Monte Carlo study of chain conformations in the swollen middle layer of onion‐skin polymeric micelles

Conformations of chains in swollen middle layers of onion‐skin micelles were studied by Monte Carlo simulations on a tetrahedral lattice under conditions that mimic real systems of swollen onion‐skin micelles. Polymer blocks are modeled as tethered self‐avoiding chains, enclosed in a narrow spherical layer. Average density of segments, 〈g_S〉 ca. 0.6, corresponds to swollen micellar systems. Only the excluded volume effect was taken into account since it plays the most important role in dense polymer systems. Individual chains are described by equivalent ellipsoids of gyration. Distributions of the ellipsoid half‐axes were calculated during simulations. Results based on a large series of simulations indicate that the middle layer‐forming blocks may be described as prolonged ellipsoids oriented preferentially perpendicular to the radial direction. Analysis of the data concerning the orientations of end‐to‐end vectors and distributions of segments within one chain indicates that individual chains are strongly interpenetrated and the multi‐chain system is fairly disordered.     

Micellar Solubilization and In Vitro Release of Silymarin in the self-Aggregates of an Amphiphilic Derivative of Chitosan

Summary: The amphiphilic derivatives of chitosan, (2-hydroxyl-3-butoxyl)-propylcarboxymethyl-chitosan (HBP-CMCHS), can form micelles with the inner core of hydrophobic segments and the outer shell of hydrophilic segments. The typical poor water-soluble drug silymarin was solubilized in the HBP-CMCHS micellar by physical entrapped method. Results showed that the solubilizing capacity was enhanced by increasing the concentration of HBP-CMCHS with the same dosage of silymarin. Silymarin-loaded micellar system of HBP-CMCHS was characterized by TEM and DLS. TEM photograph revealed that the micelles were spherical and silymarin was solubilized in the cores of the spherical polymeric micelles. DLS showed that after solubilization the size of the micelles became bigger. In vitro tests showed that silymarin was slowly released from micellar solution and the release lasted up to 40 h by means of the dialysis method.     

Triblock Polyphiles through Click Chemistry: Self‐Assembled Thermotropic Cubic Phases Formed by Micellar and Monolayer Vesicular Aggregates

Three series of triblock polyphiles consisting of a rigid 4‐phenyl‐1,2,3‐triazole or 1,4‐diphenyl‐1,2,3‐triazole core with three lipophilic and flexible alkoxyl chains at one end and a polar glycerol group at the opposite end were synthesized by copper‐catalyzed azide–alkyne click reactions. Their mesophase behavior was studied by polarizing optical microscopy, differential scanning calorimetry, and XRD. Depending on alkyl chain length and core length, a transition from hexagonal columnar to ${{\it Pm}\bar 3n}$ ‐type cubic phases was observed. In the cubic phases, the molecules are organized as spherical objects. Remarkably, compounds with a longer core unit have a higher tendency to form these cubic phases, and their stability is strongly enhanced over those of the compounds with a shorter core, despite longer cores having a smaller cone angle and therefore being expected to disfavor the formation of spherical objects. There is a large difference in the number of molecules involved in the spherical aggregates formed by compounds with long and short cores. Whereas the aggregates in the cubic phases of the compounds with short rod units are small and could be regarded as micellar, the long‐core compounds form much larger aggregates which are regarded as a kind of monolayer vesicular aggregate.     

水溶液中Pluronic嵌段共聚物聚集行为的介观模拟

通过介观动力学方法(MesoDyn)研究了低浓度下的三嵌段共聚物PEO27PPO61PEO27 (P104)水溶液的聚集行为, 讨论了聚合物浓度、模拟时间对P104水溶液相行为的影响. 在聚合物浓度较低(φ<35%)的情况下, 可以形成三种不同的胶束聚集体：球形胶束(spherical micelle)、胶束簇(micellar cluster)和盘状胶束(disk-like micelle). (1) 球形胶束(5%-10%, φ), 模拟的胶束结构表明疏水的PPO嵌段形成球形内核(micellar core), 而亲水的PEO嵌段形成核壳(micellar corona), 并有水分子存在内核和核壳之中；(2) 胶束簇(11%-15%, φ), 由于球形胶束之间的缔合, 形成直径明显高于球形胶束的聚集体, 其半径比球形胶束大1 nm左右；(3) 盘状胶束(16%-25%, φ), 胶束簇核壳PEO嵌段之间的相互缠绕, 形成了成串的类似盘状的胶束. 模拟中有序参数随浓度的变化证明了这种结构划分的合理性.     

A small-angle X-ray scattering study of the structure of lysozyme-sodium dodecyl sulfate complexes

The structure of lysozyme-sodium dodecyl sulfate (SDS) complexes in solution is studied using small-angle X-ray scattering (SAXS). The SAXS data cannot be explained by the necklace and bead model for unfolded polypeptide chain interspersed with surfactant micelles. For the protein and surfactant concentrations used in the study, there is only marginal growth of SDS micelles as they complex with the protein. Being a small and rather rigid protein, lysozyme can penetrate the micellar core which is occupied by flexible and disordered paraffin chains and also the shell occupied by the hydrated head groups. A partially embedded swollen micellar model seems appropriate and describes well the scattering data. The SAXS intensity profiles are analyzed by considering the change in the electron scattering length density of the micellar core and shell due to complexation with protein and treating the intermicellar interaction using rescaled mean spherical approximation (RMSA) for charged spheres.     

Redox-controlled micellization of organometallic block copolymers

Polystyrene-block-polyferrocenylsilane (PS-b-PFS) diblock copolymers were stoichiometrically oxidized in solution using salts of the one-electron oxidant tris(4-bromophenyl)ammoniumyl. Due to a redox-induced polarity change for the PFS block, self-assembly into well-defined spherical micelles occurs. The micelles are composed of a core of partially oxidized PFS segments and a corona of PS. When the micellar solutions were treated with the reducing agent decamethylcobaltocene, the spherical micelles disassemble and regenerate unassociated and pristine PS-b-PFS free chains.     

Measurement of Solubilization Location in Micelles Using Anchored Aggregation‐Induced Emission Donors

Solubilization locations play a critical role in developing advanced surfactants and improving solubilization power in micelle‐based applications. However, the current polarity‐based techniques for measuring solubilization locations could come to conflicting conclusions. The key challenge is the unpredictable polarities in the micellar microenvironment. Now, an approach that is independent of micellar polarities is used to measure solubilization locations by covalently linking tetraphenylethylene (TPE) to the alkyl chain end of cationic surfactants. The solubilization locations of solubilized acceptors in the TPE‐cored spherical micelles were accurately measured by calculating the Förster resonance energy transfer distance between anchored TPE donors and solubilized acceptors. Solubilization locations of solubilized substances in the micellar interior and at the micellar surface depend on their size and hydrophobicity, respectively.     

Micellar solubilization of tributylphosphate in aqueous solutions of Pluronic block copolymers. Part II. Structural characterization inferred by 1H NMR

The solubilization of tributylphosphate (TBP), a polar oil, in various micellar solutions of Pluronic has been investigated by (1)H NMR spectroscopy. Partial phase diagrams of the three components systems (Pluronic-TBP-water) have shown two characteristic temperatures, called CPT and SMT, which control the phase behavior (see Part I); Both temperature depend on the copolymer structure and, interestingly, are directly related to the TBP concentration in the medium. Monophasic microemulsions are observed only when the temperature ranges between the SMT and the CPT. Moreover, the evolution of the CPT with the TBP content clearly indicated the occurrence of a structural change of the microemulsions which allows higher quantities of TBP to be solubilized. In this second part, (1)H NMR studies of TPB/micellar systems have essentially focused on elucidating the nature of the interactions between TBP and micelle, or on the location of the solubilized species, mainly from the dependence of chemical shifts or linewidths on TBP concentration. Especially, the NMR spectra of the microemulsions before and after the structural change have been compared with those obtained for pure solution of Pluronic in D(2)O at different temperatures and in CDCl(3). The analysis of the (1)H NMR chemical shifts suggests a structural transformation of the TBP-Pluronic micelles in the sense of an hydrophobic TBP-PPO core becoming more and more dense as the TBP concentration increases. Especially, (1)H NMR data evidence an evolution of the hydration state of the hydrophobic core following addition of TBP in the micellar solutions. During the addition of TBP, the microemulsion structure turns from spherical swelled micelles to nanodroplets of pure TBP stabilized by the Pluronic (pure nanophase of TBP stabilized by the copolymer). It is shown that the structural change strongly depends on the temperatures (CPT and SMT, see Part I) and on the copolymer structure.     

Solubilization of triglycerides in liquid crystals of nonionic surfactant

The solubilization of triglycerides [1,2,3-tributanoylglycerol (TBG) and 1,2,3-trihexanoylglycerol (THG)] in water/octa(oxyethylene) dodecyl ether (C(12)EO(8)) systems has been investigated. Oil-induced changes in the structure of liquid crystals in water/C(12)EO(8) system have been studied by optical observation and small-angle X-ray scattering (SAXS) measurements. In the water/C(12)EO(8)/oil systems, solubilization of THG and TBG induces a transition between H(1) (hexagonal) and L(alpha) (lamellar) liquid crystals at high C(12)EO(8) concentrations, whereas at low surfactant concentrations a H(1)-I(1) (discontinuous micellar cubic phase) transition occurs. This anomalous behavior is attributed to the partitioning of solubilized oil in the micelles. At low surfactant concentrations THG is mainly solubilized into the hydrophobic cores of the surfactant micelles, indicating high swelling or low penetration tendency, resulting in a steep increase in the radius of the aggregates (r(H)), thereby inducing a rod-sphere transition. At high surfactant concentrations, THG is not mainly solubilized into the core but distributed between the palisade layer and the core of the aggregates. The TBG is considerably solubilized into the surfactant palisade layer, indicating a high penetration tendency, resulting in an increase in the effective cross-sectional area per surfactant molecule, a(s). The thermal stability of the I(1) phase increases with the solubilization of THG into the aggregate cores. The percentage deviation of the experimental interlayer spacings (P(d)) from complete swelling was also evaluated for different triglycerides in the H(1) and L(alpha) phases or different surfactant concentrations. It is found that the penetration tendency of triglycerides could be used as a tuning parameter for I(1) phase formation depending on the surfactant concentration and the molecular weight of the oil.     

Self-assembly of an amphiphilic derivative of chitosan and micellar solubilization of puerarin

A kind of amphiphilic derivatives of chitosan (2-hydroxyl-3-butoxyl)-propylcarboxymethyl-chitosan (HBP-CMCHS), has been synthesized, and the critical micelle concentration (cmc) of HBP-CMCHS was detected by the fluorescence method. The puerarin-loaded HBP-CMCHS micellar system was prepared by physical entrapped method. Result showed that when adding the same amount of puerarin, the solubilizing capacity was enhanced by increasing the concentration of HBP-CMCHS and temperature. Puerarin-loaded micellar system of HBP-CMCHS was characterized by TEM and DLS. TEM photograph revealed that the micelles were spherical and puerarin was solubilized in the cores of the spherical polymeric micelles. DLS showed that after solubilization the size of the micelles became bigger. In vitro tests showed that puerarin was slowly released from micellar solution and the release lasted up to 60 h by means of the dialysis method.     

Simulation of GPC‐Distribution Coefficients of Linear and Star‐Shaped Molecules in Spherical Pores

Simulations of the distribution coefficients of linear and star‐shaped polymers in spherical pores were performed in order to predict the GPC‐elution behavior of star‐shaped polymers relative to that of linear polymers. Self avoiding walks were generated on a tetrahedral lattice to simulate good solvent conditions. It was found that neither the molecular weight nor the mean squared radius of gyration of the polymer serves as a universal factor to determine the distribution coefficient. However, the calculated distribution coefficients correlate well with the calculated hydrodynamic radii even for different topologies. For molecules at same elution volume the ratios of molecular weights of star and linear polymer agree well with exact calculations for Gaussian chains. These ratios are nearly independent of pore geometry (spherical or cylindrical).     

Structure of a hydrophilic-hydrophobic block copolymer and its interactions with salt and an anionic surfactant

The aggregation of a hydrophilic-hydrophobic diblock copolymer consisting of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(methyl methacrylate) (PMMA) in aqueous solution has been investigated by small-angle neutron scattering. This polybase is extensively protonated at low pH and forms micelles with a dense core of PMMA and a diffuse coronal layer of cationic PDMAEMA. Addition of salt induced micellar growth, brought about by charge screening and more efficient packing of the chains. As a result, the aggregation number increased from 8 up to 31. A similar effect was observed at low concentrations of an anionic surfactant, sodium dodecyl sulfate (SDS) since the net cationic charge in the hydrophilic coronal layer was reduced due to surfactant binding. However, at higher surfactant concentrations, a drastic structural reorganization occurred, as the PMMA became solubilized into the SDS micellar cores and the PDMAEMA chains interacted with the surfactant micelles, resulting in a "pearl-necklace" structure. The presence of the cationic polyelectrolyte significantly increased the population of SDS micelles by effectively lowering the critical micelle concentration of this anionic surfactant.     

Effect of stiffness on the micellization behavior of model H4T4 surfactant chains

The micellization behavior of a series of model surfactants, all with four head and tail groups (H4T4) but with different degrees of chain stiffness, was studied using grand canonical Monte Carlo simulations on a cubic lattice. The critical micelle concentration, micellar size, and thermodynamics of micellization were examined. In all cases investigated, the critical micelle concentration was found to increase with increasing temperature as observed for nonionic surfactants in apolar or slightly polar solvents. At a fixed reduced temperature and increasing chain stiffness, in agreement with previous observations, it was found that the critical micelle concentration decreased and the average micelle size increased. This behavior is qualitatively consistent with that experimentally observed when comparing hydrogenated and homologous fluorinated surfactants. Thermodynamic considerations based on the analysis of the temperature dependence of the critical micelle concentration indicated that both effects could be interpreted as arising from an increased number of heterocontacts between hydrophobic portions of stiff surfactants and a lower entropic cost on packing rigid chains. Structural analysis that was also based on considering the inner micellar radial dependence of the surfactant head and tail site fraction distributions suggested that, on stiffening the molecular backbone, the resulting micellar aggregates grew, without appreciable deviations from spherical symmetry. Stiffer surfactants led to a slightly denser micellar core because of better packing.     

Linear aggregation revisited: Rods,rings and worms

The problem of ring formation in solutions of cylindrical micelles is reinvestigated theoretically, taking into account a finite bending rigidity of the self‐assembled linear objects. Transitions between three regimes are found when the scission energy is sufficiently large. At very low densities only spherical and very short, rod‐like micelles form. Beyond a critical density, mainly rings but also worm‐like chains appear in (virtually) fixed relative amounts. Above a second transition both the length of the linear chains and the relative amount of material taken up by them increase rapidly with increasing concentration. The mass accumulated into long, semi‐flexible worms then overwhelms that in rings. The ring‐dominated regime is very narrow for semi‐flexible chains, confirming that the presence of rings may be difficult to observe in many micellar systems, and indeed disappears completely for sufficiently low scission energy and/or large persistence length.     

A quantitative study of tethered chains in various solution conditions using Langmuir diblock copolymer monolayers

This article summarizes our investigations of tethered chain systems using Langmuir monolayers of poly(dimethylsiloxane)‐polystyrene (PDMS‐PS) diblock copolymers on organic liquids. In this system, the PDMS block adsorbs strongly to the air surface while the PS block dangles into the subphase liquid. The air surface can be made either repulsive or attractive for the tethered PS chain segments by choosing a subphase liquid which has a surface tension less than or greater than that of PS, respectively. The segment profile of the PS block is determined by neutron reflection as a function of the surface density, the molecular weights of the PS and PDMS blocks, and the solution conditions. We cover the range of reduced surface density (Σ ) characteristic of the large body of data in the literature for systems of chains tethered onto solid surfaces from dilute solution in good or theta solvent conditions (Σ < 12). We emphasize quantitative comparisons with analytical profile forms and scaling predictions. We find that the strong‐stretching limit assumed in analytical self‐consistent field calculations (SCF) and scaling theories is not valid over this Σ range. On the other hand, over a large portion of this range (Σ ⪇ 5) tethered chain profiles are well described by a renormalization group theory for weakly interacting or noninteracting chains. Simultaneous with the study of the profile form, the free energy of the tethered chains is examined through the surface tension. A strong increase in the surface pressure is observed with increasing surface density which determines the maximum surface density which can be achieved. This effect is attributed to a combination of higher order osmotic interactions and configurational constraints. This effect may explain several outstanding discrepancies regarding the adsorption of end‐functionalized chains and diblock copolymers onto solid surfaces.     

Formation of Core‐Shell‐Corona Micellar Complexes through Adsorption of Double Hydrophilic Diblock Copolymers into Core‐Shell Micelles

Summary: The complexation between polystyrene‐block‐poly(acrylic acid) (PS‐b‐PAA) micelles and poly(ethylene glycol)‐block‐poly(4‐vinyl pyridine) (PEG‐b‐P4VP) is studied, and a facile strategy is proposed to prepare core‐shell‐corona micellar complexes. Micellization of PS‐b‐PAA in ethanol forms spherical core‐shell micelles with PS block as core and PAA block as shell. When PEG‐b‐P4VP is added into the core‐shell micellar solution, the P4VP block is absorbed into the core‐shell micelles to form spherical core‐shell‐corona micellar complexes with the PS block as core, the combined PAA/P4VP blocks as shell and the PEG block as corona. A model is suggested to characterize the core‐shell‐corona micellar complexes.

Schematic formation of core‐shell‐corona (CSC) micellar complexes by adsorption of PEG‐b‐P4VP into core‐shell PS‐b‐PAA micelles.     

Aggregation processes in self-associating polymer systems: Computer simulation study of micelles in the superstrong segregation regime

We present the results of extensive molecular dynamics and Monte Carlo studies of the self-organization in the solution of short polymer chains with strongly attracting head groups at their end. The formation of micelles (multiplets) is studied in detail. Both two dimensional (2d) and three-dimensional (3d) systems are considered. The off-lattice and lattice models under study incorporate physical factors which control micelle structure and growth in the so-called superstrong segregation regime. These factors include (i) conformational effects associated with short-range excluded-volume interaction between the tails of flexible-chain molecules and (ii) very strong attraction of head groups. Our computer simulations of 3d micelles, constructed a priori from chains with strong attraction of head groups (with the characteristic energy ≈ 10 k_BT), show that size and shape of the micellar core depends crucially on the radius r_c of the interaction of head-groups. If the value of r_c is comparable with chain length, then micelles of nearly spherical shape emerges. The decrease of r_c can induce a sharp polymorphic transition from the micellar core which is spheric in shape to a disk-like (bilayer-shaped) aggregate. Such molecular organization differs from the commonly held notion of a radially symmetric micellar core. On the other hand, these findings fall into line with a recent theory of the super strong segregation regime. When the starting configuration is a random one (i.e., no micelles were a priori formed) the type of final microstructures, emerging as a result of micellization in the superstrong segregation regime, also depends essentially on the radius of head-head attraction. In the case of three-dimensional systems and/or short range attractive potentials we always obtain many small spherically shaped aggregates which, once formed at initial stages of micellization, remain stable for all time scales. Such a behavior is due to both the strong head-head attraction and the screening (repulsive) action of micellar shells creating insurmountable potential barriers. As a result, we deal with kinetically “frozen-in” microstructures which are not reversible and cannot exchange molecules with one another. In dense systems, we observe the formation of a (quasi) periodic pattern of alternating microdomains.     

2,5-双取代烷基苯磺酸钠胶束微结构的1H NMR研究

利用核磁共振化学位移变化, 自旋-自旋弛豫和2D NOESY(two-dimensional nuclear Overhauser enhancement spectroscopy)研究了一系列新合成的双取代烷基苯磺酸盐的胶束化. 结果表明, 邻位取代的是正烷烃链, 间位取代的是支烷烃链. 而且, 邻位取代的烷烃链越长, 参与形成胶束疏水核表面层的亚甲基个数越多. 因此, 每个分子在饱和吸附的油水界面上的面积越大. 间位取代的分支链在胶束疏水核中堆积得没有邻位取代的正烷烃链紧密. 分支链越短, 堆积得越不紧密. 描述了胶束中分子的相对排列.     

聚丙烯酸接枝辛基酚聚氧乙烯醚的合成、表征和胶束化

通过自由基聚合, 丙烯酸(AA)与辛基酚聚氧乙烯醚丙烯酸酯活性大单体(C8PhEO10Ac)共聚,合成了以聚丙烯酸为主链、C8PhEO10Ac 为支链的水溶性两亲接枝共聚物(PAA-g-C8PhEO10Ac), 用凝胶渗透色谱(GPC)测定其相对数均分子质量为4.37×10⁴, 用FTIR和1H-NMR表征了共聚物的结构和组成, 共聚物分子中丙烯酸单体与活性大单体的摩尔比为9:1, 每个共聚物分子中平均约有32个C8PhEO10侧链. 用表面张力法、荧光探针法和透射电子显微镜对共聚物在水溶液中的自组装行为进行了初步研究, 结果表明, 共聚物分子在第一临界胶束浓度cmc1)和第二临界胶束浓度(cmc2)时都形成了球型胶束. 与cmc1时相比, cmc2时溶液表面张力进一步降低, 胶束内部极性进一步减小, 而且胶束粒径增大、结构紧密. 氯化钠的加入可使共聚物溶液的表面张力和胶束内部极性降低.     

Monte Carlo simulation of the structure of mono- and bidisperse polyethylene nanocomposites

The structure of bidisperse polyethylene(PE) nanocomposite mixtures of 50:50(by mole) of long and short chains of C160H322/C80H162 and C160H322/C40H82 filled with spherical nanoparticles were investigated by a coarse-grained, on lattice Monte Carlo method using rotational isomeric state theory for short-range and Lennard-Jones for long-range energetic interactions. Simulations were performed to evaluate the effect of wall-to-wall distance between fillers(D), polymer-filler interaction(w) and polydispersity(number of short chains in the mixture) on the behavior of the long PE chains. The results indicate that long chain conformation statistics remain Gaussian regardless of the effects of confinement, interaction strength and polydispersity. The various long PE subchain structures(bridges, dangling ends, trains, and loops) are influenced strongly by confinement whereas monomer-filler interaction and polydispersity did not have any impact. In addition, the average number of subchain segments per filler in bidisperse PE nanocomposites decreased by about 50% compared to the nanocomposite system with monodisperse PE chains. The presence of short PE chains in the polymer matrix leads to a reduction of the repeat unit density of long PE chains at the interface suggesting that the interface is preferentially populated by short chains.