chemical structure and polymer properties (chapter 4 in ... · pdf filechemistry 5861 -...

Chemistry 5861 - Polymer Chemistry 1

Chemical Structure and Polymer Properties (Chapter 4 in Stevens)1

I Introduction - Materials & Fabrication Methods in Antiquity

Traditional Materials & Their Modern Analogues A)

1)

a)

b)

Rigid Thermoset Materials

Antiquity:

i)

ii)

iii)

iv)

i)

ii)

Stone

Nature’s Thermoset

Pottery/Ceramics

Made from natural/modified class

Most common & useful materials from antiquity to archeology

Concrete

Coliseum

Pilings

Plaster

Modern:

Phenol/Urea-Formaldehyde Resins

Bakelite

Plywood, Particle Board, Oriented Strand Board

Epoxy Resins

household

aircraft

1 The graphics in these notes indicated by “Figure/Table/Equation/Etc., x.x in Stevens” are taken from our lecture text: “Polymer Chemistry: An Introduction - 3rd Edition” Malcolm P. Stevens (Oxford University Press, New York,

©2002, Dr. Allen D. Hunter, Youngstown State University Department of Chemistry


2)

a)

b)

3)

a)

b)

Rigid Thermoplastic Materials

Antiquity:

i)

ii)

i)

ii)

i)

ii)

Metal

much more widely used than once suspected - recycling

Glass

much more widely used than once suspected - recycling

natural glass

obsidian

man-made glass

Modern:

Highly crosslinked polymers

Some high Tg polymers

Flexible Thermoset Materials

Antiquity:

Natural & Semi-synthetic materials

Bone & Horn

Sinew

Leather

Hair

Wood

Man-made materials

Animal glue based composites

Modern:

1999).



i)

i)

i)

Wire

Thermoset elastomers (medium and low crosslink density)

4)

a)

b)

B)

Flexible Thermoplastic Materials

Antiquity:

True Thermoplastics ???

Horn

Wood

Modern:

Most commercial thermoplastics

Traditional Processing Methods & Their Modern Analogues

1)

a)

b)

c)

2)

a)

b)

3)

a)

b)

c)

4)

5)

Molding:

Forging/Hammering/Compression

Injection/Reaction Injection

Blow Molding

Extrusion/Drawing:

Pipe

Casting:

Ceramics

Concrete

Reaction Injection

Machining of Preformed Objects:

Relative Energy, Labor, etc., Costs



II Fabrication Methods

A) Challenges With Polymers

1)

a)

b)

2)

a)

3)

B)

Viscosity

back pressure

shear

Melting

heat transfer without decomposition at edges

Throughput Considerations

Compression Molding

1)

2)

a)

b)

3)

a)

b)

Figure 4.1 in Stevens

Flow induced by applied pressure & heat

may use lower molecular weight pre-

polymer (B-Staging)

Common Uses

Thermoset polymers

i)

i)

cannot be melted once crosslinked

Composite (Fiber Reinforced) materials

e.g., fiberglass/epoxy & graphite/epoxy resins



C) Injection Molding

1)

2)

3)

a)

b)

D)


Thermoplastics

Screw Feed

⇒ pressure

⇒ heat

i)

ii)

heating coils

friction

Reaction Injection Molding, RIM

1)

2)

3)

E)


Thermoset Polymers

Polymerization-

Crosslinking occur in-situ

Blow Molding

1)

2)

3)


Thermoplastic

Common for both plastic and

glass bottles



F) Foam Production

1)

a)

b)

2)

a)

3)

4)

G)

Foaming Agent

Physical Blowing Agents

i)

ii)

iii)

i)

Dissolved liquid/gas in early stage

Gas in later stage

temperature change &/or

pressure change

Examples

halocarbons

hydrocarbons

N2, CO2, etc.

Chemical Blowing Agents

decompose and give off gas

growing gas bubbles

surface tension

Hard vs. Soft Foams

Open vs. Closed Cells

Filament Production & Winding

1)

2)

a)

b)


Extrusion of fiber

molten

in solution



c)

3)

a)

b)

c)

III

reaction in situ

i)

ii)

i)

ii)

iii)

i)

i)

ii)

crosslinks

fiber formation via monomer pyrolysis

graphite

Winding ⇒

increased alignment

increased crystallinity

increased strength/stiffness

Mechanical Properties

Dependence of Mechanical Properties on Molecular Weight A)

1)

a)

2)

a)

b)


trends in:

Viscosity/Processability

Properties

Working Range

MW Effects

end group concentration

negligible above ≈ 15,000

interchain forces

increase with MW

utility of increased chain length drops after total interchain forces are

greater than backbone strengths



3)

a)

B)

Chemical Structure effects

depend on type(s) of interchain bonding ⇒ required chain length

Mechanical Properties

1)

a)

b)

2)

a)

b)

c)

d)

3)

a)

b)

Strength & Modulus

Strength (Stress)

i)

i)

i)

i)

a measure of how much force must be applied to get the sample to change

by the desired amount (Strain)

Modulus

the resistance to deformation (≡ Stress/Strain)

Important Mechanical Properties

Tensile Strength & Modulus

Compressive Strength & Modulus

Flexural Strength & Modulus

Impact Resistance

toughness

Failure

Graceful vs. Catastrophic Failure

Fatigue

effects of repeated application of stress



C) Tensile Properties

1)

a)

b)

c)

2)

a)

b)

c)

3)

a)

b)

c)

4)

Tensile Stress, σ

a measure of how much force must be applied to get the sample to change by the

desired amount (Strain)

Tensile Stress ≡ Applied Force/Cross Sectional Area

σ = F/A

Tensile Strain, ε

a measure of sample stretching

Tensile Strain ≡ Change in Sample Length/Original Length

ε = ∆l/l

Tensile Modulus, E

the resistance to deformation

Tensile Modulus ≡ Tensile Stress/Tensile Strain

E = σ/ε

Stress-Strain Curves

a) Figure 4.7 & 4.8 in

Stevens

b)

c)

d

Brittle Material

i)

i)

high modulus

reversible before breakage

Fiber

lower or high modulus initially

epending on



degree of crystallinity

chemical structure

etc.

⇒ elongation

reversible

ii)

iii)

iv)

i)

ii)

iii)

iv)

critical stress

⇒ flow of polymer chains & untangling of chains

⇒ rapid lengthening

∴ drop in applied stress felt

irreversible

highly crystalline fiber

d)


often yields breakage soon after yield point

induced crystallinity in previously amorphous or low crystalline fiber

⇒ increased modulus

as in fiber drawing

often yields breakage soon after yield point

Elastomer

typically lower modulus overall

reversible flow

yield point

irreversible flow



5)

a)

Temperature Dependence of Modulus

Amorphous Thermoplastic

i)

ii)

iii)

i)

ii)

iii)


Tg

b)

rapid decrease of

modulus

mp

rapid decrease of modulus

Other Materials

Figure 4.10 in

Stevens

Regions of Curves

D)

Brittle-Glassy region

Rubbery/Plastic region

Liquid Region

Effects on Curves

MW Effects

Crosslink Density Effects

Crystallinity Effects

Time Dependent properties

1)

a)

Creep

cold flow



2)

E)

stress relaxation

Influences of Polymer Structure

1) Tables 4.1 & 4.2 in Stevens



IV Thermal Stability

A) Origins of Decomposition

1)

a)

b)

2)

3)

a)

b)

B)

Bond breakage

⇒ decreased materials properties

⇒ depolymerization with many polymers

i) this is especially true with vinyl polymers

Aromatization upon heating

Thermal Stability Criterion

decompose at ≥ 400 °C

retain useful properties near this

temperature

Thermally Stable Polymers

1)

a)

2)

a)

b)

c)

3)

Table 4.3 in Stevens

Inert atmosphere values

Primarily a function of bond energy

i.e., increased bond vibration ⇒

bond rupture

aromatics have advantage in that breaking one bond does not disrupt structure

this is especially true with ladder type polymers where there are no single atom-

atom interactions that solely hold together the polymer backbone

Thermooxidative Stability values



a)

b)

V

initial onset may be lower than those in inert atmospheres and the TGA curve will

typically be different

initial decomposition is still via bond rupture then new mechanism involving

oxidation may come into play

Flammability and Flame Resistance

A) Flammability Issues

1)

2)

a)

b)

c)

3)

a)

b)

4)

a)

b)

c)

d)

5)

a)

Mechanical Failure

Heat Induced Damage

direct heat effects on skin

high temperature gasses

hot “molten” materials on skin

Toxicity

evolved gasses

particulates

Relative Importance Varies for

Structural Components

Coatings

Fabrics

etc.

Terminology

Nonflammable

i) e.g., PVC



b)

c)

B)

Self-Extinguishing

i)

i)

ii)

iii)

iv)

v)

i)

e.g., polycarbonates

Flammable

Mechanism of Flames

1)

2)

a)

b)

c)

3)

a)

b)

e


Flame Components

Pyrolysis Zone

Diffusion Zone

Combustion Zone

Kinetic & Thermodynamic Processes

Heat Transfer/Loss

generated by ignition source

generated by combustion

causes pyrolysis

heating of gasses

radiative loss

Mass Transfer

diffusion of fuel molecules

CO

C2H2

H2

tc.



ii)

iii)

i)

ii)

iii)

i)

ii)

iii)

diffusion of oxygen molecules

diffusion of combustion products

4)

a)

b)

CO2

H2O

etc.

Flame resistance/retardent strategies

starve flame for fuel

decrease pyrolysis

insulating/impermeable crusts

char formation

phosphorous containing materials in cellulosic fibers

cool pyrolysis zone

Al2O3.3H20

NaHCO3

inhibit heat flow to pyrolysis zone

must slow combustion

Free Radical Scavengers (Radical Traps)

halogens

toxicity

phosphorous compounds

increased amounts of spectator gasses

their heat capacity lowers flame temperature



VI Chemical Resistance

A) Morphology Influence

1)

a)

b)

c)

2)

a)

b)

VII

For most reactions, chemical reagents must diffuse into bulk polymer

∴ deduced diffusion ⇒ reduced reactivity

decrease pore size/volume

make polymer and chemical incompatible so that they repel each other

i)

i)

ii)

i)

i)

ii)

iii)

e.g.,

hydrophobic polymers

fluoropolymers

Chemical Reactivity

Polymers react similarly to their discrete molecule analogues

albeit often slower

∴ choose polymers that chemically are expected to be unreactive

e.g.,

Ozone converts C=C into carbonyls

Electrical Conductivity

Band Structure & Conductivity A)

1)

a)

Relationship of Molecular Orbitals to Bands

Hückel Orbitals for Aromatics

Benzene

Naphthalene

Anthracene



iv)

i)

ii)

iii)

iv)

i)

ii)

iii)

i)

ii)

Graphite

b)

2)

a)

b)

c)

B)

Structural Isomers of PAHs

Anthracene & Phenanthrene

changes to HOMO-LUMO Gap

Nature of HOMO

etc.

Classes of Conductors

Metallic Conductors

Semiconductors

thermal population

photoconductors

n-type & p-type doped semiconductors

Insulators

Polymeric Conductors/Semiconductors

1)

a)

Structural Features for Conductivity

Delocalization usually required

conjugation of π system

Molecules 20-23 in Stevens

poly(sulfur nitride)

polyacetylene

exceptions via charge transfer between pendant groups

poly(N-vinyl-cabazole), 19



b)

c)

2)

a)

b)

c)

poly(vinyl ferrocene)

Dopants

Morphology

Mechanism of Conductivity

Interchain Electron Transfer!

Valence & Conduction band populations

Solitons

i)

i)

ii)

i)

ii)

i)


d) Doping

Molecules 23a in Stevens

3)

a)

b)

p-type doping

n-type doping

Property Changes on Doping

thermal stability

oxidative stability

processability

Measuring Conductivity

Units of conductivity, σ

siemens (S)/cm

siemen = 1/ohm

Classes

Insulators

σ < 10-8 S/cm



ii)

iii)

i)

ii)

Semiconductors

c)

10-8 < σ < 102 S/cm

Conductors

σ . 102 S/cm

Examples

poly(sulfur nitride), 20

100 S/cm

superconductive at about 0.3 K

polyacetylene (undoped)

1.7 x 10-9 S/cm for cis, 22

4.4 x 10-5 S/cm for trans, 23

doped materials are conductive in metallic range

Molecules 23a in Stevens

doping converts cis to trans isomers

C) Conjugated Organic Polymeric Conductors

1) Examples of Conjugated Organics

a) Molecules 24-28 in Stevens

b) polyaniline, 24

c) polypyrole, 25

d) polythiophene, 26

e) poly(para-phenylene), 27

f) poly(para-phenylenevinylene), 28



2) Experimental Conductivities

a)

b)

3)

a)

b)

c)

d)

e)

4)


Typically less than those of polyacetylene

i)

ii)

i)

ii)

iii)

sufficient for commercial application

better stabilities

Applications

anti-static materials

light weight batteries

thin film transistors

light-emitting diodes

molecular electronics

Ionic Conductors/Electrolytes

a) Molecules 29 & 30 in Stevens

b) poly(ethylene oxide), 29

c)

d)

polyphosphazenes, 30

Key properties

highly amorphous

low Tg

⇒ highly mobile ions

e.g., Li+



VIII Nonlinear Optical, NLO, Properties

Non-Linear Optical Behavior A)

1)

a)

2)

a)

b)

c)

3)

a)

b)

4)

a)

b)

c)

1st Order (linear) behavior

refraction, etc., typically see in our lives

2nd Order behavior, χ2

frequency doubling

requires molecular dipoles on Chromophores

i)

ii)

i)

ii)

iii)

i)

increases with

extended conjugation

π donor/acceptor strength

called β values in solution

requires that they be aligned

Poling of polymeric solids

non-centrosymmetric single crystals

host-guest complexes

3rd Order behavior, χ3

frequency tripling

requires polarizable materials on Chromophores

2nd & 3rd order effects

interaction of electric fields of light with those of sample

⇒ electro-optic interactions

⇒ electrical-light signal converters

⇒ optical-optical interactions



i)

i)

ii)

iii)

iv)

v)

vi)

i)

⇒ all optical device elements

IX Additives

Turn Polymers into Plastics A)

1)

2)

a)

b)

3)

a)

b)

B)

Found in almost all commercial plastics

Purposes

Alter polymer properties

pigments

oderants

stability

mechanical property modification

crosslinking agents

etc.

Improve processability

lubricants

Interactions with Polymers

Miscible

Immiscible

Commercial Additives

109 kg used per year in US 1)

2) Classes

a) Table 4.6 in Stevens




3) Examples

a)

b)


Plasticizers

i)

ii)

iii)

permanence

“new car smell”

solubility parameters

toxicity

di-2-ethylhexyl phthalate, 33

chemical structure and polymer properties (chapter 4 in ... · pdf filechemistry 5861 -...

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