Elasticity and kinetics of vulcanized rubber
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Elasticity and kinetics of vulcanized rubber
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[post_date] => 2025-01-09 07:37:40
[post_date_gmt] => 2025-01-09 12:37:40
[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Vulcanization is a process which adds more elasticity to natural rubber. This is achieved by chemically cross-linking the cis-polyisoprene strands that make up natural rubber with sulfur, as shown in figure 1. These bridges of sulfur remove natural rubber’s inherent plasticity to create a more elastic and consistent structure. Non-vulcanized rubber is easily deformable due to the polymer strands moving independently from one another. By introducing sulfur bridges, the rubber may still deform under strain, but upon release the rubber will return to its previous shape.
Varying the amount of sulfur and temperature involved with vulcanization can affect the overall durability of the rubber product: ranging from the mostly non-vulcanized rubber latex gloves used in medicine, to the hard, heavily cross-linked rubber used in bowling balls. To test the amount of cross-linking achieved in a polymer, scientists can use what is known as a swelling experiment, in which a set volume of liquid is added within a polymer container. The swelling the polymer undergoes can then be used to calculate the amount of cross-linking.
When more sulfur is used, more cross-links are created, which leads to a harder rubber product. Generally, higher temperatures will also result in higher cross-link density as well. Vulcanization can be performed at temperatures anywhere between 120°–180°C. Improper maintenance of these variables can result in an inferior product. This is especially important for medical supplies such as tubing and latex gloves. Much effort has gone into testing the vulcanization process of latex gloves to prevent glove failure during surgery. Table 1 shows the various temperatures at which gloves are vulcanized compared to the force required to cause glove rupture.
Figure 1. Sulfur cross-links between cis-polyisoprene strands (blue and green)
Table 1. Temperatures of latex glove vulcanization and their tensile strength limit
Attributions: Chemical structure of vulcanized rubber from public domain.
Information related to vulcanization process from: Rabindra Mukhopadhyay, Sadhan K. De, S.N. Chakraborty Effect of vulcanization temperature and vulcanization systems on the structure and properties of natural rubber vulcanizates Polymer Volume 18, Issue 12, December 1977, Pages 1243–1249
Latex 2006: Frankfurt, Germany, 24-25 January 2006 By Rapra Technology Limited
[post_title] => Elasticity and kinetics of vulcanized rubber
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[question] => In trials, researchers were able to stretch a latex glove 10 cm using 5N of force. Assuming the glove elasticity matches that of an ideal spring, what is the spring constant (k) of this glove?
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[answer] => 5
[description] => Reason for the Correct Answer:
Hooke’s law states that the proportion of force and displacement determine the spring constant, as given in the formula F= kx.
Plugging in the values, we would need to solve: 5= k (10cm)
Solving for k, we would get a spring constant of 0.5 N/cm
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[answers] => Array
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[each_answer] => A. 50 N/cm
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[each_answer] => B. 2 N/cm
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[each_answer] => C. 5 N/cm
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[each_answer] => D. 0.2 N/cm
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[each_answer] => E. 0.5 N/cm
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[quiz_unique_key] => 3873426850
[question] => Which of the following would be true regarding vulcanized rubber?
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[answer] => 1
[description] => Reason for the Correct Answer:
Free roaming valence electrons are found in metallic solids, not rubber.
The sulfur bonds between cis-polyisoprene strands are covalent interactions, not electrostatic.
The lattice of covalent bonds formed in cross-linking with sulfur result in a network solid, in which all atoms of a substance are inter-connected. You can think of this as a single unified molecule.
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[each_answer] => A. Vulcanized rubber is a network solid, and essentially acts as a single giant molecule
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[1] => Array
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[each_answer] => B. Intermolecular forces contribute most to the increased elasticity of vulcanized rubber
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[each_answer] => C. Free roaming valence electrons allows for rubber to be malleable
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[each_answer] => D. The electrostatic interactions between cations and anions maintain the cross-linked structure
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => Assuming all other variables are held constant, and a glove with a spring constant of 3N/cm is stretched to 3cm without breaking. Using table 1, at what temperature could you conclude the glove was vulcanized?
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[answer] => 1
[description] => Reason for the Correct Answer:
A glove with a spring constant 3N/cm would reach 3cm of stretching at 9N.
If the glove can be stretched by 9N without breaking, then it must be vulcanized at a high enough temperature to prevent this.
Since latex vulcanized at 130°C can withstand 9.9N, then this glove must have been vulcanized at a lower temperature than this, meaning our only choice is 120°C.
A ‘quick’ way to think through this problem is to see that the higher the given temperatures of vulcanization, the more force the glove was able to withstand, therefore the lowest temperature listed as an answer must be the weakest.
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[each_answer] => A. 120°C
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[each_answer] => B. 180°C
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[each_answer] => C. 160°C
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[each_answer] => D. 140°C
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[quiz_unique_key] => 2261298308
[question] => How would vulcanization influence the degree of swelling for a latex glove?
[value] => Array
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[answer] => 1
[description] => Reason for the Correct Answer:
The swelling experiment would be pointless if the swelling of the container is constant.
While the temperature of the fluid may influence the outcome of a swelling experiment, the rate of filling would only influence how fast the swelling occurs.
While vulcanization does increase the elasticity of rubber, increased elasticity would actually decrease the amount of swelling, due to the higher tensile strength of the polymer.
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[each_answer] => A. Increased cross-linking would decrease the swelling of the container due to its higher tensile strength
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[each_answer] => B. Increased cross-linking would increase the swelling of the container, due to increased elasticity
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[each_answer] => C. Increased cross-linking wouldn’t affect the swelling of the container, as the amount of volume the container could hold is constant
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[each_answer] => D. Increased cross-linking could increase or decrease the swelling, depending on the rate of filling and temperature of the fluid
)
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[quiz_unique_key] => 2261298308
[question] => If vulcanization occurs too slowly, side-chain rings of sulfur can form instead of sulfide bridges, as seen in figure 1. Which of the following would not help to prevent the formation of the cyclic side structures?
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[answer] => 4
[description] => Reason for the Correct Answer:
Decreasing the activation energy will increase the rate of the reaction
Increasing the temperature of the reaction mixture will allow the reaction to proceed at a faster rate.
Increasing the concentration of cis-polyisoprene strands would help to prevent sulfur from reacting with itself.
Lowering the ΔG of the reaction would influence the thermodynamic favorability of the reaction, but not the rate of the reaction.
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[answers] => Array
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[each_answer] => A. Increasing the concentration of cis-polyisoprene strands
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[each_answer] => B. Adding accelerants to lower the activation energy of vulcanization
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[each_answer] => C. Increasing the temperature of the vulcanization process
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[each_answer] => D. Adding activators to lower the ΔG of the reaction
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Figure 1. Sulfur cross-links between cis-polyisoprene strands (blue and green)
