Stabilization of polymer blend structure by diblock copolymers

The stabilizing (emulsifying) effect of a symmetric diblock copolymer in the mixture of two immiscible homopolymers is considered. The equilibrium value of the interfacial area per copolymer chain is calculated via minimization of the free energy of the mixture for a fixed number of copolymer chains adsorbed to the interface. The size and concentration of droplets of the minor component are determined for the equilibrium state. The particles' radius is shown to be inversely proportional to the copolymer concentration, the coefficient of proportionality being dependent on the Flory-Huggins parameter and chain length. The penetration of homopolymer segments into the copolymer layer on the interface is taken into account and proved to be important for stabilization of the droplets by symmetric copolymers. The conditions of the validity of the presented approach are discussed in detail.     

Phase behaviors of cyclic diblock copolymers

A spectral method of self-consistent field theory has been applied to AB cyclic block copolymers. Phase behaviors of cyclic diblock copolymers, such as order-disorder transition, order-order transition, and domain spacing size, have been studied, showing good consistency with previous experimental and theoretical results. Compared to linear diblocks, cyclic diblocks are harder to phase separate due to the topological constraint of the ring structure. A direct disorder-to-cylinder transition window is observed in the phase diagram, which is significantly different from the mean field phase diagram of linear diblock copolymers. The domain spacing size ratio between cyclic and linear diblock copolymers is typically close to 0.707, indicating in segregation that the cyclic polymer can be considered to be made up of linear diblocks with half of the original chain length.     

Light-scattering studies on phase separation in a binary blend with addition of diblock copolymers

Time-resolved light-scattering measurements have been conducted to investigate the influence of a diblock copolymer additive on the phase boundaries and the kinetics of the phase separation of a polymer blend. The blend studied was a polystyrene-d₈/polybutadiene (PSD/PB) mixture with a diblock copolymer composed of the same homopolymers. It was observed that the critical temperature of the blend, which has an upper critical solution temperature (UCST), decreased with increasing copolymer content and the kinetics of the phase separation via a spinodal decomposition mechanism slowed down in the presence of the copolymer. The features of the spinodal peak position and intensity as a function of time with and without copolymer additive were analyzed for near and off-critical compositions in various temperature jumps. The intermediate and late-stage growth rates do not follow a universal scaling function with the addition of diblock copolymers. © 1995 John Wiley & Sons, Inc.     

Gold nanoparticles protected with pH and temperature-sensitive diblock copolymers

Aqueous dispersions of gold nanoparticles protected with a stimuli-sensitive diblock copolymer were studied as a function of pH and temperature. Poly(methacrylic acid)-block-poly(N-isopropylacrylamide), PMAA-b-PNIPAM, copolymer was synthesized using the RAFT technique. A one-pot method utilizing the dithiobenzoate functionalized polymer was used to prepare gold nanoparticles protected with PMAA-b-PNIPAM. The gold nanoparticles coated with block copolymers, with the PNIPAM block bound to the particle surface and PMAA as an outer block form stimuli-sensitive aggregates in water. The changes in the absorption maxima of the surface plasmon resonance, SPR, of the gold particles and in the size of the aggregates were investigated as a function of pH and temperature. pH was observed to affect the size of the aggregates, whereas the effect of temperature was moderate. However, a blue shift in the SPR was observed both with decreasing pH and increasing temperature. Whereas the PMAA blocks control the colloidal stability of the particles and their aggregates, the thermo-sensitive PNIPAM blocks have a noticeable effect on the polarity of the immediate surroundings of the particles.     

Recognition-mediated assembly of nanoparticles into micellar structures with diblock copolymers

Polystyrene-based diblock copolymers, featuring diaminotriazine functionality on one of the blocks were used to assemble complementary uracil-functionalized nanoparticles into micellar aggregates. The size of these self-assembled aggregates was controlled by block length, as determined in solution (using dynamic light scattering), and in thin films (using transmission electron microscopy).     

Geometric frustration phases of diblock copolymers in nanoparticles

The geometric frustration phases are investigated for diblock copolymers in nanoparticles with neutral surfaces using real-space self-consistent field theory. First, a rich variety of geometric frustration phases with specific symmetries are observed in the polymer nanoparticles with invariable diameters by constructing the phase diagrams arranged as the volume fraction and Flory-Huggins interaction parameter. Most of the space in the phase diagram is filled with phases with strong symmetries, such as spherical or cubic symmetries, while a number of asymmetric or axisymmetric phases are located in a narrow space in the diagram. Then the geometric frustration phases are examined systematically for the diblock copolymers with special polymer parameters, and a rich variety of novel frustration phases with multilayered structures are observed by varying the diameters of the nanoparticles. Furthermore, the investigations on the free energies indicate that the transitions between these frustrated phases are first-order, and the formation mechanism of the frustration phases is reasonably elucidated.     

Phase behavior of diblock copolymers; pressure and temperature dependence studied by small-angle neutron scattering

The structure factor of a poly(ethylene-propylene)poly(dimethylsiloxane) diblock copolymer has been measured by SANS as a function of temperature and pressure. In contradiction to the random phase approximation the conformational compressibility exhibits a pronounced maximum at the order-disorder phase transition. The phase boundary shows an unusual shape: with increasing pressure it first decreases and then increases. Its origin is an increase in, respectively, the entropic and the enthalpic part of the Flory-Huggins interaction parameter. The Ginzburg parameter describing the limit of the mean-field approximation is not influenced by pressure.     

A theoretical and simulation study of the self-assembly of a binary blend of diblock copolymers

Pure diblock copolymer melts exhibit a narrow range of conditions at which bicontinuous and cocontinuous phases are stable; such conditions and the morphology of such phases can be tuned by the use of additives. In this work, we have studied a bidisperse system of diblock copolymers using theory and simulation. In particular, we elucidated how a short, lamellar-forming diblock copolymer modifies the phase behavior of a longer, cylinder-forming diblock copolymer. In a narrow range of intermediate compositions, self-consistent field theory predicts the formation of a gyroid phase although particle-based simulations show that three phases compete: the gyroid phase, a disordered cocontinuous phase, and the cylinder phase, all having free energies within error bars of each other. Former experimental studies of a similar system have yielded an unidentified, partially irregular bicontinuous phase, and our simulations suggest that at such conditions the formation of a partially transformed network phase is indeed plausible. Close examination of the spatial distribution of chains reveals that packing frustration (manifested by chain stretching and low density spots) occurs in the majority-block domains of the three competing phases simulated. In all cases, a double interface around the minority-block domains is also detected with the outer one formed by the short chains, and the inner one formed by the longer chains.     

Solution behavior of linear-dendritic rod diblock copolymers in methanol

The solution behavior of spherical dendrimers as well as hybrid-linear dendritic diblock copolymers has been extensively studied, and the size, shape, and ability of these polymers to encapsulate small molecules have led to their comparison with traditional micelles. We have recently reported the synthesis of a new dendritic copolymer architecture, the linear-dendritic rod diblock copolymer, and in this work, we examine the solution behavior of these unique polymers in methanol at 25 degrees C, using dynamic light scattering and intrinsic viscosity measurements. The diblock copolymers consist of a linear poly(ethylene oxide)-poly(ethylene imine) diblock copolymer backbone around which poly(amido amine) branches have been divergently synthesized from the poly(ethylene imine) block. The hydrodynamic radii and the viscometric radii of the polymers were found to increase slowly with increasing generation up to generation 3.5; however, after generation 3.5, the radii were found to increase very rapidly. This increase can be explained by an elongation of the dendritic block into a more rodlike configuration and a corresponding breakdown of the spherical approximation used to calculate the radii. The intrinsic viscosity of the amine and ester terminated polymers was found to follow two very different trends at low generation; however, at higher generations, they followed similar, yet slightly different, curves with the values for the amine terminated polymers only a little larger than those of the ester terminated polymers. At low generations, the chemistry of the end groups and its interaction with the solvent were found to be more important, whereas at higher generations, the highly branched nature of the dendritic block was the more important factor. For the ester terminated polymers, a maximum in the intrinsic viscosity occurred at generation 1.5. Since this maximum occurred at a much lower generation number than is traditionally seen for spherical dendrimers, new scaling relations for the intrinsic viscosity of dendritic rod polymers were developed and were found to support this observation. A minimum in the intrinsic viscosity was also observed at generation 3.5 for the ester terminated polymers and a minimum or leveling off in the intrinsic viscosity at generation 4.0 was found for the amine terminated polymers, which can be attributed to the transitioning of the polymers to a more elongated, rodlike shape and the increased influence of the shape factor on the intrinsic viscosity.     

Aggregation behavior of cyclic rod-coil diblock copolymers in selective solvents

The aggregation behavior of cyclic rod-coil (RC) diblock copolymers in dilute solutions is investigated through dissipative particle dynamics simulation. By varying the rod length and coil length, cyclic RC copolymers in selective solvents exhibit various morphologies, including spherical micelle, vesicle, bilayer disc, and ribbon bundle structure. Compared with the equivalent linear RC copolymer, only spherical micelle and barrel bundle phase are observed. Rod length is the major factor that controls the liquid-crystalline behavior of RC copolymer systems, while the coil length has a secondary effect on the aggregate morphology. The size of rod bundle varies with the coil length, especially for the end-toend ribbon bundle and side-by-side barrel bundle, which are assembled by cyclic and linear RC copolymer solutions. This finding indicates that the ribbon bundle or nanofiber-like structure in cyclic RC copolymers can be obtained by controlling the rod length and coil length, and thus the optical and electrical properties of RC copolymer would be further controlled and optimized. Results illustrate that cyclization of a linear RC copolymer induces remarkable differences in the rod arrangement and aggregation behavior, thereby indicating the competition between interfacial energy, rod orientational entropy, coil stretching entropy, and packing constraints.     

Solvent effect on the aggregation behavior of rod-coil diblock copolymers

The water-induced aggregation behavior of rod-coil diblock copolymers based on poly(ethylene oxide) (PEO) and poly{(+)-2,5-bis[4'-((S)-2-methylbutoxy)phenyl]styrene} (PMBPS), PEO104-b-PMBPS53, was investigated in the common solvent THF and in the selective solvent dioxane. Before adding water, PEO104-b-PMBPS53 stayed as single polymer chains no matter what conformation the PEO block took (i.e., either the random coil conformation in THF or the compact globule conformation in dioxane). The critical water content ( approximately 6 wt %) at which PEO104-b-PMBPS53 began to aggregate was also similar in both solvents, indicating that PMBPS dominated the aggregation process. However, the size, the size distribution, and the morphology of aggregates in THF/water were quite different from those in dioxane/water. Narrowly distributed spheres with Rh approximately 20 nm were observed in dioxane, whereas in THF, a bimodal distribution peaked at 3 and approximately 300 nm, was observed. The results from 2D wide-angle X-ray diffraction and polarized optical microscopy demonstrated that the PMBPS blocks packed in a parallel pattern upon aggregation in dioxane/water. The anisotropic disclike structures observed in THF/water also indicated the orientation of the PMBPS blocks upon forming aggregates in dilute solution.     

The effects of nanoparticles on the lamellar phase separation of diblock copolymers

The phase separation of diblock copolymers containing some energetically neutral/biased nanoparticles is studied by means of large-scale dissipative particle dynamics (DPD) simulations. The effects of the volume fraction of nanoparticles, the size of nanoparticles, and the interaction strength between nanoparticles and blocks on the lamellar phase separation of diblock copolymers are investigated. When these effects are up to a critical value, the diblock copolymer nanocomposites can form a new bicontinuous morphology, which is well consistent with the experimental results. It is also found that the degree of order of phase separation for a given system increases slightly and then decreases abruptly until the bicontinuous morphology is formed as the volume fraction of nanoparticles increases. Furthermore, we discuss the microphase transition through the position distributions of nanoparticles and present a phase diagram in terms of the nanoparticle volume fraction, size, and surface interaction strength.     

侧基带有L-苯丙氨酸的手性两亲嵌段共聚物的合成及其自组装行为

N3-苯丙氨酸与嵌段共聚物聚乙二醇-b-聚炔丙基缩水甘油(MPEO-b-PGPE)发生click反应,合成了具有光学活性的两亲嵌段共聚物聚乙二醇-B-聚L-苯丙氨酸三唑基缩水甘油(MPEO-b-PGTP),用1H-NMR和元素分析对其结构和组成进行表征.并对其自组装行为进行研究,滴体积法测定MPEO-b-PGTP溶...     

Concentration fluctuation effects on the phase behavior of compressible diblock copolymers

A Hartree analysis has been performed for compressible diblock copolymers of incompatible pairs to investigate the concentration fluctuation effects on their microphase separation behavior. The free energy in the Hartree analysis is obtained from the self-consistent correction to its mean-field cousin, which was recently formulated for such copolymer systems. The mean-field phase diagram is shown to be significantly affected by the fluctuation effects as the copolymer chain size N is lowered. An effective interaction chi(cRPA), which carries not only the change in contact interactions but also the compressibility difference between block components, plays a key role in understanding of the phase behavior and the pressure responses of various thermodynamic transitions for the copolymers with finite sizes. In particular, a symmetric copolymer at disorder-to-lamella transition is found to satisfy Nchi(cRPA)(q*)=10.495+41.022N(-1/3) when evaluated at a characteristic wave number q* for ordered microphases.     

Nanoreactors based on self-assembled amphiphilic diblock copolymers for the preparation of ZnO nanoparticles

A self-assembled diblock copolymer containing styrene (S), methyl methacrylate and a certain percentage of hydrophilic segment of poly(methacrylic acid) (i.e., poly(styrene)-block-poly(methyl methacrylate/methacrylic acid) was synthesized via the ATRP method in two steps. This was followed by a partial hydrolysis of the methyl ester linkages of the methyl methacrylate block under acidic conditions. The resultant block copolymer had a narrow molecular weight dispersity (Р< 1.3) and was characterized using FT-IR and Raman spectroscopy as well as size exclusion chromatography. The block copolymer was used as a nanoreactor for inorganic nanoparticles (ZnO). The incorporation of a single source precursor, such as ZnCl₂, into the self-assembled copolymer matrix and the conversion into ZnO nanostructures was carried out in the liquid phase using wet chemical processing techniques. We report the synthesis and characterization of nanocomposites with dual characteristics due to the functionalities incorporated into the matrix. The optical properties were determined by UV–Vis and fluorescence, the crystallinity was studied using X-ray diffraction, and the thermal stability and studies of the cyclic voltammetry were obtained by thermogravimetric analyzes and potentiodynamic electrochemical measurements, respectively. The structural, topographical and morphological characterizations of the ZnO composite in relation to the precursor block copolymer were analyzed via scanning electron microscopy, transmission electron microscopy and atomic force microscopy.     

Simple synthesis of zwitterionic diblock copolymers for the application on metal nanoparticles preparation

AB diblock copolymers of poly(2-(dimethylamino)ethyl metharylate-block-potassiurn acrylate) were prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. The structure of the block polymer was determined by the nuclear magnetic resonance (NMR) spectroscopy and the gel permeation chromatography. Moreover, it has also been shown that the diblock copolymers exhibit aggregate as function of the pH according to the result of ¹H-NMR spectroscopy, FT-IR absorption spectra, UV-vis transmittance spectroscopy, transmission electron microscopy and ultrasonic particle size analyzer. The result was attributed that such AB diblock copolymers were tailored to undergo pH-induced self-assembly. Furthermore, the aggregate can be as template of metal nanoparticles preparation, and the sizes of the aggregate, in turn, strongly control nanoparticle sizes.     

Novel fluoroalkyl end-capped amphiphilic diblock copolymers with pH/temperature response and self-assembly behavior

A series of fluoroalkyl end-capped diblock copolymers of poly[2-(N,N-dimethylamino)ethyl methacrylate] (PDMAEMA or PDMA) and poly[2-(N,N-diethylamino)ethyl methacrylate] (PDEAEMA or PDEA) have been synthesized via oxyanion-initiated polymerization, in which a potassium alcoholate of 4,4,5,5,6,6,7,7,7-nonafluoro-1-heptanol (NFHOK) was used as an initiator. The chemical structures of the NFHO-PDMA-b-PDEA and NFHO-PDEA-b-PDMA depended on the addition sequence of the two monomers and the feeding molar ratios of [DMA] to [DEA] during the polymerization process. These copolymers have been characterized by (1)H NMR and (19)F NMR spectroscopy and gel permeation chromatography (GPC). The aggregation behavior of these copolymers in aqueous solutions at different pH media was studied using a combination of surface tension, fluorescence probe, and transmission electron microscopy (TEM). Both diblock copolymers exhibited distinct pH/temperature-responsive properties. The critical aggregation concentrations (cacs) of these copolymers have been investigated, and the results showed that these copolymers possess excellent surface activity. Besides, these fluoroalkyl end-capped diblock copolymers showed pH-induced lower critical solution temperatures (LCSTs) in water. TEM analysis indicated that the NFHO-PDMA(30)-b-PDEA(10) diblock copolymers can self-assemble into the multicompartment micelles in aqueous solutions under basic conditions, in which the pH value is higher than the pKa values of both PDMA and PDEA homopolymers, while the NFHO-PDEA(10)-b-PDMA(30) diblock copolymers can form flowerlike micelles in basic aqueous solution.     

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 copolymersare 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(фC), copolymer profiles broaden while фC≥ 0.4, a lamellar phase is formed and by further increasing фC, more thinner layers are observed. Moreover, the results show that, with the increase of фC, 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(ε_(AB)), the copolymer chains not only get stretched in the direction perpendicular to the interface, but also are oriented. The simulations also revealthat the difference between symmetric and asymmetric copolymers is negligible in statistics if the lengths of two blocksare comparable.     

Comparison of complex coacervate core micelles from two diblock copolymers or a single diblock copolymer with a polyelectrolyte

With light scattering titrations, we show that complex coacervate core micelles (C3Ms) form from a diblock copolymer with a polyelectrolyte block and either an oppositely charged polyelectrolyte, a diblock copolymer with an oppositely charged polyelectrolyte or a mixture of the two. The effect of added salt and pH on both types of C3Ms is investigated. The hydrodynamic radius of mixed C3Ms can be controlled by varying the percentage of oppositely charged polyelectrolyte or diblock copolymer. A simple core-shell model is used to interpret the results from light scattering, giving the same trends as the experiments for both the hydrodynamic radii and the relative scattering intensities. Temperature has only a small effect on the C3Ms. Isothermal titration calorimetry shows that the complexation is mainly driven by Coulombic attraction and by the entropy gain due to counterion release.