Polymers

What is a polymer?

Poly mermany repeat unit

C C C C C C

HHHHHH

HHHHHH

Polyethylene (PE)

ClCl Cl

C C C C C C

HHH

HHHHHH

Polyvinyl chloride (PVC)

HH

HHH H

Polypropylene (PP)

C C C C C C

CH3

HH

CH3CH3H

repeat

unit

repeat

unit

repeat

unit

Ancient Polymer History

• Originally many natural polymers were used

– Wood – Rubber

– Cotton – Wool

– Leather – Silk

• Oldest known uses of “Modern Polymers”

– Rubber balls used by Incas

– Noah used pitch (a natural polymer)

for the ark – as had all ancient mariners!

Polymer Composition

Most polymers are hydrocarbons– i.e. made up of H and C

(we also recognize Si-H ‘silicones’)

• Saturated hydrocarbons

– Each carbon bonded to four other atoms

C C

H

H HH

HH

CnH2n+2

Unsaturated Hydrocarbons

• Double & triple bonds relatively reactive – can form new bonds

– Double bond – ethylene or ethene - CnH2n

• 4-bonds, but only 3 atoms bound to C’s

– Triple bond – acetylene or ethyne - CnH2n-2

C C

H

H

H

H

C C HH

Isomerism

• Isomerism

– two compounds with same chemical formula can have quite

different structures

Ex: C8H18

• n-octane

• 2-methyl-4-ethyl pentane (isooctane)

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3=

H3C CH

CH3

CH2 CH

CH2

CH3

CH3

H3C CH2 CH3( )6

Chemistry of Polymers

• Free radical polymerization

• Initiator: example - benzoyl peroxide

C

H

H

O O C

H

H

C

H

H

O2

C C

H H

HH

monomer(ethylene)

R +

free radical

R C C

H

H

H

H

initiation

R C C

H

H

H

H

C C

H H

HH

+ R C C

H

H

H

H

C C

H H

H H

propagation

dimer

R= 2

Chemistry of Polymers

• Free radical polymerization (addition polymerization)

• Initiator: example - benzoyl peroxide

C

H

H

O O C

H

H

C

H

H

O2

C C

H H

HH

monomer(ethylene)

R +

free radical

R C C

H

H

H

H

initiation

R C C

H

H

H

H

C C

H H

HH

+ R C C

H

H

H

H

C C

H H

H H

propagation

dimer

R= 2

Condensation Polymerization

• Some of the original monomer’s materials are shed

(condensed out) during polymerization process

• Process is conducted in the presence of a catalyst

• Water, CO2 are commonly condensed out but other

compounds can be emitted including HCN or other acids

Water is “Condensed out”

during polymerization of Nylon

Bulk or Commodity Polymers

NOTE: See Table 15.3 for commercially important

polymers – including trade names

MOLECULAR WEIGHT

molecules of #total

polymer of wttotalnM

iiw

iin

MwM

MxM

Mw is more sensitive to

higher molecular

weights

• Molecular weight, Mi: Mass of a mole of chains.

Lower M higher M

Adapted from Fig. 14.4, Callister 7e.

Molecular Weight Calculation

Example: average mass of a class

N i M i x i wi

# of students mass (lb)

1 100 0.1 0.054

1 120 0.1 0.065

2 140 0.2 0.151

3 180 0.3 0.290

2 220 0.2 0.237

1 380 0.1 0.204

M n M w

186 lb 216 lb

iiw MwM

iin MxM

i ii

i i

all i

N Mw

N M

ii

i

all i

Nx

N

Degree of Polymerization, n

n = number of repeat units per chain

ii

wiiw

niin

mfm

m

m

Mnwn

m

Mnxn

unit repeat of weightmolecular average where

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

C C C C

H

H

H

H

H

H

H

H

H( ) ni = 6

mol. wt of repeat unit iChain fraction

• Covalent chain configurations and strength:

Direction of increasing strength

Molecular Structures

Branched Cross-Linked NetworkLinear

secondarybonding

Polymers – Molecular Shape

Conformation – Molecular orientation can be

changed by rotation around the bonds

– note: no bond breaking needed

Adapted from Fig.

14.5, Callister 7e.

Polymers – Molecular Shape

Configurations – to change must break bonds

• Stereoisomerism

EB

A

D

C C

D

A

BE

mirror

plane

C C

R

HH

HC C

H

H

H

R

or C C

H

H

H

R

Tacticity

Tacticity – stereoregularity of chain

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

C C

H

H

H

R

C C

H

H

H

R

C C

H

H

H

R R

H

H

H

CC

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

syndiotactic – R groups

alternate sides

atactic – R groups random

cis/trans Isomerism

C C

HCH3

CH2 CH2

C C

CH3

CH2

CH2

H

cis

cis-isoprene

(natural rubber)

bulky groups on same

side of chain

trans

trans-isoprene

(gutta percha)

bulky groups on opposite

sides of chain

Copolymers

two or more monomers polymerized together

• random – A and B randomly vary in chain

• alternating – A and B alternate in polymer chain

• block – large blocks of A alternate with large blocks of B

• graft – chains of B grafted on to A backbone

A – B –

random

block

graft

alternating

Polymer Crystallinity

Ex: polyethylene unit cell

• Crystals must contain the

polymer chains in some way

– Chain folded structure

• Thermoplastics:-- little crosslinking

-- ductile

-- soften w/heating

-- polyethylene

polypropylene

polycarbonate

polystyrene

• Thermosets:-- large crosslinking

(10 to 50% of mers)

-- hard and brittle

-- do NOT soften w/heating

-- vulcanized rubber, epoxies,

polyester resin, phenolic resin

Thermoplastics vs. Thermosets

Callister, Fig. 16.9

T

Molecular weight

Tg

Tmmobile liquid

viscousliquid

rubber

tough plastic

partially crystalline solid

crystalline solid

Polymer Additives

Improve mechanical properties, processability, durability, etc.

• Fillers

– Added to improve tensile strength & abrasion resistance, toughness & decrease cost

– ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc.

• Plasticizers

– Added to reduce the glass transition

temperature Tg

– commonly added to PVC - otherwise it is brittle

Polymer Additives

• Stabilizers

– Antioxidants

– UV protectants

• Lubricants

– Added to allow easier processing

– “slides” through dies easier – ex: Na stearate

• Colorants

– Dyes or pigments

• Flame Retardants

– Cl/F & B

Processing of Plastics

• Thermoplastic –

– can be reversibly cooled & reheated, i.e. recycled

– heat till soft, shape as desired, then cool

– ex: polyethylene, polypropylene, polystyrene, etc.

• Thermoset

– when heated forms a network

– degrades (not melts) when heated

– mold the prepolymer then allow further reaction

– ex: urethane, epoxy

Polymer Types: Elastomers

Elastomers – rubber

• Crosslinked materials

– Natural rubber

– Synthetic rubber and thermoplastic elastomers

• SBR- styrene-butadiene rubberstyrene

– Silicone rubber

butadiene

Recycling

Most plastics are produced from crude oil, coal or gas.

Many of them are not biodegradable and have become a visible part of our environmental litter.