Strategies and Activities
Since this is a finale to my course, I will expect students to tie together much of what they have learned this year. Throughout the year, I use MACRO, NANO and SYMBOLIC as a means of looking at chemical principles from several angles. The main idea is that students connect what they can see with the atomic/molecular arrangement. The nano or atomic level should represent what can be seen (macro level). Finally, in the symbolic section students may draw a graph or give a chemical equation or other conventions used to represent processes. The students will be familiar with this system and I would like them to apply the Macro/Nano part when they link properties with structural arrangement. Since polymer chemistry is complex, much of the time I will have students matching pictures of structural arrangements with properties or sketching possible arrangements. They will not be giving a lot of detail about what is doing the crosslinking but using a more general approach to decide if there is crosslinking, branching, alignment, winding, etc.
Activity 1: I will start the unit with a demonstration which shows the absorption of water by sodium polyacrylate, the reaction of water to PolySnow T M, the reaction of water to Magic Sand, and the formation of nylon. These demonstrations are all explained in Flinn Scientific LabTopic book on polymers. These demonstrations will serve to introduce the topic by discussing materials and the purposes that they serve. Sodium polyacrylate is used in diapers and to provide controlled water release to plants. Magic Sand, sand coated with a hydrophobic material, was designed to clean up oil spills. Nylon, we use everyday to provide strong but flexible fibers. The challenge of designing a cutting-edge material will be presented to the students, so that they will be thinking about their project and cueing into the topics that will be important.
Activity 2: After a basic description of a polymer and some possibilities of structure, the students will conduct a series of investigations of various plastics and their properties. The lab will be modeled on the "Structure and Properties of Polymers" found in the Flinn Polymer book (there will be some modifications, some of which may be described in other sections of the book). This is an activity stations lab, where students move from station to station.
Station 1: They will take a hardened glue strip, made from Elmers glue that was left out for several days, and observe how the shape of the stick can be changed when it is immersed in warm water. This new shape can be retained by placing it in cold water. The glue strip is an example of an amorphous polymer. The temperature at which a polystyrene fork loses its shape will also be found.
Station 2: Compare what happens when polystyrene and polyethylene are exposed to polarized light. If the two filters are placed at right angles to each other, the polymer that is amorphous will appear dark while any semi-crystalline areas will appear as brightly colored areas. Students will observe the order within the CD case and the initial lack of order in a polystyrene baggie before it is stretched. The stretching will cause the chains to become straighter and some light or order will be observed.
Station 3: Students will compare bags made from LDPE (zip-type bag) and HDPE (grocery bags). They will take a pencil and insert it through a zip-bag partially filled with water and observe that there is no leakage. The pencil pushes the molecules together, and forms a temporary seal. Strips, cut lengthwise and widthwise, and compared in terms of stretch ability. The material will stretch more, in the direction that the molecules are lined up. Students will crack a polystyrene cup and notice how it cracks.
Station 4: Students will find the approximate density of HDPE, polystyrene, and PVC by adding pellets to solutions of water (d=1.0 g/mL); 10% NaCl solution (d=1.06 g/mL), 35% ethyl alcohol solution (d=0.94 g/mL), corn oil (d=0.91 g/mL), glycerin (d=1.25 g/mL). The float and sink method will be used to give density ranges.
Station 5: Add weights to a rubber band and measure strain versus the stress.
Students will be asked to record their observations and then try to relate some of the properties to molecular arrangements. Small groups will try to come to agreement and develop a potential picture of what these materials look like at the nano scale, so that they can present their pictures. As groups present their conclusions, they will explain how they believe their pictures explain the properties observed. Other groups can challenge their assumptions. There may be some considerations that the groups have not raised and the teacher can challenge them with some other depictions. This whole class discussion will be followed by the teacher presenting some of the actual molecular arrangements of common plastics and comparing the student pictures and discussing the similarities and differences. The difficulty of getting a material where all the molecules are the same will be addressed by looking at the chain termination process and atactic and isotactic possibilities, so that students will understand the difficulty in controlling the production of these polymers. Plastics are hydrophobic, a property that is important for their many applications. The polymer, sodium polyacrylate, which was used in the opening demonstrations, will be further explained as described in the background. The chemical reaction of nylon will be used to explain a condensation polymer. Activity 3: Many students do not really know what we mean by a property and there generally is not a common understanding for the meaning of what we might mean by a property such as "strength". There are many interpretations for this and students need to determine how you might measure a particular property. As an example to challenge their thinking, snap a piece of chalk and then ask a student to stand on a board that is supported by four pieces of chalk. Ask students to then define what "strong" means. As a homework assignment, students will be given a list of properties such as, strength, flexibility, stretchability, permeability, wearability, adhesion, rigidity, toughness, and they will provide a working definition for each property and decide how you could measure this property. Answers will be discussed until the class reaches a consensus. During this class, students will also be provided a list of properties and asked to match these with possible molecular arrangements.
Activity 4: Students will read Primo Levi's story "The Spider's Secret" for homework and the following night read the Scientific American article "Spider Webs and Silk" by Vollrath. In class, we will discuss the advantages of synthetic polymers versus natural polymers: the consistency of production, the greater complexity, and the environmental impact. The students will be shown the peptide linkage and we will renew their understanding of proteins from biology. In particular, we will discuss why proteins might form helixes or pleat. Then as a class activity, students will read a description of a natural polymer such as collagen, keratin, spider silk and try to represent the arrangement using everyday items such as strings, ropes, Slinky's and LEGOS. They will then present these models to the class.
Activity 5: Watching the DVD "Making Stuff Stronger" will tie together many of the concepts presented. The concept of strength is explored. Some materials featured are spider silk, steel, Kevlar and bulletproof vests, nanotubes, composites, and using biotechnology to grow materials. The animations of molecular arrangements are good and the demonstrations of the strength and toughness are very impressive. In the course of discussing very strong materials, the program talks about nanotubes and this is an excellent segue into the remainder of the unit.
Activity 6: Nano molecules are often used to encapsulate drugs so that they can be delivered to the correct site. We cannot see molecules at the nano level but, to demonstrate this concept, sodium alginate containing food coloring is added drop wise to a calcium chloride solution. The periphery of the droplets is immediately cross-linked with Ca 2 + ions to form spherical capsules. The dye will slowly leak out within a few minutes. The diffusion of the color demonstrates controlled release. This demonstration is described in the article cited in the bibliography.(Journal of Chem Ed. Vol. 82 No.7)
Carbon is a great example of the relationship between properties and molecular, or in this case, atomic arrangements. Carbon is also at the heart of much of the nano-technology. So using molecular models of diamond and graphite, students will once again be asked to link the correct model to its name. They will be asked to give written explanations to justify their choices. The teacher will then describe the discovery of buckyballs and graphene. Activity 7:Try to make some graphene using tape to pull a layer from a graphite pencil 'lead'. Then keep folding tape over the layer that was removed and try to remove several more layers. To further understand relative sizes and especially a nano scale, students choose an object to represent an atom and then find other everyday analogies to accurately give the relative size of graphene (thickness), nanotube (length), water molecule, polymer molecule, and protein.
Activity 8:To demonstrate that the properties of materials change when only a few atoms or molecules are together, a colloid of gold is prepared. Colloidal gold consists of gold particles that range in size from 5-50 nm and are uniformly dispersed in water. The interaction of light on these particles is very different than when many more atoms are in one place. Whereas normal or 'bulk' gold is bright, shiny, metallic yellow, colloidal gold nanoparticles are red or blue, and not at all shiny. Depending on the size and shape of the particles, the color of the gold particles varies from red to purple. The optical properties of gold nanoparticles are not only unique, they are also useful, providing the basis for commercial products such as medical diagnostic kits for HIV detection, biosensors for DNA analysis, lasers, and optical filters. The colloid is primarily gold(III) ions stabilized by the citrate ions absorbed on the surface of the particles. Adsorption of the citrate ions gives the gold particle an overall negative charge and this is the primary factor responsible for the formation of the stable colloid. This demonstration can be purchased from Flinn Scientific.
Activity 9: Again, watching the video "Making Stuff Smaller", will tie many of the concepts together. This episode shows 'machines' being replaced by materials that can perform the same function but take up minimal room. Some materials or devices discussed are graphene, materials which can be manipulated by magnets to deliver medicine, stained glass windows where artists knew that particle size of metals would influence the color of the glass, and encapsulated bee venom as a possible treatment of cancer.
Activity 10: Dr. Alex Star, located at the University of Pittsburgh, is developing many techniques to use nanotubes. He is an enthusiastic speaker and will be invited to discuss his research with the students. Students will be asked to prepare questions before the talk.
Students will then be asked to complete the final assignment. They will need to describe what the new material will accomplish. They will then need to list the required properties of the material in order to accomplish the final function. Using this list, they must describe the types of molecules involved, their molecular properties (such as polar, ionic, nonpolar), the types of intermolecular forces and their impact on molecular arrangement, and finally give a hierarchy for the final arrangement. An explanation will be needed to link the property to the molecular arrangement. Students will be evaluated on the thoroughness and accuracy of the possible properties, possible molecular arrangements and reasoning.
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