Adsorption Of Polymer Chains Research Articles

Flexible star polymer chain adsorption by a flat surface: a molecular dynamics simulation

In this work, we have studied the adsorption of a single flexible star polymer chain by a flat surface. The molecular dynamics simulation has been validated for the case of a free star polymer chain using the diffusion experiment. The scaling laws found are in agreement with the Flory theory predictions and the Rouse model. For the adsorption, preliminary results were obtained for chains of different sizes N=31 to N=199 and different functionalities (f=3,4,6,8,10). For the case of semi-flexible star polymer chains, further investigation is needed to locate the critical point of adsorption when varying the potential interaction strength between the chain and the surface.

Flattening Process of Polymer Chains Irreversibly Adsorbed on a Solid.

We report the structural relaxation process of irreversibly adsorbed polymer chains via thermal annealing that lie flat on a solid ("flattened chains"). Amorphous polystyrene and quartz, which together constitute a weakly attractive system, was used as a model where the local chain conformations of the flattened chains were investigated by sum frequency generation spectroscopy (SFG). Two different film preparation processes (i.e., spin coating and dip coating methods) were utilized to create different initial chain conformations. The spin-coated and dip-coated PS thin films were annealed at a temperature far above the bulk glass transition temperature to reach the "quasiequilibrium" state and subsequently rinsed with chloroform to uncover the buried flattened chains. The SFG results revealed that the backbone chains (constituted of CH and CH2 groups) of the flattened PS chains preferentially orient to the weakly interactive substrate surface via thermal annealing regardless of the initial chain conformations, while the orientation of the phenyl rings becomes randomized. We postulate that increasing the number of surface-segmental contacts (i.e., enthalpic gain) is the driving force for the flattening process of the polymer chains, even onto a weakly interactive solid to overcome the conformational entropy loss in the total free energy.

Molecular dynamics simulation of the adsorption of polymer chains on CNTs, BNNTs and GaNNTs

Molecular dynamics simulations are used here to study the adsorption of polymer chains on the nanotube surface. Considering three nanotubes, including carbon, boron nitride and gallium nitride nanotubes, the effect of nanotube types on the polymer/nanotube interactions are investigated. Aramid, poly(phenylene sulfide), poly(phenylene oxide) and polycarbonate are selected as the polymer chains. It is seen that the π-stacking of these polymer chains results in large interaction energy between the nanotubes and polymer chains. Comparing the interaction energies between different polymer chains and nanotubes, it is shown that boron nitride nanotubes can reinforce polymer matrices more effectively than carbon and gallium nitride nanotubes. Besides, the effect of temperature on the polymer/nanotube interaction is studied. It is shown that the polymer chains have more expanded shapes at larger temperature which leads to more π-π interactions and larger interaction energies.

On the Interfacial Properties of Polymers/Functionalized Single-Walled Carbon Nanotubes

Molecular dynamics (MD) simulations is used to study the adsorption of polyethylene (PE) and poly(ethylene oxide) (PEO) on the functionalized single-walled carbon nanotubes (SWCNTs). The effects of functionalization factor weight percent on the interaction energies of polymer chains with nanotubes are studied. Besides, the influences of different functionalization factors on the SWCNT/polymer interactions are investigated. It is shown that for both types of polymer chains, the largest interaction energies associates with the random O functionalized nanotubes. Besides, increasing temperature results in increasing the nanotube/polymer interaction energy. Considering the final shapes of adsorbed polymer chains on the SWCNTs, it is observed that the adsorbed conformations of PE chains are more contracted than those of PEO chains.

Synergistic toughening of nanocomposite double network hydrogels by physical adsorption and chemical bonding of polymer chains to inorganic nanospheres and nanorods: a comparative study

This comparative study reveals a synergistic toughening of double network hydrogels by covalent bonding and physical adsorption of polymer chains to nanospheres or nanorods.

IPAINT: a general approach tailored to image the topology of interfaces with nanometer resolution.

Understanding interfacial phenomena in soft materials such as wetting, colloidal stability, coalescence, and friction warrants non-invasive imaging with nanometer resolution. Super-resolution microscopy has emerged as an attractive method to visualize nanostructures labeled covalently with fluorescent tags, but this is not amenable to all interfaces. Inspired by PAINT we developed a simple and general strategy to overcome this limitation, which we coin 'iPAINT: interface Point Accumulation for Imaging in Nanoscale Topography'. It enables three-dimensional, sub-diffraction imaging of interfaces irrespective of their nature via reversible adsorption of polymer chains end-functionalized with photo-activatable moieties. We visualized model dispersions, emulsions, and foams with ∼20 nm and ∼3° accuracy demonstrating the general applicability of iPAINT to study solid/liquid, liquid/liquid and liquid/air interfaces. iPAINT thus broadens the scope of super-resolution microscopy paving the way for non-invasive, high-resolution imaging of complex soft materials.

Nanoscale adsorbed structures as a robust approach for tailoring polymer film stability.

The stability or wettability of thin polymer films on solids is of vital interest in traditional technologies as well as in new emerging nanotechnologies. We report here that nanoscale structures of polymer chains adsorbed onto a solid surface play a crucial role in the thermal stability of the film. In this study, polystyrene (PS) spin-cast films (20 nm in thickness) with eight different molecular weights prepared on silicon (Si) substrates were used as a model. When low molecular weight (Mw≤ 50 kDa) PS films were subjected to thermal annealing at temperatures far above the bulk glass transition temperature, dewetting occurred promptly, while high molecular weight (Mw≥ 123 kDa) PS films were stable for at least 6 weeks at 150 °C. We reveal a strong correlation between the film stability and the two different interfacial structures of the adsorbed polymer chains: their opposing wettability against chemically identical free polymer chains results in a wetting-dewetting transition at the adsorbed polymer-free polymer interface. This is a unique aspect of the stability of polymer thin films and may be generalizable to other polymer systems regardless of the magnitude of solid-polymer attractive interactions.

Effect of Acid–Base Interactions on Conformation of Adsorbed Polymer Chains

Sum frequency generation spectroscopy (SFG) and attenuated-total-reflection IR (ATR-IR) were used to investigate polymer adsorption on solid surfaces in CCl4 (neutral), CHCl3 (acidic), and acetone (basic) solvents. Fowkes showed that the adsorbed amount of the polymer from acidic and basic solvents is less than that from a neutral solvent (Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 3-7). Here, we show that besides the differences in adsorbed amount, chains adsorbed from an acidic solvent adopted a flat conformation with a much smaller ratio of segments of loops and tails to trains compared to those adsorbed from a neutral solvent. Sapphire (Al2O3) surfaces were saturated by train segments at 1.3 × 10-5 volume fraction for both CCl4 and CHCl3 solutions, with a large fraction of the surface sites occupied by the PMMA segments, which was different from what was expected based on Fowkes' experiment. In contrast, PMMA segments were not able to replace acetone molecules from the surface in a time period of 2 h. Surface interaction parameters alone were unable to predict the differences in conformation of chains adsorbed from acidic or neutral solvents.

Investigation of the adsorption of polymer chains on amine-functionalized double-walled carbon nanotubes

Molecular dynamics (MD) simulations were used to study the adsorption of different polymer chains on functionalized double-walled carbon nanotubes (DWCNTs). The nanotubes were functionalized with two different amines: NH2 (a small amine) and CH2-NH2 (a large amine). Considering three different polymer chains, all with the same number of atoms, the effect of polymer type on the polymer-nanotube interaction was studied. In general, it was found that covalent functionalization considerably improved the polymer-DWCNT interaction. By comparing the results obtained with different polymer chains, it was observed that, unlike polyethylene and polyketone, poly(styrene sulfonate) only weakly interacts with the functionalized DWCNTs. Accordingly, the smallest radius of gyration was obtained with adsorbed poly(styrene sulfonate). It was also observed that the DWCNTs functionalized with the large amine presented more stable interactions with polyketone and poly(styrene sulfonate) than with polyethylene, whereas the DWCNTs functionalized with the small amine showed better interfacial noncovalent bonding with polyethylene.

Scaling of polymer dynamics at an oil-water interface in regimes dominated by viscous drag and desorption-mediated flights.

Polymers are found near surfaces and interfaces in a wide range of chemical and biological systems, and the structure and dynamics of adsorbed polymer chains have been the subject of intense interest for decades. While polymer structure is often inferred from dynamic measurements in bulk solution, this approach has proven difficult to implement at interfaces, and the understanding of interfacial polymer conformation remains elusive. Here we used single-molecule tracking to study the interfacial diffusion of isolated poly(ethylene glycol) molecules at oil-water interfaces. Compared to diffusion in dilute aqueous solution, which exhibited the expected dependence of the diffusion coefficient (D) upon molecular weight (M) of D ∼ M(-1/2) for a Gaussian chain, the behavior at the interface was approximately D ∼ M(-2/3), suggesting a significantly more expanded polymer conformation, despite the fact that the oil was a poor solvent for the polymer. Interestingly, this scaling remained virtually unchanged over a wide range of oil viscosity, despite the fact that at low viscosities the magnitude of the diffusion coefficient was consistent with expectations based on viscous drag (i.e., Stokes-Einstein diffusion), and for high viscosity oil, the interfacial mobility was much faster than expected and consistent with the type of intermittent hopping transport observed at the solid-liquid interface. The dependence on molecular weight, in both regimes, was consistent with results from both self-consistent field theory and previous Monte Carlo simulations, suggesting that an adsorbed polymer chain adopted a partially swollen (loop-train-tail) interfacial conformation.

Thermoresponsive PDMAEMA Brushes: Effect of Gold Nanoparticle Deposition.

The paper addresses the effect of gold nanoparticle (Au-NP) deposition on the thermoresponsive volume phase transition of the weak polyelectrolyte poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes. PDMAEMA brushes were synthesized via surface-initiated atom transfer radical polymerization (SI-ATRP). The PDMAEMA/Au-NP composite brushes were fabricated by immersing the brush modified wafer in the Au-NP suspension. Atomic force microscopy (AFM), ellipsometry, and scanning electron microscopy (SEM) have been employed to characterize the neat PDMAEMA brushes and PDMAEMA/Au-NP composite brushes. All neat PDMAEMA brushes swelled below the volume phase transition temperature and collapsed with increasing temperature over a broad temperature range independent of the initial brush thickness. Water uptake of the brushes is also independent of initial brush thickness. The adsorption of the charged Au-NPs significantly affects the degree of swelling and the thermoresponsive properties of the brushes. PDMAEMA/Au-NP composite brushes do not exhibit any noticeable phase transition at the experimental temperature range irrespective of the initial brush thickness. The reason for this behavior is attributed to a combination of the following: the decreased conformational entropy of the Au-NP adsorbed polymer chains, the increased hydrophilicity of the system due to the charged Au-NPs, and the ≈13 nm diameter Au-NPs causing steric hindrance. We have also shown that the AFM full-indentation method can be successfully applied to determine the polymer brush thicknesses.

Nanorheology of adsorbed polymer chains immersed in pure solvent.

Long linear chains of polybutadiene are adsorbed on the two surfaces of a surface force apparatus and immersed in pure tetradecane. The hydrodynamic force was measured by drainage experiments and by frequency sweeps at constant distances. We related the hydrodynamic thickness to the chain dimension. The complex modulus encompasses the shear modulus and, at distances lower than the hydrodynamic thickness, a compression modulus. The compression term was related to the static force which appears when the two adsorbed layers are overlapped. The complex shear modulus was interpreted by a two-components hydrodynamic model proposed by P. Sens et al. We first complemented the theoretical model. Then, our experimental data fit the proposed viscoelastic expressions in the entire range of distances. The storage modulus is supposed to be affected by a residue of free chains and by the dispersion of the loop lengths.

Effect of CO2 on a Mobility Gradient of Polymer Chains near an Impenetrable Solid

We report a mobility gradient of polymer chains in close proximity of a planar solid substrate in compressed carbon dioxide (CO2) gas. A series of bilayers composed of bottom hydrogenated polystyrene (h-PS) and top deuterated PS (d-PS) layers were prepared on Si substrates. A high-pressure neutron reflectivity (NR) technique was used to study the diffusive motion at the h-PS/d-PS interface as a function of the distance from the substrate interface. The results reveal that the interdiffusive chain dynamics gets strongly hindered compared to the bulk when the distance from the substrate is less than 3Rg (Rg is the radius of polymer gyration of the h-PS). At the same time, by utilizing rapid quench of CO2 and subsequent solvent leaching, we reveal the presence of the CO2-induced polymer adsorbed layer on the substrate. We postulate that loop components in the adsorbed polymer chains provide a structure that can trap the neighboring polymer chains effectively, hence reducing the chain mobility in the close vi...

Investigation of the polyvinyl alcohol stabilization mechanism and adsorption properties on the surface of ternary mixed nanooxide AST 50 (Al2O3-SiO2-TiO2).

A new adsorbent consisting of fumed, mixed alumina, silica, and titania in various proportions (AST 50) was investigated. The studied material was prepared by chemical vapor deposition method. The diameter of AST 50 primary particles was equal to about 51 nm which denotes that it can be classified as a nanomaterial. In the presented paper, the adsorption properties of polyvinyl alcohol on the ternary oxide were investigated. The polymer macromolecules were characterized by two different molecular weights and degree of hydrolysis. The polymer adsorption reaches the maximum at pH 3 and decreases with the solution pH rise. The reduction of the adsorbed PVA macromolecules is related to the electrostatic repulsion forces occurring in the studied system. The AST 50 point of zero charge (pHpzc) obtained from the potentiometric titration is equal to 4.7. Due to the nonionic character of the analyzed macromolecular compound, the polymer attendance has an insignificant effect on the AST 50 surface charge density. In the case of the adsorbent particles zeta potential, the obtained dependencies are different in the absence and presence of PVA. The shift of the slipping plane and displacement of the counter-ions from Stern layer by the adsorbed polymer chains have the greatest effect on the ζ potential value. The stability measurements indicate that the AST 50 suspensions in the presence of the background electrolyte at pH 3 and 6 are unstable. In turn, in an alkaline medium the mixed oxide suspensions exhibit the highest durability, which is a result of a large number of the negative charges on the AST 50 surface. The addition of PVA 100 significantly improves the suspension stability at pH 3 and 6; at higher pH value, the polymer presence does not influence the system durability. It is related to the steric and electrosteric stabilization of the colloidal particles by the adsorbed polyvinyl alcohol macromolecules.

Ordered structures of nanorods induced by the helixes of semiflexible polymer chains

Self-assembly of nanoparticles, such as nanospheres, nanorods (NRs), and nanotubes, in polymer systems is one of the most prominent and promising candidates for the development of novel materials with high mechanical, optical, and electrical performances. A most concerned topic on the nanoparticle/polymer composites is the spatial arrangement and distribution of nanoparticles in the nanocomposites, which is controlled by the competition between the entropic packing constraints related to the incompatibility between species with different sizes and geometries, and the enthalpic consequences of a variety of polymer-nanoparticle interactions. The studies on the nonspherical nanoparticles, such as NRs, are of more challenging than on spherical nanoparticles, because both positional and orientational ordering of anisotropic nanoinclusion have an important influence on the morphology of nanocomposition, while those studies are necessary for applications of nanoscopic anisotropic objects in photovoltaic and filled emission devices. When low-volume fractions of NRs are immersed in a binary, phase-separating blend, the rods can self-assemble into needle-like, percolating networks and this special structure can enhance the macroscopic electrical conductivity and mechanical property of the material. When an electric field is applied, the phase separations of ligand-functionalized NRs in a polymer matrix and densely packed hexagonal arrays of NRs are produced. In this paper, by employing the coarse-grained model and molecular dynamics simulation, we explore the structures of nanocomposites in which a small number of NRs bind with semiflexible polymer chain. The morphology of NRs/polymer mixture is greatly affected by the bending energy b of semiflexible polymer and the binding energy D0 between NRs and semiflexible polymer. If the binding energy D0 is less than 1.1kBT, the NRs are almost free and a gas-like phase is observed. For a suitably large value of D0, three completely different morphologies of NRs/polymer mixtures are identified, namely, the side-to-side parallel aggregation of NRs, the end-to-end parallel aggregation of NRs, and the dispersion of NRs. For the flexible polymer chain (i.e., small bending energy b), the sideto- side parallel aggregation structure of NRs and the disordered conformation of adsorbed polymer chain are observed. In general, a typical equilibrium conformation of free flexible polymer chain is random coil, the binding energy between NRs and polymer can lead to the collapse of a random coil for flexible polymer chain, and the NRs aggregate in the manner of the side-to-side parallel to each other because the enthalpy is maximized through sharing the more polymer monomers between neighbor NRs. That is to say, the local aggregation of NRs can be found because the orientational entropy can make the aggregated NRs arrange in the side-to-side parallel manner. In the rigid polymer chain limit (very large bending energy), the rigid polymer chain is stretched and the NRs are well dispersed. As the rigid polymer holds a long persistence length, the NRs can move freely along the stretched polymer chain, and the dispersed conformation of NRs is formed. For the semiflexible polymer chain with a moderate bending energy, the NRs are aggregated in the end-to-end parallel arrangement. Meanwhile, the polymer monomers wrap around those NRs in a well-defined helical structure. The above discussion indicates that the morphologies of NRs are closely related to the conformations of polymer chains. In fact, when a semiflexible polymer chain binds with a large rigid surface, such as nanotube, the helical structure will be formed and it is driven by entropy. The formation of helical structures for a semiflexible polymer chain can induce NRs to form an end-to-end parallel aggregation. The formation of end-to-end parallel arrangement of NR aggregation is driven by the helical structure of semiflexible polymer chain. For the moderate binding energy, the entropy can drive the semiflexible polymer chain to form local helical structure around the NRs. When more NRs are added to the semiflexble polymer chain/NR mixtures, more local helical structures around NRs are formed. Because the movements of NRs binding with the semiflexible chain are nearly free and an end-to-end parallel arrangement of NRs can form more helical structures than the dispersed NRs, the self-assembly of NRs into an end-to-end parallel structure is expected. That is to say, the formation of end-to-end parallel aggregation of NRs is induced by the helix of semiflexible polymers because it can gain more entropies. The self-assembly of a small number of NRs can be well controlled by varying the stiffness of adsorbed polymer chain. This investigation may provide a new pathway to develop smart medium to manipulate the aggreagtion behavior of a few NRs and to construct novel materials with high performance.

Biomineralization on polymer-coated multi-walled carbon nanotubes with different surface functional groups

Substrate-controlled mineralization from simulated body fluid (SBF) has been studied as a model for biomineralization and for the synthesis of bioinspired hybrid materials. The mineralization procedure is complex and the features of final minerals are affected by many factors. Surface functional groups are among them and play important roles in inducing nucleation, crystal growth and transformation. In this study, multi-walled carbon nanotubes (MWCNTs) were surface-modified with poly(acrylic acid), polyacrylamide or poly(hydroxyethyl methylacrylate), and used as templates for biomineralization. The polymer coating was gained via photo-initiated polymerization of monomers and adsorption of polymer chains onto MWCNTs in solution. Then, the modified MWCNTs with different surface functional groups were incubated in 1.5 times SBF for different times to compare the effect of carboxyl, acylamino and hydroxyl group on calcium phosphate formation. The study involved various characterizations such as morphology observation, weight increase, chemical and crystal structures of deposited minerals at different soaking time points. In all cases, carbonated calcium-deficient hydroxyapatite (CDHA) was identified after 7 days immersion. The continuously growing mineral crystals would wrap MWCNTs into spherical composite particles ultimately. However, the rates of nucleation and crystal growth depended on the type of surface functional groups, in an order of COOH>CONH2>OH. And their different charge characteristics led to different Ca/P ratios in initially formed minerals. It revealed that acylamino group, which demonstrated the lowest Ca/P ratio in nucleation stage, was helpful to obtain c-axis preferentially oriented morphology resembling the HA structure in natural bone tissue.

Molecular dynamics simulations of the adsorption of polymer chains on graphyne and its family

Molecular dynamics simulations are used to study the adsorption of polypropylene chains on the graphene, graphyne and its family. The effects of nanosheet side length, aspect ratio, temperature and number of repeat units are studied. It is shown that the nanosheets with more acetylene bonds have weaker bonding with polymer chains. Besides, the effects of side length and aspect ratio variation on the polymer/graphene interactions are lower than that of polymer/other studied nanosheet. Investigating the effect of temperature on the polypropylene/nanosheet interactions, it is shown that adsorption of polymer chain on the graphyne and its family is approximately independent of temperature. Moreover, it is seen that due to uniform structure of graphene, polymer chains with different repeat units are adsorbed on its surface by uniform shapes.

Friction of 316L stainless steel on soft-tissue-like poly(vinyl alcohol) hydrogel in physiological liquid

The tribological behavior of medical SUS316L stainless steel on soft-tissue-like poly(vinyl alcohol) (PVA) hydrogel with and without dimethyl sulfoxide (DMSO) was investigated in physiological condition. In lower sliding velocity region, the maximum friction coefficient (μmax) is observed at certain velocity due to the adsorption of polymer chains on SUS316L. The presence of plasma proteins into liquid largely reduced μmax of SUS316L. Among plasma proteins tested, we found that mostly Human Serum Albumin was adsorbed on SUS316L, inhibiting the adsorption of PVA chains onto SUS316L.

Micro-friction of diazoresin/polyacrylic acid self-assembly films in water with atomic force microscopy

Ultrathin films composed of diazoresin (DR) and polyacrylic acid (PAA) were fabricated. The surface morphology of the films in water was measured using an atomic force microscopy (AFM). The self-assembly technique makes the surface rather flat and uniform. The friction force and its dependence on the velocity differ from the surface charge of the thin films. The friction force of repulsive DR/PAA film increases linearly with velocity and has lower values than that of attractive DR film over the full range of velocity. As the velocity increases, the attractive friction of DR film first decreases to a minimum at a velocity of 2 line/s and then increases all the way. When the surface is repulsive to the friction substrate, the friction of thin films that is determined by hydrated lubrication of polymer chains that is ultralubricated; when it is adhesive to the friction substrate, the friction is mainly contributed from the elastic deformation of adsorbed polymer chains in the low velocity region and from viscous sliding in the presence of hydrated-layer lubrication of the polymer chains in the higher velocity region.

Global analysis of the ground-state wrapping conformation of a charged polymer on an oppositely charged nano-sphere

We investigate the wrapping conformations of a single, strongly adsorbed polymer chain on an oppositely charged nano-sphere by employing a reduced (dimensionless) representation of a primitive chain-sphere model. This enables us to determine the global behavior of the chain conformation in a wide range of values for the system parameters including the chain contour length, its linear charge density and persistence length as well as the nano-sphere charge and radius, and also the salt concentration in the bathing solution. The structural behavior of a charged chain-sphere complex can be described in terms of a few distinct conformational symmetry classes separated by continuous or discontinuous transition lines which are determined by means of appropriately defined (order) parameters. Our results can be applied to a wide class of strongly coupled polymer-sphere complexes including, for instance, complexes that comprise a mechanically flexible or semiflexible polymer chain or an extremely short or long chain and, as a special case, include the biologically relevant example of DNA-histone complexes.