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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
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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
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1. Chain growth (addition polymerization)
Rapid chain reaction
2. Step growth (condensation polymerization)
Chemical reaction between pairs of reactive monomers
Much slower
Two distinct ways for the process of polymerization
Chain growth
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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
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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.
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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.
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The length of of the polymeric molecule
= lL m
l: the length of a single bond in the backbone of the hydrocarbon chain
m: the number of bonds
The root-mean-square length L
The degree of polymerization
A polymeric molecule ( C2H4 )n
n is termed the degree of polymerization (DP)
The extended length Lext
Lext = ml sin(109.5º/2)
For typical bifunctional linear polymer, m = 2n
Bend, coil, kink
Intertwining, entanglement
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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
Molecular configurations – the side groups
Schematic representations of (a) linear, (b) branched, (c) crosslinked, and (d) network (three dimensional) molecule structures. Circles designate individual mer units.
Polymers structures
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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
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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
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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
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Polymer Crystallinity
Polymers rarely 100% crystalline• Too difficult to get all those chains
aligned
• % Crystallinity: % of material that is crystalline.-- TS and E often increase
with % crystallinity.-- Annealing causes
crystalline regionsto grow. % crystallinityincreases.
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
amorphousregion
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Chain-folded model
Polymer Crystal Forms
Spherulitesurface
Adapted from Fig. 14.13, Callister 7e.
• Spherulites – fast growth –forms lamellar (layered) structures
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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 load-deflection 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
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Dynamic modulus of elasticity
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
Some polymers, especially the elastomers, are used in structuresfor 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.
Mechanical properties of polymers-Stress- strain behaviours (p207)