POLYMER STRUCTURES

Materials of Engineering

ENGR 151

LEARNING OBJECTIVES

Understand different molecular and crystal

structures of polymers

What are the general structural and chemical

characteristics of polymer molecules?

What are some of the common polymeric

materials, and how do they differ chemically?

How is the crystalline state in polymers

different from that in metals and ceramics ?

POLYMER

A solid, nonmetallic (typically organic)

compound of high molecular weight

Consisting of small repeat units (mer)

WHAT IS A POLYMER?

Poly mer many repeat unit

Adapted from Fig. 14.2, Callister & Rethwisch 9e.

C C C C C C

H H H H H H

H H H H H H

Polyethylene (PE)

Cl Cl Cl

C C C C C C

H H H

H H H H H H

Poly(vinyl chloride) (PVC)

H H

H H H H

Polypropylene (PP)

C C C C C C

CH3

H H

CH3 CH3 H

repeat

unit

repeat

unit

repeat

unit

POLYMERS

Naturally occurring

wood, rubber, cotton, wool, leather, silk

Proteins, enzymes, starches, cellulose

Production polymers are synthetic

Inexpensive

Superior properties

HYDROCARBON MOLECULES

Organic molecules

Many are hydrocarbons (contain H and C)

Hydrocarbons

Covalent bonds (sharing electrons)

Carbon (4 valence e-)

Hydrogen (1 valence e-)

MOLECULES

Unsaturated: molecules with double/triple bonds

Each C atom is not bonded to max four other atoms

More atoms or group of atoms can attach to C atom

Saturated: molecules with single bonds only

No new atoms can join without removal of other atoms

POLYMER COMPOSITION Most polymers are hydrocarbons

– i.e., made up of H and C

Saturated hydrocarbons

Each carbon singly bonded to four other atoms

Example:

Ethane, C2H6

C C

H

H HH

HH

HYDROCARBONS

Double bond between carbon atoms

Sharing of two pairs of electrons

Triple bond between carbon atoms

Sharing of three pairs of electrons

UNSATURATED HYDROCARBONS

Double & triple bonds somewhat unstable – can form new bonds Double bond found in ethylene or ethene - C2H4

Triple bond found in acetylene or ethyne - C2H2

C C

H

H

H

H

C C HH

SIMPLE HYDROCARBONS

Paraffin family (chain-like structure):

Methane CH4

Ethane C2H6

Propane C3H8

Butane C4H10

Pentane C5H12

Hexane C6H14

Strong bonds within molecule, weak with other molecules (low

melting point)

Boiling temperatures increase with molecular weight

ISOMERISM

Molecules having different atomic

arrangements

Example: Butane

Normal butane boils at -0.5°C

Isobutane boils at -12.3°C

Butane Isobutane

ISOMERISM

Isomerism

two compounds with same chemical formula can have

quite different structures

for example: C8H18

normal-octane

2,4-dimethylhexane

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

POLYMER MOLECULES

Polymer molecules are relatively large in size

(macromolecules) compared to hydrocarbon

molecules

Backbone of carbon chain polymer molecules

are carbon strings (long and flexible)

Side bonding allowed

Radical positioning

OTHER ORGANIC GROUPS

Contain radicals (groups of atoms that remain

intact during chemical reactions)

Alcohols (radical connected to OH)

Ethers (2 radical connected to O)

Acids (radical, OH, C/O double bond)

Aldehyde (radical, H, C/O double bond)

MER UNITS

Mer: successively repeated units along chain

Polymer: many mers

Monomer: stable molecule – small molecule from

which polymer is synthesized

THE CHEMISTRY OF POLYMER MOLECULES

Ethylene (C2H4): polyethylene formed from temperature and pressure (catalytic reaction)

PE is solid polymer

Active mer (PE unit) formed from reaction

Catalyst and ethylene

Causes chain-like formation with unpaired electron

POLYMERIZATION AND

POLYMER CHEMISTRY

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(ethy lene)

R +

f ree 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

propagation

CHEMISTRY AND STRUCTURE OF

POLYETHYLENE Adapted from Fig.

14.1, Callister &

Rethwisch 9e.

Note: polyethylene is a long-chain hydrocarbon

- paraffin wax for candles is short polyethylene

POLYETHYLENE

Angle between single bonded carbon atoms ~

109° (zig-zag pattern)

Bond length ~ 0.154 nm

Replace H with Fl

Polytetraflouroethylene, PTFE

Teflon

Flourocarbon

POLYVINYL CHLORIDE (PVC)

Variant from PE

Every fourth hydrogen replaced with Cl atom

Replace each Cl with CH3 (polypropylene, PP)

Table 14.3

Polystyrene (PS)

nylon, polyester (complex polymeric structure)

23

BULK OR COMMODITY POLYMERS

24

BULK OR COMMODITY POLYMERS (CONT)

25

Bulk or Commodity Polymers (cont)

CHEMISTRY OF POLYMERS

Homopolymer: repeating units of the same type along a chain

Copolymer: chains composed of two or more mer units

Monomer:

Bifunctional: may bond with two other units to form 2-D chain-like molecular structure

Trifunctional: may bond with three other units to form 3-D network structure

MOLECULAR WEIGHT

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

Low M

high M

Not all chains in a polymer are of the same length

— i.e., there is a distribution of molecular weights

MOLECULAR WEIGHT

Large chains = large molecular weight

Average molecular weight:

Number-average molecular weight ( ):

Divide chains into different size ranges

Determine number of fraction chains within each size

range

Mi = mean molecular weight of size range i

xi = fraction of number of chains within size range

nM

n i iM x M

MOLECULAR WEIGHT

Weight-average molecular weight ( ):

Mi = mean molecular weight of size range i

wi = weight fraction of molecules within

same size interval

wM

w i iM w M

DEGREE OF POLYMERIZATION

n = average number of mer units in a chain

Number-average degree of polymerization (nn):

Weight-average degree of polymerization (nw):

m = mer molecular weight

ww

Mn

m

nn

Mn

m

EXAMPLE PROBLEM 14.1 (PG 542)

EXAMPLE PROBLEM 14.1 (PG 542)

EXAMPLE PROBLEM 14.1 (PG 542)

EXAMPLE PROBLEM 14.1 (PG 542)

MOLECULAR WEIGHT

Melting temperature increases with increased

molecular weight (up to 100,000 g/mol)

Polymers with short chains (100 g/mol) are

liquids/gases at room temp

1000 g/mol (paraffin wax, soft resin)

10,000-several million g/mol (solid polymers)

POLYMERS – MOLECULAR SHAPE

Molecular Shape (or Conformation) – chain

bending and twisting are possible by rotation

of carbon atoms around their chain bonds

note: not necessary to break chain bonds to alter

molecular shape

Adapted from Fig.

14.5, Callister &

Rethwisch 9e.

CHAIN END-TO-END DISTANCE, R

Fig. 14.6, Callister &

Rethwisch 9e.

MOLECULAR SHAPE

Single chain molecule can have a zigzag

structure due to range of 109°

Responsible for large elastic extension of rubber,

also mechanical and thermal characteristics

Bending, coiling and kinking properties

Double bonds are rotationally rigid

LEARNING OBJECTIVES

Understand different molecular structures of

polymers

Analyze different molecular configurations of

polymers

Discuss thermoplastic & thermosetting

polymers

Understand different copolymers and review

polymer crystals

MOLECULAR STRUCTURE

Polymer Characteristics

Defined by:

Molecular weight

Shape

Structural differences in chains

Linear polymers

Branched polymers

Crosslinked polymers

Network polymers

MOLECULAR STRUCTURE

Each circle represents a repeat unit

Adapted from Fig. 14.7, Callister & Rethwisch 9e.

MOLECULAR STRUCTURES FOR

POLYMERS

B ranched Cross-Linked Network Linear

secondary bonding

LINEAR POLYMERS

Mer units are joined end to end in single chain

Flexible, spaghetti-like

Van der Waals & hydrogen bonding between chains

Examples:

polyethylene, polyvinyl chloride, polystyrene, polymethyl

methacrylate, nylon, flourocarbons

BRANCHED POLYMERS

Branches connected to main chain

Caused by side reaction during polymer synthesis

More branches = lower density

Linear polymers can be synthesized to branch

CROSSLINKED POLYMERS

Linear chains joined together

Covalent bonds maintain joints

Synthesis or non-reversible chemical reactions at

high temperature

Additive molecules/atoms (covalently bonded to chains)

Rubber materials (elastic)

Formation of crosslinks (vulcanization)

NETWORK POLYMERS

Three dimensional network

Formed from mers with three covalent bonds

Highly crosslinked polymer = network polymer

Epoxies (unique bonding characteristic)

14.8 MOLECULAR CONFIGURATIONS

Properties influenced by:

Regularity & symmetry of side group arrangement (two or more side

atoms/groups)

Head-to-tail configuration

Alternating

More likely to occur

Head-to-head configuration

(R) groups adjacent to each other

Polar repulsion occurs

ISOMERISM

Recall: different atomic configuration for same composition (butane/isobutane)

Stereoisomerism

Geometrical isomerism

STEREOISOMERISM

Isotactic (® groups on same side) Syndiotactic (® alternates sides)

Atactic (random)

ROTATION NOT ALLOWED! (must sever first)

Polymer can exhibit more than one configuration

3-D image

STEREOISOMERISM – ISOTACTIC

Isotactic – R groups on same side of the polymer chain

STEREOISOMERISM – SYNDIOTACTIC

Syndiotactic – R groups on alternate sides of the polymer chain

STEREOISOMERISM – ATACTIC

Atactic – R groups randomly positioned along the polymer chain

ISOMERISM

Geometrical Isomerism

Double bonds between C atoms

C atoms bonded to other atom or radical (side group)

Cis structure (same side, cis-polyisoprene)

Natural rubber

Trans structure (opposite sides, trans-polyisoprene)

Conversion from cis to trans not possible as bond rotation is not allowed – double bond is rigid

Cis Trans

POLYMER STRUCTURE CLASSIFICATION

Chemistry

Molecular size

Molecular weight

Molecular shape

Degree of twisting, coiling, bending

Molecular structure

The way units (mers) are joined together

POLYMER STRUCTURE CLASSIFICATION

THERMOPLASTIC & THERMOSETTING POLYMERS

Further classify polymers

Based on response to rising temperature

Thermoplasts:

Soften when heated, eventual liquefaction

Harden when cooled

Reversible, repeatable

Typically linear polymer, branched polymer

Fabricated using heat and pressure

THERMOSETTING POLYMERS

Thermosets:

Permanently hard when heat is applied

No softening with heat

During heating, covalent crosslinks are formed

Crosslinks serve as anchors

10%-50% crosslinking

Harder & stronger that thermoplastics

Examples:

Vulcanized rubbers & epoxies,

COPOLYMERS

Consists of two mer units

Different sequencing possible:

Random copolymer

Alternating copolymer

Block copolymer

Graft copolymer

COPOLYMERS

Two or more monomers polymerized together

random – A and B randomly positioned along chain

alternating – A and B alternate in polymer chain

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

graft – chains of B units grafted onto A backbone

A – B –

random

block

graft

alternating

POLYMER CRYSTALLINITY

Packing of chains to form ordered atomic array

Polyethylene

Small molecule polymers:

Completely crystalline (solid)

Completely amorphous (liquid)

Typical polymers:

Semi-crystalline

CRYSTALLINITY IN POLYMERS

Ordered atomic

arrangements involving

molecular chains

Crystal structures in terms

of unit cells

Example shown

polyethylene unit cell

DEGREE OF CRYSTALLINITY

Range of complete amorphous to 95% crystalline

ρs = density of specimen

ρa = density of totally amorphous polymer

ρc = density of perfectly crystalline polymer

( )% 100

( )

c s a

s c a

crystallinity

DEGREE OF CRYSTALLINITY - EXAMPLE

DEGREE OF CRYSTALLINITY

Depends on rate of cooling during solidification

Chains must assume ordered configuration for crystallinity

Not favored for complex mers

Not easily prevented for simple mers

Linear polymers (crystalline)

Branch polymers (mostly amorphous)

Network/crosslink polymers (amorphous)

CRYSTALLINITY

Stereoisomerism

Atactic (amorphous)

Isotactic/syndiotactic (crystallize easier due to regularity)

Bulky side-bonding groups = less crystallization

Copolymers

Alternating and block (more likely to crystallize)

Random and graft (not as much likelihood)

CRYSTALLINITY

Crystalline polymers resistant to diffusion and

softening by heat

POLYMER CRYSTALS

Fringed-Micelle model

Chain molecule can pass through crystallite and amorphous regions

Chain-Folded model

Chains fold back on each other to form platelets

Spherulites (like metal grains)

Crystalline lamellae separated by amorphous material

POLYMER CRYSTALLINITY Crystalline regions

thin platelets with chain folds at faces

Chain folded structure

Fig. 14.12, Callister

& Rethwisch 9e.

≈ 10 nm

HOMEWORK

HW (Due Monday, May 15th)

14.3, 14.5, 14.7, 14.25, 14.26