Hydrophobic recovery of UV/ozone treated poly(dimethylsiloxane): adhesion studies by contact mechanics and mechanism of surface modification

Silicone elastomers (Sylgard 184 and 170), based on poly(dimethylsiloxane) (PDMS), were surface treated by a combined exposure to UV and ozone. The effects of the treatments were analyzed as a function of time elapsed after stopping the treatments using different standard surface characterization techniques, such as water contact angle measurements, XPS and atomic force microscopy (AFM). However, the primary focus of this study was to apply the Johnson–Kendall–Roberts (JKR) contact mechanics approach to investigate PDMS samples prior to and following UV/ozone surface treatment. A gradual formation of a hydrophilic, silica-like surface layer with increasing modulus was observed with increasing UV/ozone exposure. A subsequent hydrophobic recovery after UV/ozone exposure was observed, as indicated by increasing contact angles. This supports the hypothesis that the hydrophobic recovery is mainly caused by the gradual coverage of a permanent silica-like structure with free siloxanes and/or reorientation of polar groups. PDMS containing a homogenously dispersed filler (Sylgard 184), exhibited a decreasing surface roughness (by AFM) when the oxidized surface region “collapsed” into a smooth SiO_x layer (final surface roughness <2 nm). PDMS containing heterogeneously distributed, aggregated filler particles (Sylgard 170), exhibited an increasing surface roughness with treatment dose, which was attributed to the “collapse” of the oxidized surface region thus exposing the contours of the underlying filler aggregates (final surface roughness ∼140 nm). A dedicated device was designed and built to study the contact mechanics behavior of PDMS prior to, and following surface treatment. The value of the combined elastic modulus obtained for PDMS lens and semi-infinite flat surface system showed an increase in full agreement with the formation of a silica-like layer exhibiting a high elastic modulus (compared with untreated PDMS). The work of adhesion observed in JKR experiments exhibited an increasing trend as a function of treatment done in agreement with contact angle data. JKR experiments showed hydrophobic recovery behavior as anticipated from contact angle measurements. Single pull-off force measurements by JKR and numerical analysis of full-approach JKR curves were in quantitative agreement regarding practical work of adhesion values.     

接触角滞后现象的理论分析

在本文的研究中，考虑表面粗糙的影响，通过引入附加“磨擦力”的概念，分别用力学方法和热力学方法导出固体表面上液滴平衡时接触角应满足的条件；并得到了液滴系统自由能－固液接触面积曲线；分析了前进接触角和后退接触角的物理意义；由此给出了接触角滞后现象的一种合理解释．这对于进一步认识接触角的滞后现象，无疑是有积极意义的．     

Interfacial debonding behavior of a rigid particle-filled polymer composite

Glass beads, non-modified and modified with coupling agent, were filled separately into high density polyethylene to obtain composite materials with different interfacial adhesion strengths. In situtensile tests reveal the damage mechanisms, which are mainly induced by the interfacial debonding. The interfacial debonding process is observed and studied. The debonding stress is found to be linearly related to the opening angle formed at two poles of the particles. Initial and final opening angles, in addition to the corresponding debonding stresses, are measured. The interfacial fracture energy obtained by using the Griffith fracture theory is found to be 0.028 J m^-2 and 0.058 J m^-2 for mechanical anchorage and physical entanglement across the interface, respectively. The stronger the interfacial adhesion, the smaller is the maximum opening angle and greater the debonding stress.     

Mechanical Characterization Measurement of Polymer Material under Stress by Birefringence Microscope

Some polymer materials under tensile stress provide us interesting information. Internal structure changes of polymer materials must be observed by external stress. Birefringence shows the orientation characteristic in molecular orientation. A microscopic measurement system for birefringence distribution is proposed to analyze the relation between applied stress and birefringence distribution of an internal structure. Birefringence distributions of gelatin such as phase difference and azimuthal angle are shown in cases of loading and unloading stress as a demonstration. The phase difference increased nonlinearly with each process, and change of birefringence in the elastic domain was different from that in the plastic domain.     

Numerical simulation of micro-scratch tests for coating/substrate composites

In this paper the micro-scratch test is simulated by ANSYS finite element code for thin hard coating on substrate composite material system. Coulomb friction between indenter and material surface is considered. The material elastic-plastic properties are taken into account. Contact elements are used to simulate the frictional contact between indenter and material surfaces, as well as the frictional contact after the detachment of coating/substrate interfaces has taken place. In the case of coating/substrate interfaces being perfectly bonded, the distributions of interfacial normal stress and shear stress are obtained for the material system subjected to normal and tangential loading. In the case of considering the detachment of interfaces, the length of interfacial detachment and the redistribution of stresses because of interfacial detachments are obtained. The influences of different frictional coefficients and different indenter moving distances on the distributions of stresses and displacements are studied. In the simulation, the interfacial adhesion shear strength is considered as a main adhesion parameter of coating/substrate interfaces. The critical normal loading from scratch tests are directly related to interfacial adhesion shear strengths. Using the critical normal loading known from experiments, the interfacial adhesion shear strength is obtained from the calculation. When the interfacial adhesion shear strength is known, the critical normal loading is obtained for different coating thicknesses. The numerical results are compared with the experimental values for composite materials of thin TiN coating on stainless steel substrate.     

Johnson-Kendall-Roberts theory applied to living cells

Johnson-Kendall-Roberts (JKR) theory is an accurate model for strong adhesion energies of soft slightly deformable material. Little is known about the validity of this theory on complex systems such as living cells. We have addressed this problem using a depletion controlled cell adhesion and measured the force necessary to separate the cells with a micropipette technique. We show that the cytoskeleton can provide the cells with a 3D structure that is sufficiently elastic and has a sufficiently low deformability for JKR theory to be valid. When the cytoskeleton is disrupted, JKR theory is no longer applicable.     

有限杆中不可逆相边界的传播规律及其应用

 采用一种简单混合物相变本构模型,对不可逆相变材料有限杆中的宏观相边界传播规律进行了研究。结果表明,界面和加卸载条件对杆中相边界的传播有重要影响,不同界面条件下杆中新相体积含量的最终分布呈现出不同的形态。还解释了相变Taylor杆碰撞实验中发现的新现象,提出了利用冲击方法制备对称型梯度材料的可能性。     

高压、高应变率与低压、高应变率实验的本构关联性

 指出Johnson-Cook（J-C）、Zerilli-Armstrong（Z-A）、Bodner-Parton（B-P）本构方程在一定条件下的适用性,表明对于低压、高应变率实验,单一曲线假定似乎可以采用。通过等效应力、等效应变,可以将不同应力状态下的流动应力函数采用统一的方程描述。然而,这些本构方程的确立,并不包括平面冲击波实验。对适合于平面冲击波实验的Steinberg-Cochran-Guinan（SCG）本构方程,讨论了其方程中所包含的高压与高应变率耦合效应。指出,以剪切模量度量的流动应力具有应变率相关性。基于温度效应的新发现以及直接测量平面冲击波流动应力的新进展,分别用J-C本构及SCG本构方程估算了钨材料在高压、高应变率加载下的流动应力。结果表明,采用J-C本构估算的流动应力仅在压力为10 GPa以下才能与实验数据相近,当压力高于10 GPa时,流动应力只能采用SCG本构估算。也指出了高压、高应变率本构方程与低压、高应变率本构方程所对应的不同物理背景。     

盐水中强载重超疏水钢网小船的制备.

通过简单的涂覆方法,结合宏观粗糙的表面和处理的低表面能材料制备超疏水钢薄片．海水的接触角高达130.16o,新的卡西-巴克斯特方程从理论上预测了这种新型材料的接触角,得到的预测 结果与实验吻合．表征了超疏水钢网小船的装载能力．最大载重约为17.50 g,这种微型钢网小船经过含量为2%的三甲氧基硅烷溶液处理．小船优异的负载能力可能归因于钢丝网表面的空气膜的作用．     

Experimental investigations of the normal loading of elastic spherical and conical indenters on to elastic flats

Results obtained from a series of experimental investigations are described in which an elastic polyisoprene hemisphere and elastic rubber cones of included angles 60°, 90°, 120° and 150° were loaded normally on to smooth blocks of soda–lime glass and polydimethylsiloxane (PDMS) containing 10% and 20% by volume of the curing agent. The load versus displacement data were continuously recorded with an instrumented indentation machine. It is shown that, whereas the loading behaviour of the hemisphere on to the blocks of the soda–lime glass and PDMS closely follows the theory of Hertz (see equation (1)), the load versus displacement behaviour of the rubber cones of included angles 60°, 90° and 120° could not be fitted by the Sneddon equation (see equation (5)) for rigid conical indenters loading on to an elastic half-space or by the modified Sneddon equation (see equation (6)) employing the combined moduli of the indenter and the half-space. The discrepancy between the predictions of the modified Sneddon equation and the experimental measurements is very significant, thus confirming our recent concern about the validity of using the modified Sneddon equation for analysing the experimental data obtained from nanoindentation experiments. Estimates of the errors caused by the use of the modified Sneddon equation have been made to further illustrate our contention. On the other hand, the behaviour of the 150° included angle cone loading on to the blocks of rubber, PDMS (1?:?10) and PDMS (1?:?20), has been shown to be particularly striking, as the rubber cone behaved as if it were rigid; moreover, the experimental data are well fitted by the Sneddon equation corresponding to a 150° rigid cone loading on to an elastic half-space. Finally, it has been proposed that, in order to determine the elastic modulus of a very stiff solid (i.e. Young's modulus close to that of the indenter) correctly using the technique of instrumented indentation, including nanoindentation, the included angle of the indenter, made of diamond, should be 150° and the measured load versus displacement data should be analysed using the Sneddon equation corresponding to a rigid cone of an included angle of 150°.     

Preliminary study on different technological tools and polymeric materials towards superhydrophobic surfaces for automotive applications

Nature-inspired fabrication of micro-structured superhydrophobic plastic film was aimed in this work in order to achieve smart materials with self-cleaning properties. Replicas of silicon masters were fabricated from different mixtures of base elements and by different processes. Corresponding microstructures were investigated by contact angle measurements, scanning electron microscopy and spectrophotometric analysis. Independently of the technology employed, the obtained films exhibited high contact angle value (larger than 150°), but while the acrylic polymers presented strong demoulding drawbacks, the polydimethylsiloxane (PDMS) films had good properties in terms of both contact angle and optical transparency. The results showed that most of the patterns realized by replica moulding and hot-embossing (on PDMS and polypropylene (PP), respectively) produced superhydrophobic self-cleaning surfaces.     

Influence of Mass Ratio of Polyols on Properties of Polycaprolactone-Polyethylene Glycol/Methylene Diphenyl Diisocyanate/Diethylene Glycol Hydrogels

Polyurethane (PU) hydrogels with good hydrophilicity and biocompatibility have been applied as biomedical materials. A series of polyurethane prepolymers based on methylene diphenyl diisocyanate (MDI), polycaprolactone (PCL) and polyethylene glycol (PEG), using diethylene glycol (DEG) as the chain-extender, were synthesized; then the polyurethane hydrogels were obtained from the prepolymers using benzoyl peroxide (BPO) as a cross-linker by free radical polymerization. The influences of the ratio of polyols (PCL and PEG) on the contact angle, swelling ratio and morphology of the polyurethane hydrogel were investigated. The loading capacity and release behavior of chloramphenicol from the PCL-PEG/MDI/DEG hydrogels with different compositions were also studied. The contact angle and swelling degree results showed that the PCL-PEG/MDI/DEG hydrogel with PCL/PEG mass ratio of 3:1 had higher hydrophilicity than that with PCL/PEG mass ratios of 1:1 and 1:3. All PCL-PEG/MDI/DEG hydrogels showed three dimensional porous structures; however, the pore size increased with increasing PEG content. The chloramphenicol release kinetics from PCL-PEG/MDI/DEG hydrogels indicated Fickian diffusion, and the drug release rate increased with increasing PEG content in the PU hydrogels.     

Dielectric relaxation in dipolar solid rotator phases

Dielectric relaxation in orientationally disordered dipolar solids often exhibits exotic features, such as a strong Cole-Cole relaxation for simple molecular solids. However, there does not seem to exist a detailed molecular theory of such phenomena. In this article, a molecular hydrodynamic theory of dielectric relaxation in solid rotator phases, such as plastic crystals, is presented.     

Adhesive contact of rough surfaces: Comparison between numerical calculations and analytical theories

The authors have employed a numerical procedure to analyse the adhesive contact between a soft elastic layer and a rough rigid substrate. The solution to the problem, which belongs to the class of the free boundary problems, is obtained by calculating Green’s function which links the pressure distribution to the normal displacements at the interface. The problem is then formulated in the form of a Fredholm integral equation of the first kind with a logarithmic kernel. The boundaries of the contact area are calculated by requiring the energy of the system to be stationary. This methodology has been employed to study the adhesive contact between an elastic semi-infinite solid and a randomly rough rigid profile with a self-affine fractal geometry. We show that, even in the presence of adhesion, the true contact area still linearly depends on the applied load. The numerical results are then critically compared with the predictions of an extended version of Persson’s contact mechanics theory, which is able to handle anisotropic surfaces, as 1D interfaces. It is shown that, for any given load, Persson’s theory underestimates the contact area by about 50% in comparison with our numerical calculations. We find that this discrepancy is larger than for 2D rough surfaces in the case of adhesionless contact. We argue that this increased difference might be explained, at least partially, by considering that Persson’s theory is a mean-field theory in spirit, so it should work better for 2D rough surfaces rather than for 1D rough surfaces. We also observe that the predicted value of separation is in agreement with our numerical results as well as the exponents of the power spectral density of the contact pressure distribution and of the elastic displacement of the solid. Therefore, we conclude that Persson’s theory captures almost exactly the main qualitative behaviour of the rough contact phenomena.     

Simulation of triaxial response of granular materials by modified DEM

A modified discrete element method(DEM)with rolling effect taken into consideration is developed to examine macroscopic behavior of granular materials in this study.Dimensional analysis is firstly performed to establish the relationship between macroscopic mechanical behavior,mesoscale contact parameters at particle level and external loading rate.It is found that only four dimensionless parameters may govern the macroscopic mechanical behavior in bulk.The numerical triaxial apparatus was used to study their influence on the mechanical behavior of granular materials.The parametric study indicates that Poisson’s ratio only varies with stiffness ratio,while Young’s modulus is proportional to contact modulus and grows with stiffness ratio,both of which agree with the micromechanical model.The peak friction angle is dependent on both inter-particle friction angle and rolling resistance.The dilatancy angle relies on inter-particle friction angle if rolling stiffness coefficient is sufficiently large.Finally,we have recommended a calibration procedure for cohesionless soil,which was at once applied to the simulation of Chende sand using a series of triaxial compression tests.The responses of DEM model are shown in quantitative agreement with experiments.In addition,stress-strain response of triaxial extension was also obtained by numerical triaxial extension tests.     

Local structural transformations in the fcc lattice in various contact interaction. Molecular dynamics study

The work is a molecular dynamics study of the peculiarities of local structural transformations in a copper crystallite at the atomic level in contact interaction of various types: shear loading of perfectly conjugate surfaces, local shear loading and nanoindentation. Interatomic interaction is described in the framework of the embedded atom method. It is shown that initial accommodation of the loaded crystallite proceeds through local structural transformations giving rise to higher-rank defects such as dislocations, stacking faults, interfaces, etc. In further plastic deformation, the structural defects propagate from the contact zone to the crystallite bulk. The egress of structural defects to a free surface causes deformation of the model crystallite. The deformation pattern can evolve, depending on the loading conditions, with a change in crystallographic orientation of the crystallite near the contact zone, generation of misoriented nano-sized regions, and eventually formation of a stable nanostructural state. The obtained results allow conceptually new understanding of the nature of defect generation in a crystalline structure during the nucleation and development of plastic deformation in loaded materials.     

A new method for determining deformation amplitudes in ultrasonic fatigue systems

An experimental procedure is described, which allows determination of the deformation parameters of materials excited in resonance with high amplitude ultrasound. The theory concerning resonance in the case of hysteresis phenomena is outlined briefly. By this procedure the maximum stress in the sample and the plastic strain can be determined from measurements of strain and particle velocity obtained in the region where the sample remains elastic. Experimentally obtained data on aluminium samples are discussed and compared with respect to predicted theory.     

Small size plasma tools for material processing at atmospheric pressure

A small size radiofrequency plasma jet source able to produce cold plasma jets at atmospheric pressure is presented. The surface modification of polyethylene terephtalate, polyethylene and polytetrafluorethylene foils is performed by using a scanning procedure. The contact angle measurements reveal that the treatment leads to hydrophilicity increase. The roughening of surface, specific to each material is noticed. A significant improvement of adhesion is obtained as result of atmospheric plasma treatments.