polymer properties -...
TRANSCRIPT
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
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
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
10 molecules : 400,000 Dalton
4 molecules : 100,000 Dalton
2 molecules : 50,000 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
000,000,10niMi
92
55
102
44
112
33
102
22
122
11
105
104
1016
10245
102
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