Polymers
1. What are polymers
2. Polymerization
3. Structure features of polymers
4. Thermoplastic polymers and
thermosetting polymers
5. Additives
6. Polymer crystals
7. Mechanical properties of polymers
8. Processing of polymers
1
Periodic table with the elements associated
with commercial polymers in color.
2
Characteristics of polymers (plastics)
1. Organic materials
2. Long-chain molecule composed of many ”mers” bonded
together
3. ”mer” is building block of the long-chain (e.g. -C2H4- in
polyethylene)
4. Compound of hydrogen and carbon, and/or O, N, F and Si
5. Extensive formability and ductility
6. Light weight, low cost
7. Low strength compared with metals; lower melting point and
higher chemical reactivity compared with ceramics
Polymerization
1. The critical feature of a monomer in
polymerization:
The presence of reactive sites double bonds
2. A saturated hydrocarbon
All bonds are single bonds,
3. Unsaturated monomer
double or triple covalent bonds
3
Two distinct ways for the process of polymerization
1. Chain growth (addition polymerization)
Rapid chain reaction
2. Step growth (condensation polymerization)
Chemical reaction between pairs of reactive monomers
Much slower
Chain growth
4
An initiator
An initiator: hydroxyl free radical in Fig. 13.2
A free radical: a reactive atom or group of atoms containing an
unpaired electron
A terminator
A terminator: another hydroxyl free radical
Form a stable molecule with n mer units
An initiator - terminator pair
Hydrogen-peroxide H2O2 fi 2OH •
Recombination: the termination step
Hydrogen abstraction: obtaining a hydrogen aton with unpaired electron
Disproportionation: obtaining a monomerlike double bond
Copolymer
Block copolymer
Regular, along a single carbon-bonded chain
Blend copolymer
irregular
5
The polymerization of formaldehyde
to form polyacetal
6
Step growth (condensition polymerization)
Extensive polymerization requires this three molecule reaction to be
repeated for each unit increase in molecule length. The time required
for this process is substantially greater than that for the chain
reaction or chain growth.
Bifunctional
a linear molecule structure,
Softer than the network polymer
Polyfunctional – has several potential points of contact ;
a three dimensional network molecule structure
Fig. 13.7 After several
reaction steps like that in Fig.
13.6, polyfunctional mers
form a three-dimensional
network molecule structure.
7
8
continued
continued
9
Structure features of polymers
Each bond angle between three adjacent C atoms is near
109.5º and the angle can be rotated freely in space.
10
The length of of the polymeric molecule
The degree of polymerization
A polymeric molecule ( C2H4 )n
n is termed the degree of polymerization (DP)
The root-mean-square length
L
L =l m
l: the length of a single bond in the
backbone of the hydrocarbon chain
m: the number of bonds
The extended length Lext
Lext = ml sin(109.5º/2)
For typical bifunctional linear
polymer, m = 2n
Bend, coil, kink
Intertwining, entanglement
11
Molecular configurations – the side groups
R: the large side group
As the side groups become larger
and more irregular, rigidity and
melting point tend to rise, because:
The side groups serve as
hindrances to molecule sliding;
The side groups lead to greater
secondary bonding forces
Polymers structures
Schematic representations of (a) linear, (b) branched, (c)
crosslinked, and (d) network (three dimensional) molecule
structures. Circles designate individual mer units.
12
Thermoplastic polymers
1. Become soft and deformable upon heating
2. Linear polymers including those that are branched but not
cross-linked
3. High-temperature plasticity – due to the ability of the
molecules to slide past one another (thermally activated or
Arrhenius process
4. The ductility of thermoplastic polymers is reduced upon
cooling
5. High temperature: for polymers ~100ºC, for metals can be
~1000ºC
Engineering polymers (see Table 13.1)
Retaining good strength and stiffness up to 150-175ºC
The “ general-use” polymers,
e.g. textile fiber nylon, polyester (textile fiber)
Polyethylene
Low-density polyethylene (LDPE)
High-density polyethylene (HDPE)
Ultra-high molecule-weight polyethylene (UHMWPE)
Thermoplastic elastomers,
With mechanical behaviour analogous to natural rubbers,
Synthetic rubbers, vulcanization
13
14
Thermosetting polymers
1. Becoming hard and rigid upon heating, the opposite of
thermoplastics
2. This phenomenon is not lost upon cooling
3. With network molecule structure, formed by the step-growth
mechanism, the chemical reaction are enhanced by high
temperatures and are irreversible
4. Commen thermosetting polymers (see Table 13.2)
Thermosets
With significant strength and stiffness
Being common metal substitutes
Not being recyclable
Elastomers
15
16
Additives
A plasticizer
To soften a polymer
Blending with a low-molecular-weight polymer
A filler
To strengthen a polymer by restricting chain mobility
Inert materials are used, e.g. short-fiber cellulose and asbestos,
carbon black
Stabilizers
To reduce polymer degradation, e.g. To retard the room temperature
oxidation by adding complex phenol group
Flame retardants
To reduce the inherent combustibility
Halogens e.g. Cl atoms, by terminating free-radical chain reaction
Colorants
To provide colour to a polymer
Pigments (insoluble), and dyes (soluble and provide transparent
colour)
Polymer crystals
17
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. 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.)
18
Chain-folded model
Polymer Crystal Forms
•
Spherulites – fast growth –
forms lamellar (layered)
structures
Spherulite
surface
Adapted from Fig. 14.13, Callister 7e.
19
Flexural modulus or modulus of elasticity in bending
Eflex = L3m / (4bh3)
m: the slope of the tangent to the initial straight-line portion of the loaddeflection curve
Describe the combined effects of compressive deformation and tensile
deformation (on the opposite side of the specimen)
The tensile and compressive moduli differ significantly
20
Dynamic modulus of elasticity
Some polymers, especially the elastomers, are used in structures
for the purpose of isolation and absorption of shock and vibration.
For such applications a ”dynamic” elastic modulus is more useful
to characterizethe performance of the polymerunder an oscillating
mechanical load.
Edyn = CIf2
C: a constant, dependent upon test geometry
I: the moment of interia (kg •m2) of the beam and weights
used in the dynamic test
f: the frequency of vibration (in cycles) for the test
Mechanical properties of polymersStress- strain behaviours (p207)
21
22
23