Micro Structures in Polymers
Chapter 3
Professor Joe Greene
CSU, CHICO
1
MFGT 041
September 20, 1999
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
– Steric (shape) effects
2
Polymer Length
• Polymer Length
– Polymer notation represents the repeating group
• Example, -[A]-n where A is the repeating monomer and n
represents the number of repeating units.
• Molecular Weight
– Way to measure the average chain length of the polymer
– Defined as sum of the atomic weights of each of the
atoms in the molecule.
• Example,
– Water (H2O) is 2 H (1g) and one O (16g) = 2*(1) + 1*(16)= 18g/mole
– Methane CH4 is 1 C (12g) and 4 H (1g)= 1*(12) + 4 *(1) = 16g/mole
3
– Polyethylene -(C2H4)-1000 = 2 C (12g) + 4H (1g) = 28g/mole * 1000
=
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 (C2H4) with different lengths; some
longer than others.
• Example,
– Polyethylene -(C2H4)-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 28,000 g/mole .
4
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.
Frequency
Histogram of Heights of Students
25
20
15
10
5
0
Series1
60
70
Height, inches
80
5
Molecular Weight
• Molecular Weight Distribution
– Count the number of molecules of each molecular weight
– The molecular weights are counted in values or groups that have
similar lengths, e.g., between 100,000 and 110,000
• For example,
– Group the heights of students between 65 and 70 inches in one group,
70 to 75 inches in another group, 75 and 80 inches in another group.
• The groups are on the x-axis and the frequency on the y-axis.
• The counting cells are rectangles with the width the spread of
the cells and the height is the frequency or number of molecules
• Figure 3.1
• A curve is drawn representing the overall shape of the plot by
connecting the tops of each of the cells at their midpoints.
6
• The curve is called the Molecular Weight Distribution (MWD)
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
7
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
8
Gel Permeation Chromatography
9
Number Average Molecular Weight, Mn
• M   N i M i  N1 M 1  N 2 M 2  N 3 M 3  ...
n
N
i
N1  N 2  N 3  ...
• 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
 N i M i  N1 M 1  N 2 M 2  N 3 M 3  ...
M

n
– 50,000
1
N1  N 2  N 3  ...
 Ni
– 100,000 4
1(50 K )  4(100 K )  5(200 K )  3(500 K )  1(700 K )
Mn 
– 200,000 5
(1  4  5  3  1)
M n  260,000
– 500,000 3
10
– 700,000 1
Molecular Weight
• Number Average Molecular Weight. Figure 3.2
– 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, M0, for example for polyethylene, M0 = 28 g/mole
– The number of repeating units, n, is often called the
Mn
degree of polymerization, DP.
n
– DP relates the amount of
M0
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monomer that has been converted to polymer.
Weight Average Molecular Weight, Mw
2
N
M
 i i
N1 M 12  N 2 M 22  N 3 M 32  ...
Mw 

 N i M i N1 M 1  N 2 M 2  N 3 M 3  ...
• Weight Average Molecular Weight, Mw
– Favors large molecules versus small ones
– Useful for understanding polymer properties that relate to
the weight of the polymer, e.g., penetration through a
membrane or light scattering.
– Example,
• Same data as before would give a higher value for the
Molecular Weight. Or, Mw = 420,000 g/mole
12
Z- Average Molecular Weight
Mz 
3
N
M
 i i
2
N
M
 i i
N 1 M 13  N 2 M 23  N 3 M 33  ...

N 1 M 12  N 2 M 22  N 3 M 32  ...
– Emphasizes large molecules even more than Mw
– Useful for some calculations involving mechanical
properties.
– Method uses a centrifuge to separate the polymer
13
Molecular Weight Distribution
• Molecular Weight Distribution represents the
frequency of the polymer lengths
• The frequency can be Narrow or Broad, Fig 3.3
• 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.
14
Physical and Mechanical Property
Implications of MW and MWD
• Higher MW increases
• Tensile Strength, impact toughness, creep resistance, and
melting temperature.
– Due to entanglement, which is wrapping of polymer
chains around each other.
– Higher MW implies higher entanglement which yields
higher mechanical properties.
– Entanglement results in similar forces as secondary or
hydrogen bonding, which require lower energy to break
than crosslinks.
15
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
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• 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
* Decomposition
17
Example of High Molecular Weight
• Ultra High Molecular Weight Polyethylene (UHWMPE)
• Modifying the MWD of Polyethylene yields a polymer with
– Extremely long polymer chains with narrow distribution
– Excellent strength
– Excellent toughness and high melting point.
• Material works well in injection molding (though high melt T)
• Does not work well in extrusion or blow molding, which
require high melt strength.
• Melt temperature range is narrow and tough to process.
• Properties improved if lower MW polyethylene
– Acts as a low-melting lubricant
– Provides bimodal distributions, Figure 3.5
– Provides a hybrid material with hybrid properties
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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
19
Melt Index and Viscosity
• Melt index is similar to 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
20
Melt Index and Molecular Weight
• Melt index is related closely with average molecular weight
• High melt index = high flow = small chain lengths = low Mn
• Low melt index = slow flow = long chain lengths = high Mn
• Table 3.1 Melt Index and Average Molecular Weight
Mn
Melt Index* (g/10min)
• 100,000
10.00
• 150,000
0.30
• 250,000
0.05
* Note: PS at T= 200C
and mass= 5.0Kg
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States of Thermoplastic Polymers
• Amorphous- Molecular structure is incapable of forming
regular order (crystallizing) with molecules or portions of
molecules regularly stacked in crystal-like fashion.
• A - morphous (with-out shape)
• Molecular arrangement is randomly twisted, kinked, and
coiled
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Amorphous Materials
•
•
•
•
•
•
•
•
•
PVC
Amorphous
PS
Amorphous
Acrylics
Amorphous
ABS
Amorphous
Polycarbonate Amorphous
Phenoxy
Amorphous
PPO
Amorphous
SAN
Amorphous
Polyacrylates Amorphous
23
States of Thermoplastic Polymers
• Crystalline- Molecular structure forms regular order
(crystals) with molecules or portions of molecules regularly
stacked in crystal-like fashion.
• Very high crystallinity is rarely achieved in bulk polymers
• Most crystalline polymers are semi-crystalline because
regions are crystalline and regions are amorphous
• Molecular arrangement is arranged in a ordered state
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Crystalline Materials
•
•
•
•
•
•
•
•
•
•
•
LDPE
HDPE
PP
PET
PBT
Polyamides
PMO
PEEK
PPS
PTFE
LCP (Kevlar)
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
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Factors Affecting Crystallinity
•
•
•
•
Cooling Rate from mold temperatures
Barrel temperatures
Injection Pressures
Drawing rate and fiber spinning:
Manufacturing of thermoplastic fibers
causes Crystallinity
• Application of tensile stress for
crystallization of rubber
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Form of Polymers
• Thermoplastic Material: A
material that is solid, that possesses
significant elasticity at room
temperature and turns into a viscous
liquid-like material at some higher
temperature. The process is reversible
Temp
Melt
Tm
• Polymer Form as a function of
temperature
– Glassy: Solid-like form, rigid, and
hard
Rubbery
Tg
Glassy
Polymer
Form 27
Glass Transition Temperature, Tg
• Glass Transition Temperature, Tg: The temperature
by which:
– Below the temperature the material is in an immobile
(rigid) configuration
– Above the temperature the material is in a mobile
(flexible) configuration
• Transition is called “Glass Transition” because the
properties below it are similar to ordinary glass.
• Transition range is not one temperature but a range
over a relatively narrow range (10 degrees). Tg is
not precisely measured, but is a very important 28
characteristic.
Glass Transition Temperature, Tg
• Glass Transition Temperature, Tg: Defined as
– the temperature wherein a significant the loss of modulus
(or stiffness) occurs
– the temperature at which significant loss of volume
occurs
Modulus
(Pa)
or
(psi)
Vol.
Tg
-50C 50C 100C 150C 200C 250C
Temperature
Tg
Tg
-50C 50C 100C 150C 200C 250C
Temperature
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Crystalline Polymers: Tm
Melt
• Tm: Melting Temperature
Tm
Rubbery
Temp
Tg
Glassy
Polymer Form
• T > Tm, The order of the molecules is random (amorphous)
• T < Tm >Tg, Crystallization begins at various nuclei and the order
of the molecules is a mixture of crystals and random polymers
(amorphous). Crystallization continues as T drops until maximum
crystallinity is achieved. The amorphous regions are rubbery and don’t
contribute to the stiffness. The crystalline regions are unaffected by
temperature and are glassy and rigid.
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• T < Tg, The amorphous regions gain stiffness and become glassy
Crystalline Polymers Tg
• Tg: Affected by Crystallinity level
– High Crystallinity Level = high Tg
– Low Crystallinity Level = low Tg
Modulus
(Pa)
or
(psi)
High Crystallinity
Medium Crystallinity
Low Crystallinity
Tg
-50C
50C
100C
150C
200C
250C
Temperature
31
Temperature Effects on Specific
Volume
•T > Tm, The amorphous polymer’s volume decreases linearly with T.
•T < Tm >Tg, As crystals form the volume drops since the crystals are
significantly denser than the amorphous material.
•T < Tg, the amorphous regions contracts linearly and causes a change in
slope
Specific
Volume
Tg
-50C
50C
100C
150C
Temperature
Tg
200C
250C
32
Thermal Properties
• Table 3.2 Thermal Properties of Selected Plastics
33