The Role of Adhesion Promoters on the Molecular and Mesoscopic Structure of the Interphase

New experimental results are described from Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS) and Atomic Force Microscopy (AFM) on the molecular orientation, chemical composition, and topography of PMDA/ODA Polyimide (PI) deposited by the Langmuir-Blodgett (LB) technique onto Si (100) surfaces covered with a native oxide (SiO_x) and onto SiO_x substrates pre-treated with a variety of alkylsilanes. The alkylsilane monolayers used as adhesion promoters were found to be chemically and structurally stable during temperature treatments used to imidize the polymer precursor films. On both the oxidized silicon surface and on the silane-treated surfaces, we find a thickness-dependent, preferential orientation of the pyromellitimide unit in PI, but randomly-oriented oxydianiline fragments. We can not identify any specific chemical bonds between the polymer and the aminosilane film or, in the case of the untreated surface, the SiO_x substrate. However, a quantitative analysis of the NEXAFS data reveals a pronounced deficit in oxydianiline in the monolayer (ML) films, which decreases with film thickness, and a higher degree of imidization on the 3-aminopropyltrimethoxysilane (APS) surface indicating that imide linkages are formed between the polymer and the aminosilane. The AFM images show a heterogeneous surface topography with nucleation of PI polycondensates on both the SiO_x and aminosilane-treated surfaces. On the aminosilane surface additional nuclei are detected, believed to be polycondensates of aminosilane formed during imidization of the PI precursor. Nucleation of aminosilane polycondensates is not observed in the absence of polyamic acid. As suggested by a comparison of Lateral Force Microscopy (LFM) and Atomic Force Microscopy measurements, these aminosilane nuclei are covered by PI with increasing film thickness and, hence, provide mechanical connections between the polymer and substrate on a mesoscopic scale. Delamination experiments of 2 μm thick PI films spun onto ω-amino and methyl terminated alkylsilanes indicate that bond fracture always occurs in the aminosilane/PI and alkylsilane/PI and interphase. We conclude that the adhesion promotion effect of LB-deposited polyimide films by aminosilanes can be explained by chemical stabilization of the interface against decomposition and mechanical interlinking surface via polycondensates of aminosilane on a mesoscopic length scale. Our study is not conclusive as to whether covalent bonding of the polymer to the substrate via the silane is important for macroscopic adhesion.     

Quality enhancement of copper oxide thin film synthesized under elevated gravity acceleration by two-axis spin coating

Two-Axis spin coating as a new modified technique is employed to enhance the quality and surface leveling of thin films. The modified technology utilizes a synthetic centrifugal force perpendicular to the surface which generates an elevated gravity acceleration while spreading the coating on the entire wafer surface. In this paper, copper acetate sol-gel is coated by conventional and Two-Axis spin coating techniques. The coated layers are sintered in an air furnace at 275 °C. The fabricated layers are characterized by GIXRD, EDX, AFM and SEM devices. Wettability and Surface Free Energy (SFE) of sintered films using the contact angle technique are measured, and evaluated by the Owens-Wendt method. XRD and EDX spectra show a higher intensity of copper oxide phase using Two-Axis spin coating technology. AFM micrographs showed an improvement in the surface leveling within the Two-Axis spin coated layer. A comparison between the SFE of conventional and Two-Axis spin coated layers shows an increase in SFE of the layer synthesized under 200g artificial gravity acceleration.     

Controlling the surface wettability of poly(sulfone) films by UV‐assisted treatment: benefits in relation to plasma treatment

Hydrophilic and superhydrophilic surfaces of poly(sulfone) (PSU) thin films were prepared by UV irradiation in the presence of O₂ or acrylic acid (AA) vapor. Treated surfaces were then investigated by water contact angle measurements, Fourier transformed IR spectroscopy in attenuated total reflectance mode (FTIR‐ATR), X‐ray photoelectron spectroscopy (XPS), near‐edge X‐ray absorption fine structure (NEXAFS) and AFM. Water contact angle values of treated PSU films using either O₂ or AA vapor as the reactive atmosphere reached about 6° after more than 120 min of irradiation. FTIR‐ATR, XPS and NEXAFS analysis showed incorporation of oxygenated groups onto the surface that led to its hydrophilic characteristics. In addition, when AA vapor was used as the reactive atmosphere, a photopolymerization process of poly(acrylic acid) onto the surface of the PSU was observed. AFM analysis showed a very low level of roughness after the treatments. A comparison of UV‐assisted surface modifications of PSU films with traditional plasma treatments showed excellent qualitative agreement between the two techniques. Our results show that UV‐assisted treatments in the presence of AA vapor or O₂ are efficient ways of controlling the surface wettability and functionalities grafted on the surface of PSU films. This treatment can be considered as a permanent dry grafting method that resists aging and uses a simple experimental setup. © 2012 Society of Chemical Industry     

Surface modification of poly(tetrafluoroethylene-co-hexafluoropropylene) with vacuum UV radiation from rotating helium dc arc plasmas

Treatment of poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) with vacuum ultraviolet (VUV) radiation from high pressures of helium in rotating dc arc plasmas was investigated. X-ray photoelectron spectroscopy (XPS) detected defluorination and appearance of the functional groups (C=O, O–C=O and C–O) on the surface. Fourier transform infrared (FT-IR) spectroscopy showed the appearance of a band at 1884 cm^?1 indicating the formation of the carboxylic acid fluoride moiety, –(C=O)–F. Improvement in film wettability was observed by contact angle measurements while SEM micrographs showed an increased amount of cracking on the surface with VUV exposure. Adhesion measurements of Cu sputter-coated onto the photo-modified surfaces resulted in failure within the fluoropolymer (cohesive failure) and not at the Cu-fluoropolymer interface. Cohesive failure occurred with shorter treatment times than for VUV exposure downstream from low pressure Ar microwave plasmas.     

Gold Nanolayers on Plasma-Treated Polypropylene

This work deals with characterization of polypropylene (PP) exposed to plasma discharge and gold layers deposited on the plasma modified PP. PP foils were exposed to Ar plasma and subsequently metallized with sputtered Au layer. Chemical structure of the plasma modified PP was studied using X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering (RBS). Wettability of the plasma modified PP and its changes during sample aging were determined by goniometry. Surface morphology of PP and deposited Au layers was measured with atomic force microscopy (AFM). Continuity of Au layers was characterized by measuring their sheet resistance. With increasing exposure time in the plasma discharge the water contact angle decreases and the polymer surface becomes more hydrophilic. During the aging of the plasma treated samples the contact angle increases again. Plasma treatment leads to a decrease of the PP surface roughness and to generation of oxygen-containing polar groups on PP surface. During sample aging the concentration of the oxygen-containing groups decreases.     

亲水性无定型TiO_2/SiO_2薄膜及其强化传热性能研究

采用溶胶-凝胶技术在传热管表面涂覆TiO2/SiO2复合溶胶涂层,并通过优化热处理温度和干湿交替处理工艺,提升涂层的亲水性能和强化传热性能。试验结果显示,在热处理温度为200℃时,凝胶涂层以无定型结构为主;热处理试样经过6次干湿交替处理后,即可使表面接触角降低到10°左右;传热管表面较好的润湿性能,可显著提升降膜式换热器的换热效率。     

A surface acoustic wave sensor study of polyimide thin film surface treatments - effect on water uptake

The effects of surface treatments on the water uptake in thin (1 μm) polyimide (PI) films were studied using a surface acoustic wave (SAW) sensor, X-ray photoelectron spectroscopy (XPS), external reflectance infrared (ERIR) spectroscopy, and contact angle measurements. Surface modification of PI films can affect film properties such as water uptake and adhesion. These properties, in turn, affect the performance and reliability of the devices in which these films are used. The ability to nondestructively study the results of various surface modification techniques in situ, prior to deposition of a metal layer for example, would be of particular benefit in the fabrication process. The results of this work indicate that the SAW sensor can measure extremely small amounts (< 0.003 μg) of water uptake in thin (1.2 μm) PI films. Also, that the water uptake of PI films, as measured by the SAW sensor, is particularly sensitive to sputter cleaning, sputtering/ KOH, and Teflon AF surface treatment. The SAW, XPS, ERIR, and contact angle studies of the Teflon AF treated PI indicate that the concentration of Teflon AF is very high in the surface region of the PI and decreases into the bulk of the film. This work suggests utility of the SAW sensor as a nondestructive and in situ method for monitoring the surface properties of thin polymers in process control applications.     

Surface properties and moisture behaviour of pine and heat-treated spruce modified with alkoxysilanes by sol–gel process

The surface morphology and moisture behaviour of pine (Pinus sylvestris) sapwood and heat-treated spruce (Picea abies) deposited with two types of silane-based sol–gel coatings were studied by atomic force microscopy (AFM) and water contact angle measurement. The chemical composition and distribution of sol–gel coatings on wood surfaces were investigated by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The AFM images revealed that the sol–gel coatings applied by spreading covered the fine structure of the wood substrates. The surface roughness analysis of the AFM topographical images indicated that the sol–gel coatings, especially the one with short aliphatic chain, had a tendency to smooth the wood surface. The XPS results confirmed that the sol–gel coatings had successfully deposited onto pine sapwood and heat-treated spruce changing their surface chemistries. ToF-SIMS images showing Si ion distribution on treated surfaces revealed that the coatings fully covered pine sapwood surfaces. The thin coating layers formed on heat-treated spruce surfaces followed the original wood surface structure. The contact angle measurements indicated that the water repellent properties of both pine and heat-treated spruce were improved to certain extent by the sol–gel coatings.     

Preparation and characterization of thin organosilicon films deposited on SPR chip

The paper reports on the preparation and characterization of organosilicon thin polymer films deposited on glass slides coated with 5 nm adhesion layer of titanium and 50 nm of gold. The polymer was obtained by the decomposition of 1,1,3,3-tetramethyldisiloxane precursor (TMDSO) premixed with oxygen induced in a N₂ plasma afterglow using remote plasma-enhanced chemical vapor deposition (PECVD) technique. The film thickness was controlled by laser interferometry and was 9 nm. The chemical stability of the gold substrate coated with the organosilicon polymer film (p-TMDSO) was studied in different acidic and basic solutions (pH 1-14). While the gold/polymer interface showed a high stability in acidic media, the film was almost completely removed in basic solutions. The resulting surfaces were characterized using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), water contact angle measurements, cyclic voltammetry, and surface plasmon resonance (SPR).     

Silicon and carbon substrates induced arrangement changes in poly(styrene-b-isoprene-b-styrene) block copolymer thin films

Poly(styrene-b-isoprene-b-styrene) (SIS) block copolymer ordering in thin films was studied using two selective substrates as carbon and silicon. Atomic force microscopy (AFM) and contact angle measurements were employed to examine the affinities between domains and surrounding interfaces. The surface morphology was examined by AFM using different amplitude ratios. Results showed polyisoprene (PI) domain layer formation in the outermost film layer. On the other hand, the layer close to substrate adopted different arrangements on silicon and carbon substrates. Topographical and phase images revealed that in both substrates with the thickest films, the interactions between substrate and block domains were not enough to induce surface ordering being the morphology independent of employed substrate. However, decreasing film thickness, SIS thin films displayed a variety of arrangements such as perforated lamellae and cylindrical morphologies. Depending on substrate, these morphologies were achieved in different film thicknesses. Finally, the thinnest film did not adjust to characteristic domain spacing commensurability and terraces formation was observed. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Surface Dynamics vs. Adhesion in Oxygen Plasma Treated Polyolefins

Polyethylene (PE) and polypropylene (PP) were oxygen plasma treated and aged in carefully reproducible conditions. The effect of aging on the surface chemistry, wettability and adhesion were studied using a combination of techniques: contact angle measurements, XPS, SSIMS, adhesion tests (shear and pull).

PE was found to be relatively insensitive to aging both in terms of wettability and adhesion, due to crosslinking during plasma treatment, which is likely to reduce macromolecular mobility within the surface layer.

In the case of PP, dramatic decreases of wettability occur with time, due to macromolecular motions leading to minimization of oxygen-containing functions at the surface. This behavior was shown to affect the adhesion performance of treated PP.     

丙烯酸树脂/TiO2有机-无机杂化材料的表面性质研究

采用预水解的二氧化钛（TiO2)溶胶与丙烯酸树脂共混或原位聚合的方法成功地制备了均匀透明的丙烯酸树脂／TiO2有机—无机杂化材料。通过原子力显徽镜(AFM)、X-射线光电子能谱(XPS)和表面接触角研究了杂化材料涂层的表面性质。结果发现在杂化材料中添加TiO2后，涂层的表面粗糙度、表面Ti元素含量和表面自由能增加，接触角下降。     

Vapor deposition polymerization of synthetic rubber thin film in a plasma enhanced chemical vapor deposition reactor

Rubber is one of the most commonly used industrial materials worldwide. However, there is a gap in the literature on the production of rubber thin films in nanoscale. When the rubber thin films are produced in nanoscale, they can be used in high-tech applications where bulk rubbers have never been used before. This study is one of the first investigations to focus on the vapor-based production of the polyisoprene (PI), which is an important member of the synthetic rubber class. For this purpose, a single-step, rapid and environmentally friendly method based on plasma enhanced chemical vapor deposition (PECVD) was employed to produce PI thin films using 2-methyl-1,3-butadiene (isoprene). The high-vapor pressure of isoprene makes it a promising monomer for the production of chemical vapor deposition polymers. The effect of plasma processing parameters on the PI deposition rate was investigated. The deposition rate of PI thin film as high as 40 nm/min was achieved and the contact angle of PI coated bamboo surface was found to be 146.8°. The mechanical durability and laundering tests of PI thin films were performed. Based on this study results, PI thin films produced by PECVD can be used in a number of potential applications.     

Hydrophilic modification of polypropylene microporous membranes by grafting TiO2 nanoparticles with acrylic acid groups on the surface

Hydrophilic microporous membranes were prepared based on polypropylene (PP) cast films blended with a commercial acrylic acid grafted polypropylene (PP-g-AA) via melt extrusion followed by grafting titanium dioxide (TiO₂) nanoparticles on its surface, annealing and stretching. ATR-FTIR, XPS and EDS analyses showed that the hydrophilic segments of an amphiphilic modifier (PP-g-AA) acted as surface functional groups on the film surface. The results indicated that the presence of the modifier was very important for grafting TiO₂ nanoparticles on the film surface. Compared to PP and PP/PP-g-AA blend films, the water contact angle decreased by a factor of 2.5 after grafting TiO₂ on the surface of the films, meanwhile the water vapor permeability of the microporous membranes prepared from those films increased by a factor of 1.5. All these results indicated that the hydrophilicity of the modified PP membranes was improved.     

Effects of surface modification by Ar+ irradiation on wettability of surfaces of poly(ethylene terephthalate) films

Surfaces of poly(ethylene terephthalate); PET, films were irradiated with Ar⁺ at 1 keV using various ion doses (ID) from 10¹⁴ to 10¹⁷ ions/cm² (isc) with and without an O₂ environment. The wettability of the modified surfaces of PET was determined by measuring the contact angle between water droplets and the modified surfaces. The modified surfaces were also characterized by AFM (atomic force microscopy) and XPS (X-ray photoelectron spectroscopy) for changes in the surface morphology, and the chemical composition and molecular structure, respectively. The contact angle decreased from 70° for unmodified surfaces to 45° for modified surface with ID = 10¹⁴ isc without O₂ and remained relatively constant with higher ID. The contact angle, however, reached a minimum value of 8° for modified surfaces with ID = 10¹⁶ isc with O₂. The improved wettability may be due to a combination of the formation of hydrophilic groups, chemical and molecular structural changes, physical structural or morphological changes, and increased roughness of the surface. The wettability of the modified surfaces also depended on the time of exposure to air. The wettability worsened with exposure time to air, but was revived by immersing the films into water. Possible mechanisms for the change of the wettability of the modified surfaces are given.     

Preparation and characterization of TiO2–SiO2 nano-composite thin films

TiO₂ nanocrystalline particles dispersed in SiO₂ have been prepared by the sol-gel method using titanium- and silicon-alkoxides as precursors. Nano-composite thin films were formed on the glass substrates by dip-coating technique and heat treated at temperatures up to 500 °C for 1 h. The size of the TiO₂ nanocrystalline particles in the TiO₂–SiO₂ solution ranged from 5 to 8 nm. The crystalline structure of TiO₂ powders was identified as the anatase phase. As the content of SiO₂ increased, the anatase phase tended to be stabilized to higher temperature. TEM results revealed the presence of spherical TiO₂ particles dispersed in a disk-shaped glassy matrix. Photocatalytic activity of the TiO₂–SiO₂ (1:1) thin films showed decomposition of 95% of methylene blue solution in 2 h and a contact angle of 10°. The photocatalytic decomposition of methylene blue increased and the contact angle decreased with the content of TiO₂ phase. TiO₂–SiO₂ with the molar ratio of 1:1 showed a reasonable combination of adhesion, film strength, and the photocatalytic activity.     

The role of chemical surface modification for structural adhesive bonding on polymers - Washability of chemical functionalization without reducing adhesion

This study investigates the role of surface functionalization to increase adhesion forces on polymers. The effects of two different physical pre-treatment methods (oxygen low-pressure plasma - LPP, vacuum-UV - VUV) are investigated on four different polymer matrices (polyetheretherketone - PEEK, polyetherimide - PEI, polyethersulfone - PES and the epoxy resin RTM6). Polymer surfaces were additionally washed after surface treatment with different polar solvents. Surface chemistry, wettability, and topography were investigated before pre-treatment, after pre-treatment and after washing of the samples using x-ray photoelectron spectroscopy (XPS), contact angle measurements and atomic force microscopy (AFM).The results show, that washing of the samples after pre-treatment lead to a chemical surface condition similar to the initial surface. Interestingly, the tensile bond strength of centrifugal adhesion tensile test (CATT) specimens however remained high. In consequence, the thus far widely accepted understanding of surface functionalization as the dominating factor for adhesion promotion on polymers has to be re-evaluated.     

Preparation of 4A zeolite coated polypropylene membrane for lithium-ion batteries separator

This study aims to improve wettability and thermal resistance of lithium-ion batteries separators. For this purpose, a commercial polypropylene (PP) separator was coated by 4A zeolite using poly(vinylidene fluoride) as binder and effects of the separators' zeolite content was investigated. All the coated separators showed lower contact angles, higher electrolyte uptakes, and less thermal shrinkages compared to the neat commercial separator. The coated PPA8 separator (zeolite to binder ratio of 8) showed the lowest wettability (contact angle of 0°) and electrolyte uptake (270%) due to its surface porosity resulting from the zeolite particles interstitial cavities as well as their internal cavities. Also, the PPA8 separator ion conductivity was found as 2.25 mS cm⁻¹ and C-rate and cycling performance of its assembled battery were higher compared to those of the commercial PP separator assembled battery. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47841.     

Surface modification of poly(propylene) by photoinitiators: Improvement of adhesion and wettability

Different benzophenone‐type photoinitiators were photografted onto poly(propylene) (PP). The polymer surfaces were analyzed by means of contact angle measurements, UV spectroscopy, and FTIR‐ATR. The modified samples showed a better wettability and higher surface energies, increasing from 26 mN/m for pure PP to 36 mN/m for the modified samples. The UV spectrum of the modified PP films showed two absorption bands that could be related to the grafted initiator. The effect of irradiation time and photoinitiator concentration was investigated. Different acrylates were grafted efficiently onto the modified polymer surfaces. FTIR‐ATR and contact angle measurements confirmed the presence of the grafted chains. The surface energy of the grafted surfaces of samples increased to 70 mN/m, depending on the type of acrylate used. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2341–2350, 2004     

Long Term Effects of Air Dielectric Barrier Discharge Treatment on the Surface Properties of Ethylene Vinyl Acetate (EVA)

This paper discusses the long term effects of dielectric barrier discharge (DBD) treatment on the surface properties of ethylene vinyl acetate (EVA) film. The EVA surface was characterised using contact angle measurements, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and 180° peel tests. EVA subjected to two different treatment times was compared to as-received and solvent cleaned film. The long term stability of the surface modification induced by the DBD treatment was studied over a period of 466 days. On initial application of DBD treatment to the EVA surface an increased wettability was observed, evident from a decreased water contact angle, improved peel strength when bonded, and an increased level of carbon–oxygen moieties measured using XPS. However, over the storage period of 466 days the material reverted to almost its original state with the contact angle being only ～3° lower than that of as-received EVA compared to a difference ～25° directly after treatment. AFM measurements showed that the treatment had a slight smoothing effect on the surface topography.