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MS211

Ch. 13 Polymers

Spring 2009

Seokwoo Jeon

Department of Materials Science and Engineering

KAIST

Movie

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Polymers = many mers

= plastic (due to plastic deformation)

Chemical category

Metal and ceramics : inorganic material

Polymerization: the process by which long-chain or

network molecules are made from relatively small organic

molecules. Types of Polymerization

Chain rowth additional ol merization

Step growth (condensation polymerization)

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Chain reaction of chemically activated monomers (i.e. from

additional polymerization by adding monomers

Ex 1. V in l ch loride ol vin l chloride

Polymerization is the joining of individual. ., , 2 3

form a polymer [(C2H3Cl)n] consisting of

manymers (again, C2H3Cl).

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Ex 2 . ethylenepolyethylene H2O2 2OH-

Until ethylene monomers are fully

Detailed mechanism of polymerization by a chain growthprocess (addition polymerization). In this case, a molecule ofy rogen perox e, 2 2, prov es wo y roxy ra ca s,

which serve to initiate and terminate. The polymerization ofethylene (C2H4) to polyethylene -(C2H4)-n. (The large dotnotation re resents an un aired electron. The oinin orpairing of two such electrons produces a covalent bond,represented by a solid line(-).]

terminator

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A polymer derived from two or more monomers

Alternating copolymers

Periodic copolymerse. . A-B-A-B-B-A-A-A-A-B-B-BRandom copolymersBlock copolymersGraft co ol mers

vinyl chloride is analogous to asolid-solution metal alloy.

Ablend of polyethylene andpolyvinyl chloride is analogous toa metal alloy with limited solidsolution

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Prof. S. Kims work at KAIST

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Polymerization by reactive functional group (slower compared to chain growth)Formation of phenol-formaldehydeTwo phenol molecules are linked by the formaldehyde moleculeBifunctional linear molecular structurePolyfunctional network molecular structure

Polyfunctional monomer

3-D network molecular structure

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Degree of polymerization length of the polymeric molecule, size of molecule, howmany mers, Viscosity, light scatteringy g gIn case of 1D linear polymerRoot-mean-square length mlL =Extended length

2

5.109sinmlL

o

ext=

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Higher degree of polymerization rigidity and melting point increasesNetwork molecular structure rigid, brittleLinear molecular structure soft (due to Van der Waals bet. Chains)Symmetry of chain

As the side groups become

Isotacticarger an more rregu ar,

Rigidity and melting pointincreases.

SyndiotacticWhy?

- Hindrance to molecularSyndiotacticsliding

- secondary bonding force

Atactic

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Branching Cross-linking

Branching involves adding a polymeric

structure. Vulcanization.

molecule to the side of the mainmolecular chain.

Accidently found by Charles Goodyear.

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Thermoplastic Polymers

Soft and deformable upon heatingLinear or branched polymeric moleculesGeneral-use polymerL d it l th l (LDPE)ow-density polyethylene (LDPE)Engineering polymerhigh-density polyethylene (HDPE) ultra-high molecular-weight polyethylene(UHMWPE) acrylonitrile butadiene styrene (ABS)UHMWPE), acrylonitrile-butadiene-styrene (ABS)Thermoplastic elastomersRigid elastomeric domains in a relatively soft matrix

Thermosetting Polymers

Hard and rigid upon heatingNetwork molecular structure formed by the step-growth mechanismEnhanced by high temperature and irreversibleHigh strength and stiffness, not being recylableTraditional elastomersAdhesives

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1. Plasticizer

to soften polymer, low molecular weight

.to strengthen polymer, volume replacement to reduce cost.

short-fiber cellulose, asbestos, carbon black (tire rubber)

3. Stabilizerto reduce polymer degradation by oxidation, thermal, or ultraviolet

.

5. Colorantsgmen : nso u e ~ 2, 2 3- 2

Dyes : soluble ~ organic colorants