wk13_HANDOUT_polymer-mechanics_f23

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MSE 235: Mechanical and Thermal Properties of Polymers 1 Instructions: The lab will meet in ARMS 2130. Be sure to bring your safety glasses and wear proper safety attire. Before your lab session, read this lab handout, the sections listed below from your textbook, and answer the “pre- activity discussion questions” on pg. 2-3. Background Reading: Sections 15.1-15.5; 15.7-15.9; 15.12-15.14 in W. D. Callister, Jr. and David G. Rethwisch, Materials Science and Engineering: An Introduction , 8 th Edition, John Wiley & Sons, Inc., Hoboken, NJ (2010). Objectives: Students will determine how molecular-level structure and thermal properties affect the macro-scale mechanical response of different types of polymeric materials deformed in tension. PRE-ACTIVITY READING Plastic materials are composed of a tangled collection of polymer molecules or “chains”. Each polymer chain is made of a series of repeating units that are connected by covalent (chemical) bonds in an end-to-end fashion to form one long, flexible, string-like polymer molecule (see Fig. 1). 1 Plastic objects contain millions of polymer chains, tangled together in a similar fashion to a tangled collection of spaghetti noodles (see Fig. 2). When a force is applied to the collection of chains, the chains can move and reorient in response to the applied force. 2 If the applied force is great enough in magnitude, the long axis of the molecules can reorient in the direction of the applied force (as shown in Fig. 2). Chain reorientation takes less energy and is thus more likely to occur when the plastic is heated. The long axis of the aligned molecules can store elastic energy within its covalently bonded “backbone”, resulting in a strong mechanical response from the deformed collection of aligned chains. Thus, a plastic material in which the chains are aligned is very strong in response to forces applied parallel to the chain alignment direction but can behave in a mechanically weak manner in response to forces applied perpendicular to the chain alignment direction. 3 This is because only the relatively weak Van der Waals interaction forces between the different chains act to resist forces applied perpendicular to the alignment direction as compared to the relatively strong covalent bonds in the chains’ backbones that resist forces applied parallel to the alignment direction. When a block of plastic is heated to a high temperature and becomes mechanically soft, it can be molded and processed into a variety of different physical forms, such as plastic cups, plastic forks, and plastic cords. 3 A quick online search for “How plastic forks (or cups Figure 1: Simple schematic of a polymer chain (left), illustrating the chemical structure of PETE (right) and the relative length scales. [created by K. Erk, 2015] Figure 2: Simple schematic of the microstructure of a plastic material, composed of a tangled collection of polymer chains (left). When the chains are exposed to a tensile force (red arrows), the chains can reorient and align in the direction of the applied force. [created by K. Erk, 2015] Mechanical Properties of Polymers

MSE 235: Mechanical and Thermal Properties of Polymers 2 or cords) are made” will result in a number of videos that illustrate different industrial plastic processing techniques, such as sheet extrusion, injection molding, compression molding, and thermoforming. Focusing on disposable plastic cups, cups are commonly manufactured by a process known as thermoforming (see Fig. 3). 4 In Step 1, a heated film of plastic is positioned above a cooled metal mold containing a cup-shaped cavity. In Step 2, a metal punch is brought into contact with the hot plastic by applying a downward force and the plastic subsequently deforms around the punch. Typically, this process is also assisted by a vacuum to aid in mold-filling. In Step 3, the punch is pushed further into the plastic, causing the plastic film to stretch and deform, ultimately filling the mold and creating the cup. After the cup is formed in Step 3, it is cooled and removed from the mold. There are excellent videos of this process at the industrial scale available online. References 1. W. D. Callister, Jr. and D. G. Rethwisch, Materials Science and Engineering: An Introduction , 8 th Ed., John Wiley & Sons, Inc., Hoboken, NJ (2010). 2. L.H. Sperling, Introduction to Physical Polymer Science , John Wiley & Sons, Hoboken, NJ (2001). 3. N.G. McCrum, C.P. Buckley, and C.B. Bucknall, Principles of Polymer Engineering , 2 nd Ed., Oxford University Press, New York, NY (1997). 4. P.W. Klein, Fundamentals of Plastics Thermoforming , Morgan & Claypool Publishers, San Rafael, CA (2009). PRE-ACTIVITY DISCUSSION QUESTIONS (1) Draw the general shape of a stress-strain curve from a polymeric material and define all the key features of the curve. Figure 3: Simple schematic (side-view) illustrating a thermoforming process that is used to create disposable plastic cups. [created by K. Erk, 2015] Mechanical Properties of Polymers

MSE 235: Mechanical and Thermal Properties of Polymers 3 (2) What happens to a polymeric material at temperatures above and below its glass transition temperature (T g )? (3) Report the T g values for the following polymeric materials (and include your sources): • PETE • LDPE • PP • Nylon 6,6 • Poly(methyl methacrylate), PMMA • PS (4) How would you expect a dog-bone sample of PETE to behave when deformed in tension in a room temperature lab? How would PP behave in comparison to PETE? Explain. Mechanical Properties of Polymers

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MSE 235: Mechanical and Thermal Properties of Polymers 4 LAB ACTIVITY INSTRUCTIONS Material and Equipment List: clear plastic cups, including PETE and PP (purchased from Payless, West Lafayette); scissors; permanent markers; dog-bone style template for tracing of the specimens; digital calipers; mechanical testing machine; laptop or in-classroom computer with spread-sheet software for data recording and analysis. Tasks: Your TA will help you to perform the following tasks during this lab activity: (1) Determine the mechanical response of different specimens of plastic cups deformed in tension: “parallel-cut” and “perpendicular-cut” specimens (see Fig. 4). (2) Relate the mechanical response of the different specimens to the molecular-level structure and thermal properties of the plastic cups. Notes: With help for your TA, you will prepare the specimens from each cup, measure and record the specimens’ dimensions, perform the mechanical tests, and analyze the resulting data by making stress-strain plots, calculating the Young’s modulus, and determining the yield stress and strain at failure (%-elongation) for each specimen. Make sure you keep an accurate list of what specimens were tested (what type of polymer and what orientation) and the testing conditions (such as temperature and strain rate). Also make sure you consider how many significant figures you should report in your data. Discussion Questions: Ø Comparing the stress-strain response of the parallel-cut sample to the perpendicular-cut sample, which sample displayed a more ductile response and which displayed a more brittle response? Why? Ø How does the mechanical responses of the PP samples differ from the PETE samples? Why? Ø If you could “zoom-in” and directly view the molecular-level structure of each specimen BEFORE deformation, what would the structure look like? How would the structure change DURING deformation but before failure? It may be helpful to draw some sketches to answer these questions. Figure 4: Schematic illustrating the two types of specimen that will be tested. [created by K. Erk, 2015] Mechanical Properties of Polymers

MSE 235: Mechanical and Thermal Properties of Polymers 5 LAB REPORT INSTRUCTIONS To prepare your report, a docx template is available on Blackboard; you should download and use/edit this document and then submit this document as a PDF through Blackboard using the appropriate assignment link. The report will be due prior to the start of your next lab session and is worth a total of 60 points . Please note following: Ø all figures and tables should be properly labeled and include a descriptive caption, Ø all reported numerical values have an appropriate number of significant figures, Ø data is analyzed/summarized using appropriate statistical methods, Ø any primary source or database that was consulted should be properly cited and included in the reference list (see pg. 1-2 for examples of this), Ø keep in mind the attributes of a well-designed, mindful figure that clearly communicates the required information (remember it is good practice to combine relevant data into the same plot), and Ø written text should be free from spelling and grammar errors, and please keep the elements of technical writing in mind (you may wish to review your lecture notes on voice, tense, conciseness and clarity or consult your Reporting Results textbook). Experimental Methods (10 pts): Write a short paragraph (less than 250 words) that fully describes the materials, machines, and methods that you used/performed during this lab activity. Results (30 pts): Please illustrate the differences in the PP and PETE samples that were tested by: Ø Creating graphs of engineering stress-strain data for the samples. (10 pts) Ø Creating a data table that concisely summarizes the most important results. (10 pts) Ø Write a short paragraph (less than 250 words) that summarizes any relevant in-lab observations from the experiments as well as clearly describes the important trends that are present in your data sets. Within this paragraph, you should directly refer to your figures and tables. (10 pts) Discussion (10 pts): Write a short paragraph (less than 250 words) that explains the underlying reason for the trends that you observed in your tensile data (e.g., why, at the molecular level, are the mechanical responses of the samples different? how does the polymer’s thermal properties impact its resulting mechanical response? Etc.). Also, in your paragraph, report some reference values and describe how your data compares to those reference values (note: determining the percentage of your data that is within ± 5% of the reference value could be a useful and concise way to communicate this information). Be sure to properly cite your sources for any scientific information as well as your reference values. Please note that your overall technical writing ability will be assessed out of 10 points . Mechanical Properties of Polymers BrightSpace BrightSpace