lecture 7 - polymers

7/27/2019 Lecture 7 - Polymers

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1

Molecular Shape (orConformation)

Chain bending and twisting are possible by rotation ofcarbon atoms around their chain bonds

Not necessary to break chain bonds to alter molecular

shape


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2

Molecular Structures for

Polymers

secondarybonding

Linear Polymers- Mer units are joined together end to end

- Polyethylene, polystyrene etc

Branched Polymers

-Side branches are connected to main ones- Lowering density

Cross linked Polymeres

-Adjacent linear chains are joined to one

another at various positions by covalentbonds

Network Polymers

- polymerization of monomers having two or

more functional groups

-may be made by cross-linking the polymers


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Isomerism

Mmolecules with the same chemical formula and

often with the same kinds ofbonds betweenatoms, but in which the atoms are arrangeddifferently

Propanol (C3

H8

O) is an example

Propan-1-ol (n-propyl alcohol) and Propan-2-ol (isopropyl alcohol)

Molecular Configurations for

Polymers

Configurations to change must break bonds
http://en.wikipedia.org/wiki/Chemical_formulahttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Image:Structural_isomers.pnghttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Chemical_formula


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4

Molecular Configurations for

Polymers

Stereo Isomers Bond structure is the same but the geometrical

positioning of atoms and functional groups in spacediffers

EB

A

DC C

D

A

BE

mirror plane

C C

R

HH

H

C C

H

H

H

R

or C C

H

H

H

R

Stereoisomers are mirror images cant superimpose without breaking a

bond


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Tacticity

Tacticity stereoregularity or spatial arrangement ofR unitsalong chain

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

isotactic all R groups on

same side of chain

C C

H

H

H

R

C C

H

H

H

R

C C

H

H

H

R R

H

H

H

CC

syndiotacticR groups

alternate sides


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Tacticity (cont.)

atacticR groups randomlypositioned

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC


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cis/trans Isomerism

C C

HCH3

CH2 CH2

C C

CH3

CH2

CH2

H

cis

cis-isoprene

(natural rubber)

H atom and CH3 grouponsame side of chain

trans

trans-isoprene

(gutta percha)

H atom and CH3 group onopposite sides of chain


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VMSE: Stereo and Geometrical

Isomers

8

Manipulate and rotate polymer structures in 3-dimensions


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10

Copolymers

two or more monomerspolymerized together

random A and B randomlypositioned along chain

alternating A and B

alternate in polymer chain block large blocks of A

units alternate with largeblocks of B units

graft

chains of B unitsgrafted onto A backbone

A B

random

block

graft

Adapted from Fig.

4.9, Callister &

Rethwisch 4e.

alternating

Sketch the repeat structure of Poly(Styrene-butadine)alternate polymer


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Several Repeat Units in Rubbers


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Thermoplastic and

Thermosetting Polymers

Thermoplastic Polymers

Becomes soft and formable when heated

Hardened when cooled significantly below their softening

point reversible process

Readily recycled Produced in one step and then made into products in a

subsequent process

Thermosetting Polymers

Do not soften upon heating cannot be shaped or formed to any great extent

Produced and formed in the same step


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14

Polymer Crystallinity

Crystalline regions thin platelets with chain folds at faces (Lamellae)

Chain foldedstructure

10 nm

Chain-folded model


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15

Polymer Crystallinity (cont.)

Polymers rarely 100% crystalline

Difficult for all regions of all chains tobecome aligned

Degree of crystallinity

expressed as % crystallinity.-- Some physical properties

depend on % crystallinity.

-- Heat treating causes

crystalline regions to grow

and % crystallinity toincrease.

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.)

crystallineregion

amorphous

region


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Polymer Crystallinity

Packing of molecular chains so as to produce anordered array

Much more complex than metals / ceramics

Not just atomic or molecular arrangements, rather

molecular chains involved

s is the density of a specimen for which % crystallinity is tobe determined

a is the density of the totally amorphous polymer

c is the density of the totally crystalline polymer

100*)(

)(%

acs

ascitycrystallin


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Example

lecture 7 - polymers

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