Wetting of Alkanes on Water from a Cahn-Type Theory: Effects of Long-Range Forces

We apply the phenomenological wetting theory of Cahn to fluids with van der Waals forces, and in particular to the wetting of pentane on water. Taking into account explicitly the long-range substrate–adsorbate interaction allows us to reproduce the experimentally observed critical wetting transition, which arises from the vanishing of the Hamaker constant at T53°C. This transition is preceded by a first-order transition between a thin and a thick film at a (much) lower temperature. If long-range forces are neglected, this thin–thick transition is the only wetting transition and critical wetting is missed. Our study focuses on the development of useful theoretical tools, such as phase portraits and interface potentials adapted to systems with van der Waals forces.     

Stability of thermoresponsive methylcellulose thin film: x‐ray reflectivity study

Methylcellulose (MC), a thermoreversible polymer, was fabricated as thin films into silicon substrates and characterized by x‐ray reflectivity (XRR) measurements for its stability with time and heating. XRR data from the as‐is thin films showed good agreement with the single‐layer model on top of a substrate from Parratt's formalism. Data fitting showed that the density of the thin films is slightly higher than the reported value by manufacturers. Interface roughness values indicate good wetting of the polymer onto the substrate. Heating the thin films at the phase transition temperatures and quenching them to room temperature showed no significant changes in the thin film parameters before and after heating. This showed the thermal stability and/or thermoreversibility of the film. Diffuse scattering measurements also showed no significant changes in the lateral structure of the film with heating and quenching. XRR measurements done on fabricated thin films stored for a month showed a slight increase in the film thickness which could be due to the hygroscopic nature of the polymer. Vacuum heating of the stored thin films at 100 °C for 1 h slightly decreased the thickness, but it has been found that other parameters such as density and surface/interface roughness show good thermal stability. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface glass transition in bimodal polystyrene mixtures

Using the cluster-embedding method of V. Zaporojchenko et al. (Macromolecules 34, 1125 (2000)), we measured the glass transition temperature T _g at the polystyrene/vacuum interface of bimodal mixtures of monodisperse polystyrenes of 3.5k and 1000k. Embedding of ≈ 1 nm Au clusters was monitored in situ by X-ray photoelectron spectroscopy (XPS). The clusters were formed by evaporation of Au onto the polymer surface. Only one glass transition was observed in the mixtures. The surface glass transition temperatures are correlated to but are below the bulk values of the mixtures and obey the Gordon-Taylor equation. The results suggest that the earlier reported molecular-weight dependence of the surface glass transition is not due to segregation of short chains to the surface.     

Reversible sphere-to-lamellar wetting transition at the interface of a diblock copolymer system

We use ellipsometry to investigate a transition in the morphology of a sphere-forming diblock copolymer thin-film system. At an interface the diblock morphology may differ from the bulk when the interfacial tension favours wetting of the minority domain, thereby inducing a sphere-to-lamella transition. In a small, favourable window in energetics, one may observe this transition simply by adjusting the temperature. Ellipsometry is ideally suited to the study of the transition because the additional interface created by the wetting layer affects the polarisation of light reflected from the sample. Here we study thin films of poly(butadiene-ethylene oxide) (PB-PEO), which order to form PEO minority spheres in a PB matrix. As temperature is varied, the reversible transition from a partially wetting layer of PEO spheres to a full wetting layer at the substrate is investigated.     

On the Localization Transition of Random Copolymers Near Selective Interfaces

In this note we consider the (de)localization transition for random directed (1+1)–dimensional copolymers in the proximity of an interface separating selective solvents. We derive a rigorous lower bound on the free energy. This yields a substantial improvement on the bounds from below on the critical line that were known so far. Our result implies that the critical curve does not lie below the critical curve set forth by Monthus (Eur. Phys. J. B 13, 111–130, 2000) on the base of a renormalization group analysis. We discuss this result in the light of the (rigorous and non rigorous) approaches present in the literature and, by making an analogy with a particular asymptotics of a disordered wetting model, we propose a simplified framework in which the question of identifying the critical curve, as well as understanding the nature of the transition, may be approached.     

Preparation of P(AN-MMA) gel electrolyte for Li-ion batteries

Phase inversion technique was used to prepare poly(acrylonitrile-methyl methacrylate) [P(AN-MMA)]-based microporous gel electrolyte with addition of SiO₂ via in-situ composition for Li-ion batteries. The P(AN-MMA) was synthesized by emulsion polymerization and was dissolved into N,N-dimethylformamide (DMF) to form a uniform solution, while tetraethyl orthosilicate (TEOS) was added into the solution and was hydrolyzed by catalysis of alkali ammonia solution to form SiO₂. Then the solution was cast onto a glass plate using a doctor blade and exposed to humidified atmosphere produced by ultrasonic humidifier, followed by washing, rinsing, and drying, successively. The gel electrolyte was obtained by putting the P(AN-MMA) microporous membrane in a liquid electrolyte. The gelled microporous membrane sucked with 755 wt% of liquid electrolyte vs the dried membrane. It had a porosity of 70%, about 1∼5 μm of pores, and presented an ionic conductivity of 0.94 × 10⁻³ S/cm at room temperature. Electrochemical stability window of the porous gel polymer electrolyte was determined by running a linear sweep voltammetry. The decomposition voltage of the polymer electrolyte exceeds 4.5 V vs Li. The coin test cell with the microporous gel electrolyte showed a good cycling performance. The discharge capacity retention was above 87% at 0.1 C for 45 cycles.     

The mean-field adsorption on a sinusoidally corrugated substrate revised

The fluid system at the bulk liquid–gas coexistence in a presence of a sinusoidally corrugated substrate exhibits not only the wetting transition, but additionally a first-order, thin–thick transition. The mean-field analysis of this transition based on a simple effective Hamiltonian is valid only in long wavelength limit. In this case the filling transition occurs so close to the wetting temperature, that the behavior of the interface is dominated by fluctuations, therefore the mean-field approach breaks down. We analyze the filling transition with the help of Hamiltonian evaluated from Landau theory. The applicability of our Hamiltonian is not restricted only to the vicinity of the wetting point. We obtain the phase diagram valid beyond the temperature range corresponding to the strong fluctuations regime. It displays more complex dependence on different length scales of the system and includes the old one as a particular case.     

Effects of counterion fluctuations in a polyelectrolyte brush

We investigate the effect of counterion fluctuations in a single polyelectrolyte brush in the absence of added salt by systematically expanding the counterion free energy about Poisson-Boltzmann mean-field theory. We find that for strongly charged brushes, there is a collapse regime in which the brush height decreases with increasing charge on the polyelectrolyte chains. The transition to this collapsed regime is similar to the liquid-gas transition, which has a first-order line terminating at a critical point. We find that, for monovalent counterions, the transition is discontinuous in theta solvent, while for multivalent counterions, the transition is generally continuous. For collapsed brushes, the brush height is not independent of grafting density as it is for osmotic brushes, but scales linear with it.Received: 26 November 2003, Published online: 11 May 2004PACS: 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 61.20.Qg Structure of associated liquids: electrolytes, molten salts, etc.     

Investigation of failure mechanisms in polymer composites by simultaneous measurement of ultra-small-angle scattering and acoustic emission during the deformation. II. Evaluation of the interface strength

The failure mechanism in polystyrene (PS) filled with glass beads was investigated with the methods of ultra-small-angle X-ray scattering (USAXS) and acoustic emission (AE) analysis simultaneously. The interface in the composites was tailored using carboxyl-terminated PS (PS-COOH). Monolayers of PSCOOH were prepared on the glass surface in a wide range of grafting densities, from single chains to a polymer brush. The layers were characterized by ellipsometry, scanning probe microscopy, and the wetting method. The morphology of the layers is in good agreement with the theoretical predictions. USAXS experiments identify submicrovoids (SMVs) at the interface in the composites even before a tensile deformation. The uniaxial loading of the specimens is followed by a debonding (dewetting) process that is identified by simultaneous increase of integral scattering intensity and AE signals. The stress of debonding is proportional to the grafting density of PS-COOH. The amplitude of the AE     

Self-healing slip pulses along a gel/glass interface

We present experimental evidence of self-healing shear cracks at a gel/glass interface. This system exhibits two dynamical regimes depending on the driving velocity: steady sliding at high velocity (>V(c) approximately 100--125 microm/s), characterized by a shear-thinning rheology, and periodic stick-slip dynamics at low velocity. In this last regime, slip occurs by propagation of pulses that restick via a "healing instability" occurring when the local sliding velocity reaches the macroscopic transition velocity V(c). At driving velocities close below V(c), the system exhibits complex spatiotemporal behavior.     

Superconductor–insulator transition induced by over-deposited Ge in the insulating ultrathin a-Nb film

We have observed the superconductor–insulator (S–I) transition induced by over-deposited Ge in the insulating ultrathin amorphous Nb (a-Nb) film. The experiments are performed by depositing Ge onto the insulating a-Nb film with a thickness of 1.04 nm. For , where d_Ge was a thickness for Ge film, the reduction of electrical sheet resistances was seen at low temperature region, suggesting superconductor. The normal sheet resistance at 8 K decreased monotonically with increasing d_Ge. The simplest explanation of this S–I transition is that the electron localization effect is weakened due to the addition of Ge film.     

Critical Properties and Finite-Size Estimates for the Depinning Transition of Directed Random Polymers

We consider models of directed random polymers interacting with a defect line, which are known to undergo a pinning/depinning (or localization/delocalization) phase transition. We are interested in critical properties and we prove, in particular, finite-size upper bounds on the order parameter (the contact fraction) in a window around the critical point, shrinking with the system size. Moreover, we derive a new inequality relating the free energy F and an annealed exponent μ which describes extreme fluctuations of the polymer in the localized region. For the particular case of a (1+1)-dimensional interface wetting model, we show that this implies an inequality between the critical exponents which govern the divergence of the disorder-averaged correlation length and of the typical one. Our results are based on the recently proven smoothness property of the depinning transition in presence of quenched disorder and on concentration of measure ideas.     

Ionic noise measurement in polymer electrolytes

Electrical noise associated with ion transport (termed as “ionic noise”) has been measured at different temperatures, using a lock-in amplifier and dynamic signal analyzer for a polymer electrolyte PEO:NH₄I and its CdS dispersed composite. The ionic noise suddenly increases as the polymer passes through its phase transition at T _g and T _m. The T _g-peak in the noise measurement appears more clearly than what it does in DTA/DSC or conductivity measurements. Therefore, we suggest the noise technique as a good probe for studying phase transitions in ion conducting solid electrolytes. Further, the present noise measurements also confirm the known results of DTA/DSC studies that both T _g and T _m of polymer electrolytes shift on the formation of composites.     

Corner wetting in a far-from-equilibrium magnetic growth model

The irreversible growth of magnetic films is studied in three-dimensional confined geometries of size L×L×M, where M≫L is the growing direction. Competing surface magnetic fields, applied to opposite corners of the growing system, lead to the observation of a localization-delocalization (weakly rounded) transition of the interface between domains of up and down spins on the planes transverse to the growing direction. This effective transition is the precursor of a true far-from-equilibrium corner wetting transition that takes place in the thermodynamic limit. The phenomenon is characterized quantitatively by drawing a magnetic field-temperature phase diagram, firstly for a confined sample of finite size, and then by extrapolating results, obtained with samples of different size, to the thermodynamic limit. The results of this work are a nonequilibrium realization of analogous phenomena recently investigated in equilibrium systems, such as corner wetting transitions in the Ising model.     

Adsorption of a spherical nanoparticle in polymer brushes: Brownian dynamics investigation

We use Brownian dynamics simulations to study the adsorption behavior of a nanosized particle in polymer brushes. The adsorption process, the dynamic behavior of the nanoparticle in the brush, the penetration depth, the diffusion coefficient of the nanoparticle in different depths of the brush, and the forces exerted on the nanoparticle by the surrounding brush are all investigated for different grafting densities.     

嵌段共聚物受限于软孔内的自组装

利用自洽场方法研究两嵌段共聚物受限于接枝均聚物链(聚合物刷)圆孔中的自组装相形貌.研究表明,当圆孔内径一定时,嵌段比f和聚合物刷C的体积分数φ_C是调控嵌段共聚物相形貌的主要因素,聚合物刷的弹性熵也起着重要作用.当f＝0.7时,在聚合物刷的浸润下,贴近刷表面处AB嵌段共聚物构成环层状结构,随着φ_C的减小这种结构会周而复始地出现.当f处于0. 关键词： 嵌段共聚物 圆孔受限 聚合物刷 自洽场     

熵驱动下柱状高分子刷螺旋结构的研究

自然界中广泛存在螺旋结构,在特定情形下熵能驱动高分子链形成螺旋结构.本文采用分子动力学方法研究柱状高分子刷吸附在无限长圆柱表面时的构象行为.发现其构象与嫁接支链条数、柱状高分子刷与圆柱表面之间的吸附能密切相关.在较弱的吸附能下,具有较多支链条数的柱状高分子刷能形成完整的螺旋结构,其本质就是熵驱动下形成的螺旋结构.该研究有助于加深对生物大分子螺旋结构的理解.     

Replication technology for optical microsystems

Replication technology is playing an increasingly important role in the production of optical microsystems and micro-optical elements. Hot embossing, injection moulding and UV-embossing all can produce high-quality optical elements in very cost-effective processes. New sol–gel materials allow the combination of replication with lithography to leave selected areas material-free for sawing and bonding. The development of wafer-scale replication technology using UV-curable sol–gel and polymer materials enables refractive and diffractive micro-optical elements as well as micro-mechanical alignment features to be replicated directly onto glass substrates or onto semiconductor device wafers. Grating nanostructures with linewidths less than 100 nm have been replicated into polymer and sol–gel materials for the cost-effective fabrication of large area subwavelength structures for applications such as polarisers and buried grating security features.     

Copolymer at a selective interface and two-dimensional wetting: a grand canonical approach

We consider two different problems involving the localization of a single polymer chain: (i) a periodic AB copolymer at a selective fluid-fluid interface, with the upper (resp. lower) fluid attracting A (resp. B) monomers (ii) a homopolymer chain attracted to a hard wall (wetting). Self avoidance is neglected in both models, which enables us to study their localization transition in a grand canonical approach. We recover the results obtained in previous studies via transfer matrix methods. Moreover, we calculate in this way the loop length distribution functions in the localized phase. Some finite size effects are also determined and tested numerically. Received 13 April 2000     

Cross-Over Between First-Order and Critical Wetting at the Liquid-Vapour Interface of n-Alkane/Methanol Mixtures: Tricritical Wetting and Critical Prewetting

A simple mean-field theory is presented which describes the basic observations of recent experiments revealing rich wetting behaviour of n-alkane/methanol mixtures at the liquid-vapour interface. The theory, qualitative and in part heuristic, is based on a microscopic lattice-gas model from which a Cahn–Landau approach is distilled. Besides the physics associated with the short-range components of the intermolecular interactions, effects of the long-range tails of the net van der Waals forces between interfaces are also taken into account. Further, gravitational thinning of the wetting phase is incorporated. The calculation of the spreading coefficient S is extended to the experimentally relevant situation in which the bulk adsorbate is slightly away from two-phase coexistence due to gravity. Analysis of this novel approximation to S for systems with short-range forces leads to the conclusion that the surface specific heat exponents _s =1,1/2, and 0, for first-order wetting, tricritical wetting and critical wetting, respectively, are robust with respect to (weak) gravitational thinning, consistently with experiment. For three different systems the adsorption is calculated as a function of temperature and compared with the experimentally measured ellipticity. Including weak long-range forces which favour wetting in the theory does not visibly alter the critical wetting transition for the nonane/methanol mixture, in contrast with the generic expectation of first-order wetting for such systems, but in good agreement with experiment. For decane/methanol weak long-range forces bring the transition very close to the prewetting critical point, leading to an adsorption behaviour closely reminiscent of short-range tricritical wetting, observed experimentally for alkane chain length between 9.6 and 10. Finally, for undecane/methanol the transition is clearly of first order. First-order wetting is also seen in the experiment.