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Polymers

• Macromolecule that is formed by linking of repeating units through covalent bonds in the main backbone

• Properties are determined by molecular weight, length, backbone structure, side chains, crystallinity

• Resulting macromolecules have huge molecular weights

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Polymers

• Terminology:– mer: a unit– monomer: one unit– dimer: two units– trimer: three units– tetramer: four units– polymer: many units– pre-polymer: growing

towards being a polymer– oligomer: few units fixed in

size– homopolymer: polymer of

fixed mer type

HOMOPOLYMER

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Polymers• Terminology (contn):

– copolymer: polymers of two mer types• random · · ·-B-A-B-A-B-B-A-· · ·• alternating· · ·-A-B-A-B-A-B-A-· · · • block · · ·-A-A-A-A-B-B-B-· · ·

– heteropolymer: polymers of many mer types

COPOLYMER

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Polymers: Molecular Weight

• i: degree of polymerization (# of monomer units)Mi = i x Mm

Mi: molar mass of polymer molecule iMm: molecular weight of monomer

• Typically all chains are not equally long but display a variation– monodisperse: equal chain lengths is specific to proteins– polydisperse: unequal length specific to most synthetic

molecules• Therefore we need to define an “average” molecular

weight– number average, Mn

– weight average, Mw

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Polymers: Molecular Weight

• number average, Mn

• weight average, Mw

Ni: # of molecules with degree of polymerization of i

Mi: molecular weight of i

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Polymers: Molecular Weight

• Ratio of Mw to Mn is known as the polydispersity index (PI)

• PI is a measure of the breadth of the molecular weight• PI = 1 indicates Mw = Mn, i.e. all molecules have equal

length (monodisperse)• PI = 1 is possible for natural proteins whereas synthetic

polymers have 1.5 < PI < 5• At best PI = 1.1 can be attained with special techniques• EXERCISE: Draw the molecular weight distribution for PI

= 1, PI = 2, and PI = 4

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Polymers: Molecular Weight

• Biomedical applications: 25,000<Mn<100,000 and 50,000<Mw<300,000

• Increasing molecular weight increases physical properties; however, decreases processibility

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Polymers

• Types of polymers:– Thermoplastic: Polymers that flow when heated; thus,

easily reshaped and recycled. This property is due to presence of long chains with limited or no crosslinks. (polyethylene, polyvinylchloride)

– Thermosetting: Decomposed when heated; thus, can not be reformed or recycled. Presence of extensive crosslinks between long chains induce decomposition upon heating and renders thermosetting polymers brittle. (epoxy and polyesters)

– Elastomers: Intermediate between thermoplastic and thermosetting polymers due to presence of some crosslinking. Can undergo extensive elastic deformation (natural rubber, silicone)

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Polymers

Structure Source-Based Name Application

R = -H Polyethylene Plastic

R = -CH3 Polypropylene Rope

R = -Cl Poly(vinyl chloride) "Vinyl"

X = -H, R = -C2H5 Poly(ethyl acrylate) Latex paints

X = -CH3, R = -CH3 Poly(methyl methacrylate) Plastic

R = -H Polybutadiene Tires

R = -CH3 Polyisoprene Tires

X = -F, R = -F Polytetrafluoroethylene Teflon®

—C—C—

H H

H R

—C—C—

H H

H R

—C—C—

H H

H R

• • • • • • • •

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Polymers

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Polymers• Polymers can be either amorphous or semi-crystalline

• Tacticity, i.e. arrangements of substituents around the backbone, determines the degree of crystallinity

• Atactic polymers are amorphous

• Isotactic and syndiotactic may crystallize

• Crytallinity depends on:–size of side groups (smaller, ↑crystallinity)–regularity of chain

• Increased crystallinity enhances mechanical properties

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Polymer Synthesis

• Two common methods of polymerization– Condensation polymerization (or stepwise addition)– Addition reaction (or chain polymerization)

• Condensation: Two monomers react to form a covalent bond usually with elimination of a small molecule such as water, HCl, methanol, or CO2. Reaction continues until one type of reactant is used up.

• Addition: Monomers react through stages of initiation, propagation, and termination.– initiators such as free radicals, cations, anions opens the double

bond of the monomer which becomes active and bonds with other such monomers

– rapid chain reaction propagates in this fashion– reaction is terminated by another free radical or another polymer

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Polymer Synthesis: Condensation

• phenol-formaldehyde: results in condensation of a water molecule

• nylon (polyamide): an organic acid reacts with an amine to form an amide. HCl condenses

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Polymers Synthesis: Addition

Termination may occur by:

•two radicalized polymers reacting

•another radicalized monomer

•one initiator (alkoxy radical, •OR, in this case)

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Condensation vs. Addition• Addition:

– Difficult to control molecular weight– Undesirable branching products

• Condensation:– Molecular weight closely controlled– Polydispersity ratios close to unity can be obtained

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Polymers: Deformation

I. Chain unfolding, unwinding, unwrapping or uncoupling (low energy)

II. Chain sliding (low energy)

III. Bond stretching, side group ordering (high energy)

IV. Bond breaking (high energy)

I II III IV

Str

ess

Strain

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Polymers: Deformation

•Lower elastic modulus, yield and ultimate properties

•Greater post-yield deformability

•Greater failure strain

Str

ess

Strain

Polymers

MetalsCeramics

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Polymers: Viscoelasticity

• Dependency of stress-strain behavior on time and loading rate

• Due to mobility of chains with each other• Crosslinking may affect viscoelastic response

Str

ess

Strain

increasing loading rate

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Polymers: Thermal Properties

• In the liquid/melt state enough thermal energy for random motion (Brownian motion) of chains

• Motions decrease as the melt is cooled• Motion ceases at “glass transition temperature”

• Polymer hard and glassy below Tg, rubbery above Tg

Str

ess

Strain

decreasing temperature or increasing crystallinity

linear amorphous

log(

Mod

ulu

s)

Temperature

semicrystalline

crosslinked

TmTg

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Viscoelastic Deformation

Glassy Materials and Tg

Rigid Brittle Solid below Tg

Viscous Deformable liquid above Tg

F

A

xvd

d

η is the viscosity of the fluid o exp

Q

R T

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Viscoelastic Modulus versus Temperature

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Fracture of Polymers

•Thermoset and Thermoplastic materials below Tg behave as brittle solids and fail by cracking. The cracks are sharp.

•Crystalline Thermoplastic Resins above Tg yield and undergo ductile failure.

•Noncrystalline Thermoplastic resins above Tg undergo Crazing and Cracking.

• Crazing - Orientation of polymer chains across the opening of a “crack-like” feature. The work of crazing is 1000 times larger than the surface work to create a crack.

•Cracking then occurs down the middle of the craze.

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Craze Formation

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Crack Propagation Through Crazed Area

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Cracking Example in Polymers