Presentation on theme: "ENS 205 Materials Science I Chapter 13: Polymers"— Presentation transcript:
1 ENS 205 Materials Science I Chapter 13: Polymers
2 Basics Ivinyl chlorideA polymer is a substance composed of molecules with large molecular mass composed of repeating structural units (called monomers), connected by covalent chemical bonds.Poly (vinyl chloride)
3 Basics II linear polymer crosslinked polymer star polymer branched polymerdendrimer
4 Polymer Structure IWhy most? Because a few types of polymers may do under certain circumstances (we will see how)semi-crystallinepolymeramorphouspolymerMOST CRYSTALLINE POLYMERS ARE NOT ENTIRELY CRYSTALLINE !!!They are either amorphous or semi-crystalline.The degree of crystallinity varies (From 0 to %).
5 Polymer Structure II Shish-kebab morphology SEM image of a spherulite SEM image of a chain having shish-kebab morphology
6 Polymer Structure III Crystallinity of polymers is affected by: Chemical structureStereochemistryMolecular weightTemperatureProcessing conditionsAll about the chain propertiesHmm…External effects
7 Is polystyrene flat like this? Polymer Structure IVSTEREOCHEMISTRY(Tacticity)This is the white, plastic coffee cup used everywhere like Fassane. Strafor kopukIs polystyrene flat like this?Absolutely NO!
8 Polymer Structure V STEREOCHEMISTRY (Tacticity) In the previous picture you see all the phenyl groups are located on the same side of the polymer chain. But they don't have to be this way. To illustrate let's look at a chain of polystyrene from above. You can see that the pendant phenyl groups can be either on the right or left side of the chain.All phenyl groups on the same sidePhenyl groups on alternating sidesPhenyl groups distributed randomly
9 Polymer Structure V STEREOCHEMISTRY (Tacticity) The question is, how tacticity helps crystallinityCRYSTALLINITY ↔ LONG RANGE ORDER ↔ PACKINGSyndiotactic polystyrene: Highly crystallineAtactic polystyrene: Highly amorphousSimilarly, a linear polymer can pack well, whereas a branched isomer cannotHighly crystallineHighly amorphous
10 Intermolecular forces and crystallinity Polymer Structure VIIntermolecular forces and crystallinity(Aromatic ring stacking)
11 Polymer Structure VII Fibers Can you stretch this structure??? Polymers with regular structure can align themselves very closely for effective utilization of the secondary intermolecular bonding forces.Already stretched out fully, up to 95% crystallinityHigh symmetry, high intermolecular forces.Characterized by high modulus, high tensile strength, and moderate extensibilitiesCan you stretch this structure???High density polyethylenePolypropyleneNylonPolyesterKevlar and NomexPolyacrylonitrileCellulosePolyurethanes
12 Polymer Structure VIII Elastomers (rubbers)Polymers with irregular structure, weak intermolecular attractive forces and flexible chains.Can undergo local mobility, but gross mobility of chains is restricted.Characterized by high extensibility, low initial modulus in tension but they stifen when strecthed.stretchleaveENTROPY WORK!
13 Polymer Structure IX Plastics Fall between the elastomers and fibers. However there is no exact boundaryHarder to stretch than elastomers (Because of crystalline regions?). But preserve their shape when stretched unlike elastomers (Strain induced crystallization, stiff chains)They are pliable, that is, they can be shaped and molded easilyThermoplastics: Melt when heated and can be melted again after coolingThermosets: Undergoes crosslinking when heated, so does not melt again, decomposes if heated furtherFlexible plastics: Plastics above their Tg. Flexible, softRigid plastics: Plastics below their glass transition temperature (Tg). Brittle, hardWhat are Tg, crosslinking and “melting for polymers” ?
14 Polymer Structure X Glass transition temperature (Tg) Different polymers have different segments on their backbones. The ease of movement of these segments (portions of the chain) depends on the structure, physical environment of the chain etc. of the segment.Any movement of these segments require energy which is kinetic in this case, right? Then each different polymer would have different energy requirement for the movement of these segments (different polymer = different structure, different physical environment of the chain etc).Below glass transition temperature, these segments do not have sufficient energy to move. So, if you apply some stress, say if you try to bend a polymer which is below its Tg then the segments won’t be able to move into new positions to relieve the stress which you have placed on them; which will make the polymer brittle. Above Tg they would, so they would be flexible.Always keep this in mind: Tg IS A PROPERTY RELATED WITH THE AMORPHOUS REGIONS OF THE POLYMER, NOT CRYSTALLINE!So it should now be obvious that elastomers are elastomers above their Tg. Below, they are not elastomers, they are glassy, because they are not flexible anymore (Remember my experiment with rubber glove and liquid nitrogen during the lecture).
15 Polymer Structure XI Melting Melting is a transition which occurs in crystalline polymers.Melting happens when the polymer chains fall out of their crystal structures, and become a disordered liquid.Always keep this in mind: MELTING IS A PROPERTY RELATED WITH THE CRYSTALLINE REGIONS OF THE POLYMER! So do you think you can melt atactic polystyrene? (No, because it is not crystalline)Question: What if I see both melting and glass transition in the differential scanning calorimeter (DSC) spectrum of a polymer sample???It is absolutely OK. Remember, most polymers are semi-crystalline, i.e. have both amorphous and crystalline regions
16 Crosslinking with sulphur Polymer Structure XIICrosslinkingPoly (1,4-butadiene)Crosslinking with sulphur(vulcanization)Synthetic rubberThis is the tire of your carThus, it is possible to produce elastomers via crosslinking!(In fact, it is not only possible but also the very common way of making elastomers, i.e. rubber)
17 Mechanical properties Now, I am sure that you can rationale the mechanical behavior of various types of polymers shown in this image.
18 Poly(dimethyl siloxane) How to make polymersStep growth(Condensations)Chain growth(Addition)MonomerMonomerMonomer (ethylene)InitiatorH2O outMonomerPolyethyleneThis is the plastic bag given in the supermarket(PET)Free radicalIonicThis is the bottle of your 1 lt cokeRing openingThis is the breast implant of your favorite female modelPoly(dimethyl siloxane)
19 Molecular weightNot all of the chains of a polymer are of same length. Their size differ most of the time.Remember: A chain is a polymer molecule (in fact the chain is the polymer itself), so the molecular weight of a polymer should in fact be the molar mass of a single polymer chain. However, since chains have different lengths (that is why we call polymers as polydisperse) we can only talk about averagesNumber average molecular weight =Weight average molecular weight =Polydispersity index = Mw / MnNi is the number of chains having molecular weights Mi
20 Configuration (Chain Structure) Copolymer(repeating units are more than one monomer type)homopolymer(repeating unit is always same monomer)