CHM 585 490

1
CHM 585 / 490

• Chapter 7
• Properties

2
Chapter 7

• Molecular weight
• Tg
• Melting Point

3
Chain Entanglement
4
The degree of polymerization (DP)

• DP in a polymer molecule is the number, n, of
  repeating units in the polymer chain.
•  
• The molecular weight of a particular polymer
  molecule is a product of the degree of
  polymerization and the molecular weight of the
  repeating unit. For instance a particular
  polyethylene molecule with DP 1000 will have a
  molecular weight of 28,000.

5
Averaging

• Because polymeric materials do not consist of
  strictly identical molecules, but instead are a
  mixture of many species having different values
  of DP, the DP of the material only is an average
  value, . Hence, measured polymer molecular
  weights are only average values.

6
Weight Average / Number Average

• The number average is the simple arithmetic mean,
  representing the total weight of the molecules
  present divided by the total number of molecules
• The probability factor in a weight-average
  emphasizes the mass of the molecules so that the
  heavier molecules are more important.

7
The ratio of the weight-average and
number-average molecular weights,     , is a
measure of the polydispersity of a polymer
mixture - how widely distributed the range of
molecular weights are in the mixture. A ratio
that is around 1.0 indicates that the range of
molecular weights in the mixture is narrow a
high ratio indicates that the range is wide.
8
Suppose that you want to travel from Atlanta to
Los Angeles, visiting your friends along the way.
Unfortunately your friends live only on the
eastern and western coasts, leading to a somewhat
unconventional journey.
9
The number-average of the miles traveled in the
four legs of the journey is 928 miles. This is
the average distance traveled in each leg of the
journey. It places equal emphasis on each
leg. The mass-average of the miles traveled in
the four legs of the journey is 2216 miles. This
average places a greater emphasis on the leg of
the journey with the largest "mass" - that is the
fourth leg in which you travel 2790 miles. It is
more representative of the major part of your
journey.
10
(No Transcript)
11
(No Transcript)
12
If you look at the numbers you can see that the
average person doesn't live in a town of a
population of 180,000. Take a look there. most of
the people in the combined populations of the
four towns live in Memphis, a town with a lot
more than 180,000 people. So how do we calculate
the size of town that the average person lives
in, if the simple average doesn't work? What we
need is a weighted average.
13
Take Memphis. It has a population of 700,000. The
total population of our four cities is 723,500.
So the fraction of people who live in Memphis
is...
14
(No Transcript)
15
2 ChainsOne at 10,000One at 100,000

• Mn (100,000 10,000) /2 55,000
• Mw
• 100,000 / 110,000 .91
• 10,000 / 110,000 .09
• (100,000 x .91) (10,000 x .09) 91,900
• Mw 91,900
• Polydispersity Mw/Mn 91,900/55,000 1.7

16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20
Glass Transition Tg
The polymer samples may be thought of as a crowd
of people on a dance floor. While each whole body
tends to stay in the same spot, various arms,
legs, and whatnot are changing position a lot.
When the temperature drops below the Tg, for
polymers the party's over, and the long-range
segmental motion grinds to a halt. When this
long-range motion ceases, the glass transition
occurs, and the polymer changes from being soft
and pliable to being hard and brittle.
21
How easily the chains move.
A polymer chain that can move around fairly
easily will have a very low Tg, while one that
doesn't move so well will have a high one. This
makes sense. The more easily a polymer can move,
the less heat it takes for the chains to commence
wiggling and break out of the rigid glassy state
and into the soft rubbery state.
22
For polysulfone Tg 500C flexible ether
linkages increase mobility and bring the Tg to
190C
23
(No Transcript)
24
(No Transcript)
25
Plasticizers

• Sometimes, a polymer has a Tg that is higher than
  we'd like. That's ok, we just put something in it
  called a plasticizer. This is a small molecule
  which will get in between the polymer chains, and
  space them out from each other. We call this
  increasing the free volume. When this happens
  they can slide past each other more easily.

26
DOP
27
Tg versus Melting Point.

• 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.
• The glass transition is a transition which
  happens to amorphous polymers that is, polymers
  whose chains are not arranged in ordered
  crystals, but are just strewn around in any old
  fashion, even though they are in the solid state.
  
• But even crystalline polymers will have a some
  amorphous portion. This portion usually makes up
  40-70 of the polymer sample. This is why the
  same polymer can have both a glass transition
  temperature and a melting temperature.