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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