"Contact" of nanoscale stiff films

We investigated the contact behaviors of a nanoscopic stiff thin film bonded to a compliant substrate and derived an analytical solution for determining the elastic modulus of thin films. Microscopic contact deformations of the gold and polydopamine thin films (<200 nm) coated on polydimethylsiloxane elastomers were measured by indenting a soft tip and analyzed in the framework of the classical plate theory and Johnson-Kendall-Roberts (JKR) contact mechanics. The analysis of this thin film contact mechanics focused on the bending and stretching resistance of thin films and is fundamentally different from conventional indentation measurements where the focus is on the fracture and compression of the films. The analytical solution of the elastic modulus of nanoscopic thin films was validated experimentally using 50 and 100 nm gold thin films coated on polydimethylsiloxane elastomers. The technical application of this analysis was further demonstrated by measuring the elastic modulus of thin films of polydopamine, a recently discovered biomimetic universal coating material. Furthermore, the method presented here is able to quantify the contact behaviors of nanoscopic thin films, effectively providing fundamental design parameters, the elastic modulus, and the work of adhesion, crucial for transferring them effectively into practical applications.     

Impact of chain architecture (branching) on the thermal and mechanical behavior of polystyrene thin films

The modulus and glass transition temperature (T_g) of ultrathin films of polystyrene (PS) with different branching architectures are examined via surface wrinkling and the discontinuity in the thermal expansion as determined from spectroscopic ellipsometry, respectively. Branching of the PS is systematically varied using multifunctional monomers to create comb, centipede, and star architectures with similar molecular masses. The bulk‐like (thick film) T_g for these polymers is 103 ± 2 °C and independent of branching and all films thinner than 40 nm exhibit reductions in T_g. There are subtle differences between the architectures with reductions in T_g for linear (25 °C), centipede (40 °C), comb (9 °C), and 4 armed star (9 °C) PS for ≈ 5 nm films. Interestingly, the room temperature modulus of the thick films is dependent upon the chain architecture with the star and comb polymers being the most compliant (≈2 GPa) whereas the centipede PS is most rigid (≈4 GPa). The comb PS exhibits no thickness dependence in moduli, whereas all other PS architectures examined show a decrease in modulus as the film thickness is decreased below ～40 nm. We hypothesize that the chain conformation leads to the apparent susceptibility of the polymer to reductions in moduli in thin films. These results provide insight into potential origins for thickness dependent properties of polymer thin films. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Surface stress of polydimethylsiloxane networks

For thin elastic films of crosslinked polydimethylsiloxane (PDMS), the tensile modulus was found to be an increasing function of reciprocal thickness over the whole range of elongations. PDMS films between 0.052 and 0.018 mm were investigated. With decreasing film thickness, surface properties may be expected to increasingly contribute to the measured modulus. For small elongations, surface tension is expected to have no effect or to decrease the measured modulus compared with that of a bulk sample. If a surface layer with a modulus greater than that of the bulk modulus is assumed to exist, then the observed increase in modulus with decreasing film thickness can be explained. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2391–2396, 1997     

Surface dilational moduli of poly (ethylene oxide), poly (methyl methacrylate), and their blend films

Surface dilational moduli of poly (ethylene oxide) (PEO), poly (methyl methacrylate) (PMMA), and compatible PEO/PMMA blend films spread at the air-water interface were investigated as a function of surface concentration. The surface dilational modulus of an expanded PEO film increased as the surface concentration increased to 0.4mg/m(2), which corresponds to the limiting surface area of PEO. After peaking at this value, the surface dilational modulus decreased with an increase in the PEO concentration. Lissajous orbits of PEO films exhibited positive hysteresis loops for all surface concentration ranges. On the other hand, the surface dilational modulus of a condensed PMMA film steeply increased as the surface concentration increased. Lissajous orbits of PMMA films changed from positive hysteresis loops to negative loops at the surface concentration at which the surface pressure reached in the plateau region. The magnitude of the surface dilational modulus of PMMA was larger than that of PEO at a fixed surface concentration. The surface dilational moduli of the PEO/PMMA blend films increased with the total surface concentration and their magnitudes were less than those of the individual PMMA films and larger than those of the individual PEO films at fixed surface concentrations. Lissajous orbits of the PEO/PMMA blend films also changed from positive hysteresis loops to negative loops beyond the surface concentration at which the plateau surface pressure of PEO was attained.     

基底界面效应对聚苯乙烯薄膜分子运动行为的影响

本文研究了Si／Si02、Si／Si—H基底与聚苯乙烯（Ps）之间的界面相互作用对Ps薄膜的玻璃化转变及相关力学性能的影响．结果显示,无论何种基底,Ps薄膜的玻璃化转变温度（L）都随其厚度降低而降低．但相同厚度（〈110nm）下,以Si／Si-H为基底时Ps薄膜的瓦比以Si／Si02为基底的PS薄膜高．Si／Si02表面Ps薄膜疋开始下降的临界厚度为110nm,远高于以Si／Si—H为基底时的40nm．对Ps薄膜的膨胀系数和弹性模量进行研究,也得到相似的临界厚度．另外,与Si／Si02基底相比,在Si／Si-H上的Ps薄膜具有更低的膨胀系数以及较高弹性模量．可能原因是Si／Si-H与Ps具有较强的相互作用,限制了该界面分子的运动能力,导致基底／PS界面效应对薄膜分子运动的影响力增强,造成该薄膜瓦的厚度依赖性下降,并呈现出相对较硬的力学特征．     

Moisture absorption and absorption kinetics in polyelectrolyte films: influence of film thickness

Specular X-ray reflectivity (XR) and quartz crystal microbalance (QCM) measurements were used to determine the absorption of water into thin poly(4-ammonium styrenesulfonic acid) films from saturated vapor at 25 degrees C. The effect of film thickness on the absorption kinetics and overall absorption was investigated in the range of thickness from (3 to 200) nm. The equilibrium swelling of all the films irrespective of film thickness was (0.57+/-0.03) volume fraction. Although the equilibrium absorption is independent ofthickness, the absorption rate substantially decreases for film thickness < 100 nm. For the thinnest film (3 nm), there is a 5 orders of magnitude decrease in the diffusion coefficient for water.     

Wetting-dewetting transition line in thin polymer films

Thin polymeric films are increasingly being utilized in diverse technological applications, and it is crucial to have a reliable method to characterize the stability of these films against dewetting. The parameter space that influences the dewetting of thin polymer films is wide (molecular mass, temperature, film thickness, substrate interaction) and a combinatorial method of investigation is suitable. We thus construct a combinatorial library of observations for polystyrene (PS) films cast on substrates having orthogonal temperature and surface energy gradients and perform a series of measurements for a range of molecular masses (1800 g/mol < M < 35 000 g/mol) and film thicknesses h (30 nm < h < 40 nm) to explore these primary parameter axes. We were able to obtain a near-universal scaling curve describing a wetting-dewetting transition line for polystyrene films of fixed thickness by introducing reduced temperature and surface energy variables dependent on M. Our observations also indicate that the apparent polymer surface tension gamma(p) becomes appreciably modified in thin polymer films from its bulk counterpart for films thinner than about 100-200 nm, so that bulk gamma(p) measurements cannot be used to estimate the stability of ultrathin films. Both of these observations are potentially fundamental for the control of thin film stability in applications where film dewetting can compromise film function.     

Instabilities and pattern miniaturization in confined and free elastic-viscous bilayers

We present an analysis of the instabilities engendered by van der Waals forces in bilayer systems composed of a soft elastic film (<10 microm) and a thin (<100 nm) viscous liquid film. We consider two configurations of such systems: (a) Confined bilayers, where the bilayer is sandwiched between two rigid substrates, and (b) free bilayers, where the viscous film is sandwiched between a rigid substrate and the elastic film. Linear stability analysis shows that the time and length scales of the instabilities can be tuned over a very wide range by changing the film thickness and the material properties such as shear modulus, surface tension, and viscosity. In particular, very short wavelengths comparable to the film thickness can be obtained in bilayers, which is in contrast to the instability wavelengths in single viscous and elastic films. It is also shown that the instabilities at the interfaces of the free bilayers are initiated via an in-phase "bending" mode rather than out-of-phase "squeezing" mode. The amplitudes of deformations at both the elastic-air and elastic-viscous interfaces become more similar as the elastic film thickness decreases and its modulus increases. These findings may have potential applications in the self-organized patterning of soft materials.     

Fabrication of polyelectrolyte multilayer films comprising nanoblended layers

Polyelectrolyte multilayer thin films were prepared via the alternate deposition of poly(allylamine hydrochloride) (PAH) and a blend of poly(acrylic acid) (PAA) and poly(styrenesulfonate) (PSS). When the pH of the blend solution was 3.5, the presence of PAA in this solution significantly increased the total film thickness. With only 10 wt % PAA in the blend adsorption solution, a large increase in film thickness was observed (92 nm cf. 18 nm). It was also demonstrated that the total amount of PSS adsorbed was enhanced by the presence of PAA in the blend solution, showing that the blend solution composition influenced that of the multilayer films. Thin films prepared with nanoblended layers also showed improved pH stability, because they exhibited reduced film rearrangement upon exposure to acidic conditions (pH = 2.5).     

Brillouin scattering studies of polymeric nanostructures *

For a range of applications, polymers are now being patterned into nanometer‐sized features. In these applications, the robust mechanical properties of the nanostructures are critical for performance and stability. Brillouin light scattering is presented as a nondestructive, noncontact tool used to quantify the elastic constants in such nanostructures. We demonstrate this through a series of thin films and parallel ridges and spacings (gratings) with ridge widths ranging from 180 to 80 nm. For the set of films and structures presented here, the room‐temperature elastic moduli did not change with decreasing film thickness or grating ridge width, and this implied that one‐dimensional and two‐dimensional confinement‐induced changes of the mechanical properties were not significant down to feature sizes of 80 nm. Additionally, Brillouin spectra of submicrometer gratings revealed new modes not present in the spectra of thin films. The origin of these new modes remains unclear. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1106–1113, 2004     

Surface dilatational moduli of poly(vinyl acetate) (PVAc) and PVAc-poly(n-hexyl isocyanate) (PHIC) blend films at the air-water interface

Surface dilatational moduli of poly(vinyl acetate) (PVAc) film and blend films of PVAc and poly(n-hexyl isocyanate) (PHIC) were measured at the air-water interface. PVAc formed a film that was looser and also more stable against strain than the PHIC film. The apparent surface dilatational modulus and surface pressure of the blend films were superimposed on the lower concentration of PVAc, irrespective of the composition of PVAc. However, the additivity rule was not applicable to the apparent surface dilatational modulus and surface pressure. The scaling exponents of the apparent surface dilatational modulus against the added surface concentration decreased with an increase in the proportion of PVAc, suggesting that blend films gradually change from glass material to expanded films.     

PMMA/超支化聚(酯-酰胺)共混物薄膜中柱状相结构的形成

利用相差显微镜、三维形貌测量仪对聚甲基丙烯酸甲酯 超支化聚 (酯 酰胺 ) (PMMA HBP)共混物薄膜在玻璃基板作用下的相分离行为进行了研究 .结果表明 ,不同组分比的共混物薄膜会呈现不同的相形态和相分离过程 .当薄膜厚度在 5 0 0nm左右 ,HBP为低组分时 ,发现了一种特殊的分散相为圆柱状的相形态 ,并对该相形态出现的条件进行了研究 .认为基板与组分之间的相互作用和薄膜厚度决定了圆柱状结构形成 .     

Time-scale- and temperature-dependent mechanical properties of viscoelastic poly(3,4-ethylenedioxythiophene) films

The viscoelastic properties of thin films of poly(3,4-ethylenedioxythiophene) (PEDOT) have been studied using the method of acoustic impedance. The films were deposited on the Au electrodes of 10 MHz AT-cut quartz thickness shear mode resonators and exposed to acetonitrile solutions of 0.1 M TEABF4 and LiClO4. For p-doped films, admittance spectra as a function of potential (E), temperature (T), and time scale (frequency, via harmonics, in the range 10-110 MHz) were acquired. Shear modulus components extracted from these responses surprisingly showed virtually no variation with E (and thus film solvation) or with T, but the variation with frequency was dramatic. This qualitative behavior and the numerical values of the shear moduli contrast strongly with recently reported data for the related poly(3-hexylthiophene) system, which shares the same conducting spine but differs substantially in the substitution pattern. Accordingly, the models and interpretation for PEDOT are quite different: film dynamics are determined by free-volume effects, and side-chain motion is not a significant factor. Qualitatively similar potential and time-scale effects were seen for n-doped PEDOT, but the scope of the measurements was limited by film stability.     

Structural and nanomechanical properties of sol–gel prepared (K,Na)NbO3 thin films

Perovskite (K, Na)NbO₃ (KNN) thin films (~100 nm) were prepared by sol–gel/spin coating process on Pt/SiO₂/Si substrates and annealed at 650 °C. The structural properties of KNN films were confirmed by X‐ray diffraction analysis (XRD), Raman spectroscopy and scanning electron, transmission electron and atomic force microscopy (SEM, TEM and AFM) analysis. Pure perovskite phase of K_0.65Na_0.35NbO₃ in nonstoichiometric composition with monoclinic symmetry in film was revealed. Uniform homogeneous microstructure of KNN film with the roughness (~6.9 nm) contained spherical particles (~50–90 nm). Nanoindentation technique was used to characterize the mechanical properties of KNN films. Elastic modulus and hardness of Pt, SiO₂ and KNN thin films were calculated from their composite values of KNN/Pt/SiO₂/Si film/substrate system. The modulus and hardness of KNN film (71 and 4.5 GPa) were lower in comparison with SiO₂ (100 and 7.5 GPa). Pt film (~30 nm) did not influence the composite modulus, but had effect on hardness of KNN film. Copyright © 2015 John Wiley & Sons, Ltd.     

Mechanical behavior and wrinkling patterns of phase-separated binary polymer blend film

The wrinkling of phase-separated binary polymer blend film was studied through combining the Monte Carlo (MC) simulation for morphologies with the lattice spring model (LSM) for mechanical properties. The information of morphology and structure obtained by use of MC simulation is input to the LSM composed of a three-dimensional network of springs, which allows us to determine the wrinkling and the mechanical properties of polymer blend film, such as strain, stress, and Young’s modulus. The simulated results show that the wrinkling of phase-separated binary polymer blend film is related not only to the structure of morphology, but also to the disparity in elastic moduli between polymers of blend. Our simulation results provide fundamental insight into the relationship between morphology, wrinkling, and mechanical properties for phase-separated polymer blend films and can yield guidelines for formulating blends with the desired mechanical behavior. The wrinkling results also reveal that the stretching of the phase-separated film can form the micro-template, which has a wide application prospect.     

Local dynamic mechanical properties in model free-standing polymer thin films

High-frequency sinusoidal oscillations of a coarse-grained polymer model are used to calculate the local dynamic mechanical properties (DMPs) of free-standing polymer thin films. The storage modulus G(') and loss modulus G(") are examined as a function of position normal to the free surfaces. It is found that mechanically soft layers arise near the free surfaces of glassy thin films, and that their thickness becomes comparable to the entire film thickness as the temperature approaches the glass transition T(g). As a result, the overall stiffness of glassy thin films decreases with film thickness. It is also shown that two regions coexist in thin films just at the bulk T(g); a melt-like region (G(')G(")) in the middle of the film. Our findings on the existence of a heterogeneous distribution of DMPs in free-standing polymer thin films provide insights into recent experimental measurements of the mechanical properties of glassy polymer thin films.     

Effects of polymerization media on the nanoscale conductivity and current-voltage characteristics of chemically synthesized polyaniline films

A current sensing atomic force microscope (CS-AFM) was used to probe the conducting homogeneity and band structures of fully doped polyaniline (PANI) films prepared from in situ chemical polymerization/deposition of aniline on indium tin oxide in various inorganic acids. The charge transport properties of PANI films depend on the film thickness as well as polymerization medium. Fluctuations in conductivity are observed on all acid-doped PANI films and the conducting homogeneity was dependent on the film thickness: the conductivity of thick film is more uniform. The current-voltage (I-V) characteristics of all thick (>200 nm) films displayed a metal-like behavior and conductivity as high as 40 S/cm was detected in high conducting regions of film thicker than 400 nm. Whereas thin (<120 nm) films revealed insulating, semiconducting, and semimetal conducting, wide distribution in conductivity and interband distances (estimated from the I-V ordI/dV-V curves) was found. The interband distances is 0-1.35, 0-1.0, and 0-0.78 eV for thin PANI film prepared from HCl(aq), HClO(4)(aq), and H2SO4(aq), respectively. PANI film (260 nm) prepared from H2SO4(aq) revealed fiberlike morphology, and compared to PANI films prepared from HCl(aq) and HClO4(aq) with similar thickness, it has higher average nanoscale conductivity but lower bulk conductivity. This result could be direct evidence which supports that the bulk conductivity of PANI depended on the carriers hopping between the conducting domains.     

Thermal change of organic light-emitting ALQ3 thin films

A series of Alq3 thin films with the thicknesses of 50, 100, and 200 nm was deposited on Si substrates at room temperature using the thermal evaporation method. The thermal crystallization process of Alq3 thin films, especially 50 nm thick films, was successfully examined using high-temperature X-ray diffraction (HT-XRD) with the in-plane scan mode. Film thickness, density, and changes in surface roughness while heating were determined using X-ray reflectometry (XRR). The decreased density and increased surface roughness, which were accompanied by sublimation, indicate the instability of the Alq3 film. Thus, thermal instability is a major factor for device failure.     

Surface morphology of poly(cyano-p-xylylene) thin films

The surface morphology of poly(cyano-p-xylylene) thin films of different thicknesses (25–1500 nm or more than 5 μm) that were synthesized by vapor-deposition polymerization on the substrate surface in the temperature range from −22 to +35°C has been studied by atomic force microscopy. The surface topography is quantified through analysis of the height-height correlation function. The surface of all films is characterized by a similar granular morphology with a transverse size of granules of 50–500 nm. The surface morphology changes with the polymerization temperature (the substrate temperature) and the film thickness. The effect of film annealing on its surface morphology is considered. It has been established that annealing at 200°C leads to a change in the surface morphology of the films. Original Russian Text ? A.I. Buzin, D.S. Bartolome, K.A. Mailyan, A.V. Pebalk, S.N. Chvalun, 2006, published in Vysokomolekulyamye Soedineniya, Ser. A, 2006, Vol. 48, No. 9, pp. 1640–1646. This work was supported by the Russian Foundation for Basic Research (project nos. 03-03-32665 and 03-03-32634) and the Russian Science Support Foundation.     

Density and Young’s modulus of thin TiO2 films

Densities, ρ, of thin TiO₂ layers, produced by reactive evaporation (RE) and ion plating (IP) have been analyzed by means of grazing incidence X-ray reflectometry (GIXR). Depending on the deposition conditions, the layers are amorphous or polycrystalline, with densities between 2.9 g/cm³ and 3.9 g/cm³. Young’s moduli, E, have been analyzed for 280 nm and 500 nm thick layers by means of surface acoustic wave spectroscopy (SAWS) and vary between 65 GPa for RE films and 147 GPa for IP layers. The values are independent of film thickness, but correlate with the density. A phase transition of the TiO₂ films from the amorphous state to anatase occurs at temperatures above 210°C and increases the Young’s modulus significantly, whereas the density remains unchanged.