Frågor Flashcards
How can we relate mechanical properties to the nano scaled manufacturing of conducting polymers?
The tensile modulus of nano scaled conducting polymers is strongly dependent on the thickness. The thicker the nanotube the higher the tensile strength modulus becomes.
Common for many 1D- conducting polymer nano composite systems is the semi conducting behavior of decreasing conductivity with temperature. What is most likely the cause of this?
This can most often be attributed to increased charge carrier scattering between composite constituent nanoparticles.
Why could polymers be a better choice for insulators than for example ceramics or glasses?
Answer: Much tougher, lighter, easier installation, and therefore cheaper to work with.
What properties are characterizing an isolator?
Answer: Dielectric strength ,mechanically strong, high insulation and able to be used in hazardous environments. The electrons need to be highly bonded to the atom and within the material to resist the flow of electrons.
Why are polymers used as thermal insulators and what types of polymers are commonly used forthis application?
Most polymers do not have many free electrons, meaning that the system is saturated.This results in low electrical conductivity and that heat transfer mainly occurs due to vibration of themolecules and atoms. These small movements and how the vibrations move through the material aredescribed as phonons, which are directly related to the thermal conductivity of solids without electricalconductivity. For amorphous polymers, the chain length affects the thermal conductivity. Longer chainsand more branches will result in lower thermal conductivity in the polymer. The thermal conductivity isalso affected by the crystallinity of the polymer, and is generally increasing with higher crystallinityPolymeric foams are most commonly used for these applications.
What is the main reason behind the low thermal conductivity of polymer foams in comparison to solid polymers?
The main reason behind the low thermal conductivity is the cellular structure of polymer foams. The cells trap oxygen or other low conductive gases that relative to the polymer itself have a very low thermal conductivity.
The Polytetrafluoroethylene(PTFE) is one of the most frequently used polymers in applications which require high wear resistance. What chemical characteristic of the PTFE molecule is the main reason for the low friction coefficient of pure PTFE polymer?
The chemical formulaof Polytetrafluoroethylene(PTFE) is (CF2-CF2). The spacing between the fluorine atoms in the molecular chain are very small, due to the high electronegativity of fluorine. This contributes to a smooth and cylindrical surface which other molecules easilycan slide over. For this reason, the PTFE polymers are considered as a self-lubrication material with good sliding behaviour and a low friction coefficient.
Explain why it is suitable to use a blend of PTFE and PEEK polymers based on wear resistant properties?
Experiments have shown that blending PTFE and PEEK leads to reduction of friction and wear-rate of the polymer composite. The mechanism behind the decrease of friction is the ability for the polymer to transfer a film of PEEK and PTFE on thehard counterface, where the PTFE provides lubricity. The reduction in wear is also related to the transfer-film formation. The low shear-strength PTFE transfers to the hard counterface during sliding, adheres to the surface and creates a thin film. The wear mechanisms transform from ploughing of asperities into sliding of two soft polymers.
How can we obtain a polymer with high wear resistance and low friction coefficient at the same time?
Quoted from the text above “It has been shown that a combination of nano-filler and traditional microfiber-fillers provides a great wear resistance and reduction in friction coefficient, since they provide different properties to the polymer matrix. The fibers give strength, stiffness, and wear resistance (since they are often carbon or glass fibers)while the particles give good thermal properties (since they are often ceramics) and reduces friction.”
In what type of applications can ultra-high and high-performancepolymers be seen in? Explain why.
Ultra-Highand high-Performance Polymers and especially composites made with these polymers seem to have typical areas of application in, the aerospace and automotive industries where weight, temperature and wear are big factors.Other typical areas of applicationsfor these types of polymers are in the area of oil and gas exploration and also in corrosive industrial environments.Some examples of more specific areas of application would be:In some high working temperature and high velocity ball bearings the retaining ring is made fromUltra-High-or High-Performance polymers.Another use that has been seen for this type of polymer is the addition of a PEEK composite into a bushing where the solid lubricants in the polymer makes it usable without the need of adding oil and grease, and these busing are seeing an increasing use in higher temperature and pressure environment.Even though metals and metal compositesoften have better mechanical properties to withstand high heat and high wear environments theformability and low weight of these polymers make them an attractive choice
Describe the main two classes of liquid crystals and the control parameters whichdetermine their phases.
Thermotropic liquid crystals are obtained from certain materials by simply keepingthe temperature of the material between the melting point Tm and the clearing pointTc. This state of matter is an intermediate phase between solid and liquid called themesophase. The main characteristic of the thermotropic class is that its phases aredependent on temperature. The closer the temperature of the mesophase is to themelting point, the higher the positional order of the crystals are.The other class is called lyotropic. Lyotropic liquid crystals are obtained when a highenough concentration of amphiphilic molecules forms ordered structures in a solvent.These structures are called micelles which make up the crystals in this class. Micellesstart to form at a concentration called the critical micelle concentration. The lyotropicphases are dictated by the concentration of amphiphile. A higher concentration ofamphiphile provides a greater positional order.
Describe why the thermotropic smectic phase has a higher viscosity than thethermotropic nematic phase of the same material.
The nematic phase is the simplest liquid crystal phase with the most disorderedmolecules. In a nematic liquid crystal, the molecules are long-range orientationalorder in one direction. This direction is along with the director n.In the case ofposition, the molecules are not ordered in any dimension.Smectic phases have the same orientational order as nematic phases with the rodspointing along with the director. Also, smectic phases have one dimension ofplacement order since the molecules are divided into planes.Smectics have more solid-like properties as a solid have positional order in all threedimensions and have a high order of orientational order. Nematic phases have moreliquid-like properties since liquids have neither positional nor orientational order.Therefore, the smectic phase will have a higher viscosity than the nematic phase.
What attributes make liquid crystals suitable for display applications?
The optical properties of liquid crystals change depending on the orientation ofthe liquid crystal molecules. The orientation of the crystals can be changed by applying avoltage over the liquid crystals.
Why are liquid crystals suitable for thermometer application?
Because the nematic liquid crystal is temperature dependent, the observed color isa function of temperature and liquid crystals can therefore be used as a thermometer.
Why do thermosetting acrylic polymers have higher hardness, toughness andchemical resistance compared to their thermoplastic counterparts?
The reason for this is because the thermoplastic resins form a hardened coatingsimply through solvent evaporation whereas the thermoset resins form a crosslinked structurethrough chemical reactions with itself or other types of resins such as epoxies and melamineformaldehyde (MF) or polyisocyanates.
