Copolymers In Dilute Solution Research Articles

Universal properties of branched copolymers in dilute solutions

We analyze the universal conformational properties of complex copolymer macromolecules, based on two topologies: the rosette structure containing fc linear branches and fr closed loops grafted to the central core, and the symmetric pom-pom structure, consisting of a backbone linear chain terminated by two branching points with functionalities f. We assume that the constituent strands (branches) of these structures can be of two different chemical species a and b. Depending on the solvent conditions, the inter- or intrachain interactions of some links may vanish, which corresponds to Θ-state of the corresponding polymer species. Applying both the analytical approach within the frames of direct polymer renormalization and numerical simulations based on the lattice model of polymer, we evaluated the set of parameters characterizing the size properties of constituent parts of two complex topologies and estimated quantitatively the impact of interactions between constituent parts on these size characteristics.

Carbazole-Fluorene Copolymers with Various Substituents at the Carbazole Nitrogen: Structure-Properties Relationship.

Carbazole derivatives, carbazole-containing polymers and iridium complexes are of interest due to many possible applications in photonics, electronics and biology, particularly as active or hole-transporting layers in organic as well as perovskite devices due to their interesting properties. Here, a series of carbazole-fluorene conjugated copolymers with various substituents at the N-carbazole position (2-methoxycarbonylethyl, 2-carboxyethyl, 2-ethylhexyl, and nonan-2,4-dionatoiridium(III)bis(2-phenylpyridine-N,C2')-9-yl) was prepared by Suzuki coupling. Their photophysical, electrochemical and electroluminescence (EL) properties were studied. Effects of molecular weight and substituents attached to carbazole unit on their properties are reported. The carbazole-fluorene copolymers in dilute solutions exhibited intense photoluminescence (PL) emission in the blue spectral region with high PL quantum yields (78-87%) except for the copolymer with the iridium complex (23%). Similar PL spectra were observed in dilute solutions. More pronounced differences were found in thin film PL and EL properties due to excimer/aggregate formation. Light-emitting devices (LEDs) made of copolymers with 2-ethylhexyl as N-carbazole substituent exhibited efficient EL emission with the best performance and the lowest EL onset voltages (3-4 V), while the LEDs made of copolymers with other substituents were not as efficient, but showed anomalous behavior and memory effects in current-voltage characteristics promising also for bio-inspired electronics.

Molecular Properties of Comb-Shaped Maleimide Copolymers in Dilute Solutions: Effect of Alkyl Side Chains

Molecular Properties of Comb-Shaped Maleimide Copolymers in Dilute Solutions: Effect of Alkyl Side Chains

Association of amphiphilic block copolymers in dilute solution: With and without shear forces

The goal of this study is to investigate the self-assembly of block copolymers in aqueous solution and on-line demonstration of the morphology transition of micelles under shear flow upon changing the temperature via rheological measurements. To this end, a series of poly(ethylene glycol)-block-polycaprolactone copolymers with variable block ratios and degree of polymerization was synthesized via ring opening polymerization of ε-caprolactone in the presence of methoxy poly(ethylene glycol) macroinitiator (M¯n∼2000g/mol), and the ensuing nanoobject assemblies were scrutinized by small-angle neutron scattering. We take advantage of solution rheology control to understand the micellar transition route of amphiphlies.

Synthesis and Properties of Alternating Copolymers Based on 4H-Cyclopenta[2,1-b:3,4-b']dithiophene and 4H-Dithieno[3,2-b:2',3'-d]silol

A number of copolymers based on 4H-cyclopenta[2,1-b:3,4-b']dithiophene and 4H-dithieno[3,2-b:2',3'-d]silol are synthesized via direct С–Н arylation and Suzuki cross-coupling, and their properties are compared. The chemical structure of the copolymers is investigated by 1Н and 13С NMR spectroscopy, and their molecular-mass characteristics are determined by GPC. The study of absorption spectra of the copolymers in dilute solutions and thin films shows that they absorb light in a wide visible spectral range (300–800 nm). On the basis of the cyclic voltammetry data, the levels of boundary molecular orbitals—the highest occupied and the lowest unoccupied—are estimated and the width of the energy gap is shown to be in the range of 1.4–1.9 eV. Using the copolymers as donor components of the active layer, the samples of solar photocells are manufactured and their photovoltaic properties are investigated.

Co-assembly behaviour of Janus nanoparticles and amphiphilic block copolymers in dilute solution.

Nanoparticles can co-assemble with amphiphilic block copolymers (ABPs) in solution to generate nanoaggregates with unique properties, yet the mechanism of such a co-assembly behaviour for Janus nanoparticles (JPs) and ABPs remains unclear. Here, the self-assembly behaviour of JP/ABP mixtures in dilute solution was studied via theoretical simulations. Two kinds of ABPs with different volume fractions fA of hydrophilic blocks were considered: one is symmetric copolymers with fA = 0.5, and the other is asymmetric ABPs with fA = 0.3. In the first case, mixtures of spheres and rods, connected networks and vesicles were formed sequentially as the volume fraction cJP of nanoparticles increases. In the second case, vesicles were constantly formed. For both cases, at lower cJP values, the nanoparticles were located at the core-corona interfaces. By contrast, at higher particle loadings, a large number of particles were involved in clusters embedded in the vesicle walls. Based on the simulation results, a morphological diagram in the space of cJP and fA was constructed to indicate the stability regions of different nanostructures. Specifically, it was found that the vesicles formed by JPs and ABPs with short hydrophilic blocks are stimuli-responsive. By changing the interaction parameters between hydrophobic blocks, controllable pores in the vesicle walls could be created. Our findings not only provide insights into the co-assembly behaviour of Janus nanoparticles and amphiphilic block copolymers in solution, but also offer a novel strategy to prepare nanoreactors with permeable membranes.

RAFT polymerization and dually responsive behaviors of terpyridine-containing PNIPAAm copolymers in dilute solutions

Introduction of new components is an effective way to gain new functionalities and to regulate the properties of responsive polymers. In the present work, a RAFT polymerization approach to terpyridine-containing poly(N-isopropyl acrylamide) (PNIPAAm) copolymers was developed and the dually responsive behavior of the obtained polymer to temperature variation and metal ions in dilute solutions were investigated. By utilizing RAFT polymerization, the molecular weight polydispersity index of the polymer can be reduced compared with the traditional free radical polymerization method. Due to the coexistence of thermosensitive component and terpyridine, the synthesized polymer is thermosensitive and metal ion sensitive. Further titration investigation revealed that this polymer may be utilized as a water-soluble and colorimetric sensor for Fe2+ and Fe3+ and the obvious color change from a colorless state to a purple state makes the polymer a potential sensor to detect Fe2+ and Fe3+ macroscopically by naked eye. Overall, the terpyridine-containing poly(N-isopropyl acrylamide) copolymers may enrich the family of terpyridine-containing polymers and provide some clues for developing new multi-responsive and multi-functional polymers.

Charged Star Diblock Copolymers in Dilute Solutions: Synthesis, Structure, and Chain Conformations

We present a systematic investigation of a novel series of star polymers consisting of arms made up from poly(N-isopropylacrylamide)-b-poly(2-acrylamido-2-methylpropanesulfonate) (PNIPAAM-block-PAMPS) block copolymers. The polymers were synthesized as a 3-arm and 2-arm (i.e., a tetrablock copolymer) using a “core-first” method and a sequential atomic transfer radical polymerization (ATRP) protocol. Using asymmetric flow field-flow fractionation (AFFFF), Zetasizer, and small-angle X-ray scattering (SAXS), the phase behavior and nanostructure of the system in dilute solutions are studied in detail. While AFFFF equipped with a light scattering and refractive index detectors provides distribution of molecular weight and overall sizes in solution, we use SAXS combined with theoretical modeling to elucidate the inter- and intramolecular interactions of the star polymers. In particular, by employing a detailed model for a star-diblock copolymer assuming Gaussian chain statistics, we extract the chain conformation for each polymer block separately. We find that the radii of gyration, Rg, for both PNIPAAM and PAMPS are very similar to the expected dimension of free chains in solution. By adding salt, we show that the strong interstar repulsion found in water is dramatically reduced after adding as little as 0.025 M NaCl. Further increase of NaCl up to 0.2 M shows that the system essentially behaves as neutral polymers in a good solvent. Concerning the chain conformations, addition of NaCl seems to have a small effect on the Rg of the different blocks.

Phase Diagrams of Polynorbornene Amphiphilic Block Copolymers in Solution

We report phase diagrams for amphiphilic block copolymers prepared via ring-opening metathesis polymerization (ROMP). A library of 30 block copolymers with variable hydrophilic functionality, block ratios, and degrees of polymerization was prepared, and the resulting assemblies were analyzed by small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). A phase diagram of the self-assemblies was constructed for each of the various copolymer systems screened, representing the first of its kind for polynorbornene block copolymers in dilute solutions. Furthermore, we take advantage of kinetic control in the preparation of an array of particle morphologies accessed from the same polymer structure.

Online rheological investigation on ion-induced micelle transition for amphiphilic polystyrene-block-poly(acrylic acid) diblock copolymer in dilute solution.

The ion-induced micellar transition is online-investigated by the time dependence of the viscosity of the solution under shear flow for the first time. During the morphological transition, the change in the micellar structure can be tracked by the change in viscosity. Adding HCl or CaCl2 into pre-prepared spherical micelle solution from the self-assembly of polystyrene-block-poly(acrylic acid) (PS144-b-PAA22) in the N,N-dimethylformamide (DMF)/water mixture, the micellar structures change into short cylinders, long, entangled cylinders, and then lamellae or vesicles, corresponding to the viscosity increasing first and then declining. When HCl or CaCl2 is added to the pre-prepared spherical micelle solution formed by PS144-b-PAA50 in the dioxane/water mixture, the micellar structures are quickly transformed into cylinders or lamellae before carrying out the rheological measurement and then are turned to vesicles or spheres under the shearing, corresponding to a gradual decline in viscosity. This study shows that the rheology can be a very simple and effective online method on the investigation of the micellization, which plays an important role in understanding the micellization mechanism and micellar transition pathway of block copolymers in dilute solution.

Dissipative Particle Dynamics Simulation on Aggregation of Rod-Coil-Rod Triblock Copolymer in Dilute Solution

采用耗散粒子动力学方法模拟研究了rod-coil-rod三嵌段共聚物在稀溶液中的聚集行为。分别考察了rod-coil嵌段的相互作用、溶剂性质、共聚物浓度以及coil嵌段长度对聚集体形貌的影响。模拟结果发现，随着rod-coil相互排斥作用的增加，共聚物由球形转变成洋葱状、笼形和柱状结构。随着coil嵌段疏水性的增加，笼形转变成洋葱状和补丁状结构。给出了聚集体形貌随共聚物浓度和coil长度变化的相图。当浓度较小和coil嵌段较长时，共聚物形成笼状聚集体，反之，则有利于洋葱状结构的形成。

Thermo-/pH-dual responsive properties of hyperbranched polyethylenimine grafted by phenylalanine

Novel thermo- and pH-dual responsive amphiphilic copolymers were synthesized based on hyperbranched polyethylenimine (PEI) by grafting l-phenylalanine. The phenylalanine-modified PEI exhibited lower cytotoxicity than commercial PEI. These copolymers showed the phenomena of phase transitions in response to pH and temperature. The dilute copolymer solution at lower pH displayed the higher LCST. Furthermore, LCST increased with the increasing of phenylalanine grafting density. LCST of these copolymers were tunable from 7.2 to 59.6 °C by the degree of amidation and pH of solution. DLS and TEM experiments certified that the copolymer chains aggregated to form small size particles as increasing the temperature above LCST. For these reasons, the obtained smart copolymers were considered to be potential gene/drug carriers in biomedical field.

Enhanced understanding and implementation of the self-assembly of fluorosilicone double-hydrophobic diblock copolymers in dilute solutions from thermodynamic perspective: The effect of different preparation factors

The aggregate properties of the fluorosilicone double-hydrophobic poly(dimethylsiloxane)-b-poly(2,2,3,3,4,4,4-heptafluorobutyl methacrylate) diblock copolymers (PDMS-b-PHFBMA) in organic solvents were investigated experimentally in this article. Different factors such as the composition of the solvents, polymer concentration, copolymer block length as well as the removal of the common solvent in the system were investigated. Besides, a classical thermodynamic model for surfactant self-assembly was applied to describe the mechanism of the formation and transition between different morphologies. It was found that the transfer free energy, the aggregate core-solvent interfacial free energy, the deformation free energy of the non-soluble block as well as the soluble block steric interactions had their specific contributions to the formation and transition of different morphologies induced from the self-assembly behavior of PDMS-b-PHFBMA. As a whole, the qualitative description of the mechanism of self-assembly and morphology transition behavior in macromolecular system can be easily achieved using the classical thermodynamic approach for surfactant self-assembly, while it still needs to be further improved.

Kinetic Analysis of Quasi-One-Dimensional Growth of Polymer Lamellar Crystals in Dilute Solutions

Flexible polymers are featured with two-dimensional growth of metastable chain-folded lamellar crystals in quiescent dilute solutions. Recently, a massive cylindrical micelle with quasi-one-dimensional (quasi-1D) growth driven by confined crystallization of diblock copolymers in dilute solutions raised a new challenge. We performed dynamic Monte Carlo simulations to investigate the kinetics of quasi-1D growth of lamellar crystals in two typical cases of dilute but not very dilute polymer solutions. We found that in both cases the growth kinetics is dominated by the surface-nucleation-controlled mechanism. Moreover, in the first case corresponding to few and small crystals grown under almost constant polymer concentrations in the huge bulk of solutions, the driving-force term in the kinetic equation dominates a linear concentration dependence of crystal growth rates in the high-concentration region, and the nucleation-barrier term dominates their nonlinear deviation in the low-concentration region. In the second case corresponding to massive crystals grown under depleting polymer concentrations in a limited volume of solutions, the crystal growth rates decay with time, but at the early stage, they follow exactly with the linear-concentration-dependent growth rates of the first case. Therefore, the growth size at the early stage of the second case can be described as an exponential-decay function of time, which provides a theoretical model to the data analysis of corresponding experimental observations.

Self-assembly structures of amphiphilic multiblock copolymer in dilute solution

We study the influence of solvent conditions and the chain backbone stiffness of an amphiphilic multiblock copolymer on its self-assembly structures in dilute solution with Brownian dynamics simulations. Various interesting structures, such as single-flower micelle, multi-flower micelle, and single or multi-bridge structures, are observed. In general, highly hydrophobic components benefit the formation of flower micelle structures, while semi-flexible chains are prone to forming bridge structures. We also study the influence of the ratio between hydrophobic and hydrophilic components on self-assembly structures. From our phase diagram, we find that a high component ratio for the hydrophobic blocks favors the formation of micelles with various structural patterns. A single to multi-flower transition is observed by increasing the chain length of the multiblock copolymer. Well-defined multicompartment wormlike micelles can be obtained from pre-assembled flower micelles in a solvent that is poor for both components.

Study of hierarchical microstructures self-assembled by π-shaped ABC block copolymers in dilute solution using self-consistent field theory

Microstructures self-assembled by amphiphilic ABC π-shaped block copolymers in dilute solution have been investigated by self-consistent field theory. The effects of architectural parameters and the interaction strength among the three blocks have been studied systematically. Our calculation results show that the distance of the two graft blocks has stronger effect than the length of graft blocks and the position of the first graft point on the phase behavior. The interaction strength among the three blocks is another important factor in controlling the resulting microstructures. Compound-core, multicompartment, and multicore micelles are observed in the case of π-shaped ABC block copolymers with hydrophilic backbone block A and hydrophobic graft blocks B and C. Core–shell–corona, incomplete skin-layered and hamburger micelles are formed when graft block C is hydrophilic and blocks A and B are hydrophobic. The wormlike multicore micelles have drawn our attention. We find that the morphology of wormlike multicore micelle can be controlled by changing the distance of the two graft blocks of the π-shaped block copolymers. In all of the wormlike multicore micelles, the streamline wormlike micelle is more stable than other wormlike micelles from the free energy analysis.

Brownian dynamics simulation study on the self-assembly of incompatible star-like block copolymers in dilute solution

We study the self-assembly of symmetric star-like block copolymers (A(x))(y)(B(x))(y)C in dilute solution by using Brownian dynamics simulations. In the star-like block copolymer, incompatible A and B components are both solvophobic, and connected to the center bead C of the polymer. Therefore, this star-like block copolymer can be taken as a representative of soft and deformable Janus particles. In our Brownian dynamics simulations, these "soft Janus particles" are found to self-assemble into worm-like lamellar structures, loose aggregates and so on. By systematically varying solvent conditions and temperature, we build up the phase diagram to illustrate the effects of polymer structure and temperature on the aggregate structures. At lower temperatures, we can observe large worm-like lamellar aggregates. Upon increasing the temperature, some block copolymers detach from the aggregate; this phenomenon is especially sensitive for the polymers with less arms. The aggregate structure will be quite disordered when the temperature is high. The incompatibility between the two parts in the star-like block copolymer also affects the self-assembled structures. We find that the worm-like structure is longer and narrower as the incompatibility between the two parts is stronger.

Mechanistic Insights for Block Copolymer Morphologies: How Do Worms Form Vesicles?

Amphiphilic diblock copolymers composed of two covalently linked, chemically distinct chains can be considered to be biological mimics of cell membrane-forming lipid molecules, but with typically more than an order of magnitude increase in molecular weight. These macromolecular amphiphiles are known to form a wide range of nanostructures (spheres, worms, vesicles, etc.) in solvents that are selective for one of the blocks. However, such self-assembly is usually limited to dilute copolymer solutions (<1%), which is a significant disadvantage for potential commercial applications such as drug delivery and coatings. In principle, this problem can be circumvented by polymerization-induced block copolymer self-assembly. Here we detail the synthesis and subsequent in situ self-assembly of amphiphilic AB diblock copolymers in a one pot concentrated aqueous dispersion polymerization formulation. We show that spherical micelles, wormlike micelles, and vesicles can be predictably and efficiently obtained (within 2 h of polymerization, >99% monomer conversion) at relatively high solids in purely aqueous solution. Furthermore, careful monitoring of the in situ polymerization by transmission electron microscopy reveals various novel intermediate structures (including branched worms, partially coalesced worms, nascent bilayers, "octopi", "jellyfish", and finally pure vesicles) that provide important mechanistic insights regarding the evolution of the particle morphology during the sphere-to-worm and worm-to-vesicle transitions. This environmentally benign approach (which involves no toxic solvents, is conducted at relatively high solids, and requires no additional processing) is readily amenable to industrial scale-up, since it is based on commercially available starting materials.

Nanostructure formation in aqueous solution of amphiphilic copolymers of 2-( N, N-dimethylaminoethyl)methacrylate and alkylacrylate: Characterization, antimicrobial activity, DNA binding, and cytotoxicity studies

Three amphiphilic random copolymers poly(2-(dimethylaminoethyl)methacrylate-co-alkylacrylate) (where, alkyl = hexyl, octyl, dodecyl) with 16 mol% hydrophobic substitution were synthesized. Surface tension, viscosity, fluorescence probe, dynamic light scattering (DLS), as well as transmission electron microscopic (TEM) techniques were utilized to investigate self-assembly formation by the hydrophobically modified polymers (HMPs) in pH 5. Formation of hydrophobic domains through inter-polymer chain interaction of the copolymer in dilute solution was confirmed by fluorescence probe studies. Average hydrodynamic diameter of the copolymer aggregates at different polymer concentration was measured by DLS studies. The copolymer with shorter hydrophobic chain exhibits larger hydrodynamic diameter in dilute solution, which decreased with either increase of concentration or increase of hydrophobic chain length. TEM images of the dilute solutions of the copolymers with shorter as well as with longer hydrophobic chain exhibit spherical aggregates of different sizes. The antimicrobial activity of the copolymers was evaluated by measuring the minimum inhibitory concentration value against one Gram-positive bacterium Bacillus subtilis and one Gram-negative bacterium Escherichia coli. The copolymer with the octyl group as pendent hydrophobic chain was found to be more effective in killing these microorganisms. The interaction of the cationic copolymers with calf-thymus DNA was studied by fluorescence quenching method. The polymer-DNA binding was found to be purely electrostatic in nature. The hydrophobes on the polymer backbone were found to have a significant influence on the binding process. Biocompatibility studies of the copolymers in terms of cytotoxicity measurements were finally performed at different concentrations of the HMPs to evaluate their potential application in biomedical fields.

An experimental and theoretical study of the carbazole to carbazole intramolecular energy transfer in N‐vinylcarbazole/vinyl tert‐butyl‐benzoate copolymers of different molar compositions

AbstractFluorescence depolarization and quenching measurements of N‐vinyl carbazole/vinyl tert‐butyl‐benzoate copolymers in dilute solution of several fluid solvents and in a rigid poly(methyl methacrylate) matrix were performed to study the effect of copolymer composition on the intramolecular energy transfer (IET) between carbazole groups (Cz) along the polymer chain. IET efficiency, as the amount of CzCz intramolecular excimer formation, monotonically increases with Cz content up to a number average sequence length of carbazole of ca 12 (Cz molar content ≥ 90%). After this value, IET efficiency remains almost constant. Molecular dynamics simulations on isotactic and syndiotactic copolymer fragments of several monomer compositions were used for obtaining different parameters related to the efficiency for the IET process between Cz units along the polymer chain. Copyright © 2011 Society of Chemical Industry