Classroom Demonstrations of Polymer Principles Part I. Molecular Structure and Molecular Mass F. Rodriguez School of Chemical Engineering. Olin Hall. Cornell Unlverslty. Ithaca. NY 14853 L. J. Mathias Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406 J. Kroschwitz John Wiley 8 Sons, Inc., 605 3rd Ave., New York, NY 10158 C. E. Canaher, Jr. Florida Atlantic University, Boca Raton, FL 33431 The principles of polymer chemistry and physics are often taught in lecture format. However, most of us would like to see the subiect come alive. to see before our very eyes the faxinatingoand intimate details of polymer str"ct"re and behavior. This is the role of the classroom demonstration whether it is conducted by the lecturer or undertaken by the student. Most of us love to have a magician explain his tricks, even if we cannot emulate him. Similarly, students want to know the whys and wherefores of the behavior of the polymers that makr up a large part of our environment. A cvmpendium of p o l p e r experiments, demonstrations, and rrvicw arricles that have heen ~ublish6.din this J ~ ~ u r n u l appeared in 1983 (1).Another collekion forms a chapter in a polymer samrecent book on demonstrations (2).Sources of . ples also have been tabulated (3). A consideration of molecular size is essential to understanding the behavior of large molecules. It is, in fact, the key to most of the physical properties that make polymers useful. Dimensional rigidity or elasticity, strength, and durability that we associate with plastics, fibers, rubber, coatings, and adhesives all depend on the interactions of molecules that are orders of magnitude larger than familiar ones such as ethanol, benzene, or acetic acid. Of course, polymers themselves are even more familiar in some cases. Garbage bags and other packaging materials of polyethylene, home siding of poly(viny1 chloride), and foamed polystyrene cups for hot drinks are encountered daily. What is more, the technical, generic names are also well known as the trade names for many of these items. 'The term polymer implies the rrpetition o i a single unit, the mer or monomer. This is the iimplest case tc~use as an example in the classroom. More than one unit may be involved as in the case of copolymers and terpolymers. The term macromolecule can he used for any large molecule including those which are composed of complex subunits and those with branches and cross-links. ~
Size of Macromolecules There is no universal agreement on what constitutes "high-molecular-weipht" materials. Most certainly polyethylene with a molecular weight greater than 2000 and the highly cross-linked network typical of phenolic resins qualify. While the existence of high-molecular-weight polymers is most easily illustrated with linear polymers (as in the first several examples to follow), branched and network polymers should be included early in the student's experience in order to complete the picture. Pop-It Bead Models An effective model to illustrate the qualitative difference in molecular weight between familiar organic molecules and 72
Journal of Chemical Education
Figure 1.When a string of several thousand pap-it beads is carefully fed into an ordinaiy plastic pail, the entirestring will ''siphon" out after an end with about a hundred beads is started over the side. polymers is t h e pop-it bead used in costume jewelry or in children's toys. When each bead is used to represent a carbon atom (without the hydrogens) in a polyethylene chain, assemblies of one to eight heads can represent the alkane series from methane to octane, all of which have everyday use (natural gas, LPG, lighter fluids, gasoline, etc.). A polymer "string" of 1000 to 4000 beads is a striking contrast. If the string is "fed" into a beaker or bucket with a rounded lip, it can be made to siphon out by hanging one end over the side and holding the container well above the floor or table. The sight and sound of the heads siphoning out and the time it takes for all the beads to appear invariably rivet the attention of the audience. A string of 4000 heads, each 10 mm in diameter, fits into a 10-quart pail (Fig. 1).The molecular weight i t represents is realistic (about 56,000 if each bead represents CH2) compared to the average molecular weight of the polyethylene from which the beads (and perhaps even the bucket) are made. Thread and Wire Models A length of thread or flexible wire also can convey the high length-to-diameter ratio that is typical of linear polymers. Because of the high rigidity compared to a jointed model such as the beads, even the thread does not entangle with itself as much as the pop-it bead model. On the other hand, some rigid molecules may be very well represented by wire models. The triple helix of the protein collagen is anexample of an assembly that retains the rigid dimensions of a rod even in aaueous. mildlv acidic solutions. Collaeen is a maior e hAix constituent protein of skin and tendons (4). ~ g triple has three molecules, each with a molecular weight of about
Figure 2. A collagen model showing the three linear molecules intertwined in a helix with "loose" pations at chain ends.
100,000 in rigid assembly that is about 280 nm long and 1.5 nm in diameter. A piece of 22-gauge copper wire, often used in laboratories for fastening rubber hoses on condensers, has a diameter of 0.65 mm and need he only 120 mm long to simulate the collagen assembly. Three strands, each 260 mm long, can he twisted together in a right-handed helix for an even more graphic picture (Fig. 2). The actual triple helix has a pitch of 10.4 nm (6.9 diameters).
Figure 3. Pop-It bead models. (a) Cmsr-section of bifunctional beads, (b) hifunctional bead anached by straight pin (bent over and cut off), and (c) tetrafunctional bead made by drilling undersized hale through one bead. Ti% attaching beads have been truncated to allow room inside the drilled bead.
Spherical Models
Although less dramatic than the length of extended molecules, the space-occupying capacity of polymers is a useful concept. A simple calculation using the known density and molecular weight yields a molecular volume. For example, the diameter, D, of a sphere of polystyrene of one million molecular weight and density of 1.05 g/mL (with, of course, Avogadro's number of molecules/mol) is:
Figure 4. A short section of model showing the snucture of a typical graft copolymer.
This is large enough to he seen in the electron microscope ouite clearlv. Let the stvreue monomer (with a molecular weight of 164, density o: 0.91 g/mL3) he'represented by a marble or bead which is 15 mm in diameter. Then a ~ o l v m e r molecule with a molecular weight of one million (ddnsity of 1.05) has about 8.700 times the volume of the monomer corresponding to about 20 times the diameter of the monomer and can he represented by a soccer hall or a basketball. Copolymers When a macromolecule is made from two monomers, the resulting copolymer can he put in one of four categories: the random conolvmer in which monomer units are added se" quentially to a growing chain according to certain rules of probability, the perfectly alternating copolymer, the block copolymer, which contains long sequences of each monomer, and the craft copolvmer. which results when blocks of one monomer are added as branches to a preformed chain of a second monomer. Linear copolymers can he simulated using pop-it heads of two colors. The random copolymer can he constructed in class by adding heads a t random. Addition of heads can be based on coin flipping where heads adds a white head and tails a red head. The block cooolvmer . " corres~ondineto stvrene-hutadiene thermoplastic elastomer is best simulated usine each bead to represent a seauence of. sav, 20 chain atoms (10 styrenes o; five butadiknes). ~ h e n styrenea hutadiene-styrene with blocks of molecular weight equal to 15,000 for styrene and 30,000 for butadiene is built by joining two hlocks of 15 red heads onto either end of a central hlockof 100white heads. The point can he made that despite -the seeming preponderance of white (hutadiene) heads, the
.
higher molecular weight per chain atom for styrene gives approximately equal weights of the two monomers in the copolymer. Graft copolymers and other branched structures require a trifunctional unit. This can he made from the bifunctional pop-it head by removing the hall from one head and attaching the head to a second, intact head (Figs. 3,4). The pinning method is preferred since most heads are made of polyethylene for which there is no satisfactory adhesive. Examples of graft copolymers include those such as hydroxyethylcellulose in which the branches are short with one to five monomer units predominating and those such as acrylic acid erafted on . oolv(ethvlene t e r e ~ h t h a l a t e )in which the -. . granches are much larger. ~ ~ d r o ~ ~ e t h ~ l c e l is l umade l o s eby the rine-scission ~olvmerizationof ethvlene oxide in the of cellul&e -and makes the pol&er water-soluble. The acrylic graft is made by exposing a polyester fahric to a low-pressure electric discharge plasma, which generates free radicals on the polymer surface. A few seconds of exposure to acrylic acid vapor a t a low pressure (5 torr) adds about 1%to the weight of the fahric as graft copolymer. Molecular Mass Accurate measurement of molecular weiehts in dilute solutions demands the observation of smalreffects with extrapolation to infinite dilution. For Durvoses of demonstration, however, the effect of a d i s s o i ~ e d ~ ~ o l y m one rthe osmotic pressure, the light scattering, and the viscosity of comm& solvents can be exaggerated to show the principle behind the method without carrying out a real determinaVolume 64
Number 1 January 1987
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Glass tubing, L =5 O c m ID=4mm
Height
11 c o l o r e d sol'n
cm in tube of
I D=4mm 10
Dialysis t ubing between two rubber stoppers Water bath
-
0
40
20
0 Time,
minutes
Figure 7. Solution height in narrow-bare glass tube. Sucrose.0.58 M in an 8cm length of dialysis tubing (1.5 cm diameter).
Figure 5. Demonstration of osmosis
Figure 6. Sucrose. Cl2H2,O,,, formula weight = 342.
tion of molecular size. For hieh-molecular-weieht . . oolvmers . . thr two most ~ O ~ I I I RaI h ~ ~ lmethods ~ ~ t v are typified in the firit twoexamvlri. Other cdirutirt. urouerties suchas melt. . ing-point depression, vapor-pressure lowering, or hoilingpoint elevation are used, hut less frequently. The changes in these physical properties are small and hard to detect accurately for high-molecular-weight materials. For low-molecular-weight polymers, a counting of the end groups present by titration or by infrared spectroscopy represents another approach. Undoubtedly the most often employed relative measure of molecular weight is dilute solution viscosity, althouehmelt viscositv is the standard method for most nolvolefins (the "melt index") and column chromatograph; (lCgel permeation chromatography") is widely used. Osmotic Pressure
For most students, semipermeable membranes a t first seem to be on a par with Maxwell's demons who can open and close valves on a molecular scale. We prefer to describe the process as a kind of filtration. The permeability is high for small molecules and decreases to the vanishing- noint . with increasing solute size. Equilibrium is not a convenient phenomenon to illustrate easilv. In this settine we describe it a> the rapid permeation thn,uyh n mrmhrnnr bv sdvent tu dilute the concentrated soluti~mon the uther side. The demonstration is carried out using dialysis tubing (regenerated cellulose) with an inflated diameter of about 1.5-cm stoppered at both ends with No. 0 rubber stoppers 74
Journal of Chemical Education
(Fig. 5). To assemble the apparatus, a suitable length of dry tuhing, say 20 cm, is dipped in water. I t becomes limp and pliable and can be slipped over the stoppers and fixed in place with rubber bands. A length of glass tubing is fixed through the upper stopper. A solution oisucrose(tahle sugar, Fig. 6) in water with a concentration of 200 g/L (0.58 M) is suitable. Spoilage of the sugar solution can he retarded by addition of a few grams of phenol (5) per liter of solution. Under these conditions the solution can be stored for some time. A few drops of food coloring should he added to give a clearly visible solution in the dass tube. T; set up the demonstration, a funnel is attached to the top of the glass tube to facilitate addition of the sugar solution. If the dialysis tubing has dried out since it was assembled, it will become pliable again as soon as i t is rewetted. I t is strong even when wet. l t c a n be squeezed periodically during the filling to push out air bubbles. Finally, with all air removed, the level is adjusted to about 2 cm above the top stopper and the funnel is removed. The dialysis "hag" hangs from the glass tube, which is supported on a ring stand. I t can be stored for a short while before class in a beaker (or cvlinder of the suear . graduated " " solution). T o carry out the demonstration, the beaker with the sugar solution is removed, a second beaker with water is put in its place to wash off the outside of the tubing, and finally, a cvlinder of water is put in place so that the surface of the water is at the same level as the sugar solution inside the glass tube. Flow through the membrane takes place rapidly. A typical height-time plot (Fig. 7 ) shows a slight decrease in rate after 15 or 20 min probably due to polarization (a slightly lower sugar concentration a t the inside membrane surface than in the bulk of the sugar solution). Actually, common dialysis tubing is permeable to molecular weights up to several thousand so eventually the height would go through a peak and come dcwn again as the sugar diaiyeduut of the tubeand into thecylinder. T)idy4is itself. while not rht: point of this drmunitrarion, is n valuable means of separating small molecules and ions from large ones. If sugar and a polymer of molecular weight 100,000 were placed inside a dialysis tube and periodically placed in
fresh water, the sugar would diffuse out of the tube andleave only the polymer inside. The time required might he as long as several days, however. After class period, the apparatus can be emptied, flushed several times with water, and allowed to dry out. With some care, the tuhing is stable on dry storage, and it can be used again without being taken apart and reassembled. Light Scanering There are several features of molecular-weight measnrement by light scattering that can he illnstrated~ualitatively in the classroom. Usually the apparatus for laboratory characterization is rather expensive and calls for certain accessories for dust and gel removal and for measuring the change in refractive index with concentration. ~owever;theprinciples can be seen without these refinements. I t is convenient to show, in a preliminary display, that the refractive index of poly(methy1 methacrylate), 1.490, is almost the same as that of toluene, 1.497, and quite different from that of acetone, 1.359. Both are good solvents for the oolvmer a t room temoeratnre. A cast rod or some other transparent poly(methy1 methacrylate) object with smooth surfaces is diooed in each solvent in a separate beaker (or in Petri dishes bn the overhead projectorj. The rod seems to disappear in the toluene but is sharply outlined in the acetone since light is being refracted at the interface in the latter system and not in the former system. Cast materials are especially good for this illustration because their high molecular weight keeps them from dissolving appreciably during a quick dip. The acrylic sheet sold as glazing or rods or tubes used in model shops usually have molecular weights of about a million or more. For making up polymer solutions, an injection molding grade of polyfmethyl methacrylate) dissolves rapidly. For example, a sample' with molecular weight of 2 X lo5 dissolves on stirring in either solvent in less than an hour a t room temperature (1% solution). The solutions can be filtered rather easily through ordinary paper filters although finer filters can be used to remove all dust particles. The filtered solutions can be stored for long periods of time in stoonered bottles. see the light-scattering effect, asimple flashlight witha cardboard mask can be used as in Figure 8. The room must be quirr diirk for the effect to be seen. It is striking that the lirht that goes throurh the tolucme solution with onlva faint shadow appears muih brighter when it gets to theacetone solution. Since the concentration is the same, the import Molding powders are manufactured by Rohm & Haas and Du Pont. However, the cheapest source usually is a discarded, clear automobile back-up light lens. One typical lens (1973 Ford) has a M, =
tance of having a difference in refractive index becomes aonarent even thoueh " the nolvmer molecules are in true solution. Of course, almost any latex will scatter light very effectivelv even at hieh - .(0.01%)dilutions. However.. the ~. o .l v mer is not in solution in that case hut suspended as small (0.01-10 um diameter) ~articles. noth her way to showthe scattering is by focusing a beam of light from a proiector on the container. The higher intensitybf scattering in the forward direction is easily seen by having the observer walk around the container in order to see it from various angles. A third method suitable with a large group uses the overhead projector that has been masked except for a rotatable cardboard slot (Fig. 9). The shaft of light passing upward throueh the container is verv visible in a dark room. even from some distance. The attention must be directed to the container in this case. since the oroiected image does not . . indicate any unusual features.
..
. -
Dilute Solution Viscosity Although the following example can be run entirely as a demonstration, it is described here in terms of a demonstration followed by a student experiment. The student's part can he done in ahout one-half hour in the laboratory. This procedure has been used successfully in a minicourse for enginering freshmen. The usual problems with intrinsic viscosity measurement are temperature control, dust particles in the capillary, and the fragile nature of glass viscometers, especially during cleaning. These problems are circumvented to a large degree by using rather concentrated solutions. The flow rate is measured for water and for several concentrations of poly(vinyl alcohol) at room temperature. The apparatus consists of beakers, capillary tuhing, graduated cylinders, meter sticks, and a stop watch (Fig. 10 and the table). The viscosity 7 , of water is measured in a lecture demonstration with student participation. I t serves to familiarize the students with the apparatus they will use in the next period and it gives them some idea of the precision of the measurement. Using the apparatus described, a reasonable flow volume is 50 mL. Several other points can he made in the lecture using the same apparatus. If two duplicate setuos are used. the measurement of time for comparative purposes can bedispensed with. In one example, the slower flow of ice water compared to room-temperature water results in a smaller collected volume when both flows are started at the same time. In another example, the students can he polled on whether the viscosity of isopropyl alcohol is higher or lower than that of water. Since they are familiar with the volatility of rubbing alcohol, most will usually guess incorrectly. They are invari-
225,000.
I
4%
1%
1%
~
~solution ~ .
"
(torch)
center slil
(in a c e t o n e ) ( i n t o l u e n e ) Fog.% 8 Llgnl scanerlng by solulons 01 polylmelhyl rnelhacrylatel A darkened toom ir essentsa Lognt from flashllgnlPasses almost undetected lnrough a IolLene sol~tmnoul s seen in acetone solutons oecause of scmtered ljgnt
Figure 9. A beaker d polymer solution can be placed directly on an overhead projector separated only by a mask with a slit to allow a beam of light to pass
upward.
Volume 64 Number 1 January 1987
75
h
t y about 5 to 10 tiines that of water. Two dilutionsgive three data points with the final viscositv about twice that of water. Thus the error in calculating v.,;s only about twice that of each individual measurement. With a reproducibility of f2% for individual measurements, the intrinsic viscosity reported by the student usually is within 10% of the best value when all computational errors are eliminated. Typical results for one set of students are plotted in Figure 11using a constant value for K,... in the Martin eouation. , I he intrinsic viscosity is converted to a molecular weight, M,, hy the Mark- Houwink equation (71,
head
.
.
~~
Apparatus A 300-mL beaker and a capillary tube (Fig. 10)
where K and a are constants for a given polymer-solvent comhination at a specified temperature. For poly(viny1alcohol) at room temperature, the valuesK = 6.66 X lo-' (dL/g) My-"and a = 0.64 have been used although they were determined (8)a t 30 "C. An interesting sidelight is the inability of some students to distinguish clearly between viscosity and density. The idea that polymers can greatly increase the first property without appreciably affecting the second confuses them. Various nonpolymeric examples can he cited suchas the low viscosity of mercury. However, the connection between viscosity and molecular weight does not seem to offer a problem. The role of polymers as thickeners for food products such as soft ice cream and salad dressings and the polymeric thickeners for lubricating oils provide useful examples for class discussion.
Typical Data Using Waterat 27.5'C
Literature cited
graduated cylinder Figure 10. Apparatus for viscosity measurement
Viscosity Measurement
Tube length. L = 41.3 cm Tube radius. r = 0.075 cm VOI. collected, V = 50.0 mL Time of flow, t = 32.8 s Initial head. h = 52.0 em = 50.4 cm Final head. h Average head, h = 51.2 crn Grav. aceei., g = 981 cm/s2
1. Mathias, L. J. J . Chem. Edu. 1983,60,990. 2. Direen, G. E.; Shakhsahiri, B. 2. In Chemicol Demonsfmliom, Vol. I; Shekhaahiri. B.
2.. Ed.:univ.Wia~"na;nl ,9114- ~ ~ . . ~ ~ ~Madis"" - ~ ~ 3. ~o~&ur: R.B.; Kirahenbaum, G.S. J. Chem. Edur 1981.61,161. 4. Cassei, J. M. InBiophysicolPm~erliesoftheShin,Eid~n, H.R.,Ed.; Wi1ey:New York, ,471
5. Properties of phenol are summarized in Merch Indez and by Thurman, C. In KirkOthmer Encyelopedio of Chemical Technology, 3rd ed.; Wiley: New York, 1982: Vol. 17, p 373. Properties and toxicity are discussod. 6. Rodriguez,F. PlinUples ofPolymar Systems. Hemisphere: New York. 1982: p 167. 7. R e f 6 p 164. . H.Forfachr. Horh8. Nakajima. A.; Furutate, K. quoted by Kursta. M.; Stockmayer, W
Equations
+
Average head, h = (h hS2 Kinematic viscosity, qlp = (rg&8Xh/L)(t/V) (u?/g) Head corrected for kineticenergy, h = h, Velocity, u = V/(sPd q = viswsity (poise) p = density (glmL)
polym~r-Forach. 1963.3(21.196.
-
Calculated Result u = 86.3 cm/s h = 43.6 cm q/p = 0.0084 stoke (stoke = cmZ/s). (Handbook lists 0.0085 stoke) with p = 1.00 gImL. q = 0.84 centipoise = 0.84 mPa
ably surprised to find that alcohol has over twice the viscosity of water. The larger size of the alcohol molecule compared to water makes up for its lesser tendency to form hydrogen bonds. The Martin equation (6) allows the extrapolation to dilution from solutions with relative viscosities ereater than 2.
-.
where c = concentration.. d d L .. n.. = . Msoln) . . . - n(solvent)l/ .. ?(solvent), [q] = intrinsic viscosity, dL/g, and K is a, constant for a given polymer-solvent comhination at a s~ecified temperature. ~ e sr tm d t s are ohtained when viscosfty measurements are corrected for kinetic energy .. (see table). Changes in density with concentration conventionally are ignored so kinematic viscositirs ran he used to calculate q,. It must he recoenized that the intrinsic viscositv obtained from this equation will differ slightly from that ohtained from very dilute solution data using other equations. Typically, the students are given a solution with aviscosi76
Journal of Chemical Education
Figure 11. Typical viscwity data tm poly(viny1 alcohol) solutions. A, B, and E are commercial materials. C and Dare mixtures.
