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Micro Structures in Polymers
Chapter 3
Chapter 3 Objectives
• Objectives
- Polymer length, molecular weight, molecular weight
distribution (MWD)
- Physical and mechanical property implications of
molecular weight and MWD
- Melt Index
- Amorphous and crystalline structures in polymers
- Thermal transitions in plastics (thermoplastics and
thermosets
Molecular Weight
• Average Molecular Weight
- Polymers are made up of many molecular weights or a
distribution of chain lengths.
- The polymer is comprised of a bag of worms of the same repeating unit, ethylene (C 2 H 4 ) with different lengths; some longer than others.
- Example,
- Polyethylene -(C 2 H 4 )- 1000 has some chains (worms) with 1001 repeating ethylene units, some with 1010 ethylene units, some with 999 repeating units, and some with 990 repeating units.
- The average number of repeating units or chain length is 1000 repeating ethylene units for a molecular weight of 28*1000 or 28, g/mole.
Molecular Weight
• Average Molecular Weight
- Distribution of values is useful statistical way to
characterize polymers.
- For example,
- Value could be the heights of students in a room.
- Distribution is determined by counting the number of students in the class of each height.
- The distribution can be visualized by plotting the number of students on the x-axis and the various heights on the y-axis.
5
Histogram of Heights of Students
0
(^105)
1520
25
60 70 80 Height, inches
Frequency
Series
Molecular Weight
• Average Molecular Weight
- Determined by summing the weights of all of the chains
and then dividing by the total number of chains.
- Average molecular weight is an important method of
characterizing polymers.
- 3 ways to represent Average molecular weight
- Number average molecular weight
- Weight average molecular weight
- Z-average molecular weight
Gel Permeation Chromatography
• GPC Used to measure Molecular Weights
- form of size-exclusion chromatography
- smallest molecules pass through bead pores, resulting in
a relatively long flow path
- largest molecules flow around beads, resulting in a
relatively short flow path
- chromatogram obtained shows intensity vs. elution
volume
- correct pore sizes and solvent critical
Number Average Molecular Weight, Mn
- where Mi is the molecular weight of that species (on the x-axis)
- where Ni is the number of molecules of a particular molecular species I (on the y-axis).
- Number Average Molecular Weight gives the same weight to all polymer lengths, long and short.
- Example, What is the molecular weight of a polymer sample in which the polymers molecules are divided into 5 categories.
- Group Frequency
- 50,000 1
- 100,000 4
- 200,000 5
- 500,000 3
- 700,000 1
10
1 2 3
1 1 2 2 3 3
∑
∑ N N N
N M N M N M
N
N M
M
i
i i n
260 , 000
( 1 4 5 3 1 )
1 ( 50 ) 4 ( 100 ) 5 ( 200 ) 3 ( 500 ) 1 ( 700 )
...
... 1 2 3
1 1 2 2 3 3
=
= + + + +
= = + + +
n
n
i n i i
M
M K K K K K
N N N
N M N M N M N M N M
Molecular Weight
• Number Average Molecular Weight. Figure 3.
- The data yields a nonsymmetrical curve (common)
- The curve is skewed with a tail towards the high MW
- The Mn is determined experimentally by analyzing the
number of end groups (which permit the determination of
the number of chains)
- The number of repeating units, n, can be found by the ratio
of the Mn and the molecualr weight of the repeating unit,
M 0 , for example for polyethylene, M 0 = 28 g/mole
- The number of repeating units, n, is often called the degree
of polymerization, DP.
monomer that has been converted to polymer.
11
M 0
M
n = n
Z- Average Molecular Weight
- Emphasizes large molecules even more than Mw
- Useful for some calculations involving mechanical
properties.
- Method uses a centrifuge to separate the polymer
13
2 3 3
2 2 2
2 1 1
3 3 3
3 2 2
3 1 1 2
3
N M N M N M
N M N M N M
N M
N M
M
i i
i i z
Molecular Weight Distribution
• Molecular Weight Distribution represents the
frequency of the polymer lengths
• The frequency can be Narrow or Broad, Fig 3.
• Narrow distribution represents polymers of about
the same length.
• Broad distribution represents polymers with varying
lengths
• MW distribution is controlled by the conditions
during polymerization
• MW distributions can be symmetrical or skewed.
Physical and Mechanical Property Implications of MW and MWD
- Higher MW increases tensile strength
- Resistance to an applied load pulling in opposite directions
- Tension forces cause the polymers to align and reduce the number of entanglements. If the polymer has many entanglements, the force would be greater.
- Broader MW Distribution decreases tensile strength
- Broad MW distribution represents polymer with many shorter molecules which are not as entangled and slide easily.
- Higher MW increases impact strength
- Impact toughness or impact strength are increased with longer polymer chains because the energy is transmitted down chain.
- Broader MW Distribution decreases impact strength
- Shorter chains do not transmit as much energy during impact
Thermal Property Implications of MW & MWD
- Higher MW increases Melting Point
- Melting point is a measure of the amount of energy necessary to have molecules slide freely past one another.
- If the polymer has many entanglements, the energy required would be greater.
- Low molecular weights reduce melting point and increase ease of processing.
- Broader MW Distribution decreases Melting Point
- Broad MW distribution represents polymer with many shorter molecules which are not as entangled and melt sooner.
- Broad MW distribution yields an easier processed polymer
17
Melt Index
• Melt index test measure the ease
of flow for material
• Procedure (Figure 3.6)
- Heat cylinder to desired temperature (melt temp)
- Add plastic pellets to cylinder and pack with rod
- Add test weight or mass to end of rod (5kg)
- Wait for plastic extrudate to flow at constant rate
- Start stop watch (10 minute duration)
- Record amount of resin flowing on pan during time limit
- Repeat as necessary at different temperatures and weights
• Melt index is similar to viscosityMelt Index and Viscosity
- Viscosity is a measure of the materials resistance to flow.
- Viscosity is measured at several temperatures and shear rates
- Melt index is measured at one temperature and one weight.
- High melt index = high flow = low viscosity
- Low melt index = slow flow = high viscosity
- Example, (flow in 10 minutes) Polymer Temp Mass
- HDPE 190C 10kg
- Nylon 235C 1.0kg
- PS 200C 5.0Kg