality was investigated

Morphological behavior of ASB miktoarm star block copolymers

A morphological study of three AsB, six-arm miktoarm star block copolymers is presented. The miktoarm stars are comprised of five arms of polyisoprene and one arm of polystyrene joined together at a single junction point. The strong segregation limit theory for the morphological behavior of miktoarm stars predicts that these materials should form cylindrical (two samples) and lamellar (one sample) morphologies, but only lamellar morphologies were observed by TEM and SAXS. These results are similar to previously reported discrepancies between miktoarm star morphological behavior and the predictions of the theory. Combining the data of this study with that of previous morphological studies of miktoarm star materials, we can track the increasing discrepancy between the experimentally observed morphology and theoretical predictions as the molecular asymmetry parameter, e, increases. The AsB materials in this study were also observed to form exceptionally well-ordered morphologies. © 1999 American Chemical Society

Phase behavior of I2S single graft block copolymer/homopolymer blends

This work is part of an extensive study of model nonlinear block copolymer/homopolymer blends. Effects of graft molecular architecture on the morphology of block copolymer/homopolymer blends have been examined. The single graft Y-shaped block copolymers used in the study are I2S block copolymers, which have two low polydispersity (PDI) polyisoprene arms and one low PDI polystyrene arm joint at a single junction point. Previously reported linear diblock copolymer/homopolymer blend systems showed that the order-order transitions (OOTs) occur at about the same volume fractions as in pure linear diblock copolymers. The OOT occurs at the same volume fraction regardless of the direction from which it is approached, i.e., blending homopolymer A with a diblock which forms A cylinders in a B matrix to push it toward lamella or blending B homopolymer with a lamellar diblock to push it back toward cylinders. This study shows that when a homopolymer is blended with an I2S block copolymer, the OOTs split so that they occur at different volume fractions depending up whether they are approached by blending homopolymer into the two-arm or the one-arm side of the block copolymer interface. A perforated lamellar morphology is obtained in a blend of homopolystyrene (hPS) and a lamella forming single graft block copolymer, and it is found to be stable to thermal annealing

Morphology of model graft copolymers with randomly placed trifunctional and tetrafunctional branch points

The morphologies of two series of model graft copolymers were studied using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Both series of materials have monodisperse polybutadiene (PB) backbones and monodisperse polystyrene (PS) graft blocks. In one series there are on average five trifunctional junction points randomly distributed along the PB backbone. Each junction point grafts one PS block to the backbone. In the other series there are on average four tetrafunctional junction points randomly distributed along the PB backbone. Each junction point grafts two PS blocks to the backbone. A range of overall PB and PS volume fractions was investigated for both series. These materials simulate a controlled and known degree of architectural disorder. Current theory cannot rigorously predict the morphological behavior for these complex molecular architectures. However, it is found that an approximate extension of existing theory utilizing the constituting block copolymer (fundamental building block) concept allows a rational explanation of the effect of architecture on morphology in these materials. The materials form the domain shape (spheres cylinders, or lamellae) which is predicted by theory, but spherical and cylindrical morphologies lack the long-range lattice order found in diblocks and other simpler block copolymer molecular architectures. When lamellar morphologies are formed, however, at least some long range order is always present due to the space filling requirements of the lamellar domains

Synthesis, characterization, and morphology of model graft copolymers with trifunctional branch points

Well-defined graft copolymers with polyisoprene backbones and polystyrene branches, having trifunctional branch points, of the type S2IS2 (H-shaped) and (SI)I(SI) (π-shaped) were synthesized by anionic polymerization high-vacuum techniques. The synthetic strategy involves the preparation of the outer parts of the molecules, carrying one reactive Si-Cl bond, followed by coupling with difunctional living poly(isoprenyllithium) chains. In this way, the number and placement of the branches can be precisely controlled. Molecular characterization of the fractionated samples by size exclusion chromatography with UV and RI detection, membrane osmometry, low-angle laser light scattering, and 1H-NMR spectroscopy confirmed that the materials exhibit narrow molecular weight distributions and low compositional heterogeneity. The strongly microphase-separated morphologies of these two samples were characterized using TEM and SAXS. The π architecture with a PS volume fraction of 0.21 was found to form body-centered cubic spheres, while the H architecture with a PI volume fraction of 0.64 was found to form a lamellar morphology. The observed morphology for these architectures was rationalized by formally dividing the π and H architectures into component simple (single) graft block copolymers which were mapped onto Milner's morphology diagram for simple graft copolymers

Asymmetric single graft block copolymers: Effect of molecular architecture on morphology

This paper reports on the synthesis and morphological characterization of graft block copolymers in which a single polystyrene (PS) arm was grafted at an asymmetric position along a polyisoprene (PI) backbone. These materials represent a model series of asymmetric simple graft (ASG) block copolymer structures. The synthesis of these materials was carried out with methods developed for three-arm "miktoarm" star copolymers using anionic polymerization high-vacuum techniques with cholorosilane linking agents. The three arms were two polyisoprene blocks with different degrees of polymerization and one deuterated polystyrene block, which formed an asymmetric simple graft structure (ASG). Molecular characterization was performed using size exclusion chromatography (SEC) with refractive index and UV detection, membrane osmometry, and low-angle laser light scattering. These techniques confirmed that the materials exhibited narrow molecular weight distributions and low compositional heterogeneity. The morphologies formed by these samples were characterized using transmission electron microscopy (TEM) and small-angle neutron scattering (SANS). The ASG structures were compared with structures formed by linear diblock copolymers and other samples having miktoarm structures, like I2S (symmetric simple graft with PI backbone and one PS branch grafted from the middle) and I3S (three equal PI arms and one PS arm). Comparisons of the morphologies formed and their dimensions indicated that the chain stretching and lateral crowding due to the miktoarm architecture was partially alleviated by the different lengths of PI blocks in ASG