PolymersOverview of polymers

• Carbon and hydrogen atoms are basic building blocks (in hydrocarbons); other elements include chlorine, fluorine, nitrogen, oxygen, etc.

• Backbone of hydrocarbons are C-C covalent bonds: CH4, C2H6, C2H4, C2H2

Name Boiling points

Methane

Ethane

Propane

-161 C

-89 C

-42 C

Strongervan der Waalsbonds

Poly-”mer”

C C C C C CHHHHHH

HHHHHH

Polyethylene (PE)

mer

ClCl Cl

C C C C C CHHH

HHHHHH

Polyvinyl chloride (PVC)

mer

Polypropylene (PP)

CH3

C C C C C CHHH

HHHHHH

CH3 CH3

mer

Ethane vs. Ethylene vs. Acetylene

• Each carbon has four bonds with neighboring atoms• Single, double & triple bonds• Although polyethylene has single bonds, and looks like a

long version of ethane, its starting material is ethylene

Ethane Ethylene Acetylene

Types of Polymers

• Elastomer: e.g., rubber; can be stretched by large amounts and returns to its original length when stress is removed

• Thermoplastic: e.g., polyethylene, polystyrene; becomes soft when heated so that it can be shaped, and then becomes rigid again when cooled

• Thermosets: e.g., epoxy, polyester; they become more rigid when heated, due to crosslinking of chains due to covalent bonds

Flexibility of polymers

• In many polymers, the atoms are not bonded in a 3-dimensional network, but in a linear chain; for example, the “straight” polyethylene is zigzagging back and forth along its length; chain can twist, rotate and turn in all directions as long as certain angles between bonds are maintained

• Second reason for flexibility is that there are only secondary bonds between chains which are weak and so chains can slide over each other very easily

• To strengthen the material, we have to somehow prevent these chains from moving around (or “flopping” around) easily

Stiffening Polymers

• Cross-linking– form primary bonds between chains, and restrict the relative

movement of chains– Example: “vulcanization” of rubber by adding sulphur

• Chain stiffening– Replace some of the small hydrogen atoms on the chain with

large bulky atoms or groups of atoms (e.g., benzene rings: C6H6)

– can restrict the chain rotation and chain sliding over each other because of steric hindrance

• Crystallization• Chain length increase• Copolymer

Copolymers• A copolymer is a combination of 2 or more different

monomers (or mers) in one chain

• Each circle represents a monomer

• In this example, we have 2 different types of mers in one chain

• (a) Random copolymer• (b) Alternating copolymer• (c) Block copolymer• (d) Graft copolymer

Polyethylene

• Most common polymer found in milk containers, trash cans, dish pans, pipes, grocery bags, etc.

• Low density polyethylene (LDPE) & high density polyethylene (HDPE)

• Thermoplastic softens when heated and hardened when cooled, reversibly recycleable

Synthesis of polyethylene• The formation of polyethylene from ethylene occurs by a

process called addition polymerization and involves the use of high pressures, high temperatures and catalysts

• Catalytic reaction:

Synthesis of polyethylene (contd.)• There are also other possibilities: termination of polymerization

• Chain branching: hydrogen is replaced by an ethylene mer; chain branching can be controlled by temperature, pressure and catalysts

• Short-chain branching produces:– lower density material because of inefficient packing of chains– low crystalline content as crystallinity requires long, straight and parallel chains– weaker materials less bonding between chains in shorter chains

Rubber & High Impact Polystyrene (HIPS)

• Can we take advantage of these 2 synthetic materials to make a material that is strong, hard and ductile? The answer to this is HIPS

• How can we make high impact polystyrene? Blends

Natural rubber merSoft & ductile

Styrene mer (synthetic)Brittle

Butadiene mer (synthetic)Very ductile

Polystyrene-polybutadiene blends

• … but this produces very little improvement. Cracks propagate in only polystyrene region in such a blend

Polystyrene (PS)

Polybutadiene (PB)crack

• Mix polystyrene and rubber physically …

Copolymers• A copolymer is a combination of 2 or more different

monomers (or mers) in one chain

• Each circle represents a monomer

• In this example, we have 2 different types of mers in one chain

• (a) Random copolymer• (b) Alternating copolymer• (c) Block copolymer• (d) Graft copolymer

Polystyrene-polybutadiene copolymer blends - HIPS

• In HIPS, we still have polybutadiene rubber regions in a polystyrene matrix, but now we have styrene-butadiene graft copolymer added to the mix. The graft copolymer acts to join the two components in a much stronger manner because the grafted rubber chain prefers to be in the rubber region while the polystyrene chain wants to be in the polystyrene matrix

• Now, any crack that tries to pass around polybutadiene particles has to interact with the network of rubbery branches connecting the PS matrix to the PB particles. The result is the trapping of the crack round the PB particle and the crack spreads in all directions producing crazing of the PS in this area

• HIPS becomes very strong, rigid and tough and is used in the dashboard of your car, in the lining of your refrigerator, in computer and telephone housing, etc.

polystyrene

polybutadiene

Thermal Properties (Heat transfer)Subjecting a material to heat has 3 consequences:• Heat transfer or conduction• Thermal expansion• Too much heat will result in bond breakage; material destroyed

Types of heat transfer• Conduction: heat transfer by vibration of atoms & electrons; occurs in solids• Convection: heat transfer by vibration of atoms; occurs in liquids & gases• Radiation: heat transfer by electromagnetic waves; occurs in vacuum

Conduction in metals• Metals are best conductors of heat; heat is carried through movement of electrons• As metals have free electrons that can freely move, heat conducted very fast & efficiently• A metal feels cold to the touch because heat is transferred from our hand to the metal very

fast• Mechanism: heat is a form of energy; when exposed to a warm object, the electrons in a

metal start vibrating (kinetic energy); through collisions, this kinetic energy is passed on from electron to electron

• Any obstruction to the electron motion (such as defects) will cause heat to travel slowly; this is the reason stainless steel which contains a large number of alloying elements such as nickel, carbon, chromium conduct heat less than pure aluminum or copper

Conduction in other materials• Other materials with no free electrons (e.g., insulators – ceramics,

polymers) also conduct heat but not as well as metals

• Mechanism: vibration of atoms instead of electrons

• Vibration of atoms is much more restricted than vibration of electrons

• Vibration of atoms result in “waves” or “phonons” due to such restrictions

• Ceramics are more robust to heat than polymers; Why? Because ceramics are bonded strongly in 3-d, whereas polymers are strongly bonded in 1-d; so polymers tend to melt or soften when exposed to too much heat; polymers expand a lot though (more about this later)

• Best insulator is vacuum; next best is air; this is used in vacuum flasks, house insulation and winter jackets; important note: the material that holds vacuum or air must not melt or soften due to heat!

Thermal expansion

• Why do material expand when exposed to heat? [Notable exception: water, due to phase change]

• Due to asymmetry of atom vibrations

• Polymers expand more than metals more than ceramics; related to rigidity of bonds

• Bimetallic strip: used in thermostats and garage door openers

Ceramics

• Ceramics brittle because: • surface cracks• difficult to move dislocations

• Toughen ceramics:• flame polishing• maintain compression• fine grain size

• Thermal stresses• Corning Ware• Tempered glasses

Corning Ware ®• Regular glass is amorphous, but Corning Ware is poly-crystalline

• How do we make them?

– A glass object (e.g., a container or pan) is made first using conventional glass-forming techniques (blowing, drawing, pressing)

– A nucleating agent (normally TiO2) is added to the glass formula before the glass forming process

– Heat treatment – devitrification (removal of glassy state) – to induce crystallization. TiO2 acts as nuclei so that very fine SiO2 crystals can form. The result is 99% of small crystals and the remaining 1 % amorphous glass binds these crystals together to produce a solid, non-porous material

– The glass ceramic so produced has• Low thermal expansion coefficient• High fracture strength

Properties of Corning Ware

• Slightly smaller than regular glass: due to crystallinity (easy to pack efficiently if crystalline than amorphous)

• White or opaque: due to numerous crystals, light is scattered by grain boundaries and is not able to pass through efficiently

Visions Ware

• Grains even smaller than in Corning Ware, so that red light (longer wavelength than the grains) can go through, but blue light (shorter wavelength than the grains) gets scattered

• VIBGYOR small to large wavelength

• Similar to scattering of sunlight: why is sky blue away from the sun, but orange close to the setting sun?

Corelle ® glass: toughening & tempering for shatter proofing

• We make a 3-layer sandwich of two different glasses such that, on cooling, the one in the middle shrinks much more than the one forming the outer layers; thus, outer layers are held in compression, so cracks closed

• Tempered glass: made of same glass, and a simple trick is used to produce the layering effect;

– the sheet of glass is heated so that it is soft all through and the top and bottom surfaces are then cooled very quickly with a blast of cold air.

– The outer surfaces therefore contract and become hard. When they do this, the inside is still quite soft because heat does not travel quickly in these materials, i.e., the inside has not had time to solidify. It therefore flows and adjusts to the small dimensions of the outside.

– The soft inside glass then cools and also contracts, and in so doing pulls the outer layers which cooled first. As a result these outer layers are pulled into compression, resulting in crack closures and toughening

– These glasses are called safety glass and are used in sliding doors, side windows in cars, eyeglass lenses, etc.

Low thermal expansion glass

High thermal expansion glass

Low thermal expansion glass