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Chapter 4 -
CHAPTER 4:POLYMER STRUCTURES
Spherulite, rubber specimen. Chain-folded lamellar crystallites, ~10 nm thick, 30,000×
2
ISSUES TO ADDRESS...
• 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 ?
4.1 Structures of Polymers
• Introduction and Motivation– Polymers are extremely important materials (i.e. plastics)– Have been known since ancient times – cellulose, wood, rubber,
etc..– Biopolymers – proteins, enzymes, DNA …– Last ~50 years – tremendous advances in synthetic polymers
– Just like for metals and ceramics, the properties of polymers• Thermal stability
• Mechanical properties
Are intimately related to their molecular structure …
Chapter 4 - 4
4.1 Ancient PolymersOriginally natural
polymers were used:– Wood– Rubber– Cotton– Wool– Leather– Silk
Oldest known use:Rubber balls used by IncasNoah used pitch (a natural polymer) for the ark
Noah's pitchGenesis 6:14 "...and cover it inside and outside with pitch."
gum based resins extracted from pine trees
Chapter 4 - 5
4.2 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
Chapter 4 - 6
4.2 Unsaturated Hydrocarbons
• Double & triple bonds somewhat unstable• Thus, 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
Chapter 4 -
4.2 Structures of Polymers• about hydrocarbons
– Why? Most polymers are hydrocarbon (e.g. C, H) based
– Bonding is highly covalent in hydrocarbons– Carbon has four electrons that can participate in bonding,
hydrogen has only one– Saturated versus unsaturated
C C
C C
H
H
H
H
H H
Ethylene
Acetylene
C C
H
H
H
H
H
HEthane
Unsaturated Saturated
• Unsaturated – species contain carbon-carbon double/triple bonds
• Possible to substitute another atom on the carbon
• Saturated – carbons have four atoms attached
• Cannot substitute another atom on the carbon
c04eqf02
4.2 Hydrocarbon Molecules
EthyleneEthene
AcetyleneEthyne
(normal) butaneisobutane
Hydrocarbons have strong chemical bonds, but interact only weakly with one another (van der Waals’ forces)
4.2 Isomerismcompounds with same chemical formula can have quite different structures
for example: C8H18
• normal-octane
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
Isomerism – compounds of the same chemical composition but different atomic arrangements (i.e. bonding connectivity)
2,4-dimethylhexane
Chapter 4 - 10
4.2
4.3 Polymer Molecules
Molecules are giganticMacromoleculesRepeat unitsMonomer
4.3 Polymers• Polymer molecules
– what is a polymer?– Polymers are molecules (often called
macromolecules) formed from a series of building units (monomers) that repeat over and over again
*
C
C
H H
poly-ethylene
*
H H
mer unit :C
C
H H
H H
n
n is often a very large number!e.g. can make polyethylene with MW > 100,000! ~3600 mers ~7200 carbons
• polymers can have a range of molecular weights
• There are many monomers• Can make polymers with
different monomers, etc..
Chemistry of polymer moleculesExample: ethylene
• Gas at STP
• To polymerize ethylene, typically increase T, P and/or add an initiator
C C
H
H
H
H
+ R C C
H
H
H
H
R
C C
H
H
H
H
R + C C
H
H
H
H
C C
H
H
H
H
R C C
H
H
H
H
R* = initiator; activates the monomer to begin chain growth
After many additions of monomer to the growing chain…
*
C
C
H H
poly-ethylene
*
H H
n
Initiation
Propagation
C
H
H
O O C
H
H
C
H
H
O2 R= 2
Initiator: example - benzoyl peroxide
4.4 Polymer chemistry• Polymers are chain molecules. They are built
up from simple units called monomers. • E.g. polyethylene is built from ethylene units:
which are assembled into long chains:
Polyethylene or polythene (IUPAC name poly(ethene)) is a thermoplastic commodity heavily used in consumer products (notably the plastic shopping bag). Over 60 million tons of the material are produced worldwide every year.
c04eqf08
c04eqf09
Tetrafluoroethylene monomer polymerize to form PTFE or polytetrafluoroethylene
Vinyl chloride monomer leads to poly(vinyl chloride) or PVC
poly(tetrafluoroethene) or poly(tetrafluoroethylene) (PTFE) is a synthetic fluoropolymer. PTFE is the DuPont brand name Teflon. Melting: 327C
PVC: manufacturing toys, packaging, coating, parts in motor vehicles, office supplies, insulation, adhesive tapes, furniture, etc. Consumers: shoe soles, children's toys, handbags, luggage, seat coverings, etc. Industrial sectors: conveyor belts,
printing rollers. Electric and electronic equipment: circuit boards, cables, electrical boxes, computer housing.
Chapter 4 -
17
Chemistry and Structure of Polyethylene
Adapted from Fig. 4.1, Callister & Rethwisch 3e.
Note: polyethylene is a long-chain hydrocarbon- paraffin wax for candles is short polyethylene
• Polymer = many mers
C C C C C CHHHHHH
HHHHHH
Polyethylene (PE)
mer
ClCl Cl
C C C C C CHHH
HHHHHH
Polyvinyl chloride (PVC)
mer
Polypropylene (PP)
CH3
C C C C C CHHH
HHHHHH
CH3 CH3
mer
Adapted from Fig. 14.2, Callister 6e.
Polymer chemistry– In polyethylene (PE) synthesis, the monomer is ethylene– Turns out one can use many different monomers
• Different functional groups/chemical composition – polymers have very different properties!
*
C
C
H H
poly(ethylene)(PE)
*
H H
n *
C
C
F F
poly(tetrafluoroethylene)(PTFE, teflon)
*
F F
n
*
C
C
H H
poly(vinylchloride)(PVC)
*
Cl H
n *
C
C
H H
poly(styrene)(PS)
*
H
n
Monomers
C C
H
H
H
H
C C
F
F
F
F
C C
H
H
H
Cl
C C
H
H
H
Homopolymer and Copolymer
• Polymer chemistry– If formed from one monomer (all the repeat units are the
same type) – this is called a homopolymer – If formed from multiple types of monomers (all the repeat
units are not the same type) – this is called a copolymer
• Also note – the monomers shown before are referred to as bifunctional– Why? The reactive bond that leads to polymerization (the
C=C double bond in ethylene) can react with two other units
– Other monomers react with more than two other units – e.g. trifunctional monomers
Chapter 4 -
The Top 10 Bulk or Commodity
Chapter 4 - 21
4.5 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
Chapter 4 -
– The properties of a polymer depend on its length
– synthesis yields polymer distribution of lengths
– Define “average” molecular weight
– Two approaches are typically taken• Number average molecular weight (Mn)
• Weight-average molecular weight (Mw)
Molecular weight
Chapter 4 - 23
xi = number fraction of chains in size range i
molecules of #total
polymer of wttotalnM
MOLECULAR WEIGHT DISTRIBUTION
iiw
iin
MwM
MxM
Adapted from Fig. 4.4, Callister & Rethwisch 3e.
wi = weight fraction of chains in size range i
Mi = mean (middle) molecular weight of size range i
Molecular weightAre the two different? Yes, one is essentially based on mole fractions, and the other on weight fractions
They will be the same if all the chains are exactly of the same MW! If not Mw > Mn
Get Mn from this
Get Mw from this
Molecular weight– Other ways to define polymer MW
– Degree of polymerization
• Represents the average number of mers in a chain. The number and weight average degrees of polymerization are
m
Mn
n
n m
Mn
w
w
m is the mer MW in both cases. In the case of a copolymer (something with two or more mer units), m is determined by jjmfm
fj and mj are the chain fraction and molecular weight of mer j
Example Problem 4.1– Given the following data determine the
• Number average MW• Number average degree of polymerization• Weight average MW
How to find Mn?1. Calculate xiMi
2. Sum these!
molgM n /150,21
Number average MW (Mn)
MW range (g/mol) Mean (M i)
Min Max x i (g/mol)
5000 10000 0.05 750010000 15000 0.16 1250015000 20000 0.22 1750020000 25000 0.27 2250025000 30000 0.20 2750030000 35000 0.08 3250035000 40000 0.02 37500
x iM i (g/mol)
37520003850607555002600750
c04tf04a
Example Problem 4.1Number average degree of polymerization
– (MW of H2C=CHCl is 62.50 g/mol)
m
Mn
n
n
MW range (g/mol) Mean (M i)
Min Max wi (g/mol)
5000 10000 0.02 750010000 15000 0.10 1250015000 20000 0.18 1750020000 25000 0.29 2250025000 30000 0.26 2750030000 35000 0.13 3250035000 40000 0.02 37500
How to find Mw?1. Calculate wiMi
2. Sum these!
molgM w /200,23
wiM i (g/mol)
15012503150652571504225750
10.1/150,21
/200,23
molg
molg
M
M
n
w
338/50.62
/150,21
molg
molg
Weight average molecular weight (Mw)
c04tf04b
Chapter 4 - 30
Degree of Polymerization, DP
DP = average number of repeat units per chain
iimfm
m
:follows as calculated is this copolymers for
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( ) DP = 6
mol. wt of repeat unit iChain fraction
m
MDP
n
Chapter 4 - 31
4.6 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. 4.5, Callister & Rethwisch 3e.
– C-C bonds are typically 109° (tetrahedral, sp3 carbon)– If you have a macromolecule with hundreds of C-C bonds, this
will lead to bent chains
Chapter 4 -
Structures of Polymers• Molecular shape
– Taking this idea further, can also have rotations about bonds• Leads to “kinks”, twists
• “the end-to-end distance of a polymer chain in the solid state (or in solution) is usually much less than the distance of the fully extended chain!
• This is not even taking into account that you have numerous chains that can become entangled!
4.7 Molecular structure Physical properties of polymers depend
not only on their molecular weight/shape, but also on the difference in the chain structure
Four main structures• Linear polymers• Branched polymers• Crosslinked polymers• Network polymers
Chapter 4 - 34
Adapted from Fig. 4.7, Callister & Rethwisch 3e.
4.7 Molecular Structures for Polymers
Branched Cross-Linked NetworkLinear
secondarybonding
– polymers in which the mer units are connected end-to-end along the whole length of the chain
These types of polymers are often quite flexible• Van der waal’s forces and H-bonding are the two
main types of interactions between chains• Some examples – polyethylene, teflon, PVC,
polypropylene
Linear polymers
Branched polymers• Polymer chains can branch:
• Or the fibers may aligned parallel, as in fibers and some plastic sheets.
• chains off the main chain (backbone)– This leads to inability of chains to pack very closely together
» These polymers often have lower densities
• These branches are usually a result of side-reactions during the polymerization of the main chain
– Most linear polymers can also be made in branched forms
Crosslinked polymers • Molecular structure
– adjacent chains attached via covalent bonds• Carried out during polymerization or by a non-reversible reaction after
synthesis (referred to as crosslinking)• Materials often behave very differently from linear polymers• Many “rubbery” polymers are crosslinked to modify their mechanical
properties; in that case it is often called vulcanization
• Generally, amorphous polymers are weak and cross-linking adds strength: vulcanized rubber is polyisoprene with sulphur cross-links:
Network polymers– polymers that are “trifunctional” instead of bifunctional– There are three points on the mer that can react– This leads to three-dimensional connectivity of the polymer
backbone• Highly crosslinked polymers can also be classified as network
polymers
• Examples: epoxies, phenol-formaldehyde polymers
2
• Covalent chain configurations and strength:
Direction of increasing strength
Adapted from Fig. 14.7, Callister 6e.
POLYMER MICROSTRUCTURE
Branched Cross-Linked NetworkLinear
secondarybonding
Classification scheme for the characteristics of polymer molecules
4.8 Molecular configurations
isomerism – different molecular configurations for molecules (polymers) of the same compositionStereoisomerismGeometrical Isomerism
4.8 Molecular Configurations
Repeat unitR = Cl, CH3, etc
C CR
HH
HC C
H
H
H
R
or C C
H
H
H
R
Stereoisomers are mirrorimages – can’t superimposewithout breaking a bond
Configurations – to change must break bonds
EB
A
DC C
D
A
BE
mirror plane
Head to-head
Head to-tail Typically the head-to-tail configuration dominates
Structures of Polymers
• Stereoisomerism– Denotes when the mers are linked together in the same way
(e.g. head-to-tail), but differ in their spatial arrangement– This really focuses on the 3D arrangement of the side-chain
groups– Three configurations most prevalent
• Isotactic
• Syndiotactic
• Atactic
ISOTACTIC• Stereoisomerism
– Isotactic polymers– All of the R groups are on the same side of the chain
C
C
C
C
C
C
C
R R R R
H H H
H H H H
HHH
Isotactic configuration
• Note: All the R groups are head-to-tail• All of the R groups are on the same side of the chain
• Projecting out of the plane of the slide• This shows the need for 3D representation to understand
stereochemistry!
SYNDIOTACTIC• Stereoisomerism
– Syndiotactic polymers– The R groups occupies alternate sides of the chain
Syndiotactic configuration
• Note: The R groups are still head-to-tail• R groups alternate – one of out of the plane, one into the plane
C
C
C
C
C
C
C
R H R H
H H H
H R H R
HHH
ATACTIC• Stereoisomerism
– Atactic polymers– The R groups are “random”
Atactic configuration
• R groups are both into and out of the plane, no real registry• Two additional points
• Cannot readily interconvert between stereoisomers – bonds must be broken
• Most polymers are a mix of stereoisomers, often one will predominate
C
C
C
C
C
C
C
R R H R
H H H
H H R H
HHH
Stereoisomerism—Head-to-tail
Syndiotactic conformation
Atactic conformation
isotactic configuration
Chapter 4 - 48
cis/trans Isomerism
C CHCH3
CH2 CH2
C CCH3
CH2
CH2
H
cis
cis-isoprene (natural rubber)
H atom and CH3 group on same side of chain
trans
trans-isoprene (gutta percha)
H atom and CH3 group on opposite sides of chain
c04eqf18
Geometrical Isomerism
4.9 Plastics• variety of properties due to their rich chemical makeup
• They are inexpensive to produce, and easy to mold, cast, or machine.
• Their properties can be expanded even further in composites with other materials.
Glass-rubber-liquid• Amorphous plastics have a complex thermal profile with
3 typical states:
Log(stiffness)Pa
Temperature
3
9
6
7
8
4
5
Glass phase (hard plastic)
Rubber phase (elastomer)
Liquid
Leathery phase
THERMOPLASTICS• Thermosetting and thermoplastic polymers
– Another way to categorize polymers – how do they respond to elevated temperatures?
– Thermoplastics – these materials soften when heated, and harden when cooled – this process is totally reversible
• This is due to the reduction of secondary forces between polymer chains as the temperature is increased
• Most linear polymers and some branched polymers are thermoplastics
THERMOSETS• Thermosetting and thermoplastic polymers
– Thermosets – these materials harden the first time they are heated, and do not soften after subsequent heating
• During the initial heat treatment, covalent linkages are formed between chains (i.e. the chains become cross-linked)
• Polymer won’t melt with heating – heat high enough it will degrade
• Network/crosslinked polymers are typically thermosets
• Polymers which irreversibly change when heated are called thermosets.
• Most often, the change involves cross-linking which strengthens the polymer (setting).
• Thermosets will not melt, and have good heat resistance.
• They are often made from multi-part compounds and formed before setting (e.g. epoxy resin).
• Setting accelerates with heat, or for some polymers with UV light.
Thermoplastics• Polymers which melt and solidify without chemical
change are called thermoplastics. • They support hot-forming methods such as injection-
molding and FDM.
3
• Thermoplastics: --little cross linking --ductile --soften w/heating --polyethylene (#2) polypropylene (#5) polycarbonate polystyrene (#6)
• Thermosets: --large cross linking (10 to 50% of mers) --hard and brittle --do NOT soften w/heating --vulcanized rubber, epoxies, polyester resin, phenolic resin
Callister, Fig. 16.9
T
Molecular weight
Tg
Tmmobile liquid
viscous liquid
rubber
tough plastic
partially crystalline solid
crystalline solid
Adapted from Fig. 15.18, Callister 6e. (Fig. 15.18 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.)
THERMOPLASTICS VS THERMOSETS
4.10 Structures of Polymers• Copolymers
– Idea – polymer that contains more than one mer unit– Why? If polymer A has interesting properties, and polymer B has
(different) interesting properties, making a “mixture” of polymer should lead to a superior polymer
“Random” copolymer – exactly what it sounds like
“Alternating” copolymer – ABABABA…
Structures of Polymers
• Copolymers– Idea – polymer that contains more than one mer unit– Why? If polymer A has interesting properties, and polymer B has
(different) interesting properties, making a “mixture” of polymer should lead to a superior polymer
“Block” copolymers. Domains of “pure” mers
“Graft” copolymers. One mer forms backbone, another mer is attached to backbone and is a sidechain (it is “grafted” to the other polymer)
Chapter 4 - 58
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
Adapted from Fig. 4.9, Callister & Rethwisch 3e.
alternating
Chapter 4 -
Copolymers
• Polymers often have two different monomers along the chain – they are called copolymers.
• With three different units, we get a terpolymer. This
gives us an enormous design space…
4.11 Polymer structure• The polymer chain layout determines a lot of material
properties:
• Amorphous:
• Crystalline:
Chapter 4 - 61
Crystallinity in Polymers
• Ordered atomic arrangements involving molecular chains
• Crystal structures in terms of unit cells
• Example shown
– polyethylene unit cell
Adapted from Fig. 4.10, Callister & Rethwisch 3e.
– Polymers can be crystalline (i.e. have long range order)
– However, given these are large molecules as compared to atoms/ions (i.e. metals/ceramics) the crystal structures/packing will be much more complex
Structures of Polymers
• Polymer crystallinity– (One of the) differences between small molecules and
polymers– Small molecules can either totally crystallize or
become an amorphous solid– Polymers often are only partially crystalline
• Why? Molecules are very large• Have crystalline regions dispersed within the remaining
amorphous materials• Polymers are often referred to as semicrystalline
Structures of Polymers
• Polymer crystallinity– Another way to think about it is that these are two
phase materials (crystalline, amorphous)– Need to estimate degree of crystallinity – many ways
• One is from the density
100%
acs
ascitycrystallin
Structures of Polymers
4.11 Polymer crystallinity– What influences the degree of crystallinity
• Rate of cooling during solidification• Molecular chemistry – structure matters
– Polyisoprene – hard to crystallize
– Polyethylene – hard not to crystallize
• Linear polymers are easier to crystallize• Side chains interfere with crystallization• Stereoisomers – atactic hard to crystallize (why?); isotactic,
syndiotactic – easier to crystallize• Copolymers – more random; harder to crystallize
Chapter 4 - 65
4.11 Polymer Crystallinity (cont.)
Polymers rarely 100% crystalline• Difficult for all regions of all chains to
become aligned
• Degree of crystallinity expressed as % crystallinity. -- Some physical properties depend on % crystallinity. -- Heat treating causes crystalline regions to grow and % crystallinity to increase.
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.)
crystalline region
amorphousregion
Chapter 4 -
• Molecular weight, Mw: Mass of a mole of chains.
4
smaller Mw larger Mw
• Tensile strength (TS): --often increases with Mw. --Why? Longer chains are entangled (anchored) better.
• % Crystallinity: % of material that is crystalline. --TS and E often increase with % crystallinity. --Annealing causes crystalline regions to grow. % crystallinity increases.
crystalline region
amorphous region
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.)
4.11 MOLECULAR WEIGHT & CRYSTALLINITY
Chapter 4 -
4.12 Polymer crystals– Chain folded-model
• Many polymers crystallize as very thin platelets (or lamellae)
• Idea – the chain folds back and forth within an individual plate (chain folded model)
4.12 Polymer Crystallinity
• Crystalline regions – thin platelets with chain folds at faces– Chain folded structure
Chapter 4 - 68
4.12 Single Crystals
• Electron micrograph – multilayered single crystals (chain-folded layers) of polyethylene
• Single crystals – only for slow and carefully controlled growth rates
Adapted from Fig. 4.11, Callister & Rethwisch 3e.
Chapter 4 - 69
4.12 Semicrystalline Polymers
Spherulite surface
Adapted from Fig. 4.13, Callister & Rethwisch 3e.
• Some semicrystalline polymers form spherulite structures
• Alternating chain-folder crystallites and amorphous regions
• Spherulite structure for relatively rapid growth rates
Structures of Polymers
• Polymer crystals– More commonly, many polymers that crystallize from a melt form
spherulites• One way to think of these – the chain folded lamellae have
amorphous “tie domains” between them• These plates pack into a spherical shape• Polymer analogues of grains in polycrystalline
metals/ceramics
Chapter 4 - 71
Photomicrograph – Spherulites in Polyethylene
Adapted from Fig. 4.14, Callister & Rethwisch 3e.
Cross-polarized light used -- a maltese cross appears in each spherulite
END of chapter 4
