Low Surface Energy Substrates Research Articles

Emulsion homopolymerization of chlorotrifluoroethylene toward new waterborne fluorocarbon coatings

To solve the problems of environmental unfriendliness brought by long fluoroalkyl chain surfactant used in the homopolymerization of chlorotrifluoroethylene (CTFE) monomers as well as film forming difficulty and poor adhesion of PCTFE resins at the same time, a short fluoroalkyl capping waterborne polyurethane (F-WPU) dispersion was firstly synthesized in this work based on the processes of self-emulsification and phase inversion. The prepared fluorinated polyurethane dispersions could reduce the surface tension of water to 17.81–19.04 mN/m, showing high surface activity. Subsequently, the PCTFE (Poly(chlorotrifluoroethylene)/F-WPU composite latex was successfully prepared by emulsion homopolymerization, of which the F-WPU was used as the stabilizer and microreactor of system to emulsify CTFE monomers and then the corresponding composite coatings were further fabricated. Effects of the type and addition amount of F-WPU on emulsion homopolymerization of CTFE and the stability of composite emulsion were systematically studied. As a result, the PCTFE/F-WPU composite latex exhibits excellent wetting ability on various low surface energy substrates, and the composite coating displayed excellent adhesion and UV (Ultraviolet) resistance due to the synergistic action of F-WPU and PCTFE. The synthesis of F-WPU not only opens up a new route for the preparation of fluorinated surfactants, but also provides a new method for the emulsion homopolymerization of fluoroethylene monomers, and accelerates the wide application of fluorocarbon latex coating.

Environmentally friendly gelatin-based ink with durability on low surface energy substrates for anti-counterfeiting printing

Recent research is keen to introduce biomass materials into inks to develop "green” printing, but little attention has been given to whether these inks can be truly widely used in printing industry. There are few bio-based inks suitable for low surface energy plastic such as polyethylene glycol terephthalate (PET), which is one of the most common printing substrates. Herein, waterborne polyurethane with polyfluoroalkyl side chain (F-WPU) was designed and synthesized to prepare fluorocarbon quantum dots (F-CDs). F-WPU and F-CDs were mixed together with gelatin to obtain environmentally friendly inks which were utilized to print on a low surface energy substrate. F-WPU (30 wt%) and F-CDs (0.5 wt%) significantly improve the mechanical properties of composites by forming an interpenetrating network system and hydrogen bonds , respectively. Abundant carbon-fluorine bonds effectively reduce the surface energy of gelatin, resulting in excellent hydrophobicity and strong adhesion to the PET surface. The thermal stability of the composite could significantly enhance to meet the promising needs for the further engineering applications. Besides, F-CDs could emit blue-green fluorescence based on conjugated structure and π→π* transition under a wider excitation wavelength. Therefore, the complex exhibited good transparency under sunlight and performed significant cyan fluorescence under an ultraviolet irradiation, in which can be efficiently developed and used in anti-counterfeiting labels and printing. Furthermore, the friction, soaking and kneading tests have been measured to seriously take the considerations for the stable dry and wet fastness for the PET films, which could industriously expand the ink applications. • A durable and environmentally friendly gelatin-based ink was prepared. • Fluorinated waterborne polyurethane and carbon quantum dots were obtained. • F-WPU and F-CDs enhanced the mechanical strength and hydrophobicity of gelatin. • F-CDs contributes fluorescence characteristics and anti-counterfeiting value. • Composite gelatin is a potential anti-counterfeiting ink for PET surface printing.

Wetting of surfaces decorated by gas-phase synthesized silver nanoparticles: Effects of Ag adatoms, nanoparticle aging, and surface mobility.

The wetting state of surfaces can be rendered to a highly hydrophobic state by the deposition of hydrophilic gas phase synthesized Ag nanoparticles (NPs). The aging of Ag NPs leads to an increase in their size, which is also associated with the presence of Ag adatoms on the surface between the NPs that have a strong effect on the wetting processes. Furthermore, surface airborne hydrocarbons were removed by UV-ozone treatment, providing deeper insight into the apparent mobility of the NPs on different surfaces and their subsequent ripening and aging. In addition, the UV-ozone treatment revealed the presence of adatoms during the magnetron sputtering process. This surface treatment lowers the initial contact angle of the substrates and facilitates the mobility of Ag NPs and adatoms on the surface of substrates. Adatoms co-deposited on clean high surface energy substrates will nucleate on Ag NPs that will remain closely spherical and preserve the pinning effect due to the water nanomeniscus. If the adatoms are co-deposited on a UV-ozone cleaned low surface energy substrate, their mobility is restricted, and they will nucleate in two-dimensional islands and/or nanoclusters on the surface instead of connecting to existing Ag NPs. This growth results in a rough surface without overhangs, where the wetting state is reversed from hydrophobic to hydrophilic. Finally, different material surfaces of transmission electron microscopy grids revealed strong differences in the sticking coefficient for the Ag NPs, suggesting another factor that can strongly affect their wetting properties.

Bio-mimicking hybrid polymer architectures as adhesion promoters for low and high surface energy substrates

Thermoplastic materials like High Density Polyethylene (HDPE) and Polytetrafluoroethylene (PTFE) are promising candidates for specialty industry/aerospace sectors considering their exceptional properties and lower densities. However, the low surface energy leading to poor wetting and reduced adhesion curtails the scope of their wide-spread usage. In this work, we report adhesion promoter primers based on epoxy–catechol with and without fluorinated polymer and investigated their adhesion promotion efficiency on low/high surface energy substrates. The rational design of catechol-fluorine-epoxy hybrid network, mimicking mussel chemistry enabled a non-destructive surface activating approach resulting in profound adhesion promotion. The fluorine containing primer could effectively enhance adhesion between homo and hetero interfaces of Aluminium/HDPE/PTFE owing to the good interface interaction and could outperform common etching methods. A series of molecular level characterizations together with computational and AFM studies provided evidences for the high degree of interaction of the primer between the substrates and epoxy-based adhesives. Bond integrity endurance test using a weight of 10 and 20 kg resulted in ‘no fail’ categorization for the primed substrates during the entire 40 days test duration. The catechol-functionalized primers reported here can be a good overreaching approach to keep non-polar substrates afloat in advanced sectors.

Coefficient of restitution for silver nanoparticles colliding on a wet silver substrate

Air humidity can intervene in a particle–surface collision by forming a layer of water molecules on the surface. These water molecules act as a buffer layer that alters the collision condition. In this work, we study the effects of condensed water layer on the collision of 5-nm particles on a substrate by molecular dynamics simulations. The coefficient of restitution that is inversely proportional to the adhesion between the nanoparticle and the surface is used to characterize the collision. Results indicate that air humidity can either increase or decrease the coefficient of restitution in the studied nanoscale collisions, depending on the condensed water layer thickness and the surface energy of the substrate. The surface energy of the substrate varies with its Lennard-Jones potential. For a high surface energy substrate, the coefficient of restitution first increases then decreases by increasing the water layer thickness on the substrate when the impact velocity changes from 20 to 500 m/s. For a low surface energy substrate, the coefficient of restitution decreases because of a condensed water layer on the substrate at impact velocities less than 300 m/s. For higher impact velocities, the coefficient of restitution first increases, then decreases, with the condensed water layer thickness.

Polyethyleneimine as a surface activator for low surface energy substrates bonded with cyanoacrylate adhesives

The modification of surface properties of polypropylene (PP) substrates has attracted much attention in fundamental science and industrial applications. Primer treatment is one of the most simple and facile surface modification methods. This study used polyethyleneimine (PEI) as a surface activator for bonding polypropylene substrates with ethyl cyanoacrylate (ECA) adhesive. We have understood the effect of various factors such as the open time, solvent composition, optimum concentration and molecular weight of the primer. The bonding strength was maximum when the substrates were bonded immediately on priming. It was also observed that the solvent composition plays a minor role in the development of bond strength. The bond strength of substrates coated with a primer having a PEI concentration as low as 0.05% showed better results and was, in fact, three times better than the unprimed ones. It was also concluded that a low molecular weight PEI performs better than one having a higher molecular weight. DSC results confirmed that an improvement in strength is due to an increase in the molecular weight of the ECA polymer, which occurs in the presence of PEI molecules.

Device fabrication on curvilinear two-dimensional surfaces using polymer probes

Deposition of molecules on flat surfaces with sub-100 nm features of a plethora of combinations of inks and substrates, using flexible and hard probes on flat surfaces is extensively demonstrated in literature. However, the studies on the deposition and removal of fluorescent molecules and metal patterning on curvilinear two-dimensional substrates are scarce. In this study, conical and pyramidal probes composed of polydimethylsiloxane and polyacrylamide were used to deposit and remove thin coatings on planar and curvilinear surfaces. Fabrication of micro-photodetector devices on curved surfaces is demonstrated by fabricating microscale inter-digitated electrodes using probe-based patterning. A comprehensive characterization utilizing optical, electron and atomic force microscopy, and x-ray spectroscopic measurements of the probes and patterned substrates is presented. Differences in pattern capacity, speed, and quality of patterns based on nano-porous and non-porous probes is discussed. The inherent porosity and flexibility of polyacrylamide hydrogels allowed storage of large amounts of etchants facilitating microscale patterning of fluorescent inks and silver films on planar and curvilinear surfaces. A single polyacrylamide probe containing 0.9–1.2 nmol of rhodamine 6G stored within the polymeric matrix, allowed microscale patterning of an area of approximately 2.1 mm2 at an areal speed of ~1800 μm2/s. The modulation of size of the patterns was accomplished by changing the tip-substrate contact area. The experimental results showed a linear correlation between the displacement of the probe in the z-axis and the patterned area which agreed well with a physical model presented here. Importantly, the quality of micro-patterns was found to depend upon probe-substrate interfacial surface energy — high-quality patterns were obtained on low surface energy substrates. Finally, we demonstrated the fabrication of micro-photodetectors on two-dimensional curvilinear surfaces that posssessed a radius of curvature (k) ≈ 0.14 mm−1 by selectively removing silver from the surfaces followed by deposition of organic electron donor-acceptor pairs. In general, polymer lithography editor allows fabrication of devices on both planar and non-planar surfaces — PLE is an alternative tool to conventional microfabrication techniques.

Changing polymer catechol content to generate adhesives for high versus low energy surfaces.

Adhesive bonding is commonly used to replace mechanical fasteners in many applications. However, the surface chemistry of different substrates varies, making adhesion to a variety of materials difficult. Many biological adhesives are adept at sticking to multiple surfaces with a range of surface chemistries. Marine mussels utilize a catechol moiety within their adhesive proteins to bring about surface binding as well as cohesive cross-linking. Mimicking this functionality in synthetic polymers has yielded high strength adhesives that can attach to both high and low surface energy materials, although not equally well. Here, the amount of catechol within a copolymer system was varied for potential tailoring to specific surfaces. Structure-function studies revealed differing trends of optimal catechol content for high energy aluminum versus low energy polytetrafluoroethylene (TeflonTM) surfaces. Adhesion strengths were optimized with ∼10 mol% catechol for aluminum and ∼41 mol% for TeflonTM. Varying the catechol incorporation also resulted in changes to wettability, failure modes, and mechanics on these substrates. When considering performance of the entire bulk material, the different surfaces required an altered adhesive-cohesive balance. Tailoring the composition of polymeric adhesives for different surfaces may aid future manufacturing in cases where joining a variety of materials is required.

Engineering the Microstructure and Morphology of Explosive Films via Control of Interfacial Energy.

Physical vapor deposition of organic explosives enables growth of polycrystalline films with a unique microstructure and morphology compared to the bulk material. This study demonstrates the ability to control crystal orientation and porosity in pentaerythritol tetranitrate films by varying the interfacial energy between the substrate and the vapor-deposited explosive. Variation in density, porosity, surface roughness, and optical properties is achieved in the explosive film, with significant implications for initiation sensitivity and detonation performance of the explosive material. Various surface science techniques, including angle-resolved X-ray photoelectron spectroscopy and multiliquid contact angle analysis, are utilized to characterize interfacial characteristics between the substrate and explosive film. Optical microscopy and scanning electron microscopy of pentaerythritol tetranitrate surfaces and fracture cross sections illustrate the difference in morphology evolution and the microstructure achieved through surface energy modification. X-ray diffraction studies with the Tilt-A-Whirl three-dimensional pole figure rendering and texture analysis software suite reveal that high surface energy substrates result in a preferred (110) out-of-plane orientation of pentaerythritol tetranitrate crystallites and denser films. Low surface energy substrates create more randomly textured pentaerythritol tetranitrate and lead to nanoscale porosity and lower density films. This work furthers the scientific basis for interfacial engineering of polycrystalline organic explosive films through control of surface energy, enabling future study of dynamic and reactive detonative phenomena at the microscale. Results of this study also have potential applications to active pharmaceutical ingredients, stimuli-responsive polymer films, organic thin film transistors, and other areas.

Nonthermal Plasma Treatment Improves Uniformity and Adherence of Cyclodextrin-Based Coatings on Hydrophobic Polymer Substrates

Low surface energy substrates, which include many plastics and polymers, present challenges toward achieving uniform, adherent coatings, thus limiting intended coating function. These inert materials are common in various applications due to favorable bulk, despite suboptimal surface, properties. The ability to functionally coat low surface energy substrates holds broad value for uses across medicine and industry. Cyclodextrin-based materials represent an emerging, widely useful class of coatings, which have previously been explored for numerous purposes involving sustained release, enhanced sorption, and reversible reuse thereof. In this study, substrate exposure to nonthermal plasma was explored as a novel means to improve uniformity and adherence of cyclodextrin-based polyurethane coatings upon unreceptive polypropylene substrates. Plasma effects on substrates were investigated using contact angle goniometry and X-ray photoelectron spectroscopy (XPS). Plasma impact on coating uniformity was assessed through visualization directly and microscopically. Plasma effects on coating adhesion and bonding were studied with mechanical lap-shear testing and XPS, respectively. Substrate surface wettability and oxygen content increased with plasma exposure, and these modifications were associated with improved coating uniformity, adhesion, and interfacial covalent bonding. Findings demonstrate utility of, and elucidate mechanisms behind, plasma-based surface activation for improving coating uniformity, adherence, and performance on inert polymeric substrates.

A nucleobase-inspired super adhesive hydrogel with desirable mechanical, tough and fatigue resistant properties based on cytosine and ε-caprolactone

Adhesive hydrogels have significantly attracted scientific interest as a promising material for next-generation adhesive applications, such as tissue engineering, wound dressing, electronic skin sensor, and drug delivery. However, the acute defects of low mechanical property, limited adhesion and poor fatigue-resistant ability hinder their further applications, especially for low-surface energy materials. In this work, inspired by the advantages of cytosine from the basic units of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), the cytosine methacryloylchloride (CMA) was successfully introduced into the hydrogel and crosslink with glycerol-polycaprolactone-methacrylic anhydride (GPCL-MA) to obtain excellent mechanical, tough, adhesive and recyclable hydrogels. Surprisingly, the resultant hydrogel could repeatedly adhere to various solid materials (e.g., steel, aluminum, glass, ceramics, wood, PET and rubber) as well as skin. The maximum adhesive strength was 16.6 ± 0.38 KPa on the steel substrate, it also shows the superior capability of adhesion to the low-surface energy substrates (e.g., Polypropylene (PP) and Polytetrafluoroethylene (PTFE)). Furthermore, the hydrogel exhibited robust mechanical performance: 1065% (strain loading), 81.0 KJ/m3 (toughness), 6.3 MPa (tensile elastic modulus), and it could quickly recover its initial state after 6 cycles loading-unloading deformation. As a result, the obtained hydrogel shows promise for the future development of adhesive materials with outstanding mechanical properties and functionality simultaneously.

Property modification of a silicone acrylic pressure-sensitive adhesive with oligomeric silicone urethane methacrylate

A silicone acrylic pressure-sensitive adhesive (SPSA) was synthesized using a silicone macro-azo-initiator as an adhesive for low surface energy substrates. The SPSA had a high peel strength value on an SUS (steel use stainless) substrate due to its low surface energy; however, it had very low peel strength values on low surface energy substrates such as PP and PE. In this study, a silicone urethane dimethacrylate (SiUDMA) oligomer was synthesized as a modifier for the SPSA. The SiUDMA had a much lower viscosity and a slightly higher surface energy than the SPSA. The addition of SiUDMA increased the peel strength of the SPSA regardless of the substrate. In the case of the PP and PE substrates, the increase of peel strength with SiUDMA addition was remarkably large when compared to the SUS substrate because SiUDMA significantly reduced SPSA viscosity and improved its fluidity, consequently facilitating the wetting of the rough surfaces of PP and PE. The rough surfaces of PP and PE could also aid the spreading of SiUDMA-modified SPSA and enlarge the interfacial area. On the other hand, UV curing of SiUDMA and the SiUDMA-modified SPSA resulted in a serious decrease in the peel strength on the PP and PE substrates due to the decrease in the fluidity of the SPSA material, but the SAFT of SPSA increased with UV irradiation.

Evaluation of screen printed silver trace performance and long-term reliability against environmental stress on a low surface energy substrate

Otherwise attractive substrate materials for printed electronics may have such surface characteristics that make patterning challenging. This article focuses on the printability and performance characterization of conductive patterns on a low surface energy substrate. Surface characteristics of a hydrophobic polyphenylene ether (PPE) substrate and the effects of surface modification using chemical and physical pre-treatments were studied. In addition, silver ink performance and its reliability on this substrate were evaluated. The surface was characterized by surface energy measurements and surface profile analysis. Screen-printed test patterns were characterized to evaluate print quality and electrical and mechanical performance. A further inspection of substrate-ink interactions was conducted using environmental reliability tests. It was observed that ink adhesion could be significantly promoted by choosing a suitable surface pre-treatment method. Low sheet resistances were obtained, and thus, suitable inks for further characterization were found. In addition, it was observed that environmental stress has a significant impact on ink-substrate interactions.

Efficient Enrichment and Self-Assembly of Hybrid Nanoparticles into Removable and Magnetic SERS Substrates for Sensitive Detection of Environmental Pollutants.

A structure consisting of a low surface energy substrate and low surface tension liquid is designed and prepared by taking advantage of perfluorinated fluid infusion into the porous Teflon membrane. This slippery platform allows efficient enrichment and self-assembly of hybrid nanoparticles and the assembled structure can be detached from the membrane. A macroscale superlattice array of Au nanorods doped with magnetic Fe3O4 nanoparticles is obtained by suppressing the outward capillary flow and coffee-ring effect during evaporative self-assembly. In SERS (surface enhanced Raman scattering) detection of environmental pollutants including thiram, diquat and polycyclic aromatic hydrocarbons, the removable plasmonic superlattice array with magnetic properties enables rapid separation of analytes from the solution resulting in excellent sensitivity and detection limits down to the nanomolar level. The self-assembly strategy shows great potential in the fabrication of removable 3D plasmonic superlattice arrays for SERS detections.

Thermal stability and surface properties of acrylic PSAs modified by hexafluorobutyl acrylate

The article attempted to prepare special acrylic adhesives with preferable adhesion property and better thermal stability by introducing a fluorinated monomer. The FT-IR result showed that fluorinated monomers and acrylic monomers participated in copolymerization successfully. Furthermore, fluorinated groups performed good compatibility with acrylic resins, based on differential scanning calorimetry curve. According to the TG test under different heating rates, the activation energy of PSAs containing different content of fluorinated monomers was calculated to evaluate the effect of hexafluorobutyl acrylate on heat resistance of PSAs. Then, the findings of contact angle test revealed that the fluorinated PSAs also had rather lower surface energy than ordinary PSAs. Finally, the results of peel strength measurements indicated that the fluorinated PSAs demonstrated excellent adhesion property on various materials, especially low surface energy substrates.

Preparation and characterization of fluorinated acrylic pressure sensitive adhesives for low surface energy substrates

Abstract In this work, a series of PSAs with various fluorinated groups were synthesized using 2,2,3,4,4,4-hexafluorobutyl acrylate (F6BA) via solution polymerization. Compared to existing PSAs, fluorinated acrylic PSAs exhibited excellent performance in terms of the adhesiveness to the low surface energy materials. F6BA played a positive role in heat resistance of PSAs, resulting from the improvement of decomposition temperatures of PSAs substantially due to the addition of fluorinated groups. FT-IR and DSC results indicated that fluorinated groups were introduced to the structure of copolymer successfully and all the monomers were well copolymerized. Furthermore, it was confirmed that the incorporation of fluorine improved wettability of PSAs effectively and extended protective property of PSAs against the water corrosion.

The Evaluation of Alkyl Borane Compounds and Imidazole Borate Complexes as Initiators for Acrylic Adhesives Bonding Low Energy Substrates

Novel amido-borate initiators based on imidazolide bridged borate complexes were developed for acrylic adhesives used to bond low surface energy substrates. The focus of this development was examining the shelf life of the acrylic adhesive system as a function of the molecular architecture of the organoboron curative. The two key molecular features which significantly affect stability are: 1) the chain length of the alkyl groups on the boron atom and 2) arrangement of the imidazolide bridged borate structure. The results show that superior shelf life stability can be attained over the conventional trialkyl borane- and tetraalkyl borate-type initiators when this new family of catalysts (imidazolide borate compounds) is incorporated into the adhesive formulation without any harmful effect on cure speed.

Synthesis and Characterization of Alcohol Soluble Acrylate-Polyurethane Adhesive for Low Surface Energy Substrate

In this paper, vinyl-terminated polyurethane was synthesized with polypropylene glycol (PPG, Mn=1000) and toluene diisocyanate (TDI) as raw materials and hydroxyethyl acrylate as the blocking agent. Then the alcohol soluble acrylate-polyurethane (PU) adhesive for low surface energy substrate was acquired from vinyl acetate, acrylate and vinyl-terminated PU by solution polymerization. The resin was characterized by Infrared spectroscopy (IR), thermogravimetry analyses (TGA) and differential scanning calorimetry (DSC) respectively. Adhesion performance to biaxially oriented polypropylene (BOPP) was also studied. The results showed that the incorporation of PU can improve the viscosity, heat resistant, peel-off strength of the adhesive.

Evaluation of natural rubber latex-based PSAs containing aliphatic hydrocarbon tackifier dispersions with different softening points—Adhesive properties at different conditions

Natural rubber latex-based water–borne pressure sensitive adhesives (PSAs) have been formulated with three aliphatic hydrocarbon water-based dispersions (varying softening points) at two different resin addition levels (25% and 50%). Time–temperature superposition analysis using WLF approximations for adhesive peel has revealed that the adhesives formulated with 50% resin addition level show good adhesive behavior. It has also been determined from time–temperature superposition analysis that peel force increases systematically with softening point and peel rate. Correlation of viscoelastic behavior with adhesive properties suggests that at least 50% resin addition level is needed to bring the natural rubber-based formulations into PSA criteria as defined by Dahlquist and others. Adhesive property evaluations performed on a high surface energy substrate (stainless steel) and low surface energy substrate (LDPE) suggested that optimum tack, peel and shear properties at room temperature were obtained for a formulation containing a higher softening point dispersion (95°C) at 50% resin addition level. Adhesive peel and tack tend to follow softening point trends as well. A 25% tackifier dispersion addition level did not provide any significant adhesion. Humid aging (50°C and 100% relative humidity) evaluations of the water–borne adhesives seem to correlate well with the room temperature adhesive property observations.

Supercritical carbon dioxide swelling of fluorinated and hydrocarbon surfactant templates in mesoporous silica thin films

The penetration of compressed CO2 in hydrocarbon and fluorocarbon regions of concentrated surfactant mesophases are interpreted from differences in the CO2-processed pore expansion of mesoporous silica thin films templated by three surfactants containing varying degrees of hydrocarbon and fluorocarbon functionality. Ordered silica thin films are synthesized for the first time using the 16-carbon (C16) partly fluorinated surfactant, 11,11,12,12,13,13,14,14,15,15,16,16,16-tridecafluorocetyl pyridinium bromide (HFCPB), as a templating agent. Silica films templated with surfactants containing a 8-carbon (C8) fluorocarbon tail (3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl pyridinium chloride (HFOPC)) and a 16-carbon (C16) hydrocarbon tail (cetyl pyridinium bromide (CPB)) and HFCPB (C16) are processed in compressed CO2 (69–172bar, 25°C and 45°C) during synthesis. CO2 processing results in significant pore expansion for films templated with both fluorinated surfactants, while pore expansion is negligible for the hydrocarbon templated material suggesting that preferential CO2 penetration occurs in the ‘CO2-philic’ fluorocarbon segments of the surfactant template. The effect of substrate surface energy on the final uniformity of the dip-coated films is studied by varying the substrate from unmodified glass to a fluorocarbon-capped substrate. The ability to create dip-coated thin films on low surface energy substrates through favorable interaction of surfactant template tail with the substrate surface functional groups is demonstrated.