Polymer Structures and Properties

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Polymer Structures and Properties
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Dr. Ying-Chieh Yen ???

Ref INTRODUCTION TO POLYMERS 2nd edition, R. J.
Young and P. A. Lovell.
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Polymer Structures
Polymer Research Center Institute of
Applied Chemistry of NCTU
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Basic Definitions and Nomenclature
Polymer Research Center Institute of
Applied Chemistry of NCTU

• A polymer is a substance composed of molecules
  which have long sequences of one or more species
  of atoms or groups of atoms linked to each other
  by covalent bonds.
• The words, polymers and macromolecules are used
  interchangeably, the latter strictly defines
  molecules of which the former is composed.

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Polymer Research Center Institute of
Applied Chemistry of NCTU
Basic Definitions and Nomenclature

• Macromolecules are formed by linking together
  monomer molecules through chemical reactions, the
  process by which this is achieved being known as
  polymerization.

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Molecular Mass and Polydispersity
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Number average molecular mass ( )
• Weight average molecular mass ( )
• The ratio must be greater than unity for a
  polydisperse polymer and is known as the
  polydispersity or heterogeneity index.
• A perfectly monodisperse polymer would have
  polydispersity 1.00.

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Classification of Polymers
Polymer Research Center Institute of
Applied Chemistry of NCTU
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Classification of Polymers
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Thermoplastics It often referred to just as
  plastics (linear or branched polymers) which can
  be melted upon the application of heat.

• Crystalline Those which do crystallize
  invariably do not form perfectly crystalline
  materials but instead are semi-crystalline with
  both crystalline and amorphous regions. (Tm)

• Amorphous Many thermoplastics are completely
  amorphous and incapable of crystallization. (Tg)
  At the temperature, thermoplastics transform
  abruptly from the glass state (hard) to the
  rubbery state (soft).

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Classification of Polymers
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Elastomers are rubbery polymers (i.e. rubbery
  networks) which can be stretched easily to high
  extensions (e.g. 3x to 10x their original
  dimensions) and which rapidly recover their
  original dimensions when the applied stress is
  released.
• Thermosets normally are rigid materials and are
  network polymers in which chain motion is greatly
  restricted by a high degree of crosslinking.

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Skeletal Structures
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Linear structure A chain with two ends.
• Non-linear structures
• Branched structure Side chains, or branches, of
  significant length bonded to the main chain at
  branch points (junction points).

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Skeletal Structures
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Non-linear structures
• Network structure (crosslinked) Polymers have
  three-dimensional structures in which each chain
  is connected to all others by a sequence of
  junction points and other chains.

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Homopolymers
Polymer Research Center Institute of
Applied Chemistry of NCTU

• The formal definition of a homopolymer is a
  polymer derived from one species of monomer.
• However, it often is used more broadly to
  describe polymers whose structure can be
  represented by multiple repetition of a single
  type of repeat unit.

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Some Common Homopolymers
Polymer Research Center Institute of
Applied Chemistry of NCTU
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Some Common Homopolymers
Polymer Research Center Institute of
Applied Chemistry of NCTU
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Some Common Homopolymers
Polymer Research Center Institute of
Applied Chemistry of NCTU
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Tacticity
Polymer Research Center Institute of
Applied Chemistry of NCTU

• For polymers prepared from monomers of the
  general structure CH2CXY, where X and Y are two
  different substituent groups, there are two
  distinct configurational arrangements of the
  repeat unit.

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Tacticity
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Isotactic
• Syndiotactic
• Atactic

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Copolymers
Polymer Research Center Institute of
Applied Chemistry of NCTU

• The formal definition of a copolymer is a polymer
  derived from more than one species of monomer.
• However, it often is used more broadly to
  describe polymers whose molecules contain two or
  more different types of repeat unit.

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Copolymers
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Statistical copolymers The sequential
  distribution of the repeat unit obeys the
  statistical laws. (Markovian)
• Random copolymers A special type of statistical
  copolymer in which the distribution of repeat
  units is truly random. (Older textbooks and
  scientific papers often use the term random
  copolymer to describe both random and non-random
  statistical copolymers.)

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Copolymers
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Alternating copolymers Only two different types
  of repeat units are arranged alternately along
  the polymer chain.
• Block copolymers Linear copolymers with repeat
  units existing only in long sequences or blocks.

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Copolymers
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Graft copolymers Branched polymers with the
  branches having different chemical structure to
  that of the main chain.

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Polymer Properties
Polymer Research Center Institute of
Applied Chemistry of NCTU
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The Glass Transition
Polymer Research Center Institute of
Applied Chemistry of NCTU

• If the melt of a non-crystallizable polymer is
  cooled it becomes more viscous and flows less
  readily. If the temperature is reduced low enough
  it becomes rubbery and then as the temperature is
  reduced further it becomes a relatively hard and
  elastic polymer glass.
• The temperature at which the polymer undergoes
  the transformation from a rubber to a glass is
  known as the glass transition temperature, Tg.

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The Glass Transition
Polymer Research Center Institute of
Applied Chemistry of NCTU

• There is a dramatic change in the properties of a
  polymer at glass transition temperature. For
  example, there is a sharp increase in the
  stiffness of an amorphous polymer when its
  temperature is reduced below Tg.
• There are also abrupt changes in other physical
  properties such as heat capacity and thermal
  expansion coefficient.
• There have been attempts to analyse the glass
  transition from a thermodynamic viewpoint.

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The Glass Transition
Polymer Research Center Institute of
Applied Chemistry of NCTU

• In the first-order transition there is an abrupt
  change in a fundamental thermodynamic property
  such as enthalpy, H or volume, V, whereas in a
  second-order transition only the first derivative
  of such properties changes.
• This means that during a first-order transition,
  such as melting, H and V will change abruptly
  whereas for a second-order transition changes
  will only be detected in properties such as heat
  capacity, Cp or volume thermal expansion
  coefficient, a which are definded as

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The Glass Transition
Polymer Research Center Institute of
Applied Chemistry of NCTU

• As both of these parameters are found to change
  abruptly at the glass transition temperature it
  would appear that it may be possible to consider
  the glass transition as a second-order
  thermodynamic transition.
• At the glass transition, the molecules which are
  effectively frozen in position in the polymer
  glass become free to rotate and translate and so
  it is not surprising that the value of the Tg
  will depend upon the physical and chemical
  structure of the polymer molecules.

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The Glass Transition
Polymer Research Center Institute of
Applied Chemistry of NCTU

• The most important factor is chain flexibility
  which is governed by the nature of the chemical
  groups which constitute the main chain.

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The Glass Transition
Polymer Research Center Institute of
Applied Chemistry of NCTU

• In vinyl polymers of the type (-CH2-CHX-)n the
  nature of the side group (bulky and polar groups)
  has a profound effect upon Tg as can be seen in
  the table.

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Crystallization
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Crystallization is the process whereby an ordered
  structure is produced from a disordered phase,
  usually a melt or dilute solution, and melting
  can be thought of as being essentially the
  opposite of this process.
• Features
• (a) Polymer crystals are usually thin and
  lamellar when crystallized from both dilute
  solution and the melt.

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Crystallization
Polymer Research Center Institute of
Applied Chemistry of NCTU

• (b) The lamellar thickness is related to the
  crystallization temperature.
• (c) Chain folding is known to occur during
  crystallization.
• (d) The growth rates of polymer crystals are
  found to be highly dependent upon the
  crystallization temperature and molar mass of the
  polymer.

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Thermal Degradation Temperature
Polymer Research Center Institute of
Applied Chemistry of NCTU

• The thermal degradation temperature was
  determined from the changes in weight in relation
  to change in temperature using thermogravimetric
  analysis (TGA).

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Polymer Research Center Institute of
Applied Chemistry of NCTU
Surface Free Energy
Surface free energy from work The reversible
work required to create a unit surface area is
related to the surface free energy of the
material.
Units mJ/m2 mN/m
high surface free energy ? strong cohesion
surface tension of liquids corresponds to
surface energy of solids
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Polymer Research Center Institute of
Applied Chemistry of NCTU
The Theory of Surface Free Energy
Three-Liquid Acid-Base Method
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Polymer Research Center Institute of
Applied Chemistry of NCTU
Fluoropolymers and Silicones
The low intermolecular forces present in
fluorinated polymers have been recognized to
account for the relatively low surface free
energy.
poly(dimethylsiloxane) PDMS
polyethylene PE
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Mechanical Properties
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Stress In consideration of the mechanical
  properties of polymers we are mainly to
  interested in effect of applying surface forces
  such as stress or pressure to the material.

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Mechanical Properties
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Strain When forces are applied to a material the
  atoms change position in response to the force
  and this change is known as strain.
• Youngs modulus E of a material which for simple
  uniaxial extension or compression is given by E
  stress/strain.

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Mechanical Properties
Polymer Research Center Institute of
Applied Chemistry of NCTU

• Viscoelasticity A distinctive feature of the
  mechanical behavior of polymers is the way in
  which their response to an applied stress or
  strain depends upon the rate or time period of
  loading.
• The behavior of most polymers can be though of
  as being somewhere between that of elastic solids
  and liquids.

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Mechanical Properties
Polymer Research Center Institute of
Applied Chemistry of NCTU

• At low temperatures and high rates of strain
  they display elastic behavior whereas at high
  temperatures and low rates of strain they behave
  in a viscous manner, flowing like a liquid.
• Polymers are therefore termed viscoelastic as
  they display aspects of both viscous and elastic
  types of behavior.

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Mechanical Properties
Polymer Research Center Institute of
Applied Chemistry of NCTU

• General time-dependent behavior

Creep experiment Relaxation experiment
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Mechanical Properties
Polymer Research Center Institute of
Applied Chemistry of NCTU
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Stress-Strain Curve
Polymer Research Center Institute of
Applied Chemistry of NCTU
Brittle!!
Ductile!!
Neck occurs!!
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The EndThanks for Your Attention