Asymmetric Copolymers Research Articles

Insight into the Solution Self-Assembly of Amphiphilic Asymmetric Brush Copolymers via Computer Simulations

Insight into the Solution Self-Assembly of Amphiphilic Asymmetric Brush Copolymers via Computer Simulations

Molecular Simulation Study of the Kinetics and Mechanism of Micellization of Charged and Uncharged Block Copolymer in Aqueous Solution: Polystyrene-Block-Poly(acrylic acid) PS-b-PAA

The micellization kinetics, dynamics, and diffusional properties of poly(styrene)-block-poly(acrylic acid) copolymer chains in salt-free aqueous solution have been investigated as a function of the copolymer composition (XPS ) for the cases of un-ionized (charge density f = 0) and ionized PAA blocks (charge density f = 1), for the first time to our knowledge via atomistic molecular dynamic simulations. The micelle formation mechanism was inspected by tracking the population of unimers and clusters across the simulation trajectory; it confirmed that the asymmetric copolymer micelle formation followed a combined approach of unimer insertion and cluster fusion mechanisms, while symmetric micelle formation followed the unimer insertion method exclusively. Micelle formation took a longer time for copolymers having charged (ionized) PAA blocks (f > 0) and relatively short PS blocks (number fraction of PS repeat units in copolymer XPS < 0.5) due to the presence of a greater number of hydrophobic groups, in agreement with the micellization kinetics observed for model copolymer micelles via DPD (Dissipative Particle Dynamics) simulation studies in literature. The conformational dynamics were studied using the relaxation of backbone dihedral angles and radius of gyration, for the core and corona blocks, respectively, showed that the PS blocks relaxed slower as compared to the soluble PAA blocks in solution due to their hydrophobic nature. The conformational relaxation time increased linearly for the PS blocks and was invariant for the PAA blocks with respect to XPS. The interaction dynamics of the PS-b-PAA copolymer micelles studied via the relaxation times of the PAA-PAA inter-chain (τHB,PP) and PAA-water inter-molecular hydrogen bonds (τHB,PW) showed the increase of τHB,PP, and the decrease of τHB,PW in solution with the increase in XPS. The PAA-water H-bond relaxes slower at f = 1 than that at f = 0 due to the stronger affinity of ionized PAA units with water molecules. The diffusivity of the copolymer chains decreased exponentially with an increase in XPS value, which is in agreement with the theoretical and experimental observations described in the literature.

Self-assembly of amphiphilic asymmetric comb-like copolymers with responsive rigid side chains.

Amphiphilic asymmetric comb-like copolymers (AACCs) exhibit distinct self-assembly behaviours due to their unique architecture. However, the synthetic difficulties of well-defined AACCs have prohibited a systematic understanding of the architecture-morphology relationship. In this work, we conducted dissipative particle dynamics simulations to investigate the self-assembly behaviours of AACCs with responsive rigid side chains in selective solvents. The effects of side chain length, number of branches, and spacers on the morphology of aggregates were investigated by mapping out morphology diagrams. Besides, the numbers and surface areas of aggregates clearly depicted the morphological transitions during the self-assembly process. Moreover, the rod-to-coil conformation transitions were simulated to explore the stimuli-responsive behaviour of the AACCs with responsive rigid side chains by adjusting the bond angle parameter of the rigid chains. The results indicated that without the support of the rigid chains, the assembly structure collapsed, leading to the tube-to-channelized micelles and one-compartment-to-multicompartment vesicle morphology transformations. The simulation results are consistent with earlier experimental results, which can provide theoretical guidance for assembly toward desired nanostructures.

Stabilizing undulated lamellae by diblock copolymers confined in alternately adsorbed thin films

The self-assembly behavior of AB diblock copolymer confined in staggering alternately adsorbed thin films is studied using the self-consistent field theory (SCFT) and dissipative particle dynamics (DPD), focusing on the emergence and stability of the undulated lamella (UL) phase. Phase diagrams for the volume fraction of blocks, period of the adsorbed pattern and thickness of the confinement are constructed. Our results indicate that the UL phase can be stable in the case with a large period of adsorbed pattern, and is more favorable in the asymmetrical diblock copolymer with volume fraction around f = 0.40, which becomes cylindrical phases (C) with too small f and transforms into tilted lamellae (TL) on the contrary. Furthermore, the number of layers in the UL phases can be regulated by adjusting the period of the adsorbed pattern and the confinement degree of the thin film. In addition, the formation of the UL is verified by DPD simulations. The study demonstrates that the specially confined diblock copolymers provide an efficient route to regulate the stability of the complex UL phases.

Facile synthesis of asymmetric tetrablock copolymer by dual thermal and photochemical initiator

Vesicles obtained by the self-assembly of asymmetric tetrablock copolymers are a class of materials with fascinating properties. However, simple strategies to synthesize asymmetric tetrablock copolymers are limited. Herein, we have designed and synthesized an initiator which generates radicals by either heating or photoirradiation. Taking the advantages of organocatalyzed reversible complexation mediated radical polymerization which has lower energy requirements, milder reaction conditions, and reduced generation of hazardous byproducts, a three steps synthetic route following “photo-photo-thermal” initiation condition for the preparation of asymmetric CABC tetrablock copolymers with controllable molecular weight, precise block composition and low polydispersity was developed.

Methyl Group in Asymmetric DPP n-Type Copolymers Impedes Aggregation and Charge Transport Anisotropy

Methyl Group in Asymmetric DPP n-Type Copolymers Impedes Aggregation and Charge Transport Anisotropy

Topological structure-induced self-assembly of highly asymmetric PLLA-b-QPDMAEMA block copolymers in solution: Linear versus bottlebrush

During self-assembly process of block copolymers in solution, when the volume fraction of solvophobic blocks is too low, usually below 10%, the copolymers generally tend to dissolve, which has caused trouble. Herein, a bottlebrush block copolymer composed of poly(L-lactic acid) (PLLA) and quaternized poly(N,N-dimethylaminoethylmethacrylate) (QPDMAEMA), whose content of solvophobic block PLLA was below 2 wt%, was prepared. Owing to the rigidity derived from the advantages of topological structure of bottlebrush-like molecules, the interfacial curvature between two incompatible microphases is maintained, and this highly asymmetric copolymer unexpectedly spontaneously assembles into uniform spherical micelles with diameters of tens of nanometers. The morphology of aggregates was verified by dynamic light scattering, transmission electron microscopy and atomic force microscope. This study expands the understanding of traditional theory about copolymers self-assembly in solution, which helps to design, prepare, and apply polymer micelles with various structures from a broader perspective.

Regioregular Asymmetric Diketopyrrolopyrrole Copolymers with Good Molecular Ordered Assembly Ability for High-performance Polymer Transistors

Regioregular Asymmetric Diketopyrrolopyrrole Copolymers with Good Molecular Ordered Assembly Ability for High-performance Polymer Transistors

Liposome-based measurement of light-driven chloride transport kinetics of halorhodopsin

We report a simple and direct fluorimetric vesicle-based method for measuring the transport rate of the light-driven ions pumps as specifically applied to the chloride pump, halorhodopsin, from Natronomonas pharaonis (pHR). Previous measurements were cell-based and methods to determine average single channel permeability challenging. We used a water-in-oil emulsion method for directional pHR reconstitution into two different types of vesicles: lipid vesicles and asymmetric lipid-block copolymer vesicles. We then used stopped-flow experiments combined with fluorescence correlation spectroscopy to determine per protein Cl- transport rates. We obtained a Cl− transport rate of 442 (±17.7) Cl−/protein/s in egg phosphatidyl choline (PC) lipid vesicles and 413 (±26) Cl−/protein/s in hybrid block copolymer/lipid (BCP/PC) vesicles with polybutadine-polyethylene oxide (PB12PEO8) on the outer leaflet and PC in the inner leaflet at a photon flux of 1450 photons/protein/s. Normalizing to a per photon basis, this corresponds to 0.30 (±0.07) Cl−/photon and 0.28 (±0.04) Cl−/photon for pure PC and BCP/PC hybrid vesicles respectively, both of which are in agreement with recently reported turnover of ~500 Cl−/protein/s from flash photolysis experiments and with voltage-clamp measurements of 0.35 (±0.16) Cl−/photon in pHR-expressing oocytes as well as with a pHR quantum efficiency of ~30%.

Effects of Asymmetric Molecular Architecture on Chain Stretching and Dynamics in Miktoarm Star Copolymers

We use broad-band dielectric spectroscopy (BDS) and small-angle X-ray scattering (SAXS) to investigate the impact of architectural asymmetry in the miktoarm star copolymer on the dielectric polymer relaxations. The miktoarm copolymers studied contain one or two polystyrene (PS) chains of constant molecular weight and two identical poly(cis-1,4-isoprene) (PI) chains with varied molecular weights. Using the chains in the PI block as dielectric probes, we find that the architecturally asymmetric miktoarm star copolymer systems (PSPI2) feature distributions in chain relaxation times and dielectric relaxation strengths that are not dependent on molecular weight or morphology, in stark contrast to the effects of morphological confinement observed for symmetric diblock systems (PS-b-PI or PS2PI2). Along with evidence from the SAXS measurements regarding phase separation, these results are attributed to influences from chain stretching within the framework of the Gaussian chain model for block copolymer systems. As such, the molecular architecture in block copolymers happens to be a versatile handle to control polymer chain dynamics and, ultimately, the macroscopic physicochemical properties in architecturally complex polymers.

Asymmetric Siloxane Functional Side Chains Enable High-Performance Donor Copolymers for Photovoltaic Applications.

In this work, three benzodithiophene-benzotriazole alternated wide band gap copolymers attaching symmetric or asymmetric conjugated side chains, namely, PDBTFBTA-2T, PBDTFTBA-TSi, and PBDTFBTA-2Si, were developed for efficient nonfullerene polymer solar cells. The symmetry effect of the side chains was investigated in detail on the overall properties of these donor polymers. The results demonstrated that the introduced siloxane functional groups showed less effect on the absorption and frontier orbital levels of the prepared polymers but had a significant effect on the miscibility between these polymer donors and the nonfullerene acceptor. When increasing the content of siloxane functional groups, the miscibility of the polymer donors and Y6 would be improved, leading to the decreased domain size and more mixed domains. Interestingly, the active blend based on PBDTFTBA-TSi with asymmetric side chains exhibited more balanced miscibility, carrier mobility, and phase separation, benefiting exciton diffusion and dissociation. Therefore, a champion power conversion efficiency (PCE) of 14.18% was achieved finally in PBDTFTBA-TSi devices, which was 20.6 and 19.0% higher than the symmetric counterparts of PBTFBTA-2T devices (PCE = 11.76%) and PBDTFBTA-2Si devices (PCE = 11.92%), respectively. This work highlights that the asymmetric side-chain engineering based on siloxane functional groups is a promising design strategy for high-performance polymer donor semiconductors.

Gradient and asymmetric copolymers: the role of the copolymer composition profile in the ionization of weak polyelectrolytes

The ionization of weak polyelectrolytes can be altered by controlling the composition profile of the comonomers along the chain.

Block copolymer microparticles comprising inverse bicontinuous phases prepared via polymerization-induced self-assembly.

Traditionally, post-polymerization processing routes have been used to obtain a wide range of block copolymer morphologies. However, this self-assembly approach is normally performed at rather low copolymer concentration, which precludes many potential applications. Herein, we report a facile method for the preparation of block copolymer particles exhibiting complex internal morphology via polymerization-induced self-assembly (PISA). More specifically, a series of diblock copolymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) alternating copolymerization of styrene (St) with N-phenylmaleimide (NMI) using a poly(N,N-dimethylacrylamide) (PDMAC) stabilizer as a soluble precursor. Conducting such PISA syntheses in a 50 : 50 w/w ethanol/methyl ethyl ketone (MEK) mixture leads directly to the formation of micrometer-sized PDMAC-P(St-alt-NMI) diblock copolymer particles at 20% w/w solids. Adjusting the degree of polymerization (DP) of the core-forming P(St-alt-NMI) block to target highly asymmetric copolymer compositions provides convenient access to an inverse bicontinuous phase. TEM studies of intermediate structures provide useful insights regarding the mechanism of formation of this phase. SEM studies indicate that the final copolymer particles comprise perforated surface layers and possess nanostructured interiors. In addition, control experiments using 1,4-dioxane suggest that the high chain mobility conferred by the MEK co-solvent is essential for the formation of such inverse bicontinuous structures. One-pot PISA formulations are reproducible and involve only cheap, commercially available starting materials, so they should be readily amenable to scale-up. This augurs well for the potential use of such nanostructured micrometer-sized particles as new organic opacifiers for paints and coatings.

Asymmetric Copolymers: Synthesis, Properties, and Applications of Gradient and Other Partially Segregated Copolymers.

Asymmetric copolymers are a class of materials with intriguing properties. They can be defined by a distribution of monomers within the polymer chain that is neither strictly segregated, as in the case of block copolymers, nor evenly distributed throughout each chain, as in the case of statistical copolymers. This definition includes gradient copolymers as well as block copolymers that contain segments of statistical copolymer. In this review, different methods to synthesize asymmetric copolymers are first discussed. The properties of asymmetric copolymers are investigated in comparison to those of block and random counterparts of similar composition. Finally, some examples of applications of asymmetric copolymers, both academic and industrial, are demonstrated. The aim of this review is to provide a perspective on the design and synthesis of asymmetric copolymers with useful applications.

Stereocomplex Crystallization between l- and d-Configured Staggered Asymmetric Random Copolymers Based on 2-Hydroxyalkanoic Acids

Stereocomplex (SC) crystallization in staggered asymmetric 2-hydroxyalkanoic acid-based random copolymer blends of l-configured poly(l-2-hydroxybutanoic acid-co-l-lactic acid) [P(L-2HB-co-LLA)] (50/50) and d-configured poly(d-2-hydroxybutanoic acid-co-d-2-hydroxy-3-butanoic acid) [P(D-2HB-co-D-2H3MB)] (50/50) composed of monomers with three different chemical structures was reported for the first time. The wide-angle X-ray diffractometry profiles and interplanar distance values of SC crystallites of the staggered asymmetric random copolymer blends were similar to those of poly(l-2-hydroxybutanoic acid)/poly(d-2-hydroxybutanoic acid) SC crystallites. The melting temperature values of SC of copolymer blend samples were much higher than those of P(L-2HB-co-LLA) and P(D-2HB-co-D-2H3MB) individual crystallites of the nonblended P(L-2HB-co-LLA) and P(D-2HB-co-D-2H3MB) samples (90.5 and 96.2–126.0 °C, respectively). Moreover, this article reports for the first time that the 2-hydroxyalkanoic acid-based random co...

Temperature-Selective Dual Radical Generation from Alkyl Diiodide: Applications to Synthesis of Asymmetric CABC Multi-Block Copolymers and Their Unique Assembly Structures.

Temperature-selective radical generation from a newly designed alkyl diiodide (I-R2 -R1 -I) was studied. R1 -I and I-R2 had different reactivities for generating alkyl radicals in the presence of a tetraoctylammonium iodide (ONI) catalyst. Taking advantage of the temperature selectivity, we used the alkyl diiodide as a dual initiator in ONI-catalyzed living radical polymerization to uniquely synthesize CABC non-symmetric multi-block copolymers. Because of their non-symmetric structure, CABC multi-block copolymers form unique assemblies, that is, Janus-type particles with hetero-segment coronas and flower-like particles with hetero-segment petals.

Morphology and linear rheology of comb-like copolymer melts with high grafting density: Ⅱ. Heterografted PVSt-g-(PS/PE) comb-like copolymer with short backbone and mixed side chains

A series of heterografted PVSt-g-(PS/PE) based on polystyrene (PS) backbone with mixed PS and polyethylene (PE) side chains was obtained, in which the branching density of PVSt-g-(PS/PE) was above 72%. Because of the extremely short backbone, long side chains and high branching density in these comb-like copolymers, the topological structure of PVSt-g-(PS/PE) was similar to asymmetrical star copolymers. By adjusting the molar ratio of PS and PE side chains, the total PS (PVSt backbone + side chain PS) volume fraction in the graft copolymers could be adjusted. PVSt-g-(PS/PE) showed a microphase-separated structure at the melt state, depending on the composition of the copolymers. When the volume fraction of PS changed from 42.7% to 76.9%, the microphase-separated structures changed from lamellar phase structure to cylindrical morphology. Furthermore, all the PVSt-g-(PS/PE) samples with the microphase-separated structure showed a typical rheological behavior of cross-linked network-like structure, but the change trend of melt elasticity of the copolymer with increasing temperature depended on the microphase-separated structure.

Nanostructured Polymer Particles as Additives for High Conductivity, High Modulus Solid Polymer Electrolytes

For the next generation of safe and high energy rechargeable lithium metal batteries, we introduce nanostructured polymer particles of asymmetric miktoarm star copolymers as additives to liquid electrolytes for use as solid polymer electrolytes (SPE). The mechanical properties of the resulting SPEs are dramatically improved compared to the pure liquid electrolyte (the elastic modulus increased by up to 8 orders of magnitude), while the ionic conductivity was maintained close to that of the pure liquid electrolyte. In particular, the addition of 44 wt % miktoarm stars, composed of ion conducting poly(ethylene oxide), PEO, arms that complement stiff insulating polystyrene arms, PS ((PS)n(PEO)n, where n = 30 the number of arms), in a low molecular weight PEO doped with lithium bis(trifluoromethane)sulfonamide (LiTFSI), resulted in SPEs with a shear modulus of G′ ∼ 0.1 GPa and ion conductivity σ ∼ 10–4 S/cm. The SPEs show a strong decoupling between the mechanical behavior and the ionic conductivity as G′ rem...

Phase behavior and interfacial properties of diblock copolymer-homopolymer ternary mixtures: Influence of volume fraction of copolymers and interaction energy

We use a Monte Carlo method to study the phase and interfacial behaviors of A-b-B diblocks in a blend of homopolymers, A and B, which are confined between two asymmetric hard and impenetrable walls. Our results show that, when the interaction strength is weak, the block copolymers are uniformly distributed in the ternary mixtures under considered concentrations. Under strong interaction strength, distribution region of the block copolymers changes from a single smooth interface to a curved interface or multi-layer interface in the ternary mixtures. Furthermore, our findings show that with increasing volume fraction of A-b-B diblock copolymer (ϕ C), copolymer profiles broaden while ϕ C ≥ 0.4, a lamellar phase is formed and by further increasing ϕC, more thinner layers are observed. Moreover, the results show that, with the increase of ϕ C, the phase interface first gradually transforms from plane to a curved surface rather than micelle or lamellar phase while with the increase of the interaction between A and B segments (e AB), the copolymer chains not only get stretched in the direction perpendicular to the interface, but also are oriented. The simulations also reveal that the difference between symmetric and asymmetric copolymers is negligible in statistics if the lengths of two blocks are comparable.

Asymmetric medium bandgap copolymers and narrow bandgap small-molecule acceptor with over 7% efficiency

To further investigate non-fullerene polymer solar cells based-thieno[2,3-f]benzofuran (TBF) polymers, we designed and synthesized two medium bandgap TBF-based polymers, namely TBF-BDD and TBF-BT, containing alkoxyphenyl substituted TBF electron-donor unit, 1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) and 4,7-di(thiophen-2-yl)-5,6-dioctyloxybenzo[c][1,2,5]thiadiazole (BT) electron-acceptor segment, respectively. When blended with ITIC (a n-type small molecule acceptor), two polymer:ITIC blends possess better complementary absorption than the absorption with PC71BM. The properties, including charge mobilities, and morphologies have been intensively investigated. The polymer solar cells based on TBF-BDD:ITIC and TBF-BT:ITIC (1:1,w/w) exhibited promising power conversion efficiencies (PCEs) of 7.13% and 7.03%, respectively, under the illumination of AM 1.5 G, 100 mW/cm2, which are higher in comparison with fullerene-based devices. Up to now, these PCEs are the highest among TBF-based polymer photovoltaic devices.