3 Linear polymer molecules Polyethylene is typical of a large number of polymers. They have a long “chain” backbone, with side groups attached to the backbone. Each molecule is like a long fiber.Lubrication applicationsTeflon, (PTFE)
5 PolystyreneBenzene ring, or phenol group:Cheap, clear plasticdrink cups
6 Other polymer backbones Some linear polymers have more complex backbones:NylonFabrics, ropes,structural plasticsPolycarbonate, PCShatter-resistantclear plastic
7 Degree of Polymerization/Molecular Weight One of the most important features associated with polymer structures is the size of the molecule. Most useful polymers have huge molecule sizes.Molecular weights of 25,000 g/mol are not uncommon.This means that there are a large number of mers in the backbone.Degree of Polymerization is average number of mers in a chain.
8 Shape of moleculesC-C bond angle is 109o, but there is rotational freedom.This means that the molecules are not straight, and willform random 3-D messes, like a plate of spaghetti.
9 Polyethylene Spaghetti Note: 2-D representation.Will actually wander in3-D (into and out ofpaper) as well.
11 Thermosets and thermoplasts Thermoplastic polymers soften and melt when heated. They may be recycled.Thermosetting polymers stay hard, and eventually burn when heated. They may not be recycled.Many thermosetting polymers are formed by a chemical reaction called condensation polymerization, where two chemicals are added to form the polymer. A common example is epoxy, which is formed by combining a resin and a hardener.Thermosets are often highly cross-linked.
12 CopolymersCopolymers are like polymer alloys. Different mers are joined to form a mixture in the backbone.Example: ABSAcrylonitrile-butadiene-styrene copolymerFootball helmetsCopolymers may be tailoredto obtain specific properties.
13 Crystalline polymersAll the polymers we have talked about so far are ordered at the atomic scale (C-C bond angle, etc). But they are amorphous (no long-range order) at the scale above atomic bonding.By processing, we can imposeorder on the polymer by regularlyarranging the chains. We call thiscrystallization, even though it doesnot look very much like the metaland ceramic crystals from Ch 3.
14 CrystallinityFold the polymer chainsover on each other in anordered way.
16 GlassesA glass (amorphous material) is quite different than a crystalline metal or ceramic. In a glass, there is no long-range crystalline order.Therefore, the molecular structures of liquid and solid glasses are not very different, and amorphous materials are often called super-cooled liquids.Mechanical behavior changes gradually and continuously, for example, the viscosity (related to the ability to blow a glass) changes smoothly with temperature.
18 Glass transitionThe temperature above which the glass becomes soft and viscous enough to work is related to the glass transition temperature, Tg.Below the glass transition temperature, the material is relatively hard and stiff; above it, it becomes more viscous.This shows up in the volume/temperature curve.
19 Mechanical Properties Mechanical behavior of amorphous and semi-crystalline polymers is strongly affected by the glass transition temperature.In general (although there are exceptions):Polymers whose glass transition temperature is above the service temperature are strong, stiff and sometimes brittlee.g. Polystyrene (cheap, clear plastic drink cups)Polymers whose glass transition temperature is below the service temperature are weaker, less rigid, and more ductilePolyethylene (milk jugs)If the service temperature changes, and Tg is crossed, the behavior can change drastically.
21 FibersWe see that both nylon and polyester have glass transition temperatures that are above room temperature. So in bulk form they are stiff and relatively brittle. Many plastic gears and bushings are made of nylon.We also know that many clothing items, which are very flexible, yet very resistant to tearing, are made of nylon and polyester. This is accomplished by making the material in the form of a fiber.A fiber is a long, thin strand of material. Since the fiber is so thin, it is flexible.
22 Typical properties of selected materials Material UTS (ksi) E (ksi) Density (g/cc)Low densitypolyethylenePolyethylenefiber (Spectra 900) ,000 17075 Aluminum ,4340 Steel Q+T ,
23 Specific StrengthWe see that the fibers have an excellent combination of low density, high stiffness, and high strength. We quantify these combinations by using specific properties.Specific strength = strength/densitySpecific stiffness = modulus/densityThe higher these properties are, the better is the performance of the material concerning light-weight design.
24 Specific propertiesMaterial Specific Strength Specific stiffnessLow densitypolyethylenePolyethylenefiber (Spectra 900) ,0007075 Aluminum ,5704340 Steel Q+T ,840units are ksi/(g/cc) (should clean this up!)