Influence of polymer structure on adsorption behavior at solid–liquid interface by Monte Carlo simulation

Monte Carlo simulations for the adsorption of polymers including random copolymer, homopolymer, diblock copolymer and two kinds of triblock copolymers, respectively, in nonselective solvent at solid–liquid interface have been performed on a simple lattice model. The effect of polymer structure on adsorption properties was examined. In simulations, all polymeric molecules are modeled as self-avoiding linear chains composed of two segments A and B while A is attractive to the surface and B is non-attractive. It was found that for all polymers, the size distribution of various configurations is determined by the linked sequence of segments and the interaction energy between segment and surface. The results of simulation show that the adsorbed amount always increases with increasing bulk concentration but the adsorption layer thickness is mostly dependent on the adsorption energy at a fixed fraction of segments A. On the other hand, diblock copolymer has always the highest surface coverage and adsorbed amount, while random copolymers and homopolymers give generally the smallest surface coverage and adsorbed amount. It is shown that the sequence of polymer chains, i.e. molecular structure, is the most important factor in affecting adsorption properties at the same composition and interaction between segment and surface. The results also show that the adsorption behavior of random copolymers is remarkably different from that of block copolymers, but acting like homopolymer.     

Kinetically controlled nanostructure formation in self-assembled globular protein-polymer diblock copolymers

Aqueous processing of globular protein-polymer diblock copolymers into solid-state materials and subsequent solvent annealing enables kinetic and thermodynamic control of nanostructure formation to produce block copolymer morphologies that maintain a high degree of protein fold and function. When model diblock copolymers composed of mCherry-b-poly(N-isopropylacrylamide) are used, orthogonal control over solubility of the protein block through changes in pH and the polymer block through changes in temperature is demonstrated during casting and solvent annealing. Hexagonal cylinders, perforated lamellae, lamellae, or hexagonal and disordered micellar phases are observed, depending on the coil fraction of the block copolymer and the kinetic pathway used for self-assembly. Good solvents for the polymer block produce ordered structures reminiscent of coil-coil diblock copolymers, while an unfavorable solvent results in kinetically trapped micellar structures. Decreasing solvent quality for the protein improves long-range ordering, suggesting that the strength of protein interactions influences nanostructure formation. Subsequent solvent annealing results in evolution of the nanostructures, with the best ordering and the highest protein function observed when annealing in a good solvent for both blocks. While protein secondary structure was found to be almost entirely preserved for all processing pathways, UV-vis spectroscopy of solid-state films indicates that using a good solvent for the protein block enables up to 70% of the protein to be retained in its functional form.     

Bilayers of nucleosome core particles

Among the multiple effects involved in chromatin condensation and decondensation processes, interactions between nucleosome core particles are suspected to play a crucial role. We analyze them in the absence of linker DNA and added proteins, after the self-assembly of isolated nucleosome core particles under controlled ionic conditions. We describe an original lamellar mesophase forming tubules on the mesoscopic scale. High resolution imaging of cryosections of vitrified samples reveals how nucleosome core particles stack on top of one another into columns which themselves align to form bilayers that repel one another through a solvent layer. We deduce from this structural organization how the particles interact through attractive interactions between top and bottom faces and lateral polar interactions that originate in the heterogeneous charge distribution at the surface of the particle. These interactions, at work under conditions comparable with those found in the living cell, should be of importance in the mechanisms governing chromatin compaction in vivo.     

Interaction of nonionic PEO-PPO diblock copolymers with lipid bilayers

The relationship between the molecular architecture of a series of poly(ethylene oxide)-b-poly(propylene oxide) (PEO-PPO) diblock copolymers and the nature of their interactions with lipid bilayers has been studied using small- and wide-angle X-ray scattering (SAXS and WAXS) and differential scanning calorimetry (DSC). The number of molecular repeat units in the hydrophobic PPO block has been found to be a critical determinant of the nature of diblock copolymer-lipid bilayer association. For dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-based biomembrane structures, polymers whose PPO chain length approximates that of the acyl chains of the lipid bilayer yield highly ordered, expanded lamellar structures consistent with well-integrated (into the lipid bilayer) PPO blocks. Shorter diblock copolymers produce mixed lamellar and nonlamellar mesophases. The thermotropic phase behavior of the polymer-doped membrane systems is highly influenced by the presence and molecular architecture of the diblock copolymer, as evidenced by shifting of the main phase transition to higher temperatures, broadening of the main transition, and the appearance of other features. Studies of temperature-induced changes in the mesophase structure for compositions prepared with well-integrated PEO-PPO polymers indicate that they undergo reversible changes to a nonlamellar structure as the temperature is lowered. Increasing either the number of repeat units in the PEO block or the polymer concentration promotes a greater degree of structural ordering.     

A spreading technique for forming film in a captive bubble.

Mechanisms underlying the surface properties of lung surfactant are extensively studied in in vitro systems such as the captive-bubble surfactometer (CBS), the pulsating-bubble surfactometer, and the Wilhelmy balance. Among these systems, the CBS is advantageous when a leakproof system and high cycling rates are required. However, widespread application of the CBS to mechanistic studies of dynamic surfactant protein-phospholipid interactions of spread film and to comparative studies between spread and adsorbed film is hampered because spreading of film is difficult. In addition, when film is formed by adsorption, the amount of material required is fairly large. We have developed an easy spreading technique that allows routine formation of film by spreading of small amounts of surfactant components at the air-water interface of an air bubble in a CBS. The technique is reliable, precise, and accurate, and the biophysical activity of film formed by spreading is similar to that of film formed by adsorption. This method will be useful for mechanistic studies of surfactant components under dynamic conditions and for comparative studies of spread films and adsorbed films.     

Adhesion of microorganisms to bovine submaxillary mucin coatings: effect of coating deposition conditions

The adhesion of Staphylococcus epidermidis, Escherichia coli, and Candida albicans on mucin coatings was evaluated to explore the feasibility of using the coating to increase the infection resistance of biomaterials. Coatings of bovine submaxillary mucin (BSM) were deposited on a base layer consisting of a poly(acrylic acid-b-methyl methacrylate) (PAA-b-PMMA) diblock copolymer. This bi-layer system exploits the mucoadhesive interactions of the PAA block to aid the adhesion of mucin to the substratum, whereas the PMMA block prevents dissolution of the coating in aqueous environments. The thickness of the mucin coating was adjusted by varying the pH of the solution from which it was deposited. Thin mucin coatings decreased the numbers of bacteria but increased the numbers of C. albicans adhering to the copolymer and control surfaces. Increasing the mucin film thickness resulted in a further lowering of the density of adhering S. epidermidis cells, but it did not affect the density of E. coli. In contrast, the density of C. albicans increased with an increase in mucin thickness.     

Vascular smooth muscle cells on polyelectrolyte multilayers: hydrophobicity-directed adhesion and growth

Polyelectrolyte multilayer films were employed to support attachment of cultured rat aortic smooth muscle A7r5 cells. Like smooth muscle cells in vivo, cultured A7r5 cells are capable of converting between a nonmotile "contractile" phenotype and a motile "synthetic" phenotype. Polyelectrolyte films were designed to examine the effect of surface charge and hydrophobicity on cell adhesion, morphology, and motility. The hydrophobic nature and surface charge of different polyelectrolyte films significantly affected A7r5 cell attachment and spreading. In general, hydrophobic polyelectrolyte film surfaces, regardless of formal charge, were found to be more cytophilic than hydrophilic surfaces. On the most hydrophobic surfaces, the A7r5 cells adhered, spread, and exhibited little indication of motility, whereas on the most hydrophilic surfaces, the cells adhered poorly if at all and when present on the surface displayed characteristics of being highly motile. The two surfaces that minimized cell adhesion consisted of two varieties of a diblock copolymer containing hydrophilic poly(ethylene oxide) and a copolymer bearing a zwitterionic group AEDAPS, (3-[2-(acrylamido)-ethyldimethyl ammonio] propane sulfonate). Increasing the proportion of AEDAPS in the copolymer decreased the adhesion of cells to the surface. Cells presented with micropatterns of cytophilic and cytophobic surfaces generated by polymer-on-polymer stamping displayed a surface-dependent cytoskeletal organization and a dramatic preference for adhesion to, and spreading on, the cytophilic surface, demonstrating the utility of polyelectrolyte films in manipulating smooth muscle cell adhesion and behavior.     

Self-assembled hydrophobin protein films at the air-water interface: structural analysis and molecular engineering

Hydrophobins are amphiphilic proteins produced by filamentous fungi. They function in a variety of roles that involve interfacial interactions, as in growth through the air-water interface, adhesion to surfaces, and formation of coatings on various fungal structures. In this work, we have studied the formation of films of the class II hydrophobin HFBI from Trichoderma reesei at the air-water interface. Analysis of hydrophobin aqueous solution drops showed that a protein film is formed at the air-water interface. This elastic film was clearly visible, and it appeared to cause the drops to take unusual shapes. Because adhesion and formation of coatings are important biological functions for hydrophobins, a closer structural analysis of the film was made. The method involved picking up the surface film onto a solid substrate and imaging the surface by atomic force microscopy. High-resolution images were obtained showing both the hydrophilic and hydrophobic sides of the film at nanometer resolution. It was found that the hydrophobin film had a highly ordered structure. To study the orientation of molecules and to obtain further insight in film formation, we made variants of HFBI that could be site specifically conjugated. We then used the avidin-biotin interaction as a probe. On the basis of this work, we suggest that the unusual interfacial properties of this type of hydrophobins are due to specific molecular interactions which lead to an ordered network of proteins in the surface films that have a thickness of only one molecule. The interactions between the proteins in the network are likely to be responsible for the unusual surface elasticity of the hydrophobin film.     

Depletion phenomenon in diblock copolymer films: a dissipative particle dynamics simulation

A tentative simulation study has been carried out on the depletion phenomenon in diblock copolymer films through dissipative particle dynamics technology. Results indicate that a depletion layer appears in nearly all the systems with strong interaction between different components, accompanied with weak interaction between the component and the boundary. The system temperature plays a dominant role in the thickness of the depletion layer, on which the component fraction also has an effect to a certain extent. The findings can give support to relevant application processes.     

Spin-on end-functional diblock copolymers for quantitative DNA immobilization

We demonstrate a simple means to covalently bond DNA to both hard (i.e., glass and silicon wafers) and soft (i.e., polymeric) substrates that provides quantitative and precise control of the DNA areal density. The approach is based on spin coating an alkyne-end-functional diblock copolymer, alpha-alkyne-omega-Br-poly( tBA- b-MMA), that self-assembles on both types of substrates as an ordered monolayer and thereby directs alkyne groups to the surface. Azido-functionalized DNA is covalently linked to the alkyne functionalized substrates by means of a "click" reaction between azide and alkyne groups. The density of immobilized DNA can be quantitatively controlled by varying the parameters used for spin-coating the copolymer film, that is, solution concentration and rotational speed, or by varying the copolymer molecular weight. We find the yield of the DNA coupling reaction to be dependent on the nature of the polymer underlying the reactive alkyne functional groups, being higher for more hydrophilic polymers.     

Biodegradable interpolyelectrolyte complexes based on methoxy poly(ethylene glycol)-b-poly(alpha,L-glutamic acid) and chitosan

We synthesized methoxy poly(ethylene glycol)-b-poly(alpha,L-glutamic acid) (mPEGGA) diblock copolymer by ring-opening polymerization of N-carboxy anhydride of gamma-benzyl-L-glutamate (NCA) using amino-terminated methoxy polyethylene glycol (mPEG) as macroinitiator. Polyelectrolyte complexation between mPEGGA as neutral-block-polyanion and chitosan (CS) as polycation has been scrutinized in aqueous solution as well as in the solid state. Water-soluble polyelectrolyte complexes (PEC) can be formed only under nonstoichiometric condition while phase separation is observed when approaching 1:1 molar mixing ratio in spite of the existence of hydrophilic mPEG block. This is likely due to mismatch in chain length between polyanion block of the copolymer and the polycation or hydrogen bonding between the components. Hydrodynamic size of primary or soluble PEC is determined to be about 200 nm, which is larger than those reported in some literatures. The increase in polyion chain length of the copolymer leads to the increase in the hydrodynamic size of the water-soluble PEC. Formation of spherical micelles by the mPEGGA/CS complex at nonstoichiometirc condition has been confirmed by the scanning electron microscopy observation and transmission electron microscopy observations. The homopolymer CS experiences attractive interaction with both mPEGA and PGA blocks within the copolymer. Competition of hydrogen bonding and electrostatic force in the system or hydrophilic mPEG segments weakens the electrostatic interaction between the oppositely charged polyions. The existence of hydrogen bonding restrains the mobility of mPEG chains of the copolymer and completely prohibits crystallization of mPEG segments. In vitro culture of human fibroblasts indicates that mPEGGA/CS-based materials have potential in biomedical application, especially in tissue engineering.     

Theory of lipid polymorphism: application to phosphatidylethanolamine and phosphatidylserine

We introduce a microscopic model of a lipid with a charged headgroup and flexible hydrophobic tails, a neutral solvent, and counter ions. Short-ranged interactions between hydrophilic and hydrophobic moieties are included as are the Coulomb interactions between charges. Further, we include a short-ranged interaction between charges and neutral solvent, which mimics the short-ranged, thermally averaged interaction between charges and water dipoles. We show that the model of the uncharged lipid displays the usual lyotropic phases as a function of the relative volume fraction of the headgroup. Choosing model parameters appropriate to dioleoylphosphatidylethanolamine in water, we obtain phase behavior that agrees well with experiment. Finally we choose a solvent concentration and temperature at which the uncharged lipid exhibits an inverted hexagonal phase and turn on the headgroup charge. The lipid system makes a transition from the inverted hexagonal to the lamellar phase, which is related to the increased waters of hydration correlated with the increased headgroup charge via the charge-solvent interaction. The polymorphism displayed upon variation of pH mimics that of the behavior of phosphatidylserine.     

Synthesis, characterization, and bioavailability of mannosylated shell cross-linked nanoparticles

Saccharide-functionalized shell cross-linked (SCK) polymer micelles designed as polyvalent nanoscaffolds for selective interactions with receptors on Gram negative bacteria were constructed from mixed micelles composed of poly(acrylic acid-b-methyl acrylate) and mannosylated poly(acrylic acid-b-methyl acrylate). The mannose unit was conjugated to the hydrophilic chain terminus of the amphiphilic diblock copolymer precursor, from which the SCK nanoparticles were derived, by the growth of the diblock copolymer from a mannoside functionalized atom transfer radical polymerization (ATRP) initiator. Mixed micelle formation between the amphiphilic diblock copolymer and mannosylated amphiphilic diblock copolymer was followed by condensation-based cross-linking between the acrylic acid residues present in the periphery of the polymer micelles to afford SCK nanoparticles. SCKs presenting variable numbers of mannose functionalities were prepared from mixed micelles of controlled stoichiometric ratios of mannosylated and nonmannosylated diblock copolymers. The polymer micelles and SCKs were characterized by dynamic light scattering (DLS), electrophoretic light scattering, atomic force microscopy (AFM), transmission electron microscopy (TEM), and analytical ultracentrifugation (AU). Surface availability and bioactivity of the mannose units were evaluated by interactions of the nanostructures with the model lectin Concanavalin A via DLS studies, with red blood cells (rabbit) via agglutination inhibition assays and with bacterial cells (E. coli) via TEM imaging.     

Mesoscale simulation on patterned nanotube model for amphiphilic block copolymer

Self-assembly of AB diblock copolymer confined in concentric-cylindrical nanopores was studied using MesoDyn simulation. Our calculation shows that in this confined geometry a zoo of exotic structures can be formed. These structures include bicontinuous phases like carbon nanotube, imperfect single helixes and double helixes. Moreover, the dependence of the chain conformation on the volume fraction, concentration, the interactions between blocks and the diameter of the cylindrical pore are investigated. The results of these simulations can be used to predict the diblock copolymer morphologies confined in concentric-cylindrical nanopores and should be helpful in designing polymeric nanomaterials in the future.     

表面压与紫膜含量对紫膜LB膜质量的影响

本文报导了用表面轮廓测量仪测量了不同表面压和不同紫膜含量下制备的紫膜LB膜中紫膜碎片的厚度。实验结果表明:单个紫膜碎片在紫脂LB膜中的厚度为50左右,相当数量的紫膜碎片之间有重叠。当表面压为30mN/m或紫膜碎片与大豆磷脂之重量比为20:1时,紫膜碎片容易进入到水相或碎片之间相互重叠变得更加严重。     

Computer simulation of polymer and biopolymer self-assembly for drug delivery

Molecular simulation is an emerging tool to bridge relevant time- and length-scales in self-assembly and interfacial processes in soft matter and biological systems. In this review, we highlight mesoscale and coarse-grained molecular dynamics simulation techniques as applied to poly(ethylene oxide)-based diblock copolymer self-assembly. Moreover, we review recent progress pertaining to diblock copolymer and biopolymer self-assembly, stability, and finally, interactions of hydrophobic drugs with polymer membranes. We expect that these computational investigations should provide a useful complement to experimental studies that address open questions in the field of polymeric drug delivery.     

Protein-protein interactions in concentrated electrolyte solutions

Protein-protein interactions were measured for ovalbumin and for lysozyme in aqueous salt solutions. Protein-protein interactions are correlated with a proposed potential of mean force equal to the free energy to desolvate the protein surface that is made inaccessible to the solvent due to the protein-protein interaction. This energy is calculated from the surface free energy of the protein that is determined from protein-salt preferential-interaction parameter measurements. In classical salting-out behavior, the protein-salt preferential interaction is unfavorable. Because addition of salt raises the surface free energy of the protein according to the surface-tension increment of the salt, protein-protein attraction increases, leading to a reduction in solubility. When the surface chemistry of proteins is altered by binding of a specific ion, salting-in is observed when the interactions between (kosmotrope) ion-protein complexes are more repulsive than those between the uncomplexed proteins. However, salting-out is observed when interactions between (chaotrope) ion-protein complexes are more attractive than those of the uncomplexed proteins.     

Change in fibroblast polarization during change in contractility of the actin cytoskeleton

The aim of this work was to study role of the contractility in the process of fibroblast spreading. We investigated the morphology and cytoskeleton of cells seeded in the medium containing 2,3 butanedione monoxime (BDM), an inhibitor of myosin II and myosin-ATPase. Time-lapse video observation and immunofluorescence microscopy were used. BDM caused delay in spreading and blocked cell polarization, that led eventually to the conservation of disk-like cell morphology. The actin-myosin cytoskeleton was also BDM-changed. The number and thickness of stress-fibers decreased. Myosin II orientation was dramatically disturbed to obtain a difuse pattern in the cytoplasm. Paxillin-containing focal adhesions decreased in length and their distribution was changed. The movement of concanavalin A receptors and concanavalin A-coated beads on the lamellar cell surface was also BDM inhibited. It indicates an obvious depression of the lamellar cytoplasm activity and points to the damage of the actin-myosin cytoskeleton. Thus, the change in contractility of the latter alters significantly the morphogenesis of fibroblast spreading.     

Mesoscale Modelling

Abstract

Increasingly, industrial materials are being designed to have structure on length scales of tens to thousands of nanometers. These structures are crucial to achieving a particular desired material property. Such structures, however, may depend on the underlying chemistry of the material for their existence. For example, a thousandfold increase in the ionic conductivity of a polymer blend may only occur in a narrow region of a hugely complex phase diagram, the location' of which region can be expected to depend on the molecular chemistry and physics from the monomer scale to the coil size.

Traditional Computational Chemistry has proved incapable of dealing with the length and time scales involved in the formation of these ‘Mesoscale’ structures. On the other hand, traditional Computational Physics has proved incapable of consistently incorporating the necessary chemical detail for modelling real industrial materials. In this paper we present two novel methods which successfully address both the chemistry and the physics of mesophase formation. The methods, described in detail, are MesoDyn and Dissipative Particle Dynamics (DPD).

Unlike phenomenological theories of materials, such as the Landau models which one finds in much of the computational physics literature, the two models mentioned incorporate molecular geometry and connectivity explicitly. We discuss each of the methods briefly.

We then give an overview of how these methods are being used in industry to optimise materials and processes. We discuss previous simulation results for triblock Pluronic surfactants in solution studied with MesoDyn, and for diblock copolymers studied with DPD, where the known experimental changes in morphology from micellar to hexagonal to bicontinuous to lamellar have been successfully reproduced. We also present new results for several systems, including binary and ternary blends, where the third component in the latter system is a diblock copolymer, which acts as a compatibiliser. We discuss the effects of changing solvent character on the material properties of these systems, as well as the effects of an externally imposed shear flow.     

Study of the stability of long-range-ordered lamellar structures for directed self-assembly lithography,performed using dissipative particle dynamics

We performed dissipative particle dynamics (DPD) simulations to obtain long-range-ordered lamellar structures for directed self-assembly lithography. The self-assembled structure of diblock copolymers (DBCs) depends on the length of the different blocks and the difference in their solubility parameters. In the DPD simulations, the DBCs were formed from coarse-grained particles, and the difference between the solubility parameters was represented by a repulsion parameter. We examined the phase separation morphology of the DBCs, which were confined using a trench model system. The repulsion parameter for the assembly of the lamellar structures from the DBC particles was chosen from six types of parameters. The orientation of the lamellar structure was controlled by the repulsion parameter that described the repulsion between the particles and the wall of the system. We changed the width of the trench, and examined the probability for the formation of the lamellar structure. The lamellar structure could not be obtained by increasing the width. To increase the probability, we placed a ridge at the centre of the bottom wall. It was found that the presence of the ridge increased the probability for the formation of the long-range-ordered lamellar structures.