Materials Engineering Lecture 12: Polymers

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

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

Molecular structure and shape Macromolecule: polymer molecule containing a large number of mers and having the molecular weight of 105-109 and the dimensions of some microns. Linear polymer Branched polymer Cross-linked polymer Network (3D) polymer Branched polymer (x15,000)

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)

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

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

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.

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

Mechanical properties: stress-strain behavior

Viscosoelastic deformation

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

Viscosoelastic fracture

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

Melting and cooling of polymers

Polymer synthesis Polymerization Polycondensations Pressure Heating Initiators Polycondensations

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

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 (600-800º) (-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

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