Chapter 4: Polymer Structures

1
Chapter 4 Polymer Structures

• Polymer molecules are very large, and primarily
  with strong covalent bonds.
• In a solid polymer, the molecules are held
  together by entanglement and van der Waals
  forces.
• Polymer solids tend to be mostly or entirely
  amorphous.
• In this chapter, we deal with polymers made from
  organic compounds, which are the most common.
• Cover molecular structures, molecular weight,
  crystallinity.
• Typical properties of solids made from organic
  polymerslow density, low strength, inexpensive,
  unable to withstand high temperature.
• But strength-to-weight ratio may be high,
  particularly for fibers.
• On-line information
• General http//en.wikipedia.org/wiki/Polymer
• History http//cen.acs.org/articles/91/i36/Plasti
  c-Planet.html
• Biopolymers http//en.wikipedia.org/wiki/Biopolym
  er
• Silicones (based on Si) http//en.wikipedia.org/
  wiki/Silicone

2
Applications of organic polymers

• Originally, natural polymers were used. For
  example
• Wood Rubber
• Cotton Wool
• Leather Silk
• Oldest known uses
• Building materials
• Clothing
• Rubber balls used by Mesoamericans 3,600 years
  ago
• Amber used for jewelry 13,000 years ago
• Applications for synthetic organic polymers are
  increasing
• When low weight is required
• Often strengthened by addition of fibers,
  typically glass or carbon (composites)
• Automobiles, aircraft, even jet engine components

3
Simplest Example of a Polymer Polyethylene
Industrial catalytic method
? Free-radical mechanism
? VMSE
4
Reaction between different chemicals to form a
polymer

• Example polyethylene terephthalate (PET)
  http//en.wikipedia.org/wiki/Polyethylene_terephth
  alate
• One process for makingn C6H4(CO2H)2 n
  HOCH2CH2OH ? (CO)C6H4(CO2CH2CH2O)n 2n
  H2Oterephthalic acid ethylene glycol

5
Many types of polymer molecules

• Linear repeat units joined end to end in single
  chains.
• Since rotation occurs about single bonds, the
  molecules are not straight.

• Isomers Same molecular weight repeat unit, but
  different arrangement
• For example, 1,4-polyisoprene ?VMSE
• Natural rubber consists primarily of
  cis-1,4-polyisoprene (https//en.wikipedia.org/wik
  i/Natural_rubber ). Doesnt crystallize, more
  elastic.
• Gutta percha consists primarily of
  trans-1,4-polyisoprene and has different
  properties (http//en.wikipedia.org/wiki/Gutta-per
  cha ). Tends to crystallize.

6
Tacticity

• Spatial arrangement of R units along the chain

7
Copolymers
More than 1 repeat unit, e.g. A? and B? random
A and B randomly positioned alternating
alternate A and B block large blocks of A
alternate with large blocks of B graft
chains of B grafted onto A backbone
random
alternating
block
http//en.wikipedia.org/wiki/Copolymer
Example Styrene-butadiene rubber(SBR,
originally GRS) http//en.wikipedia.org/wiki/Styre
ne-butadiene
graft
8
Many possible molecular shapes
http//en.wikipedia.org/wiki/Polymer
9
Polymerization produces a wide range of molecular
sizes
Average MW

• Mi mean molecular weight of size range i
• xi number fraction of molecules in size range
  i
• wi weight fraction of molecules in size range
  i
• See Example Problem 4.1 for calculations

10
Degree of Polymerization, DP

• DP average number of repeat units per chain

DP 6
11
Polymer Crystallinity

• Polymers are rarely 100 crystalline
• Difficult for all parts of all molecules to
  become aligned.

12
Polymer Crystallinity

• Polymer solids are semicrystalline with tiny
  crystals surrounded by amorphous material.
• The crystals tend to be thin platelets with chain
  folds at their faces, i.e. in chain-folded
  structures

http//en.wikipedia.org/wiki/Crystallization_of_po
lymers
13
Crystallinity (continued)

• The degree of crystallinity depends on the
  polymer and how it's produced.
• Crystallization easier when molecules are linear
  and the same.
• More difficult or even impossible with side
  branches.
• Raising the temperature causes the crystals to
  grow and the degree of crystallinity to increase.
• Some properties depend on the degree of
  crystallinity, such as density and mechanical
  properties. Become more dense with increasing
  crystallinity.

14
Polymer Crystals

• Can get single small single crystals only by slow
  growth, usually from a solution of the polymer in
  a solvent (although finding a solvent can be
  difficult or even impossible). Some fibers, e.g.
  of PET, almost.
• Semi-crystalline polyethylene is shown below.
  The image was obtained using an electron
  microscope. The crystals consist of chain-folded
  layers. Notice the micron scale.

15
Polyetheylene unit cell

• Contains portions of 4 molecules.

16
Spherulitic solidification

• Some polymers and other organic molecules form
  spherulite structures
• Alternating crystallites and amorphous regions
• Occurs at relatively rapid freezing rates

Cross-polarised light micrograph of spherulites
in polyhydroxybutyrate http//www.doitpoms.ac.uk/t
lplib/polymers/spherulites.php
17
Cross linking of polymershttps//en.wikipedia.org
/wiki/Cross-link

• Can form strong chemical bonds between polymer
  molecules.
• Cross-linked polymers can withstand larger
  forces without breaking, particularly at higher
  temperatures.
• Cross-links can be formed by chemical reactions
  that are initiated by heat, pressure, change in
  pH, or radiation.
• A prime example is vulcanization of natural
  rubber (polyisoprene) using S

• Highly cross-linked polymers are relative rigid
  and cannot be molded by application of force.
  Dont readily crystallize, if at all.
• If heating does not cause crosslinking, then the
  polymer can be repeatedly molded into shapes at
  high temperature. Such "thermoplastic" polymers
  can be easily recycled. http//en.wikipedia.org/wi
  ki/Thermoplastic
• Polymers that cross link when heated are called
  thermosetting.Example vulcanization of
  natural rubber while forming into
  tires.http//en.wikipedia.org/wiki/Thermosetting_
  plastic

18
Summary

• An unlimited number of types of polymers
  possible, with new ones being developed all the
  time.
• The polymer produced in a chemical reaction
  depends on
• Composition of the reaction mixture.
• Catalyst used.
• Temperature.
• Pressure.
• Time.
• Many additives used to modify the final products
  fibers, particles, plasticizers, etc.
• Applications http//en.wikipedia.org/wiki/List_o
  f_synthetic_polymers
• VMSE http//higheredbcs.wiley.com/legacy/college/
  callister/1118061608/vmse/mer.htm
• Recycling http//en.wikipedia.org/wiki/Plastic_r
  ecycling