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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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Periodic table with the elements associated with commercial polymers in color.

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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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The polymerization of formaldehyde to form polyacetal

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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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continued

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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)

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