POLYMERS-Part 2

Properties and Production

MSE 226-Engineering Materials

Lecture-12

Molecular Structure

Direction of increasing strength

B ranched Cross-Linked Network Linear

secondary bonding

The physical properties of polymers are affected from the structure of

polymer chains

Polyethylene,

PVC, nylon Rubber Epoxies,

polyurethane

Branches lower polymer density

i.e. High density polyethylene (HDPE): linear polymer

Low density polyethylene (LDPE) : brached polymer

Wan der walls

or hydrogen

Adapted from Fig. 14.13,

Callister & Rethwisch 8e.

Polymer Crystallinity

Many bulk polymers that are crystallized from melt are semicrystalline

and forma spherulite structure. (composed of lamellar crystal and

amorhous material)

(Polyethylene, PVC, PP, nylon)

Mechanical Properties

Metals vs. Polymers

Polymers have very low elastic modulus and tensile strength

FS (fracture strength) of polymer 10% that of metals

Polymers elongate more than 1000% possible, but metals rarely

plastically elongate more about 100%.

Polymers become more softer and ductile as the rate of deformation

increases.

Mechanical properties of polymers are much sensetive to temperature

changes near room temperature.

Stress-strain behavior of polymers

brittle polymer

plastic

Elastomer (totally elastic)

Adapted from Fig. 15.1,

Callister 7e.

Mechanical Properties

Yield

point

Tensile strength

corresponds to stress

at fracture

brittle failure

plastic failure

(MPa)

e

x

x

crystalline

regions

slide

fibrillar

structure

near

failure

crystalline

regions align

onset of

necking

Initial

Near Failure

semi-

crystalline

case

aligned,

cross-

linked case

Networked

case

amorphous

regions elongate

unload/reload

Tensile Responce: Brittle&Plastic

Deformation of semicrystalline polymer (spherulitic structure)

Epoxies,

polyurethane

(Polyethylene, PVC, PP, nylon)

Results of drawing:

• increases the elastic modulus (E) in the stretching direction

• increases the tensile strength (TS) in the stretching direction

• decreases ductility (%EL)

Predeformation by Drawing

In plastic deformation region, crystalline blocks and tie chain

align in the direction of tensile axis. This process is called

DRAWING. Used for improving the mechanical properties

of polymer fibers and films. i.e. Monofilament fishline

Annealing after drawing...

• Specimen is heated to high temp., near its melting point

• The material will recrystallize to form spherulitic structure

• Reverses effects of drawing.

• Comparable to cold working in metals!

Factors Effecting the Mechanical Properties

of Semi-crystalline Polymers

1- Temperature and Strain rate

2- Molecular weight

3- Degree of crystallinity

4- Predeformation by drawing

5- Heat treating

• Decreasing temperature

-- increases E

-- increases TS

-- decreases %EL

20

4 0

6 0

8 0

0 0

0.1 0.2 0.3

4°C

20°C

40°C

60°C

(MPa)

e

Data for the semicrystalline polymer: PMMA (Plexiglas)

Temperature and Strain Rate:

Factors Effecting the Mechanical Properties

of Semi-crystalline Polymers

• Increasing strain rate...

-- same effects as decreasing T.

Molecular weight and Degree of Crystallinity

Factors Effecting the Mechanical Properties

of Semi-crystalline Polymers

TS increases with increasing molecular weight

Crystallinity affect the extent of the intermolecular secondary bonding.

Increasing crystallinity: Strength is enhanced but polymer become

brittle

For example: for polyethylene modulus increases an order of magnitude

as crystallinity is raised from 0.3 to 0.6

Predeformation by Drawing

Strength and modulus can be increased by deformation in tension

(drawing)

Molecular chains slip past one another and become highly oriented

Employed in the production of FIBERS and FILMS

Factors Effecting the Mechanical Properties

of Semi-crystalline Polymers

Annealing

Causes increase in the %crystallinity, crystallite size

For undrawn polymer annealing heat treatment causes;

1-an increase in tensile modulus and yield strength

2-reduction in ductility

SHRINK-WRAP POLYMER FILMS

PVC, PE or polyolefin plastically deformed

about 20-300% to provide aligned film. Then

film is wrapped around object to be packaged.

When heated to about 100-150oC, this

prestrecthed film shrinks to recover 80-90% of

its initial deformation, which gives tightly

strecthed film.

Stress-strain curves

adapted from Fig. 15.1,

Callister 7e. Inset

figures along elastomer

curve (green) adapted

from Fig. 15.15, Callister

7e. (Fig. 15.15 is from

Z.D. Jastrzebski, The

Nature and Properties of

Engineering Materials,

3rd ed., John Wiley and

Sons, 1987.)

(MPa)

e

initial: amorphous chains are kinked, cross-linked.

x

final: chains are straight,

still cross-linked

elastomer

Deformation is reversible!

brittle failure

plastic failure x

x

Tensile Response: Elastomer Case

Elastomer: Rubber like elasticity

Ability to deform to quite large deformations and elastically spring back to their

original shape

Cross-linked

polymer

Unstrained Case: Elastomer is amorphous and

composed of crosslinked molecular

chains (twisted, linked and coiled)

Elastic Deformation: Chains elongate in the stress

direction and uncoiling,

untwisting occurs

Release of load: Chains spring back

Tensile Response: Elastomer Case

For a polymer to be elastomeric it mustn’t easily crystallize (they are

amorphous)

Elastomer behavior seen in cross-linked polymers. Cross-linking occurs

during vulcanization process.

Vulcanization process: Sulphur atoms are added to heated polymer

Chains of sulphur atoms bond with adjacent polymer backbone chains and

crosslink them.

ε

unvulcanized

vulcanized Elastic modulus, tensile strength

and oxidation resistance

increase by vulcanization

Polymer Fracture

Thermosetting Polymers (heavily cross-linked network):

The mode of fracture is brittle

Thermoplastic Polymers

Capable of experiencing ductile to brittle transition

Both ductile and brittle modes are possible

Thermoplastics are brittle when

- temp.< Tg (glass transition temp)

- strain rate increased

- there is sharp notch

- Specimen thickness is increased

Ductile in the vicinity of Tg

fibrillar bridges microvoids crack

alligned chains

Adapted from Fig. 15.9,

Callister 7e.

Crazing Griffith cracks in metals

Crazing of Thermoplastic Polymers

- Crazes are regions of very localized plastic deformation, which lead to the

formation of small and interconnected microvoids

– spherulites plastically deform to fibrillar structure

– microvoids and fibrillar bridges form

Polymer Fracture

What factors affect Tm and Tg?

• Both Tm and Tg increase with increasing chain stiffness

• Chain stiffness increased by

1. Bulky sidegroups

2. Polar groups or sidegroups

3. Double bonds or aromatic chain groups

• Regularity (tacticity) – affects Tm only

Adapted from Fig. 15.18,

Callister 7e.

Melting and Glass Transition Temp.

Melting and glass transition temperature determine the upper and

lower temperature limits for numerous applications, especially for

semicrystalline polymers

• Thermoplastics: -- little crosslinking

-- ductile

-- soften w/heating

-- polyethylene

polypropylene

polycarbonate

polystyrene

• Thermosets: -- large crosslinking

(10 to 50% of mers)

-- hard and brittle

-- do NOT soften w/heating

-- vulcanized rubber, epoxies,

polyester resin, phenolic resin

Adapted from Fig. 15.19, Callister 7e. (Fig. 15.19 is from F.W. Billmeyer,

Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc.,

1984.)

Callister, Fig. 16.9

T

Molecular weight

Tg

Tm mobile liquid

viscous liquid

rubber

tough plastic

partially crystalline solid crystalline

solid

Melting and Glass Transition Temp.

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

Polymer Additives

• 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

Polymer Additives

Polymer Types

Depending on the end use polymers are groupped as;

- Plastics

- Elastomers (or rubbers)

- Fibers

- Coatings

- Adhesives

- Foams

- Films

• Compression and transfer molding

– thermoplastic or thermoset

Adapted from Fig. 15.23,

Callister 7e. (Fig. 15.23 is from

F.W. Billmeyer, Jr., Textbook of

Polymer Science, 3rd ed.,

John Wiley & Sons, 1984. )

Processing of Plastics: Molding

Plastics:

- Have some structural rigidity

- Usually have high degree of crystallinity

- If amorphous: use below Tg; if semicrystalline: use below Tm

- Thermoplastics or thermosets (PE, PP, PVC, epoxies, phenolics)

• Injection molding

– thermoplastic & some thermosets Adapted from Fig. 15.24,

Callister 7e. (Fig. 15.24 is from

F.W. Billmeyer, Jr., Textbook of

Polymer Science, 2nd edition,

John Wiley & Sons, 1971. )

Processing of Plastics: Molding

Adapted from Fig. 15.25,

Callister 7e. (Fig. 15.25 is from

Encyclopædia Britannica, 1997.)

Processing of Plastics: Extrusion

Elastomers – rubber

• Crosslinked materials

– Natural rubber

– Synthetic rubber and thermoplastic elastomers

• SBR- styrene-butadiene rubber styrene

– Silicone rubber

butadiene

Polymer Types (acc. to end use)

Fibers - length/diameter >100

• Textiles are main use

– Must have high tensile strength

– Usually highly crystalline & highly polar

• Formed by spinning

– ex: extrude polymer through a spinnerette

• Pt plate with 1000’s of holes for nylon

• ex: rayon – dissolved in solvent then pumped through

die head to make fibers

– the fibers are drawn

– leads to highly aligned chains- fibrillar structure

Polymer Types: Fibers

• Coatings – thin film on surface – i.e. paint, varnish

– To protect item

– Improve appearance

– Electrical insulation

• Adhesives – produce bond between two adherands

– Usually bonded by:

1. Secondary bonds

2. Mechanical bonding

• Films – blown film extrusion

• Foams – gas bubbles in plastic

Polymer Types

Blown-Film Extrusion

Adapted from Fig. 15.26, Callister 7e.

(Fig. 15.26 is from Encyclopædia

Britannica, 1997.)

Blown Film- Extrusion

• Ultrahigh molecular weight

polyethylene (UHMWPE)

– Molecular weight

ca. 4 x 106 g/mol

– Excellent properties for

variety of applications

• bullet-proof vest, golf ball

covers, hip joints, etc.

UHMWPE

Adapted from chapter-

opening photograph,

Chapter 22, Callister 7e.

Advanced Polymers

The Stem, femoral head, and the AC socket are made from Cobalt-chrome metal alloy or ceramic, AC

cup made from polyethylene

• General drawbacks to polymers: -- E, y, Kc, Tapplication are generally small.

-- Deformation is often T and time dependent.

-- Result: polymers benefit from composite reinforcement.

• Thermoplastics (PE, PS, PP, PC):

-- Smaller E, y, Tapplication -- Larger Kc

-- Easier to form and recycle

• Elastomers (rubber):

-- Large reversible strains!

• Thermosets (epoxies, polyesters):

-- Larger E, y, Tapplication

-- Smaller Kc

Summary