Other experiments can be made with the material. For example, in 1824 Joseph Nicéphore Niepce, a French physicist and amateur Egyptologist, coated a glass plate with the same mixture of oil and tar and exposed it to a brightly lighted scene with a camera obscura that he constructed according to the design of Leonardo da Vinci. When Niepce subsequently washed the plate with oil of lavender, the unexposed tar dissolved but the light-struck portions, which were photocrosslinked, adhered to the glass, forming an image of the scene. The plastic film served as a lithographic surface for greasy inks, thus yielding the first permanent photograph.

Photocrosslinking has subsequently become a highly developed branch of photochemistry, as has photopolymerization: the use of light for triggering chemical chain reactions in which the small molecules of certain liquids join end to end to form the giant molecules of solid plastic. In both phenomena light causes fluids to solidify. An innovator in the field of photopolymerization and photocrosslinking is Gerald Oster, professor of biophysics at the Mount Sinai School of Medicine of the University of the City of New York. He has recently developed several new chemical systems that demonstrate both effects. One of his systems enables the amateur to make a finished photograph of grainless texture in less than a second. Oster explains how the new materials work and describes a few of the many experiments that can be made with them:

"All plastics consist of very large molecules that are made up of smaller molecules repetitively joined together. The small units are known as monomer (mono, meaning one, plus meros, meaning part) and the large ones as polymer (many parts). Molecules consisting of two small units are called dimers and those consisting of three units trimers. Most common polymer materials, such as polyethylene and Lucite, are made by heating monomer together with a catalyst. The reaction proceeds slowly. Several hours may pass before the monomers are fully linked.

"It turns out, however, that light can trigger almost instantaneous chain reactions in certain organic fluids, with the result that the fluids solidify in less than a second. If you expose a solution of this kind to light passed through a photographic transparency, you get a three-dimensional image in plastic. You can stop the reaction at any time by turning off the light. The resulting photograph can be fixed (made insensitive to light) by washing off the unreacted monomer. The image forms during the exposure and so requires no chemical development as in ordinary photography.

"The effect is nicely demonstrated by monomer in the form of a water solution of calcium acrylate. This material is prepared by neutralizing acrylic acid with calcium hydroxide. The neutralizing procedure is a bit tricky, because heat that is liberated as the chemicals combine must be removed by controlled cooling to maintain the compound at a constant temperature. Moreover, the acid is toxic prior to its reaction with the alkali. For these reasons the amateur is advised to buy the monomer ready-made. It is available, along with other chemicals and materials that are required for the experiment, from the Edmund Scientific Co., 101 East Gloucester Pike, Barrington, N.J. 08007.

"Calcium acrylate monomer is made into polymer by a combination of both photopolymerization and photocrosslinking. In effect units of monomer, each roughly in the shape of a T, are fastened together to form a ladder-like structure by chemical 'nails.' The crossbars of the T's are joined end to end to form the sides of the ladders. This is an example of polymerization. The rungs of the ladders-the crosslinks-are atoms of calcium.

"The 'nails' are electrons supplied by another chemical, triethanolamine, which must be mixed with the calcium acrylate solution. Triethanolamine is known as an electron donor, but it must be forced to make its donations. The necessary force is derived by means of a third chemical, a dye that must also be added to the solution. The dye absorbs energy in the form of light. Dyes of various colors can be used, but a convenient one for making experiments indoors is methylene blue, which absorbs red light, the color emitted most strongly by incandescent lamps.

"For the initial experiment put about 20 milliliters of monomer solution in a transparent container such as a cup made of clear plastic, add two drops of triethanolamine and two drops of methylene blue and swirl the container to mix the solutions. Hold the container a few inches from an incandescent lamp. Within seconds the clear blue mixture will become cloudy on the side that faces the lamp and will change into an opaque, blue-white film attached to the wall of the container. The film is a plastic: polycalcium acrylate. It will continue to grow in thickness as the dye absorbs light; within seconds it will fill the container with solid polymer. The mass can be dissolved by adding a mild acid. The resulting fluid is a solution of polyacrylic acid.

"The reaction has several steps. When the dye acquires energy by absorbing light, it exerts an electrical force on a neighboring molecule of triethanolamine and thereby attracts an electron to itself. The dye is now in an electron-rich condition. It carries a charge, is highly reactive chemically and is called a free radical. The free radical attaches itself to a monomer and thereby makes the monomer a free radical. This free radical in turn attaches itself to a neighboring monomer. Now we have a dimer. Again, the free radical makes the dimer a free radical that reacts with still another neighboring monomer to make a trimer radical, and so on. In other words, we have a chain reaction. Similar chain reactions occur throughout the solution, but the majority of them are triggered in regions where the light is brightest. The action stops when two growing chains come together and unite.

"Under appropriate conditions the dye will in time reject the electron and revert to its normal state, as can be demonstrated by experiment. Place five or 10 milliliters of triethanolamine in a test tube and to the clear fluid add a drop or two of methylene blue. Expose the mixture to an incandescent lamp or to sunlight. Within 20 seconds or so, depending on the intensity of the light, the bright blue solution will become as clear as water. Molecules of dye have now absorbed electrons; as chemists would say, the dye is reduced.

"The reduced dye can also be oxidized. Darken the test tube for a minute or two by putting it in your pocket or even in the shade. As oxygen from the air diffuses into the solution the dye loses its borrowed electron and the blue color returns. You can speed the recovery by shaking the container. The experiment can be repeated several times until the triethanolamine is exhausted.

"Both original photographs and prints can be made with the monomer mixture. Prints are easier to make. Coat a glass plate with solution and project an image onto the coating. A film of plastic that varies in thickness with the intensity of the light in the image will form on and adhere to the glass. The resulting picture will appear to be either a photographic positive or a negative, depending on how you light it. For example, if you made the exposure with a negative transparency, the plastic film can be projected on a screen as a positive image. Highlights in the original scene become dark areas on the negative that shields the plastic from the light. In these areas the plastic is thin, and so it transmits projected light to the screen. Conversely, dark areas in the original scene are represented by thick areas of plastic that absorb light and result in dark areas on the screen. On the other hand, the plastic, when it is examined by reflected light against a dark background, appears to be a photographic negative. Thick regions of plastic reflect blue-white light, whereas the dark background appears through thin regions. Hence highlights in the original scene appear as shadows and vice versa.

"The exposures can be made in various ways. I improvise a shallow container for holding the monomer by pressing a strip of caulking compound around the edges of a glass plate. If the glass is clean, the plastic image will adhere to the plate as a conventional photographic emulsion does.

"Before applying the caulking compound I wash the plate with a kitchen detergent, scrub the washed surface with absorbent cotton saturated with strong ammonia solution and rinse it with distilled water. After drying the glass with clean absorbent paper I apply the caulking compound. Monomer is poured onto the plate to a depth of a millimeter or two, depending on the nature of the picture. As in ordinary photography, the production of good prints is more an art than a science. The best depth of monomer to use for a given picture must be found by experiment. The results improve with practice.

"Because the photosensitive material is a fluid, the glass plate must be kept horizontal when the exposure is made. The plate can be placed on the level base of a vertical enlarging projector, as in conventional projection printing. Polymer forms initially at the upper surface of the fluid and grows downward. It may not adhere firmly to the glass. For this reason I prefer to make exposures through the bottom of the glass by projecting the light upward with a conventional 35-millimeter projector and a mirror mounted at an angle of 45 degrees [see Figure 2]. An ordinary hand mirror can be used, but some light is reflected by the unsilvered front surface, which tends to cause a ghost image. Photographs of the best quality require the use of a front-surface mirror. Mirrors of this kind are available from the Edmund Scientific Co.

"Reproductions of opaque illustrations, such as printed text and halftones, can be made with an opaque projector. Light reflected by the opaque object is brought to a focus on the monomer by a projection lens. Direct prints can be made by sandwiching a negative or any other transparency between a sheet of clear glass and the plate that supports the monomer. The exposure is made through the bottom of the sandwich with projected light. Do not use a bare incandescent lamp because the scattered rays will form a fuzzy image.

"The thickness of the plastic film, and hence the density of the photographic reproduction, varies with the intensity of the light, the exposure time and the temperature of the monomer solution. The sensitivity of the solution to light depends in part on the proportions of dye and electron-donor solution. In general the speed of the polymerizing reaction increases, up to a point, as the proportion of dye and electron-donor solution is increased.

"For the first experiment I suggest a mixture consisting of 30 milliliters of calcium acrylate, two drops of methylene blue and four drops of triethanolamine. If this solution is exposed at room temperature by a 500-watt, 35-millimeter projector that contains a transparency of average density that is enlarged to an image approximately five inches square, polymerization will occur in the highlights within about 20 seconds, and the image will be fully developed within a minute or two. To stop the development, turn off the light. Undeveloped monomer solution can be poured into a container for reuse. I usually immerse the developed plate in a tray of distilled water in order to wash off the film of unreacted monomer, and then let the polymer dry in the air.

"The reaction can be speeded by any or all of four procedures: preexposing the monomer solution briefly to light, adding dye, adding electron-donor solution and increasing the intensity of the light. For example, you can make instant plastic by carrying out the exposure with an ordinary photographic flash lamp. Similarly, plastic will form instantly in the beam of a helium-neon laser at distances of up to 50 feet, depending on the power of the laser.

"Numerous other experiments are possible. For instance, any plastic image n can be used as a printing plate. Bring the dry plastic into contact with an ink pad and transfer the adhering ink to white paper. The quality of the first print may leave something to be desired, experience. Transparencies of line drawings are the easiest to reproduce with this technique.

"You can also incorporate pigments and other materials into plastic pictures. For example, moisten a tablespoon of fine, clear beach sand with monomer mixture, stir to make a thin paste, spread the paste on a glass plate and expose it to a transparency that has coarse details. After washing the polymerized material you will have a photograph in sand. The effect can be varied by substituting for the sand other substances such as powdered glass and fine beads. By polymerizing a mixture of monomer and powdered fluorescent chalk you can make pictures that glow when they are exposed to ultraviolet light. In the same way you can make a permanent record of a magnetic field by mixing iron filings with monomer, placing a horseshoe magnet against the bottom of the plate and 'freezing' the pattern with a flash of intense light.

"It is easy to prove that the mixture discussed so far responds only to red light. Cover several areas of the monomer with gelatin color filters of various hues and make the exposure through the filters. You might use red, yellow and blue filters. Polymer will appear only in the region that is exposed to light through the red filter. By the same token exposures made with color transparencies respond only to shades of red.

"Other colors can be reproduced by substituting appropriate dyes for methylene blue. For example, the monomer mixture can be made sensitive to blue by using acridine yellow and sensitive to green by substituting rose bengal for methylene blue. Rose bengal, a red dye, is a derivative of fluorescein. A print in full color can be made as a composite of three plastic films, each exposed for one of the primary colors, by means of a negative color transparency. The negative is essential because methylene blue responds to red, acridine yellow to blue and rose bengal to green-the reverse of the colors in the negative. These dyes are commonly used for staining biological materials and can be bought from distributors of chemical supplies, such as the Fisher Scientific Company, 52 Fadem Road, Springfield, N.J. 07081.

"Even better photographic reproductions can be made in transparent plastic by means of another chemical system that demonstrates photopolymerization. The photographic resolution that can be achieved with this system is limited principally by the wave nature of light and the quality of the optical system used for making the exposure. The monomer is acrylamide, a solid organic material that dissolves in water at room temperature in the proportion of about one part monomer to four parts water plus 1 percent or less of the crosslinking methylene bis acrylamide. The solution is clear and becomes sensitive to light by the addition of a small percentage of a dye, such as riboflavin. This particular dye functions both as the light absorber and as the electron donor. The polymer forms an essentially grainless, three-dimensional image that does not scatter light. After the unreacted monomer has been washed away the image has the form of a transparent plastic film. The thickness of the film varies roughly in proportion to the square root of the intensity of the light. In effect the polymer becomes a device for converting the intensity modulation of light into optical phase modulation with obvious applicability to the production of holograms. The monomers are about as toxic as ordinary photographic chemicals, and the same precautions should be observed when working with them."

Modifications in the physical properties of many substances, including the polymerization of plastics, are accompanied either by changes in vapor pressure or by changes in volume, which can be transformed into variations in pressure by enclosing the substance in a sealed container. Substantial changes in pressure can be measured fairly accurately by a simple manometer: a U-shaped tube of glass containing a fluid of known density, such as water.

One arm of the tube is connected to the vessel containing gas under unknown pressure. If the gas pressure is higher than atmospheric pressure, fluid moves downward in that arm and upward in the other arm. The pressure is equal to the difference in the height of the fluid in the alms multiplied by the specific weight of the fluid. The height can be measured with reasonable accuracy to within .5 millimeter. In the case of water .5 millimeter is equal to .0014 pound per square inch.

By a simple modification of the manometer the sensitivity of the instrument can be increased by a factor of several thousand. Details of the modification are explained by Kipling Adams, who is associated with the General Radio Company in West Concord, Mass. Adams writes:

"The sensitive manometer consists of two cylindrical containers interconnected near the bottom by a pair of tubes. One of them is a capillary made of glass or clear plastic and therefore transparent. Each tube contains a stopcock. The assembly is filled with fluid. The movement of the fluid is indicated by a bubble of air trapped in the middle of the capillary.

"One cylinder is closed at the top by a gastight lid containing a pipe nipple through which the manometer is connected to the source of unknown pressure. The top of the second cylinder is exposed to the air in the room [Figure 6]. The principle of the device is as simple as its construction. A difference in pressure that acts on the surface of fluid in the containers causes fluid to flow through the capillary toward the container of lesser pressure, as is indicated by the movement of the bubble.

"The dimensions are not critical, but, they must be known as accurately as possible for calibrating the instrument. For instance, assume that the cylindrical containers have a diameter of five inches and that the capillary has a bore of .05 inch. If we now apply a pressure of 36 millionths of a pound per square inch to the surface of the water in the closed container, the water level will fall .00.5 inch and will flow through the transparent tube until the level rises an equal amount in the opposite container. (A cubic inch of water weighs about .036 pound.) The ratio of the cross-sectional area of the cylindrical containers in this example is 10,000 times the cross-sectional area of the bore of the capillary. The bubble will therefore move five inches toward the left. Hence a bubble displacement of 1/8 inch indicates a change in pressure of a millionth of a pound per square inch.

"A practical device can be made with a pair of coffee cans, two brass stopcocks, some rubber tubing and the capillary. Glass tubes with a bore of from .01 inch to several inches are available from distributors of scientific supplies. Assemble the apparatus as indicated in the accompanying illustration. Place it in operation by closing the stopcock connected to the glass tube and opening the other stopcock. Fill the cans about three-quarters full of water. When the levels have equalized, close that stopcock and open the other one. With a medicine dropper add a few drops to one reservoir until water pushes most of the air out of the glass tube. A few drops added to the second reservoir will bring the air bubble back to the center of the tube. If the reservoirs are five inches in diameter, each drop will move the bubble about 1.5 inches in a tube with a bore of .05 inch.

"The multiplying factor of 10,000 may prove to be too large for some measurements and the pressure range too small. A reduction factor of, say, 100 may be required. You can make this reduction in sensitivity by shrinking the effective diameter of the open reservoir from five inches to .5 inch. Reducing the diameter this substantially is not as difficult as it might seem. You can accomplish it by putting a nonfloating cylinder 4.741 inches in diameter in the open reservoir. This 'dividing' plug does not need to be placed concentrically in the reservoir. It will work in any position, provided that it does not block the outlet tubes and provided also that the level of the water in the reservoir does not reach the top or bottom of the plug.

"The instrument can also be used as a highly sensitive tilt indicator. The 10,000-to-1 sensitivity figure applies to the liquid levels, so that raising one reservoir from the balance position 10 millionths of an inch displaces the bubble almost 1/16 inch. To demonstrate the sensitivity of the device, place a piece of wood two feet long and two by four inches in cross section on supports two feet apart. Stand one reservoir on the center of the board and the other reservoir above one of the supports. The weight of a finger placed on the board near the center will cause the wood to bow downward about .0001 inch and will displace the bubble .5 inch!"

Bibliography

PHOTOPOLYMERIZATION AND PHOTOCROSSLINKING. Gerald Oster in Encyclopedia of Polymer Science and Technology: Vol. X, edited by Herman F. Mark, Norman G. Gaylord and Norbert M. Bikales. Interscience Publishers, 1969.

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