Polymer Structure and Properties

1. ENR116 – Mod. 4- Slide No. 1
ENR116 Engineering Materials
Module 4 Non-metals and Corrosion
Dr Louise Smith
School of Advanced Manufacturing and Mechanical Engineering

2. ENR116 – Mod. 4- Slide No. 2
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3. ENR116 – Mod. 4- Slide No. 3
Polymer structure and
properties

4. ENR116 – Mod. 4- Slide No. 4
Intended Learning Outcomes
At the end of this section, students will be able to:
• Identify crystalline structures in polymers and how they
differ from metals.
• Understand how the tensile properties of polymers are
affected by microstructure.
• Describe the effects of temperature on polymers.

5. ENR116 – Mod. 4- Slide No. 5
Thermoplastic and thermosetting
polymers
Thermoplastics
Thermosets
• Soften when heated and harden when cooled (reversibly).
• Temperature is raised, secondary bonding forces are
diminished – molecules easily move against each other.
• Thermoplastics are relatively soft.
• Network polymers, covalent cross-links between adjacent
chains.
• Generally harder and stronger than thermoplastics and have
better dimensional stability.

6. ENR116 – Mod. 4- Slide No. 6
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.
A – B –
Copolymers
random
block
graft
alternating
Adapted from Fig.
14.9, Callister &
Rethwisch 8e.

7. ENR116 – Mod. 4- Slide No. 7
Styrene–butadiene rubber (SBR):
Common random copolymer from
which automobile tires are made.
Copolymers
Nitrile butadiene rubber (NBR):
Automotive transmission belts,
hoses, O rings, gaskets, synthetic
leather....

8. ENR116 – Mod. 4- Slide No. 8
Polymer crystallinity
Polymers can form crystalline (or
semicrystalline) structures.
Example: polyethylene unit cell
Adapted from Fig.
14.10, Callister &
Rethwisch 8e.
Molecular chains ‘pack’ to produce
an ordered atomic array.
Degree of crystallinity can range
from completely amorphous to
almost entirely crystalline
Density of a crystalline polymer
will be greater than an amorphous
one

9. ENR116 – Mod. 4- Slide No. 9
Degree of crystallinity
rs is the density of a specimen for which the percent
crystallinity is to be determined.
ra is the density of the totally amorphous polymer.
rc is the density of the perfectly crystalline polymer.
May be determined from accurate density measurements.
rc(rs - ra)
rs(rc - ra)
% crystallinity = X 100

10. ENR116 – Mod. 4- Slide No. 10
Crystallinity depends on:
Degree of crystallinity
• The rate of cooling during solidification – sufficient time to
allow chains to move and align.
• The nature of the repeat unit. Simple repeat units
crystallise easier
Atactic polymers are difficult to crystallise; isotactic and
syndiotactic polymers crystallise more readily
Bulky / large side groups limit crystallisation

11. ENR116 – Mod. 4- Slide No. 11
Polymer crystals
Semicrystalline polymer consists of small crystalline regions
(crystallites).
Chain folded model
Adapted from Fig.
14.12, Callister &
Rethwisch 8e.
Adapted from Fig. 14.11, Callister 6e. (Fig. 14.11 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.)
crystalline
region
amorphous
region

12. ENR116 – Mod. 4- Slide No. 12
Spherulites: Fast growth –
forms lamellar (layered)
structures.
Polymer crystals
Spherulite
surface
Photomicrograph of polyethylene using
cross polarised light.
Adapted from Fig. 14.13, Callister & Rethwisch 8e.
Adapted from Fig. 14.14, Callister & Rethwisch 8e.

13. ENR116 – Mod. 4- Slide No. 13
Polymer mechanical properties
Expressed in terms of modulus of
elasticity, and yield and tensile
strengths from the stress–strain test.
Sensitive to:
• Temperature
• The rate of deformation (strain rate)
• The chemical nature of the environment
(the presence of water, oxygen, organic
solvents, etc.)

14. ENR116 – Mod. 4- Slide No. 14
Stress-strain behavior:
elastic modulus
less than metal
Polymer mechanical properties
Adapted from Fig. 15.1,
Callister & Rethwisch 8e.
Brittle polymer – fractures when
deformed elastically.
Plastic – initially elastic
deformation followed by
plastic.
Elastomer – large
recoverable strains
produced at low stress
levels.

15. ENR116 – Mod. 4- Slide No. 15
Polymer mechanical properties
Adapted from Fig. 15.02,
Callister & Rethwisch 8e.
σy
TS

16. ENR116 – Mod. 4- Slide No. 16
Influence of T on the stress–strain characteristics of
poly(methyl methacrylate) (PMMA).
With an increase in
temperature:
Polymer mechanical properties:
Temperature
Adapted from Fig. 15.03,
Callister & Rethwisch 8e.
• Decrease in
elastic modulus.
• Reduction in
tensile strength.
• Enhancement
in ductility.
X
X
X

17. ENR116 – Mod. 4- Slide No. 17
Polymers can behave as glasses, a rubbery solid or
viscous liquid as a function of temperature.
Polymer mechanical properties:
Viscoelasticity
Viscoelasticity: The behaviour of the polymers as rubbery
solids at an intermediate temperature.
Time, t
Strain
ta tr

18. ENR116 – Mod. 4- Slide No. 18
Deformation of semicrystalline
polymers
Mechanism of Elastic Deformation:
Chain molecules in amorphous regions elongate and align in
the direction of the applied tensile stress.
Adapted from Fig. 15.12,
Callister & Rethwisch 8e.

19. ENR116 – Mod. 4- Slide No. 19
Deformation of semicrystalline
polymers
Mechanism of Plastic Deformation:
Adapted from Fig. 15.13,
Callister & Rethwisch 8e.

20. ENR116 – Mod. 4- Slide No. 20
• Degree of Crystallinity:
Tensile strength increases significantly.
Material tends to become more brittle.
Factors that influence the mechanical
properties of semicrystalline polymers
• Temperature
• Heat treating (or annealing):
Can increase the percent crystallinity and size of crystallites
• Molecular weight
For some polymers – TS increases with Mn

21. ENR116 – Mod. 4- Slide No. 21
Summary
• Polymers may exhibit varying degrees of crystallinity.
• Many semicrystalline polymers form spherulites
• Polymers fall into three general categories of stress
strain behaviour; brittle, plastic and highly elastic.
• The deformation of polymers is often both time and
temperature dependent.

22. ENR116 – Mod. 4- Slide No. 22
Thank you