Micro Structures in Polymers Chapter 3

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Micro Structures in PolymersChapter 3
Professor Joe Greene CSU, CHICO
September 20, 1999
MFGT 041
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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

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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
• Polyethylene -(C2H4)-1000 2 C (12g) 4H (1g)
  28g/mole 1000 28,000 g/mole

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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 281000 or 28,000 g/mole .

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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.

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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.
• The curve is called the Molecular Weight
  Distribution (MWD)

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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

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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

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Gel Permeation Chromatography
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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

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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 degree of polymerization, DP.
• DP relates the amount of
• monomer that has been converted to polymer.

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Weight Average Molecular Weight, Mw

• 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

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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

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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.

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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.

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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

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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

Mechanical Properties
Melting Point
Decomposition
MW
MW
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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

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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

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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

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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 Crystalline
• HDPE Crystalline
• PP Crystalline
• PET Crystalline
• PBT Crystalline
• Polyamides Crystalline
• PMO Crystalline
• PEEK Crystalline
• PPS Crystalline
• PTFE Crystalline
• LCP (Kevlar) 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
• Polymer Form as a function of temperature
• Glassy Solid-like form, rigid, and hard
• Rubbery Soft solid form, flexible, and elastic
• Melt Liquid-like form, fluid, and elastic

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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 characteristic.
• Tg applies to all polymers (amorphous,
  crystalline, rubbers, thermosets, fibers, etc.)

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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

Vol.
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Crystalline Polymers Tm
Melt
Tm

• Tm Melting Temperature
• T gt Tm, The order of the molecules is random
  (amorphous)
• T lt Tm gtTg, 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 dont
  contribute to the stiffness. The crystalline
  regions are unaffected by temperature and are
  glassy and rigid.
• T lt Tg, The amorphous regions gain stiffness and
  become glassy

Temp
Rubbery
Decreasing Temp
Tg
Glassy
Polymer Form
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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
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Temperature Effects on Specific Volume

• T gt Tm, The amorphous polymers volume decreases
  linearly with T.
• T lt Tm gtTg, As crystals form the volume drops
  since the crystals are significantly denser
  than the amorphous material.
• T lt Tg, the amorphous regions contracts linearly
  and causes a change in slope

Temperature
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Thermal Properties

• Table 3.2 Thermal Properties of Selected
  Plastics