Chapter 15 Flashcards
The fracture strength of polymers is approximately ___ of metals.
10%
Ref: Lecture Notes CH15
Deformation strains of polymers may exceed ____. But for most metals, deformation strains remain typically ____ of polymers.
1000%
<10%
Ref: Lecture Notes CH15
In figure 15.1 in the textbook the material that has the highest slope is the _____.
Brittle polymer.
Ref: Pg 513
For plastic polymers (curve B in fig 15.1), the yield point is taken as a ____ on the curve.
Maximum
Ref Pg 513
The maximum value on the linear-elastic portion of the stress vs strain curve is referred to as _____.
The yield strength.
Ref: Pg 513
The point of fracture on the stress-strain curve of a plastic polymer (not a metal) is known as the ______.
Tensile Strength (this is different from metals)
Ref: Pg 513
True or False
Polymeric materials are much more sensitive to temperature changes as opposed to metals.
True
Ref: Pg 513
True or False
Maximum tensile strength for polymeric materials is about the same as metals.
False
Ref: Pg 513
Increasing temperature has what three effects on a polymer undergoing a tensile test?
- Decrease in Elastic Modulus
- Reduction in Tensile Strength
- Enhancement in ductility
Ref: Pg 513
True or False
Generally speaking, a decrease in temperature make a polymer more brittle.
True
Extra Notes: See fig 15.3 in the textbook
Ref: Pg 513
The onset of elastic deformation for semi crystalline polymers results from chain molecules in ______ elongating in the direction of the applied tensile stress.
amorphous regions
Ref: Pg 523
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.
Both amorphous and lamellar crystalline regions.
Strong chain covalent bonds
Ref: Pg 523
Plastic deformation occurs during stage III. Adjacent chains in the lamellae slide past one another; this results in ____.
The tilting of the lamellae so that the chain folds and becomes more aligned with the tensile axis.
Ref: Pg 524
What occurs during stage four of polymer elongation (plastic region)?
Crystalline block segments are separated from the lamallae.
Ref: Pg 524
What occurs during stage five of polymer elongation (plastic deformation to fracture)?
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
What are the four main factors that influence the mechanical properties of semi-crystalline polymers?
- Molecular weight
- Degree of Crystallinity
- Pre-deformation by drawing
- Heat treatments
Ref: Pg 524-527
True or False
The modulus of elasticity of a polymer increases as both the secondary bond strength and chain alignment increase.
True
Ref: Pg 524
True or False
The magnitude of the tensile modulus can be directly influenced by molecular weight.
False, the molecular weight has no effect on the modulus. It does however effect the tensile strength.
Ref: Pg 524
Explain why the degree of crystallinity can have a significant influence on the mechanical properties of a polymer.
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
Why is secondary bonding much less prevalent in amorphous regions?
The chain misalignment provides fewer opportunities for atoms to covalently bond.
Ref: Pg 526
Generally speaking, increasing the crystallinity of a polymer enhances it’s ______; however, it also makes the material more _____.
Strength
Brittle
Ref: Pg 526
What is the most common way that a given polymeric material’s tensile modulus and strength can be improved?
By Drawing.
Ref: pg 526
True or False
The properties of drawn polymers are highly anisotropic.
True
Ref: Pg 527
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?
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
Why does heat treating lead to desirable mechanical changes in a polymer.
Because it increases the percent crystallinity. This strengthens the material.
Ref: Pg 527
Why does an elastomer spring back to it’s original form?
Cross-links in the polymer provide a force to restore the chains to their undeformed state.
Ref: Pg 528
True or False
In an unstressed state, an elastomer is amorphous and composed of crosslinked molecular chains that are highly twisted, kinked, and coiled.
True
Ref: Pg 528
With respect to elastomers, what is one large driving force for elastic deformation in general.
Entropy
Ref: Pg 528
Describe the two major effects of entropy on the elongation of an elastomer.
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
What are the criteria for a polymer to be considered an elastomer?
- Must not easily crystallize.
- Chain bond rotations must be relatively free for the coiled chains to readily respond to an applied force.
- 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
Polymers that display a brittle nature on the stress strain curve have what kind of internal structure?
Networked
Ref: Lecture Notes
Summarize the three main benefits of drawing.
- Increase in of the modulus of elasticity in the direction of stretching.
- Increases the tensile strength in the stretching direction.
- Decreases ductility
Ref: Lecture Notes
What does an annealing treatment do to a material that has been drawn?
It reverses the effects of the drawing process. Annealing in polymers is analogous to cold working in metallic alloys.
Ref: Lecture Notes
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?
- Increases E
- Increases TS
- Decreases %EL (ductility)
Ref: Lecture Notes
Define the melting temperature with respect to polymers.
The temperature at which a polymer goes from a solid to a liquid state.
Ref: Lecture Notes
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.
True
Ref: Lecture Notes
Define the Glass Transition Temperature (Tg)
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
Both Tm and Tg increase with ____.
Increasing chain stiffness.
Ref: Lecture Notes
Chain stiffness increases with the presence of what four items?
- Bulky sidegroups
- Polar groups or sidegroups
- Chain double bond
- Aromatic Chain groups
Ref: Lecture Notes
Does the regularity of repeat unit arrangements affect Tm or Tg?
Tm only
Ref: Lecture Notes
True or False
Thermoplastics can be…
1. Reversibly cooled and reheated.
2. Heat softens and cooling hardens.
1=True
2=True
Ref: Lecture Notes
Give three examples of Thermoplastics and their abbreviations.
- Polyethylene (PE)
- Polypropylene (PP)
- Polystyrene (PS)
Ref: Lecture Notes
True or False
When thermosets are heated, they form a molecular network.
True
Ref: Lecture Notes
Give two examples of thermosets.
- Silicone
- Epoxy
Ref: Lecture Notes
What are the three main ways of plastic manufacturing?
- Compression molding
- Injection Molding
- Extrusion
Ref: Lecture Notes
What are the three main limitations of polymers?
- The modulus of Elasticity (E) and the yield strength is less than metals.
- Mechanical properties can fluctuate with temperature.
- Deformation is often time and temperature dependent.
Ref: Lecture Notes
What are the two major benefits of Thermoplastics?
- Easy to form and recycle
- Can be modified by temperature adjustments.
Ref: Lecture Notes
What is the major drawback of thermoplastics?
Low modulus of elasticity (E) and yield strength.
Ref: Lecture Notes
Which typically has a larger modulus of elasticity; thermosets or thermoplastics?
Thermosets.
Ref: Lecture Notes
