engineering materials Flashcards
What are some structures of molecules and what do each structure mean for mechanical properties of that polymer
Linear, branched, cross-linked, Network polymer.
Linear chains are connected from end of a repeat unit to the other end forming long chains.
Branched polymers are when side branched chains are connected to one continuous long body chain.
Cross linked is when the two chains are bonded by covalent bonds at various points along the chain.
Network polymers. They form three or more active covalent bonds making three dimensional structures. Generally harder and stronger, more dimensional stability and higher melting point.
What is stereoisomerism and explain the types of it.
Stereoisomerism is an arrangement where the atoms are arranged in the same order but differ in spatial arrangement. Isotactic configuration all the R groups are situated on one side of the chain in regular intervals. Syndiotacatic configurations is when the R groups are place in alternate sides of the chain repeatedly.
Atactic is for random positioning.
When does geometrical Isomerism occur and give an example of it.
They occur within the repat units of double bond between chain carbon atoms. the groups bonded to each side of the Double Carbon atoms can alternate. cis(structure) is when the CH3 group and the H group are positioned on the same side. The trans structure has H on the opposite side.
what is and Give examples of thermoplastic and thermoset.
Thermoplastic softens when heated and are able to be extruded or shaped and hardens when cooled again. Occurs mainly in linear and some branched flexible polymers. e.g. polyethylene, PVC Thermosets once cured (hardened ) by a chemical reactin, do not soften while heated since they usually have networked structures which resist vibrational and rotational motion during heating, but at excessive amounts of heat, the bonds can be severed. Generally they have brittle fracture. Cracks form at areas of stress concentration and which is then propagated and eventually failure. e.g. vulcanized rubber, epoxies
What are elastomers and thermoplastic elastomers?
Elastomers are polymeric materials that can undergo large and irreversible deformations. They are linear and occasionally cross-linked polymers, which mean the secondary bonds have already melted in room temperature. This allows for easy deformation. Relatively free-chain bond rotations. The cross links make a memory that the material can return to once the load is removed. Thermoplastic elastomer, acts like elastomers but is actually thermoplastic.
Describe cold drawing in macroscopic polymer deformation.
Because the polymers are semi-crystalline, i.e. they have amorphous regions within areas of crystallinity. First, elongation of amorphous regions, which is still under the yield point cuz this is elastic deformation. 2nd, increase in lamellar crystal thickness due to stretching and bending or crystallite regions. 3rd seperation of crystallite block regions. finally, orientation of the block segments and tie segments parallel to the direction of tensile.
What is crazing
Thermoplastics undergo crazing which is very localize plastic deformation which leads to microvoids forming. If load is sufficient, the micrvoids connect and causing a crack to form.
What is viscoelasticity
It is a property of a material where it exhibits mechanical properties of viscous flow and elastic deformation - that is it is a rubbery solid. At lower temperatures, the amorphous polymer behaves like a glass. Above glass transition temperatures, it behaves like rubbery solid. At higer temperatures, it is viscous or liquid.
What is a viscoelastic creep? Experimental measurements of the stress relaxation modulus?
It is the time-dependent deformation of a polymer under sustained load. Stress relaxation is dependent on both time and temperature. So a specimen is initially rapidly strained and held at constant strain and the stress required to keep that amount of strain is measure over time under constant temperature. the Stress is found t
What are the three typical stress strain graphs?
One has that of a brittle material in lower temperatures. usually 0.75 glass temperature. Another is yield point is maximum similar to that of a metal. third is like an elastomer when the material is far above the glass tranisition temperature.
Describe the process of crystallization and why is it so important.
understanding polymer crystallization is important because the degree of crystallinity influences mechanical properties. For polymer, upon cooling, through the melting temperature nuclei form, wherein the small regions of the tangled and random molecules become ordered and aligned in the manner of the chain-folded layer. Nuclei grow by continued ordering and alignment of additional molecular chain segments.
Describe what happens to polymers around the glass transition temperature.
The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials), from a hard and relatively brittle “glassy” state into a viscous or rubbery state as the temperature is increased. The crystalline regions will experience melting.
What is the importance of the glass transition temperature and the melting temperature.
Important parameter relative to the in service applications of polymers. Also influence the fabrication and processing techniques.
Factors that influence the Glass transition temperature and the melting temperature?
Magnitudes of Tm and Tg will increase with chain stiffness. Chain stiffness is enhanced by the presence of double bonds and side groups which are either bulky or polar hence restricting the motion and ease of rotation.
At low molecular weights, Tm and Tg increase with molecular weight. The degree of branching affects the melting temperature. The introduction of branches introduces defects into the crystalline structure and lowers the melting point.
Note : polymers have a range of melting temperatures because each polymer will be composed of molecules each having different molecular weights.
Describe the two processes of polymerization
Addition and condensation.
Addition is a process by which the monomer units are attached one at a time in chainlike fashion to form a linear macromolecule. the composition of the resulting product molecul is an exact multiple for that of the original reactant monomer. Three distinct stages, initiation, propagation and termination. Initiation is when the catalyst starts the reaction. Propagation involves linear growth of polymer chain by sequential addition of monomer units. Termination - possibly it ends at the start forming on continous loop.
Condensation polymerization is the formation of polymers through a step wise intermolecular chemical f that may involve more than one monomer species. There is usually a small molecular weight by product such as water. No reactant species has the same chemical formula of final repeat units.
What are some polymer additives and give examples? and why do we use them?
Polymer additives can be used to modify and enhance the properties of polymers thus render a polymer more serviceable. 1) Fillers they are added to improve tensile and compressive properties. e.g. such as wood flour, silica flour, sand, and glass.
2) plasticizers improve the flexibility, the ductility and the toughness. Their presence also produces reductions in hardness and stiffness.
3) stabilizers, some polymers under normal environmental conditions are subject to rapid deterioration, stabilizers precisely counteract this.
4) colorants - just dye giving color which diffuses into the polymer.
5) Flame retardants. Most polymers are flammable in their pure form. Flame retardants can increase resistance to flammability.
How are Fibers and films fabricated?
The process by which fibers are formed from bulk polymer material is termed spinning. Most often the fibers spin from the molten state called melt spinning. The material to be spun is heated first until it forms a relatively viscous liquid. Next it is pumped through a plate called a spinneret whcih contains numerous small holes which forces them into single fibers. This is rapidly cooled by air blowers or a water bath. The crystallinity of the spun fiber will depend on the rate o cooling. The strength of the fibers are improved by drawing which simply is permanent mechanica elongation of a fiber in the direction of the axis such that the tensile strength, modulus of elasticity and toughness are improved. There’s dry spinning and wet spinning, which just differs by the solvent used.
Many films are simply extruded though a thin die slit, this may be followed by rolling or a drawing operation that serves to reduce the thickness and improve strength.
What are the forming techniques used for either thermoplastic or thermosets?
If thermosplastic, it is formed above Tg if amorphous. If semi-crystalline, above Tm. The pressure must be maintained as it cools to retain shape. Thermosplastics can be recycled.
For thermosets, usually in two stage,
1) prep of prepolymer in form of liquid with low molecular weight.
2) converted into the hard stiff shape whilst curing
this can happen with a catalyst or an increase in temperature. This is when the cross links or network structures form. Thermoset polymers can be removed from the mould because they are geometrically stable. Difficult to recycle since they dont melt easily at higher temperatures.
What are the moulding processes, briefly explain each type
Most common forms are compression, transfer, injection, extrusion, blow. For all it is when a finely pellitized plastic is forced at an elevated temperature and pressure to flow into and fill the cavity of the mould.
Compression and transfer : When a pellet is placed directly into a heated metal mould which softens it, and is then forced to conform to the shape of the mold
Injection - most common, fastest can also be applied to thermosets. The melted viscous liquid is forced by an injector through a nozzle to fill a mold cavity.
Extrusion takes place as the molten mass is forced through a die orifice ( a hole or whatever shape is needed) this results in one continuous extruded length of the polymer forming.
Blowing : the hollow piece is formed by blowing air or steam under pressure into the parison forcing the tube walls to conform into the contours of the mold.
Define compositea and what are their phases. What determines the properties of the composite.
Generally speaking composites are defined as to be any multiphase material that gives a better combination of mechanical properties of both of the phases. The two phases that make up a composite are the matrix phase which is continous and surrounds the other phase. Diffused phase which is surrounded by the matrix. Properties of the composite are determine by the relative amount of each phase, properties of the constituent phases, and the geometry of the diffused phase (i.e the shape, size and the orientation)
The diagram ? Composites classification ?
Revision pptx slide 2
Describe the large particle strengthened composites. Give some examples.
Large is used to indicate that the interactions between the diffused phase and the matrix phase is not at the atomic or molecular scale. Generally, the particulate phase is harder and stiffer, which restrains the movement of the matrix phase near each particle. This means matrix transfers some of the load to the particles. The degree or reinforcement of the material property depends on the strong bonding at the particle-matrix phase.
Examples are CuW alloys, - Tungsten particles dispersed in copper alloy. High heat resistance, high heat/electrical conductivity.
Cermet - ceramic and metal composites. Extremely hard particles of a carbide ceramic in a metal matrix. Used extensively in cutting tools. Toughness is enhance by the metal matrix even though the carbide on its own is very brittle. Isolates carbide particles from each other hence prevents crack propagation. Carbon black, in car tyres. Concrete, reinforced concrete.
