Diblock Copolymer Solutions Research Articles

Shear flow behaviors of rod-coil diblock copolymers in solution: A nonequilibrium dissipative particle dynamics simulation

We employed the nonequilibrium dissipative particle dynamics method to study the shear flow behaviors of rod-coil diblock copolymers in solutions. The effects of copolymer concentrations and molecular architecture on the rheology are investigated. The simulated results show that the shear flow behaviors change from Newtonian to non-Newtonian when the morphologies transform from micelles to gels by increasing the copolymer concentrations. For the non-Newtonian systems, it was found that the curve of the viscosity versus shear rate is divided into three regions, that is, shear thinning region I, platform region II, and shear thinning region III. From the physical origin, the three-region behavior is governed by the distinct flow behaviors of the rod and coil blocks and their different time scale in response to the shear field. Additionally, by tuning the molecular architectures, the simulated results reveal that the slopes in region I and region III are influenced by the length of rod and coil blocks, respectively. The present research revealed the microscopic origin of the complex rheological properties of rod-coil diblock copolymers in solutions and could provide useful information for preparing functional materials based on block copolymers.

A correlation between structural features of an amphiphilic diblock copolymer in solution and the structure of the porous surface in an integral asymmetric membrane

A correlation between the pore size of isoporous block copolymer membranes produced via the combination of self-assembly with non-solvent induced phase separation (SNIPS) on one side and the macromolecular dimensions of the block copolymer in the casting solution on the other side is presented. Dilute solutions of a polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer and corresponding polystyrene (PS) and poly(4-vinylpyridine) (P4VP) homopolymers similar to the respective blocks of the diblock copolymer are investigated separately by static and dynamic light scattering (SLS, DLS) in solvent mixtures of tetrahydrofuran/N,N-dimethylformamide (THF/DMF). These measurements provide information about the size of the individual polymers in solution. Solutions of the diblock copolymer at higher concentrations are studied furthermore by small-angle X-ray scattering (SAXS) and cryo-scanning electron microscopy (cryo-SEM) in order to estimate the size of self-assembled structures in concentrated solution, as well as by scanning electron microscopy (SEM) in the final membrane. The pore radius of the selective layer in the resulting membrane obtained by SNIPS is shown to be similar to the hydrodynamic diameter of the pore forming block, as determined in dilute solution.

Control of pH- and temperature-responsive behavior of mPEG-b-PDMAEMA copolymers through polymer composition

ABSTRACTA series of poly(ethylene glycol) monomethyl ether-block-poly(2-(dimethylamino)ethyl methacrylate) (mPEG-b-PDMAEMA) diblock copolymers were synthesized using atom transfer radical polymerization to achieve controlled polymer molecular weight and narrow molecular weight distribution. The thermoresponsive properties of the mPEG-b-PDMAEMA diblock copolymers in aqueous buffered solutions were determined using UV-Visible spectroscopy and dynamic light scattering. The cloud point, a soluble-to-insoluble transition, was observed for all mPEG-b-PDMAEMA diblock copolymer solutions. Increasing either the mPEG or PDMAEMA molecular weight resulted in a decrease in observed cloud points as a function of pH and polymer concentration. Changing the mPEG molecular weight determined whether a second, higher temperature, thermal transition was observed as a function of pH and polymer concentration. Controlling the thermoresponsive properties of mPEG-b-PDMAEMA diblock copolymers through polymer composition, concentration, and pH enables the tailoring of these copolymers for applications ranging from non-viral gene delivery to use as a strengthening agent in paper.

Polyion Complex Vesicles with Solvated Phosphobetaine Shells Formed from Oppositely Charged Diblock Copolymers.

Diblock copolymers consisting of a hydrophilic poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) block and either a cationic or anionic block were prepared from (3-(methacrylamido)propyl)trimethylammonium chloride (MAPTAC) or sodium 2-(acrylamido)-2-methylpropanesulfonate (AMPS). Polymers were synthesized via reversible addition-fragmentation chain transfer (RAFT) radical polymerization using a PMPC macro-chain transfer agent. The degree of polymerization for PMPC, cationic PMAPTAC, and anionic PAMPS blocks was 20, 190, and 196, respectively. Combining two solutions of oppositely charged diblock copolymers, PMPC-b-PMAPTAC and PMPC-b-PAMPS, led to the spontaneous formation of polyion complex vesicles (PICsomes). The PICsomes were characterized using 1H NMR, static abd dynamic light scattering, transmittance electron microscopy (TEM), and atomic force microscopy. Maximum hydrodynamic radius (Rh) for the PICsome was observed at a neutral charge balance of the cationic and anionic diblock copolymers. The Rh value and aggregation number (Nagg) of PICsomes in 0.1 M NaCl was 78.0 nm and 7770, respectively. A spherical hollow vesicle structure was observed in TEM images. The hydrodynamic size of the PICsomes increased with concentration of the diblock copolymer solutions before mixing. Thus, the size of the PICsomes can be controlled by selecting an appropriate preparation method.

Self-assembly of AB diblock copolymer solutions confined in cylindrical nanopores

The self-assembly of AB diblock copolymer solutions confined in a cylindrical nanopore is investigated systematically via Monte Carlo simulations.

Amphiphilic Diblock Co-polymers Bearing a Cysteine Junction Group: Synthesis, Encapsulation of Inorganic Nanoparticles, and Near-Infrared Photoresponsive Properties.

Encapsulation of inorganic nanoparticles (NPs) in the interfaces of amphiphilic vesicles is a challenging task. The traditional strategy is to use amphiphilic triblock co-polymers, which possess two outer blocks for building the walls and coronas of the vesicles, and one middle NP binding block for localizing NPs at the vesicle interfaces. In this manuscript, we describe the design and synthesis of an amphiphilic diblock co-polymer, that is, PEG-SH-b-PS (PEG=poly(ethylene glycol), PS=polystyrene) bearing a cysteine junction with one free pendant thiol group at the center point between the hydrophilic poly(ethylene glycol) block and the hydrophobic polystyrene block. The amphiphilicity-driven self-assembly in aqueous solution of the pure linear diblock co-polymer PEG-SH-b-PS and the corresponding amphiphilic PEG-SH-b-PS/gold NPs (GNPs) nanocomposites is examined. From TEM observations of the self-assembled samples containing the conjugated GNPs, it can be concluded that most of the GNPs are dispersed at the interfaces of the formed vesicles. In addition, near-infrared (NIR)-absorbing copper monosulfide (CuS) NPs are also encapsulated into the PEG-SH-b-PS vesicles. Due to the photothermal heating effect of the CuS NPs, the corresponding PEG-SH-b-PS/CuSNPs vesicles can disassemble and release the embedded cargos under NIR illumination, which endows this nanocomposite material with potential in biomedical applications, such as cancer imaging, photothermal therapy, and drug delivery.

Aggregation Behavior of Doubly Thermoresponsive Polysulfobetaine-b-poly(N-isopropylacrylamide) Diblock Copolymers

A 2-fold thermoresponsive diblock copolymer PSPP430-b-PNIPAM200 consisting of a zwitterionic polysulfobetaine (PSPP) block and a nonionic poly(N-isopropylacrylamide) (PNIPAM) block is prepared by successive RAFT polymerizations. In aqueous solution, the corresponding homopolymers PSPP and PNIPAM feature both upper and lower critical solution temperature (UCST and LCST) behavior, respectively. The diblock copolymer exhibits thermally induced “schizophrenic” aggregation behavior in aqueous solutions. Moreover, the ion sensitivity of the cloud point of the zwitterionic PSPP block to both the ionic strength and the nature of the salt offers the possibility to create switchable systems which respond sensitively to changes of the temperature and of the electrolyte type and concentration. The diblock copolymer solutions in D2O are investigated by means of turbidimetry and small-angle neutron scattering (SANS) with respect to the phase behavior and the self-assembled structures in dependence on temperature and el...

Template-Free Bottom-Up Method for Fabricating Diblock Copolymer Patchy Particles.

Patchy particles are one of most important building blocks for hierarchical structures because of the discrete patches on their surface. We have demonstrated a convenient, simple, and scalable bottom-up method for fabricating diblock copolymer patchy particles through both experiments and dissipative particle dynamics (DPD) simulations. The experimental method simply involves reducing the solvent quality of the diblock copolymer solution by the slow addition of a nonsolvent. Specifically, the fabrication of diblock copolymer patchy particles begins with a crew-cut soft-core micelle, where the micelle core is significantly swelled by the solvent. With water addition at an extremely slow rate, the crew-cut soft-core micelles first form a larger crew-cut micelle. With further water addition, the corona-forming blocks of the crew-cut micelles begin to aggregate and eventually form well-defined patches. Both experiments and DPD simulations indicate that the number of patches has a very strong dependence on the diblock copolymer composition-the particle has more patches on the surface with a lower volume fraction of patch-forming blocks. Furthermore, particles with more patches have a greater ability to assemble, and particles with fewer patches have a greater ability to merge once assembled.

Poly(lactic acid)/poly(ethylene glycol) supramolecular diblock copolymers based on three-fold complementary hydrogen bonds: Synthesis, micellization, and stimuli responsivity

Thymine-terminated poly(lactic acid) (PLA-THY) and diaminotriazine-terminated poly(ethylene glycol) (PEG-DAT) are synthesized and they can form the supramolecular amphiphilic diblock copolymers in solution through the thymine/diaminotriazine complementary hydrogen bonding interactions. Such supramolecular copolymers self-assemble into the stimuli-responsive micelles in aqueous solution. Micelle size, crystalline state, stimuli responsibility, drug loading and release behavior of the supramolecular micelles can be readily tuned from the block length, stereostructure, homo and stereocomplex crystallizations of hydrophobic PLA blocks. The enantiomerically-mixed supramolecular copolymers with both poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) blocks assemble into the stereocomplexed micelles, while those having the isotactic PLLA (or PDLA) and atactic poly(d,l-lactic acid) (PDLLA) blocks form the homocrystalline and amorphous micelles, respectively. The stereocomplexed and amorphous micelles exhibit smaller size than their homocrystalline analogs, all of which are much larger than the micelles formed from the conventional covalently-bonded diblock copolymers. The supramolecular micelles are sensitive to the external stimuli such as pH and ion. The homocrystalline and stereocomplexed micelles undergo fast aggregation in the acidic and salted solutions due to the disassociation of complementary hydrogen bonds. The stereocomplexed micelles exhibit larger drug loading content and slower drug release rate than the amorphous and homocrystalline ones, because of the larger polymer/drug interactions and tighter chain packing inside the micelle cores.

Synthesis of Amphiphilic Diblock Copolymer Using Heterobifunctional Linkers, Connected by a Photodegradable N-(2-Nitrobenzyl)imide Structure and Available for Two Different Click Chemistries

Abstract Two photodegradable heterobifunctional linkers (PHBL) connected by a N-(2-nitrobenzyl)imide structure have been developed as tools to obtain photodegradable diblock copolymers with precisely controlled molecular weight. They can undergo two different click reactions, i.e., Huisgen cycloaddition and Michael addition. One linker has an alkyne moiety and maleimide protected as adduct with furan. Reaction of azido-terminated polystyrene (PS), poly(ε-caprolactone) (PCL) or poly(ethylene glycol) (PEG) with this PHBL followed by deprotection affords the corresponding maleimide-terminated polymer, which was coupled with thiol-terminated PEG or poly(N-isopropylacrylamide) (PNIPAM) to synthesize PS-b-PHBL-b-PEG, PCL-b-PHBL-b-PEG, and PEG-b-PHBL-b-PNIPAM. The other linker has maleimide and an alkyne protected by a trimethylsilyl group. This affords diblock copolymers by the reverse route, i.e., maleimide reacts with thiol-terminated PS to obtain a product by Michael addition, and then after deprotection, Huisgen cycloaddition of the generated alkyne- and azido-terminated PEG affords PEG-b-PHBL-b-PS. The resulting photosensitive diblock copolymers were characterized by UV spectroscopy in solution before and after photoirradiation. The energy needed for photolysis of the diblock copolymers in solution ranged from 12 to 17 J cm−2, and was independent of polymer type and molecular weight.

Electric-Field-Induced Order–Order Transition from Hexagonally Perforated Lamellae to Lamellae

Block copolymers form a variety of microphase morphologies due to their ability to phase separate. The hexagonally perforated lamellar (HPL) morphology represents an unusually long-lived, nonequilibrium transient structure between lamellar and cylindrical phases. We present a detailed study of a concentrated, HPL-forming poly(styrene-b-isoprene) diblock copolymer solution in toluene in the presence of an electric field. We will show that this phase is readily aligned by a moderate electric field and provide experimental evidence for an electric-field-induced order–order transition toward the lamellar phase under sufficiently strong fields. This process is shown to be fully reversible as lamellar perforations reconnect immediately upon secession of the external stimulus, recovering highly aligned perforated lamellae.

Fibril Crystal Growth in Diblock Copolymer Solutions Studied by Dynamic Monte Carlo Simulations

Quasi-one-dimensional fibril crystal growth of diblock copolymers is a fundamental issue in the investigation of nanotechnology and neurodegenerative diseases. We performed dynamic Monte Carlo simulations of lattice polymers to study the crystallization-driven fibril crystal growth of diblock copolymers under two circumstances of solutions: sporadic crystals with the feeding mode of constant polymer concentrations, and massive crystals with the depleting mode of decaying polymer concentrations. We confirmed anisotropic driving forces as a prerequisite of steady growth of fibril crystals. The lamellar crystal width is confined by the noncrystalline block below a critical concentration that shifts down with the decrease of the noncrystallizable block fractions. In the depleting mode, the long-axis sizes at the early stage of fibril crystal growth can be fitted well by an exponential-decay function of time, and the growth rates decrease linearly with polymer concentrations by following the growth rates in the feeding mode, appearing as consistent with our previous simulation results of homopolymer solutions.

Synthesis and self-assembly in aqueous solution of amphiphilic diblock copolymers containing hyperbranched polyethylene

In this paper, the well-defined amphiphilic diblock copolymers, hyperbranched polyethylene-b-poly(ethylene oxide) (HBPE-b-PEO) and hyperbranched polyethylene-b-poly(acrylic acid) (HBPE-b-PAA), were synthesized by combing chain walking ethylene polymerization in the presence of 4-vinylbenzyl chloride catalyzed with Pd-α-diimine and click chemistry or atom transfer radical polymerization (ATRP). The self-assembly behavior in water, a selective solvent for PEO and PAA, and the micelle morphology of HBPE-b-PEO and HBPE-b-PAA were investigated by laser light scattering (LLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The resultant diblock copolymers can self-assemble into spherical vesicles as demonstrated by LLS and TEM, which become collapsed on mica surface during drying as showed by AFM.

Cononsolvency of Water/Methanol Mixtures for PNIPAM and PS-b-PNIPAM: Pathway of Aggregate Formation Investigated Using Time-Resolved SANS

We investigate the cononsolvency effect of poly(N-isopropylacrylamide) (PNIPAM) in mixtures of water and methanol. Two systems are studied: micellar solutions of polystyrene-b-poly(N-isopropylacrylamide) (PS-b-PNIPAM) diblock copolymers and, as a reference, solutions of PNIPAM homopolymers, both at a concentration of 20 mg/mL in D2O. Using a stopped-flow instrument, fully deuterated methanol was rapidly added to these solutions at volume fractions between 10 and 20%. Time-resolved turbidimetry revealed aggregate formation within 10–100 s. The structural changes on mesoscopic length scales were followed by time-resolved small-angle neutron scattering (TR-SANS) with a time resolution of 0.1 s. In both systems, the pathway of the aggregation depends on the content of deuterated methanol; however, it is fundamentally different for homopolymer and diblock copolymer solutions: In the former, very large aggregates (>150 nm) are formed within the dead time of the setup, and a concentration gradient appears at their surface in the late stages. In contrast, the growth of the aggregates in the latter system features different regimes, and the final aggregate size is ∼50 nm, thus much smaller than for the homopolymer. For the diblock copolymer, the time dependence of the aggregate radius can be described by two models: In the initial stage, the diffusion-limited coalescence model describes the data well; however, the resulting coalescence time is unreasonably high. In the late stage, a logarithmic coalescence model based on an energy barrier which is proportional to the aggregate radius is successfully applied.

Fabrication of honeycomb-patterned porous films from PS-b-PNIPAM amphiphilic diblock copolymers synthesized via RITP

Poly(styrene)-b-poly(N-isopropylacrylamide) (PS-b-PNIPAM) amphiphilic diblock copolymers were synthesized via reverse iodine transfer polymerization (RITP) firstly. And then the honeycomb structured porous films were fabricated by drop the amphiphilic diblock copolymer solutions on glass substrates. The porous films were characterized using field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM), respectively. The results showed that the regular honeycomb patterned porous film was achieved successfully by employing the block copolymer with a significantly larger polystyrene block proportion into the solution and the pore diameter of the film decreases obviously with the polymer concentration increased. The effects of airflow speed and Ag particles on porous films are also discussed in this study.

Crowding effect induced phase transition of amphiphilic diblock copolymer in solution

The crowding agent induced phase transition of amphiphilic block copolymers in solution was explicitly considered. The influence of the size and the volume fraction of the crowding agent on the phase separation of amphiphilic diblock copolymers is investigated by using self-consistent field theory (SCFT) method. The concentration of the disorder to order transition of the block copolymer decreases when the size of the crowding agent is larger than that of the solvent. The higher volume fraction of the crowding agent will induce the transition of the block copolymer from disorder to order state at a lower concentration. The relation between the size and the volume fraction of the crowding agent is elucidated. When the size of the crowding agent is larger, its volume fraction of the disorder to order transition of the block copolymer will be lower. The conformation of the crowding agent considered as a polymer chain is also studied and compared.

A Universal Scaling Analysis of Nonisothermal Kinetics in Block Copolymer Phase Transitions.

Recently, Farjas and Roura (FR) have proposed a universal scaling law to describe nonisothermal crystallization kinetics based on a modification of the conventionally used Avrami model. In this letter, we apply the approach of Farjas and Roura to analyze the kinetics of an order-order phase transition in a diblock copolymer solution. We present an analysis of kinetics of the hexagonally packed cylinders (HEX) to gyroid transformation in polystyrene-b-polyisoprene (SI) diblock copolymer solutions in dimethyl phthalate using time-resolved small-angle X-ray scattering (SAXS) measurements. By shifting and scaling time in terms of the time at which the transformation rate is maximal, data for samples at two different concentrations at different ramp rates collapse onto the single universal curve predicted by Farjas and Roura. The activation energy for this process was estimated by fitting to the FR model. An estimate of the activation energy was also obtained by Avrami analysis of temperature jump experiments on the same sample. These two estimates differ by a factor of 2, suggesting that the two methods probe different stages of the phase transformation.

Synthesis of azobenzene‐containing side chain liquid crystalline diblock copolymers using RAFT polymerization and photo‐responsive behavior

ABSTRACTWell‐defined azobenzene‐containing side chain liquid crystalline diblock copolymers composed of poly[6‐[4‐(4‐methoxyphenylazo)phenoxy]hexyl methacrylate] (PAzoMA) and poly(glycidyl methacrylate) (PGMA) were synthesized by a two‐step reversible addition–fragmentation chain transfer polymerization (RAFT). The thermal liquid‐crystalline phase behavior of the PGMA‐b‐PAzoMA diblock copolymers in bulk were measured by differential scanning calorimetry (DSC) and polarized light microscopy (POM). The synthesized diblock copolymers exhibited a smectic and nematic liquid crystalline phase over a relatively wide temperature range. With increasing the weight fraction of the PAzoMA block, the phase transition temperatures, and corresponding enthalpy changes increased. Atomic force microscope (AFM) measurements confirmed the formation of the microphase separation in PGMA‐b‐PAzoMA diblock copolymer thin films and the microphase separation became more obvious after cross‐linking the PGMA block. The photochemical transition behavior of the PGMA‐b‐PAzoMA diblock copolymers in solution and in thin films were investigated by UV–vis spectrometry. It was found that the trans–cis isomerization of diblock copolymers was slower than that of the corresponding PAzoMA homopolymer and the photoisomerization rates decreased with increasing either the length of PAzoMA block or PGMA block. The photo‐induced isomerization in solid films was quite different with that in CHCl3 solution due to the aggregation of the azobenzene chromophore. The cross‐linking structures severely suppressed the photoisomerization of azobenzene chromophore. These results may provide guidelines for the design of effective photo‐responsive anisotropic materials. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2165–2175, 2013

Effect of Heating Rate on the Pathway for Vesicle Formation in Salt-Free Aqueous Solutions of Thermosensitive Cationic Diblock Copolymers

Amphiphilic diblock copolymers composed of a thermo-responsive poly(2-isopropyl-2-oxazoline) (PIPOZ) block (Mn = 7000 g/mol) linked to a poly(3-acrylamidopropyl)trimethylammonium chloride) (PAMPTMA) block (Mn = 3500 or 8200 g/mol) were prepared by cationic ring-opening polymerization (PIPOZ block) and subsequent atom transfer radical polymerization (PAMPTMA block). Salt-free dilute aqueous solutions of the diblock copolymers underwent a reversible phase transition upon heating past 40 °C that led to the formation of vesicles, as established by light scattering (LS) and transmission electron microscopy (TEM) studies of solutions heated to 60 °C. The pathway toward vesicles varied depending on the heating rate. A slow heating rate (0.2 °C/min) led to the formation of small spheres as the temperature reached 40 °C; the spheres grew in size as the temperature increased and eventually converted into vesicles by diffusion of some hydrophilic charged blocks toward the center. Upon application of a fast heating rate (0.7 °C/min or faster), the copolymers formed rod-like and/or worm-like micelles at 40 °C and, at higher temperature, oblate membranes that closed up into vesicles. The kinetic control of the pathway toward vesicles may be attributed to the existence of several metastable states in the system, by analogy with studies of the isothermal self-assembly of diblock copolymers in selective solvents.

Computer simulations of surface deposition of amphiphilic diblock copolymers driven by solvent evaporation

Large-scale molecular dynamics simulations are used to study thin films of diblock-copolymer solutions drying on a flat, smooth surface. The solution is represented by coarse-grained models of the polymers dissolved in an explicit ‘atomic’ solvent. The adsorption of polymers on to a flat surface is examined as the solvent slowly evaporates from the thin film. The polymer structures during and after the evaporation are compared with experimental data from atomic force microscopy measurements [E. Glynos, et al., Macromolecules, 2008, 41, 4313–4320]. Because such processes are ‘slow’, we have used a bespoke molecular dynamics code utilising GPU acceleration in order to simulate large system sizes over long timescales. The simulations show excellent qualitative agreement with experiments, and yield significant microscopic insights on the coupling between drying and adsorption.