1. ENGINEERING MATERIALS ENT 112/4
LECTURE 3 (continue)
3.4 Structure of Polymer

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

3. 2.5.1 INTRODUCTION 2 Categories- Naturally occurring polymers which
derives from plants and animals: wood, natural rubber, cotton, silk
Other natural polymer-proteins, enzymes, starches, cellulose
Synthetic polymers
i.e. plastics, synthetic rubbers, and fibrous materials
Properties of polymers are intricately related to the structural elements of materials

4. 2.5.2 Hydrocarbon Molecules
Most polymeric materials are composed of very large molecules-mostly composed of hydrogen and carbon
Intramolecular bonds are covalent
Saturated hydrocarbon -all bonds are single
These molecules have strong covalent bond but weak hydrogen & van der Waals bonds between molecules
Unsaturated hydrocarbon -have double & triple covalent bond.
Possibility of other atom or group of atoms to attached to original molecule

5. Saturated Hydrocarbon
Most polymers are hydrocarbons
– i.e. made up of H and C
Saturated hydrocarbons
Each carbon bonded to four other atoms
CnH2n+2

7. 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

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

9. 2.5.3 Polymer Molecules
Polymers are referred as macromolecules due to their giant size
The molecules are in the form of long & flexible chains
The backbone consist from string of carbon atoms, many times each carbon atom singly bonds to two adjacent carbon atoms on either side

10. Monomer refers to stable molecule where the polymer is synthesized
The long molecules are composed of structural entities called repeat unit which are successively repeated along the chain.
Repeat unit also called a mer.
Mer originates from Greek word meros, which mean parts, thus, polymer means many parts or many mers/repeat unit
Monomer refers to stable molecule where the polymer is synthesized
Adapted from Fig. 14.2, Callister 6e.

11. 2.5.4 The Chemistry of Polymer Molecules
Consider ethylene(C2H4) gas at ambient temperature & pressure.
How this hydrocarbon monomer transformed into polyethylene, which is a solid polymeric material?

12. 2.5.4 The Chemistry of Polymer Molecules
The process happens when the initiator/catalyst react with the ethylene mer unit
(a) (b)
The polymer chain then forms by the sequential addition of polyethylene monomer units to this active initiator-mer center.
The active site or unpaired electron is transfered to each successive end monomer. The final result is the polyethylene molecules

13. Chemistry of Polymers Note: polyethylene is just a long HC
Adapted from Fig. 14.1, Callister 7e.
The angle between the singly bonded carbon atoms is not 180° but rather close to 109°.
The C – C bond length being nm.
Polymer- can have various lengths depending on number of repeat units
Note: polyethylene is just a long HC
- paraffin is short polyethylene

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

17. Other Terms
Homopolymer-when all the repeating units along a chain are of the same type
Copolymers-Polymers’ chains may be composed of two or more different mer units
Bifunctional-they have 2 active bonds which form 2 dimensional molecular structure
Trifunctional-have 3 active bonds, which results in 3 dimensional molecular structure

18. 2.5.5 MOLECULAR WEIGHT
During polymerization, not all polymer chains have a same length which results in distribution of molecular weights
An average molecular weight is specified from various physical properties
The no. average molecular weight
Mn=ΣxiMi
The weight average molecular weight
Mw=ΣwiMi

19. MOLECULAR WEIGHT • Molecular weight, Mi: Mass of a mole of chains.
Lower M
higher M
Fraction of the total number of chains
The mean molecular weight within the size range
Simple for small molecules
All the same size
Number of grams/mole
Polymers – distribution of chain sizes
The weight fraction of molecules
Adapted from Fig. 14.4, Callister 7e.

20. Degree of Polymerization, n
- An alternative way of expressing average chain size of polymer
n = number of repeat units per chain
ni = 6
Chain fraction
mol. wt of repeat unit i

21. Computation of Average Molecular Weights & Average Degrees of Polymerization
Assume that the molecular weight distributions are for poly (vinyl chloride):
(a)
(b)
Compute: a) the no. average molecular weight
b) the no. average degree of polymerization
c) the weight average molecular weight

22. The data taken is from (a) and presented in the above table, the summation of all xiMi products yields the no. average molecular weight, which is 21,150 g/mol
No average degree of polymerization is computed from mer molecular weight. For PVC each mer has-2 carbon atoms, 3 hydrogen atoms, 1 chlorine atoms.

23. Atomic weights: C= 12, H= 1, Cl=35.5, thus
m= 2(12 g/mol)+3(1 g/mol)+35.5 g/mol
= 62.5 g/mol
&
nn =Mn/m = (21,150 g/mol)/(62.5 g/mol)
= 338
(c)
The data taken is from (b), the summation of all wiMi products yields the weight average molecular weight, which is 23,2000 g/mol

24. 2.5.6 MOLECULAR SHAPE
Polymer chain molecules are illustrated to be straight for easy understanding
A more accurate model is shown with 109o between carbon atoms that forms a zigzag pattern
Figure (b) shows a straight chain segment resulted when the chain atoms are well positioned

25. 2.5.7 Molecular Shape
Chain bending & twisting are possible when there is chain atoms rotation into other positions as depicted in (c)
This could result as in picture below:
Polymers consist of large no. of molecular chains, each of which may bend, coil & kink as in the picture

26. End to End Distance, r
This distance is much smaller than the total chain length
Adapted from Fig. 14.6, Callister 7e.

27. The shape of the polymer chain molecule are responsible for a number of important characteristics
Some mechanical & thermal characteristics of polymer are a function of the ability of the chain segments to experience rotation in response to applied stress or thermal vibrations
Rotational flexibility is dependent on mer structure& chemistry
For example: The region of a chain segment that has a double bond (C=C) is rotationally rigid
Also, the introduction of a bulky or large side group of atoms restricts rotational movement

28. 2.5.7 Molecular Structure Four general molecular chain structures:
Linear polymers
Branched polymers
Cross-linked polymers
Network polymers

29. Linear Polymers
The mer units are joined together end to end in single chains represents as a mass of spaghetti
May have extensive van der Waals & hydrogen bonding between chains
Examples: PE, PVC, polystyrene, PMMA

30. Branched Polymers Side branch chains are connected to the backbone
The branches resulted from side reactions that occur during the polymer synthesis
The chain packing efficiency is reduced by the formation of side branches which lowers the polymer density

31. Cross-linked Polymers
Adjacent linear chains are joined one to another at various positions by covalent bonds
Crosslinking process is achieved either by synthesis or nonreversible chemical reaction carried out at elevated temperature
Is accomplished by additive atoms or molecules covalently bonded to the chains

32. Network Polymers
Trifunctional mer units, having 3 active covalent bonds to form 3-D networks
A polymer that is highly crosslinked maybe classified as a network polymer
These materials have distinctive mechanical & thermal properties
Polymers can have more than one distinctive structural type

33. 2.5.8 MOLECULAR CONFIGURATIONS
Consider the mer unit where R
represents an atom or side
group other than H (e.g., Cl, CH3)
A possible arrangement when
the R side groups of successive
mer units bond to alternate carbon
atoms. This is designated as a head
-to-tail configuration
Its complement the head-to-head
configuration occurs when R groups
bond to adjacent chain atoms

34. ISOMERISM-polymer molecules with same composition but different atomic arrangements
STEREOISOMERISME-denotes the situation in which atoms are linked together in the same order (head-to-tail) but different in their spatial arrangement
ISOTACTIC CONFIGURATION
All the R groups are situated on the same side of the chain

35. Syndiotactic Configuration
The R groups alternate sides of the chain
Atactic Configuration
Random positioning of R groups
Conversion of one stereoisomer to another will involve severing of bonds reformation after appropriate rotation

36. GEOMETRICAL ISOMERISM
Also possible within mer units having double bond between chain carbon atoms
Bonded to the carbon atoms participating with double bond is a single sided bonded atom or radical
Let’s say the CH3 group & the H atom on the same side. This is a CIS structure
For the trans structure, the CH3 & H reside on opposite chain sides. This isomerism also can’t be convert by simple rotation because the chain double is too rigid
cis-isoprene (natural rubber)
trans-isoprene (gutta percha) – has properties that are distinctly different from natural rubber

37. 2.5.9 THERMOPLASTIC & THERMOSETTING POLYMERS
Classification scheme based on the materials behavior with rising temperature
Thermoplastic polymer have linear & branched structures; they soften when heated & harden when cooled.
The process reversible & can be repeated
It is not reversible if the temperature is raised to a point where molecular vibrations are violent enough to break the primary covalent bonds

38. 2.5.9 THERMOPLASTIC & THERMOSETTING POLYMERS
Thermosetting polymer, are cross linking and network polymer
Become permanently hard during their formation, and do not soften upon heating.
At initial heat treatment, covalent cross-links are formed between adjacent molecular chains to anchor the chains together which resist vibrations & rotations at high temperature
Crosslinking is usually extensive
Thermoset polymers are harder & stronger than thermoplastics

39. 2.5.10 COPOLYMERS Composed of two repeat units
Have different sequencing arrangement along the polymer chain
a) Random copolymer – two different units are randomly dispersed along the chain
b) Alternating copolymer – the 2 mer units alternate chain positions
c) Block copolymer – the identical mers are clustered in blocks along the chain
d) Graft copolymer – homopolymer side branches of one type maybe grafted to homopolymer main chains that are composed of a different mer

40. Styrene-butadiene rubber (SBR) is a common random copolymer which use to make automobile tires.

41. 2.5.11 POLYMER CRYSTALLINITY
Atomic arrangement in polymer crystals is more complex than in metals or ceramics (unit cells are typically large and complex).
Polymer crytallinity is the packing of molecular chains to produce an ordered atomic array
More crystallinity: higher density, more strength, higher resistance to dissolution and softening by heating!
Polyethylene

42. Degree of crystallinity is determined by:
Rate of cooling during solidification: time is necessary for chains to move and align into a crystal structure
Mer complexity: crystallization less likely in complex structures, simple polymers, such as polyethylene, crystallize relatively easily
Chain configuration: linear polymers crystallize relatively easily, branches inhibit crystallization, network polymers almost completely amorphous, crosslinked polymers can be both crystalline and amorphous
Isomerism: isotactic, syndiotactic polymers crystallize relatively easily - geometrical regularity allows chains to fit together, atactic difficult to crystallize
Copolymerism: easier to crystallize if mer arrangements are more regular - alternating, block can crystallize more easily as compared to random and graft.

43. How to calculate crystallinity?

44. POLYMER CRYSTALS
Polymer molecules are often partially crystalline (semicrystalline), with crystalline regions dispersed within amorphous material.
Crystalline
Amorphous
Fringed-micelle model

45. Polymer Crystallinity
Polymers rarely 100% crystalline
Too difficult to get all those chains aligned
crystalline
region
• % Crystallinity: % of material that is crystalline.
-- TS and E often increase
with % crystallinity.
-- Annealing causes
crystalline regions
to grow. % crystallinity
increases.
amorphous
region
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.)

46. Recently : Studies focus on Polymer single crystals grown from dilute solution
Each platelet will consist of a number of molecule; however the average chain length is much greater than the thickness of the crystallite

47. Photomicrograph of
spherulite structure
of polyethylene
Spherulites: Aggregates of lamellar crystallites ~ 10 nm
thick, separated by amorphous material. Aggregates
approximately spherical in shape.

48. Schematic representation of the detailed
structure of a sperulite.