materials of engineering engr 151 polymer structures · 2017-04-19 · learning objectives...
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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
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