Abstract Six or seven common polymers (four or five "unknown" and two "known") are characterized by their visual appearance, some of their thermal properties, and their flame characteristics. One goal

of the lab is to attempt to identify the unknown polymers. A more important goal is to compare the properties of the polymers to one another and to the expected behavior of polymers in general. Background This lab will introduce the behavior of polymers through the use of several simple lab techniques - ones that require relatively little time and sophisticated equipment and only small amounts of material. (As an aside, these kinds of investigations are useful for the scientist who has just synthesized a new polymer and wishes to quickly learn about it, or for the scientist or engineer who is faced with quickly identifying an unknown polymer. For example, if your failure analysis involves a plastic component, you might wish to use some of the techniques from this lab to help identify it.) 1- Visual Examination It is possible to learn quite a bit about an unknown polymer sample just by looking at it. For example, an optically clear polymer sample is almost certainly highly amorphous (that is, it possesses a low degree of crystallinity). This does NOT mean that this particular polymer cannot be processed so that is it highly crystalline, however. Polymers are tricky that way. A translucent sample, by contrast, is likely to be highly crystalline. If the polymer has been filled or colored, of course, that will complicate the visual examination. For this lab, it is likely that none of the polymers we will investigate has been filled (blended with a powdered substance like wood flour, glass fibers, mica, or calcium carbonate) or colored (blended with a pigment or dye). If one or more of this year's polymers has been filled or colored, you will be so informed in lab. 2- Thermal Characteristics The behavior of polymers as they are heated and cooled is critical both to their performance in service and to their ability to be manufactured into shapes. This thermal behavior is also typically quite different from other engineering materials (metals and ceramics). This is due to the macro molecular nature of polymers. Thermoplastics versus thermosets One common way to categorize polymers is according to whether they may be reversibly softened upon heating. Most of you have probably seen what happens when you get a plastic spoon too close to the heating element on an electric range! The spoon softens and flows - evidence that it is made from a thermoplastic polymer. Thermoplastic polymers are composed of long molecular chains. The bonds within the chain are strong covalent ones. The bonds among chains, however, are weak secondary ones. These weak secondary bonds allow the material to show thermoplastic behavior. Most common everyday "plastic" items are made from thermoplastic polymers. Food containers of HDPE, PP, PET, or PS, for example, are all thermoplastic. A thermoset polymer, on the other hand, cannot be reversibly softened by heating. At the molecular level, a thermoset also consists of long, covalently bonded chains. However, in a thermoset, the bonds among chains are also covalent ones. A thermoset is said to have a three-dimensional network of strong covalent bonds. Thus, when a thermoset is heated, the chains never gain enough energy to slide past one another the way they do in thermoplastics. Eventually, if you heat a thermoset hot enough, the network of covalent bonds will be destroyed, and the material with change color, char, give off smoke ,etc. Examples of thermosets are epoxies (used as adhesives and as the "matrix" for advanced composite materials like carbon fiber golf clubs or airplane parts) and phenolics (used for the handles on coffeepots, etc.). The Glass Transition All commercial polymers are at least partially amorphous (non-crystalline). Some polymers, like polystyrene and polycarbonate, are virtually 100% amorphous. Other polymers, like high-density polyethylene, are highly (but never 100%) crystalline. They are thus a blend of both crystalline and amorphous material. The amorphous fraction of a polymer (or any other amorphous material, like plate glass) undergoes a completely reversible phenomenon known as the glass transition. This is a bit of a slippery concept - but a very real one. The glass transition is usually taken to occur at a single temperature, known as Tg, the glass transition temperature. As a polymer is heated through the glass transition temperature, "large segments" of the polymer chains in the amorphous fraction of the material gain the ability, through thermal energy, to cooperatively rotate. A "large segment" might consist of 10-15 repeat units of a single chain. Below Tg, such large segments of chains are sort of locked in place, while above Tg, these chain segments can rotate and wiggle around quite a bit. DSC If all of this sounds really vague and nebulous, consider that Tg can be observed physically many different ways. Some of them require specialized equipment, but some do not. One specialized technique for determining Tg is called DSC, or Differential Scanning Calorimetry. DSC scans of some of this year's polymers will be provided to you as part of this lab. If you are careful, you should be able too observe and record Tg in this experiment for several of the polymer samples in the thermal observations(or "cooking") part of the lab, and then compare your results to the DSC results you will be given. Briefly, DSC measures the heat flow (in watts per gram, positive or negative) that is necessary to heat up a small sample of polymer at a fixed rate - say 5°C/min. When the polymer reaches a temperature (such as Tg or Tm) at which its heat capacity changes, the heat flow vs. temperature curve will change slope ,and/or show a positive or negative peak. These points on the DSC curve will sometimes (but not always!) correlate reasonably well to the visual observations you make for the same polymer during the" cooking" portion of this lab .In lab, you will be provided with DSC curves (measured at TU on the Chemistry department's DSC instrument) for some of the polymers tested in lab this year. We will discuss these DSC results in lab. Below Tg, an amorphous polymer is said to show "glass-like" behavior. A striking example of this is observed in elastomeric polymers (rubbers). If you take a rubber hose and chill it in liquid nitrogen, it will shatter like glass when you slap it against a tabletop. What you have done to the rubber is cool it below its Tg. Above Tg (at room temperature), elastomers are in the rubbery state - thus they can be stretched - because at that temperature, the molecular chains have the ability to cooperatively rotate -since they are above Tg. Not all amorphous polymers show this type of dramatic transition in behavior above and below Tg .Amorphous polymers like polystyrene, which is NOT an elastomer, are sometimes, but not always, brittle below Tg. Above Tg, these polymer lose almost all their useful mechanical properties - the "Tg effect" is quite dramatic. Polycarbonate, also a fully amorphous thermoplastic, has a Tg of 150°C. At room temperature (and way below it), polycarbonate is well below Tg, but it is also quite tough (for reasons that we won't go into here). Thus, just because a polymer is below Tg does not mean that it is" as brittle as glass. "Highly crystalline polymers like HDPE show much different behavior with respect to the glass transition. HDPE has a Tg that is well below 0°C. But remember that the glass transition only influences the amorphous portion Tg, HDPE, unlike the amorphous thermoplastics, retains very useful properties. HDPE is in fact one of the polymer. Thus, at room temperature and above, even though it is well above the most successful synthetic polymers in the world.