In the Laboratory
Polymer Mechanical Properties via a New Laboratory Tensile Tester T. Carter Gilmer* and Matthew Williams Department of Chemistry, University of Michigan-Dearborn, 4901 Evergreen Road, Dearborn, MI 48128-1491 Polymers, or macromolecules, are gaining interest in the technical and scientific communities. There is an abundance of new and useful polymeric structures that generate corresponding new, interesting, and potentially useful (mechanical) properties. Table 1 shows a way of categorizing natural and synthetic polymers (1). The subcategorization that will be emphasized divides solid material oriented polymers into: • • •
Table 1. Categories of Polymers Natural (Bio) Organic
Synthetic Organic
Inorganic
Proteins hair silk wool
Fibers Nylon – carpets Antron – upholstery Orlon – wood substitute Kevlar – nylon based composite, bullet proof vests
SiO2 – based glass
Vulcanized (natural rubber) tires
Nucleic Acids (DNA/RNA)
Plastics Plexiglas Contact lens Corian – marble substitute
Silicones
Genetically engineered DNA
Polysaccharides wood starch
Elastomers Rubber Bands Spandex – swimwear
fibers (low elongation, high stiffness and modulus) plastics (intermediate elongation, generally intermediate stiffness) elastomers or rubbers (high elongation, low modulus)
Examples of natural fibers are wool, silk, and wood. Common synthetic fibers are Nylon® (polyamides) and Rayon®. Rubbers are more readily identifiable. Natural rubber (polyisoprene) comes from trees. Rubber bands and tires are common examples of (hybrid) synthetic elastomers. But plastics are all synthetic, originating with Bakelite discovered by Leo Bakeland in the early 1900s. Synthetic plastics are ubiquitous now. Eyeglass lenses are often made of polycarbonates. Trash bags are often polyethylene. Cookware is coated with a nonstick, high surface energy Teflon® or tetrafluoropolyethylene. Many demands are placed on each specific polymer, polymer blend, or polymeric composite. Essentially all materials, be they substitutes for brick, wood, glass, or other structurally demanding materials, must have and reasonably maintain specific mechanical properties. One way of measuring a range of some mechanical properties is to perform tensile and/or compression testing. Discussions here will refer to determination of tensile properties in a uniaxial mode. That is, stress is applied along one axis, as when you stretch a rubber band by extending your hands. Figure 1 shows three representative stress/strain curves of a typical fiber, a plastic and an elastomer, which have undergone tensile testing (2, 3). As described in the attached Tensile Properties Experiment, five common parameters are obtained from each curve: 1. Tensile strength (in lb/sq in. or MPa) (force/area) 2. Elongation-to-break (% increase in length) 3. Elastic modulus or Young’s modulus or modulus (measure of stiffness, or stress/strain—same units
as tensile strength) 4. Yield (applies primarily to plastics but also to rubbers, where stress/strain slope equals zero; polymer chains are disentangling and rearranging causing no additional stress with increasing strain), determined at a specific stress or strain point 5. Toughness or energy-to-break, area under the stress/strain curve to the break point (material must be strained to failure)
The following experiment on tensile properties describes a simple laboratory version of a common sophisticated tensile machine, namely, Instron’s Tensile Testers. A polymeric material of known dimensions (width and thickness primarily and length secondarily) is attached to and suspended from a laboratory clamp. Attached to the bottom of the sample is a bucket with a handle, into which precise weights can be placed. As individual weights are added to the bucket, a specific load is applied, which upon using the known cross-sectional area of the sample is converted to stress (force/unit area). By measuring the initial length of the sample and its incremental increase with each new load, strain is also recorded. Upon completing the process of adding weights and measuring each new length, all necessary data for a standard stress/strain curve are generated. Some samples will break with the maximum weight applied; others may not break under these conditions. During the tensile testing basically three events occur that can lead to fracture or breakage: •
• •
*Corresponding author. Present address: Department of Chemistry, Bowling Green State University, Bowling Green, OH 43403-0213.
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Bio/Synthetic, or Hybrids Organic
disentanglement and stretching of the polymer: random coils stretch and align in a somewhat parallel fashion, and crystalline polymers in a lamellar arrangement unalign and go towards a stretched-out, somewhat linear configuration breakage of covalent bonds break under the applied stress (see Fig. 2) chain slippage (where secondary bonds, e.g. dipole– dipole interactions, are broken; see Fig. 2)
In tensile testing it is critical to use uniform samples
Journal of Chemical Education • Vol. 73 No. 11 November 1996
In the Laboratory
Figure 1. Stress–strain behavior of three types of polymeric materials.
Figure 2. Schematic representation of fracture mechanisms in polymeric materials: (a) bond breakage; (b) chain slippage.
with minimal surface defects or flaws. If a sample is nonuniform in terms of density or cross-sectional area, the least dense sections or the thinnest will have a lower effective cross-sectional area, which under a given load (weight) will experience the most stress. As cross-sectional area decreases, stress increases, assuming the load is the same. So the thinner or less dense sections, or sections with flaws, become the locations most likely to fail first. In effect these locations are the weak links in the chain. Accordingly, tensile measurements such as strength, elongation-to-break, and toughness will be biased towards the low side if defects and/or thin sections exist. To get the most accurate and precise tensile data the sample must be uniform in dimensions and contain minimal flaws, especially surface flaws. All of the discussion so far has implied tensile measurements done at room temperature or 20 °C. Often polymers exhibit tremendous change in mechanical properties with temperature (4). This is readily illustrated in Figure 3 for poly(methyl methacrylate) (PMMA). As shown in Figure 3, PMMA appears to be a brittle plastic (resembling a fiber) at room temperature or 277 K. PMMA’s modulus steadily decreases as temperature increases, with corresponding changes in other mechanical properties. At 333 K, the stress/strain curve shape for PMMA resembles that of an elastomer. On a macroscopic scale tensile data are quite informative as they reflect the collective effect of many polymer chains. With more in-depth analysis they can be useful in obtaining microscopic information as well. For example, polymers that exhibit extensive H-bonding generally are stiffer (higher modulus) than those with less H-bonding. Conformation of polymers (tacticity, for example) also affects their morphology (shape), which affects entanglements, density, and in turn mechanical properties. Many polymers are solids. Mechanical properties of all solids are definable to some extent and extremely useful information can be obtained from those studies. Again, one difficulty in obtaining meaningful data is assuring that an adequate sample is prepared that is uniform and free of significant surface defects. Apparatus and Experimental Procedure Tensile measurements are readily determined by individual students in a small laboratory. Only common and quite inexpensive tools and devices are needed. Beginning with a sample suspended from a laboratory rack and attached to a load-carrying bucket, stress versus strain curves are determined as described below.
Procedure for Tensile Properties Determinations (of a Fiber, a Plastic, and an Elastomer) Materials • • • • •
Figure 3. Variation of the stress–strain behavior of poly(methylmethacrylate) with temperature.
• • • •
Ninety 32.0-g bolts (or an equivalent constant-mass material) 1 light-weight basket with removable handle (model is 129 g with aluminum mesh screen) Regular typing paper, known thickness; 4.5 mil (0.001 in.) is fine Rubber bands (measure thickness and width) 1 elastic strap (about 8 in. long) with metal hooks at ends (often called bike strap or bungy cord) 1 roll of polyethylene film (0.8 mil thickness) Scotch tape 1 tape measure (or meter stick) Micrometer
Vol. 73 No. 11 November 1996 • Journal of Chemical Education
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In the Laboratory
Figure 4. Stress/strain plot for polyethylene. Figure 5. Modulus of polyethylene.
Procedure 1. Attach one hook of the elastic strap to a lab clamp. 2. Adjust clamp location so that with strap extended and attached to the basket with its handle, the bottom of the basket is about 10 in. from bench top. 3. Measure length of the strap excluding metal hooks. 4. Incrementally add specific loads (maybe 5 bolts) (weights) to the basket. 5. After each additional load measure the new length and record the total weight. 6. Continue steps 3–5 until the material breaks or the basket is full with all 90 bolts or 2880 g. 7. Using paper strips 1 in. wide and 12 in. long, reinforce the top and bottom 4 in. with Scotch tape. 8. Wrap the top taped portion of the paper around the support (lab clamp) and secure it by reinforcing that portion with more tape, making a loop around the clamp. 9. Repeat step 8 for the bottom portion and slip this loop through the handle of the basket. 10. Measure the length of the paper (approximately 4 in.), excluding the portions that have been reinforced by Scotch tape. 11. Repeat steps 4–6. 12. Using the polyethylene, which has been cut with a razor blade or scissors to 1/2 in. (or any other known thickness from about 1/16 to 1 in.) and 12 in. long, attach top and bottom portions to clamp and reinforce with transparent tape as in step 7. 13. Continue from step 12 with steps 8–11. 14. Stress–strain data can be collected for all other materials (such as the rubber bands) in a way similar to that described in steps 7–11. 15. Plot load (force or weight added) versus strain for each material. 16. Plot stress (force/cross-sectional area) versus strain for each material. Strain = (new length – initial length)/initial length. 17. From plots in 16 above, determine tensile strength (peak stress obtained), elastic modulus (slope of linear portion of stress/strain curve), elongation-tobreak (strain at break × 100), and toughness (or energy-to-break), area under the stress/strain curve. Sample must break for toughness measurement. Tensile strength in MPa (Pa = N/m 2); modulus in MPa; toughness in MPa (or kJ/m3). 1 MPa = 145 psi; 1 N = 0.2248 ft-lb; N = (meter) (kg)/s2; F (force) = ma (mass × acceleration).
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Results As the procedure above describes, four types of polymeric materials were chosen for demonstration purposes: a fiber (paper), a plastic (polyethylene), an elastomer (rubber band) and a composite (bike strap). Using this laboratory apparatus, a stress/strain curve for a commercially available polyethylene (PE) was generated (Fig. 4). Subsequently, this curve was used to calculate: • • • •
tensile strength elastic modulus % elongation-to-break (%E) toughness (area under stress/strain curve)
11.9 MPa 41 MPa 366% 34 MPa
Figure 5 shows an expanded scale (x-axis) to look more closely at the linear portion of the stress/strain curve and more accurately determine modulus. A similar stress/strain curve is shown for a rubber band in Figure 6. Here, the percent elongation-to-break (432%) is higher than for the PE, and the modulus and Table 2. Tensile Properties of Polyethylenesa
LDPEb
Tensile Strength (MPa) 6.9 – 17.2
Modulus (MPa) 138 – 310
Elongation-to-break (%) 100 – 700
LLDPEb
14 – 21
137 – 186
200 – 1200
HDPEb
18.6 – 30.3
—
100 – 1000
UHMWPEb
19.9 – 41.4
110,000
300
LMDPEb
14 – 24
—
200 – 1200
"LLDPE"c
12
41
360
a
LDPE= low-density polyethylene (PE); LLDPE = linear low density PE; HDPE = high density PE; UHMWPE = ultra-high molecular weight PE; LMDPE = linear medium density PE. b Values from ref 6. c Values via Gilmer’s Laboratory Tensile Device. “LLPDE” is Brute Kitchen Bags™ – a blend of LLDPE (linear low-density PE); also contains TiO 2.
Table 3. Precision of Weight of a Set of "Constant" Weight Objects (Bolts) Number weighed 20 Range
22.24 – 22.40 g
Mean
22.32 g
Standard Deviation
0.046 g
% Standard Deviation
0.21%
Journal of Chemical Education • Vol. 73 No. 11 November 1996
In the Laboratory
Figure 6. Stress vs. strain of rubber band.
Figure 7. Stress vs. strain for elastic strap.
tensile strength of the rubber are much lower—1.52 MPa and 4.17 MPa, respectively. When PE and the rubber bands were run in duplicate tests, the results were very reproducible, although in some cases %E and tensile strength varied. This variation is normal in tensile testing, since failure (breakage) is so dependent on flaws in the test specimen. Samples with more flaws or larger flaws fail sooner. Thus, reproducibility is most dependent on sample uniformity and sample preparation. Since there are several grades of PE, we decided to compare mechanical data for our “Brute Kitchen Bags” PE generated by our apparatus to other values in the literature. As shown in Table 2, tensile properties for various grades of PE vary significantly between grades and show wide ranges in values even within a specific type or grade. In measuring tensile properties of a composite sample (in this case a bike strap or bungy cord), the stress-strain curve shows two separate response regions for the two components of the composite. Those two components are (i) the interior elastic rubber and (ii) the exterior fibrous woven cord. See Figure 7. As stress is applied the rubber matrix responds in an elastic manner, absorbing the stress while the outer woven cord simply expands to a strain of ca. 1.2. At this point the cord is fully stretched, and it starts to respond as a stiff fiber. Thus with additional stress there is little strain. This region has a high modulus. Two regions exist for the composite: (i) a low modulus region early when the rubber is carrying the load, and (ii) a high modulus region where the outer stretched cord is carrying the load. As the data above indicate, well-defined stress/strain curves are generated using this apparatus. Mention is made of using “constant”-weight objects, such as bolts, available at any hardware store. Table 3 shows the precision in weights of one set of bolts that we used. One can routinely expect a standard deviation of ca. 0.2%. Thus in adding weights consecutively as described in the procedure, we can assume all additions to provide equivalent weight. This makes these measurements quite straightforward and fast.
Acknowledgments The helpfulness of following individuals is gratefully acknowledged: Jean Strong at UWSP, Beth McGraw at RPI, and Janet Lewis at USM for secretarial support and oversight of preparations for the summer workshops; S. Czysz, Jason Waldkirch, S. Baird, T. Maeder, J. Opsteen, Russ Cartwright, Stephanie Fredrickson, Chris Petersen, C. Roberts, G. Tullos, student assistants at the three sites; and S. Krause (RPI), Paul Hladky (UWSP), and Members of the Advisory Panel for their helpfulness. This project was supported by National Science Foundation Division of Undergraduate Education Grants DUE #91-50497 and 92-54351 and by POLYED, the joint polymer education committee of the ACS Polymer Divisions. This experimental procedure was developed as part of the 1993 NSF-POLYED Scholars Program directed by John P. Droske of the University of Wisconsin–Stevens Point. The author was appointed an NSF-POLYED Scholar for 1993 and gratefully acknowledges the financial support of NSF in grants DUE #91-50497 and #92-54351. Most of the confirming data and refinement of the method are due to the efforts of Matthew Williams, the primary contributor to this work, and an undergraduate Engineering student at the University of MichiganDearborn. J.P. Droske and one of his students, Stephanie Fredrickson, assisted in generating some of the confirming stress–strain data. These experiments were prepared as part of the NSF-POLYED Scholars Program. This collaborative effort involves the following individuals working at three regional sites:
Conclusion
Literature Cited
A simple, inexpensive, and quite instructive apparatus has been designed to measure tensile properties. It should be applicable to undergraduate and graduate courses where polymers, principles of engineering materials, and conversions of units involving English to metric, force to energy, mass to pressure, etc., are of concern.
University of Wisconsin–Stevens Point: John P. Droske, UWSP, Project Director; Jerry P. Jasinski†, Keene State College, NH; Joe Young‡, Chicago State University. Rensselaer Polytechnic Institute: Gary Wnek, RPI Site Director; Karen S. Quaal†‡, Siena College, NY; ChangNing Wu †‡ , University of Massachusetts–Dartmouth. University of Southern Mississippi: Lon Mathias, USM Site Director; T. Carter Gilmer†, University of Michigan–Dearborn; Guy Mattson‡, University of Central Florida;E. Ann Nalley†‡ , Cameron University, OK. † NSF-POLYED Scholar, 1993; ‡NSF-POLYED Scholar, 1992.
1. Young, R. J.; Lovell, P. A. Introduction to Polymers; Chapman and Hall: London, 1991. 2. Sperling, L. H. Introduction to Physical Polymer Science; Wiley: New York, 1986. 3. Handbook of Plastic Materials and Technology; Rubin, I. I., Ed.; Wiley: New York, 1990; p 355. 4. Van Krevelen, D. W., Properties of Polymers; Elsevier: New York, 1972. 5. Rodriguez, R. J. Chem. Educ. 1990, 67, 784. 6. Rubin, I. I., Ed. Handbook of Plastic Materials and Technology, Wiley: New York, 1990.
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