Polymers

CHAPTER

11 Polymers

8-1

Introduction

• What is a polymer? Poly (many)

8-2

mer (repeat unit)

repeat unit

repeat unit

H H H H H H C C C C C C H H H H H H

H H H H H H C C C C C C H Cl H Cl H Cl

Polyethylene (PE)

Polyvinyl chloride (PVC)

repeat unit

H C H

H H C C CH3 H

H H C C CH3 H

Polypropylene (PP)

H C CH3

• Originally natural polymers were used – Wood – Cotton – Leather

– Rubber – Wool – Silk

Polymeric Materials Polymers

Plastics

Thermoplastics Can be reheated and formed into new materials

Elastomers

Thermosetting Plastics Cannot be reformed by reheating. Set by chemical reaction.

•

Wide range of properties. • Minimum finishing. • Minimum lubrication.

Remote Control

• Good insulation. • Light weight. • Noise Reduction.

Wafer bands Air intake manifold Figure 7.1

Polymer Composition

Most polymers are hydrocarbons – i.e. made up of H and C • Saturated hydrocarbons – Each carbon bonded to four other atoms H

H C

H

H C H

CnH2n+2

H

Polymer Composition

Unsaturated hydrocarbons •

Double & triple bonds relatively reactive – can form new bonds – Double bond – ethylene or ethene - CnH2n

H

H C C

H

H

H C C H • 4-bonds, but only 3 atoms bound to C’s – Triple bond – acetylene or ethyne - CnH2n-2

Isomerism

• Isomerism – two compounds with same chemical formula can have quite different structures Ex: C8H18 • n-octane H H H H H H H H H C C C C C C C C H

= H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3

H H H H H H H H

⇓ H3C ( CH2 ) CH3 6

• 2-methyl-4-ethyl pentane (isooctane) CH3

H3C CH CH2 CH CH3 CH2 CH3

Chemistry of Polymers

• Free radical polymerization R

+

H H H H

H H

C C

R C C

H H monomer (ethylene)

free radical

R C C

H H

+

initiation

H H

H H

H H H H

C C

R C C C C

H H

H H H H

propagation

dimer

• Initiator: example - benzoyl peroxide H H H C O O C H

H

2

C O H

=2R

Adapted from Fig. 14.1, Callister 7e.

Note: polyethylene is just a long HC - paraffin is short polyethylene

Bulk or Commodity Polymers

Molecular Weight • Molecular weight, Mi: Mass of a mole of chains. Lower M

total wt of polymer Mn = total # of molecules

M n = Σx i M i M w = Σw i M i Mw is more sensitive to higher molecular weights

higher M

•

Average molecular weight determined by special physical and chemical techniques.

Mm = •

∑ fi M i ∑ fi

Example:

M m = average molecular weight of thermoplastics. Mi = Mean molecular weight of each molecular range selected. fi = Weight fraction of the material having Molecular weights of a selected molecular Weight range.

Mm

= 19,550 1 = 19,550 g/mol

Molecular Weight Calculation

Example: average mass of a class Ni

Mi

# of students

mass (lb)

1 1 2 3 2 1

100 120 140 180 220 380

xi

wi

0.1 0.1 0.2 0.3 0.2 0.1

0.054 0.065 0.151 0.290 0.237 0.204

Mn 186 lb

Mw 216 lb

M n = ∑ xi Mi M w = ∑ w i Mi

Degree of Polymerization, n

n = number of repeat units per chain H H H H H H H H H H H H H C C (C C ) C C C C C C C C H

ni = 6

H H H H H H H H H H H H

Mn nn = ∑ x i ni = m

Mw nw = ∑ w i ni = m

where m = average molecular weight of repeat unit m = Σfi mi Chain fraction

mol. wt of repeat unit i

Molecular Structures

• Covalent chain configurations and strength:

secondary

bonding

Linear

Branched

Cross-Linked

Network

Direction of increasing strength Adapted from Fig. 14.7, Callister 7e.

Polymers – Molecular Shape

Conformation – Molecular orientation can be changed by rotation around the bonds – note: no bond breaking needed

Polymers – Molecular Shape

Configurations – to change must break bonds • Stereoisomerism H

H C C

H

H H

H R or

C C R

C C

H R

H H

A

A

C B

E

C

E

D

D mirror plane

B

Tacticity

Tacticity – stereoregularity of chain isotactic – all R groups on same side of chain

syndiotactic – R groups alternate sides

H H H H H H H H C C C C C C C C H R H R H R H R H H H R H H H R C C C C C C C C H R H H H R H H H H H H H R H H

atactic – R groups random

C C C C C C C C H R H R H H H R

cis/trans Isomerism

CH3

H C C

CH2

CH2

CH3

C C

CH2

CH2 H

cis

trans

cis-isoprene (natural rubber)

trans-isoprene (gutta percha)

bulky groups on same side of chain

bulky groups on opposite sides of chain

Copolymers

two or more monomers polymerized together • random – A and B randomly vary in chain • alternating – A and B alternate in polymer chain • block – large blocks of A alternate with large blocks of B • graft – chains of B grafted on to A backbone

random

alternating block

graft

Polymer Crystallinity

Ex: polyethylene unit cell • Crystals must contain the polymer chains in some way – Chain folded structure 10 nm

Polymers rarely 100% crystalline • Too difficult to get all those chains crystalline aligned region

• %

Crystallinity: % of material

that is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases. amorphous region

Polymer Crystal Forms

• Single crystals – only if slow careful growth

• Spherulites – fast growth – forms lamellar (layered) structures

Spherulite surface Nucleation site

Spherulites – crossed polarizers Maltese cross

Polymerization •

Chain growth polymerization: Small molecules covalently bond to form long chains (monomers) which in turn bond to form polymers. • Example: Ethylene H n

H Heat

C

C

H

H

Pressure Catalyst

n=degree of Polymerization (DP). (range: 3500-25000

H

H

C

C

H

H n Molecular mass of polymer(g/mol)

•

DP = Mass of a mer (g/mer) Functionality: Number of active bonds in a monomer.

Chain Polymerization - Steps •

Initiation:  A Radical is needed.  Example H2O2

In General

• One of free radicals react with ethylene molecule to form new longer chain free radical.

•

Propagation: Process of extending polymer chain by addition of monomers. R

• •

CH2

CH2 + CH2

CH2

R CH2 CH2 CH2 CH2

Energy of system is lowered by polymerization. Termination: By addition of termination free radical.  Combining of two chains  Impurities.

R(CH2 CH2)m + R’(CH2 CH2)n

R(CH2 CH2)m R (CH2

Coupling of two chains

CH2)n R’

•

Stepwise Polymerization: Monomers chemically react with each other to produce linear polymers and a small molecule of byproduct.

• Network polymerization: Chemical reaction takes place in more than two reaction sites (3D network).

Industrial Polymerization Raw Materials: Natural gas, Petroleum and coal

Granules, pellets, Polymerization powders or liquids.

•Bulk polymerization : Monomer and activator mixed in a reactor and heated and cooled as desired • Solution polymerization: Monomer dissolved in non-reactive solvent and catalyst. • Suspension polymerization: monomer and catalyst suspended in water. • Emulsion polymerization: Monomer and catalyst suspended in water along with emulsifier.

Figure 7.12

Polymer Additives

Improve mechanical properties, processability, durability, etc. • Fillers – Added to improve tensile strength & abrasion resistance, toughness & decrease cost – ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc. • Plasticizers – Added to reduce the glass transition temperature Tg – commonly added to PVC - otherwise it is brittle

• Stabilizers – Antioxidants – UV protectants • Lubricants – Added to allow easier processing – “slides” through dies easier – ex: Na stearate • Colorants – Dyes or pigments • Flame Retardants – Cl/F & B

Thermoplastics •

Polyethylene, polyvinyl chloride (PVC) polypropylene and polyesters account for most plastic materials sold.

Polyethylene • •

Clear to whitish translucent thermoplastic. Types  Low density  High Density  Linear low density Table 7.3

Figure 7.28

• Applications: containers, insulation, chemical tubing, bottles, water pond liners etc.

Polyvinyl Chloride and Copolymers • PVC is amorphous, does not recrystallize. • Chlorine atoms produce large dipole moments and also hinder electrostatic repulsion. • PVC homopolymer has high strength (7.5 to 9 KSI) and is brittle. • Compounding of PVC: Modifies and improves properties.  Plasticizers: Impart flexibility. Eg – Phthalate.  Heat Stabilizers: Prevent thermal degradation. Eg – lead and tin compounds.  Lubricants: Aid in melt flow of PVC. Eg – Waxes and fatty esters.  Fillers: Lower the cost. Eg – Calcium Carbonate.  Pigments : Give color.

Polypropylene H

H

C

C

• Methyl group substitute every other carbon atom in carbon polymer chain. • High melting (165-1770Cand heat deflection temperature.

H CH3 n •

Low density, good chemical resistance, moisture resistance and heat resistance. • Good surface hardness and dimensional stability. • Applications: Housewares, appliances, packaging, laboratory ware, bottles, etc.

Polystyrene H

H

C

C

H

•

n

• Phenyl ring present on every other carbon atom. • Very inflexible, rigid, clear and brittle. • Low processing cost and good dimensional stability. • Poor weatherability and easily attacked by chemicals.

Applications: Automobile interior parts, dials and knobs of appliances and housewares.

Polyacrylonitrile and Styrene-Acrylonitrile (SAN)

Polyacrylonitrile H C

H C

Does not Melt.

H C N n • High strength. • Good resistance to moisture and solvents. • Applications: sweaters and blankets. Commoner for SAN and ABS resins.

SAN •

Random amorphous copolymer of styrene and acrylonitrile. • Better chemical resistance, high heat deflection temperature, toughness and load bearing characteristics than polyester alone. • Applications: Automotive instrument lenses, dash components, knobs, blender and mixer bowls.

ABS •

ABS = Acrylonitrile + Butadiene + Styrene (Three monomers).

Table 7.4

• Applications: Pipe and fittings, automotive parts, computer and telephone housings etc.

Figure 7.31

Polymethyl Methacrylate (PMMA) •

An acrylic commonly known as Plexiglas. H CH3 C

C O

H •

C

• Rigid and relatively strong. • Completely amorphous and very transparent.

CH3 n

Applications: Glazing of aircraft, boats, skylights, advertising signs etc.

Fluoroplastics • •

Monomers have one or more atoms of fluorine. Polytetrafluoroethylene(PTFE): F F • Exceptionally resistant to Melting chemicals. Point C C • Useful mechanical properties 0 170 C at a wide temperature range. F F n • High impact strength but low tensile strength. • Good wear and creep resistance.

• Applications: Chemically resistant pipe, parts, molded electrical components, nonstick coating etc.

Polychlorotrifluroethylene (PCTFE) F

F

C

C

F

Cl n

Melting Point 2180C

• Chlorine atom substitutes for every fourth fluorine atom. •Can be extruded and mold easily.

Applications: Gaskets, chemical processing equipments, seals and electric components.

Engineering Thermoplastics • •

Low density, low tensile strength. High insulation, good corrosion resistance. Table 7.5

Polyamides (Nylons) •

Main chain structure incorporates repeating amide O H group. Amide linkage C N

• Processed by injection molding. • Examples:

Properties of Nylon •

High strength due to hydrogen bonding between molecular chain.

Figure 7.35

•

Flexibility of carbon chain contributes to molecular flexibility, low melt viscosity and high lubricity. • Applications: Electrical equipments, gears, auto parts, packaging etc.

Polycarbonate • High strength, toughness and dimensional stability. • Very high impact strength. • high heat deflection temperature. • Resistance to corrosion. •

Applications: Precision parts, cams, gears, helmets, power tool housings and computer terminals.

Phenyl Oxide Based Resins •

Produced by oxidative coupling of phenolic monomers.

• High rigidity, strength, chemical resistance, dimensional stability and heat deflection temperature. • Wide temperature range, low creep • High modulus. •

Applications: Electric connectors, TV tuners, small machine housing, dashboards and grills.

Acetals •

Strongest (68.9 Mpa) and stiffest (2820 Mpa) thermoplastics. 2 Types H • Homopolymers Polyoxymethylene • Copolymers mp: 1750C C O

• Excellent long term load carrying capacity n and dimensional stability. • Homopolymer is harder and rigid than copolymer. • Low wear and friction but flammable. • Applications: Fuel systems, seat belts, window handles of automobiles, couplings, impellers, gears and housing. H

Thermoplastic Polyesters • Phenylene ring provides rigidity. • Good strength and resistant to most chemicals. Good insulator: independent of temperature and humidity. • Applications: Switches, relays, TV tuner components, circuit boards, impellers, housing and handles.

Polysulfone and Polyphenylene Sulfide. •

Polysulfone: Phenylene ring provides high strength and rigidity. • Can be used for long time at high temperature.

•

Applications: Electrical connectors, cores, circuit boards, pollution control equipments. • Polyphenylene Sulfide:Mp: 2880C • Rigid and strong. S • Highly crystalline. n • •

No chemical can dissolve it below 2000C. Applications: Chemical process equipment, emission control equipment, electrical connectors.

Polyetherimide and Polymer Alloys • Polyetherimide:

• High heat and creep resistance and rigidity. • Good electric insulation. • Applications: High voltage circuit breaker housing, coils etc. • Polymer alloys: Mixture of structurally different homopolymers or copolymers optimizes properties. • Some degree of compatibility needed. • Example:- Bayblend MC2500 (ABS/Polycarbonate)

Thermosetting Plastics •

High thermal and dimensional stability, rigidity, resistance to creep, light weight.

Table 7.7

Phenolics • • • • • • •

Low cost, good insulating and mechanical properties. Produced by polymerization of phenol and formaldehyde. General purpose compounds: Usually wood flour filled to increase impact resistance. High impact strength compounds: Filled with cellulose and glass fibers. High electrical insulating compounds: Mineral (Mica) filled. Heat resistant compounds: Mineral filled. Applications: Wiring devices, auto transmission parts, plywood lamination, adhesives, shell molding.

Epoxy Resins •

Good adhesion, chemical resistance and mechanical properties. O Epoxide CH2 C group H

•

High molecular mobility, low shrinkage during hardening. • Applications: Protective and decorative coating, drum lining, high voltage insulators and laminates.

Unsaturated Polyesters • Have reactive double Carbon-Carbon covalent bonds. •

Low viscosity and can be reinforced with low viscosity materials. • Open mold lay up or spray up techniques are used to process many small parts. • Compression molding is used for big parts. • Applications: Automobile panels and body parts, boat hulls, pipes, tanks etc.

Amino Resins (Ureas and Melamines) •

•

Formed by reaction of formaldehydes with compounds having –NH2 group.

Combined with cellulose fillers to produce low cost products with good mechanical properties. • Applications: Electrical wall plates, molded dinnerware, buttons, control buttons, knobs, flooring etc.

Elastomers (Rubbers) •

Natural rubber: Produced from latex of Havea Brasiliensis tree. H C H

CH3 C

H C

H C H

n

• Vulcanization: Heating rubber with sulfur and lead carbonate. • Increases tensile strength. • Restricts molecular movement by crosslinking of molecules. Figure 7.41

Natural Rubber - Properties

Synthetic Rubbers • • • • • • • •

Styrene-Butadiene rubber (SBR): Most widely used. Greater elasticity than natural rubbers. Tougher and stronger, war Figure 7.44 resistant. Absorbs organic solvents and swell. Nitrile Rubbers: 55-82% Butadiene and 45-18% acrylonitrile. Resistance to solvents H Cl H H and wear. Less flexible. Polychloroprene: Increased resistance C C C C to oxygen, ozone, heat and weather. Low temperature flexibility, high cost. H Hn

Vulcanization of Polychloroprene Elastomers

2ZnCl2 + MgO OH H2O 2Zn + MgCl Cl X Silicone Rubbers: Wide temperature Si range. • Used in gaskets, X electric insulation etc.

CH3

• •

Example

O

Si n

CH3

O n

Mechanical Properties

• i.e. stress-strain behavior of polymers brittle polymer σFS of polymer ca. 10% that of metals plastic elastomer elastic modulus – less than metal

Strains – deformations > 1000% possible (for metals, maximum strain ca. 10% or less)

Tensile Response: Brittle & Plastic Near Failure

σ(MPa)

fibrillar structure

x brittle failure onset of necking

near failure

plastic failure

x Initial unload/reload

ε aligned, networked crosscase linked case

crystalline regions slide semicrystalline case

amorphous regions elongate

crystalline regions align

Tensile Response: Elastomer Case σ(MPa) x brittle failure

x

plastic failure

x

elastomer

ε initial: amorphous chains are kinked, cross-linked.

final: chains are straight, still cross-linked

Deformation is reversible!

• Compare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers)

Predeformation by Drawing • Drawing…(ex: monofilament fishline) -- stretches the polymer prior to use -- aligns chains in the stretching direction • Results of drawing: -- increases the elastic modulus (E) in the stretching direction -- increases the tensile strength (TS) in the stretching direction -- decreases ductility (%EL) • Annealing after drawing... -- decreases alignment -- reverses effects of drawing.

• Compare to cold working in metals!

Thermoplastics vs. Thermosets • Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene

T viscous liquid

mobile liquid

• Thermosets: -- large crosslinking (10 to 50% of mers) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin

crystalline solid

Callister, rubber Fig. 16.9 tough plastic

partially crystalline solid

Molecular weight

Tm Tg

T and Strain Rate: Thermoplastics • Decreasing T... -- increases E -- increases TS -- decreases %EL

• Increasing strain rate... -- same effects as decreasing T.

σ(MPa) 80 4°C 60

20°C

40

Data for the semicrystalline polymer: PMMA (Plexiglas)

40°C

20 0

60°C 0

0.1

0.2

ε

to 1.3 0.3

Deformation of Thermoplastics •

Below Tg Elastic deformation. Above Tg Plastic deformation. Elastic deformation

Elastic or plastic deformation

Plastic deformation

Melting vs. Glass Transition Temp.

What factors affect Tm and Tg? • •

Both Tm and Tg increase with increasing chain stiffness Chain stiffness increased by 1. 2. 3.

•

Bulky sidegroups Polar groups or sidegroups Double bonds or aromatic chain groups

Regularity – effects Tm only

Time Dependent Deformation • Stress relaxation test: -- strain to εο and hold. -- observe decrease in stress with time. tensile test

εo

for T5 > Tg. 10

rigid solid (small relax)

Er (10s) 3 in MPa 10

transition region

1

10

strain

10-1

σ(t)

10-3 (large relax)

time

• Relaxation modulus: σ(t ) E r (t ) = εo

• Data: Large drop in Er

viscous liquid

(amorphous polystyrene) Adapted from Fig. 15.7, Callister 7e. (Fig. 15.7 is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.)

60 100 140 180 T(°C) Tg

• Sample Tg(°C) values: PE (low density) PE (high density) PVC PS PC

- 110 - 90 + 87 +100 +150

Selected values from Table 15.2, Callister 7e.

Effects of Temperature on Strength • •

Thermoplastics soften as temperature increases. Strength dramatically decreases after Tg.

Figure 7.50

• Thermosets also become weaker but not viscous. • Thermosets are more stable at high temperature than thermoplastics.

Fracture of Polymers •

Thermosetting plastics Primarily brittle mode. • Thermoplastics ductile or brittle depending on the temperature.

Polymer Fracture Crazing ≅ Griffith cracks in metals – spherulites plastically deform to fibrillar structure – microvoids and fibrillar bridges form alligned chains

fibrillar bridges

microvoids

crack

Strengthening of Thermoplastics • Increasing average molecular mass increases strength upto a certain critical mass. • Degree of crystallinity increases strength, modulus of elasticity and density. • Chain slippage during permanent deformation can be hindered by introduction of pendant atomic groups to main carbon chain. • Strength can be increased by bonding highly polar atoms on the main carbon chain.

Strengthening of Thermoplastics (Cont..) •

Strength can be increased by introduction of oxygen and nitrogen atoms into main carbon chain. • Introduction of phenylene ring into main polymer chain with other elements increases strength. • Adding plastic fibers increases the strength. Figure 7.49 • Thermosetting plastics can be strengthened by reinforcements and creation of covalent bonds by chemical reaction during setting.

Processing of Plastics

• Thermoplastic – – can be reversibly cooled & reheated, i.e. recycled – heat till soft, shape as desired, then cool – ex: polyethylene, polypropylene, polystyrene, etc. • Thermoset – when heated forms a network – degrades (not melts) when heated – mold the prepolymer then allow further reaction – ex: urethane, epoxy

Processing Plastics - Molding

• Compression and transfer molding – thermoplastic or thermoset

Processing Plastics - Molding

• Injection molding – thermoplastic & some thermosets

Processing Plastics – Extrusion