The aggregation behavior between anionic carboxymethylchitosan and cetyltrimethylammonium bromide: MesoDyn simulation and experiments

The aggregation behavior between carboxymethylchitosan (CMCHS) and cetyltrimethylammonium bromide (CTAB) is investigated by MesoDyn simulation and experimental techniques, for increasing CTAB concentrations. Mixed CMCHS/CTAB bulk aggregates are formed in the solution. Simulation results give the morphologies of aggregates clearly and illustrate the two stages for the formation of aggregates: the first stage is CTAB molecules aggregating on the CMCHS chain and the second stage is the equilibrium stage. A viscosity maximum and a hydrodynamic radius minimum at a certain CTAB concentration reveal the bridging structure of the polymer chains by the micelles. Transmission electron microscopy (TEM) images give the bridging structure clearly. At higher surfactant concentrations, light scattering and TEM show the existence of larger structures, whose size increases with CTAB concentration. According to the simulation and experimental results, the process of aggregate formation and aggregation mechanism are analyzed. Initially CMCHS and CTAB form network structure due to the bridge action of CTAB micelles, while the network structure disappears gradually and is replaced by ellipsoidal CMCHS/CTAB aggregate structure with CTAB concentration increasing.     

Experimental research and DPD simulation on the interaction between an ethoxy-modified trisiloxane and F127

The interactions between surfactants and polymers are widely investigated due to favorable changes on properties in their mixtures. Silicone surfactants and pluronic copolymers, both having low toxicity, are used in the detergent, cosmetics, medical, and pharmaceutical fields. Their mixture may gain better performance in their further applications. Therefore, we investigated the interaction between an ethoxy-modified trisiloxane (a silicone surfactant named Ag-64) and a block polyether F127 in this paper. From aggregation behavior of Ag-64 and F127, the formation mechanism and conformation of the aggregates were proposed based on experiments and dissipative particle dynamics (DPD) simulation. The surface activity and aggregation behavior of Ag-64 are affected by F127 in aqueous solutions. As the amounts of added Ag-64 increase, two types of aggregates (Ag-64/F127 aggregate with F127 as skeleton and the “pearl- necklace” aggregate in which Ag-64 micelles are strung along F127 chain) form successively. At higher polymer concentration, F127 twists together to form a coil/cluster aggregate with Ag-64. The results of DPD simulation approve that two main factors, the hydrophobic association and twist of F127 coil, contribute to the formation of different aggregates of Ag-64 and F127.     

A colloidal route to fabricate hierarchical sticky and non-sticky substrates

Ginsenoside Ro (Ro), a natural anionic biosurfactant derived from ginseng, has been found to markedly increase the solubility of saikosaponin a (SSa), which is the active ingredient of Radix Bupleuri. SSa is minimally soluble in water. To determine the mechanism by which Ro solubilizes SSa, the self-assembly behavior of Ro and the phase behavior of blended Ro and SSa systems were studied by mesoscopic dynamics (MesoDyn) and dissipative particle dynamics (DPD) simulations. The simulation results show that Ro can form vesicles via the closure of oblate membranes. At low concentrations, SSa molecules are solubilized in the palisade layer of the Ro vesicles. At high concentrations, they interact with Ro molecules to form mixed vesicles with Ro adsorbing on the surfaces of the vesicles. The evaluation of the SSa solubilization process reveals that, at low concentrations, Ro aggregates preferentially to form vesicles, which then absorb SSa into themselves. However, at high concentrations, SSa first self-aggregates and then dissolves. This is because the solubilization behavior of Ro shifts the precipitation-dissolution equilibrium in the direction of dissolution. These results of the simulations are consistent with those of transmission electron microscopy (TEM) and dynamic light scattering (DLS).     

杂臂星型嵌段共聚物在溶液中自组装形成囊泡结构的计算机模拟

利用耗散粒子动力学模拟方法, 研究了杂臂星型嵌段共聚物A_m(B_n)₂在溶液中自组装形成囊泡的行为. 主要分析了自组装过程、亲水分枝和疏水分枝的长度及分子构型对组装结构的影响. 结果表明, 杂臂星型聚合物在溶液中会自组装形成碟状胶束, 之后弯曲闭合形成囊泡. 当亲水部分的分枝较短时, 易于形成囊泡结构; 在可形成囊泡结构的条件下, 双分子层囊泡膜的厚度随分枝长度的增加而增加. 与构成相近的线型嵌段共聚物相比, 杂臂星型嵌段共聚物更易形成囊泡结构, 且形成的囊泡结构较薄.     

Phase diagrams,mechanisms and unique characteristics of alternating-structured polymer self-assembly via simulations

Alternating-structured polymers(ASPs), like alternating copolymers, regular multiblock copolymers and polycondensates, are very important polymer structures with broad applications in photoelectric materials. However, their self-assembly behaviors,especially the self-assembly of alternating copolymers, have not been clearly studied up to now. Meanwhile, the unique characteristics therein have not been systematically disclosed yet by both experiments and theories. Herein, we have performed a systematic simulation study on the self-assembly of ASPs with two coil alternating segments in solution through dissipative particle dynamics(DPD) simulations. Several morphological phase diagrams were constructed as functions of different impact parameters. Diverse self-assemblies were observed, including spherical micelles, micelle networks, worm-like micelles, disklike micelles, multimicelle aggregates, bicontinuous micelles, vesicles, nanotubes and channelized micelles. Furthermore, a morphological evolutionary roadmap for all these self-assemblies was constructed, along with which the detailed molecular packing models and self-assembly mechanisms for each aggregate were disclosed. The ASPs were found to adopt a folded-chain mechanism in the self-assemblies. Finally, the unique characteristics for the self-assembly of alternating copolymers were revealed especially, including(1) ultra-fine and uniform feature sizes of the aggregates;(2) independence of self-assembled structures from molecular weight and molecular weight distribution;(3) ultra-small unimolecular aggregates. We believe the current work is beneficial for understanding the self-assembly of alternating structured polymers in solution and can serve as a guide for the further experiments.     

Photosensitive cross-linked block copolymers with controllable release

We intend to form photosensitive block copolymer micelles for controllable release of encapsulated substances. Here, we designed and synthesized a new photocleavable cross-linker (2-nitrophenyl ethylene glycol dimethacrylate) for methyl methacrylate (MMA) atom transfer radical polymerization. Four different ratios (0:1, 1:26, 1:16, 1:8.8) of the photocleavable cross-linker to MMA monomer were used and four block copolymers (P0, P1, P2, P3) were synthesized with PEO-Br as the macroinitiator. Gel permeation chromatography and (1) H NMR studies showed that linear polymer molecules could be cross-linked by the photocleavable linker. The fluorescence studies of the encapsulated Nile Red (NR) showed that there were lower critical micelle concentrations for the polymer P1, P2 and P3 than polymer P0. And dynamic light scattering and SEM confirmed the formation of polymer micelles. Photolysis experiments demonstrated that NR encapsulated in the polymer micelles could be released upon UV irradiation (365 nm, 11 mW cm(-2)) due to the breakage of the photocleavable linker and the generation of more hydrophilic acid moieties, which destabilized polymer micelles. Our study shows a new strategy for the possibility of photocontrollable drug release for hydrophobic drugs.     

Molecular aggregates in aqueous solutions of bile acid salts. Molecular dynamics simulation study

The aggregation behavior of two bile acid salts (i.e., sodium cholate and sodium deoxycholate) has been studied in their aqueous solutions of three different concentrations (i.e., 30, 90,and 300 mM) by means of molecular dynamics computer simulations. To let the systems reach thermodynamic equilibrium, rather long simulations have been performed: the equilibration period, lasting for 20-50 ns, has been followed by a 20 ns long production phase, during which the average size of the bile aggregates (regarded to be the slowest varying observable) has already fluctuated around a constant value. The production phase of the runs has been about an order of magnitude longer than the average lifetime of both the monomeric bile ions and the bonds that link two neighboring bile ions together to be part of the same aggregate. This has allowed the bile ions belonging to various aggregates to be in a dynamic equilibrium with the isolated monomers. The observed aggregation behavior of the studied bile ions has been found to be in good qualitative agreement with experimental findings. The analysis of the results has revealed that, due to their molecular structure, which is markedly different from that of the ordinary aliphatic surfactants, the bile ions form rather different aggregates than the usual spherical micelles. In the lowest concentration solution studied, the bile ions only form small oligomers. In the case of deoxycholate, these oligomers, such as the ordinary micelles, are kept together by hydrophobic interactions, whereas in the sodium cholate system, small hydrogen-bonded aggregates (mostly dimers) are also present. In the highest concentration systems, the bile ions form large secondary micelles, which are kept together both by hydrophobic interactions and by hydrogen bonds. Namely, in these secondary micelles, small hydrophobic primary micelles are linked together via the formation of hydrogen bonds between their hydrophilic outer surfaces.     

INTERACTIONS BETWEEN CATIONIC STARCH AND ANIONIC SURFACTANTS III RHEOLOGY AND STRUCTURE OF THE COMPLEX PHASE

This paper reports on studies of the rheological properties of cationic starch (CS)/ surfactant systems. The degree of substitution of the CS was 0.1 - 0.8. Surfactants investigated were sodium dodecyl sulfate (SDS), potassium octanoate (KOct), sodium decanoate (NaDe)potassium dodecanoate (KDod), sodium oleate (NaOl) and sodium erucate (NaEr). Aggregation of surfactant micelles with the polymer produces a hydrophobic and pseudoplastic gel-like complex phase with low water content and high viscosity. The rheological behavior of the gels is described by the Herschel-Bulkley model. In dilute aqueous solution the CS/surfactant aggregate structure resembles a randomly coiled polymer network, in which polymer molecules are linked by micelles. The rheological data for the gel are compatible with the assumption that the surfactants form liquid crystalline structures with the polymer anchored to the surfactant aggregates, as recently suggested for analogous systems. However, this conjecture needs to be corroborated by more direct determinations of the structure.     

ARGET ATRP 与ROP 结合制备pH 响应两亲性聚合物分子刷及其自组装研究

结合电子转移活化剂再生-原子转移自由基聚合(ARGET ATRP)和开环聚合(ROP)法合成了一种具有无规疏水/ pH 响应结构的两亲性聚合物分子刷聚(甲基丙烯酸聚丙交酯酯-co-甲基丙烯酸)-b-聚甲基丙烯酸单甲氧基聚乙二醇酯 [P(PLAMA-co-MAA)-b-PPEGMA]. 通过核磁共振氢谱(¹H NMR)和凝胶渗透色谱(GPC)表征了聚合物的结构、分子量及分子量分布. 优化了反应条件并合成出分子量可控、分子量分布窄的聚合产物. 采用动态光散射法(DLS)、扫描电子显微镜(SEM)研究了聚合物分子刷在水溶液中自组装胶束的粒径、形貌及pH 响应行为. P(PLAMA-co-MAA)-b-PPEGMA 自组装形成粒径分布均匀的球形胶束. 且随着溶液pH 值从7 降低至3, 胶束中的PMAA 逐渐去离子化, 溶胀的胶束逐渐收缩, 粒径由200～300 nm 减小至150 nm 左右; 但当pH 值减小到2 以下, 胶束表面电荷量非常小, 胶束聚集, 使得粒径增大.     

Dissipative particle dynamics simulation on a ternary system with nanoparticles, double-hydrophilic block copolymers, and solvent

Dissipative particle dynamics (DPD) simulations are performed to study the aggregation of hydrophobic nanoparticles in the presence of double-hydrophilic block copolymer (DHBC). A single compact spherical nanoparticle aggregate is formed in the absence of DHBC. The response of the aggregate to a continuous increase in the concentration of DHBC has been investigated in detail. We observe the evolvement from single spherical aggregate, through single ellipsoidal aggregate, single platelike aggregate, single long and curly rod, dispersed aggregates, then to hexagonally packed cylinders, and ultimately to ordered lamellar structures upon slow addition of DHBC chains. However, when nanoparticles and DHBCs are added into the system simultaneously at the beginning of simulation, we only obtain single spherical aggregate, dispersed aggregates, hexagonally packed cylinders, and ordered lamellar structures at different concentrations of DHBC. Phase diagrams of structures against concentration of DHBC are presented for these two methods, and the stabilities of structures obtained with the two methods are compared.     

两亲性含糖三嵌段共聚物的合成与自组装研究

以二(2-溴异丁酸)4,4′-联苯酯(BiBBP)为引发剂,采用顺序加料的方法,用原子转移自由基聚合法(ATRP)合成了一系列窄分子量分布的甲基丙烯酸甲酯(MMA)和6-O-甲基丙烯酰基-1,2;3,4-双-O-亚异丙基-α-D-吡喃半乳糖苷(MAIPGal)的三嵌段共聚物.用GPC和1H-NMR表征了聚合物的相对分子量和链段组成.结果表明,通过改变投料顺序可以合成ABA和BAB型2种三嵌段共聚物,改变投料比和控制单体的转化率可以调控聚合物的链段组成.聚合物脱保护后得到两亲性含糖共聚物.用TEM考察了聚合物在水溶液中的自组装行为,表明具有不同链段组成的共聚物可以形成不同形态的聚集体,含糖段组成高的聚合物易于形成大尺寸的胶团,含糖段组成低的聚合物易于形成结构清晰的囊泡、胶束,其中,含糖段在两边的BAB型共聚物易于形成囊泡,含糖段在中央的ABA型共聚物易于形成胶束.     

Self-aggregation of the SDS surfactant at a solid-liquid interface

Molecular dynamics simulations of sodium dodecyl sulfate (SDS) molecules on a graphite surface are presented. The simulations were conducted at low and high surface coverage to study aggregation at the water/graphite interface. Results showed that at low surface coverage, the SDS molecules form hemicylindrical aggregates, in agreement with AFM experiments, whereas at high surface coverage, the surfactants form full cylinders. The latter aggregates have not been reported in systems of SDS on hydrophobic substrates, such as graphite. The unexpected results are explained in terms of a water layer adsorbed at the solid surface which was the responsible for the formation of these aggregates. Moreover, the SDS tails in the full cylindrical configuration became straighter than those of the hemicylindrical aggregate. Hydrogen bond formation between water and surfactant head groups was also studied, and it was found that they did not depend on the surfactant concentration.     

Morphology transformation of hybrid micelles self-assembled from rod-coil block copolymer and nanoparticles

Hybrid polymeric micelles self-assembled from a mixture containing poly(γ-benzyl-L-glutamate)-block-poly(ethylene glycol) (PBLG-b-PEG) block copolymer and gold nanoparticles (AuNPs) were prepared. The effect of AuNPs on the self-assembly behavior of PBLG-b-PEG was studied both experimentally by transmission electron microscopy, scanning electron microscopy, and laser light scattering and computationally using dissipative particle dynamics (DPD) simulations. It was found that, the pure PBLG-b-PEG block copolymer self-assembles into long cylindrical micelles. By introducing AuNPs to the stock block copolymer solution, the formed aggregate morphology transforms to spherical micelles. The DPD simulation results well reproduced the morphological transformations observed in the experiments. And the simulation revealed that the main reason for the aggregate morphology transformation is the breakage of ordered packing of PBLG rods in micelle core by the added nanoparticles. Moreover, from the DPD simulations, the distribution information on nanoparticles was obtained. The nanoparticles were found to prefer to locate near the core/shell interface as well as in the core center of the micelles. The combination of experimental and simulation methods lead to a comprehensive understanding of such a complex self-assembly system.     

Directed self-assembly of surfactants in carbon nanotube materials

The self-assembly of surfactant molecules on crossing carbon nanotubes has been investigated using a bead-spring model and implicit solvent dissipative particle dynamics simulations. Adsorption is directed to the nanotube crossing by its higher hydrophobic potential which is due to the presence of two surfaces. As a consequence of the tendency of surfactant molecules to self-assemble into micelles, the adsorbed molecules form a "central aggregate" at the crossing, thus, confining the molecules to the immediate vicinity of the crossing. Adsorption on the remaining nanotube surface becomes significant only at higher surfactant concentrations, where the molecules self-assemble to hemimicelles which grow continuously to full micelles upon increase of the (bulk) surfactant concentration. Our results allow two conclusions for the rational design of nanostructured materials: (i) the size of the central aggregate can not be much larger than that of a bulk micelle and (ii) control of the adsorbed structures is conveniently possible via the (bulk) surfactant concentration.     

Poorly water-soluble drug nanoparticles via an emulsion-freeze-drying approach

In this paper, we examine, by dissipative particle dynamics (DPD) simulation, the interactions between nanoparticles and block copolymer bilayer membranes. The bilayer has a hydrophobic core and hydrophilic head groups on both sides of the core. Nanoparticles without or with a grafted homopolymer are considered. For the conditions investigated, the single nanoparticles and small aggregates are located at the interfaces of the membrane, namely the interfaces between the hydrophilic domains of the membrane and the solvent as well as at the interface between the hydrophobic and hydrophilic domains of the membrane. The large aggregates are located in the hydrophilic domains. By increasing the length of the homopolymer grafted on the nanoparticles, the size of the aggregates in the membrane decreases. At relatively short DPD step times, the particles aggregate in the solvent. As the time increases, the single particles and aggregates penetrate into the membrane.     

Rational design of light-controllable polymer micelles

Amphiphilic block copolymer (BCP) micelles are nanocarriers that hold promise for controlled delivery applications. This account highlights our recent works on light-dissociable BCP micelles. We have designed and developed light-responsive amphiphilic BCPs whose micellar aggregates (core-shell micelles and vesicles) can be disrupted by light exposure. The basic strategy is to incorporate a chromophore into the structure of the hydrophobic block, whose photoreaction can result in a conformational or structural change that shifts the hydrophilic/hydrophobic balance toward the destabilization of the micelles. Using various chromophores including azobenzene, pyrene and nitrobenzene, we have achieved both reversible and irreversible dissociation of BCP micelles upon illumination with UV/visible or near infrared light. The demonstrated rational design principle based on light-changeable or light-switchable amphiphilicity is general and can be applied to many polymer/chromophore combinations. This opens the door to developing photocontrollable polymer nanocarriers offering control over when and where the release of loaded agents takes place.     

无规共聚物自组装球形胶束表面亲水性的耗散粒子动力学模拟

建立了含不同亲疏水粒子比的双亲性无规共聚物粗粒化模型. 采用耗散粒子动力学方法模拟了两亲性无规共聚物选择性溶剂自组装球形胶束表面的亲水性能. 模拟结果表明, 无规共聚物在选择性溶剂中自组装得到实心球形胶束, 球形胶束表面的亲水性与聚合物链亲水粒子含量、溶剂的选择性有关. 随着聚合物链所含亲水粒子增加, 球形胶束表面的亲水性增强. 球形胶束表面的亲水性随着疏水粒子与溶剂粒子间的排斥参数增大而增强, 模拟结果与实验结论一致. 该模拟方法给出的胶束微结构信息可以为双亲无规共聚物分子设计及自组装双亲胶束制备提供一定的理论指导.     

Preparation of well-defined core-shell particles by Cu2+-mediated graft copolymerization of methyl methacrylate from bovine serum albumin

Small well-defined core-shell poly(methyl methacrylate)-bovine serum albumin (PMMA-BSA) particles have been prepared in a direct one-step graft copolymerization of MMA from BSA at 75 degrees C in water with a trace amount of Cu2+ (5 microM). Initially, BSA generates free radicals and acts as a multifunctional macroinitiator, which leads to the formation of an amphiphilic PMMA-BSA grafting copolymer. Such formed copolymer chains act as a polymeric stabilizer to promote further emulsion polymerization of MMA inside, resulting in surfactant-free stable core-shell particles, confirmed by a transmission electron microscopic (TEM) analysis. The PMMA-BSA copolymers as well as PMMA homopolymer inside the particles were isolated by Soxhlet extraction and characterized by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetry (TG). The highest grafting efficiency was approximately 80%. Effects of the reaction temperature, the MMA/BSA ratio, and the concentrations of Cu2+ and BSA on such core-shell particle formation have been systematically studied. Due to their inert PMMA core and biocompatible BSA shell, these small polymer particles are potentially useful in biomedical applications.     

Synthesis and dissipative particle dynamics simulation of cross-linkable fluorinated diblock copolymers: self-assembly aggregation behavior in different solvents

Developing microstructures, such as low molecular aggregates, spherical micelles and multi-compartment micelles, is an expanding area of research in Materials Science. By applying an atom transfer radical polymerization (ATRP) process to cross-linkable fluorinated diblock copolymers and analyzing the data we are able to demonstrate the potential for developing films with different micro-structures for additional biological research. Applying the Dissipative Particle Dynamic (DPD) Method, Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) techniques to cross-linkable fluorinated diblock copolymers of (methyl methacrylate-co-hydroxyethyl methacrylate-co-butyl methacrylate)-b-2-(perfluoroalkyl)ethyl methacrylate (MMA-co-HEMA-co-BMA-b-FMA) we were able to analyze the structures and their relationships to the aggregation of various microstructure formations through the use of various solvents in the process. For the self-assembly of the cross-linkable diblock copolymer in solutions, the DPD simulation results are only in qualitative agreement with experimental data of aggregate morphologies and sizes. This suggests an improved approach to creating materials and methods necessary for studying microstructures in films used in other research areas. Our work examines whether using selective solvents can be easily extended to prepare aggregates with different morphologies, which is an effective shortcut to obtain films with different microstructures. DPD simulation can be considered as an adjunct to experiments and provides other valuable information for the experiment.     

Fate of excited probes in micellar systems

This article presents studies on the photophysical and photochemical behavior of probes within micellar systems: organized emulsifier/polymer aggregates; the intra- and interpolymer association of amphiphilic polymers; monomer-swollen micelles (microdroplets); and the interfacial layer. Pyrene (Py) as a probe is particularly attractive because of its ability to measure the polarity of its microenvironment. Dipyme yields information on the microviscosity of micellar systems. Probes such as laurdan and prodan can be used to explore the surface characteristics of micelles or microdroplets. The dansyl group has a special photophysical property that gives information about the local polarity and mobility (viscosity) of the microenvironment. The organized association of amphiphilic polymer and emulsifier introduces a heterogeneity in the local concentration of the reactants. This heterogeneity also results from the attractive interaction between hydrophilic monomer and emulsifier in the case when the monomer carries a positive charge and the counterpart a negative one, and vice versa. Some emulsifiers can bind to the amphiphilic copolymers by simple partitioning between the aqueous phase and the polymer--non-cooperative association. The interaction between micelles (microdroplets) and charged polymers leads to the formation of mixed micelles. Binding emulsifiers to these polymers was detected at emulsifier concentrations much below the critical micellar concentration (CMC). Emulsifiers often interact cooperatively with polymers at the critical aggregation concentration (CAC) below the CMC, forming micelle-like aggregates within the polymer. The CAC can be taken as a measure of interaction between the emulsifier and polymer. A decrease in the monomer fluorescence intensity of probe-labeled polymer results from increased excimer formation, or higher aggregates within the unimolecular polymeric micelles. An increase in the monomer fluorescence intensity of probe-labeled polymer within the micellar system can be ascribed to shielding of the probe chromophores by emulsifier micelles. The quenching of probe emission by (un)charged hydrophilic monomer depends on partitioning of the monomer between the aqueous phase and the micelles. Penetration of reactants into the interfacial layer determines the quenching of the hydrophobic probe by hydrophilic quencher, or vice versa. Quenching depends on the thickness, density and charge of the interfacial layer. Compartmentalization prevents the carbonyl compound and unsaturated monomer from coming into sufficiently close contact to allow singlet or triplet-monomer interaction. All negatively charged carbonyl probe molecules are quenched with significantly lower rates than the parent neutral hydrophobic benzophenone molecules, which were located further inside the aggregates. This results from the different conformation and allocation of reactants within the micellar system. In the reverse micelles, quenching depends on the amount of water in the interfacial layer and the total area of the water/oil interface.