Simulation of grafted polymers on nanopatterned surfaces

Reversible Binding Interfaces Made of Microstructured Polymer Brushes

The polymer brush architecture of the end-tethered polymer molecules is one of the most widely used efficient methods to regulate interfacial interactions in colloidal systems found in live matter and manufactured materials. Emerging applications of polymer brush structures require solutions to new tasks in the control of interfacial interactions. The rapid development of live cell manufacturing relies on scalable and efficient cell harvesting methods. Stimuli-responsive surfaces made of surface-grafted poly(N-isopropylacrylamide) (PNIPAM) can bind and detach the adherent cell upon changes in temperature and have been used for cell growth and harvesting. The applications are limited by the requirement to satisfy a range of PNIPAM coating characteristics that depend on the dimensions of the integrin complex in the cell membrane and the basal surface. The analysis of the microstructured surfaces, when adhesive and disjoining functions of the microdomains are decoupled, shows that many limitations of PNIPAM one-component coatings can be avoided by using a much broader range of structural characteristics of the microstructured interfaces composed of alternating disjoining PNIPAM domains and adhesive polymeric domains with cell-affinity functional groups. Temperature-controlled reversible adhesion to such microstructured interfaces is studied here experimentally with model systems of solid spherical particles and by employing simulations for solid and soft membranes interacting with the microstructured surfaces to mimic interactions with soft and solid disk-like particles.

Investigation of the Effect of Molecular Weight, Density, and Initiator Structure Size on the Repulsive Force between a PNIPAM Polymer Brush and Protein

This paper focuses on the effect of degree of polymerization (N), density ( $\sigma$ ), and pattern size ( $x$ ) on the interaction force between a periodically patterned Poly(N-isopropylacrylamide) (PNIPAM) brush and protein. The hydrophobic interaction, the Van der Waals attractive force, and the steric repulsive force were expressed in terms of $N$ , $\sigma$ , and $x$ . The osmotic constant (k1) and the entropic constant (k2) were determined from the fit of the steric repulsive force to an experimentally obtained force distance curve. The osmotic constant was 0.105, and the entropic constant was 0.255. Using these constants, the steric repulsive force was plotted as a function of the separation distance(s) between the substrate and the protein. The forces were determined at a separation distance equal to 0.3 nm, where L0 is the equilibrium thickness of the PNIPAM brush. At this separation distance, the value of the steric repulsive force was much higher than the value of the sum of the hydrophobic interaction and the Van der Waals attractive force for large degree of polymerization ( $N>100$ ) and density ( $\sigma >0.2$ chains/nm2). However, the repulsive force was comparable to the sum of the hydrophobic interaction and the Van der Waals attractive force for a small degree of polymerization ( $N<100$ ) and density ( $\sigma =0.2$ ). Furthermore, the steric repulsive force was plotted as a function of pattern size $x$ . The plot indicated that the steric repulsive force becomes nearly zero for all degrees of polymerization and density when the value of the initiator structure size was less than 200 nm. In addition to the steric repulsive force, the lateral extension of the chains in the periodically patterned PNIPAM brush was calculated by scaling low and compared with the experimental data taken from previously published literatures. The polymer brush structure was modelled as if the immediate bare substrate is so wide that even a stretched polymer segment cannot reach to the next polymer brush structure. In such models, the value of the lateral extension was equal to the thickness of the homogenous brush. It was independent of the pattern size. However, when the polymer brush structure was modelled as if there is another polymer brush structure at a distance half of the size of the period, the lateral extension was found to be dependent on the size of the initiator structure size due to chain bridging. This was witnessed by the patterning of polymer brushes using the interferometric patterning of PNIPAM brushes and an atomic force microscopy imaging of the polymer brush structures both in air and in water. The polymer brush structure resolution in water was much lower than the resolution in air, which indicates the lateral extension of the polymer chains in water. For such kind of periodic polymer brush structures, the gap between them was calculated, and it was found dependent on the degree of polymerization, density, and initiator structure size.

Surface science of cosmetic substrates, cleansing actives and formulations

The development of shampoo and cleansing formulations in cosmetics is at a crossroads due to consumer demands for better performing, more natural products and also the strong commitment of cosmetic companies to improve the sustainability of cosmetic products. In order to go beyond traditional formulations, it is of great importance to clearly establish the science behind cleansing technologies and appreciate the specificity of cleansing biological surfaces such as hair and skin. In this review, we present recent advances in our knowledge of the physicochemical properties of the hair surface from both an experimental and a theoretical point of view. We discuss the opportunities and challenges that newer, sustainable formulations bring compared to petroleum-based ingredients. The inevitable evolution towards more bio-based, eco-friendly ingredients and sustainable formulations requires a complete rethink of many well-known physicochemical principles. The pivotal role of digital sciences and modelling in the understanding and conception of new ingredients and formulations is discussed. We describe recent numerical approaches that take into account the specificities of the hair surface in terms of structuration, different methods that study the adsorption of formulation ingredients and finally the success of new data-driven approaches. We conclude with practical examples on current formulation efforts incorporating bio-surfactants, controlling foaming and searching for new rheological properties.

Spatiotemporal control of polymer brush formation through photoinduced radical polymerization regulated by DMD light modulation

Spatially arranged polymer brushes provide the essential capability of precisely regulating the surface physicochemical and functional properties of various substrates. A novel and flexible polymer brush patterning methodology, which is based on employing a digital mirror device (DMD)-based light modulation technique to spatiotemporally regulate a surface-initiated photoinduced atom transfer radical polymerization (photo-ATRP) process, is presented. Various characterization techniques confirm that the spatially and/or temporally controlled brush formation results in complex PEG-derived brush patterns in accordance with a customized digital image design. A series of step-and-exposure strategies, including in situ multiple exposure, dynamic multiple exposure and dynamic sequential exposure, are developed to implement spatiotemporal regulation of the photo-ATRP process, leading to complex patterned and gradient brushes featuring binary functionalities, pyramid nanostructures and radial directional chemical gradients. Moreover, tunable and radial directional concentration gradients of various biomacromolecules (e.g., streptavidin) are obtained through preparation of height gradients of azido-functionalized brushes and subsequent orthogonal chemical activation aimed at specific protein immobilization. Finally, a unidirectional concentration gradient of fibronectin, surrounded by non-fouling PEG brushes, is fabricated and applied for human umbilical vein endothelial cell (HUVEC) adhesion experiments, whose preliminary results indicate gradient-dependent cell adhesion behavior in response to the concentration gradient of fibronectin. The presented fabrication technique could be integrated with microfluidic devices for sensors and bio-reactors, paving the way for novel approaches for lab-on-a-chip technologies.

Towards the description of adsorption of water in slit-like pores with walls covered by molecular brushes

The density profiles, adsorption isotherms, and phase behavior of a water model in a slit-like pore with walls modified by pre-adsorbed tethered chain molecules have been studied in the framework of a density functional theory. Each chain is bonded to the surface by its terminal segment, and the surface density of grafted chains is the same for each wall. The model for water taken from the work of Clark et al. [Mol. Phys. 104, 3561 (2006)] reproduces successfully a bulk equation of state. The mean field approach has been used to describe the effects of attractive interactions. The chemical association effects are taken into account by using the first-order thermodynamic perturbation theory of Wertheim. We have found that the presence of molecular brushes on the pore walls has important consequences for the adsorption and phase behavior of confined water. If the brush segments do not attract water molecules strongly, the vapor-liquid coexistence envelope shrinks upon increasing brush density, but the critical temperature is weakly affected. Alteration from capillary condensation to evaporation is observed with changes in the brush density, number of segments of tethered chains, and/or chemical identity of segments. The crossover temperature is affected by all these factors. Moreover, we have shown that affinity of water to segments of tethers is an important factor determining adsorption of water vapor and the entire phase diagram.

Efficient configurational-bias Monte-Carlo simulations of chain molecules with "swarms" of trial configurations

The pruned-enriched Rosenbluth method (PERM) is a popular and powerful Monte-Carlo technique for sampling flexible chain polymers of substantial length. In its original form, however, the method cannot be applied in Markov-chain Monte-Carlo schemes, which has rendered PERM unsuited for systems that consist of many chains. The current work builds on the configurational-bias Monte-Carlo (CBMC) method. The growth of a large set of trial configurations in each move is governed by simultaneous pruning and enrichment events, which tend to replace configurations with a low statistical weight by clones of stronger configurations. In simulations of dense brushes of flexible chains, a gain in efficiency of at least three orders of magnitude is observed with respect to CBMC and one order of magnitude with respect to recoil-growth approaches. Moreover, meaningful statistics can be collected from all trial configurations through the so-called "waste-recycling" Monte Carlo scheme.

Coordinated Responsive Arrays of Surface-Linked Polymer Islands-CORALs

The concept of co-ordinated responsive arrays of surface-linked islands (polymer CORALs) is introduced. This study targets a responsive system capable of revealing or covering the substrate surface in response to environmental changes in a reversible way. A convenient method of fabrication of polymer CORALs is proposed. It is based on microphase separation that occurs in thin films of supramolecular assemblies of block copolymers with reactive blocks. Such blocks form nanometer-size domains that may serve as anchors for surface-linked polymer islands. Two characteristics of the islands are critically important for the switching function: high grafting density within the islands and small lateral separation that allows interactions between polymer chains grafted to the neighboring islands. This combination permits complete coverage of the substrate surface upon exposure to a good solvent (relaxed state). In a weak solvent, the chains collapse within the islands, thus revealing the substrate (compact state). The morphology of the CORALs in both states and some details of the switching process were studied with atomic force microscopy, grazing incidence small-angle scattering, and coarse-grained molecular dynamic simulations.

Nanotribological Investigation of Polymer Brushes with Lithographically Defined and Systematically Varying Grafting Densities

Following controlled photodeprotection of a 2-nitrophenylpropyloxycarbonyl-protected (aminopropyl)triethoxysilane (NPPOC-APTES) film and subsequent derivatization with a bromoester-based initiator, poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes with various grafting densities were grown from planar silicon substrates using atom transfer radical polymerization (ATRP). The grafting density correlated closely with the extent of deprotection of the NPPOC-APTES. The coefficient of friction for such PMPC brushes was measured by friction force microscopy in water and found to be inversely proportional to the grafting density due to the osmotic pressure that resists deformation. Deprotection of NPPOC-APTES via near-field photolithography using a range of writing rates enabled the fabrication of neighboring nanoscopic polymeric structures with dimensions ranging from 100 to 1000 nm. Slow writing rates enable complete deprotection to occur; hence, polymer brushes are formed with comparable thicknesses to macroscopic brushes grown under the same conditions. However, the extent of deprotection is reduced at higher writing rates, resulting in the concomitant reduction of the brush thickness. The coefficient of friction for such polymer brushes varied smoothly with brush height, with lower coefficients being obtained at slower writing rate (increasing initiator density) because the solvated brush layer confers greater lubricity. However, when ultrasharp probes were used for nanotribological measurements, the coefficient of friction increased with brush thickness. Under such conditions, the radius of curvature of the tip is comparable to the mean spacing between brush chains, allowing the probe to penetrate the brush layer leading to a relatively large contact area.

Sequential Adsorption of Nanoparticulate Polymer Brushes as a Strategy To Control Adhesion and Friction

This work investigates surface forces that result from adsorbed layers of silica nanoparticles with grafted pH-responsive, cationic poly(2-(dimethylamino)ethyl methacrylate) brushes (SiO₂-g-PDMAEMA) and how adhesive bridging forces can be suppressed and friction forces reduced by "backfilling" these heterogeneous adsorbed layers with nonionic poly(ethylene oxide) star copolymers (Star PEO₄₅MA). Adsorption of SiO₂-g-PDMAEMA and Star PEO₄₅MA to silica is measured as a function of pH by quartz crystal microbalance with dissipation (QCM-D) in order to evaluate the electrostatically driven adsorption of SiO₂-g-PDMAEMA and hydrogen-bonding-driven adsorption of Star PEO₄₅MA. Force measurements performed using colloidal probe force microscopy show the strong role that limited surface coverage plays in adhesive bridging forces between silica with adsorbed SiO₂-g-PDMAEMA, while Star PEO₄₅MA adsorption produces purely repulsive steric interactions. Bridging between SiO₂-g-PDMAEMA-coated surfaces produces frictional forces that tend to be larger than those acting between bare surfaces at similar normal loads, while friction is consistently decreased by Star PEO₄₅MA adsorption. Sequential adsorption of SiO₂-g-PDMAEMA and Star PEO₄₅MA generates high-coverage mixed nanoparticulate brush layers with uniformly repulsive normal forces and reduced friction forces. Adsorption and force measurements reveal that Star PEO₄₅MA not only adsorbs to silica but also binds to SiO₂-g-PDMAEMA. The latter allows sequential adsorption of the two components to produce mixed multilayers. The mixed SiO₂-g-PDMAEMA/Star PEO₄₅MA multilayers exhibit larger layer thicknesses, no bridging, and sustained smooth friction, highlighting their potential usefulness as aqueous boundary lubricant layers.

Nanopatterned polymer brushes by reactive writing

Polymer brush patterns were prepared by a combination of electron beam induced damage in self-assembled monolayers (SAMs), creating a stable carbonaceous deposit, and consecutive self-initiated photografting and photopolymerization (SIPGP). This newly applied technique, reactive writing (RW), is investigated with 1H,1H,2H,2H-perfluorooctyltriethoxysilane SAM (PF-SAM) on silicon oxide, which, when modified by RW, can be selectively functionalized by SIPGP. With the monomer N,N-dimethylaminoethyl methacrylate (DMAEMA), we demonstrate the straightforward formation of polymer brush gradients and single polymer lines of sub-100 nm lateral dimensions, with high contrast to the PF-SAM background. The lithography parameters acceleration voltage, irradiation dose, beam current and dwell time were systematically varied to identify the optimal conditions for the maximum conversion of the SAM into a carbonaceous deposit. The results of this approach were compared to patterns prepared by carbon templating (CT) under analogous conditions, revealing a dwell time dependency, which differs from earlier reports. This new technique expands the range of CT by giving the opportunity to not only vary the chemistry of the created polymer patterns with monomer choice but also vary the chemistry of the surrounding substrate.

Nanopatterned polymer brushes: conformation, fabrication and applications

Surfaces with end-grafted, nanopatterned polymer brushes that exhibit well-defined feature dimensions and controlled chemical and physical properties provide versatile platforms not only for investigation of nanoscale phenomena at biointerfaces, but also for the development of advanced devices relevant to biotechnology and electronics applications. In this review, we first give a brief introduction of scaling behavior of nanopatterned polymer brushes and then summarize recent progress in fabrication and application of nanopatterned polymer brushes. Specifically, we highlight applications of nanopatterned stimuli-responsive polymer brushes in the areas of biomedicine and biotechnology.

Nanopatterned smart polymer surfaces for controlled attachment, killing, and release of bacteria

Model surfaces with switchable functionality based on nanopatterned, thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) brushes were fabricated using interferometric lithography combined with surface-initiated polymerization. The temperature-triggered hydration and conformational changes of nanopatterned PNIPAAm brushes reversibly modulate the spatial concealment and exposure of molecules that are immobilized in the intervals between nanopatterned brushes. A biocidal quaternary ammonium salt (QAS) was used to demonstrate the utility of nanopatterned PNIPAAm brushes to control biointerfacial interactions with bacteria. QAS was integrated into polymer-free regions of the substrate between nanopatterned PNIPAAm brushes. The biocidal efficacy and release properties of these surfaces were tested against Escherichia coli K12. Above the lower critical solution temperature (LCST) of PNIPAAm, desolvated, collapsed polymer chains facilitate the attachment of bacteria and expose QAS moieties that kill attached bacteria. Upon a reduction of the temperature below the LCST, swollen PNIPAAm chains promote the release of dead bacteria. These results demonstrate that nanopatterned PNIPAAm/QAS hybrid surfaces are model systems that exhibit an ability to undergo noncovalent, dynamic, and reversible changes in structure that can be used to control the attachment, killing, and release of bacteria in response to changes in temperature.

Nanopatterned polymer brushes as switchable bioactive interfaces

We report convenient methods for synthesis of nanopatterned, thermally responsive brushes of poly(N-isopropyl acrylamide) over large areas (e.g., 1 cm(2)) to form model, dynamic, biofunctional surfaces. The new nanopatterned brush structure can be used to control (i) the rate of both nonspecific and biospecific adsorption processes at the polymer-graft-free regions of the substrate, and (ii) the rate of cell detachment. These capabilities have potential implications in a number of areas of biotechnology including biosensing, separations and cell culture.

Highly sensitive, patternable organic films at the nanoscale made by bottom-up assembly

Nanoscale patterning of organic thin films is of great interest for next-generation technologies. To keep pace with the demands of state-of-the-art lithography, both the sensitivity and resolution of the patternable thin films need to be improved. Here we report a highly sensitive polyurea film grown by bottom-up assembly via the molecular layer deposition (MLD) technique, which allows for high-resolution patterning at the nanoscale. The MLD process used in this work provides an exceptionally high degree of control over the film thickness and composition and also offers high coating conformality. The polyurea film was formed by urea coupling reactions between 1,4-diisocyanatobutane and 2,2'-(propane-2,2-diyldioxy)diethanamine precursors and deposited in a layer-by-layer fashion. Acid-labile ketal groups were incorporated into the backbone of the polymer chains to ensure chemically amplified cleaving reactions when combined with photoacid, which was generated by electron-beam activation of triphenylsulfonium triflate soaked into the polyurea film. With electron-beam lithography, sub-100 μC/cm(2) sensitivity and sub-100 nm resolution were demonstrated using this new bottom-up assembly approach to resist fabrication.

Polymer nanogels grafted from nanopatterned surfaces studied by AFM force spectroscopy

Nanopatterned cross-linked polymers are important for applications with controlled mechanical properties. Grafted linear and cross-linked polydimethylacrylamide gels on micro- and nanopatterns were created using iniferter-driven quasi-living radical polymerization combined with conventional photolithography and nanosphere lithography. Micropatterned linear polymers reproduce the expected scaling behavior at moderate grafting density. The addition of cross-linker to the polymerization solution leads to an increased tendency of early termination as determined by AFM force spectroscopy. Similarly, nanopatterned linear polymers show reduced thickness in agreement with the expected scaling relationship for nanoisland grafts that have reduced lateral confinement. The addition of cross-linker reintroduces some of the lateral confinement for the length of polymers reported here. The mechanical properties of both the micro- and nanopatterned linear as well as cross-linked polymers were analyzed using an algorithm to objectively determine the contact point in AFM force spectroscopy and two independent Hertz-based analysis approaches. The obtained Young's moduli are close to those expected for homogeneous thick polymer films and are independent of pattern size. Our results demonstrate that polymeric nanopillars with controlled elastic modulus can be fabricated using irreversible cross-linkers. They also highlight some of the factors that must be considered for successful fabrication of grafted nanopillars of defined mechanical and structural properties.

Patterned polymer carpets

For the development of polymer carpets as active devices for micro- and nanotechnology, a control of the polymer carpet morphology and especially control of the stimulus responsive polymer brush is needed. Here, we report on the first example for the fabrication of patterned polymer carpets. On a two-dimensional framework of fully crosslinked and chemically patterned nanosheets, polymer brushes of styrene and 4-vinyl pyridine were grafted by self-initiated surface photopolymerization and photografting (SIPGP). It was found that polymer grafting by SIPGP occurred over the entire nanosheets but with a preferred grafting on the amino functionalized nanosheet areas. This results in continuous polymer carpets with an intact nanosheet framework but with amplification of the chemical patterning into a three dimensional topography of the grafted polymer brush. In the case of negative patterned nanosheets, the patterned carpet could be prepared as freestanding ultrathin membranes. Furthermore, swelling experiments with poly(4-vinyl pyridine) carpets showed that the patterns induces a directional buckling of the flexible polymer carpet. This may open the possibility of the development of micro- or nanoactuator devices with anisotropic responds upon environmental changes.

The role of the interplay between polymer architecture and bacterial surface properties on the microbial adhesion to polyoxazoline-based ultrathin films

Surface platforms were engineered from poly(L-lysine)-graft-poly(2-methyl-2-oxazoline) (PLL-g-PMOXA) copolymers to study the mechanisms involved in the non-specific adhesion of Escherichia coli (E. coli) bacteria. Copolymers with three different grafting densities α (PMOXA chains/Lysine residue of 0.09, 0.33 and 0.56) were synthesized and assembled on niobia (Nb₂O₅) surfaces. PLL-modified and bare niobia surfaces served as controls. To evaluate the impact of fimbriae expression on the bacterial adhesion, the surfaces were exposed to genetically engineered E. coli strains either lacking, or constitutively expressing type 1 fimbriae. The bacterial adhesion was strongly influenced by the presence of bacterial fimbriae. Non-fimbriated bacteria behaved like hard, charged particles whose adhesion was dependent on surface charge and ionic strength of the media. In contrast, bacteria expressing type 1 fimbriae adhered to the substrates independent of surface charge and ionic strength, and adhesion was mediated by non-specific van der Waals and hydrophobic interactions of the proteins at the fimbrial tip. Adsorbed polymer mass, average surface density of the PMOXA chains, and thickness of the copolymer films were quantified by optical waveguide lightmode spectroscopy (OWLS) and variable-angle spectroscopic ellipsometry (VASE), whereas the lateral homogeneity was probed by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Streaming current measurements provided information on the charge formation of the polymer-coated and the bare niobia surfaces. The adhesion of both bacterial strains could be efficiently inhibited by the copolymer film only with a grafting density of 0.33 characterized by the highest PMOXA chain surface density and a surface potential close to zero.

Patterning of polymer brushes. A direct approach to complex, sub-surface structures

We report a unique method to directly fabricate complex polymer brush structures with nanometer scale features by means of electron beam lithography. Polymer brushes for direct patterning were grown from surface-anchored initiator sites using atom transfer radical polymerization. Selected monomers (poly(2-hydroxyethyl methacrylate) and poly(methyl methacrylate)) were used based on their ability to readily scission when exposed to radiation. Single step direct patterning of polymer brushes is attractive as this eliminates many process steps, reducing the possibility of contamination and possibly improving resolution. In addition, we report a method to form subsurface polymer brush channels with nanometer-scale features. With the chains tethered to a surface, a diblock copolymer brush with a negative tone upper layer (polystyrene) and a positive tone under layer (poly(methyl methacrylate)) or (poly(2-hydroxyethyl methacrylate) were patterned to create channels. In the work presented, the direct electron beam patterning behavior of the brushes was studied and fabrication of nanochannels was demonstrated. Imaging of the nanopatterned surfaces was carried out using atomic force microscopy and fluorescence microscopy.

Competitive adsorption and assembly of block copolymer blends on nanopatterned surfaces

By employing off-lattice Monte Carlo simulations, the competitive adsorption and assembly of block copolymer blends on a nanopatterned surface were investigated. The segment distributions and polymer configurations are examined by varying the chemical structures of polymers, the interactions between segments and adsorbing stripe domains of the nanopatterned surface, and the width of stripe domains in the nanopatterned surface. The simulation results show that by modulating the affinities between a copolymer and the adsorbing stripe domain, one can adjust the density distributions and adsorption properties of block copolymer blends. With decorating the chemical structure of a surface, the targeted molecules would be actively recognized and separated. This offers a versatile way for novel separation materials and for the fabrication of nanomaterials.

Direct patterning of intrinsically electron beam sensitive polymer brushes

The fabrication of patterned polymer brushes has attracted considerable attention as these structures can be exploited in devices on the nano- and microscale. Patterning of polymer brushes is typically a complex, multistep process. We report the direct patterning of poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), poly(isobutyl methacrylate) (PIBMA), poly(neopentyl methacrylate) (PNPMA), and poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) brushes in a single step by electron beam (e-beam) lithography, to obtain nanopatterned polymer brush surfaces. PMMA, PHEMA, PIBMA, PNPMA, and PTFEMA brushes were grown on silicon substrates via surface-initiated atom transfer radical polymerization. Surface analysis techniques including ellipsometry, contact angle goniometry, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to characterize the thickness, hydrophilicity, roughness, and chemical composition of the polymer brushes. Tapping-mode AFM imaging confirmed the successful electron beam patterning of these brushes. Using this direct patterning method, highly resolved nanostructured polymer brush patterns down to 50 nm lines were obtained. This direct patterning of brushes eliminates the need for complex lithographic schemes. The sensitivity of these polymer brushes toward direct patterning with e-beam was studied and compared. The sensitivity curves indicate that the structure of the e-beam degradable methacrylate polymer has a significant effect on the sensitivity of the polymer brush toward e-beam patterning. In particular, the effect of the chemical functionality at the beta-position to the carbonyl group on the polymer brush sensitivity toward direct patterning was studied using groups of varying size and polarity.

Surface-initiated polymerization on laser-patterned templates: morphological scaling of nanoconfined polymer brushes

Nonlinear laser processing of silane-based monolayers is used to fabricate nanostructured chemical templates for the selective growth of polymer brushes in confined domains via surface-initiated polymerization (SIP). Upon varying the laser parameters, reactive domains with lateral dimensions from several micrometers down to the sub-100-nm range are fabricated. This provides a versatile means for studying the morphological scaling behavior of confined polymer brushes. Here, the surface-initiated growth of a stimuli-responsive polymer, poly(N-isopropylacrylamide) (PNiPAAm), via atom transfer radical polymerization (ATRP) is investigated. Polymer chains at the domain boundaries extend into the surrounding polymer-free areas. For this reason the width of confined polymer brushes is significantly larger than that of the underlying domains. Within experimental error, though, the excess width does not depend on the domain size. In contrast, the brush height decreases more and more when the domain size falls below a certain value. Simple considerations point to a geometrical scaling relation between height and width of the polymer brushes. These results are considered as essential for implementation of SIP routines in laser-assisted fabrication schemes targeting micro- and nanofluidic applications.

Polymer brushes on periodically nanopatterned surfaces

Structural properties of polymer brushes tethered on a periodically nanopatterned substrate are investigated by computer simulations. The substrate consists of an alternating succession of two different types of equal-width parallel stripes, and the polymers are end-tethered selectively on every second stripe. Three distinct morphologies of the nanopatterned brush have been identified, and their range of stability has been determined in terms of a single universal parameter that combines the grafting density, the polymer length, and the stripe width. We propose scaling relations for the average brush height and for the architectural properties of the outer surface of the nanopatterned brush under good solvent conditions. Our analysis provides guidelines for fabricating well-defined and tunable nanopatterned polymeric films.

Effect of nanoconfinement on the collapse transition of responsive polymer brushes

Nanopatterned brushes of a thermo-responsive polymer, poly(2-(2-methoxyethoxy)ethyl methacrylate) (PMEO2MA), displaying a collapse temperature in the physiological range were synthesized for grafting diameters from a few micrometers down to 35 nm. The reversible collapse transition of the nanobrushes was studied in water as a function of their lateral confinement, down to ensembles of brushes containing only approximately 300 chains. The confinement results in a considerable broadening of the collapse transition and in an increase of the degree of vertical swelling, which can be explained by the internal structure of the nanodroplets derived from a theoretical model of dry nanobrushes. These results enable the rational design of responsive surfaces having a tunable topography engineered at the nanometer scale, which is of direct interest for the development of soft nanoactuators and new substrates for cell adhesion studies.

Density functional approach to adsorption of simple fluids on surfaces modified with a brush-like chain structure

A density functional theory to describe adsorption of a simple fluid from a gas phase on a surface modified with pre-adsorbed chains is proposed. The chains are bonded to the surface by one of their ends, so they can form a brush-like structure. Two models are investigated. According to the first model all but the terminating segment of a chain can change the configuration during the adsorption of fluid species. The second model assumes that the chains remain "frozen", and the system is considered as a nonuniform quenched-annealed mixture. We apply simple form of interactions to study adsorption phenomena, microscopic structure, and layering transitions. Our principal findings show that new layering phase transitions can occur because of a chemical modification of the substrate under certain conditions, in comparison with nonmodified surfaces. However, opposite trends, that is, smoothing the adsorption isotherms, can also be observed, depending on the surface density of the grafted chains.

Assembly of copolymer blend on nanopatterned surfaces: a molecular simulation study

We report a molecular simulation study on the assembly of an (A7B5)5/A7B5 copolymer blend on nanopatterned surfaces. The density distributions, anisotropic radii of gyration, and conformations of both copolymers are quantitatively characterized. As the width of stripes on the surface decreases, the shape and thickness of the assembled film are found to be in qualitative agreement with those from experiments. The simulation results indicate that the shape and conformation of ordered film can be modulated by tuning the adsorption energy between the surface and the polymer or by adjusting the width of the stripes on the surface. We can regulate the width of the stripes to obtain a desired polymer conformation without altering the assembled film. In remarkable contrast to the pure copolymer, the radii of gyration of the blend in three directions are consistently smaller. The simulation reveals that the addition of a short chain during assembly is of central importance in restructuring the conformations of the long chain.

Morphology control of structured polymer brushes

The surface-initiated photopolymerization (SIPP) of vinyl monomers on structured self-assembled monolayers, as defined by two-dimensional (2D) initiator templates for polymer growth, is investigated. The 2D templates are prepared by electron-beam chemical lithography (EBCL) of 4'-nitro-4-mercaptobiphenyl (NBT) and chemical conversion to an asymmetric azo initiator (4'-azomethylmalonodinitrile-1,1'-biphenyl-4-thiol). Ex situ kinetic studies of the SIPP process reveal a linear increase in the thickness of the polymer layer with the irradiation/polymerization time. The effect of the applied electron dosage during the EBCL process upon the final thickness of the polymer layer is also studied. The correlation between the electron-induced conversion of the 4'-nitro to the 4'-amino group and the layer thickness of the resulting polymer brush indicates that the polymer-brush grafting density can be directly controlled by the EBCL process. NBT-based template arrays are used for the combinatorial study of the influence of the lateral structure size and the irradiation dosage on the morphology of the resulting polymer-brush layer. Analysis of the array topography reveals the dependence of the thickness of the dry polymer layer on both electron dosage and structure size. This unique combination of EBCL as a lithographic technique to locally manipulate the surface chemistry and SIPP to amplify the created differences allows the preparation of defined polymer-brush layers of controlled morphologies.

Viscoelastic modeling of highly hydrated laminin layers at homogeneous and nanostructured surfaces: quantification of protein layer properties using QCM-D and SPR

The adsorption of proteins at material surfaces is important in applications such as biomaterials, drug delivery, and diagnostics. The interaction of cells with artificial surfaces is mediated through adsorbed proteins, where the type of protein, amount, orientation, and conformation are of consequence for the cell response. Laminin, an important cell adhesive protein that is central in developmental biology, is studied by a combination of quartz crystal microbalance with dissipation (QCM-D) and surface plasmon resonance (SPR) to characterize the adsorption of laminin on surfaces of different surface chemistries. The combination of these two techniques allows for the determination of the thickness and effective density of the protein layer as well as the adsorbed mass and viscoelastic properties. We also evaluate the capacity of QCM-D to be used as a quantitative technique on a nanostructured surface, where protein is adsorbed specifically in a nanopattern exploiting PLL-g-PEG as a protein-resistant background. We show that laminin forms a highly hydrated protein layer with different characteristics depending on the underlying substrate. Using a combination of QCM-D and atomic force microscopy (AFM) data from nanostructured surfaces, we model laminin and antibody binding to nanometer-scale patches. A higher amount of laminin was found to adsorb in a thicker layer of a lower effective density in nanopatches compared to equivalent homogeneous surfaces. These results suggest that modeling of QCM-D data of soft viscoelastic layers arranged in nanopatterns may be applied where an independent measure of the "dry" mass is known.

Imaging energy landscapes with concentrated diffusing colloidal probes

The ability to locally interrogate interactions between particles and energetically patterned surfaces provides essential information to design, control, and optimize template directed self-assembly processes. Although numerous techniques are capable of characterizing local physicochemical surface properties, no current method resolves interactions between colloids and patterned surfaces on the order of the thermal energy kT, which is the inherent energy scale of equilibrium self-assembly processes. Here, the authors describe video microscopy measurements and an inverse Monte Carlo analysis of diffusing colloidal probes as a means to image three dimensional free energy and potential energy landscapes due to physically patterned surfaces. In addition, they also develop a consistent analysis of self-diffusion in inhomogeneous fluids of concentrated diffusing probes on energy landscapes, which is important to the temporal imaging process and to self-assembly kinetics. Extension of the concepts developed in this work suggests a general strategy to image multidimensional and multiscale physical, chemical, and biological surfaces using a variety of diffusing probes (i.e., molecules, macromolecules, nanoparticles, and colloids).

Molecular dynamics investigation into the effect of temperature on the structure and properties of methyl methacrylate thin films on a Au(111) surface

An atomistic modeling approach is performed to investigate the effect of temperature on the structural properties of the MMA (methyl methacrylate) thin film and a Au(111) surface. The density profile and orientation of the MMA molecule in the thin film have been analyzed. We found that there is a significant effect on the density profile and orientation of the MMA molecule in the region near the interface between the thin film and the Au substrate. As the temperature increases, the density clearly decreases in this region; in addition, the orientation of the MMA molecule also changes. Next, we calculated and examined the relationships between the stress, surface tension, and free energy and the density profile. Finally, we analyzed the influence of temperature on the interaction strength between the MMA molecules and between the MMA molecules and the Au(111) surface. We found that the influence of the interaction strength is more significant between MMA molecules than between MMA molecules and the Au(111) surface.

Density functional approach to the adsorption of spherical molecules on a surface modified with attached short chains

A density functional and Monte Carlo simulation study of end-grafted polymers immersed by simple fluids is presented. The polymer molecules are modeled as freely jointed tangent hard spheres with the end segments linked to the surface. The authors analyze an influence of the chain length, the grafting density, and a nature of solvent on the brush structure. Adsorption of hard-sphere mixtures on the modified surface is also discussed. The theory precisely approximates simulation data.

Nanostructured polymer brushes

Nanopatterned polymer brushes with sub-50-nm resolution were prepared by a combination of electron-beam chemical lithography (EBCL) of self-assembled monolayers (SAMs) and surface-initiated photopolymerization (SIPP). As a further development of our previous work, selective EBCL was performed with a highly focused electron beam and not via a mask, to region-selectively convert a SAM of 4'-nitro-1,1'-biphenyl-4-thiol to defined areas of crosslinked 4'-amino-1,1'-biphenyl-4-thiol. These "written" structures were then used to prepare surface-bonded, asymmetric, azo initiator sites of 4'-azomethylmalonodinitrile-1,1'-biphenyl-4-thiol. In the presence of bulk styrene, SIPP amplified the primary structures of line widths from 500 to 10 nm to polystyrene structures of line widths 530 nm down to approximately 45 nm at a brush height of 10 or 7 nm, respectively, as measured by scanning electron microscopy and atomic force microscopy (AFM). The relative position of individual structures was within a tolerance of a few nanometers, as verified by AFM. At line-to-line spacings down to 50-70 nm, individual polymer brush structures are still observable. Below this threshold, neighboring structures merge due to chain overlap.