A Closer Look at Polymers and Polymeric Nanofibers

Transcription

1 A Closer Look at Polymers and Polymeric Nanofibers Introduction Polymers, polymers, polymers they re everywhere!! Right now, the clothes you are wearing are made of polymers, you drive to school in a car that is composed of polymers, and the food you eat is composed of polymers. In fact, life as we know it in America would be drastically different if it wasn t for polymers. Plastics, one of type of polymer, are ubiquitous. Plastics versatility allow it to be used in everything from car parts to doll parts, from soft drink bottles to the refrigerators they are stored in. Now - its time for you to learn more about polymers. The following website is broken up into five main levels. Use each level to answer questions about polymers. Level One Take a walk around level one, and while you are shopping, find five items that are composed of polymers that you have or will use today. List them below as well as the type of polymer that it is made of. Item Polymer Level Two This level shows you the structures of the different polymers, as well as giving some of their uses. Remember - polymers are really looooong chains made of single units called monomers. (In the game Monkeys in a Barrel, the individual monkeys are the monomers, and a chain of monkeys would be the polymer.) When polymers form, they do it in two ways: condensation polymerization and addition polymerization. Adam Held, New Trier High School; 2004 Page 1 of 11

2 Addition Polymerization CH 2 =CH 2 + CH 2 =CH 2 -> CH 2 =CH-CH 2 -CH 3 A polymer reaction is an addition reaction if the entire monomer becomes part of the polymer. You can see in the above example the monomer is preserved. Go to this website. Click on addition polymerization on the left side. At this page, find the section labeled Mechanism of Addition Polymerization and then click on the restart button. You be able to see a simulation of an addition reaction. Cool! Condensation Polymerization A polymer reaction is a condensation reaction if a small molecule(usually water) is given off as the monomers combine. Go to this website. Click on condensation polymerization on the left side. At this page, find the section labeled Mechanism of Condensation Polymerization and then click on the restart button. You be able to see a simulation of an condensation reaction. Very cool! For the following polymers, go back to level 2 and identify the monomer(s)(using a picture) and whether it is an addition or condensation reaction. Polymer Monomer(s) Type of reaction Poly(vinyl acetate) Polyurethane Protein Adam Held, New Trier High School; 2004 Page 2 of 11

3 Level Three When you enter level three you ll see a lot of new words dealing with polymers. Some of these words are important for our lab tomorrow. Using the information in level three, answer the following questions about polymers. 1. How does crosslinking affect the physical characteristics of a polymer? How does it affect the strength of the polymer? 2. What is the tensile strength of a polymer and how is it different from the compressional strength? 3. Why would it be incorrect to assign a specific molecular weight to a polymer? 4. What is viscosity and how do the properties of a polymer affect it? Adam Held, New Trier High School; 2004 Page 3 of 11

4 Synthesis of Polymers Lab In today s lab you will be studying four different polymers. You will be producing three of them and watching Mr. Held produce the fourth. Make good observations and answer the questions for each polymer. Polymer #1- Polyurethane Materials 1- A cup (contains polyurethane foam part A) 1- B cup (contains polyurethane foam part B) stirring rod food coloring (optional) newspaper Procedure 1. Go to fume hood and acquire a cup marked A and a cup marked B. 2. If desired, add several drops of food coloring to one of the cups and stir thoroughly to mix. 3. Spread a paper towel or newspaper flat on the table and place one of the cups in the center of the paper towel. 4. Pour the contents from the second cup into the cup on the paper towel and stir thoroughly until you see the foam beginning to expand. Remove the stirring rod. 5. Observe the foam as it expands to about 30 times its original volume. The cup will get warm, indicating an exothermic reaction. Do not touch the foam until it is completely hardened. 6. For disposal the cups may be thrown in the trash once they have cooled down. Observations Describe the following physical properties of polyurethane 1) Response to agitation Knead the polymer as forcefully as you can. How does it respond? Set it back on the lab bench for a while. How does it respond? 2) Stretchability Try stretching the polymer quickly. How does it respond? Try stretching it slowly. How does it respond? Questions 1) Based on your observations, do you think the polyurethane you ve made exhibits crosslinking? Why or why not? 2) How would polyurethane be different if it didn t have this crosslinking? Uses List two uses of polyurethanes 1) 2) Repeating Unit (Draw Structure/Give Name) Adam Held, New Trier High School; 2004 Page 4 of 11

5 Polymer #2 - Polyvinyl Alcohol Materials Polyvinyl alcohol, 4% solution, 25 ml Sodium Borate, 4% Solution, 3 ml Disposable Cup 2 Graduated Cylinders, 1-10 ml,1-50 ml Wooden Splint Food Coloring Procedure 1) Place 25 ml of 4% polyvinyl alcohol into the disposable cup. Add a couple drops of food coloring, if desired, and stir with a wooden stick. 2) Pour 3 ml of sodium borate solution into the cup while stirring. The mixture will gel almost immediately but keep stirring until smooth. Observations - Describe the following physical properties of the slime 1) Response to agitation Knead the slime as forcefully as you can. How does it respond? Set it back on the lab bench for a while. How does it respond? 2) Stretchability Try stretching the slime quickly. How does it respond? Try stretching it slowly. How does it respond? 3) Resilience Shape the slime into a ball and drop it on the lab bench. Can you get it to bounce? *When finished with your slime, put it back in the baggie and write your name on the baggie with a marker. Turn the slime in this will count as the results part of your lab grade. You may come back at the end of the day to collect your slime. If you take it home, keep it away from carpets, wood furniture, and little ones who may think it looks tasty. (Eating it is a very bad idea.) Wash your hands after you are done handling the slime.* Questions 1) What happened when you tried to stretch the slime quickly? When you tried to stretch it slowly? Explain why fast and slow stretching might cause different reactions in the slime. 2) If you had to choose one or the other, would you categorize your slime as a solid or a liquid? Justify your answer. Adam Held, New Trier High School; 2004 Page 5 of 11

6 Polymer #3 - Sodium Polyacrylate Materials 4 Styrofoam cups Sodium polyacrylate,.5 grams Distilled water, 100 ml Salt, sugar and baking soda Procedure 1) Place about.5 grams of sodium polyacrylate in the Styrofoam cup. 2) Add 100 ml of water to the cup. 3) Feel the jelly like mass inside the cup. Turn the cup upside down. 4) Place a blob of jelly into a clean Styrofoam cup. Add a little salt and stir. Repeat in different cups with sugar and baking soda. Observations - Describe the following physical properties of the sodium polyacrylate. 1) Response to agitation Knead the gel as forcefully as you can. How does it respond? Set it back on the lab bench for a while. How does it respond? 2) Stretchability Try stretching the gel quickly. How does it respond? Try stretching it slowly. How does it respond? 3) Resilience Shape the gel into a ball and drop it on the lab bench. Can you get it to bounce? Questions 1) Describe the jelly-like material inside the cup. Was their any water inside the cup? 2) What happened when you added the salt to the jelly? What about sugar? Baking soda? 3) What are some consumer products that might use sodium polyacrylate? Adam Held, New Trier High School; 2004 Page 6 of 11

7 Polymer #4 Nylon (Done as a teacher demonstration) Observations - Describe the following physical properties of the nylon. 1) Stretchability Try stretching the nylon quickly. How does it respond? Try stretching it slowly. How does it respond? 2) Resilience Shape the nylon into a ball and drop it on the lab bench. Can you get it to bounce? Questions 1) Based on your observations of the nylon reaction, would you guess it to be an addition or condensation polymer? 2) Why is important that the two chemicals used to make the nylon not be miscible? In this lab you have been introduced to polymerization, different uses of polymers and several different kinds of polymers. Based on your observations, hopefully you have a better understanding of the process of polymerization and the properties that different polymers possess. Using the knowledge gleaned during lab, answer these questions. Overview Questions 1) What would be the benefits and what would be the problems with using synthetic fibers instead of natural fibers such as carbon and wool? 2) Which polymer had the greatest tensile strength? 3) Which polymer exhibited the greatest degree of crosslinking? The least? Adam Held, New Trier High School; 2004 Page 7 of 11

8 Take a spin with me - Electrospinning Polymers Nanofibers Background Some polymers can be stretched or pulled into much longer shapes. There is a special technique, called electrospinning, which can be used to stretch a polymer out so much that it becomes a nanofiber. How small is a polymer nanofiber? Imagine taking a human hair and cutting in half lengthwise, so you have two hairs that are as long as the original, but half as wide. Take one of these two thinner hairs, and cut it again. Continue this about 1000 times, and you have reached the size of some nanofibers!! (~ 500 nanometers) (See the figure below.) What are these nanofibers used for? There are a quite a few exciting potential uses for nanofibers produced by electrospinning. Do a Google search and find two uses that seem to be promising. 1) 2) How does electrospinning work? Electrospinning uses either a polymer solution (which we will use) or a polymer that has been melted. This polymer is placed into a syringe (other devices have been used, such as a cake icing tip) and a wire is placed through the solution. This wire charges up the polymer, which is then ejected through the syringe and forms a super thin filament that collects on a grounded metal plate underneath the syringe. Electrospinning nanofibers A Human Hair compared to Nanofibers The size of the fibers produced in electrospinning can be seen in comparison to a human hair. Adam Held, New Trier High School; 2004 Page 8 of 11

9 Electrospinning Setup Materials Needed High Voltage Power Supply 0-30 KV (A local university may have one you can borrow. Also, the University of Akron has been very helpful in bringing electrospinning to the high school classroom, they might be a good place to start.) 2 - ring stands 3 - three prong extension clamps with vinyl sleeves for insulation 1 20 ml glass volumetric pipette (used to hold polymer solution) 1 disposable Pastuer pipette tubing copper wire aluminum foil or metal sheet Ethylene-co-vinyl alcohol (EVOH, 38% ethylene content, Soarus, (847) ) Isopropanol Note: This experiment should be performed on a non-conductive surface (i.e. wood) for safety reasons Figure 1. A basic electrospinning system replicated from the work of John Zeleny 1. Setting up the electrospinning apparatus Figure 1 illustrates a simple electrospinning system. The system consists of two ring stands, three 3-prong extension clamps with vinyl sleeves for electrical insulation, 20 ml glass volumetric pipette (polymer reservoir), disposable glass Pasteur pipette (nozzle), tubing, copper wire to charge the polymer solution and ground the metal collecting plate. Producing the polymer solution The polymer suggested for this simple classroom demonstration is ethylene-co-vinyl alcohol with isopropanol/water(70/30% v/v) as the solvent. The EVOH is not very soluble in this solvent at room temperature; thus the solution must be heated to 80 for 2-3 hours for complete dissolution of the polymer to occur. After cooling to room temperature, significant precipitation will occur after approximately 6-7 hours, so the demonstration should take place during this 6-7 hour window. 2 1 J. Zeleny, The Physical Review X (1), 1 (1917) 2 E. Kenawy, J. Layman, J.A. Matthews et al. Biomaterials 24, 907 (2003). Adam Held, New Trier High School; 2004 Page 9 of 11

10 A series of different polymer solution concentrations should be made to illustrate the fiber diameter and subsequent mechanical property dependence on concentration. A range of 2.5 to 10% (weight/volume, w/v) is suggested. You should produce about 50 ml of each solution. Beginning to Spin Start with the 10% (w/v) solution to set up the process parameters. The first step in the procedure is to fill the polymer reservoir and insert the wire electrode. Next, connect the positive output from the high voltage power supply to the wire contained within the polymer reservoir/solution. Adjust the height of the reservoir so that the solution drips slowly from the nozzle tip. At this time, switch on the power supply and adjust voltage until jet/fiber formation is observed. Fibers should begin to accumulate as a mesh on the grounded metal plate. Allow the process to continue for several minutes until the build-up is substantial enough to peel a sheet from the collector. Repeat this process with all of the processing conditions constant, except vary the polymer concentration, i.e. refill the polymer reservoir with a new polymer solution of different concentration. 3 After the spin After electrospinning, examine the structure with a light microscope. Use the tables to describe the fibers produced from the different polymer solutions. Also allow the students to manually manipulate and stretch the different structures, this will help them get an idea of the structural stability of the fabricated materials. Table 1: Apperance of nanofiber mats Under microscope Concentration of Polymer Solution Physical Description of Nanofiber Mats Formed Table 2: Structural properties of nanofibers mats Concentration of Polymer Solution Description of Structural Stability 3 Bowlin, G. et al, Biomedical Nanoscience: Electrospinning Basic Concepts, Applications, and Classroom Demonstrations Adam Held, New Trier High School; 2004 Page 10 of 11

11 Questions/Discussion 1. How did polymer concentration affect the physical properties (such as fiber diameter) of the nanofiber mats? 2. How did the polymer concentration of the solution affect the strength of the polymers produced? 3. Based on what you ve seen of electrospinning, would you say to someone who claimed nanotechnology was expensive, hard to do, and had no practical uses? 4. Think of a unique use for the nanofibers that were produced. Adam Held, New Trier High School; 2004 Page 11 of 11