1. CHAPTER 14/15: POLYMER STUDIES
Issues:
• What are the basic microstructural features of a polymer?
• How are polymer properties affected by molecular weight?
• How do polymeric materials accommodate the polymer chain?
• What are the tensile properties of polymers and how are they affected by basic microstructural features?
• Changing Polymer Properties: Hardening, anisotropy,
and annealing in polymers.
• How does the elevated temperature mechanical response of polymers compare to ceramics and metals?
• What are the primary polymer processing methods?

2. Chapter 14 – Polymers What is a polymer? Poly mer many repeat unit C H
Polyethylene (PE) Cl C H
Polyvinyl chloride (PVC) H
Polypropylene (PP) C
CH3
Adapted from Fig. 14.2, Callister 7e.

3. Ancient Polymer History
Originally many natural polymers were used
Wood – Rubber
Cotton – Wool
Leather – Silk
Oldest known uses of “Modern Polymers”
Rubber balls used by Incas
Noah used pitch (a natural polymer) for the ark – as had all ancient mariners!

4. Polymer Composition Most polymers are hydrocarbons
– i.e. made up of H and C
(we also recognize Si-H ‘silicones’)
Saturated hydrocarbons
Each carbon bonded to four other atoms
CnH2n+2

6. Unsaturated Hydrocarbons
Double & triple bonds relatively reactive – can form new bonds
Double bond – ethylene or ethene - CnH2n
4-bonds, but only 3 atoms bound to C’s
Triple bond – acetylene or ethyne - CnH2n-2

7. Isomerism
Isomerism
two compounds with same chemical formula can have quite different structures
Ex: C8H18
n-octane
2-methyl-4-ethyl pentane (isooctane) 

8. Chemistry of Polymers Free radical polymerization
Initiator: example - benzoyl peroxide

9. Chemistry of Polymers Initiator: example - benzoyl peroxide
Free radical polymerization (addition polymerization)
Initiator: example - benzoyl peroxide

10. Condensation Polymerization
Water is “Condensed out” during polymerization of Nylon
Some of the original monomer’s materials are shed (condensed out) during polymerization process
Process is conducted in the presence of a catalyst
Water, CO2 are commonly condensed out but other compounds can be emitted including HCN or other acids

11. Bulk or Commodity Polymers
Relatively few polymers responsible for virtually all polymers sold – these are the bulk or commodity polymers

13. NOTE: See Table 15.3 for commercially important polymers – including trade names

17. MOLECULAR WEIGHT • Molecular weight, Mi: Mass of a mole of chains.
Lower M
higher M
Simple for small molecules
All the same size
Number of grams/mole
Polymers – distribution of chain sizes
Mw is more sensitive to higher molecular weights
Adapted from Fig. 14.4, Callister 7e.

18. Molecular Weight Calculation
Example: average mass of a class

19. Degree of Polymerization, n
n = number of repeat units per chain
ni = 6
Chain fraction
mol. wt of repeat unit i

20. Molecular Structures • Covalent chain configurations and strength:B
ranched
Cross-Linked
Network
Linear
secondary
bonding
Direction of increasing strength
Adapted from Fig. 14.7, Callister 7e.

21. Polymers – Molecular Shape
Conformation – Molecular orientation can be changed by rotation around the bonds
note: no bond breaking needed
Adapted from Fig. 14.5, Callister 7e.

22. Polymers – Molecular Shape
Configurations – to change must break bonds
Stereoisomerism
mirror plane

23. Tacticity Tacticity – stereoregularity of chain
isotactic – all R groups on same side of chain
syndiotactic – R groups alternate sides
atactic – R groups random

24. cis/trans Isomerism cis trans cis-isoprene (natural rubber)
bulky groups on same side of chain
trans
trans-isoprene (gutta percha)
bulky groups on opposite sides of chain

25. Copolymers two or more monomers polymerized together
Adapted from Fig. 14.9, Callister 7e.
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
A – B –
random
alternating
block
graft

26. Polymer Crystallinity
Adapted from Fig , Callister 7e.
Ex: polyethylene unit cell
Crystals must contain the polymer chains in some way
Chain folded structure
Adapted from Fig , Callister 7e.

27. Polymer Crystallinity
amorphous
region
Polymers rarely exhibit 100% crystalline
Too difficult to get all those chains aligned
crystalline
region
• % Crystallinity: how much is crystalline.
-- TS and E often increase
with % crystallinity.
-- Annealing causes
crystalline regions
to grow. % crystallinity
increases.
Adapted from Fig , Callister 6e.
(Fig is from H.W. Hayden, W.G. Moffatt,
and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)

28. Mechanical Properties
i.e. stress-strain behavior of polymers
brittle polymer
FS of polymer ca. 10% that of metals
plastic
elastomer
elastic modulus – less than metal
Adapted from Fig. 15.1, Callister 7e.
Strains – deformations > 1000% possible (for metals, maximum strain ca. 100% or less)

29. Tensile Response: Brittle & Plastic
(MPa)
fibrillar structure
near
failure
Initial
Near Failure x
brittle failure
onset of
crystalline
regions align
necking
plastic failure x
crystalline
regions
slide
amorphous
regions
elongate
aligned,
cross-
linked
case
networked
unload/reload e
semi-
crystalline
case
Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from Figs & 15.13, Callister 7e. (Figs & are from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp )

30. 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
-- decreases ductility (%EL)
• Annealing after drawing...
-- decreases alignment
-- reverses effects of drawing.
• Comparable to cold working in metals!
Adapted from Fig , Callister 7e. (Fig is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp )

31. Tensile Response: Elastomer Case
(MPa)
final: chains
are straight,
still
cross-linked x
brittle failure
Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along elastomer curve (green) adapted from Fig , Callister 7e. (Fig is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)
plastic failure x x
elastomer
initial: amorphous chains are
kinked, cross-linked.
Deformation
is reversible! e
• Compare to responses of other polymers:
-- brittle response (aligned, crosslinked & networked polymer)
-- plastic response (semi-crystalline polymers)

32. Thermoplastics vs. Thermosets
Callister,
Fig. 16.9 T
Molecular weight Tg Tm
mobile
liquid
viscous
rubber
tough
plastic
partially
crystalline
solid
• 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 , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.)

33. T and Strain Rate: Thermoplastics
(MPa)
• Decreasing T...
-- increases E
-- increases TS
-- decreases %EL
• Increasing
strain rate...
-- same effects
as decreasing T. 20 4 6 8
Data for the
4°C
semicrystalline
polymer: PMMA
20°C
(Plexiglas)
40°C
to 1.3
60°C e
0.1
0.2
0.3
Adapted from Fig. 15.3, Callister 7e. (Fig is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.)

34. Melting vs. Glass Transition Temp.
What factors affect Tm and Tg?
Both Tm and Tg increase with increasing chain stiffness
Chain stiffness increased by
Bulky sidegroups
Polar groups or sidegroups
Double bonds or aromatic chain groups
Regularity (tacticity) – affects Tm only
Adapted from Fig , Callister 7e.

35. Time Dependent Deformation
• Stress relaxation test:
• Data: Large drop in Er
for T > Tg.
(amorphous
polystyrene)
Adapted from Fig. 15.7, Callister 7e. (Fig is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.) 10 3 1 -1 -3 5 60
100
140
180
rigid solid
(small relax)
transition
region
T(°C) Tg
Er (10s)
in MPa
viscous liquid
(large relax)
-- strain to eo and hold.
-- observe decrease in
stress with time.
time
strain
tensile test eo
s(t)
• Relaxation modulus:
• 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.

36. Polymer Fracture Crazing  Griffith cracks in metals
– spherulites plastically deform to fibrillar structure – microvoids and fibrillar bridges form
fibrillar bridges
microvoids
crack
alligned chains
Note: crack spreads by breaking C-C bonds
Ductile plastics have large craze zones that absorb large amounts of energy as it spreads
Adapted from Fig. 15.9, Callister 7e.

37. 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
Polymers are almost never used as a pure material
Migration of plasticizers can be a problem

38. 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

39. 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
Can be brittle or flexible & linear, branching, etc.

40. Processing Plastics - Molding
Compression and transfer molding
thermoplastic or thermoset
Male & female parts to mold
Adapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed.,
John Wiley & Sons, )

41. Processing Plastics - Molding
Injection molding
thermoplastic & some thermosets
Adapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition,
John Wiley & Sons, )
Polymer is melted in chamber & then the molten polymer is forced under pressure into the die

43. Processing Plastics – Extrusion
An extruder is a device that used a large screw to melt a polymer, compress it, & force it into a mold
Extremely commonly used
Adapted from Fig , Callister 7e. (Fig is from Encyclopædia Britannica, 1997.)

44. Polymer Types: Elastomers
Elastomers – rubber
Crosslinked materials
Natural rubber
Synthetic rubber and thermoplastic elastomers
SBR- styrene-butadiene rubber
styrene
butadiene
– Silicone rubber

45. Polymer Types: Fibers 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

46. Polymer Types 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:
Secondary bonds
Mechanical bonding
Films – blown film extrusion
Foams – gas bubbles in plastic

47. Blown-Film Extrusion
Adapted from Fig , Callister 7e. (Fig is from Encyclopædia Britannica, 1997.)

48. Advanced Polymers 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.

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

50. ABS – A Polymerized “Alloy”
ABS, Acrylonitrile-Butadiene-Styrene
Made up of the 3 materials: acrylonitrile, butadiene and styrene. The material is located under the group styrene plastic. Styrene plastics are in volume one of the most used plastics.
Properties
The mechanical properties for ABS are good for impact resistance even in low temperatures. The material is stiff, and the properties are kept over a wide temperature range. The hardness and stiffness for ABS is lower than for PS and PVC.
   
Weather and chemical resistance
The weather resistance for ABS is restricted, but can be drastically improved by additives as black pigments. The chemical resistance for ABS is relatively good and it is not affected by water, non organic salts, acids and basic. The material will dissolve in aldehyde, ketone, ester and some chlorinated hydrocarbons.
Processing
ABS can be processed by standard mechanical tools as used for machining of metals and wood. The cutting speed need to be high and the cutting tools has to be sharp. Cooling is recommended to avoid melting of the material. If the surface finish is of importance for the product, the ABS can be treated with varnish, chromium plated or doubled by a layer of acrylic or polyester. ABS can be glued to it self by use of a glue containing dissolvent. Polyurethane based or epoxy based glue can be used for gluing to other materials.

51. A Processing Movie:
Calloway Golf:

52. Summary • General drawbacks to polymers:
-- E, sy, 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, sy, Tapplication
-- Larger Kc
-- Easier to form and recycle
• Elastomers (rubber):
-- Large reversible strains!
• Thermosets (epoxies, polyesters):
-- Larger E, sy, Tapplication
-- Smaller Kc
And Remember:
Table 15.3 Callister 7e
Is a Good overview of applications and trade names of polymers.