Copyright © 1998-2005 by Pichai Rusmee
This lab will expose students to polymeric type materials. This includes a brief introduction to polymer morphology. In addition, they will be made aware of synergistic effects that can arise when a material is subjected to more than one kind of action, environment, etc. The effect of synergism will be demonstrated using the polymer subjected to a load and a corrosive environment.
What is Polymer?
Polymer is a synthetic or natural material consisting of molecules stringing together to form a long chain. The properties of a given polymer depends on the basic molecules (a "mer" unit, repeating unit) making up the polymer, the length of the chain, and the arrangement of the polymeric chains. Polyethlylene is the most simple commodity polymer available. It's repeathing unit consists of two carbons backbone each with two hydrogen atoms attached to it. The number of repeating units strings together determine whether this polymer is a gas, liquid, or solid. In solid form, the polymeric chains may pack neatly or in random fashion.
There are three basic type of polymer packing: chain, branch, and network as shown in Figure 1.1. A chain type polymer is a polymer consisting primarily of long chains of polymer without any irregularity in the chain. When a polymer chain exhibits some irregularity, i.e., some side atoms or molecules are replaced with chains of the same plymer, then it is said that this particular polymer has a branch structure. If the side branches joined up with other polymer chains, they are forming a network, hence network type structure. Sometimes, polymers chain can arrange themselves in regular, close-pack manner forming polymer crystals. Usually, only chain type polymers is capable of forming polymer crystal lattices.
Homopolymer is a polymer consist of one and only one type of monomer unit. Copolymer incorporates two or more type of monomers into its chains. Polymer blend is a polymer resulting from physically blending different kind of polymers without incorporating the different kind of polymers into the chain. In a given chain, the polymer could be either homopolymer or copolymer of some kind.
Class of polymer
There are roughly three classifications of polymer:
Thermoset plastics are usually the network type of polymer where all the polymer chains are tightly knit together. The common example of this type of polymer is the epoxy type adhesive. Epoxy adhesive is not a true glue that is water or solvent base and harden by the evaporation of solvent. Epoxy hardens by polymerization. Uncured epoxy usually consists of uncured polymer or partially cured polymer. Once the polymer is exposed to the condition "favorable" to polymerization, it will start crosslinking and formed a crosslinked network that is directly related to the strength of the adhesive. Depending on the type of adhesive, favorable condition may include air, heat, catalyst, initiator, moisture, etc.
Elastomer plastics are usually the chain, branch, or even network type. The polymer chains are very loosely intertwined. As a result, the elastomers are pliable and stretchy. Another name for elastomer is rubber. An example of this type of polymer would be silicon rubber.
Thermoplastics are usually the polymers of the chain, branch, or loosely networked type with some capable of forming crystal structure. These polymers are what commonly called plastics. They are usually meltable and formable through heating. There are different classes of thermoplastics such as polyolefins and polyamides. Some of them are: polyethlene, polypropylene, polytetrafluoroethylene (Teflon®), polycarbonate, etc.
For an abbreviate list of common polymers with their letter designations see Standardized polymer abbreviations.
Phase and properties
Polymers are highly rate and temperature dependent (Figure 1.2). They are mainly useful in the glassy region when they are solid. Passing the transition point, the solid polymer will turn rubbery and, depending on the type, will either burn or melt. This is the general trend for the amorphous type polymers. For the crystalline type polymers the transition temperature is not so obvious since they would have a crystalline structure instead of glassy structure.
One of the effect being studied in the laboratory is the effect of synergism. That is when two or more agents acting together having a larger effect than each one of them acting separately. The effect may be positively reinforced or negatively reinforced. This laboratory will demonstrate the negatively reinforced synergism effect where the material degrades much more severely than each agent acting separately.
There are three sets of data that will be presented. The first set will be the failure load vs. preload time for some fiber specimens. The second set will be the failure load vs. the time exposed to corrosive agent. The third will be the failure load vs. the preload concurrently to exposure time.
This is a dry lab due to hazardous nature of the experiment. The students only have to plot the data given in the handout and discuss them.
See Miscellaneous for comments on the graph.
Materials are continually changing, both on a molecular and macroscopic scale. Molecularly, atomic structures are inherently unstable and are continually changing to a more stable state; however, for many materials this rate is a rather slow process. Molecular changes can also occur as a result of "other" energies subjected onto (surface or boundary) and in (volume) the material properties.
Macroscopic changes can occur as a result of energy input to the material system. This can result in morphological changes that can drastically alter material behavior. For example, ultra-violet and/or gamma irradiation of polyethylene changes the macroscopic and molecular structure, such that the ultimate strength is increased and the fracture toughness is decreased. There also comes a point after a "great deal" of radiation that the polyethylene has so many cracks in it that it is rendered useless. To help us become more familiar with materials other than metals, a general overview of polymer morphology will be discussed in the lecture.
Unlike all the other labs in this course, this lab will not be a "hands-on" lab due to the dangerous nature of the experiment and laboratory time constraints. The experiment will be explained in the lecture and if opportunity arises an experiment in progress will be shown. This experiment investigates the effect of different environments on Nylon 66 fibers. Actual data obtained from the Nylon 66 degradation studies are provided for use in the report write-up. The data can be interpreted as follows:
Your introduction should include some background on material degradation and how it applies to different materials.
Polymethyl Methacrylate (Acrylic)
Drinking tumblers; faucet knobs; camera, projection, and viewer lenses; signal light devices, nameplates, automotive instrument panels, aircraft windows
SAN, PC, PS, clear ABS
Impact modified grades: Acrylic hard phase and an acrylic modifier (all acrylic). Impact modified grades: Clear styrene-butadiene modified (not as free from haze). Impact Strength: 1-2 ft . lbf/in.
Absorb water, surface left in tension after drying, leads to crazing.
Sheet PMMA is manufactured by placing initiated monomer between polished glass surfaces and allowing it to polymerize. Pellets are also produced for molding and extrusion fabrication.
(Ref. Engineered Materials Handbook: Engineering Plastics, 1988)
Not all type of graphs is appropriate for the presentation of the given data. The data contain multiple values of load for the same exposure time. A category plot where each time data is taken to be a separate name category is inappropriate. The data should be plotted as a scatter plot.
Connecting each data point together with lines is also inappropriate. Doing so implies that the order that the data is presented is significant which is not the case. Use distinct symbols for group of data in the graph instead.
Plotting a simple average of the data hides statistical trend within a time set and may skew the overall trend of the data. Use curvefitting line to indicate trends in the data. Synergism means that the sum is greater than its parts taken separately. This must be shown in the graph and clearly stated in the report.
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