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APSC 278 (Engineering Materials) > Polymers II > Flashcards

Polymers II Flashcards

1
Q

What is the typical crystalline rate of metals?

A

Metals are typically 100% crystalline

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2
Q

What is the range of crystallinity in polymers?

A

Polymers can be partially crystalline, ranging from entirely
amorphous (noncrystalline) to almost completely crystalline

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3
Q

What is the relationship between crystallinity and polymer density? Why?

A
  • For a given composition and molecular weight, increased crystallinity
    increases the density of a polymer due to increased packing
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4
Q

What bonds form the polymer chain backbone of amorphous polymers?

A

– Strong covalent bonds form the polymer
chain backbone

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5
Q

What holds the polymer strands of amorphous polymers?

A

Extensive van der Waals
interactions between chains hold the
polymer strands together, but with no
long-range order

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6
Q

What are factors that inhibit crystal formation?

A
  • Bulky side groups
  • Branching
  • Atacticity
  • Random arrangement of mers in copolymers
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7
Q

What is atacticity?

A

Random distribution of sidechain locations

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8
Q

What does an increase in crystallinity lead to?:

A

A greater degree of secondary interactions, which increases:
* Melting temperature
* Strength
* Stiffness
* Density (ie. Greater packing of polymer chains)
* Resistance to softening (polymer can be used at higher temperatures)

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9
Q

How is the degree of polymerization determined?

A

By dividing the average
molecular weight by the
the molecular weight of a mer
unit.

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10
Q

The mechanical behavior of polymers is highly sensitive to:

A
  • rate of deformation (strain rate)
  • temperature, even close to room temperature
  • environment (organic solvents,
    water)
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11
Q

What are the three types of general stress-strain
behaviour for polymers

A
  1. Brittle
  2. Plastic
  3. Highly elastic
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12
Q

What are the brittle and plastic behaviours of polymers similar to?

A

The brittle and plastic behaviors are similar to metals

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13
Q

What is the highly elastic behaviour of polymers characteristic of and not characteristic of?

A

Highly elastic behavior is only characteristic of some polymers,
not metals or ceramics

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14
Q

What is the impact of increasing the strain (deformation) rate on elastic modulus (stiffness), ductility, and ultimate tensile strength?

A

Increasing the strain (deformation) rate:
* Increases the elastic modulus (stiffness)
* Decreases ductility
* Increases the ultimate tensile strength

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15
Q

What types of deformation are present during the deformation of semi-crystalline polymers?

A

Elastic and Plastic Deformation

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16
Q

What are the steps of deforming semi-crystalline polymers under tension?

A
  1. Elongation of
    amorphous tie chains
  2. Tilting of lamellar
    chain folds
  3. Separation of crystalline
    block segments
  4. Orientation of block
    segments and tie chains
    with tensile axis
17
Q

Which steps of the deformation of semi-crystalline polymers are elastic deformation?

A
  1. Elongation of
    amorphous tie chains
  2. Tilting of lamellar
    chain folds
18
Q

Which steps of the deformation of semi-crystalline polymers are plastic deformation?

A
  1. Separation of crystalline
    block segments
  2. Orientation of block segments and tie
    chains with tensile axis
19
Q

What are the three types of polymers based on temperature and elasticity?

A

-Thermoset
-Thermoplastic
-Elastomers

20
Q

What are examples of thermoset polymers?

A
  • Eg. epoxies, most polyurethanes, formaldehyde and polyesters, protein
    molecules
21
Q

What are examples of thermoplastic polymers?

A

Polyethylene, acrylic, ABS, nylon, and polystyrenes.

22
Q

What are examples of elastomers?

A

Natural rubber, polyurethanes, neoprene, silicones.

23
Q

How does a thermoset polymer respond to heat?

A

decomposes upon heating and does not become softer

24
Q

How does a thermoplastic respond to heating?

A

Becomes more pliable or moldable upon heating

25
Q

How are thermoset polymers formed?

A

By curing (i.e. hardening) a soft solid or viscous
liquid material

26
Q

What is a thermoplastic?

A

Any resin which can be melted by heat and cooled without appreciable change
in properties

27
Q

What can elastomers withstand at ambient temperatures?

A

Large elastic strains

28
Q

Are most elastomers categorized as thermoset or thermoplastic?

A

Most elastomers are also thermoset plastic (i.e. cannot heat and melt most
elastomers).

29
Q

How can the stability of elastomers be improved?

A

May be lightly crosslinked for stability

30
Q

What happens during curing of a thermoset polymer?

A

The addition of
catalysts and/or heat result in
crosslinking of polymer chains,
so that chains cannot freely
flow or move

31
Q

What are polymer chains of thermoplastics held by and not interconnected by?

A

Polymer chains are not
interconnected by covalent
bonds, but held by weak
secondary bonds

32
Q

What are the major characteristics of thermoset polymers?

A
  • Strong and stiff
  • Can be used at higher
    temperature (although will
    decompose at high temperature)
  • More chemically inert
33
Q

What are the major characteristics of thermoplastic polymers?

A
  • Less strong and stiff
  • Cannot be used at high
    temperatures (will melt instead of
    decompose)
  • Formable via injection molding,
    vacuum forming and extrusion
34
Q

What is the relative modulus of elasticity for elastomers?

A

The modulus of elasticity is small

35
Q

What are elastomers like in its unstressed state?

A

Elastomers
are amorphous, and composed of
molecular chains that are highly
twisted, kinked and coiled

36
Q

What happens when a tensile load is applied to an elastomer?

A

There is partial uncoiling,
untwisting and straightening of
chains in the direction of stress.

37
Q

What happens upon the release of stress for an elastomer?

A

Entropy
increase restores the chains
original conformations

38
Q

How does deformation by drawing affect the mechanical properties of a polymer?

A

Deformation of a polymer via drawing enhances its mechanical properties by increasing alignment of and secondary interactions between polymer chains

39
Q

What happens prior to the deformation of semi-crystalline polymers under tension?

A

Adjacent chain-folded
lamellae, interspaced by
amorphous material

APSC 278 (Engineering Materials) (30 decks)

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