1. ME 330 Engineering Materials
Lecture 20
Polymers
Polymer Structure
Mer Structure
Molecular Structure and Shape
Copolymers
Please read Chapter 14

2. Learning Objectives (Ch. 14)
After careful study of this chapter you should be able to do the following:
Describe a typical polymer molecule in terms of its chain structure, and, in addition, how the molecule may be generated by repeating mer units.
Draw mer structures for polyethylene, polyvinyl chloride, polytetrafluoroethylene, polypropylene, and polystyrene.
Calculate number-average and weight-average molecular weights, and number-average and weight-average degrees of polymerization for a specified polymer.
Name and briefly describe:
four general types of polymer molecular structures;
three types of stereoisomers;
two kinds of geometrical isomers;
four types of copolymers.
Briefly describe the crystalline state in polymeric materials.
Briefly describe/diagram the spherulitic structure for a semicrystalline polymer.

3. Polycarbonate (LEXAN)
Natural vs. Synthetic
Natural
Synthetic
Rubber
Cellulose
Starch
Silk
Spider Web
Cotton
Wool
Leather
Wood
Proteins
PVC
PMMA (Plexiglas)
HDPE
LDPE
ABS
Nylon
Polycarbonate (LEXAN)
Epoxy
Polyester
Acrylic
Interesting Website :

4. Organic Materials Most natural and synthetic polymers are organic
Carbon based compounds
Made of most abundant elements in the periodic table
Valence - how many additional bonds needed to fill outer shell
4 - C, Si
3 - N
2 - S, O
1 - H, F, Cl, Br, I
Most polymers involve carbon backbone, focus on this ...

5. Polymer Materials
Poly-mer [many - meros (Greek, parts)] – long molecule consisting of many smaller monomers chemically joined
Built from a basic structure (monomer) joined in the same way
Often chain-like
Hydrocarbon molecule most common backbone
Strong primary bonds in chain (Covalent)
Weak secondary bonds between chains (Van der Waals)
Polyethylene C H
Ethylene: C2H4 C H

6. Hydrocarbon Review Covalent bond between carbon and hydrogen atoms
Carbon has 4 electrons to share
Hydrogen has 1 electron to share
Saturated hydrocarbon - carbon bonded to 4 other atoms
Unsaturated hydrocarbon - carbon not bonded to 4 other atoms
Secondary bonds hold chains together
Low melting temperatures that increase with molecular weight H C H C C H C H

7. From Hydrocarbons to Polymers
Saturated  all bonds are single; molecule not reactive
If unsaturated, double bond present, and catalyst (R) will react:
Often indicate polymers by:
Basic repeating structure
n > 1000! C H R* + H H
+ C H C R C C* *
+…  Polyethylene H H
Ethylene
mer
mer
Polymer C H n

8. A Few Definitions Homopolymer - all mers are of the same type
Copolymers - two or more mer units of different structure
Bifunctional - mer units with two active sites available for covalent bonding
Trifunctional - mer units with three active sites available for covalent bonding
Aromatic ring - H C

11. Molecular Weight
Molecular weight indicates the size of a polymer molecule
Measure polymer size distribution
Number-average Weight-average
Count the number of chains that are
a certain length (molecular weight)
Weigh the number of chains that are
a certain length (molecular weight)
Molecular weight (103 g/mol)
0.1
0.2
0.3
Weight fraction 10 20 30 40 w1 M1
Molecular weight (103 g/mol)
0.1
0.2
0.3
Number fraction 10 20 30 40 x1 M1

12. Degree of Polymerization
Average number of mers per chain
is the mer molecular weight
Number average degree of polymerization
Weight average degree of polymerization

13. Problem 14.4 (Callister)
(a) Compute the repeat unit molecular weight of polystyrene.
(b) Compute the number-average molecular weight for a polystyrene for which the degree of polymerization is 25,000.

15. Solution
(a) The repeat unit molecular weight of polystyrene can be calculated as follows. For polystyrene, from Table 14.3, each repeat unit has eight carbons and eight hydrogens. Thus,
m = 8(AC) + 8(AH)
= (8)(12.01 g/mol) + (8)(1.008 g/mol) = g/mol

16. Solutions (cont) We are now asked to compute the number-average
molecular weight. Since the degree of polymerization
is 25,000, using Equation 14.6

17. Prob in Callister
Molecular weight data for some polymer are tabulated here. Compute:
Compute the number-average molecular weight.
Compute the weight-average molecular weight.
(c) If it is known that this material's degree of polymerization is 710,
which one of the polymers listed in Table 14.3 is this polymer? Why?

18. Solution (a) From the tabulated data, we are asked to compute
the number-average molecular weight.
Molecular wt.
Range Mean Mi xi xiMi
15,000-30,000 22,
30,000-45,000 37,
45,000-60,000 52,
60,000-75,000 67, ,550
75,000-90,000 82, ,800
90, ,000 97, ,700
105, , ,
120, , ,
_________________________

19. (b) From the tabulated data, we are asked to compute
the weight-average molecular weight.
Molecular wt.
Range Mean Mi wi wiMi
15,000-30,000 22,
30,000-45,000 37,
45,000-60,000 52,
60,000-75,000 67, ,200
75,000-90,000 82, ,275
90, ,000 97, ,600
105, , , ,500
120, , ,
_________________________

20. We are now asked if the degree of polymerization is 710,
which of the polymers in Table 14.3 is this material?
It is necessary to compute m in Equation 14.6 as
Answer: Polysterene.

21. Molecular Structure Linear Tangled covalently bonded chains
Secondary interchain bonds
Branched
Tangled covalently bonded chains with side branches
Crosslinked
Tangled covalently bonded chains
Covalent and secondary interchain bonds
Networked
Highly crosslinked polymer

22. Polymer Classifications I
Thermosetting polymers:
Become permanently hard
Will not soften before burning
Heavily networked or crosslinked.
Hard, strong, brittle
Thermoplastic polymers:
Soften when heated, harden when cooled.
Will degrade if heat is sufficient to break primary bonds along chains.
Reversible changes in secondary bonding between chains.
Generally soft and ductile.
Elastomers:
Can be classified as either lightly crosslinked thermoset or thermoplastic
Main property: returns to reference state when load is released.
Can deform to VERY large strains.

23. Copolymers
Random:
Alternating:
Block
Graft copolymer:
Mer #1
Mer #2

24. Polymer Classifications II
Crystalline:
Periodic 3-D, repeating array of molecules
Amorphous:
Literally “without structure”
No repeating array
Semi-crystalline:
Structure containing regions of both crystalline and amorphous molecular arrangements

25. Polymer Crystallization
Akin to grain formation in metals
Involves entire molecules instead of individual atoms or ions
Consider a polymer cooling from melt at three different rates
Note inflection points at:
Melting temperature (Tm)
Glass transition temperature (Tg)
Specific Volume Tg Tm
Temperature

26. Polymer Crystallization
Fast cooling ( )
Molecules do not have time to reorient into ordered grouping (diffusion)
Change in volume TG  amorphous structure
Slow cooling ( )
Molecules have time to reorient during solidification
Change in volume at melting temperature -
More ordered upon solidificaiton,
Takes up less space
No kink at TG  crystalline structure
Moderate cooling ( )
Both crystalline and amorphous structure
Kink at Tm & Tg due to crystalline and amorphous, respectively
Degree of crystallinity:

27. Crystallographic Models

28. Grain Formation (Spherulites)

29. ME 330 Engineering Materials
Lecture 21
Polymers
Mechanical Properties
Deformation and Strengthening
Thermal Properties
Viscoelasticity
Polymerization
Processing Techniques
Read Chapter 15

30. Learning Objectives (Ch 15)
Make schematic plots of the three characteristic stress–strain behaviors observed for polymeric materials.
Describe/sketch the various stages in the plastic deformation of a semicrystalline (spherulitic) polymer.
Discuss the influence of the following factors on polymer tensile modulus and/or strength:
molecular weight
degree of crystallinity
predeformation
heat treating of undeformed materials.
List four characteristics or structural components of a polymer that affect both its melting and glass-transition temperatures.
Cite the differences in behavior and molecular structure for thermoplastic and thermosetting polymers.
Describe the molecular mechanism by which elastomeric polymers deform elastically.
Briefly describe addition and condensation polymerization mechanisms.
Name five types of polymer additives and, for each, indicate how it modifies the properties.
Cite the seven different polymer application types and, for each, note its general characteristics.
Name and briefly describe five fabrication techniques used for plastic polymers.

31. Mechanical Properties
Mechanical properties are dictated by microstructure
Microstructure depends on processing
Microstructural features to engineer:
Mer structure
Molecular weight
Degree of crosslinking/branching Degree of crystallinity 60 x
Brittle
Stress (MPa) 40 x
Plastic x 20
Elastomer 2 4 6 8
Strain

32. Comparing Properties to Metals
Much more temperature sensitive near room temperature
Increasing temperature will
Reduce E
Reduce UTS
Increase %EL
May demonstrate ductile-to-brittle transition near 25 ºC
Much more strain rate sensitivity
Slower rate similar to increasing temperature
Generally, polymers have:
Lower elastic modulus
Lower tensile strengths
Higher ductility
Lower melting temperatures
Higher thermal expansion

33. Comparing Deformation to Metals (Stable Neck Formation)

34. Deforming Semicrystalline Polymers
Elastic Behavior:
Higher crystallinity increases elastic modulus;
Elastic deformation via stretching and bending of primary (covalent) bonds
Plastic Behavior:
Higher crystallinity greatly increases strength;
Deformation via complicated process
Produces highly oriented structure
Crystalline
Amorphous

35. Stiffening Mechanisms
Raise elastic modulus (E)
Colder temperatures
Higher strain rate
More crosslinking
More secondary bonding
Higher crystallinity
Aligned polymer chains

36. Strengthening Mechanisms
Anything that hinders deformation
Colder temperatures
Higher strain rate
More crosslinking
More secondary bonding
Higher crystallinity
Longer chains (higher molecular weight)

37. Strengthening Mechanisms
Drawing of polymers
Analogous to cold work in metals, but more dramatic.
Strength and modulus can have 3x increase in drawing direction
Heat treatment
On un-drawn materials,
May lead to higher modulus, strength, and reduced ductility.
On drawn materials,
Will undo strengthening by undoing chain orientation and strain-induced crystallinity

38. Thermal Characteristics
Melting point increases with:
Chain stiffness
Double bonds
Aromatic groups
Bulky side groups
Polar side groups (more intermolecular bonding)
Longer chains (reason why there is a range of melting temperatures)
Increased linearity (fewer branches)
Glass transition temperature increases with:
Chain stiffness
Double bonds
Aromatic groups
Bulky side groups
Polar side groups
Longer chains
Increase crosslinking
Generally, Tg= ( )*Tm
Units of Kelvin

39. Glass Transition Temperatures
Temperature at which polymers transitions from rubbery to rigid (glassy) behavior
Rubbery above this temperature
Occurs only in amorphous regions of polymers Tg
-110
-100
-20 50 90
100
150 Tm
115
330
175
265
212
240
Temperature
Specific Volume Tg
Amorphous
Crystalline
Semi-Crystalline
Polyethylene
PTFE
Polypropylene
Nylon
PVC
Polystyrene
Polycarbonate

40. Creep Properties  t  t  t  t
Due to low melting temperatures, creep and stress relaxation are important at room temperatures.  t  t
Relaxation modulus:  t  t
Creep modulus:
As silly putty shows, similar viscous-elastic transition takes place by changing time taken for loading
As a viscous fluid, the behavior of a thermoplastic depends on both temperature and strain rate.

41. Viscoelasticity Polymer behavior depends on temperature
At high temperatures & slow strain rates, flow like a viscous fluid
At low temperatures & high strain rates, solid-like elastic deformation
Intermediate temperatures … combination of both … viscoelastic!
stress
time
Instantaneous elasticity followed by stress relaxation
=E
/
strain
time

42. Below Tg
stress
stress
time
strain
strain
Linear elastic behavior
time

43. Above Tg Creep Stress relaxation stress time strain time strain time

44. Material Models liquids viscoelastic liquids viscoelastic solids
Maxwell
Kelvin - Voigt

45. Maxwell Model (Stress Relaxation)
t time constant 1
0.8
0.6
0.4
stress
time
0.2 1 2 3 4

46. Kelvin – Voigt Model (Creep)
strain
time 1 2 3 4

47. Thermoplastic Modulus
10-4
10-3
10-2
10-1 1 10
102
103
104
Relaxation Modulus, MPa 80
100
120
140
160
180
200
Temperature, °C
Glassy
Leathery
Rubbery
Viscous Flow Tg

48. Effect of Crystallinity
10-4
10-3
10-2
10-1 1 10
102
103
104
Relaxation Modulus, MPa 80
100
120
140
160
180
200
Temperature, °C
Glassy
Leathery
Rubbery
Viscous Flow
Crystalline
Elastomer
Amorphous

49. Fracture, Fatigue & Impact
Thermoset fracture: brittle.
Thermoplastic fracture: brittle or ductile.
Craze in a thermoplastic
Fatigue: Polymers fatigue failure is similar to metals, but very rate sensitive
Impact: Some polymers have brittle to ductile transition, and then decreasing impact strength as thermal softening occurs

50. Overview of Processing
Mixing (thermoset) or melting/softening (thermoplastic)
Form polymer from individual mer units
Disperse additives in polymer
Achieve uniform shapable state
Shaping
Extrusion
Continuous, constant cross section
Generally not for thermosets
Molding processes
Batch process
3-D products molded and stabilized
Near net shape - often do not need secondary operations
Stabilizing
Solidifying such that it retains shape

51. Addition Polymerization
“Chain-reaction” or “free radical” polymerization
Three stages
Initiation
Propagation
Termination
Extremely exothermic reaction C H
R° 
C ° R
+ C
C +…= polyethylene
mer
polymer

52. Condensation Polymerization
Joining of two small molecules to give big molecule and biproduct
Often release water during condensation
Usually much shorter chains (lower MW) than addition
CH2 C R OH R’ O
+ H20 + =
ethylene glycol
adipic acid
polyester
water

53. Polymer Additives Usually added in melt stage Fillers:
Increase strength, abrasion resistance, toughness, etc.
Particles of wood, glass, clay, etc.
Plasticizers: low molecular weight liquids.
Molecules sit between polymer molecules and “screen out” the secondary bonding, reducing glass transition temperature.
Softens material and increases room temperature ductility.
Stabilizers:
Counteract degradation due to UV light (which breaks primary bonds)
Retarders: slows reaction
Allows more uniform chain length and more uniform properties
Slows heat build-up during addition polymerization
Colorants
Flame Retardants

54. Elastomers An elastomer is a lightly cross-linked polymer
Goodyear achieved this in poly-isoprene by adding sulfur to cross-link … vulcanization
Crosslinking forces chains to return to original position even after large deformation
Stronger, stiffer, similar ductility S S S
CH2 CH
CH2
CH2 S
CH2
CH2
CH2
CH2 S S S + S S S
CH2
CH2
CH2
CH2
CH2 CH
CH2 CH S S

55. Shaping Techniques Many, many possible techniques Must consider
Geometry of finished product
Stability of material being formed
Thermoplastic vs. Thermosetting
Thermoplastics require processing at temperature above softening
Thermosets require curing using a chemical additive or catalyst
Often inject two different liquids
Hardener
Catalyst

56. Compression Molding Polymer mix or charge placed in heated mold
Mold closes and applies pressure
Plastic becomes viscous
Conforms to mold shape
Thermoset and thermoplastics

57. Extrusion Injection molding through open ended die
As molten liquid passes through cold die
Takes form of die
Cools very quickly
Incredibly fast process
Constant cross section
Sheets
Rods
Pipe/Tube
Gutters

58. Injection Molding Pellets melted Viscous melt injected into mold
Usually in screw chamber that also pushes melt into mold
Sometimes heating chamber and hydraulic ram
Viscous melt injected into mold
Quick solidification in mold
Thermoplastics ~30 sec
Thermoset longer for cure
Widely used for thermoplastics

59. Specialty methods Blow molding Casting Fiber drawing or spinning
Force extruded tube walls to conform to mold with air injection
Often used to make bottles
Casting
Similar to metal casting
Pour liquid polymer into mold and allow to freeze
Fiber drawing or spinning

60. New Concepts Deformation and strengthening mechanisms of polymers
Viscoelasticity
Temperature effects vs. rate effects
Creep and relaxation modulus