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Micro Structures in Polymers - Plastics and Polymers - Lecture Slides, Slides of Mathematical Methods for Numerical Analysis and Optimization

The main points are: Micro Structures in Polymers, Polymer Length, Molecular Weight Distribution, Melt Index, Steric Effects, Thermal Transitions in Plastics, Polymer Length, Polymer Notation, Distribution of Chain Lengths

Typology: Slides

2012/2013

Uploaded on 04/17/2013

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Micro Structures in Polymers
Chapter 3
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Download Micro Structures in Polymers - Plastics and Polymers - Lecture Slides and more Slides Mathematical Methods for Numerical Analysis and Optimization in PDF only on Docsity!

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

  • Steric (shape) effects

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.

  • DP relates the amount of

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

  • Decomposition

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