Molecular Motion in Polymers Mechanical Behavior of Polymers near the Glass-Rubber Transition Temperature L. H. Sperling Materials Research Center #32,Lehigh University, Bethlehem, PA 18015
Polymers serve society in many ways: as protective coatings, elastomers, plastics, fibers, and adhesives. While permeability, clarity, chemical stability, etc. play greater or lesser roles in determining the use of a given polymer, the mechanical hehavior of . ~ o l.v m e r sis the one outstanding feature that distinguishes polymers from low molecular weight materials. Whether a polymer is stiff (or glassy) or soft (and either elastomeric or a viscous liquid) depends upon whether or not polvmeric chains are capable of lone ranee. the . . " . coordinated molecular motion. When the temperature is low, motion is restricted largely to vibrational modes, and a glassy type plastic results. (This experiment presumes the absence of crystallinity which is important, particularly in fiber forming polymers such as nylon.) A typical glassy polymer would he poly(methy1 methacrylate), sold under the tradenames of Plexiglas", Lucite," and others. If the chains are capahle of rotational motwns, rspccinllv long-ranye c~x~rdinated motions inr(,l\,ing 10-50 carlxm atoms, the povmer ~ s s o f tThe . temperatureat which the onset of coordinated segmental motion begins is called the glass-rubber transition temperature, T, Above this temperature, the chiun achieve-. murh greater conft~rmnthmnl irt:t:(lom. Natural rubber at r m m temperature is a guod examole of a material above its T. At T,, the polymer acquires the necessary energy to undergo long range molecular motion and softens because the chains are capable of rapid response to mechanical deformation. At this temperature imuosed mechanical forces are most easilv converted to rnolecuiar motion, and thus heat. Often referred to as dynamic mechanical spectrosco~v. ... there is a direct analogy to electromagnetic spectroscopy: when conditions are right (frequency and/or temperature) the imposed energy is absorbed. Polymers near T, are often used in noise and vibration damping applications. In the present experiment or demonstration, we will examine the response of a typical polymer to temperature variations above and below Tg.
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1 yardstick (or meter stick) 1 clock (a watch is fine) 1 hard surface, suitable for ball bouncing (most floors or desks are suitable)
First, place the yardstick vertical to the surface, and drop hall from the top height, record percent recovery (bounce).Second, drop ball into liquid nitrogen,allow 2-5 min for complete coaling (rapid boiling ceases). Remove ball, and record percent bounce immediately, and each succeeding minute (or more often)for the first 15 min, then after 20 and 25 min and each 5 min thereafter until recovery equals that first obtained. For the hollow rubber ball. first observe touehness at room temperature. lhen cool in liquid nitrogrn, then thmu, hard ngmnar the 11wwor wall. Ohsrrve [he glassy behavior of the plerw, and their he. havior as they warm up.
3
5
1 Temperature
Figure 1. Storage modulus, E', and loss modulus E" of polymers as a function of temperature. E = 3RT line indicates crosslinked rubber behavior.
Experiment Time: Actual laboratory time about 1hour Leuel: Phvsical Chemistrv principle; Illustrated: " 1. The onset of molecular motion in polymers. 2. The influence of molecular motion on mechanical hehavior. Equipment a n d Supplies: 1 solid rubber ball (a small Superballa is exce1lent)l 1 hollow rubber ball (optional) 1 Dewar flask of liquid nitrogen2,large enough to hold above rubber halls 1 lade or s l r m wrrh long handle. 10remwc f n w n hnlk rhlwrnatelv, t i t balls wxh lhng string.,
' Most toy balls wlll work flne The "Superball" s compressed durmg
crossllnklng to glve Improve0 bounce character:sf~cs. Caution: Llquld nnlrogen s dangerous because 11can freeze skm rapidly. Do not touch!
942
Journal of Chemical Education
"Time Temperature
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Figure 2. Bouncing characteristics of Cold Superballs."
Report the data obtained, plus the best explanations possible in terms of molecular behavior. Did the solid rubber ball behave as an oversized marble when cold? Where did the energy go when bounce was reduced to below lo%? Why did the hollow ball shatter? For extra credit, obtain three small dinner bells. Coat twoof them with any latex paint. (Most latex paints have T,near room temperature.) After drying, ring the bells. Plaeeone coated bell inafreezer, and compare its behavior cold to the room temperature coated hell. How can you explain the difference observed?
On warming the frozen solid rubber hall, the percent recovery (bounce) versus time will go through a minimum a t T,, as shown in Figure 2. Below T,,the hall is glassy, and bounces much like a marble. At Tgrthe bounce is at a minimum, due to conversion of kinetic energy to heat, (The hall actually warms up slightly,) r and bounce characteristics Above Tg,normal ~ h b e elasticity are observed.
Expected Results
Further Reading
The storage modulus, E,, is a measure of stiffness, and is closely related to Young's modulus. The loss modulus, E", is a measure of energy converted to heat during the experiment. Plotted against temperature, typical values are shown in Figure 1.
I l l Balmeyer, F. W.. "Textbook of Polymer Science." 2nd Ed., Wiley-lnterscience. New York, 1971.
(2) uodrique.,F.," P ~01 polymer ~ ~ systems? ~ ~ M CI C ~ ~~W - H ~N*W ~ I I ,Y W ~ 1982. , (31 seymour, R. B and camher.dr..c.E.." P ~chemistry: I ~ A" ~ ~~~ t ~ o d ~Marcel eti~~,"
Dekker. New Yurk. 1981. (4, A I I ~ ~ ~ ~ , H . R . ~ ~ ~ L ~ ~ ~ ~ . F . W . . ~ ~ C C C ~ ~ ~ ~ P O P O ~ ~ P P E ~ ~ I ~ WC O I ~ O~ F~S , N1981. J,
Volume 59
Number 11
November 1982
943
