Adhesion and friction becomes more important when the lateral size of objects decreases. This is due to the increased surface to volume ratio. At the micro and nanoscale the gravitational interaction is usually negligible compared to the van der Waals interaction and electrostatic interactions, or forces due to the formation of small (nanometer) capillary bridges. Problems related to adhesion and friction (and wear) are the most severe problems to be overcomed in the development of many useful micro or nanoscale mechanical systems, e.g., micromotors.
In this presentation I will describe a new theory of contact mechanics for solids with randomly rough surfaces. I consider the elastic contact between solids both without and with adhesion. As an illustration I will discuss in detail biological adhesion systems used for locomotion, and show how natural selection in some cases has optimized the adhesive systems from the (macroscopic) size of the adhesion pads (typically of order ~ 1 mm) to the nanoscale. As a second application of the theory I will briefly describe rubber friction on rough substrates. Finally, I will briefly discuss molecular dynamics resuls for the friction between solids consisting of (alkane) hydrocarbon chains with 20, 60, 100, 140 and 1400 carbon atoms, and describe some basic results relating to the friction on ice and snow.
The tribological system of a ski sliding on snow consists of four elements, the slider, the snow surface, the interface and the surrounding atmosphere. All four have an important influence on the glide performance of the slider. The slider base is prepared to meet the different conditions of the snow surface both with respect to base material, base structure and surface treatment. The condition of the second element, snow, is determined by the atmospheric conditions before the time of contact. Important meteorological parameters include air and snow temperature, relative humidity and net radiation. The dominating friction mechanism at the interface is determined by the conditions of the base, the snow surface and the atmosphere. Since snow, at the temperatures at which skiing is performed, constantly is changing its properties different preparations are needed in order to meet the prevailing conditions. The paper discusses the important parameters of each element in order to decrease the friction between slider and snow.
A traditional skiwax should give sufficient kick and glide on a specific snow-surface. This means a balance between static and kinetic friction.
The meteorological conditions will vary within large limits, creating everything from tiny, sharp crystals at very low sub-freezing temperatures to a melting coarse-grained substance at several degrees above melting. A discussion of the problems caused by the conditions around freezing, also called the critical temperature of ski-waxing.
What type of raw-materials are used to match such large differences in surface conditions ? How do the blends interact with the snow surface?
As the name of the products indicate, waxes are important, but not all “ski-waxes” contain wax. A discussion on waxes, resins and additives used to-day.
Application and interaction with the UHMW-PE in the ski-bases.
A mathematical model for the thickness of a lubrication film during sliding of two surfaces is constructed and used for simulations. The film is created by melting due to friction forces on contact spots and is removed by squeezing and shear. Heat conduction into the sliders is accounted for in the model. Some analytical results are also shown.
I faststoff rakettmotorer består drivstoffet av en partikkelfylt polymer, som etter støp i isolerte motorrør herder til en fast fase. Vedheften mellom drivstoff og isolasjon er meget viktig for å gi ønsket forbrenningsforløp i rakettmotoren. Kjennskap til diffusjonskoeffisienter i både drivstoff og isolasjon er nødvendig for forståelsen av diffusjons- og vedheftsprosessene i en rakettmotor.
Isolasjonsmaterialet, som er en EPDM gummi, kan overflatemodifiseres ved hjelp av lavtrykks gassplasma. Behandlingen gir en økt funksjonalisering av overflaten, slik at den blir mer tilpasset drivstoffets overflateegenskaper, og overflateenergien kan kontrolleres. Videre øker ruheten i overflaten med plasmabehandlingstiden. Plasmabehandling av isolasjonsmaterialer øker bindingsstyrken mot polymermaterialene som brukes i drivstoff opp til det tidobbelte, men kan også redusere den. Med få unntak ble det funnet at bindingen var best når differansen i overflateenergi mellom isolasjon og drivstoff var minst mulig.
Enkapsling har vært brukt i stor utstrekning til å sperre inn flere typer kjemikalier i partikulære materialer. Typiske anvendelser finnes i biovitenskap, matvareindustri, kosmetikk, kjemisk og farmasøytisk industri.
Størrelsen til kapsler er vanligvis fra 1 µm til flere mm. Dette representerer en viktig hindring for å kunne anvende kapsler i belegg og maling.
Nanomedisin, definert som nanoteknologi anvendt på helse, er et raskt voksende nytt forskningsområde. Nanopartikler og nanokapsler vil være helt sentralt ved utviklingen av morgendagens medisiner.
Denne presentasjonen tar utgangspunkt i kompetanse innen nanoteknologi som SINTEF har bygget på gjennom flere år. Spesielt har nye materialer i form av nanopartikler og nanokapsler blitt utviklet for kontrollert frigivelse av molekyler eller kjemikalier. Flere typer eksterne stimuli så som temperatur, pH eller lys er benyttet til å slippe ut aktive komponenter. Ultra-tynne belegg og overflatemodifisering har satt oss i stand til å funksjonalisere overflater. Flere eksempler vil vises frem som illustrasjon av det store potensialet som ligger i anvendelsen av slike materialer innen biomedisin og korrosjonsmotstand.
By changing the temperature in solutions of an uncharged copolymer we will demonstrate a unique transition in the turbidity as the cloud point is approached. This transition is accompanied with special crossover effects in viscosity and dynamic light scattering. Some interesting structural changes of the nano-clusters under shear at different temperatures will be revealed with the aid of small-angle light scattering (SALS). It will be shown that temperature can induce a significant contraction of the core-shell structure at elevated temperatures. The special features of this system are governed by a delicate balance between hydrophilic and hydrophobic interactions. To gain more insight into this problem, we have also conducted a study on a similar copolymer bearing charges. In this case, the hydrophobic interactions are reduced because of the electrostatic repulsive forces and the findings facilitate a more profound understanding of the intricate interplay between different types of interactions.
There is great interest in understanding the mechanisms that regulate the generation of new blood vessels, because the future ability to manipulate this process may enable control of cancer growth, revascularization of ischemic lesions or building a functional microvasculature in tissue engineering. However, a major limitation in angiogenesis research is the lack of appropriate in vivo models to test possible players in the process of vascular development.These kinds of experiments have, traditionally, been conducted using MatrigelTM. However, the properties of this gel are not optimal for several reasons. The aim of this study was, therefore, to find a suitable hydrogel for an in vivo model. The physicochemical and the biological properties of an alginate-calcium matrix and MatrigelTM were compared. The alginate-calcium matrix showed promising physicochemical properties and could be considered as a replacement for the Matrigel. However in vitro and in vivo studies do not reveal the cell activity hoped for.
Nanosized particles and capsules will play a central role in the development of tomorrow’s medicine. Examples, where research is in fast development, may be found in “theranostics”, a new group of medicines that combines diagnosis, therapy and therapy control. Surface modification of nanoparticles is essential, both for target-specific drug delivery and to prevent clearance through phagocytosis. The nanoparticles will thus be coated with stealth structured layers, for example PEG, to prevent uptake by monocytes and subsequent blood clearance. Secondly, specific ligands will be chemically coupled to the nanoparticle surface to achieve selective targeting to the cells of interest.
The intermolecular interactions and intramolecular associations in aqueous solutions of thermoresponsive block copolymer Methoxypoly(ethylene glyco)-block-poly(N-isopropylacrylamide) or MPEG-b-PNIPAAM at different copolymer concentrations and temperatures are studied with the aid of turbidity and dynamic light scattering measurements. The results of turbidity measurement shows MPEG-b-PNIPAAM's turbidity decreases with temperature increases and then increases again, this change happens around body temperature. Dynamic light scattering experiments revealed the existence of two diffusive relaxation processes, one is ascribed to the diffusion of individual polymer coils, or small clusters of molecules, and the other is attributed to interchain aggregation where nanoparticles are formed with a hydrophobic PNIPAAM core and a hydrophilic PEG shell.
Water soluble and amphiphilic polymers may respond to various stimuli in aqueous solutions. Changes in the dimensions or the degree of aggregation may find use in several applications.
Thermally responsive polymers like poly(N-isopropylacrylamide), PNIPAM, poly(vinylcaprolactam), PVCL, and poly(vinylmethylether), PVME, have been extensively investigated during the last decade. These polymers dissolve in cold water but phase separate upon increasing temperature at a certain critical temperature. Polymers either precipitate or, in very dilute solutions, form colloidally stable particles. Crosslinked polymers (gels) collapse upon heating at the critical temperature but reswell upon cooling. PVCL microgel particles, for example, may find use as drug carriers and in controlled drug release. For this application it is beneficial to stabilise the particles against coagulation.
Polyelectrolytes respond to changes of pH and ionic strength. As an example, the synthesis and properties of amphiphilic star diblock copolymers will be described. In these stars, the outer block of the arm is a polyelectrolyte whereas the inner one is hydrophobic. Aggregation and self assembling of these polymers is affected by the number of arms, as well as by changes in the solvent composition.
Various examples of the use of size-exclusion chromatography (SEC) with multiple detectors will be given:
Thermoresponsive associations in aqueous mixtures of the anionic sodium dodecyl sulfate (SDS) surfactant and copolymers of poly(N-vinylcaprolactam) and omega-methoxy poly(ethylene oxide) undecyl alfa-methacrylate (PVCL-g-C11EO42) have been characterized using turbidimetry, rheology, and dynamic light scattering (DLS). The cloud point (CP) is shifted toward higher temperature as the percentage of surfactant increases in the PVCL-g-C11EO42/SDS mixture, and at low copolymer concentration and high level of SDS addition the system exhibits a strong polyelectrolyte effect. For semidilute polymer solutions and moderate levels of surfactant addition, the viscosity results indicate a temperature-induced compression of the polymer-surfactant clusters. At a given temperature, high surfactant concentration leads to an expansion of the complexes.
Effects of steady shear flows on intermolecular interactions in dilute and semidilute aqueous solutions of hyaluronic acid (HA) are reported. Pronounced shear thinning behavior is observed for solutions of HA at high shear rates, and no hysteresis effects are detected upon the subsequent return to low shear rates. With the aid of the asymmetric flow field-flow fractionation (AFFFF) technique, it is shown that mechanical degradation of the polymer does not take place in these shear viscosity experiments, even at high shear rates. The low shear rate viscosity of a semidilute HA solution decreases by approximately 40 % when the temperature is increased from 10 oC to 45 oC. It is shown that when a dilute HA solution is exposed to a low fixed shear rate (0.001 s-1), a marked viscosification occurs in the course of time and prominent intermolecular complexes are formed. It is argued that shear-induced alignment and stretching of polymer chains promote the evolution of hydrogen-bonded structures, where cooperative zipping of stretched chains generates a network. At a higher constant shear rate (0.1 s-1), the viscosity decreases as time goes because of the alignment of the polymer chains, but the higher shear flow perturbation prevents the chains in dilute solutions from building up association complexes. The viscosity of an entangled HA solution is not changed in the considered time window at this shear rate, but the network structures breakdown at the highest shear rate (1000 s-1), and then they are restored upon return to a low shear rate.
Nanocomposties have gained extensive interest in polymer science since a small amount of nanofiller has a large effect on the material properties, saving both material and costs for a variety of applications.
One of the most common nanofiller is silicate clays, such as montmorillonite (MMT). These clays consist of thin sheets, about 1 nm thick, with a very high surface to volume ratio. Even with small amounts of filler there will thus be a potentially large area available for interactions between the filler and the polymer matrix, especially if complete exfoliation is obtained.
The sheets are negatively charged with cations e.g. Na+ and Ca2+ holding them together by electrostatic forces. These forces are generally too strong to allow separation of the sheets by shearing forces in a polymer melt only. How to increase the interlayer spacing is one of the central questions in producing a polymer nanocomposite, another being how to make the clay compatible with the polymer.
This presentation will introduce the ongoing research at Chalmers, in collaboration with Hydro Polymers, within the field of PVC nanocomposites.
Novel information about the effects of temperature and addition of hydroxypropyl-â-cyclodextrin (HP-â-CD) or a charged â-cyclodextrin polymer (poly(â-CDN+)) on the rheological features of aqueous solutions of hydrophobically modified hydroxyethylcellulose (HM-HEC) and hydrophobically modified dextran (HM-dextran) is given. The viscosity results for the HM-HEC/poly(â-CDN+) system demonstrated that the addition of this cosolute sets up bridges between adjacent polymer chains, which lead to a strong enhancement of the viscosity. Furthermore, a shear-thickening effect is registered at fairly high shear rates, and this feature is influenced by temperature. On the other hand, addition of HP-â-CD monomers to the system generates decoupling of associations via inclusion complex formation with the polymer hydrophobic tails. For the HM-dextran solutions, the interaction with the cosolutes is weak and only a slight increase of the viscosity by addition of poly(â-CDN+) is observed. In contrast to the HM-HEC solutions, addition of HP-â-CD monomers to the aqueous solutions of HM-dextran deactivates the intramolecular hydrophobic associations and thereby the compact coil structure expands and this leads to viscosity enhancement. Elevated temperature induces higher chain mobility and the formation of weaker network associations. The bridges formed in the HM-HEC/poly(â-CDN+) system are especially sensitive to temperature changes.
Intramolecular and intermolecular associations of dilute aqueous alkali solutions of unmodified hydroxyethylcellulose (HEC) and HEC of different hydrophobicity in the presence of a chemical cross-linker agent (divinyl sulfone, DVS) at different pH values are studied with the aid of rheological methods. Results on compact dextran species are also reported. When the reaction mixtures of unmodified hydroxylethylcellulose are subjected to shear intraploymer cross-linking with contraction of the molecules is observed, and this effect is followed by an interpolymer cross-linking and the formation of aggregates at longer times. Intramolecular cross-linking is not observed on hydrophobically modified HEC and dextran.