Effect of bidispersity in grafted chain length on grafted chain conformations and potential of mean force between polymer grafted nanoparticles in a homopolymer matrix

In efforts to produce polymeric materials with tailored physical properties, significant interest has grown around the ability to control the spatial organization of nanoparticles in polymer nanocomposites. One way to achieve controlled particle arrangement is by grafting the nanoparticle surface with polymers that are compatible with the matrix, thus manipulating the interfacial interactions between the nanoparticles and the polymer matrix. Previous work has shown that the molecular weight of the grafted polymer, both at high grafting density and low grafting density, plays a key role in dictating the effective inter-particle interactions in a polymer matrix. At high grafting density nanoparticles disperse (aggregate) if the graft molecular weight is higher (lower) than the matrix molecular weight. At low grafting density the longer grafts can better shield the nanoparticle surface from direct particle-particle contacts than the shorter grafts and lead to the dispersion of the grafted particles in the matrix. Despite the importance of graft molecular weight, and evidence of non-trivial effects of polydispersity of chains grafted on flat surfaces, most theoretical work on polymer grafted nanoparticles has only focused on monodisperse grafted chains. In this paper, we focus on how bidispersity in grafted chain lengths affects the grafted chain conformations and inter-particle interactions in an implicit solvent and in a dense homopolymer polymer matrix. We first present the effects of bidispersity on grafted chain conformations in a single polymer grafted particle using purely Monte Carlo (MC) simulations. This is followed by calculations of the potential of mean force (PMF) between two grafted particles in a polymer matrix using a self-consistent Polymer Reference Interaction Site Model theory-Monte Carlo simulation approach. Monte Carlo simulations of a single polymer grafted particle in an implicit solvent show that in the bidisperse polymer grafted particles with an equal number of short and long grafts at low to medium grafting density, the short grafts are in a more coiled up conformation (lower radius of gyration) than their monodisperse counterparts to provide a larger free volume to the longer grafts so they can gain conformational entropy. The longer grafts do not show much difference in conformation from their monodisperse counterparts at low grafting density, but at medium grafting density the longer grafts exhibit less stretched conformations (lower radius of gyration) as compared to their monodisperse counterparts. In the presence of an explicit homopolymer matrix, the longer grafts are more compressed by the matrix homopolymer chains than the short grafts. We observe that the potential of mean force between bidisperse grafted particles has features of the PMF of monodisperse grafted particles with short grafts and monodisperse grafted particles with long grafts. The value of the PMF at contact is governed by the short grafts and values at large inter-particle distances are governed by the longer grafts. Further comparison of the PMF for bidisperse and monodisperse polymer grafted particles in a homopolymer matrix at varying parameters shows that the effects of matrix chain length, matrix packing fraction, grafting density, and particle curvature on the PMF between bidisperse polymer grafted particles are similar to those seen between monodisperse polymer grafted particles.     

Structure and phase transformations of DPPC lipid bilayers in the presence of nanoparticles: insights from coarse-grained molecular dynamics simulations

In this article, we investigate fluid-gel transformations of a DPPC lipid bilayer in the presence of nanoparticles, using coarse-grained molecular dynamics. Two types of nanoparticles are considered, specifically a 3 nm hydrophobic nanoparticle located in the core of the bilayer and a 6 nm charged nanoparticle located at the interface between the bilayer and water phase. Both negatively and positively charged nanoparticles at the bilayer interface are investigated. We demonstrate that the presence of all types of nanoparticles induces disorder effects in the structure of the lipid bilayer. These effects are characterized using computer visualization of the gel phase in the presence of nanoparticles, radial distribution functions, and order parameters. The 3 nm hydrophobic nanoparticle immersed in the bilayer core and the positively charged nanoparticle at the bilayer surface have no effect on the temperature of the fluid-gel transformation, compared to the bulk case. Interestingly, a negatively charged hydrophobic nanoparticle located at the surface of the bilayer causes slight shift of the fluid-gel transformation to a lower temperature, compared to the bulk bilayer case.     

Modelling of polymer aggregates in solution

The self-consistent mean field model of Scheutjens and Fleer is used to model spherical aggregates of homopolymers and monomer—polymer particles in solution. For homopolymer aggregates we found that the chain ends are preferentially located at the exterior side of the polymer/solvent interface. The distribution of the end segments may be an important parameter in latex film formation. For monomer—polymer particles a “core-shell” structure is found with an extended core containing a monomer—polymer mixture and a thin shell a few nanometres thick strongly enriched with monomer. The monomer-enriched shell seems to function as a solvating envelope for the dangling chain ends. These results are compared with other simulations based on a single chain in a spherical-cavity model.     

Nanosegregated polymeric domains on the surface of Fe3O4@SiO2 particles

Multifunctional, biocompatible, and brush‐grafted poly(ethylene glycol)/poly(ε‐caprolactone) (PEG/PCL) nanoparticles have been synthesized, characterized, and used as vehicles for transporting hydrophobic substances in water. For anchoring the polymer mixed brushes, we used magnetic‐silica particles of 40 nm diameter produced by the reverse microemulsion method. The surface of the silica particle was functionalized with biocompatible polymer brushes, which were synthesized by the combination of “grafting to” and “grafting from” techniques. PEG was immobilized on the particles surface, by “grafting to,” whereas PCL was growth by ROP using the “grafting from” approach. By varying the synthetic conditions, it was possible to control the amount of PCL anchored on the surface of the nanoparticles and consequently the PEG/PCL ratio, which is a vital parameter connected with the arrangement of the polymer brushes as well as the hydrophobic/hydrophilic balance of the particles. Thus, adjusting the PEG/PCL ratio, it was possible to obtain a system formed by PEG and PCL chains grafted on the particle's surface that collapsed in segregated domains depending on the solvent used. For instance, the nanoparticles are colloidally stable in water due to the PEG domains and at the same time are able to transport, entrapped within the PCL portion, highly water‐insoluble drugs. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2966–2975     

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.     

Cyclic voltammetry of highly hydrophilic ions at a supported liquid membrane

Cyclic voltammetry has been used to study the coupling of ion transfer reactions at a liquid membrane. The liquids are either supported by a porous hydrophobic membrane (polyvinylidene difluoride, PVDF) when the organic solvent is non-volatile (o-nitrophenyloctylether) or are merely a free standing organic solvent layer such as 1,2-dichloroethane comprised between two hydrophilic dialysis membranes supporting the adjacent aqueous phases. The passage of current across the liquid membrane is associated with two ion transfer reactions across the two polarised liquid liquid interfaces in series. It is shown that it is possible to study the transfer of highly hydrophilic ions at one interface by limiting the mass transfer of the other ion transfer reaction at the other interface. Indeed, for systems comprising an ion M in one aqueous phase and a reference ion R partitioned between the membrane and the other aqueous phase, the observed and simulated cyclic voltammograms have a half-wave potential determined by the Gibbs energy of transfer of M transferring at one interface and by the limiting mass transfer of R at the other interface. This new methodology opens a way to measure the Gibbs energy of transfer of highly hydrophilic or hydrophobic ions, which usually limits the potential window at single liquid liquid interfaces (ITIES).     

Effect of homopolymer matrix on diblock copolymer grafted nanoparticle conformation and potential of mean force: A molecular simulation study

We study the effect of homopolymer A or B matrix on the conformations and effective interactions of AB diblock copolymer grafted particles using coarse‐grained molecular dynamics simulations. In an A homopolymer matrix we observe patchy conformations within the AB diblock copolymer grafted layer, where the number of B patches is controlled by the A‐A attractive interaction strength. In a B homopolymer matrix the grafted particle takes on a core‐corona conformation, where the inner A block aggregates near the particle surface and the outer B block forms a corona that interacts with the B matrix. The potential of mean force (PMF) between two particles in an A homopolymer matrix has a long‐ranged attractive well with a minima at intermediate distances corresponding to the location of the outer B block patches. The PMF between two particles in a B homopolymer matrix has an attractive well at short interparticle distances corresponding to the size of the inner A block. We isolate the contribution of the homopolymer matrix on the PMF between the two diblock copolymer grafted particles, by deducting the PMF in the absence of a matrix, assuming the contributions of the grafted particle and matrix to the PMF to be additive. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 76–88     

Control of nanoparticle location in block copolymers

A simple procedure is described to incorporate gold nanoparticles and control their location within symmetric poly(styrene-b-2 vinyl pyridine) (PS-PVP) diblock copolymers. Gold nanoparticles coated with thiol-terminated PS and/or PVP homopolymer chains (Mn approximately 1300 and 1500 g/mol, respectively) are incorporated into alternating lamellar layers of PS and PVP (total Mn approximately 196 500 g/mol). The location of the particles is controlled by varying the composition of ligands on the particle surfaces. In particular, gold particles coated with 100% PS or PVP reside near the center of the respective polymer domains, while particles coated with a mixture of both homopolymers reside at the interfaces between the two blocks.     

Dissipative particle dynamics simulations of polymer-protected nanoparticle self-assembly

Dissipative particle dynamics simulations were used to study the effects of mixing time, solute solubility, solute and diblock copolymer concentrations, and copolymer block length on the rapid coprecipitation of polymer-protected nanoparticles. The simulations were aimed at modeling Flash NanoPrecipitation, a process in which hydrophobic solutes and amphiphilic block copolymers are dissolved in a water-miscible organic solvent and then rapidly mixed with water to produce composite nanoparticles. A previously developed model by Spaeth et al. [J. Chem. Phys. 134, 164902 (2011)] was used. The model was parameterized to reproduce equilibrium and transport properties of the solvent, hydrophobic solute, and diblock copolymer. Anti-solvent mixing was modeled using time-dependent solvent-solute and solvent-copolymer interactions. We find that particle size increases with mixing time, due to the difference in solute and polymer solubilities. Increasing the solubility of the solute leads to larger nanoparticles for unfavorable solute-polymer interactions and to smaller nanoparticles for favorable solute-polymer interactions. A decrease in overall solute and polymer concentration produces smaller nanoparticles, because the difference in the diffusion coefficients of a single polymer and of larger clusters becomes more important to their relative rates of collisions under more dilute conditions. An increase in the solute-polymer ratio produces larger nanoparticles, since a collection of large particles has less surface area than a collection of small particles with the same total volume. An increase in the hydrophilic block length of the polymer leads to smaller nanoparticles, due to an enhanced ability of each polymer to shield the nanoparticle core. For unfavorable solute-polymer interactions, the nanoparticle size increases with hydrophobic block length. However, for favorable solute-polymer interactions, nanoparticle size exhibits a local minimum with respect to the hydrophobic block length. Our results provide insights on ways in which experimentally controllable parameters of the Flash NanoPrecipitation process can be used to influence aggregate size and composition during self-assembly.     

Hexagonal-to-cubic phase transformation in composite thin films induced by FePt nanoparticles located at PS/PEO interfaces

The organization process of asymmetric poly(styrene-block-ethylene oxide) (PS-b-PEO) copolymer thin films blended with FePt nanoparticles is studied. In a first step, it is shown that FePt nanoparticles stabilized by oleic acid ligands are distributed within the PS matrix phase, whereas the same particles partially covered with short dopamine-terminated-methoxy poly(ethylene oxide) (mPEO-Dopa) are located at PS/PEO interfaces. The swelling of PS domains, induced by FePt_oleic acid nanoparticles during the solvent annealing process, results in formation of a disordered microstructure in comparison to the well-organized hexagonally close-packed (HCP) cylinder phase formed in the neat PS-b-PEO copolymer. The evolution of the microstructure of PS-b-PEO/FePt_mPEO-Dopa composite has been investigated for different solvent annealing treatments. Under high-humidity conditions during the vapor annealing process, the addition of FePt nanoparticles results in formation of spheres in the film split into terraces. The upper and lower terraces are occupied by spheres organized in an unusual square and HCP phases, respectively. Under low-humidity conditions, undulated PEO cylinders oriented parallel to substrate are formed in the presence of FePt nanoparticles. In this case, we observe that most of the nanoparticles accumulate within the core of topological defects, which induces a low nanoparticle concentration at the PS/PEO interfaces and so stabilizes an intermediate undulated cylinder phase.     

Amphiphilicity-driven organization of nanoparticles into discrete assemblies

We present a method for organizing metallic nanoparticles in solution that is based on the hydrophobic effect and does not require either hydrogen bonding or molecular recognition. When amphiphilic V-shaped molecules are attached to a gold cluster, an aggregation process ensues in aqueous solution and leads to the formation of well-defined cylindrical and vesicular nanoarrays of particles. The metallic clusters densely pack at the boundary separating the hydrophobic core from the hydrophilic corona of the hybrid micelle-like aggregates. This design allows one to assemble and disassemble the nanoparticles in a reversible manner and control the size and the morphology of the arrays by changing the conditions of the solution preparation. The versatility of this method is demonstrated by its applicability to different metals with covalently attached amphiphilic arms with various chemical compositions (PS-PEO and PB-PEO) and molecular weights.     

Dispersion of polystyrene inside polystyrene‐b‐poly(N‐isopropylacrylamide) micelles in water

The addition of mixture of polystyrene‐b‐poly(N‐isopropylacrylamide) (PS‐b‐PNIPAM) and polystyrene homopolymer (h‐PS) in tetrahydrofuran dropwise into water leads to nanoparticles with a PS core and a thermally sensitive PNIPAM shell. The effects of the ratio of the homopolymer to copolymer and temperature on the formation and stabilization of the dispersion were investigated by using a combination of static and dynamic laser light scattering. PNIPAM shell continuously collapses as temperature increases in the range 20–40 °C. Such formed particles are stable even at temperatures much higher than lower critical solution temperature (LCST ～ 32 °C) of PNIPAM. Our results reveal that the area occupied per hydrophilic PNIPAM chain on the hydrophobic PS core remains nearly a constant regardless of the amount of h‐PS in the polymer mixture. This clearly indicates that the surface area occupied per hydrophilic group is a critical parameter for stabilizing particles dispersed in water. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 749–755, 2010     

Responsive colloidal systems: reversible aggregation and fabrication of superhydrophobic surfaces

We report on a method of fabricating stimuli-responsive core-shell nanoparticles using block copolymers covalently bound to a silica nanoparticle surface. We used the "grafting to" approach to graft amphiphilic block copolymer brushes of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) and poly(styrene-b-4-vinylpyridine) onto silica nanoparticles with two different diameters: colloidal silica 200 nm in diameter and fumed silica 15 nm in diameter. We used the pH-responsive properties of the grafted brush to regulate the interactions between the particles, and between the particles and their environment. We show that this behavior can be applied for a reversible formation of particle aggregates, and can be used to tune and stabilize the secondary aggregates of particles of the appropriate size and morphology in an aqueous environment. The suspensions of the particles form a textured hydrophilic coating on various substrates upon casting and the evaporation of water. Heating above the polymer's glass transition temperature or treatment in acidic water result in back and forth switching between superhydrophobic and hydrophilic surfaces, respectively.     

Molecular dynamics study of the influence of surfactant structure on surfactant-facilitated spreading of droplets on solid surfaces

The spreading of a partially wetting aqueous drop in air on a hydrophobic surface can be facilitated by the adsorption of surfactants from the drop phase onto the air/aqueous and aqueous/hydrophobic solid interfaces of the drop. At the contact line at which these interfaces meet, conventional surfactants with a linear alkyl hydrophobic chain attached to a polar group adsorb onto the surfaces, forming monolayers which remain distinct as they merge at the contact juncture. The adsorption causes a decrease in the interfacial tensions and reduction in the contact angle but the angle remains above zero so the drop is still nonwetting. Trisiloxane surfactants with a T-shaped geometry in which the hydrophobic group is composed of a trisiloxane oligomer with a polar group attached at the center of the chain can give rise to a zero contact angle at the contact line and complete wetting (superspreading). Experimental evidence suggests the adsorption of the T-shaped molecule, in addition to significantly decreasing the tensions of the interfaces (relative to the conventional surfactants), promotes the formation of a precursor film consisting of a surfactant bilayer at the contact line which facilitates the spreading. The aim of this study is to use molecular dynamics to examine if the T-shaped structure can promote spreading by the formation of a bilayer and to contrast this case with that of the linear chain surfactant where complex assembly does not occur. The simulation models the solvent as a monatomic liquid, the substrate as a particle lattice, and the surfactants as united atom structures, with all interactions given by Lennard-Jones potentials. We start with a base case in which the solvent partially wets a substrate comprised of a lattice of particles. We demonstrate that adsorbed T-shaped surfactant monolayers can, when the interaction between the solvent and the hydrophile particles is strong enough, assemble into a bilayer, allowing the drop to extend to a thin planar film. In the case of the flexible linear chain surfactant, there is no interaction between the monolayers on the two interfaces in the case of a strong hydrophile-solvent interaction and less coordination for a weaker interaction. In either case, the monolayers remain distinct, as the surfactant only marginally improves wetting.     

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.     

Electrostatic repulsion-controlled formation of polydopamine-gold janus particles

Polydopamine (PDA)-Au Janus particles were obtained by simply adding HAuCl(4) to a PDA particle suspension, prepared via self-polymerization of dopamine in basic solution at room temperature. The structures of the PDA-Au particles are readily controlled by electrostatic repulsion between the constituent particles, which can be realized simply via adjusting the environmental pH. PDA-Au Janus particles are formed only in a narrow pH range of 2.5-3.0 due to the properly enhanced electrostatic repulsion between the Au particles growing on as-prepared PDA particles and between the Au and PDA particles. The obtained PDA-Au Janus particles can become interfacially active and self-assemble at oil/water interfaces as a result of spatially well-separated hydrophilic (PDA) and hydrophobic (Au) domains on the surfaces, reminiscent of amphiphilic molecules.     

Nanoparticle-loaded magnetophoretic vesicles

Magnetic nanoparticles have been assembled into the bilayer membrane of block copolymer vesicles. The nanoparticles decorate the hydrophobic/hydrophilic interface, which leads to bridging of adjacent bilayers and the formation of oligo-lamellar vesicles. The nanoparticle uptake of the vesicles is sufficiently high to become magnetophoretic in external magnetic fields as shown by video microscopy.     

两亲性壳聚糖衍生物的制备及其包覆量子点

以天然壳聚糖为功能性高分子的骨架,通过西弗碱还原法,以正辛醛和端醛基聚乙二醇单甲基醚(mPEG aldehyde,mPEG-ald)修饰壳聚糖,制备了具有亲疏水性质的N-辛基化-N-mPEG化壳聚糖衍生物(N-octyl-N-mPEG-chitosan,OPEGC),并且以此两亲性壳聚糖衍生物包覆量子点制备了水溶性聚合物量子点纳米粒子.用FTIR、1H-NMR对壳聚糖衍生物进行了结构表征.制备的水溶性胶束,用动态光散射(DLS)测试其流体力学直径与分布,研究了烷基链接枝比率对粒径大小的影响,研究结果表明:烷基链接枝率越高,聚合物胶束粒径越小.以芘为分子探针,通过荧光光谱法测定了壳聚糖衍生物的临界胶束浓度(CMC)为2.032×10-2mg/mL,并对水溶性聚合物量子点纳米粒子进行了紫外、荧光及形貌表征,结果表明得到了荧光发射产率高的聚合物量子点纳米粒子,且尺寸均一、水溶性好.     

Bridging across length scales: multi-scale ordering of supported lipid bilayers via lipoprotein self-assembly and surface patterning

We show that a two-step process, involving spontaneous self-assembly of lipids and apolipoproteins and surface patterning, produces single, supported lipid bilayers over two discrete and independently adjustable length scales. Specifically, an aqueous phase incubation of DMPC vesicles with purified apolipoprotein A-I results in the reconstitution of high density lipoprotein (rHDL), wherein nanoscale clusters of single lipid bilayers are corralled by the protein. Adsorption of these discoidal particles to clean hydrophilic glass (or silicon) followed by direct exposure to a spatial pattern of short-wavelength UV radiation directly produces microscopic patterns of nanostructured bilayers. Alternatively, simple incubation of aqueous phase rHDL with a chemically patterned hydrophilic/hydrophobic surface produces a novel compositional pattern, caused by an increased affinity for adsorption onto hydrophilic regions relative to the surrounding hydrophobic regions. Further, by simple chemical denaturation of the boundary protein, nanoscale compartmentalization can be selectively erased, thus producing patterns of laterally fluid, lipid bilayers structured solely at the mesoscopic length scale. Since these aqueous phase microarrays of nanostructured lipid bilayers allow for membrane proteins to be embedded within single nanoscale bilayer compartments, they present a viable means of generating high-density membrane protein arrays. Such a system would permit in-depth elucidation of membrane protein structure-function relationships and the consequences of membrane compartmentalization on lipid dynamics.     

Hydrophilic Domains Enhance Nanobubble Stability

Highly stable nanoscale gas states at solid/liquid interfaces, referred to as nanobubbles, have been widely studied for over a decade. In this study, nanobubbles generated on a hydrophobic Teflon amorphous fluoroplastic thin film in the presence and absence of hydrophilic carbon domains are investigated by peak force quantitative nanomechanics. On the hydrophobic surface without hydrophilic domains, a small number of nanobubbles are generated and then rapidly decrease in size. On the hydrophobic surface with hydrophilic domains, the hydrophilic domains have a significant effect on the generation and stability of nanobubbles, with bubbles remaining on the surface for up to three days.