polymer properties -...

Polymer Properties

Thermal Transitions: Crystallization, Melting and the Glass Transition

Polymer structure

• The polymer chain layout determines a lot

of material properties:

• Amorphous:

• Crystalline:

The Amorphous State

• Amorphous high molar mass polymers can be in the glassy state, rubbery state or melt state going from low to high temperature

• Not all volume is occupied: free volume concept

• Segmental and chain mobility strongly dependent on temperature and free volume

The Crystalline state

Many important polymers are partially crystalline

• Polyethylene

• Polypropylene

• Polyamides; Nylon 6, Nylon 6.6, Nylon 4.6

• Linear polyesters; PET

Crystallinity influences • stiffness and brittleness

• fracture strength and elongation at break

• solubility

• permeability of gases and water sorption

• many other properties

In practice, polymers are semi-crystalline

Polymer Acronym Coding Tg (ºC) Tm (ºC) Morphlgy

Low-density polyethylene LDPE 4 -130 +105 crystalline

High-density polyethylene HDPE 2 -125 +135 crystalline

Polypropylene PP 5 -27 .. -10 +165 .. +170 crystalline

Polyamide (nylon) 6,6 PA6,6 7 +55 +255 crystalline

Poly vinylchloride PVC (V) 3 +75 .. +80 -- amorphous

Polystyrene PS 6 +90 .. +100 -- amorphous

Polyethylene terephthalate

PET (PETE) 1 +67 .. +80 265 crystalline

Polycarbonate PC 7 +145 .. +150 -- amorphous

Polyurethane PU 7 +140 -- amorphous

Factors influencing crystallinity

• Cooling rate

• Chain complexity and regularity

• Side group size • Tacticity

• Cross-linking

• Branching

Ex

Polymer Structure V

a linear polymer can pack well, whereas a branched isomer cannot

Highly crystalline Highly amorphous

– Example: poly(ethylene terephthalate),

abbreviated PET or PETE, can be made with

crystalline domains of 0% to 55%.

OO

OO

n

Poly(ethylene terephthalate)

Completely amorphous PET is formed by quickly cooling the melt. PET with a low degree of crystallinity is used for plastic beverage bottles.

• Tacticity affects the physical properties

– Atactic polymers will generally be amorphous,

soft, flexible materials

– Isotactic and syndiotactic polymers will be more

crystalline, thus harder and less flexible

• Polypropylene (PP) is a good example

– Atactic PP is a low melting

– Isoatactic PP is high melting (176º), crystalline,

tough material that is industrially useful

– Syndiotactic PP has similar properties, but is very

clear. It is harder to synthesize

There are three main stereochemical classifications

for polymers.

Atactic: random orientation

Isotactic: All stereocenters have same orientation

Syndiotactic: Alternating stereochemistry

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Ex.Three possible

arrangements of nonsymmetrical

monomers: (a) isotactic,

(b)syndiotactic, and

(c)atactic.

The glass transition

temperature (Tg), is the

temperature at which the

amorphous phase of the

polymer is converted

between rubbery and glassy

states

Glass transition temperature (Tg)

: Tg IS A PROPERTY RELATED WITH THE

AMORPHOUS REGIONS OF THE

POLYMER, NOT CRYSTALLINE!

Melting

• Melting is a transition which occurs in crystalline polymers.

• Melting happens when the polymer chains fall out of their crystal structures,

and become a disordered liquid.

• Always keep this in mind: MELTING IS A PROPERTY RELATED WITH THE

CRYSTALLINE REGIONS OF THE POLYMER! So do you think you can melt

atactic polystyrene? (No, because it is not crystalline)

: What if I see both melting and

glass transition in the

differential scanning

calorimeter (DSC) spectrum of

a polymer sample???

. Remember, most polymers

are semi-crystalline, i.e. have

both amorphous and crystalline

regions. So they have Tg and

Tm

• At low temperatures, all amorphous polymers are stiff and

glassy,

• On Warming, polymers soften in a characteristic

temperature range known as the glass-rubber transition

region.

.

The following physical properties undergo a drastic change at

the glass transition temperature of any polymer:

a) hardness

b) volume

c) modulus (Young’s module) d) percent elongation-to-break

Polymer behaviour above and below Tg

Molecular Weight of Polymers

• Measurements of average molecular

weight (M.W.)

• Number average M.W. (Mn): Total

weight of all chains divided by # of

chains

• Weight average M.W. (Mw): Weighted

average. Always larger than Mn

# o f m o l e c u l e s

Mn

Mw

increasing molecular weight

Mv

Weight Average (Mw)

wi weight fraction of ni molecules having a molecular

weight of Mi

Number Average (Mn)

n1 number of molecules with a molecular weight of M1

n2 number of molecules with a molecular weight of M2,

ni number of molecules with a molecular weight of Mi.

Ex:If we have the following polymer sample:

2 molecules : 1,000,000 Dalton

5 molecules : 700,000 Dalton




1 Dalton = 1 g/mole

Ex: Calculate (Mn) and(Mw)

n1=2 M1=1,000,000 n1M1=2,000,000

n2=5 M2=700,000 n2M2=3,500,000

n3=10 M3=400,000 n3M3=4,000,000

n4=4 M4= 100,000 n4M4=400,000

n5=2 M5=50,000 n5M5=100,000

23ni 000,000,10niMi

435000

2310000000

ni

niMi

Mw

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104

1016

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Mn

Mn

Mn

Mn

Mn

122 1009,6niMi

5.609

100000001009,6 122

niMi

niMiMw

Polydispersity—We can describe the polydispersity through the width of the distribution of

molar masses.

• Mn is sensitive to the mixture of molecules of low molecular mass.

• Mw is sensitive to the mixture of molecules of high molecular mass.

• Mw always higher than Mn

• The ratio Mw/Mn is a measure of the range of molecular sizes in

the specimen it is normally in the ranges of 2 - 100 some

polymer has very small or very high value of polydispersity

index.

• This ratio is known as polydispersity or heterogeneity index.

• Monodisperse polymer would have Mw/Mn = 1.00

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