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Chapter Outline: Polymer Structures
¾ Hydrocarbon and Polymer Molecules
¾ Chemistry of Polymer Molecules
¾ Molecular Weight and Shape
¾ Molecular Structure and Configurations
¾ Copolymers
¾ Polymer Crystals
Optional reading: none
Introduction to Materials Science, Chapter 15, Polymer Structures
University Tennessee, Dept. of Materials Science and Engineering (^) 2
¾ Polymer - a large molecule consisting of (at least five) repeated chemical units (`mers') joined together, like beads on a string. Polymers usually contain many more than five monomers, and some may contain hundreds or thousands of monomers in each chain.
¾ Polymers may be natural , such as cellulose or DNA, or synthetic , such as nylon or polyethylene.
Polymers: Introduction
Many of the most important current research problems involve polymers. Living organisms are mainly composed of polymerized amino acids (proteins) nucleic acids (RNA and DNA), and other biopolymers. The most powerful computers - our brains - are mostly just a complex polymer material soaking in salty water! We are just making first small steps towards understanding of biological systems.
Silk fiber is produced by silk worms in a cocoon, to protect the silkworm while it metamorphoses in a moth.
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¾ Most polymers are organic, and formed from hydrocarbon molecules
¾ Each C atom has four e -^ that participate in bonds,
each H atom has one bonding e -
Hydrocarbon molecules (I)
Methane, CH 4 Ethane, C 2 H 6 Propane, C 3 H 8
Examples of saturated (all bonds are single ones) hydrocarbon molecules:
Introduction to Materials Science, Chapter 15, Polymer Structures
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Double and triple bonds can exist between C atoms
(sharing of two or three electron pairs). These bonds
are called unsaturated bonds. Unsaturated
molecules are more reactive
Hydrocarbon molecules (II)
Ethylene, C 2 H 4 Acetylene, C 2 H 2
H-C≡H-C
Isomers are molecules that contain the same atoms but in a different arrangement. An example is butane and isobutane:
Butane → C 4 H 10 ← Isobutane
University Tennessee, Dept. of Materials Science and Engineering (^) 7
Chemistry of polymer molecules (I)
¾ Ethylene (C 2 H 4 ) is a gas at room temp and pressure
¾ Ethylene transforms to polyethylene (solid) by forming active mers through reactions with an initiator or catalytic radical (R. )
¾ (. ) denotes unpaired electron (active site)
- Termination: When two active chain ends meet each other or active chain ends meet with initiator or other species with single active bond:
- Rapid propagation ~1000 mer units in 1-10 ms:
Polymerization:
- Initiation reaction:
Introduction to Materials Science, Chapter 15, Polymer Structures
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Chemistry of polymer molecules (II)
Replace hydrogen atoms in polyethylene: make polytetraflouroethylene (PTFE) – Teflon
Replace every fourth
hydrogen atom in
polyethylene with Cl
atom: polyvinyl
chloride
Replace every fourth
hydrogen atom in
polyethylene with CH 3
methyl group:
polypropylene
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Chemistry of polymer molecules (III)
¾ When all the mers are the same, the molecule is called a homopolymer ¾ When there is more than one type of mer present, the molecule is a copolymer ¾ Mer units that have 2 active bonds to connect with other mers are called bifunctional
¾ Mer units that have 3 active bonds to connect with other mers are called trifunctional. They form three- dimensional molecular network structures.
Phenol-formaldehyde
( trifunctional)
Polyethylene
( bifunctional)
Introduction to Materials Science, Chapter 15, Polymer Structures
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Molecular weight (I)
¾ Final molecular weight (chain length) is controlled by relative rates of initiation, propagation, termination steps of polymerization ¾ Formation of macromolecules during polymerization results in distribution of chain lengths and molecular weights ¾ The average molecular weight can be obtained by averaging the masses with the fraction of times they appear ( number-average molecular weight ) or with the mass fraction of the molecules ( weight-average molecular weight ).
Mn = ∑xiMi
Mw = ∑wiMi
number-average:
weight-average:
Mi is the mean molecular weight of range i w (^) i is weight fraction of chains of length i x (^) i is number fraction of chains of length i
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Molecular shape
¾ Molecular chains may thus bend, coil and kink
¾ Neighboring chains may intertwine and entangle
¾ Large elastic extensions of rubbers correspond to
unraveling of these coiled chains
¾ Mechanical / thermal characteristics depend on the
ability of chain segments to rotate
Introduction to Materials Science, Chapter 15, Polymer Structures
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Molecular structure
The physical characteristics of polymer material depend not only on molecular weight and shape, but also on molecular structure:
1 Linear polymers : Van der Waals bonding between chains. Examples: polyethylene, nylon.
2 Branched polymers : Chain packing efficiency is reduced compared to linear polymers - lower density
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Molecular structure
3 Cross-linked polymers : Chains are connected by covalent bonds. Often achieved by adding atoms or molecules that form covalent links between chains. Many rubbers have this structure.
4 Network polymers : 3D networks made from
trifunctional mers. Examples: epoxies, phenol- formaldehyde
Introduction to Materials Science, Chapter 15, Polymer Structures
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Isomerism
Isomerism : Hydrocarbon compounds with same
composition may have different atomic compositions.
Physical properties may depend on isomeric state
(e.g. boiling temperature of normal butane is -0.5 oC,
of isobutane - 12.3 oC)
Butane → C 4 H 10 ← Isobutane
Two types of isomerism are possible:
stereoisomerism and geometrical isomerism
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Summary: Size – Shape -Structure
Introduction to Materials Science, Chapter 15, Polymer Structures
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Copolymers (composed of different mers)
Copolymers, polymers with at least two different types of mers, can differ in the way the mers are arranged:
Random copolymer
Alternating copolymer
Block copolymer
Graft copolymer
Synthetic rubbers are copolymers
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Polymer Crystallinity (I)
Atomic arrangement in polymer crystals is more complex than in metals or ceramics (unit cells are typically large and complex).
Polymer molecules are often partially crystalline (semi- crystalline), with crystalline regions dispersed within amorphous material.
Polyethylene
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Polymer Crystallinity (II)
Degree of crystallinity is determined by:
¾ Rate of cooling during solidification : time is necessary for chains to move and align into a crystal structure
¾ Mer complexity : crystallization less likely in complex structures, simple polymers, such as polyethylene, crystallize relatively easily
¾ Chain configuration : linear polymers crystallize relatively easily, branches inhibit crystallization, network polymers almost completely amorphous, cross- linked polymers can be both crystalline and amorphous
¾ Isomerism : isotactic, syndiotactic polymers crystallize relatively easily - geometrical regularity allows chains to fit together, atactic difficult to crystallize
¾ Copolymerism : easier to crystallize if mer arrangements are more regular - alternating, block can crystallize more easily as compared to random and graft
More crystallinity: higher density, more strength, higher resistance to dissolution and softening by heating
University Ten
Polymer Crystals
Spherulites: Aggregates of lamellar crystallites ~ 10 nm thick, separated by amorphous material. Aggregates approximately spherical in shape.
Photomicrograph of spherulite structure of polyethylene
Introduction to Materials Science, Chapter 15, Polymer Structures
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Five Bakers Dancing
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Number Eighty Eight
HUMAN APOLIPOPROTEIN A-I.
Biopolymers can be complex… and nice
Introduction to Materials Science, Chapter 15, Polymer Structures
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His bark is worse than his bite
