Template-directed adsorption of block copolymers on alkanethiol-patterned gold surfaces

Functionalized alkanethiols have been self-assembled on gold to modify the wetting properties of the surface and promote or hinder the adsorption of block copolymers containing both hydrophobic and hydrophilic blocks. X-ray photoelectron spectroscopy (XPS) studies of spin-coated polyethylene-block-poly(ethylene oxide) (PE-b-PEO) copolymers on 16-mercaptohexadecanoic acid (MHDA)-, octadecanethiol (ODT)-, and 1H,1H,2H,2H-perfluorodecanethiol (PFDT)-covered surfaces have been performed. In the case of an 80 wt % PEO block copolymer, spin-coating on a gold surface precovered with MHDA results in a polymer film thick enough to completely attenuate Au 4f photoelectrons; spin-coating on the more hydrophobic ODT and PFDT monolayers leads to significantly thinner polymer films and incomplete attenuation of the gold photoelectrons. The opposite results are observed when a 20 wt % PEO block copolymer is used. Angle-resolved XPS studies of the 80 wt % PEO block copolymer spin-coated onto an MHDA-covered surface indicate that the PE blocks of the polymer segregate to the near-surface region, oriented away from the hydrophilic carboxylic acid tails of the monolayers; the surface concentration of PE is further enhanced by annealing at 90 degrees C. Microcontact printing and dip-pen nanolithography have been used to pattern gold surfaces with MHDA, and the surfaces have been backfilled with ODT or PFDT, such that the unpatterned regions of the surface are covered with hydrophobic monolayers. In the case of backfilling with PFDT, spin-coating the 80 wt % PEO copolymer onto these patterned surfaces and subsequent annealing results in the block copolymer preferentially adsorbing on the MHDA-covered regions and forming well-defined patterns that mimic the MHDA pattern, as determined by scanning electron microscopy and atomic force microscopy. Significantly worse patterning, characterized by micron-sized polymer droplets, results when the surface is backfilled with ODT instead of PFDT. Using PFDT and MHDA, polymer features having widths as small as 500 nm have been formed. These studies demonstrate a novel method to pattern block copolymers with nanoscale resolution.     

Selective hydrolysis of oxazoline block copolymers

Block copolymers, composed of a hydrophobic block [poly(N-t-butylbenzoyl ethylenimine) or poly(N-lauroyl ethylenimine)] and a hydrophilic block [poly(N-propionyl ethylenimine)], synthesized by cationic ring-opening polymerization of 2-substituted Δ2-oxazolines, were selectively deacylated by acid hydrolysis. The hydrolysis process was monitored by using ¹H-NMR. The results show that the propionyl groups could be removed from the hydrophilic block of the polymer chain without touching the hydrophobic block, if appropriate reaction conditions were used.     

Photochemical modification and patterning of polymer surfaces by surface adsorption of photoactive block copolymers

We report a simple photolithographic approach for the creation and micropatterning of chemical functionality on polymer surfaces by use of surface-active block copolymers that contain protected photoactive functional groups. The block copolymers self-assemble at the substrate-air interface to generate a surface that is initially hydrophobic with low surface tension but that can be rendered hydrophilic and functional by photodeprotection with UV radiation. The block copolymer employed, poly(styrene-b-tert butyl acrylate), segregates preferentially to the surface of a polystyrene substrate because of the low surface tension of the polyacrylate blocks. The strong adsorption of block copolymers causes a bilayer structure to form presenting a photoactive polyacrylate layer at the surface. In the example described, the tert-butyl ester groups on the polyacrylate blocks are deprotected by exposure to UV radiation in the presence of added photoacid generators to form surface carboxylic acid groups. Surface micropatterns of carboxylic acid groups are generated by UV exposure through a contact mask. The success of surface chemical modification and pattern formation is demonstrated by X-ray photoelectron spectroscopy and contact angle measurements along with imaging by optical and fluorescence microscopy methods. The resultant chemically patterned surfaces are then used to template patterns of various biomolecules by means of selective adsorption, covalent bonding and molecular recognition mechanisms. The surface modification/patterning concept can be applied to virtually any polymeric substrate because protected functional groups have intrinsically low surface tensions, rendering properly designed block copolymers surface active in almost all polymeric substrates.     

Adsorption of semiflexible block copolymers on homogeneous surfaces

We present the results of extensive numerical off-lattice Monte Carlo simulations of semiflexible block-copolymer chains adsorbed onto flat homogeneous surfaces. We have compared the behavior of several chain structures, such as homopolymers, diblocks, (A(alpha)B(alpha)) block copolymers, and random heteropolymers. In all the cases studied, we have found the adsorption process to be favored with an increase of the chain rigidity. Particularly, the adsorption of diblock structures becomes a two-step process characterized by two different adsorbing temperatures that depend on the chain stiffness kappa, the chain length N, and the adsorbing energies epsilon(A) and epsilon(B). This twofold adsorbing process changes to a single one for copolymers of reduced block size alpha. Each block of the stiff copolymer chain is found to satisfy the classical scaling laws for flexible chains, however, we found the scaling exponent phi to depend on the chain stiffness. The measurement of the radius of gyration exhibits a typical behavior of a polymer chain composed of Nl(p) blobs whose persistence length follows l(p) approximately (kappa/k(B)T)(0.5) for large stiff chains.     

Analytic density-functional self-consistent-field theory of diblock copolymers near patterned surfaces

Analytical solutions are derived for the density profiles and the free energies of compressible diblock copolymer melts (or incompressible copolymer solutions) near patterned surfaces. The density-functional self-consistent-field theory is employed along with a Gaussian chain model for bonding constraints and a random mixing approximation for nonbonded interactions. An analytical solution is rendered possible by expanding the chain distribution function around an inhomogeneous reference state with a nontrivial analytical solution, by retaining the linear terms, and by requiring consistency with the homopolymer limit. The density profiles are determined by both real and complex roots of a sixth-degree polynomial that may easily be obtained by solving a generalized eigenvalue problem. This analytical formulation enables one to efficiently explore the large nine-dimensional parameter space and can serve as a first approximation to computationally intensive studies with more detailed models. Illustrative computations are provided for uniform and patterned surfaces above the order-disorder transition. The results are consistent with the previous self-consistent-field calculations in that lamellar ordering appears near the surface above the order-disorder transition and the lamella order perpendicular or parallel to the surface depending on the commensurability between the periods of the surface pattern and the density oscillations.     

Poly(vinylpyrrolidone-b-styrene) block copolymers tethered surfaces for protein adsorption and cell adhesion regulation

Poly(vinylpyrrolidone-b-styrene) (PVP-b-PS) diblock copolymers tethered to glass surfaces were prepared, and the effects on protein adsorption and cellular behavior to the glass and the modified glass surfaces investigated. The PVP-b-PS grafting process was confirmed by water contact angle and XPS measurements. The results obtained for the water contact angles suggest that there are two phases that coexist on the PVP-b-PS block copolymer tethered surface, under aqueous conditions. Although the PVP-b-PS surface possessed, to some extent, a protein resistant property, following introduction of the PS segment to the end of tethered PVP, both fibrinogen and lysozyme adsorption were increased significantly. The PVP-b-PS modified surface, based on Western-blot analysis, appeared to have the greatest amount of surface bound vitronectin, however the conformation of the adsorbed vitronectin may have subsequently been affected by the surface tethered copolymer as was suggested by cell culture results. From these results, we proposed that protein adsorption and cell adhesion can be regulated by tuning the chemical compositions of diblock copolymers tethered to surfaces.     

Brownian dynamics simulations of polyelectrolyte adsorption onto charged patterned surfaces

The adsorption of single polyelectrolyte molecules onto surfaces decorated with periodic arrays of charged patches was studied using Brownian dynamics simulations. A free-draining, freely jointed bead-rod chain was used to model the polyelectrolyte, and electrostatic interactions were incorporated using a screened Coulombic potential with the excluded volume accounted for by a hard-sphere potential. The simulations predicted that the polyelectrolyte lies close to the adsorbing surface if the patch length, surface charge density, and screening length are sufficiently large. Chain conformations were found to be very sensitive to patch length, patch spacing, and the nature of the charge on adjacent patches. This is due both to the size of the polymer relative to patch length and spacing and to the structure of the electric field near the surface. In some cases, the component of the radius of gyration parallel to the surface can be made smaller than its free-solution value, which is contrary to what is observed for a uniformly charged surface. Isolated charged patches were also considered, and significant adsorption was observed above a critical surface charge density. The results demonstrate how polyelectrolyte conformations can be controlled by the design of the charged patches and may be useful for applications in which adsorbed polyelectrolyte films play a key role.     

Brownian dynamics simulations of polyelectrolyte adsorption onto topographically patterned surfaces

The effect of patterned surface topography on the adsorption of single polyelectrolyte molecules is explored using Brownian dynamics simulations. The polyelectrolyte is modeled as a free-draining, freely jointed bead-rod chain, and electrostatic interactions are incorporated using a screened Coulombic potential with excluded volume interactions accounted for by the repulsive part of a Lennard-Jones potential. Topography consisting of periodically spaced valleys of square cross section separated by flat hills is considered. Chain conformations are characterized for a wide range of valley widths, depths, and spacings as well as for several different types of surface charge distributions. Depending on the parameter values describing the topography, the chains are found to adopt conformations ranging from flat and extended to those associated with bridge-, brush-, or semi-bridge-like structures. The formation of these structures is rationalized on the basis of a free-energy model that takes into account the increase in free energy due to entropic confinement, excluded volume interactions, and chain stretching as well as the decrease in free energy due to bead-surface electrostatic attraction. The results of this work are expected to be useful in designing patterned surface topography to control the conformations of adsorbed polyelectrolyte molecules.     

Selective adsorption of protein molecules on phase-separated sapphire surfaces

Site-selective adsorption of protein molecules was found on sapphire surfaces that exhibit a phase separation into two domains: weakly charged hydrophobic domain and negatively charged hydrophilic one. Ferritin and bovine serum albumin molecules, which are negatively charged in a buffer solution, are adsorbed to the hydrophobic domains. Avidin molecules, which are positively charged, are adsorbed to the other domain. Fibrinogen molecules, which consist of both negative and positive modules, are adsorbed to the whole sapphire surface. Hemoglobin molecules, whose net charge is almost zero, are also adsorbed to the whole surfaces. These results indicate that electrostatic double layer interaction is the primary origin of the observed selectivity. Dependence of protein adsorption or desorption behaviors on the pH value can also be interpreted by the proposed model.     

Theory of polyelectrolyte adsorption onto surfaces patterned with charge and topography

Mean-field theory is used to derive criteria for the adsorption of a weakly charged polyelectrolyte molecule from salt solution onto surfaces patterned with charge and topography. For flat surfaces patterned with periodic arrays of charged patches, the adsorbed layer thickness predicted using mean-field theory and that found by Brownian dynamics simulations are in quantitative agreement in the strong-adsorption regime, which corresponds to sufficiently small kappa or sufficiently large |sigma(eff)q|, where kappa is the inverse Debye screening length, sigma(eff) is an effective surface charge density, and q is the charge on each segment of the polyelectrolyte. Qualitative agreement is obtained in the weak-adsorption regime, and for the case where surfaces are patterned with both charge and topography. For uniformly charged, sinusoidally corrugated surfaces, the theory predicts that the critical temperature required for adsorption can be greater than or less than the corresponding value for a flat surface depending on the relative values of kappa and the corrugation wave number. If the surface charge is also allowed to vary sinusoidally, then adsorption is predicted to occur only when the topography crests have a surface charge opposite to that of the polyelectrolyte. Surfaces patterned with rectangular indentations having charged bottoms which are separated by flat charged plateaus are investigated as well. Adsorption is predicted to occur even when the net surface charge is zero, provided that the plateaus have a charge opposite to that of the polyelectrolyte. If the charge on the plateaus and polyelectrolyte is the same, adsorption may still occur if electrostatic attraction from the indentation bottoms is sufficiently strong.     

The self-assembly of asymmetric block copolymers in films contacting a patterned surface

The preparation of ordered high-density polymer layers via the combined method of templated self-assembly is discussed. This approach combines the advantages of guided self-assembly of block copolymers and lithography on a topographical or chemical pattern. To implement the approach, a simulation has conducted for the first time through the use of the dissipative particle dynamics method in the NPAT ensemble. The pattern replication by asymmetric copolymers that form cylindrical phases in the bulk owing to their self-assembly near the patterned surface is studied. The effects of three patterns are described, i.e., hexagonal, rectangular, and triangular, which are characterized by one or two length scales. It is shown that the dense hexagonal pattern and the sparse rectangular and triangular patterns induce vertically oriented cylindrical domains in a thin film. The control of the orientation and ordering in the formed morphology heavily depends on the interaction between the minority component and the pattern. This effect is global in nature: The surface pattern propagates into the bulk of a film. In the case of rectangular and triangular patterns, two- and fourfold increases in their quantitites in the bulk are observed.     

The kinetics and saturation of reversible adsorption on patterned (heterogeneous) surfaces

We analytically examine the time-dependent adsorption of analyte (solute) on a finite-sized adsorption region as a model for sensors utilizing patterned or heterogeneous surfaces. We account for both reversible adsorption (assuming first-order reaction) and saturation of the adsorption patch that may arise either from packing constraints (finite area) or because of a finite number of binding sites (ligands). Our main conclusions include the following: (1) Saturation effects, due to either finite patch size or finite number of binding sites, become significant at extremely short times. (2) Increasing the strength of binding between the analyte and the adsorption sites increases the adsorbed amount at short times, but, at long times, the mass adsorbed on a weakly binding patch is higher than that on a strongly binding one. (3) The sensitivity of detection, as defined by the adsorption of the minimal analyte mass required for signaling, over a fixed period of time, does not scale as 1/detection time. As a result, increasing the time over which adsorption occurs increases sensitivity, but not linearly. Sensitivity of detection also increases with increasing patch area and initial binding strength.     

Diverse access to artificial superhydrophobic surfaces using block copolymers

The creation of an artificial superhydrophobic surface with micro- and nanostructures has been demonstrated using a block copolymer micelle solution and silica nanoparticles. The unique technique of a nanoparticle-supported micelle stabilization together with changes in the solvent power guarantees the precise morphology control of certain block copolymer-mediated surfaces. The approaches presented here provide a new strategy for the fabrication of a wettability-controlled organic-inorganic hybrid or organic coatings.     

Wetting of heterogeneous surfaces of block copolymers containing fluorinated segments

 The wetting of well-characterized heterogeneous surfaces of block copolymers has been studied by low-rate dynamic contact angle measurements using axisymmetric drop-shape analysis. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to investigate the roughness, the heterogeneity and the chemical composition of the surfaces. By changing the block length of polysulfone and semifluorinated polyester segments in the block copolymers, the surface heterogeneity of thin films prepared on silicon wafers could be controlled. Tapping-mode AFM measurements showed that soft, hydrophobic domains of varying size on the submicrometer length scale were obtained on these surfaces (60–250 nm). The mean roughness was of the order of several nanometers. The results of the contact angle measurements showed that neither roughness nor heterogeneity had a significant effect on the advancing contact angle of water, at the scale of the features present; however, the contact angle hysteresis increased with increasing percentage of the soft domains. We assume that liquid retention by the solid upon retraction of the three-phase line is the main cause for the observed increase in contact angle hysteresis. Concerning the molecular composition of these block copolymer surfaces, angle-resolved XPS analysis showed a surface segregation of fluorine within the surface region. A direct correlation was found between the fluorine content of the block copolymer surfaces and the advancing contact angle of water. Received: 26 May 2000 Accepted: 3 January 2001     

Tem- and neutron reflection studies on surfaces and interfaces with block copolymers

The behavior of block copolymers at various interfaces is studied by transmission electron microscopy and neutron reflection. A thin film of a symmetric diblock copolymer of styrene and methyl methacrylate forms layer structures when in contact with air and a random copolymer of styrene and acrylonitrile containing 35 wt% acrylonitrile. When the random copolymer has an acrylonitrile content of 25 wt%, a competition between layer formation and diffusion of disordered micelles takes place. Driving force for these processes are different interfacial tensions and a changing miscibility behavior as a function of acrylonitrile contents of the random copolymers. The ordering behavior of a symmetric diblock copolymer of deuterated styrene and isoprene in contact with poly(3,5-dimethyl phenylene ether) is studied by neutron reflection. Polystyrene-block-poly(ethene-co-but-1-ene)-block-polystyrene with cylindrical PS microdomains shows an interfacial phase transition to lamellae near to the interface with different polymers. The morphological studies are in agreement with adhesion data obtained by peel tests on different bilayer specimens.     

Selective surface chemistry using alumina nanoparticles generated from block copolymers

Developing orthogonal surface chemistry techniques that perform at the nanoscale is key to achieving precise control over molecular patterning on surfaces. We report the formation and selective functionalization of alumina nanoparticle arrays generated from block copolymer templates. This new material provides an alternative to gold for orthogonal surface chemistry at the nanometer scale. Atomic force microscopy and X-ray photoelectron spectroscopy confirm these particles show excellent selectivity over silica for phosphonic and carboxylic acid adsorption. As this is the first reported synthesis of alumina nanoparticles from block copolymer templates, characterizations via Fourier transform infrared spectroscopy, Auger electron spectroscopy, and transmission electron microscopy are presented. Reproducible formation of alumina nanoparticles was dependent on a counterintuitive synthetic step wherein a small amount of water is added to an anhydrous toluene solution of block copolymer and aluminum chloride. The oxidation environment of the aluminum in these particles, as measured by Auger electron spectroscopy, is similar to that of native aluminum oxide and alumina grown by atomic layer deposition. This discovery expands the library of available surface chemistries for nanoscale molecular patterning.     

Wetting films on chemically patterned surfaces

The behavior of thin wetting films on chemically patterned surfaces was investigated. The patterning was performed by means of imprinting of micro-grid on methylated glass surface with UV-light (λ=184.8 nm). Thus imprinted image of the grid contained hydrophilic cells and hydrophobic bars on the glass surface. For this aim three different patterns of grids were utilized with small, medium and large size of cells. The experiment showed that the drainage of the wetting aqueous films was not affected by the type of surface patterning. However, after film rupturing in the cases of small and medium cells of the patterned grid the liquid from the wetting film underwent fast self-organization in form of regularly ordered droplets covering completely the cells of the grid. The droplets reduced significantly their size upon time due to evaporation. In the cases of the largest cell grid, a wet spot on the place of the imprinted grid was formed after film rupturing. This wet spot disassembled slowly in time. In addition, formation of a periodical zigzag three-phase contact line (TPCL) was observed. This is a first study from the planned series of studies on this topic.