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Material Science > Chapter 15 > Flashcards

Chapter 15 Flashcards

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1
Q

The fracture strength of polymers is approximately ___ of metals.

A

10%

Ref: Lecture Notes CH15

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2
Q

Deformation strains of polymers may exceed ____. But for most metals, deformation strains remain typically ____ of polymers.

A

1000%
<10%

Ref: Lecture Notes CH15

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3
Q

In figure 15.1 in the textbook the material that has the highest slope is the _____.

A

Brittle polymer.

Ref: Pg 513

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4
Q

For plastic polymers (curve B in fig 15.1), the yield point is taken as a ____ on the curve.

A

Maximum

Ref Pg 513

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5
Q

The maximum value on the linear-elastic portion of the stress vs strain curve is referred to as _____.

A

The yield strength.

Ref: Pg 513

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6
Q

The point of fracture on the stress-strain curve of a plastic polymer (not a metal) is known as the ______.

A

Tensile Strength (this is different from metals)

Ref: Pg 513

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7
Q

True or False

Polymeric materials are much more sensitive to temperature changes as opposed to metals.

A

True

Ref: Pg 513

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8
Q

True or False

Maximum tensile strength for polymeric materials is about the same as metals.

A

False

Ref: Pg 513

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9
Q

Increasing temperature has what three effects on a polymer undergoing a tensile test?

A
  1. Decrease in Elastic Modulus
  2. Reduction in Tensile Strength
  3. Enhancement in ductility

Ref: Pg 513

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10
Q

True or False

Generally speaking, a decrease in temperature make a polymer more brittle.

A

True

Extra Notes: See fig 15.3 in the textbook

Ref: Pg 513

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11
Q

The onset of elastic deformation for semi crystalline polymers results from chain molecules in ______ elongating in the direction of the applied tensile stress.

A

amorphous regions

Ref: Pg 523

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12
Q

After the onset of elastic deformation for semi-crystalline polymers, the second stage occurs by changes in the ____ regions. Amorphous chains continue to align and become elongated; in addition, there is bending and stretching of the ______ within the lamellar crystallites.

A

Both amorphous and lamellar crystalline regions.

Strong chain covalent bonds

Ref: Pg 523

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13
Q

Plastic deformation occurs during stage III. Adjacent chains in the lamellae slide past one another; this results in ____.

A

The tilting of the lamellae so that the chain folds and becomes more aligned with the tensile axis.

Ref: Pg 524

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14
Q

What occurs during stage four of polymer elongation (plastic region)?

A

Crystalline block segments are separated from the lamallae.

Ref: Pg 524

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15
Q

What occurs during stage five of polymer elongation (plastic deformation to fracture)?

A

The blocks and tie chains become oriented in the direction of the tensile axis. This process of orientation is referred to as “drawing.”

Ref: Pg 524

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16
Q

What are the four main factors that influence the mechanical properties of semi-crystalline polymers?

A
  1. Molecular weight
  2. Degree of Crystallinity
  3. Pre-deformation by drawing
  4. Heat treatments

Ref: Pg 524-527

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17
Q

True or False

The modulus of elasticity of a polymer increases as both the secondary bond strength and chain alignment increase.

A

True

Ref: Pg 524

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18
Q

True or False

The magnitude of the tensile modulus can be directly influenced by molecular weight.

A

False, the molecular weight has no effect on the modulus. It does however effect the tensile strength.

Ref: Pg 524

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19
Q

Explain why the degree of crystallinity can have a significant influence on the mechanical properties of a polymer.

A

Because it affects the extent (magnitude) of the intermolecular secondary bonding.

Extra Notes: For crystalline regions in which molecular chains are closely packed in an ordered and parallel arrangement, extensive secondary bonding typically exists between adjacent chains segments.

Ref: Pg 526

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20
Q

Why is secondary bonding much less prevalent in amorphous regions?

A

The chain misalignment provides fewer opportunities for atoms to covalently bond.

Ref: Pg 526

21
Q

Generally speaking, increasing the crystallinity of a polymer enhances it’s ______; however, it also makes the material more _____.

A

Strength
Brittle

Ref: Pg 526

22
Q

What is the most common way that a given polymeric material’s tensile modulus and strength can be improved?

A

By Drawing.

Ref: pg 526

23
Q

True or False

The properties of drawn polymers are highly anisotropic.

A

True

Ref: Pg 527

24
Q

For an amorphous polymer that is drawn at an elevated temperature, the oriented molecular structure is retained only when the material is quickly cooled to the ambient temperature. Why?

A

If, after stretching, the polymer is held at the temperature of drawing, molecular chains relax and assume random configurations characteristic of the pre-deformed state. As a consequence, this will not alter the mechanical properties of the material.

Ref: Pg 527

25
Q

Why does heat treating lead to desirable mechanical changes in a polymer.

A

Because it increases the percent crystallinity. This strengthens the material.

Ref: Pg 527

26
Q

Why does an elastomer spring back to it’s original form?

A

Cross-links in the polymer provide a force to restore the chains to their undeformed state.

Ref: Pg 528

27
Q

True or False
In an unstressed state, an elastomer is amorphous and composed of crosslinked molecular chains that are highly twisted, kinked, and coiled.

A

True

Ref: Pg 528

28
Q

With respect to elastomers, what is one large driving force for elastic deformation in general.

A

Entropy

Ref: Pg 528

29
Q

Describe the two major effects of entropy on the elongation of an elastomer.

A

As an elastomer is deformed it’s molecular chains become more ordered (decrease in entropy) and a corresponding rise in temperature takes place.

Additionally, as the temperature rises, so too does the modulus of elasticity.

Neither of these effects occur in other common materials.

Ref: Pg 528

30
Q

What are the criteria for a polymer to be considered an elastomer?

A
  1. Must not easily crystallize.
  2. Chain bond rotations must be relatively free for the coiled chains to readily respond to an applied force.
  3. Must deform elastically to a large extent before any plastic deformation (restricting the motions of chains past one another by crosslinking accomplishes this).

Ref: Pg 529

31
Q

Polymers that display a brittle nature on the stress strain curve have what kind of internal structure?

A

Networked

Ref: Lecture Notes

32
Q

Summarize the three main benefits of drawing.

A
  1. Increase in of the modulus of elasticity in the direction of stretching.
  2. Increases the tensile strength in the stretching direction.
  3. Decreases ductility

Ref: Lecture Notes

33
Q

What does an annealing treatment do to a material that has been drawn?

A

It reverses the effects of the drawing process. Annealing in polymers is analogous to cold working in metallic alloys.

Ref: Lecture Notes

34
Q

What are the three major effects that DECREASING the temperature has on the strain rate (slope of the stress-strain curve) with respect to thermoplastics?

A
  1. Increases E
  2. Increases TS
  3. Decreases %EL (ductility)

Ref: Lecture Notes

35
Q

Define the melting temperature with respect to polymers.

A

The temperature at which a polymer goes from a solid to a liquid state.

Ref: Lecture Notes

36
Q

True or False
The Melting Temperature (Tm) can take place over a range of temperatures, which depends on the polymer molecular weight, physical and chemical makeup, etc.

A

True

Ref: Lecture Notes

37
Q

Define the Glass Transition Temperature (Tg)

A

The Temperature at which the amorphous domains of a semi-crystalline polymer go from a hard (glassy) state to a rubbery viscous state.

Extra Notes: An amorphous solid that exhibits a glass transition is referred to as a glass. The reverse transition is referred to as vitrification.

Ref: Lecture Notes

38
Q

Both Tm and Tg increase with ____.

A

Increasing chain stiffness.

Ref: Lecture Notes

39
Q

Chain stiffness increases with the presence of what four items?

A
  1. Bulky sidegroups
  2. Polar groups or sidegroups
  3. Chain double bond
  4. Aromatic Chain groups

Ref: Lecture Notes

40
Q

Does the regularity of repeat unit arrangements affect Tm or Tg?

A

Tm only

Ref: Lecture Notes

41
Q

True or False
Thermoplastics can be…
1. Reversibly cooled and reheated.
2. Heat softens and cooling hardens.

A

1=True
2=True

Ref: Lecture Notes

42
Q

Give three examples of Thermoplastics and their abbreviations.

A
  1. Polyethylene (PE)
  2. Polypropylene (PP)
  3. Polystyrene (PS)

Ref: Lecture Notes

43
Q

True or False

When thermosets are heated, they form a molecular network.

A

True

Ref: Lecture Notes

44
Q

Give two examples of thermosets.

A
  1. Silicone
  2. Epoxy

Ref: Lecture Notes

45
Q

What are the three main ways of plastic manufacturing?

A
  1. Compression molding
  2. Injection Molding
  3. Extrusion

Ref: Lecture Notes

46
Q

What are the three main limitations of polymers?

A
  1. The modulus of Elasticity (E) and the yield strength is less than metals.
  2. Mechanical properties can fluctuate with temperature.
  3. Deformation is often time and temperature dependent.

Ref: Lecture Notes

47
Q

What are the two major benefits of Thermoplastics?

A
  1. Easy to form and recycle
  2. Can be modified by temperature adjustments.

Ref: Lecture Notes

48
Q

What is the major drawback of thermoplastics?

A

Low modulus of elasticity (E) and yield strength.

Ref: Lecture Notes

49
Q

Which typically has a larger modulus of elasticity; thermosets or thermoplastics?

A

Thermosets.

Ref: Lecture Notes

Material Science (15 decks)

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