Competitive adsorption of neutral comb polymers and sodium dodecyl sulfate at the air/water interface

The interfacial behavior of aqueous solutions of four different neutral polymers in the presence of sodium dodecyl sulfate (SDS) has been investigated by surface tension measurements and ellipsometry. The polymers comprised linear poly(ethylene oxide) with low and high molecular masses (10(3) and 10(6) Dalton (Da), respectively), and two high molecular mass methacrylate-based comb polymers containing poly(ethylene oxide) side chains. The adsorption isotherms of SDS, determined by Gibbs analysis of surface tension data, are nearly the same in the presence of the high molecular mass linear polymer and the comb polymers. Analysis of the ellipsometric data reveals that while a single surface layer model is appropriate for films of polymer alone, a more sophisticated interfacial layer model is necessary for films of SDS alone. For the polymer/surfactant mixtures, a novel semiempirical approach is proposed to determine the surface excess of polymer, and hence quantify the interfacial composition, through analysis of data from the two techniques. The replacement of the polymer due to surfactant adsorption is much less pronounced for the high molecular mass linear polymer and for the comb polymers than for the low molecular mass linear polymer. This finding is rationalized by the significantly higher adsorption driving force of the larger polymer molecules as well as by their more amphiphilic structure in the case of the comb polymers.     

Chitosan-N-poly(ethylene oxide) brush polymers for reduced nonspecific protein adsorption

The possibility of using a novel comb polymer consisting of a chitosan backbone with grafted 44 units long poly(ethylene oxide) side chains for reducing nonspecific protein adsorption to gold surfaces functionalized by COOH-terminated thiols has been explored. The comb polymer was attached to the surface in three different ways: by solution adsorption, covalent coupling, and microcontact printing. The protein repellent properties were tested by monitoring the adsorption of bovine serum albumin and fibrinogen employing surface plasmon resonance and imaging null ellipsometry. It was found that a significant reduction in protein adsorption is achieved as the comb polymer layer is sufficiently dense. For solution adsorption this was achieved by adsorption from high pH solutions. On the other hand, the best performance of the microcontact printed surfaces was obtained when the stamp was inked either at low or at high pH. For a given comb polymer layer thickness/poly(ethylene oxide) density, significant differences in protein repellent properties were observed between the different preparation methods, and it is suggested that a reduction in the mobility of the comb polymer layer generated by covalent attachment favors a reduced protein adsorption.     

Surface properties of bottle-brush polyelectrolytes on mica: effects of side chain and charge densities

Surface properties of a series of cationic bottle-brush polyelectrolytes with 45-unit-long poly(ethylene oxide) side chains were investigated by phase modulated ellipsometry and surface force measurements. The evaluation of the adsorbed mass of polymer on mica by means of ellipsometry is complex due to the transparency of mica and its birefringence and low dielectric constant. We therefore employed a new method to overcome these difficulties. The charge and the poly(ethylene oxide) side chain density of the bottle-brush polymers were varied from zero charge density and one side chain per segment to one charge per segment and no side chains, thus spanning the realm from a neutral bottle-brush polymer, via a partly charged brush polyelectrolyte, to a linear fully charged polyelectrolyte. The adsorption properties depend crucially on the polymer architecture. A minimum charge density of the polymer is required to facilitate adsorption to the oppositely charged surface. The maximum adsorbed amount and the maximum side chain density at the surface are obtained for the polymer with 50% charged segments and the remaining 50% of the segments carrying poly(ethylene oxide) side chains. It is found that brushlike layers are formed when 25-50% of the segments carry poly(ethylene oxide) side chains. In this paper, we argue that the repulsion between the side chains results in an adsorbed layer that is non-homogeneous on the molecular level. As a result, not all side chains will contribute equally to the steric repulsion but some will be stretched along the surface rather than perpendicular to it. By comparison with linear polyelectrolytes, it will be shown that the presence of the side chains counteracts adsorption. This is due to the entropic penalty of confining the side chains to the surface region.     

Adsorption of model surfactantlike copolymers on nanopatterned surfaces

The adsorption of polymers, copolymers, surfactants, and biopolymers is often used to engineer surfaces. Towards improving our understanding of polymer adsorption we report simulation results for the adsorption of model copolymers, resembling surfactants, on nanoscale patterned hydrophobic surfaces at infinitely dilute concentrations. The surfactants are composed by a hydrophobic tail and a hydrophilic head. Surfactant adsorption on the hydrophobic surface occurs in the tail-down configuration in which the tail segments are in contact with the surface. We investigate how the presence of a solid hard mask, used to create the nanoscale pattern on the underlying hydrophobic surface, affects the surfactant adsorption. We find that surfactant adsorption on the underlying hydrophobic surface is prevented when the characteristic dimensions of the solid hard mask are less than twice the radius of gyration. We also show that details about mask-surfactant head effective interactions have the potential to alter the characteristics of adsorption. When the mask repels the head segments, the surfactants hardly adsorb on the underlying hydrophobic surface. When the mask strongly attracts the surfactant heads, the surfactants may preferentially adsorb on the mask rather than on the underlying hydrophobic surface. Under these latter circumstances the adsorbed surfactants in some cases assume a head-down configuration in which the head segments are in contact with the mask and the tail segments extend towards the bulk solution. We explain our results in terms of enthalpy and entropy of adsorption and discuss practical implications.     

Well-packed chains and aggregates in the emission mechanism of conjugated polymers

We synthesized dialkoxy-substituted poly[phenylene vinylene]s (dROPPV-1/1, 0.2/1, and 0/1) consisting of two repeating units with different side-chain lengths (methoxy and 3,7-dimethyloctyloxy). These polymers can serve as a model system to clarify roles of aggregates (the sites with ground-state interchain interactions) and the independent chain segments in the well-packed chains (the chain segments that are compactly packed without interaction) in the emission mechanism of conjugated polymers. Due to the packing of polymer chains, films of all of these polymers are accessible to interchain excitations, after which excitons can re-form to result in delayed luminescence. Besides, some chains form aggregates so that the delayed luminescence is no more the ordinary single-chain emission but red-shifted and less structured. Not only the re-formation of these indirect excitons but also the aggregation of chains are facilitated in the polymers with short methoxy side groups, revealing that both packing and aggregation of chain segments require a short spacing between polymer chains. However, the incorporation of other side chains such as the 3,7-dimethyloctyloxy group to dROPPVs is necessary for the formation of aggregates because these long branched side chains can reduce the intrachain order imposed by the short methoxy groups, which accounts for the absence of aggregate emission in the well-studied poly[2,5-dimethoxy-1,4-phenylene vinylene]. This study reveals that the well-packed chains do not necessarily form aggregates. We also show that the photophysical properties and the film morphology of conjugated polymers can be deliberately controlled by fine-tuning of the copolymer compositions, without altering the optical properties of single polymer chains (e.g., as in dilute solutions).     

Computer Simulation of Adsorbed Polymer Chains with a Different Molecular Architecture

Simulations of simple models of polymer chains were carried out by the means of the dynamic Monte Carlo method. The model chains were confined to a simple cubic lattice. Three different chain architectures were studied: linear, star‐branched and ring chains. The polymer model chain interacted with an impenetrable surface with a simple contact attractive potential. It was found that size parameters of all these polymers obey scaling laws. The temperatures of the transitions from weakly to strongly adsorbed chain were determined. It was shown for weakly adsorbed chains that ring polymers are always ca. 50% more adsorbed than linear and star‐branched ones. The properties of adsorbed linear and star‐branched polymers are very similar in the length of chain and the strength of adsorption studied. Strongly adsorbed ring polymers are still more adsorbed but differences between all kinds of chains become less pronounced.     

Dynamics of star branched polymers in a matrix of linear chains — a Monte Carlo study

A simple cubic lattice model of the melt of 3-arm star-branched polymers of various length dissolved in a matrix of long linear chains (n₁ = 800 beads) is studied using a dynamic Monte Carlo method. The total polymer volume fraction is equal to 0,5, while the volume fraction of the star polymers is about ten times smaller. The static and dynamic properties of these systems are compared with the corresponding model systems of isolated star-branched polymers and with the melt of linear chains. It has been found that the number of dynamic entanglements for the star polymers with arm length up to 400 segments is too small for the onset of the arm retraction mechanism of polymer relaxation. In this regime dynamics of star-branched polymers is close to the dynamics of linear polymers at corresponding concentration and with equivalent chain length. The entanglement length for star polymers appears to be somewhat larger compared with linear chains.     

Polymer surface reorientation after protein adsorption

Surface side-chain orientation changes of two polymers have been observed upon protein adsorption using sum frequency generation vibrational spectroscopy. Side-chain-deuterated poly(ethyl methacrylate) and poly(n-butyl methacrylate) were contacted with five protein solutions: albumin, fibrinogen, ubiquitin, cytochrome c, and lysozyme. The CD(3)/CD(2) symmetric stretch ratios of the surface polymer side chains in contact with these different media were compared to each other and to that of the polymer contacting air or phosphate buffered saline. The adsorption of different proteins to the surfaces resulted in polymer side-chain orientations slightly different from each other, with orientations between the air and buffer cases.     

Star-branched polymers in an adsorbing slit: a Monte Carlo study

A coarse-grained model of star-branched polymer chains confined in a slit was studied. The slit was formed by two parallel impenetrable surfaces, which were attractive for polymer beads. The polymer chains were flexible homopolymers built of identical united atoms whose positions in space were restricted to the vertices of a simple cubic lattice. The chains were regular star polymers consisted of f = 3 branches of equal length. The chains were modeled in good solvent conditions and, thus, there were no long-range specific interactions between the polymer beads-only the excluded volume was present. Monte Carlo simulations were carried out using the algorithm based on a chain's local changes of conformation. The influence of the chain length, the distances between the confining surfaces, and the strength of the adsorption on the properties of the star-branched polymers was studied. It was shown that the universal behavior found previously for the dimension of chains was not valid for some dynamic properties. The strongly adsorbed chains can change their position so that they swap between both surfaces with frequency depending on the size of the slit and on the temperature only.     

Calcium mediated polyelectrolyte adsorption on like-charged surfaces

Monte Carlo simulations within the primitive model of calcium-mediated adsorption of linear and comb polyelectrolytes onto like-charged surfaces are described, focusing on the effect of calcium and polyion concentrations as well as on the ion pairing between polymers and calcium ions. We use a combination of Monte Carlo simulations and experimental data from titration and calcium binding to quantify the ion pairing. The polymer adsorption is shown to occur as a result of surface overcharging by Ca(2+) and ion pairing between charged monomers and Ca(2+). In agreement with experimental observations, the simulations predict that the polymer adsorption isotherm goes through a maximum as the calcium or the polymer concentration is increased. The non-Langmuir isotherms are rationalized in terms of charge-charge correlations.     

Adsorption Mechanism of Comb Polymer Dispersants at the Cement/Water Interface

Comb polymers are commonly used as dispersants to stabilize highly concentrated cement suspensions. The effectiveness of such polymeric additives to stabilize these suspensions is determined to a large extent by the amount adsorbed. In this study we investigated the adsorption characteristics of various comb dispersant containing different graft densities on surfaces of cement particle. The effect of inorganic salts on their adsorption was also examined in order to elucidate their adsorption mechanism. The results show that the adsorption of comb polymer dispersants on cement surface conforms approximately to Langmuir's adsorption isotherm and the characteristic plateau A _s and adsorption free energy ΔG_ads are largely dependent on the anionic group content of the comb polymers. The A _s and ΔG_ads increase with increasing anionic group content. This information suggests that the adsorption of comb polymers on cement surfaces is dominated by electrostatic interaction between COO-groups on the comb polymers and the positive surface of the cement. This conclusion is supported by effects of inorganics such as calcium and sulfate ions, and diffuse reflectance FTIR spectroscopy. The implication of results for tuning polymers for the required performance in cement manufacture should be noted.     

Adsorption of interacting oligomers and polymers at an interface

We discuss in a qualitative way the physical background of a recently developed polymer adsorption theory, in which all the possible chain conformations for interacting chain molecules near an adsorbing interface are taken into account. Any conformation is described as a step-weighted random walk in a lattice. Each step is weighted according to a segmental weighting factor that contains the adsorption energy (for segments in contact with the surface), the entropy of mixing, and the attraction or repulsion between segments and solvent molecules. A suitable computing method is used to calculate the contribution of all chain conformations to the concentration profile, to the adsorbed amount, to the fraction of trains, loops and tails, to the layer thickness, etc. The theory is valid for any chain length and any concentration in the solution.Results for various chain lengths are given. Oligomers have a low affinity for the surface, whereas polymer adsorption isotherms are of the well known high affinity type. Three concentration regimes can be distinguished. In (extremely) dilute solutions the molecules on the surface adsorb as isolated chains (the Henry region).     

Dynamics of Adsorbed Star‐Branched Polymer Chains: A Computer Simulation Study

The simple cubic‐lattice model of polymer chains was used to study the dynamic properties of adsorbed, branched polymers. The model star‐branched chains consisted of f = 3 arms of equal lengths. The chain was modeled with excluded volume, that is, in good solvent conditions. The only interaction assumed was a contact potential between polymer segments and an impenetrable surface. This potential was varied to cover both weak and strong adsorption regimes. The classical Metropolis sampling algorithm was used for models of star‐branched polymers in order to calculate the dynamic properties of adsorbed chains. It was shown that long‐time dynamics (diffusion constant) and short‐time dynamics (the longest relaxation time) were different for weak and strong adsorption. The diffusion of weakly adsorbed chains was found to be qualitatively the same as for free nonadsorbed chains, whereas strongly adsorbed chains behaved like two‐dimensional polymers. The time‐dependent properties of structural elements such as tails, loops, and trains were also determined.

The mean lifetimes of tails, loops, and trains versus the bead number for the chain with N = 799 beads for the case of the weak adsorption ε_a = −0.3.     

高分子链在球内表面上的吸附/排空转变

应用自洽场理论(SCFT)研究了受限于球内的高分子溶液的结构,重点关注高分子链在受限壁附近的行为.根据自洽场理论数值计算结果,讨论了球半径、高分子与球限制壁的相互作用、高分子平均浓度等因素对球内高分子浓度分布的影响.从高分子浓度分布和吸附/排空层厚度可以发现,在一定的条件下,受限的高分子在受限壁上会发生吸附/排空转变.吸附/排空转变与受限球大小、高分子链长和平均浓度,以及高分子链与受限壁之间相互作用都有关系.理论预测发生吸附/排空转变时的高分子与球限制壁的临界相互作用参数与链长的倒数成线性关系,且斜率与球半径有关.限制球越小,要发生吸附/排空转变,需要高分子与球之间有更大的临界吸引能.     

Motion of a branched polymer chain in confinement: a Monte Carlo study

The aim of the study was a theoretical investigation of the polymer molecules located between two parallel and impenetrable surfaces which were also attractive for polymer segments. The chains were constructed of identical segments and were restricted to knots of a simple cubic lattice. Since the chains were at good solvent conditions the only interactions between the segments of the chain were the excluded volume. The properties of the model chains were determined by means of Monte Carlo simulations with a sampling algorithm based on the chain's local changes of conformation. The differences and similarities in the structure for different adsorption regimes and the size of the slit were shown and discussed. It was observed that at certain conditions the polymer chain was adsorbed at one of the confining surfaces, and then after a certain period of time it detached from this surface and approached the opposite wall; this switch was repeated many times. The influence of the strength of the adsorption, the size of the slit, and the chain's length on the frequency of these jumps were determined. The mechanism of the chain's motion during the switch was also shown.     

The Structure of Star-Branched Chains in a Confined Space

Summary. We studied the properties of a simplified model of star-branched polymers confined in a slit formed by two parallel and impenetrable surfaces. The chains were built of identical united atoms (segments) whose positions were restricted to vertices of a simple cubic lattice. The polymer excluded volume and polymer segment-surface contact interactions were also introduced into the model. The properties of the model chains were determined by means of Monte Carlo simulations with a Metropolis-type sampling algorithm based on local changes of chain’s conformation. The structure of star-branched chains was investigated and the influence of the confinement and the temperature on the chain dimensions and structure was studied. It was shown that for chains in the adsorbing slits their sizes do not exhibit a universal behavior contrary to confined athermal polymers. The polymers in narrow slits at higher temperatures still exhibited features of a three-dimensional chain. It was also shown that chains in small slits and at low temperatures were fully adsorbed at one of the surfaces but could also switch the surface rapidly.     

Diverse spreading behavior of binary polymer nanodroplets

Molecular dynamics simulations are used to study the spreading of binary polymer nanodroplets in a cylindrical geometry. The polymers, described by the bead-spring model, spread on a flat surface with a surface-coupled Langevin thermostat to mimic the effects of a corrugated surface. Each droplet consists of chains of length 10 or 100 monomers with approximately 350,000 monomers total. The qualitative features of the spreading dynamics are presented for differences in chain length, surface interaction strength, and composition. When the components of the droplet differ only in the surface interaction strength, the more strongly wetting component forms a monolayer film on the surface even when both materials are above or below the wetting transition. In the case where the only difference is the polymer chain length, the monolayer film beneath the droplet is composed of an equal amount of short chain and long chain monomers even when one component (the shorter chain length) is above the wetting transition and the other is not. The fraction of short and long chains in the precursor foot depends on whether both the short and the long chains are in the wetting regime. Diluting the concentration of the strongly wetting component in a mixture with a weakly wetting component decreases the rate of diffusion of the wetting material from the bulk to the surface and limits the spreading rate of the precursor foot, but the bulk spreading rate actually increases when both components are present. This may be due to the strongly wetting material pushing out the weakly wetting material as it moves toward the precursor foot.     

Control of specific attachment of proteins by adsorption of polymer layers

This study concerns the design of protein-resistant polymer adsorbed layers for the control of surface binding of biospecific recognition entities. Polymer surface layers were prepared using the adsorption of poly(allylamine hydrochloride) (PAH), poly(l-lysine) (PL), and branched and linear polyethyleneimine (PEI) and further modified by the covalent attachment of biotin for specific avidin attachment. The adsorption of PAH, PL, and PEI on silicon substrates was studied as a function of pH and ionic strength using ellipsometry. Average dry layer thicknesses of approximately 10, approximately 5, approximately 9, and approximately 3 A (+/-1 A) were obtained when polymer adsorption occurred from solutions at pH 9.5 that contained 0.5 M NaCl for PAH, PL, branched PEI, and linear PEI, respectively. These polymers showed significant differences in their efficiency to suppress nonspecific avidin adsorption. At low ionic strength, avidin adsorption occurred on all polymer-coated surfaces at basic pH values, despite the same positive electrostatic charge for protein globules and the surface. Though the net electrostatic repulsion between avidin molecules and branched PEI was efficiently screened in a protein solution of pH 7 and 0.15 M NaCl, branched-PEI coatings of high molecular weight were unique in their ability to provide avidin-resistant surfaces as a result of steric hindrance from the branched architecture of adsorbed polymer chains. All polymers studied were effective in suppressing avidin adsorption at pH 3 as a result of protonation of the avidin surface functional groups at this pH. Branched-PEI-coated surfaces were also effective for the suppression of smaller positively charged proteins such as lysozyme and ribonuclease A at pH 7 and 0.15 M NaCl. They were also resistant to the adsorption of negatively charged proteins such as BSA and fibrinogen at pH 7 and 0.75 M NaCl. Furthermore, by using PEI-modified protein-repellent surfaces, selective binding of avidin was achieved to surface-bound silver nanoparticles, which should provide a promising application for the label-free detection of biological species using surface-enhanced Raman scattering (SERS).     

Oligo(ethylene glycol) containing polymer brushes as bioselective surfaces

The nitroxide-mediated polymerization of styrenic monomers containing oligo(ethylene glycol) (OEGn) moieties was chosen for the preparation of biocompatible polymer brushes tethered to silicon oxide surfaces due to the broad range of monomer structures available and the use of a nonmetallic initiator. These surfaces were characterized by near-edge X-ray absorption fine structure and water contact angle measurements. The biocompatibility of these grown polymer brushes was studied and compared with deposited assemblies of surface-bound OEGn-terminated silanes with selected chain lengths. Grown polymer brushes with short OEGn side chains suppressed protein adsorption significantly more than the deposited assemblies of short OEGn chains, and this was attributed to higher surface coverage by the brushes. Cell adhesion studies confirmed that OEGn-containing polymer brushes are particularly effective in preventing nonspecific adhesion. Studies of protein adsorption and cell localization carried out with specific ligands on surfaces patterned demonstrated the potential of these surface-tethered polymer brushes for the formation of micro- and nanoscale devices.     

Properties of Star-Branched Polymer Chains Near a Surface

We considered two model systems of star-branched polymers near an impenetrable surface. The model chains were constructed on a simple cubic lattice. Each star polymer consisted of f = 3 arms of equal length and the total number of segments was up to 799. The excluded volume effect was included into these models only and therefore the system was studied at good solvent conditions. In the first model system polymer chain was terminally attached with one arm to the surface. The grafted arm could slide along the surface. In the second system the star-branched chain was adsorbed on the surface and the strength of adsorption was were varied. The simulations were performed using the dynamic Monte Carlo method with local changes of chain conformations. The internal and local structures of a polymer layer were determined. The lateral diffusion and internal mobility of star-branched chains were studied as a function of strength of adsorption and the chain length. The lateral diffusion and internal mobility of star-branched chains were studied as a function of strength of adsorption and the chain length. It was shown that the behavior of grafted and weakly adsorbed chains was similar to that of a free three-dimensional polymer, while the strongly adsorbed chains behave as a two-dimensional system.