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63 Cards in this Set
- Front
- Back
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What two things can cause a material to have less rotational flexibility, chemistry-wise?
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1) A region that has a chain segment of double bonds.
2) Bulky or large side group of atoms |
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At room temperature, what are the typical molecular weights of polymers that cause a material to be liquid, gas, soft solid, and hard solid?
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Liquid/Gas: 100 g/mol
Waxy solids/Soft resins: 1000 g/mol Solids: 10,000 to a few 1,000,000 g/mol |
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Does the melting/softening temperature of a polymer generally increase or decrease with a higher molecular weight?
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Increases (which will make the polymer stronger)
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What is viscosity?
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A measure of the resistance of a fluid which is being deformed by either shear stress or tensile stress.
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What roles do cations play in silicates?
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1) Contribute to charge neutrality
2) Ionically bond SiO4 tetrahedra together. |
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Differentiate between isotactic, syndiotactic, and atactic configurations for a polymer characterized by stereoisomerism.
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Isotactic - R groups line up on same side of chain.
Syndiotactic - R group alternates. Atactic - Random positioning of R. |
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high density polyethylene vs. low density polyenthylene
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HDPE is primarily a linear polymer, while LDPE is branched.
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Polymer crystallinity
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the packing of molecular chains to produce an ordered atomic array.
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Because of their size and complexity, polymers are often semi-crystalline (up to 95%), but never completely. Amorphous regions caused by kinks and coils.
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Ceramics are either totally crystalline or non-crystalline.
Metals almost always totally crystalline. |
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Crystalline polymer materials have what different properties from noncrystalline polymer materials?
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Crystalline polymer materials have a higher density and more strength.
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Crystallization does not occur in polymers with more complex repeat units as much as those with chemically simpler ones.
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Cooling to produce crystallinity in a polymer material requires time since the polymers have to align themselves with one another.
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Polymers vs. Metals
(characteristically) |
Much lower modulus, higher plastic elongation, more sensitive to temp. changes near room temperature.
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For polymers, increasing temp. near room temp. has what effects?
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1) Decreases elastic modulus
2) Reduces tensile strength 3) Enhances ductility |
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Specifying gauge length in ductility measurements isn't required for polymers. Why is this?
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Necking occurs along entire length of gauge before fracture occurs-->no localized necking.
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What is vicoelastic creep?
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Time-dependent deformation of polymers when stress level is constant over a period of time.
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Mode of fracture in:
Thermosetting polymers Thermoplastic polymers |
Thermosetting: Brittle
Thermoplastic: Ductile |
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How does drawing increase polymer strength?
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The molecular chains in a material slip past one another and align in a highly oriented fashion.
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Tensile strength increases with...?
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Increasing molecular weight.
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Tensile modulus is independent of...?
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Molecular weight.
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atraer
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to attract, to allure, to charm
attirer |
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Elastomers are very elastic because when under stress, they simple uncoil and unknot first.
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They naturally want to be coiled up-->high entropy.
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Two effects of entropy on elastomers:
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1) Temperature increases when stretched.
2) Modulus of elasticity increases with increasing temperature. |
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3 criteria of elastomers:
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1) Cannot crystallize easily
2) Chain bonds must rotate easily. 3) Delayed plastic deformation 4) Must be above its glass transition temperature. |
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What is vulcanization?
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Sulfur compounds are added to a heated elastomer where they then chain up and attach to backbones of polymer chains, forming crosslinks between them.
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Vulcanization enhances what properties of polymers?
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1) Modulus of elasticity.
2) Tensile strength 3) Resistance to degradation |
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Addition polymerization
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Monomer units are attached one at a time to form a polymer chain.
Double bonds between carbon atoms become single bonds. |
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Condensation polymerization
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Two reactant species combine to form a repeat unit. Water is a usual byproduct.
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Additives
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Foreign substances intentionally introduced to enhance or modify properties of the polymer material.
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Fillers (additive)
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Inexpensive materials added to Improve tensile and compressive strengths, abrasion resistance, toughness, and dimensional and thermal stability.
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Plasticizers (additive)
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Liquids of low vapor pressure and molecular weight. Occupy spaces b/w polymer chains increasing distance b/w said chains and decreasing crystallinity.
Improves flexibility, ductility, and toughness of a polymer while reducing hardness and stiffness. |
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Stabilizers (additive)
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Counteract deteriorative processes in polymers
Light exposure: thin film added to provide UV protection Oxidation: stabilizers added to consume oxygen before it reacts with polymer. |
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Colorants (additive)
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Impart a specific color to the polymer.
Dye: molecules dissolve in polymer Pigment: don't dissolve |
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Flame Retardants (additive)
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Interfere with the combustion process through the gas phase or initiate a reaction that generates less heat.
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Thermoplastics
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Polymers with linear and branched structures that soften when heated and harden when cooled.
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Thermosets
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Polymers with crosslinked and network structures that, once hardened, will not soften upon heating.
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Thermoplastic fabrication
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Via addition polymerization
Melt and form by injection molding |
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Thermoset fabrication
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Via condensation polymerization
Formed by mixing resins |
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What is a composite material?
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A multiphase material that exhibits substantial properties of each phase resulting in a better combination.
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What is the makeup of its phases?
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A matrix phase that surrounds a dispersed phase.
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What characteristics of the dispersed phase can change the properties of the composite?
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Size, shape, distribution, orientation, and concentration.
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Subclasses of particle-reinforced composites?
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a) Large-particle
b) Dispersion-strengthened |
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Basic properties of particle-reinforced composites
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1) the particulate phase is harder and stiffer than the matrix phase.
2) the reinforcing particles restrain movement of nearby matrix 3) degree of reinforcement depends on strong bonds between particulate and matrix phase |
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Dispersion-strengthened composite characteristics
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particles are small and interactions between phases occur at atomic/molecular level. particles impede motion of dislocations, restricting plastic deformation.
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Cermets
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Ceramic-metal composites.
Brittle but very hard particles embedded in metal matrix. Volume % of particles very high. |
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rubber reinforcement
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carbon particles in a vulcanized rubber matrix enhances toughness, tensile strength, tear and abrasion resistance.
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Concrete
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Ceramic-ceramic composites
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Portland cement concrete
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Matrix: portland cement and water
Dispersion: sand and gravel Weak and brittle, thermal shock, |
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Reinforced concrete
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Addition of steel rods, wires, bars, or mesh into the matrix. Cracks won't hinder its reinforcement.
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Why does steel make a good reinforcer for concrete?
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1) Coef. of therm. exp. nearly same
2) Immune to rapid corrosion.inside concrete 3) Strong adhesive bond w/ concrete |
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Prestressed concrete
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residual compressive stresses added. Brittle ceramics-->stronger in compression than tension. To fracture, must overcome comp. stress with appl. tension stress.
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Prestress technique 1.
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stretch steel wire in empty molds. Add concrete. Let harden. Release tension in wires. Wires contract and transfer stress to concrete through steel/concrete bond.
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Prestress technique 2 (posttensioning)
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Concrete is cast around tubes. Steel fed through resulting holes and stretched by jacks. Stress transferred to concrete. Empty spaces in holes filled with grout.
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Dispersion-strengthened composites remain strong at high temps and long periods because
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the fine, dispersed particles are chosen to be unreactive with the matrix.
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Mechanical characteristics of fiber-strengthened composites depends on:
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1) properties of the fiber itself
2) degree at which applied load is transmitted to the fiber by the matrix phase |
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As a fiber's length increases beyond its critical length, what happens?
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The reinforcement it provides during tensile loads is maximum over a greater distance of the fiber, rather than just at the center of a fiber of exact critical length.
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A fiber shorter than its critical length will...?
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Not reinforce to its max. capabilities during tension (won't transfer enough stress because maxtrix deforms around it)
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Why is composite failure for longitudinally-applied tension not catastrophic?
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All the fibers don't fracture at the same time when failure technically occurs, and the matrix surrounding the fiber doesn't fracture when the fiber fractures.
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Drift velocity depends on what?
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Magnitude of the electric field and the mobility (square meters per volt-second) of an electron
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Electrical conductivity depends on what?
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electron concentration, charge, and mobility.
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A metal's resistivity depends on:
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thermal vibrations, impurities, plastic deformation, crystallinity.
resistivity increases with more electron scattering. |
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Intrinsic semiconductors
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narrow forbidden band gap so that if excited enough, valence electrons get excited into conduction band. holes left behind also provide current.
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Extrinsic n-type semiconductors
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impure atoms with 1 extra valence electron than host atoms. this extra electron gets swept into conduction band very easily.
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Extrinsic p-type
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impure atoms with 1 less valence electron. holes get filled causing current.
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