1. Materials Engineering
Lecture 12:
Polymers

2. What is a polymer
C (+6) 2ē) 4ē) non-metal, capable of forming 4 covalent bonds C C
A polymer is natural or artificial organic compound containing repeating groups of atoms: (-CH2-CH2-)n polyethylene
Monomer is the precursor of polymer: CH2=CH2 (ethylene)
Mer is the structural unit of the polymer: (-CH2-CH2-)
Degree of polymerization: how many times the mer is repeated in the polymer molecule: n
Copolymer is a polymer obtained from two or more monomers

3. Most common polymers Natural polymers Modified polymers
Plants: wood, cotton, cotton-wool, cellulose, paper, flax;
Animals: wool, leather
Modified polymers
nitrocellulose, rubber
Synthetic polymers
Plastics
Synthetic rubbers

4. Molecular structure and shape
Macromolecule: polymer molecule containing a large number of mers and having the molecular weight of and the dimensions of some microns.
Linear polymer
Branched polymer
Cross-linked polymer
Network (3D) polymer
Branched polymer (x15,000)

5. State of aggregation Some polymers are liquid (PEG polyethyleneglycol)
Many are solid
Polymer crystallinity depends on the order of neighboring molecules
Crystallinity depends on the cooling rate (the slower the more crystalline) and on the molecular shape (crystalline structure can be easily obtained for linear polymers)

6. Semi-crystals
Semi-crystals have both crystalline and amorphous segment.
XRD analysis of polyethylene;
Both crystalline peaks (110, 200) and amorphous structure present

7. Liquid crystals
Liquid crystals are polymers which have high order even in liquids
Recommended additional reading

8. Plastics
Thermoplasts: reversibly soften on heating and harden on cooling.
Thermosets: irreversibly harden on initial heating.
Linear or branched are generally thermoplastics
Cross-linked or network are generally thermosets.

9. Elastomers (rubbers) Natural rubber: (-CH(CH3)-CH=CH-CH2-)n
Synthetic rubbers:
(-CHR-CH=CH-CH2-)n
Silicons:
(-SiR2-O-)n
Polyurethanes
(-CO-NH-R-CO-NH-R’-O-)n
Polysulfides:
(-(CH2)x-Sy-)n

10. Mechanical properties: stress-strain behavior

11. Viscosoelastic deformation

12. Creep and fracture Creep: slow deformation under constant stress
Fracture: special type of fracture: viscosoelastic fracture
Fatigue: some have fatigue limit (PMMA)

13. Viscosoelastic fracture

14. Factors influencing the mechanical properties of polymers
Molecular weight
Degree of crystallinity
Drawing
Heat treating
Elastomers: vulcanization
(sulfur addition; -S-S- bridges)

15. Melting and cooling of polymers

16. Polymer synthesis Polymerization Polycondensations Pressure Heating
Initiators
Polycondensations

17. Polymer additives Fillers: wood flour, silica flour, polymers
Plasticizers
Stabilizers (UV, oxidation)
Colorants
Flame retardants

18. The most common polymers
Very inert to aggressive media. Not for high temperatures
thermoplast
(-CH2-CH2-)n
polyethylene PE
(-CH(CH)3-CH2-)n
polypropylene PP
Very inert to aggressive media. Ideal for high temperatures ( º)
(-CF2-CF2-)n
Polytetrafluorethylene (teflon)
PTFE
Destructed by sunlight
(-CHCl-CH2-)n
polyvinylchloride
PVC
(-CHPh-CH2-)n
polystyrene PS
“Organic glass”
(-CH2-C(CH)3(COOCH3))n
polymethylmetacrylate
PMMA
Hard and stable
thermoset
(CH2-Ph(CH2-)CH2-)n
Phenolformaldehyde
Bakelite
Light, strong, but very susceptible to heat and chemicals
(-NH-(CH2)6-NH-CO-(CH2)4-CO-)n
Polyhexamethylene adipamide
Nylon
Light, strong
(-CO-Ph-COO-(CH2)2-O-)n
Poluethylene terephthalate
PET
Light, strong; organic glass
(-Ph-C(CH3)(CH3)-Ph-OOC-)n
polycarbonate
Polycarb

19. Musts of this lecture
Basic: monomer, mer, polymer, degree of polymerization
Polymer structure and state of aggregation; semicrystallines
Plastics: thermoplasts and thermosets
Elstomers
Viscosoelsticity and elasticity
Creep and fracture
Polymer synthesis: polymerization and polycondensation
Polymer additives
Main polymers