28.3.

The method you will use in this project comes right out of the journal article which introduced the word, polyester.[1] It is not a commercial polyester. It melts at too low a temperature to be used as a textile, but its low melting point makes it convenient for us to make in the laboratory without special equipment. To make a polyester you need a di-alcohol and a di-acid. One di-alcohol, ethylene glycol, is readily available from automotive suppliers as anti-freeze. When shopping for anti-freeze, check the label; you want a close to pure ethylene glycol as possible. Look for a generic, no-frills, non-environmentally-friendly, undiluted anti-freeze, the cheaper the better. You do not want pre-mixed anti-freeze because it has been mixed with water and you will be trying to eliminate water from the reaction.

Unfortunately, there is no consumer market for di-acids so you will have to get one from a chemical supply. Phthalic anhydride is the anhydrous form of phthalic acid, which is particularly convenient for condensation reactions since you are trying to eliminate water from the reaction. The reaction is:

n C8H4O3 + n C2H6O2 = (C10H8O4)n + n H2O

Of course, you will need to answer the stoichiometric questions:

Q: How many grams of phthalic anhydride will react with 1.0 g of ethylene glycol?
Q: How many grams of polyethylene phthalate should be expected from the reaction of 1.0 g of ethylene glycol?

You are probably wondering what to do with the "n" in the equation. Since the stoichiometry involves only the ratios of products and reactants, the "n" will cancel out of any stoichiometric calculation. Try it with n=1 or n=10 or n=100; you should get the same answers no matter what choice you make for "n."

Before you begin, you will need to know the weight of your empty test tube. It is convenient to place a small beaker on your balance, tare it so that it reads zero, then place the empty tube into the beaker and write down the weight of the empty tube in your notebook. The use of the beaker will allow you to weight the tube later on even when it is full of liquid. Using the methods of Appendix C, weigh your calculated amount of phthalic anhydride and 1.0 g of ethylene glycol into your test tube.

Light your spirit lamp and use it to heat the test tube. You will, of course, need to hold the test tube with a test tube holder. It would also be wise to wear a leather work glove on your test-tube hand since even the holder may get hot. As you heat the tube you want to be sure to heat it evenly from top to bottom. Hold the tube horizontally as much as is possible without losing its contents. Move the tube back and forth in the flame, turn it over, and heat the other side. Your goal it to heat the entire tube, not just a part of it. As it warms, the phthalic anhydride will begin to melt and dissolve in the ethylene glycol. You may notice that as it does so, the solution appears to boil; the water produced by the reaction is boiling away. If the mouth of the tube is cool, the water may condense and dribble back into the reaction. We heat the entire tube evenly to prevent this from happening. You might think that more heat would be better, but at this stage our goal is to boil away the water, not the ethylene glycol. To this end, you should heat your tube gently and with great skill. If the reaction begins to boil vigorously, remove the tube from the flame for a moment until it settles down.

When all of the solid anhydride has melted, the production of water will noticeably decrease. Continue to heat it gently for another minute or two and then weigh the tube, taring the beaker as before, and record the weight in your notebook. Subtract the empty weight of the tube from the full weight, record the actual yield of polyester and express it as a percentage of the theoretical yield. If this percentage is over 105%, you have not eliminated all of the bejeesical water; return the tube to the flame for another round of gentle heating. If the percentage is below 95% you have boiled away some of your ethylene glycol prematurely; you may continue, but you will have less polyester than you might have had.

When your yield is at or below 100%, return the tube momentarily to the flame until any solid material has melted once again. Then pour the hot polyester into the middle of a square of aluminum foil. Touch your glass rod to the hot, viscous polyester. A little of it will stick to the glass rod and dribble back into the puddle of molten plastic. Touch the glass rod to the polyester and you will notice that it becomes more viscous as it cools. Continue touching the rod to the polyester until you are able to draw a fiber from it, as shown in Figure 28-2. This fiber will be short in the beginning, perhaps a few cm, because the polymer chains are not very long. In other words, the "n" is some small integer. Eventually, the polymer will cool to the point that you can no longer draw a fiber from it.

Figure 28-2. Drawing a Polyester Fiber

Imagine the molecular dating scene at the beginning of the reaction. As each phthalic anhydride molecule melted, it would find itself surrounded by unattached ethylene glycols. Each one would hook up with two ethylene glycol molecules, since bigamy is the natural state for a phthalate. Toward the middle of the reaction, a melting phthalic anhydride molecule would find that many of the free glycols were already taken. No problem—each of the glycols attached to the earlier phthalates would still have had a free -OH available for grabbing. Occasionally, two short chains would find one another and hook end-to-end to form a longer chain, but toward the end of the reaction the chains would be long enough that the loose ends would almost never find one another. Furthermore two ends would have to be of the opposite "gender" to hook up properly; two glycol ends would be out of luck in the condensation department. Consequently, the longer the chains get, the harder it would be for them to grow.

If you want long fibers, you need long chains. To promote this, we can return the polymer to the flame, this time by holding the aluminum foil over the spirit lamp with gloved hands until the polyester melts once more. Continue heating gently and you will start to boil off any ethylene glycols and phthalic acids which happen to fall off the ends of their respective chains in the heat of the moment. If two glycol ends were near one another initially and one of the glycols were boiled away, the other would now be free to hook up with the now-available phthalate end of the other chain. Allow the polymer to cool and you will find that you can draw longer fibers from the melt than you could before this second heating. Carefully wind your fiber onto the glass rod to form a small cocoon, as shown in Figure 28-2. You may return foil to flame as many times as you like but be careful not to scorch the polymer; it is possible to produce fibers 10 meters long by repeated heating of the polyester.

ImportantQuality Assurance
 

Record in your notebook the percent yield and the length of your longest polyethylene phthalate fiber. Sadly, your fibers are likely to be too fragile to keep permanently; photograph your best fiber as a kind of phthalic symbol. This photograph and the polymer remaining on the foil should be taped into your notebook.

Notes

[1]

Reference [6].