Institute of Mechanics of Metal-Polymer Systems

Scanning force microscopy (SFM) was used for probing micromechanical properties of compliant polymeric materials. Classic models of elastic contacts, Sneddon's, Hertzian, and JKR, were tested for various indentation depths and for a variety of polymeric materials. We selected extremely compliant polyisoprene rubbers (Young's modulus, E = 1−3 MPa), elastic polyurethanes (E = 5−50 MPa), and hard surfaces of polystyrene (PS) and polyvinylchloride (PVC) (E = 1−5 GPa). Both Sneddon's and Hertzian elastic models gave consistent and reliable results in the range of indentation depths up to 200 nm which are close to JKR solution. Close correlation is observed between absolute values of elastic moduli determined by SFM and known values for bulk materials.

Polymer tribology is a fast growing area owing to increasing applications of polymers and polymer composites in industry, transportation, and many other areas of economy. Surface forces are very important for polymer contact, but the real origin of such forces has not been fully investigated. Strong adhesive interaction between polymers leads to an increase in the friction force, and hence, the asperities of the material may be removed to form wear particles or transfer layers on the counterface. The theory of polymer adhesion has not been completely elucidated yet and several models of adhesion have been proposed from the physical or chemical standpoints. This paper is focused on the research efforts on polymer adhesion with emphasis on adhesion mechanisms, which are very important in the analysis of polymer friction and wear.

The nanomechanical behavior of molecularly thick (8−10 nm) compliant polymeric layers with the nanodomain microstructure from poly[styrene- b -(ethylene- co -butylene)- b -styrene] (SEBS or Kraton) was probed with micromechanical surface analysis based on scanning probe microscopy. The microprobing with high lateral resolution revealed the bimodal character of the nanomechanical surface behavior with different elastic responses shown by the rubber matrix and the glassy nanodomains. High-resolution probing showed virtually constant elastic response for the compliant layer compressed to 60% of its initial thickness followed by a sharp increase of the resistance when the tip reached within 3 nm from a stiff solid substrate. Application of the double-layer model allowed the estimation of the actual elastic moduli of different nanophases within the grafted polymer monolayer: 7 ± 3 MPa for the rubber phase and 20 ± 7 MPa for the glassy domains. The relatively high elastic modulus of the rubber matrix is caused by a combination of chemical cross-linking/branching and spatial confinement within a <2 R g layer. On the other hand, the observed low modulus of the glassy nanodomains can be attributed to both the low molar weight of polystyrene segments and the presence of rubber layers in the probed volume.

Mixtures of layered silicates (vermiculite and kaolinite) and carbon fibers were investigated as filler materials for polytetrafluoroethylene. The supramolecular structure and the tribological and mechanical properties of the resulting polymer composite materials were evaluated. The yield strength and compressive strength of the polymer increased by 55% and 60%, respectively, when a mixed filler was used, which was attributed to supramolecular reinforcement of the composites. In addition, the wear resistance increased by 850 times when using vermiculite/kaolinite fillers, which was due to protection of the surface by the formation of hard tribofilms.

Scanning force microscopy (SFM) was used to probe the micromechanical properties of polyisoprene rubbers, polyurethanes, polystyrene and polyvinylchloride. Applicability of the SFM cantilevers for microprobing is presented as a convenient method for materials selection. The Hertzian model of elastic contacts was used to obtain Young's modulus at contact frequencies range 0.05–367 Hz. The absolute values of the elastic modulus were correlated with the known properties of bulk materials, with no frequency dependence detected for glassy polyvinylchloride and polystyrene in air. The time dependence of the Young's modulus of rubber and polyurethanes followed the Williams–Landel–Ferry relationship, with parameters known for bulk materials. For PVC in water, a significant reduction of the elastic modulus and strong time-dependency were recorded. This behaviour can be related to the lower local glass transition temperature because of a plastification effect of solvent molecules on the uppermost polymer layer. © 2000 Society of Chemical Industry

Two approaches to the characterization of three-dimensional (3-D) textures are presented: one based on gradient vectors and one on generalized co-occurrence matrices. They are investigated with the help of simulated data for their behavior in the presence of noise and for various values of the parameters they depend on. They are also applied to several medical volume images characterized by the presence of microtextures and their potential as diagnostic tools and tools for quantifying and monitoring the progress of various pathologies is discussed. No firm medical conclusions can be drawn as not enough clinical data are available. The gradient based method appears to be more appropriate for the characterization of microtextures. It also shows more consistent behavior as a descriptor of pathologies than the generalized co-occurrence matrix approach.

An analysis of the features of variation and monitoring of the oil viscosity during tribosystem operation is presented. Methods and means for on-line monitoring of oil viscosity are reviewed, which allow on-line monitoring of the state of machinery and equipment. It is shown that the development of means for on-line measurement of viscosity based on the method applied under laboratory conditions does not meet the requirements for miniaturization and reliability of the diagnostic devices. To solve this problem, vibration methods of monitoring based on the application of tuning forks and microcantilevers are proposed. Among devices mounted directly into oil circulation lines, and, particularly, into lubricating systems for vehicles, acoustic devices have the most promise.

In order to develop biocompatible antifriction composites based on UHMWPE the mechanical and tribotechnical characteristics of a hybrid polymer matrix in the form of a mixture (blend) of UHMWPE with polytetrafluoroethylene (PTFE) have been studied under dry friction, boundary lubrication, and abrasive wear. It has been shown that the wear rate of the material under dry sliding friction was reduced more than twofold compared with pure UHMWPE. However, the mechanical characteristics have changed insignificantly. Under the boundary lubrication (distilled water), the wear resistance of the matrix material is similar to that for dry sliding friction. Under abrasive wear, the durability of the composites differs a little from that for pure UHMWPE. The mechanisms of wear of the polymer blend UHMWPE–PTFE under dry sliding friction and abrasive wear have been discussed.

In the framework of the thermodynamic approach Landau-Ginzburg-Devonshire (LGD) combined with the equations of electrostatics, we investigated the effect of polarization surface screening on finite size effects of the phase diagrams, polar, and dielectric properties of ferroelectric nanoparticles of different shapes. We obtained and analyzed the analytical results for the dependences of the ferroelectric phase transition temperature, critical size, spontaneous polarization, and thermodynamic coercive field on the shape and size of the nanoparticles. The pronounced size effect of these characteristics on the scaling parameter, the ratio of the particle characteristic size to the length of the surface screening, was revealed. Also our modeling predicts a significant impact of the flexo-chemical effect (that is a joint action of flexoelectric effect and chemical pressure) on the temperature of phase transition, polar, and dielectric properties of nanoparticles when their chemical composition deviates from the stoichiometric one. We showed on the example of the stoichiometric nanosized SrBi2Ta2O9 particles that except the vicinity of the critical size, where the system splitting into domains has an important role, results of analytical calculation of the spontaneous polarization have a little difference from the numerical ones. We revealed a strong impact of the flexo-chemical effect on the phase transition temperature, polar, and dielectric properties of SryBi2+xTa2O9 nanoparticles when the ratio Sr/Bi deviates from the stoichiometric value of 0.5 within the range from 0.35 to 0.65. From the analysis of experimental data, we derived the parameters of the theory, namely, the coefficients of expansion of the LGD functional, the contribution of flexo-chemical effect, and the length of the surface screening.

Effects of various geometrical and physical factors, as well as the method of data reduction (analysis of experimental force–displacement curves) on the values of local interfacial strength parameters (local IFSS, τd, and critical energy release rate, Gic) determined by means of a single fiber pull-out test are discussed. Experimental results of our pull-out tests on several fiber–polymer matrix systems showed that τd and Gic weakly depended on geometrical factors. However, the pull-out test appeared to be sensitive to the conditions of specimen formation and testing, such as changing the nature of the contacting surfaces (fiber sizing) and the fiber pull-out rate. Of several methods of τd and Gic determination from a force–displacement curve, the most reliable and reproducible one is the approach based on the values of the maximum force recorded in a pull-out test and the interfacial frictional force immediately after fiber debonding.

Abstract The single filament composite (SFC) test has been uses to investigate the adhesion of carbon and glass fibers to thermoplastic matrices. A new modification of the test is proposed, consisting of SFC specimens stretched until the neck formation is complete. This makes the measuring of fiber fragment lengths much easier and allows a wider class of matrices to be investigated. The adhesion between fiber and thermoplastic matrix is strongly dependent on the contact forming conditions. With increased time of thermal treatment, interfacial bonding is improved and the failuere mode is changed. In the case of por fiber‐mitrix adhesion, interfacial failure occurs. With an increase in interfacial shear strength, the martix cracks perpendicular to the fiber at the fiber breaks.

Polymer tribology is based on the analysis of abrasion, adhesion, and fatigue of polymer materials in a friction contact. The structural features of polymers provide a variety of tribological applications of basic polymers mostly as matrices and fillers of composite materials. Recently polymer nanocomposites are used for making components of various tribosystems. A short review of polymer materials for tribosystems is presented. The main results of studies in friction and wear of polymers are given. Formation of the real area of contact is evaluated when taking account of polymer viscoelasticity and the effects of temperature and load in the contact. Adhesion of polymers and its part in friction transfer is considered. Various aspects of friction and wear tests of polymer materials for estimation of their characteristics, prediction of service life in different operational conditions are discussed. Practical examples of applications of polymer composites and nanocomposites in various branches of industry are given.

Growth of solid tribofilms between moving parts substantially reduces friction and wear, and this is true especially for boundary-lubricated sliding contacts. However, efficacy of the tribofilms relies upon antiwear additives in formulated oil, typically zinc dialkyldithiophosphate, that usually gives rise to adverse environmental impacts. It is desirable to reduce the use of environmentally harmful additives by designing materials of moving parts so that they function properly with base oil lubrication. Herein, CoAl layered double hydroxide (LDH) nanoplatelets were synthesized via a one-step hydrothermal approach. When being lubricated with polyalphaolefin base oil (PAO 4), epoxy nanocomposites reinforced with low-loading LDH exhibit excellent tribological performance. In comparison to neat epoxy, wear of the nanocomposite is reduced by an order of magnitude and friction is decreased by 32%. Our work provides direct evidence that LDH released onto a friction interface facilitates tribofilm growth compensating for lubrication insufficiency of the oil film. This work paves a route for minimizing the dependency on harmful antiwear additives by optimizing tribomaterials.

The results of the tribotesting of polyamide composites dispersely filled with glass (PA6-L-CV30-1) and carbon (UPA-6130UV) fibers, which are promising for the production of metal-polymer gears, are presented. The characteristics of their wear resistance under the sliding friction conditions are determined. The calculation estimation of the durability of the metal-polymer cylindrical spur gears with wheels made of specified antifriction materials by the criterion of extreme wear and tear is obtained.

Grafting of itaconic acid on low density polyethylene in the molten state has been investigated. Static and dynamic mixers assembled on the Brabender plastograph have been used as a blender of reagents and as a chemical reactor. Shear rates were 50 and 100 s−1. Dicumyl peroxide; 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane; 2,2-di-(tert-butyl-peroxy)-5,5,6-trimethylbicyclo-[2,2,1]-heptane; 2,2-di(3-methyl-1-butyne-3-peroxy)-5,5,6-trimethylbicyclo[2,2,1]heptane; 2,5-dimethyl-2-hydroxy-5-tert-butylperoxy-3-hexyne were used as peroxide initiators of grafting. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1493–1502, 1997

In this work, melt-spun polypropylene (PP) fibers were treated in an electric field of a corona discharge. The fibers were then characterized using the thermally stimulated current (TSC) spectroscopy. It has been shown that the electret state of corona-treated PP fibers is a result of the combination of Maxwell–Wagner polarization and charge trapping. Activation energies and relaxation times for these processes have been determined, and characteristics of trapping sites have been calculated. The electret state induced in PP fibers by the corona discharge treatment holds for a long time (several months). Our analysis of the effect of processing temperature and electric field intensity on the characteristics of the electret state in melt-spun PP fibers allows one to specify optimum technological regimes for industrial production of PP-based electret filter materials. © 2000 John Wiley & Sons, Inc. Adv Polym Techn 19: 312–316, 2000

The impact of high-intensity laser radiation on a polymer in vacuum is accompanied by the release of gaseous products of degradation and, in some cases, of clusters of the partially destroyed polymer. Polytetrafluoroethylene (PTFE) exhibits an abnormal behavior in this process: being exposed to continuous CO 2 laser radiation, it degrades at a high rate and its clusters have a fibrous form. Depending on the irradiation conditions, the fibrous fraction forms two types of product, “cotton wool” and “felt”. Polytetrafluoroethylene and its laser-modified “cotton wool“ product have a semicrystalline topological structure. The preliminary γ-irradiation of PTFE enhances the laser ablation process.

Using thermomechanical spectroscopy, the molecular-weight distribution and relaxation transitions have been investigated in commercial LDPE and grafted by itaconic acid (LDPE-g-IA). This grafting in the molten LDPE was done by reactive extrusion with varied content of reactants in the blend under alternating of a shearing rate applied onto the melt. The dependence of structural relaxation changes in LDPE is shown upon the depth of the mechanochemical transformations and the competing reactions at IA grafting, and also on the chemical crosslinking of the macromolecules. The reason for MWD bimodality for LDPE-g-IA obtained in dynamic mixing is the raised homogenization degree of the reactive blend and the higher grafted product yield compared with static mixers. The mixer type substantially affects the structure of the LDPE-g-IA amorphous phase. The data obtained reflect chemical transformations of LDPE molecules in IA's presence without an initiator of radical reactions (DCP). Most probable is the IA initiation of molecular crosslinking reactions. There could also occur IA thermodegradation and oligomerization. LDPE and IA or products of acid chemical transformations do not agree thermodynamically (the calculated solubility parameters are 16.1 (MJ/m3)0.5 for polyethylene and 26.4 (MJ/m3)0.5 for IA). From the above procedure it can be supposed that nongrafted IA (or its oligomers) exerts an antiplastifying effect on LDPE and LDPE-g-IA. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1771–1779, 1999

The dielectric properties of epoxy/MWCNT (multi-walled carbon nanotubes)/MgO hybrid composites with a fixed MWCNT amount of 0.12 vol.% (0.2 wt.%) and varying MgO concentrations up to 3 vol.% were investigated in broad frequency (20-40 GHz) and temperature (20-500 K) ranges. The composites with up to 2 vol.% MgO nanoparticles concentration showed a significant increase of DC conductivity in relation to their non MgO-containing counterparts. The optimal content of MgO was found, i.e., 0.46 vol.%, which gave up to 2.5 orders of magnitude larger DC conductivity than those of the samples prepared without MgO additives. Using various amounts of MgO, it is possible to predictably vary the broadband electromagnetic properties of the composites, even entirely eliminating the electrical percolation. Electrical transport at different temperatures can be substantially controlled by the addition of given amounts of MgO. The broadband properties are discussed in terms of the distribution of relaxation times, which are proven to be an effective, noninvasive, and simple tool for checking composite fabrication issues, such as the distribution of MWCNT aggregates within the epoxy matrix.

We present the results of microthermal studies of ultrathin polymeric films deposited on a silicon substrate. We propose the procedure for microthermal analysis of ultrathin polymer films, elucidate limits of applicability of this technique to study their microthermal properties, and observe the thermomechanical response during local heating for films with thicknesses above 10 nm. The glass transition temperature of ultrathin polystyrene (PS) films deduced from experiments decreases for film thicknesses below 200 nm. For the thinnest PS film with clearly detectable response(25nm), the glass transition temperature is 20 °C below its bulk value. Our estimation of heat dissipation in the tip–surface contact area supports the conclusion that the observed micro-thermal-analysis response for polymer films is associated mainly with temperature-induced elastic variation of the contact area.