Heteroarm Star Copolymers Research Articles

A New Simulation Algorithm with Revised “Association Criteria” for Studying the Association of Heteroarm Star Copolymers

AbstractSummary: A new computational algorithm for dynamic lattice Monte Carlo simulations of the associative behavior of heteroarm copolymers in selective solvents was developed and optimized for efficient and relatively fast simulation studies. The algorithm is based on the Siepmann and Frenkel variant of the bias self‐avoiding walk procedure. Simultaneously, a new criterion for recognition of an associate was proposed. The first results on the micellization of heteroarm star copolymers are presented. magnified image

Functionalized (X-PEO)2-(PS-Y)2 Star Block Copolymers at the Interfaces: Role of Terminal Groups in Surface Behavior and Morphology

We focus on the role of functional terminal group combinations for (X-PEO)2-(PS-Y)2 heteroarm star copolymers with respect to their interfacial behavior and surface morphology. We synthesized a series of star copolymers with combinations of bromine, amine, TBDPS, hydroxyl, and carboxylic terminal groups. The study concluded that hydrophilic functional groups attached to hydrophobic chains and hydrophobic functional groups attached to hydrophilic chains resulted in the stabilization of the spherical domain morphology, rather than cylindrical morphology predicted for the given chemical composition of star copolymers. The replacement of functional groups of hydrophobic polymer chains was found to be even more effective in creating stable and very fine circular domain morphology. In addition, for (COOH-PEO)2-(PS-NH2)2 star polymer the ionization of carboxylic terminal groups at higher pH led to greater solubility of PEO chains in the water subphase, which along with deionization of amine terminal group preven...

New insights on the solution behavior and self-assembly of polystyrene/poly(2-vinylpyridine) ‘hairy’ heteroarm star copolymers with highly asymmetric arms in polar organic and aqueous media

The solution behavior and self-assembly of a ‘hairy’ heteroarm star copolymer polystyrene/poly(2-vinylpyridine) containing 20 fairly short polystyrene and 20 long poly(2-vinylpyridine) arms was studied in 1,4-dioxane–methanol mixtures and in acidic aqueous solutions. The copolymer forms reversible micelles in 1,4-dioxane–methanol mixtures. Since the conformation of the unimolecular heteroarm star (unimer) with collapsed insoluble PS arms and stretched soluble PVP arms resembles the spherical core/shell micelle, the solubility of the copolymer in the selective solvent is fairly high and the multimolecular micelles are formed by only few heteroarm stars. Both heteroarm stars (in pure 1,4-dioxane) and micellar systems (composed of both micelles and unimolecular heteroarm stars (unimers) in 1,4-dioxane–methanol mixtures) may be transferred in acidic aqueous media by dialysis. Polyelectrolyte behavior of unimer stars was studied in detail in acidic aqueous media. Static and dynamic light scattering measurements show that heteroarm stars interact electrostatically over long distances in low ionic strength solutions. Experimental data indicate that the charged stars may form temporary shell interacting (electrostatically stabilized) aggregates at elevated concentrations.

Intramolecular Janus Segregation of a Heteroarm Star Copolymer

Conformations of heteroarm star copolymers AnBn are investigated by off-lattice Monte Carlo simulations. Intramolecular segregation is always observed in selective solvents. It can also be found in a common good solvent, which however gives block incompatibility. The degree of Janus segregation is characterized by the mean-square separation between A and B blocks, 〈(xA - xB) 2 〉, or by the orientation correlation between the two blocks, 〈xA‚xB〉, where xi denotes the relative position of the center of mass associated with the block arms i from the star core. The degree of segregation grows with increasing arm numbers while the effect of arm lengths is insignificant. The influence of the intramolecular segregation on the aggregation behavior in solution can be revealed through interactions between two star copolymers. The potential of mean force and second virial coefficient are calculated for various solvent qualities. In a selective solvent, the blockarm star copolymer can form a unimolecular micelle structure and always shows intermolecular repulsions. In contrast, the heteroarm star copolymer may display the Janus segregation, which leads to intermolecular attractions. This consequence facilitates the formation of multimolecular micelles.

Monte Carlo Study of Dilute Solutions of Heteroarm Star Copolymers in Solvents Differing in Thermodynamic Quality

Lattice Monte Carlo simulations (an original modification of the Siepmann and Frenkel simulation variant) was used to study the conformational behavior of heteroarm star copolymers star-(polystyrene; polyisoprene), PS8PI8, in a common good solvent for both types of arms (tetrahydrofuran) and in selective θ-solvents for both types of arms (in cyclohexane, i.e., in θ-solvent for PS and in 1,4-dioxane, i.e. in θ-solvent for PI). Results of simulations were compared with experimental data published by Pispas et al. The coarse graining procedure was performed to match the experimental behavior of heteroarm stars in a good solvent. The computer simulations reproduce all decisive trends of the conformational behavior for both selective solvents. The quantitative agreement between experimental and simulated size characteristics is very good. Simulations yield very detailed information on the system at the molecular level and show that the incompatible arms significantly segregate in selective solvents.

Amorphous‐Crystalline PSnPEOn Heteroarm Star Copolymers: Crystallinity and Crystallization Kinetics

AbstractSummary: The crystallization behavior and kinetics of poly(ethylene oxide) in polystyrene/poly(ethylene oxide) heteroarm star copolymers were studied by differential scanning calorimetry and optical microscopy. A comparison between star and linear amorphous‐crystalline block copolymers showed that the macromolecular architecture is an important factor affecting crystallinity. The following points were observed: the equilibrium melting point is higher in the star copolymers, the crystallinity reduces as the number of arms increases, leading to smaller and ill‐defined spherulites, and crystallization proceeds faster in linear copolymers at low supercooling.Half crystallization times, t1/2, calculated from the Avrami analysis of the latent heats, obtained during the isothermal crystallization experiments as a function of supercooling, ΔT, for all copolymers.magnified imageHalf crystallization times, t1/2, calculated from the Avrami analysis of the latent heats, obtained during the isothermal crystallization experiments as a function of supercooling, ΔT, for all copolymers.

Triblock and Radial Star‐Block Copolymers Comprised of Poly(ethoxyethyl glycidyl ether), Polyglycidol, Poly(propylene oxide) and Polystyrene Obtained by Anionic Polymerization Initiated by Cs Initiators

AbstractThe anionic polymerization of ethoxyethyl glycidyl ether (EEGE) initiated by cesium alkoxide was studied. The ring‐opening polymerization of EEGE in the presence of cesium alkoxide of 1‐methoxy‐2‐ethanol does not involve any side reactions. The presence of an additional alcohol leads to a significant increase of the initiator efficiency.Aqueous solutions of poly (ethoxyethyl glycidyl ether) (PEEGE) exhibit lower critical solution temperature (LCST), and the polymer solubility in water is extremely sensitive to its MW. Two novel types of block copolymers based on PEEGE were synthesized: triblock‐copolymers of ABA (A′:BA′) structure, where A is the PEEGE block, A′ polyglycidol (PG) and B the polypropylene oxide (PPO) block, and A2S (A′2S) and A4S (A′4S) heteroarm stars, where S is the polystyrene block.The synthesis of the ABA block was performed by polymerization of EEGE initiated by bi‐functional PPO/Cesium alkoxide macroinitiator. The PEEGE blocks were converted into PG blocks by successful cleavage of the ethoxyethyl group.Polystyrene/PEEGE and polystyrene/PG three‐ and five‐ heteroarm star copolymers were synthesized by a coupling reaction between well‐defined chain‐end‐functionalized polystyrenes carying dendritic benzyl bromide moieties with living anionic polymers of PEEGE with one cesium alkoxide terminal group. The coupling reaction proceeds quantitatively without any side reactions, and thus series of star‐branched polymers can be systematically synthesized. Polystyrenes with two or four PG arms have been obtained after the cleavage of the protecting group. The compact structure of these multi‐arm star polymers and their amphiphilic character leads to the formation of nanoparticles in aqueous solution with rather uniform size distribution and a mean diameter of 15 nm.

Single Molecules and Associates of Heteroarm Star Copolymer Visualized by Atomic Force Microscopy

Single molecule conformations and associate morphologies for polystyrene/poly(2-vinylpyridine) (PS7−P2VP7) heteroarm star copolymer, formed in controlled environment and deposited onto mica or Si wafers, have been studied by atomic force microscopy (AFM) with molecular resolution. At concentrations below 0.01 mg/mL PS7−P2VP7 exists in the molecularly dissolved state in both selective (acidic water, toluene) and common good (chloroform, tetrahydrofuran) solvents. In acid conditions PS7−P2VP7 forms either unimolecular or multimolecular micelles depending on concentration and pH. The core of the micelles is constituted of the collapsed PS arms surrounded by protonated P2VP shell. PS7−P2VP7 undergoes inverse intramolecular segregation upon addition of toluene. In this case, P2VP arms form the dense core of the micelles embedded in the swollen PS shell. The transition between those two inverse types of micelles is strongly modified by interaction with the mica substrate. The micelles deposited onto mica from a...

Monte Carlo Study of Heteroarm Star Copolymers in Good and Selective Solvents

AbstractLattice Monte Carlo simulations of conformations of hereroarm star copolymers AnBn in selective solvents were performed using a special variant of the Siepman and Frenkel algorithm. The effects of solvent quality, the number and lengths of blocks on the collapse of the insoluble block A, segregation of the two types of blocks and the behavior of the soluble blocks B were studied mainly for “hairy” stars containing high numbers of long arms. The simulation shows that insoluble blocks collapse in strongly selective solvents and the gravity centers of soluble and insoluble blocks separate which suggests the possibility of the formation of non‐spherical structures.Snapshots of star a in a very bad solvent (T = 3.00) for blocks A. There are two different views of the star with a total number of arms f = 16 and number of segments N = 300.imageSnapshots of star a in a very bad solvent (T = 3.00) for blocks A. There are two different views of the star with a total number of arms f = 16 and number of segments N = 300.

Ruthenium-Centered Heteroarm Stars by a Modular Coordination Approach: Effect of Polymer Composition on Rates of Chelation

Bipyridine end- and center-functionalized polymer chains, bpyA and bpyA2, respectively, were chelated to Ru(II) centers to form Ru(bpyAn)2Cl2 complexes 1−6 where A = polystyrene (PS), n = 1 (1), n = 2 (2); A = poly(methyl methacrylate) (PMMA), n = 1 (3), n = 2 (4); A = poly(ε-caprolactone) (PCL), n = 2 (5); and A = poly(ethylene glycol) PEG, n = 2 (6). Intermediates 1 and 5 were subsequently reacted with a second type of bpy macroligand to generate [Ru(bpyAn)2(bpyB2)]2+ heteroarm star copolymers for the following combinations: An,B = PCL2,PS (7), PCL2,PEG (8), PS,PLA (9), where PLA = poly(lactic acid), and PS,PEG (10). Rates of chelation were affected by macroligand composition, molecular weight, and architecture and paralleled the solubility parameter series for PCL, PLA, PMMA, PS, and PEG in dimethoxyethane (DME). Rates decreased with increasing Mn and were lower for bpyA2 relative to end-functionalized bpyA of comparable molecular weights within each compositional class.

Synthesis and Characterization of Amphiphilic A3B3Hetero Arm Star Copolymers: Poly(isobutylene3-star-methyl vinyl ether3)

Synthesis and Characterization of Amphiphilic A₃B₃Hetero Arm Star Copolymers: Poly(isobutylene₃-<i>s</i><i>tar</i>-methyl vinyl ether₃)

Aggregation Behavior of Polystyrene/Poly(acrylic acid) Heteroarm Star Copolymers in 1,4-Dioxane and Aqueous Media

Ionic heteroarm star copolymers bearing polystyrene (PS) and poly(acrylic acid) (PAA) arms (PSnPAAn) were prepared by quantitative hydrolysis of the poly(tert-butyl acrylate) (PtBA) arms of the corresponding PSnPtBAn star copolymer. The aggregation properties of these copolymers were studied in various solvents. In 1,4-dioxane PS12PAA12 (with nearly symmetrical PS and PAA arms) forms reverse micelles of low aggregation number (Nagg) and spherical morphology. In an 80 : 20 (v/v) 1,4-dioxane/water mixture these micelles are transformed to regular micelles with an unexpectedly high Nagg and an elongated rod-like structure. An abnormal behavior was observed in aqueous solutions of charged PS24PANa24 (with asymmetrical PS and PAA arms, WPS = 19 wt.-%) at low concentrations. A non-equilibrium physical gel is formed, characterized by a very high viscosity and an elastic response upon oscillatory shearing.

Lyotropic Liquid Crystalline Structures Formed by Amphiphilic Heteroarm Star Copolymers

Polystyrene/poly(ethylene oxide) heteroarm star copolymers (i.e., star-shaped polymers consisting of a central poly(divinylbenzene) core bearing an equal number of short polystyrene and long poly(ethylene oxide) arms, PS10PEO10) were synthesized by anionic polymerization, and their phase behavior was investigated in aqueous solutions as well as in water−oil solvent mixtures. Because of their amphiphilic character and despite their star architecture, these polymeric species self-assemble under equilibrium conditions into lyotropic liquid crystalline structures.

Heteroarm Star‐Like Micelles Formed from Polystyrene‐block‐poly(2‐vinyl pyridine)‐block‐poly(methyl methacrylate) ABC Triblock Copolymers in Toluene

Polystyrene-block-poly(2-vinyl pyridine)block-poly(methyl methacrylate) ABC triblock copolymers were synthesized by sequential living anionic polymerization. Their solution properties were investigated in toluene, which is a bad solvent for the middle block. Spherical micelles are formed, which consist of a poly(2-vinyl pyridine) dense core bearing polystytene and poly(methyl methacrylate) soluble chains at the corona. These micelles exhibit the architecture of heteroarm star copolymers obtained by living polymerization methods. The aggregation numbers strongly depend on the length of the insoluble P2VP middle block thus remarkably attecting the size of the micelles.

A Comparative Experimental and Theoretical Study between Heteroarm Star and Diblock Copolymers in the Microphase Separated State

A pronounced difference in the behavior of heteroarm star copolymers from the conventional diblock copolymers is analyzed theoretically and experimentally on the example of symmetric polystyrene-poly(2-vinylpyridine) (PS6P2VP6) star block copolymer melts. Microphase separation of PS6P2VP6 stars with volume fraction of PS varying from 0.30 to 0.75 is studied using transmission electron microscopy (TEM). The observed (lamellar, cylindrical and undefined “bicontinuous-like”) structures often turned out to be different from that expected for the corresponding diblock copolymers. The dependence of the long period of lamellar structures of heteroarm star block copolymers on block length is found to deviate in a systematic way from N2/3 law (known for diblock copolymers), especially for the stars of high functionality. The theoretical model proposed to describe an extra-stretching of star block copolymers near the lamellar interface is found to be successful in prediction of block length and star functionality d...

Polystyrene/Poly(2-vinylpyridine) Heteroarm Star Copolymer Micelles in Aqueous Media and Onion Type Micelles Stabilized by Diblock Copolymers,

Micellization of polystyrene/poly(2-vinylpyridine) heteroarm star copolymers, PSnPVPn, in aqueous media has been studied by a combination of light scattering and fluorescence techniques. Three copolymer samples differing in numbers of arms and their length form micelles with compact hydrophobic cores and polyelectrolyte poly(2-vinylpyridine) shells. The association number decreases with increasing number of water-soluble arms and their length. Micellization equilibrium is kinetically frozen in aqueous media. The micellar cores are frozen and the behavior of the micellar solutions is controlled by the polyelectrolyte behavior of the polyelectrolyte shell. In the case of the present heteroarm stars with ca. 101 arms, the soluble and insoluble arms are strongly segregated and the micellar cores are formed of pure polystyrene as with micelles formed by linear diblock copolymers. As concerns the sample with several tens of arms, a significant steric hindrance, together with a low association number (ca. 4), does not allow for a complete segregation of polystyrene and the poly(2-vinylpyridine) arms. The micellar cores formed by the very hairy star copolymer with several tens of arms contain a fraction of poly(2-vinylpyridine) arms as shown by fluorescence measurements with solubilized pyrene. Alkalimetric titration of an acidic mixture of PSnPVPn micelles with a linear poly(2-vinylpyridine)-block-poly(ethylene oxide) copolymer, PVP−PEO, yields fairly monodisperse spherical onion-skin micelles. The onion-skin micelle has a compact spherical PS core deriving from the parent PSnPVPn micelle, a fairly thin and compact middle layer formed by PVP both from the parent micelle and PVP−PEO copolymer, and a protective PEO shell. The formation of onion-skin micelles is fully reversible and occurs suddenly and very rapidly at pH values higher than 4.8. The “onions” dissociate immediately into PSnPVPn micelles and PVP−PEO copolymer below pH 4.8. Evaluation of the alkalimetric titration data shows that only ca. 40% of the PVP units in the micellar shells are protonized (even in strongly acidic solutions that contain a large surplus of a strong acid, e.g., in 0.1 M HCl). This observation is in agreement with indirect fluorometric data which indicate that in the case of a weak polyelectrolyte, such as poly(2-vinylpyridine), the inner layer of the polyelectrolyte shell close to the core is not ionized.

Hydrodynamic dimensions of heteroarm star copolymers by size exclusion chromatography

Heteroarm star copolymers, i.e. star-shaped polymers bearing A and B pure arms (type A n B n ) were synthesized by anionic polymerization and their hydrodynamic properties in a common good solvent were investigated by means of size exclusion chromatography. The proposed method allows the study of the evolution of the hydrodynamic dimensions of the heteroarm star copolymers as the second generation of the arms is growing from the cores. Different growth rates of the A n B n star size were observed which are influenced by three factors: the number of arms, the ratio of the size of the chemically different arms and the interactions between the unlike segments. The last factor is affected significantly by the selectivity of the solvent.

Amphiphile micelles formed by polystyrene/poly(2-vinyl pyridine) heteroarm star copolymers in toluene

We report the micellar behavior of polystyrene/poly(2-vinyl pyridine) heteroarm star copolymers (PSnP2VPn) in toluene which is a selective solvent of polystyrene. A series of PSnP2VPn were synthesized by anionic polymerization and their micellar characteristics such as critical micelle concentrations (cmc), aggregation numbers, hydrodynamic dimensions and core–corona sizes were determined by the means of static and dynamic light scattering, viscometry and transmission electron microscopy. All these parameters were studied as a function of the insoluble arm length and the corresponding scaling relations were established. Our system seems to belong to the class of amphiphile micelles characterized by strong dependencies of the aggregation number and the microdomain sizes on the insoluble arm length. This behavior is also characteristic of strongly segregated diblock and triblock copolymer systems.

Synthesis of Model A5B1 Heteroarm Star Copolymers Using Living Radical Polymerization Mediated by 2,2,6,6-Tetramethylpiperidine-1-oxyl

Synthesis of Model A 5 B 1 Heteroarm Star Copolymers Using Living Radical Polymerization Mediated by 2,2,6,6-Tetramethylpiperidine-1-oxyl

Polystyrene/poly(2-vinyl pyridine) heteroarm star copolymer micelles in toluene: morphology and thermodynamics

Polystyrene/poly(2-vinyl pyridine) heteroarm star copolymer (i.e. star-shaped polymers consisting of a central poly(divinyl benzene) core bearing an equal number of polystyrene and poly(2-vinyl pyridine) arms, PS 6P2VP 6) and the corresponding diblock copolymer were investigated in toluene, which is a selective solvent for PS. The star copolymers are associated in polymolecular micelles following the closed association model. The characteristics of micelles were determined by means of static light scattering, viscometry and cryo-transmission electron microscopy. The micelles adopt a core-shell structure of spherical shape with core radius almost equal to the corona thickness. Comparing their solution behaviour with that of the corresponding diblock copolymer, having the same block lengths as those of the star arms, significant differences were observed. The A n B n exhibit cmc which is three orders of magnitude higher and an aggregation number which is about one order of magnitude lower with respect to the AB copolymer. The thermodynamics of micellization were also studied. Δ G 0, Δ H 0 and Δ S 0 were found to be negative for both systems. Δ G 0 is less negative for the star copolymer implying a smaller driving force for micellization. This is due to a pronounced loss of the combinatorial entropy of the star polymer arms.