F. Rodriguez
schoolof
Chemical Engineering Cornell University
lthaca, New York 14850
Lecture Demonstrations of Polymer Structure Using Polarized Light
O v e r the past few years, we have developed in the polymer courses a t Cornell a series of demonstrations. Some of these have been borrowed by us from such well-known "demonstrators" as H. Mark, F. Billmeyer, and F. Winslow. However, the experiments discussed here owe more to light microscopy techniques used a t Cornell by C. W. Mason.' I n fact, the transition from microscope to overhead projector is mainly one of finding suitable samples. The reason for using polarized light is that it makes changes in polymer orientation apparent in a rather dramatic fashion. Actually, the bright image seen when a sample is between a polarizer and analyzer indicates only that the material has an index of refraction that is not the same in all directions. However, in almost all the cases illustrated, we can identify the double refraction (birefringence) with molecular orientation. It is easy enough to see that light would have a different velocity traveling parallel to an extended molecule than perpendicular to it. I n many polymers, the axis parallel to the extended molecules has the lower velocity for light (higher refractive index). However, when the side chains are complex enough, as in polystyrene or poly(ethy1 acrylate), orienting the main chain gives an overall structure with higher velocity along the chains, or "negative hirefringence." It is also true that when the polymer is restrained from segmental Brownian motion, as in the glassy state or when highly cross-linked, that birefringence often appears on stressing as the result of hond bending and stretching with a consequent distortion of normal electronic distribution. One way of differentiating this type of stress-birefringence from the extended molecule version is that the latter usually coincides with large strains, while the former often is associated with small strains, say less than one percent elongation.
Figure 1. Typical overhead projector with polarizer and analyzer. ples are manipulated on or just above the polorizer sheet.
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Physical Arrangement
The overhead projector allows the manipulation of macroscopic samples in a way that can be viewed simultaneously by the entire class. For the usual projector with a 10 X 10 in. table, a plastic laminate type of polarizing sheet can be used as the p o l a r i ~ e r a, ~ ~ masking around the edges if necessary to insure against "light leaks" (Fig. 1). I t is a good idea to protect the top of the polarizer from scratches and solvents with a thin sheet of glass. Small spacers below it will slow down heating of the polarizing sheet by conduction. The analyzer can be small for most projectors, a 6 X 6 in. sheet usually being enough. It should be spaced from any hot surface, also. Orientation in Melts, Relaxation Time, Rubbers, and. Liquids
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(1) About gm of low molecular weight polyisohutylene' is squeezed beforehand between two glass or methacrylate sheets (Fig. 2). When the sample then is sheared by rotating the two sheets in the plane of the interfaces, a maltese cross appears L (the quadrants at 45' to the polariz& and analyzer become bright). If the shearing stress is released, the image disappears abruptly and a. slight strain recovery is noted. If the strain is held constant after shearing, the image disappeam after a few seconds. The explanation of the phenomena is thatshearing orients the Figure 2. Flow orientation and re- polymer molecules which then laxation in rhea.. become hirefringent. The uncoiling of the random configuration proceeds by rotations about single bonds in the polymer backbone, and not by hond bending or stretching. When the shearing stress is released, the elastic energy stored in the elongated molecules gives the strain recovery as the molecules return to a random orientation. At constant strain, normal thermal energy, "segmental Brownian motion," isenough to return the molecules to a random orientation, hut the process requires time. A quantitative measurement of hirefringence is possible and is proportional to the stress in a sample. It could he used to calculate 8. relaxation time, 8. This 8 would be the time for the stress to decrease to l / e of its original value. It is sufficient to note in the demonstration experiment that, at this molecular weight, the relaxation time is on the order of seconds. (2) A sample of raw rubber (onvulcanized) in the form of a. slip about 6 X X in. is stretched at 45' to the polmieer
' CHAMOT, E. M., AND MASON,C. W., "Handbook of Chemical Microscopy," Vol. I, Jonn Wiley & Sons, Inc., New York, 1958. Edmund Scientifio Co., Barrington, N. J. a Polaroid J-Film, Polaroid Corp., Cambridge, Mass. Vistanex LM-MH, Enjay Chemical Co., New York, N. Y.
Viscoelastic P r o ~ e r t i e sof Polvmer Melts Approx. Mal. Wt.
Material polyisobutylene butyl rubber cross-linked rubber
Relaxation Time
10'
seconds minutes infinite
lo6 infinite
axis. Relaxstion is evident in most common materials (butyl rubber, cis-polybutsdiene, cis-polyisoprene) in a few seconds by the changing birefringent patterns a t constant elongation. Often flow is evident and the samnle mav break or neck. In this samnle the relaxation time, qualitkiveli is on the order of minutes. ' (3) I t becomes apparent now thrtt relaxation time is a function of molecular weight. This is made very evident by stretching a cross-linked rubber which shows almost no change in pattern with time a t constant strain. A peroxide cross-linked cis-polyisclprerre or ?D in. thick is convrnient for tl1i.i rpcrirnelit. Fillpr.; and opsqur vulrnnl,ing ingredienrs rhould he xvreided. Thr r ~ w l l *are wnrnari~cdin the tnldr. From rhe-P dnrn the lecturer mav eo on to wneralize about viscositv versus molecular wpighr, di.cntmglrment, the probnhly efiert oi rernperaturr, nod rhc diitinrtion betu~ecuvisn,n.i and rlwtir rnntrrinl-.. .\n n~mloy ro rite i, the hlanrcll element and it, hehnvior i n rrecp (Fig. 3
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fringence most often used in photoelastic stress analysis. Crosslinked epoxy resins have been formulated especially for this purpo~e.~ (2) With one end of a cellulose acetate sheet (about 12 X I/* X 0.010 in.) held in a clamp, hest the center with a "heat gun"' and pull on the free end. Once above T. (- 120DC), the sample can be stretched and cooled in the stretched, hirefringent position. Heating above T. without stress will allow relaxation of most of the strain. The "frozen" orientation is typical of m y molding process where a polymer flows rapidly above T, and is cooled before the molecules can relax. Samples used for this demonstrstion should have a relatively high molecular weight to give rubbery behavior on heating. Very thin (0.005 in.) samples may weaken rapidly in the hot air stream. Thick samples (0.080 in.) may cool too slowly to be handled easily. Injection-molded protractors made of polystyrene are available in supermarkets and dime-stores. By looking at the hirefringent pattern "frozen" in during molding, one csn trace the injection point, flow path, and "knit" lines behind obstacles. A second protractor can he heated above T. to illustrate the distortion that occurs when relaxation takes place. In a separate demonstration, one can illustratevacuum forming, an important molding process that makes use of the transition through T.. A toy which is inexpensive compared to ordinary laboratory equipment is quite satisfactory." Orientation in Crystalline Materials, Spherulites, "Drawing"
1viscous
Ion? .
exp rment
ydashp7
/ Time
-loaded not loaded
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(1) Poly(ethy1ene oxide)n with a molecular weight about 4000 can give spectacularly large spherulites, 5 to 10 mm in diameter. A film can be squeezed out between two 3'/4 X 4 projector slide cover glasses a t 65-C to a thickness of about 0.002 in. Two strips of cellophane tape can he used to limit the thickness. Usually a few tries at crystdlillizing at 30-40DC may be needed to achieve good-sized spherulites. Besides seeing the maltese cross which indicates that all the molecules in the spherulite have the same orientation towards the nucleus, one can superimpose a first order red plate to show whether the radial or the tangential direction has the higher index of refracti~n.~I n poly(ethy1ene oxide) the tangential direction has the higher index. From the fact that a drawn film of the materid has a higher index in the direction of drawing which presumably is the direction of the molecule's long dimen-
Figure 3. Mexwell analog for virceelartisity in creep. An experiment which is fast compared to the flowing tendency of the experiment sees the materid as rtn elastic solid. A long experiment may not perceive the elastic recoil and see only the viscous flow of an apparent liquid. As a non-projected demonstration of viscoelasticity, a hall of silicone "silly putty'' can be used to illustrate the time-scale effect, also. Orientation in G l a s s e s , Stress-Birefringence, "Frozen" Flow Lines ( I ) Ilavingemphnsired ~hrimportnnreofscgmcntnl Rrorninn motion, one ran pmnt out that at .mmc rhnmrtcri-tic twnprrntwe depending on chain configurations, rotation about single bonds in the polymer backbone becomes difficdt. When it becomes so difficult that, in the time scale of an experiment, segmental motion does not t?ke place, energy is stored on stressing by band bending and stretching. At this glass transition temperature, T,,a great increase in stiffness (often X 108) occurs also. Methin. can be distorted as 8. aerylate sheet about 12 X X simple beam so that the outer half is in tension and the inner half is in oompressian. The small strsin is insufficient to orient molecules but does distort the bonding electron configuration to give a hirefringent pattern. This is the type of stress bire. ~
'"Stycast 1269-A," Emerson and Cumming, Ino., Canton, Mas. "'Heat Gun" Model HG751B, Master Appliance Corp., Racine, Wisc. ' "Vac-U-Form," Mattel, Inc., Hawthorne, Calif. "Carbowax 4000," Union Carbide Corp., New York, N. Y. 'See footnote 1, chap. 12.
Figure 4. Orientation of rpherulitic polymer agation in on oriented specimen.
by drawing and tear
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sion, we surmise that molecules in the spherulite have their long dimensions tangential to the nucleus. (2) Actual growth of spherulites on the projector table is possible, but it is hard to get 1mge ones in a reasonable length of time. About 0.5 g m of the poly(ethy1ene oxide) used in (1) can be melted in an uncovered Petri dish to give a layer of about 0.010 in. before the demonstration. After showing the bright image, a quick zap with the heat gun melts the polymer without heating the glass too much. Even so, the insertion of an extra piece of methacrylate sheet between dish and polarizer is recommended to protect the latter. The spherulites appear after a minute or so, usually in profusion so that the superficial appearance of bacterial growth is obtained. The thick layer of polymer becomes crystalline more slowly than a thin one which is good for purposes of demonstration. On the other hand, the maltese cross pattern is less obvious because of the thick spherulites. The experiment does demonstrate the uniform radical growth of
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spherulites from definite nuclei. (3) Polyethylene about 0.004 in. thick can heused to illustrste the changes in structure that occur in a. spherulitio material on elongating or "drawing." The general purpose, branched polymer is suitable. Most commercial film has some initial oorientation. Strips, or, preferably, dumbbells, are cut in the machine direction and across the roll. Often one will neck and the other will not, showing that growth a t a neck is associated with a localized preorientation. When the strip or dumbbell is strained only a few percent, the hirefringent pattern that appears is somewhat reversible. However, large strains cause an unfolding from the spherulite form to an oriented crystdlite with molecules extended in the direction of stress. The oriented-crystdlite structure is typical of natural and synthetic fibers. The resemhlance can be emphasi~edby making a cut with a razor a t one end of the drawn strip, then tearing down the length of the strip, illustrating the directional variation in strength (Fig. 4).
