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
