Star Triblock Copolymers Research Articles

Thermoresponsive star triblock copolymers by combination of ROP and ATRP: From micelles to hydrogels

AbstractNovel biocompatible, biodegradable, four‐arm star, triblock copolymers containing a hydrophobic poly(ε‐caprolactone) (PCL) segment, a hydrophilic poly(oligo(ethylene oxide)475 methacrylate) (POEOMA475) segment and a thermoresponsive poly(di(ethylene oxide) methyl ether methacrylate) (PMEO2MA) segment were synthesized by a combination of controlled ring‐opening polymerization (ROP) and atom transfer radical polymerization (ATRP). First, a four‐arm PCL macroinitiator [(PCL‐Br)4] for ATRP was synthesized by the ROP of ε‐caprolactone (CL) catalyzed by stannous octoate in the presence of pentaerythritol as the tetrafunctional initiator followed by esterification with 2‐bromoisobutyryl bromide. Then, sequential ATRP of oligo(ethylene oxide) methacrylate (OEOMA475, Mn = 475) and di(ethylene oxide) methyl ether methacrylate) (MEO2MA) were carried out using the (PCL‐Br)4 tetrafunctional macroinitiator, in different sequence, resulting in preparation of (PCL‐b‐POEOMA475‐b‐PMEO2MA)4 and (PCL‐b‐PMEO2MA‐b‐POEOMA475)4 star triblock copolymers. These amphiphilic copolymers can self‐assemble into spherical micelles in aqueous solution at room temperature. The thermal responses of the polymeric micelles were investigated by dynamic light scattering and ultraviolet spectrometer. The properties of the two series of copolymers are quite different and depend on the sequence distribution of each block along the arms of the star. The (PCL‐b‐POEOMA475‐b‐PMEO2MA)4 star copolymer, with the thermoresponsive PMEO2MA segment on the periphery, can undergo reversible sol‐gel transitions between room temperature (22 °C) and human body temperature (37 °C). © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Self-Assembly of ABC Star Triblock Copolymer Thin Films Confined with a Preferential Surface: A Self-Consistent Mean Field Theory

The microphase separation and morphology of a nearly symmetric A(0.3)B(0.3)C(0.4) star triblock copolymer thin film confined between two parallel, homogeneous hard walls have been investigated by self-consistent mean field theory (SCMFT) with a pseudospectral method. Our simulation experiments reveal that under surface confinement, in addition to the typically parallel, perpendicular, and tilted cylinders, other phases such as lamellae, perforated lamellae, and complex hybrid phases have been found to be stable, which is attributed to block-substrate interactions, especially for those hybrid phases in which A and B blocks disperse as spheres and alternately arrange as cubic CsCl structures, with a network preferred structure of C block. The results show that these hybrid phases are also stable within a broad hybrid region (H region) under a suitable film thickness and a broad field strength of substrates because their free energies are too similar to being distinguished. Phase diagrams have been evaluated by purposefully and systematically varying the film thickness and field strength for three different cases of Flory-Huggins interaction parameters between species in the star polymer. We also compare the phase diagrams for weak and strong preferential substrates, each with a couple of opposite quality, and discuss the influence of confinement, substrate preference, and the nature of the star polymer on the stability of relatively thinner and thick film phases in this work.

Real-space self-consistent mean-field theory study of ABC star triblock copolymers

The phase behavior of ABC star triblock copolymers is examined using real-space self-consistent mean-field theory. The central part of the triangular phase diagram for ABC triblock copolymers with equal A/B, B/C, and C/A interactions is determined by comparing the free energy of a number of candidate ordered phases. In this region of the phase diagram, the dominant microstructures are cylinders with polygonal cross sections or two-dimensional polygon-tiling patterns. Most of the known polygon-tiling patterns observed in experiments and simulations, plus some neighboring morphologies, are considered in the construction of the phase diagram. The resulting phase behavior is consistent with experiments and computer simulations.

Stability of hierarchical lamellar morphologies formed in ABC star triblock copolymers

The stability of hierarchical lamellar morphologies formed in ABC star triblock copolymers, is studied using the self-consistent mean-field theory. The hierarchical lamellae con- sist of repeating period of the largest block A-formed layer and B/C coformed layer where B and C domains are arranged alter- natively. An angle, which is used to characterize the shifting magnitude between two neighbor B/C coformed layers, varies from 0 to 180 degrees. By comparing the free energy among the lamellar morphologies with various shift angle, their rela- tive stability is analyzed. Our results show that the morphology with larger shift has lower entropic energy and higher internal energy. In general, the morphology with the largest shift of 180- degree is stable compared with those with smaller shift as the entropic energy dominates the internal energy. However, the rel- ative stability can be tuned by the interactions among the three components as well as their relative compositions. PACS num- bers: 61.25.Hq, 64.60.Cn, 64.75.+g. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1101-1109, 2010

Self-Assembly of ABC Star Triblock Copolymers under a Cylindrical Confinement

Self-assembly of ABC star triblock copolymers confined in cylindrical nanopores is studied using real-space self-consistent mean-field theory. Specifically, the investigation focuses on the confined self-assembly of a triblock copolymer which forms hierarchical lamellae in the bulk. Generically, the hierarchical lamellae can be parallel or perpendicular to the pore surfaces. Concentric rings of A and B/C lamellae are formed in the parallel case. The B/C layers further form B/C domains. The number of B/C domains is controlled by the pore size. In the perpendicular case, the B/C layers are arranged alternatively along the pore axis. The stability of these observed structures is analyzed.

Synthesis and characterization of novel resorcinarene‐centered amphiphilic star‐block copolymers consisting of eight ABA triblock arms by combination of ROP, ATRP, and click chemistry

AbstractNovel amphiphilic eight‐arm star triblock copolymers, star poly(ε‐caprolactone)‐block‐poly(acrylic acid)‐block‐poly(ε‐caprolactone)s (SPCL‐PAA‐PCL) with resorcinarene as core moiety were prepared by combination of ROP, ATRP, and “click” reaction strategy. First, the hydroxyl end groups of the predefined eight‐arm SPCLs synthesized by ROP were converted to 2‐bromoesters which permitted ATRP of tert‐butyl acrylate (tBA) to form star diblock copolymers: SPCL‐PtBA. Next, the bromide end groups of SPCL‐PtBA were quantitatively converted to terminal azides by NaN3, which were combined with presynthesized alkyne‐terminated poly(ε‐caprolactone) (A‐PCL) in the presence of Cu(I)/N,N,N′,N″,N″‐pentamethyldiethylenetriamine in DMF to give the star triblock copolymers: SPCL‐PtBA‐PCL. 1H NMR, FTIR, and SEC analyses confirmed the expected star triblock architecture. The hydrolysis of tert‐butyl ester groups of the poly(tert‐butyl acrylate) blocks gave the amphiphilic star triblock copolymers: SPCL‐PAA‐PCL. These amphiphilic star triblock copolymers could self‐assemble into spherical micelles in aqueous solution with the particle size ranging from 20 to 60 nm. Their micellization behaviors were characterized by dynamic light scattering and transmission electron microscopy. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2905–2916, 2009

Complex Crystal Structures Formed by the Self-Assembly of Ditethered Nanospheres

We report the results from a computational study of the self-assembly of amphiphilic ditethered nanospheres using molecular simulation. As a function of the interaction strength and directionality of the tether-tether interactions, we predict the formation of four highly ordered phases not previously reported for nanoparticle systems. We find a double diamond structure comprised of a zinc blende (binary diamond) arrangement of spherical micelles with a complementary diamond network of nanoparticles (ZnS/D), a phase of alternating spherical micelles in a NaCl structure with a complementary simple cubic network of nanoparticles to form an overall crystal structure identical to that of AlCu2Mn (NaCl/SC), an alternating tetragonal ordered cylinder phase with a tetragonal mesh of nanoparticles described by the [8,8,4] Archimedean tiling (TC/T), and an alternating diamond phase in which both diamond networks are formed by the tethers (AD) within a nanoparticle matrix. We compare these structures with those observed in linear and star triblock copolymer systems.

Phase behavior of ditethered nanospheres

We report the results from a computational study of the self-assembly of amphiphilic ditethered nanospheres using molecular simulation. We explore the phase behavior as a function of nanosphere diameter, interaction strength, and directionality of the tether-tether interactions. We predict the formation of seven distinct ordered phases. We compare these structures with those observed in linear and star triblock copolymer systems.

Self-Assembly of Star ABC Triblock Copolymer Thin Films: Self-Consistent Field Theory

Microphase separation and morphology of star ABC triblock copolymers confined between two identical parallel walls (symmetric wetting or dewetting) are investigated with self-consistent field theory (SCFT) combined with the "masking" technique to describe the geometric confinement of the films. In particular, we examine the morphology of confined near-symmetric star triblock copolymers under symmetric and asymmetric interactions as a function of the film thickness and the surface field. Under the interplay between the degree of spatial confinement, characterized by the ratio of the film thickness to bulk period, and surface field, the confined star ABC triblock copolymers are found to exhibit a rich phase behavior. In the parameter space we have explored, the thin film morphologies are described by four primary classes including cylinders, perforated lamellae, lamellae, and other complex hybrid structures. Some of them involve novel structures, such as spheres in a continuous matrix and cylinders with alternating helices structure, which are observed to be stable with suitable film thickness and surface field. In particular, complex hybrid network structures in thin films of bulk cylinder-forming star triblock copolymers are found when the natural domain period is not commensurate with the film thickness. Furthermore, a strong surface field is found to be more significant than the spatial confinement on changing the morphology of star triblock copolymers in bulk. These findings provide a guide to designing novel microstructures involving star triblock copolymers via geometric confinement and surface fields.

Evolution of Multicompartment Micelles to Mixed Corona Micelles Using Solvent Mixtures

Miktoarm star triblock copolymers mu-[poly(ethylethylene)][poly(ethylene oxide)][poly(perfluoropropylene oxide)] self-assemble in dilute aqueous solution to give multicompartment micelles with the cores consisting of discrete poly(ethylethylene) and poly(perfluoropropylene oxide) domains. Tetrahydrofuran is a selective solvent for both the poly(ethylethylene) and poly(ethylene oxide) blocks, and thus in tetrahydrofuran mixed corona micelles are favored with poly(perfluoropropylene oxide) cores. The introduction of tetrahydrofuran into water induces an evolution from multicompartment micelles to mixed corona [poly(ethylethylene) + poly(ethylene oxide)] micelles, as verified by dynamic light scattering and nuclear magnetic resonance spectroscopy. A mixed solvent containing 60 wt % tetrahydrofuran corresponds to the transition point, as verified by analysis of a poly(ethylethylene)-poly(ethylene oxide) diblock copolymer in the same solvent mixtures. Furthermore, cryogenic transmission electron microscopy suggests that, as the poly(ethylethylene) block transitions from the core to the corona, the micelle morphologies evolve from disks to oblate ellipsoid micelles (with some vesicles), with worms and spheres evident at intermediate compositions.

Mesoscopic dynamics of inhomogeneous polymers based on variable cell shape dynamic self-consistent field theory

In this paper, we combine variable cell shape method with dynamic self-consistent field theory and extend to study structure and dynamics under shear for triblock copolymer melts. Due to shear, the calculation cell shape is variable and no longer orthogonal. Pseudospectral method is employed to solve the diffusion equation for chain propagator on the nonorthogonal coordinate and the shear periodical condition can be easily designed in terms of the variable cell shape method. By using this strategy, the shear induced morphology evolution is investigated for topologically complex polymeric systems such as linear and star triblock copolymers; the morphology of linear ABC triblock copolymers is more shear sensitive than that of star triblocks. In particular, once the chain propagator is obtained, the microscopic elastic stress and spatial stress distribution can be derived and thus the dynamic mechanical property can be calculated under shear. By imitating the dynamic storage modulus G' corresponding to any given morphology in the oscillatory shear measurements, we explore the relationship between the morphology and the storage modulus G' and extend to study the mechanism of phase separation dynamics as well as order-disorder transition (ODT) for linear and star triblock copolymers. The results show that the chain architecture can be easily distinguished by investigating the ODT, though the systems such as AB symmetric diblock and ABA triblock copolymers by coupling AB precursors almost exhibit similar microstructures. In addition, the storage modulus G' and loss modulus G" can be simultaneously determined in frequency sweeps of oscillatory shear measurements and the dependence of the moduli on phase separated patterns and the chain topology is investigated. The simulation findings are in qualitatively agreement with the experimental results.

Multicompartment Micelles from Star and Linear Triblock Copolymer Blends

Dissipative particle dynamics simulations were performed on multicompartment micelles formed by blending star and linear triblock copolymers, in which the influences of blending options and blending ratio as well as copolymer chain compositions were studied systematically. The results show that blending of copolymers with different architectures is a promising strategy to control the morphology and structure of multicompartment micelles. This work revealed several new morphologies of multicompartment micelles by blending star and linear triblock copolymers, and the dynamic processes were elucidated at the molecular level by tracing the motions of copolymer chains. The results of this work provide deep insight into micro/mesoscopic details of the underlying mechanisms, contributing to a more complete understanding of multicompartment micelle formation and structural control.

Self-Assembly of Amphiphilic ABC Star Triblock Copolymers and Their Blends with AB Diblock Copolymers in Solution: Self-Consistent Field Theory Simulations

The self-assembled morphologies of amphiphilic ABC star triblock copolymers consisting of hydrophilic A blocks and hydrophobic B and C blocks and the blends with their counterpart linear AB diblock copolymers in solution are investigated by 2D real-space implementation of self-consistent field theory (SCFT) simulation. The star triblock copolymers self-assemble in solution to form various micellar structures from hamburger, to segmented wormlike, to toroidal segmented micelles, and finally to vesicles with simultaneously increasing hydrophobic lengths of blocks B and C. When the length of hydrophobic blocks B and C is asymmetric, specific bead-on-string worm micelles are found. Particularly, when the star ABC triblock copolymer is in a strong segregation regime and both B and C blocks are strongly hydrophobic, quite long segmented wormlike micelles are obtained, which had not been found in previously investigated diblock and linear ABC triblock copolymers solution. Additionally, raspberry micelles with beads dispersed on the core also occur in the strong segregation regime of bulk star ABC triblock copolymers. Furthermore, the aggregate morphology of ABC star triblock copolymers is strongly influenced by the addition of linear AB diblock copolymers. The most significant feature is that the long segmented worms will become shorter, to form hamburger micelles with the addition of AB diblock copolymers. These simulations are in good agreement with the experimental findings by Lodge's group.

Multicompartment Micellar Solutions in Shear: A Dissipative Particle Dynamics Study

AbstractSummary: Dissipative particle dynamics simulations were performed to study the effect of shear on the rheological behavior of multicompartment micellar solutions, demonstrating that both shear thickening and thinning can occur, and the macroscopic behavior was elucidated at a molecular level. In addition, a novel shear‐induced morphology of “sphere‐on‐rod” was observed. This work provides useful information towards a complete understanding of the properties and morphologies of multicompartment micelles that is useful for future rational synthesis of novel micelles.Shear‐induced morphological transitions and rheological behavior in multicompartment micellar solutions formed from star triblock copolymers.magnified imageShear‐induced morphological transitions and rheological behavior in multicompartment micellar solutions formed from star triblock copolymers.

Self-assembled pattern formation of block copolymers on the surface of the sphere using self-consistent field theory

The spherical surface is spatially discretized with triangular lattices to numerically calculate the Laplace-Beltrami operator contained in the self-consistent field theory (SCFT) equations using a finite volume method. Based on this method we have developed a spherical alternating-direction implicit (ADI) scheme for the first time to help extend real-space implementation of SCFT in 2D flat space to the surface of the sphere. By using this method, we simulate the equilibrium microphase separation morphology of block copolymers including AB diblocks, ABC linear triblocks and ABC star triblock copolymers occurred on the spherical surface. In general, two classes of microphase separation morphologies such as striped patterns for compositionally symmetric block copolymers and spotted patterns for asymmetric compositions have been found. In contrast to microphase separation morphology in 2D flat space, the geometrical characteristics of a sphere has a large influence on the self-assembled morphology. For striped patterns, several of spiral-form and ring-form patterns are found by changing the ratio of the radius of a sphere to the averaging width of the stripes. The specific pattern such as the striped and spotted pattern with intrinsic dislocations or defects stems from formed periodic patterns due to microphase separation of block copolymers arranged on the curved surface.

Dissipative Particle Dynamics Study of the Formation of Multicompartment Micelles from ABC Star Triblock Copolymers in Water

AbstractSummary: Dissipative particle dynamics simulation was performed to study the formation of multicompartment micelles from ABC star triblock copolymers in water. The study revealed some new morphologies that had not been observed before and also provided a direct visualization of the evolution of wormlike multicompartment micelles that follows the fusion process. Thus, this work provides molecular understanding of multicompartment micelles which will be useful for the future rational synthesis of novel micelles.Multicompartment micelles formed from ABC star triblock copolymers in water by DPD simulations.magnified imageMulticompartment micelles formed from ABC star triblock copolymers in water by DPD simulations.

Study of Morphology and Phase Diagram of π-Shaped ABC Block Copolymers Using Self-Consistent-Field Theory

Using a combinatorial screening method based on the self-consistent-field theory for polymers, we study the bulk morphology and the phase behavior of π-shaped ABC block copolymers, in which A is the backbone and B and C are the two grafts. By systematically varying the positions of the graft points, the π-shaped block copolymer can change from a star block copolymer to a linear ABC block copolymer. Thus, the corresponding order−order phase transition due to the architecture variation can be investigated. At two given compositions, we find seven different morphologies (“three-color” lamellar phase, “three-color” hexagonal honeycomb phase, lamellae with beads inside, dodecagon−hexagon−tetragon, hexagon−hexagon, lamellae with alternating beads, and octagon−octagon−tetragon). The hexagon−hexagon morphology has not been reported previously for linear and star triblock copolymers in the bulk state. The phase diagram of the π-shaped ABC block copolymer with symmetric interactions among the three species is const...

Synthesis of an Amphiphilic Star Triblock Copolymer of Polystyrene, Poly(ethylene oxide), and Polyisoprene Using Lysine as Core Molecule

A new amphiphilic ABC star triblock copolymer, with polystyrene (PS) and polyisoprene (PI) as hydrophobic segments and poly(ethylene oxide) (PEO) as hydrophilic segment, is successfully prepared using lysine as a core molecule. The mPEO with blocked methoxyl group at one end and hydroxyl at another end, PS, and PI with hydroxyl end groups are obtained by the anionic technique. The hydroxyl groups of mPEO-OH and PS-OH are converted into succinimidyl carbonate first and then coupled with e and α amino groups of lysine, respectively, to yield an intermediate mPEO-PS-Lys-COOH. The PI-OH is also converted into PI carbonate (PI-SC) and then PI-NH2 and reacted with a carboxyl group of lysine at the junction of PS and mPEO arms of mPEO-PS-Lys-COOH in the presence of N,N‘-dicyclohexylcarbodiimide (DCC). The molecular weight distribution of the final product mPEO-PS-PI is rather narrow (<1.15). The structure of intermediates and final products was characterized by NMR, IR, UV, and SEC in detail. This method is prom...

Synthesis and Characterization of Triptych μ-ABC Star Triblock Copolymers

We describe a general procedure for the synthesis of miktoarm star triblock copolymers with a hydrocarbon, a fluoropolymer, and a hydrophilic segment. Several μ-(polyethylethylene)(poly(ethylene oxide))(poly(perfluoropropylene oxide)) [μ-(PEE)(PEO)(PFPO)] star triblock copolymers were prepared using two successive anionic polymerization steps and one polymer−polymer coupling reaction. Initially, living polybutadienyllithium chains were end-capped with 2-methoxymethoxymethyloxirane forming a heterobifunctional 1,2-polybutadiene (PBD) precursor, with a hydroxyl group and a protected hydroxyl group at one chain end. Catalytic hydrogenation of this PBD gave the corresponding polyethylethyene (PEE) while preserving the end group structures. Transformation of the terminal hydroxyl group in the PEE precursor to a potassium alkoxide followed by addition of ethylene oxide and subsequent end-capping with ethyl bromide generated polyethylethylene−poly(ethylene oxide) (PEE−PEO) diblock copolymers with a protected hydroxyl group at the junction. Deprotection of the methoxymethyl group followed by coupling with acid chloride end-capped PFPO yielded well-defined μ-(PEE)(PEO)(PFPO) star triblock copolymers. Detailed molecular characterization of these products and their precursors confirmed the composition and architecture of these new star block copolymers. This modular strategy represents a new, straightforward, and versatile methodology for the preparation of mixed arm star block copolymers.

Morphology and Phase Diagram of Complex Block Copolymers: ABC Star Triblock Copolymers

Microphases and triangle phase diagrams of ABC star triblock copolymers are investigated on the basis of a real-space implementation of the self-consistent field theory (SCFT) for polymers. For the sake of numerical tractability, the calculations are carried out in two dimensions (2D). Nine stable microphases are uncovered, including hexagonal lattice, core-shell hexagonal lattice, lamellae, and lamellae with beads at the interface as well as a variety of complex morphologies that are absent in linear ABC triblocks, such as a three-color hexagonal honeycomb phase, knitting pattern, octagon-octagon-tetragon phase, lamellar phase with alternating beads, and decagon-hexagon-tetragon phase. We have found that when the volume fractions of the three species are comparable the star architecture of the polymer chain is a strong topological constraint that regulates the geometry of the microphases formed. However, when at least one of the volume fractions of the three species is low, the influence of the star architecture on the morphology is not significant. Our calculations reasonably agree with previous theoretical and experimental results and can be used to guide the design of novel microstructures involving star triblock copolymers.