How do the properties change when T < Tg? When T > Tg?
g
Plasticizers
•
Small organic molecules that act as lubricants
•
Lower Tg, make material flexible
•
Dialkyl phthalates are commonly used as plasticizers
•
Due to safety and health questions they have been banned in children’s toys
Fibers
•
Thin threads produced by extruding molten polymer through small holes in a die, or spinneret
•
Fibers are cooled and drawn out orienting the crystallite regions along the axis of the fiber
•
Contain considerable tensile strength
•
Extend – gets stronger = more ordered
Elastomers
•
Amorphous polymers that
stretch out and spring back to
original shapes
•
Entropy driving force for its return.
•
Low T
g
values and small amount of cross-linking
Molecular weight: M
Mass of a mole
chains
Not all chains in a polymer are the same length because there is a distribution of molecular weights.
Degree of Polymerization
: average number of repeat unit per chains
M=
the molecular weight of the repeating units.
Number Average Molecular weight:
the sum of the products
of
the molar mass of each fraction multiplied by its mole
fraction.
Where
x
i
is the mole fraction of molecules of length
i
, the ratio of the number of molecules of length
i
,
n
i
, to
the total number of molecules,
n
.
m is the mass of each species:
M
w
≥ M
n
Molecular weight Distribution curve:
Synthetic polymers have molecules that vary in molecular weight:
Natural products such as proteins and
peptides have a single defined
molecular weight and are called
monodisperse
1
1
2
2
3
3
1
1
2
3
1
. . .
. . .
n
i
i
i
n
n
i
i
n
M
n
M
n
M
n
M
M
n
n
n
n
=
=
+
+
+
=
=
+
+
+
å
å
The relative viscosity
rev
:
the ration of the viscosity of the solution
, to the velocity of the pure solvent
o
.
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- Spring '17
- Polymer, Erythromycin
