Author: Mark Rogers
Because polymers are such large molecules, the intermolecular forces between polymer chains are stronger than most other organic substances. Rubber, despite our general suppositions on how phases of matter should behave, is a liquid at room temperature (it will flow over time). Due to the strong intermolecular forces present (van der Waal interactions), rubber is extremely viscous and seems to be a solid. If rubber is to be used in a commercial setting (such as tire manufacturing), the liquid properties of water will reveal themselves over time and with fluctuations in temperature while travelling. The solution to this problem was solved by Charles Goodyear. Through experimentation, Goodyear discovered that the addition of sulfur to a heated sample of rubber changed the properties of the rubber, making the rubber a rigid solid that did not flow over time. Today, we understand that Goodyear had discovered vulcanization, the process of linking polymer chains to one another covalently. Rather than flowing over time, vulcanized (or “cross-linked”) rubber contains polymer chains that are “locked” into place and, as a result, is useful for the production of tires that are expected to withstand high temperatures and degradation over time without deforming. The vulcanization of rubber revolutionized the production of tires and, with the later addition of nanofillers such as carbon black, has resulted in reliable tires that withstand the harsh conditions through which we expect the tires to survive.
This lesson introduces the concept of cross-linking rubber and how cross-linking affects the properties of a rubber sample. The lesson itself can be accomplished in 2 days. The first day should be spent introducing polymers and their properties/uses. The second day should be spent on exploration into cross-linking, the Borax/glue demonstration, and beginning the swell test. The swelling test itself will take between 10 and 14 days (undisturbed). This lesson is excellent to begin just before a holiday break, to give the test time to complete while students are not attending class daily.
What should students know as a result of this lesson?
What should the students be able to do as a result of this lesson?
This lesson will best serve students if it is performed after students have had a brief introduction to polymers and their importance in nature as well as commercially, with an emphasis placed on physical properties and the role of intermolecular forces (van der Waal interactions).
To introduce rubber, the video below is an excellent basic intro: https://www.youtube.com/watch?v=rHhD6YhsGk0
Natural rubber is a (extremely) viscous liquid and often it is assumed to be a solid. The solid-like properties of rubber are a result of the attractive van der Waal interactions, which increase in all molecular compounds as the molar mass/size of the molecules increase (so in the case of polymers, which are large molecules with a high molar mass, they play a large role in rubber’s observed properties such as high viscosity).
Once students have a (somewhat) decent grasp on the observed properties of natural and synthetic rubber, a classic demonstration can be used to illustrate how polymers can be manipulated. School glue, which is a solution of polyvinyl acetate (PVA), can be cross-linked by the borate ion, BO32- (contained in Borax laundry detergent). When the glue is mixed with a solution containing the borate ion, the PVA “coagulates” to form a mass with obviously different properties. There are many variations to this demonstration that can be utilized to obtain different consistencies of the product (slime vs. putty).
Students should be broken into groups of 2-3 and each group should be given a plastic cup, a plastic spoon, roughly 50 mL of school glue, and roughly 20 mL of a prepared borax solution (made by combining roughly 1 g of Twenty Mule Team Borax laundry detergent to roughly 25 mL of water). Students should add the glue to an empty plastic cup and stir the glue with a plastic spoon, making observations about the properties of the glue. Students should then add about 20 mL of the prepared Borax solution and stir, making observations of any changes to the properties of the glue. If you don’t mind a bit of a mess, allowing students to manipulate the mass that forms with their hands is perfectly safe and quite enjoyable by the students.
After allowing the students to manipulate their newly cross-linked polymer, students should be allowed time to hypothesize what may have occurred with the addition of the Borax solution. The concept of cross-linking can be introduced as the explanation.
Cross-linking polymers is an important process in manufacturing and is a great example of how these macromolecules can be manipulated to meet commercial needs, such as with tire production. Other than the observed differences in properties, polymer scientists are also interested in how to determine the degree of cross-linking that has occurred (cross-link density).
There are several ways to test the presence of cross-links using expensive and unavailable equipment, but the simplest method to determine not only if cross-linking has occurred, but also the density of these cross-links, is a process called a swelling test. Samples of the cross-linked rubber are submerged in an organic solvent for a specified amount of time (determined by the type of rubber in the sample).
FOR TEACHER’S EYES ONLY – [The masses of the rubber sample before and after submersion in the solvent are compared and, if the rubber sample has substantial cross-linkage (as with cured Holden’s HX-80 latex), the mass should increase, due to the absorption of the solvent and structural “integrity” of the sample. If there is no cross-linkage between the rubber chains (as with rubber cement), the polymer chains should begin to dissolve into the solvent and, thus, the sample should decrease in mass over time.]
The organic solvents typically used in a lab setting (hexane, toluene, chloroform, etc.) are not safe for a school setting, but mineral oil is a suitable, though less optimal, solvent for this swelling test. It is important to remind the students why mineral oil will be used as the solvent rather than just water (polymers are generally nonpolar, therefore a nonpolar solvent will be necessary to interact with the rubber samples).
The instructor should introduce swell testing and inform students that it is an excellent way for a scientist to determine whether or not a sample of rubber has been cross-linked. They should also be informed that Holden’s Latex contains cross-linking agents and rubber cement does not. To increase the level of critical thought and application of the concept, the students should not be told what results to expect, but be allowed to predict the results and later hypothesize the explanation for their results themselves.
Both Holden’s HX-80 Latex and rubber cement contain the polymer polyisoprene. Holden’s latex contains the necessary cross-linking agents when it is bought, but must be allowed to cure and vulcanize first. To do this, a layer of the latex can be poured onto any surface (baking tray, plate, sheet of aluminum foil, etc.). Spread the latex evenly so that a (relatively) consistent thickness is obtained. The latex should be allowed to stand at room temperature for 5-7 days to cure. (To cure in a shorter amount of time, the latex can be heated in an oven at 110°F for 4 hours or placed in boiling water for 2 hours.)
The rubber cement can be dried in a similar way, however it is advised to pour the layer of rubber cement onto a sheet of aluminum foil, as it may be slightly difficult to remove all of the sample after it has dried.
Groups of 2-3 students should obtain a roughly 1 g sample of both types of rubber. The samples may be provided by the teacher or be obtained by students by cutting a single piece from the larger sample using scissors. It is not necessary that the samples be exactly 1 g, but the students should record the exact mass of their sample before beginning the test. Students should make and record observations of the samples and note any similarities or differences between each type of rubber.
Once the masses of both samples have been recorded and observations have been made, the samples should be placed in separate clean glass containers that can be covered. The containers should be able to hold at least 60 mL of liquid. Mineral oil should then be poured into each container until the rubber samples are completely submerged and there is 10-15 mL of extra oil on top. The containers should then be covered with a lid, aluminum foil, or anything else that will prevent contamination of the samples.
The containers should be labeled with the type of rubber present, the initial mass of each sample, the start date of the test, and the names of the students to which each sample belongs. They should then be placed out of the way and allowed to sit, undisturbed, for 10-14 days. For comparison, a group may use a heating pad to warm the solvent during swelling, making sure to monitor the samples so they do not become hot.
After 10-14 days, the samples have had the opportunity to interact with the mineral oil solvent. The samples should be removed from the solvent and any observed changes should be recorded. The samples should be dried of any remaining oil with a paper towel, and placed on a balance to record the new mass. The latex should be quite easy to remove using forceps, however the rubber cement will have undergone significant changes and may be more difficult to remove, as it has either dissolved completely or is in the process of dissolving.
Students should note the changes that occurred and be given the opportunity to brainstorm within their groups or in a class discussion why those changes occurred. While the name of the test itself may have been a giveaway as to what to expect, the students may need probing to arrive at the actual answer. The cross-linked latex increased in mass because the sample absorbed the solvent, which is similar in polarity. The presence of the cross-linkages allowed the sample to retain its structure without dissolving. The rubber cement, however, dissolved in the solvent due to the absence of any cross-linkages to hold the rubber molecules together.
Students should have a basic understanding of the science of polymers before beginning the cross-linking lesson. This can be accomplished by lecture, class discussion, or many videos available on the internet.
Students should also have an understanding of intermolecular forces such as van der Waal interactions, what causes these attractions to increase, and the differences in properties one would expect between a substance with strong attractive forces (rubber, etc.) vs weak attractive forces (methane, propane, etc.).
Students should be assessed both informally (during class discussions) and formally. Possible questions for students to answer: