SOLAR ENGINES AND POLYMER
ENTROPY

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Because the polyethylene chains are very long, some chains pass through several crystallites and serve to tie these small crystalline areas together. When a thin strip of polyethylene is pulled (cold-drawn) the crystallites tend to spread apart, and the tie molecules become almost taut:

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If this stretched film is heated, the increased thermal energy causes the taut tie molecules to move vigorously and become more coiled and tangled, much like they were before stretching. This increased coiling (increased conformational entropy) causes the crystallites to be drawn together. When the strip is cooled the tie molecules return to their stretched out (taut) conformation, and the crystallites again spread a part. When the strip is viewed as a whole, heating is observed to contract the strip and cooling is observed to expand the strip. This behavior is the exact opposite of what is observed for most solid materials: they expand when they are heated and contract when cooled. Similar behavior can be observed by heating and cooling a stretched rubber band. In this case, however, crystallites are generally not present in the polymer (natural rubber is amorphous) and essentially all of the molecules are "tie molecules". The large thermal contractions and expansions characteristic of rubbery polymers and plastics can be harnessed to produce useful mechanical work. Because the polyethylene strips resemble our own muscles during contraction, they have been nicknamed "Solar Muscles."
 

MATERIALS NEEDED                                              TOP OF PAGE

• 1 - 1/4" diameter dowel 12" long
• 2 - 16 oz. polystyrene foam cups
• 1 - 1/4" thick polystyrene foam plate (such as fresh meat is packaged in)
• 2 - straight pins
• 16 - Solar Muscle strips (prepared by stretching 1" wide strips cut from a black polyethylene trash bag)
• 1 - 6 or 7" diameter plastic disk (such as the lid of a tub of margarine
• 2 - pint cans
• 32 - paper clips
• epoxy cement
• contact cement

TOOLS NEEDED                                                      TOP OF PAGE

• single edge razor blade
• sharp pencil
• flat file compass
• scissors
• ruler
• one sheet medium sandpaper
• 1/4" drill bit

MAKING SOLAR MUSCLES                                          TOP OF PAGE

CONSTRUCTING A SOLAR ENGINE                              TOP OF PAGE

1. With the compass, draw two circles on the plastic foam plate with a diameter equal to the inside diameter of the tops of the plastic foam cups.  Cut out the circular disks using the single-edge razor blade. Using the sharp pencil with a twisting motion from both disk sides, punch holes in the centers of the disks so that each fits snugly over the dowel. The dowel should have its ends slightly rounded with the sandpaper.
2. Sand down the edges of the foam disks so that each fits snugly recessed 1/4" below each cup end. This will make both cups rigid. Using the 1/4" drill bits, carefully drill by hand a 1/4" hole in the center of the bottom of each cup.
3. Find the centers of the ends of the dowel and insert the straight pins 1/4" into the centers. Take care that the pins provide a centered spin-axis.
4. Mark the center of the dowel length. Assemble the rotor as shown in the drawing below. Ensure a good quick-set epoxy bonding of all parts, but do not bond one of the disks to the dowel.

 

 
 
 

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5. Coat 1/4" of the inner lip of each cup with quick-set epoxy. This provides a surface for gluing the Solar Muscle (SM) strips to the cup lips with contact cement. Otherwise, the contact cement will dissolve the plastic foam (why?).
6. After the epoxy has thoroughly set, cut a 1/2" radius center hole in the disk that was not glued to the dowel. Remove the cut-out disk from the dowel. This allows one of the cups to wobble about the dowel axis.
7. Now take one of the SM strips you stretched previously. Hold it against both cup ends, and cut it to length so that 3/8" extends beyond each cup end. Cut 16 strips this way. Apply contact cement to the epoxy surface on the inner lip of each cup and to the ends of the SM strips. Keep all of the contact cement well inside the epoxy-coated surface to avoid dissolving the foam.
8. Attach the SM strips symmetrically around the cups, parallel to the dowel, and with each end of each strip cemented to the cup's inner lips. No two strips should touch along the rotor, and the spacing between the strips should not exceed the strips' width. In attaching the SM strips, take up all the slack in them, but apply only slight tension to flatten the strips.
9. File a notch in the rim of each pint can in which the pins can turn freely. Weight the cans with sand or dirt so they won't easily tip.
10. Place the rotor on its pint can friction bearings in a sunny window, and by hand, rotate the motor slowly until the SM tightens to assume its natural tension. About five minutes of slow turning in bright sun will complete the process.
11. Cut a two-inch diameter hole at the center of the plastic lid.  Apply epoxy cement to the rim of the wobbly cup. Attach the plastic lid flywheel to the wobbly cup rim, being careful to center it on the axle (dowel). Balance the rotor by attaching paper clips to the rim of the flywheel.
12. Place the solar engine in a sunny window, and it will turn at about 50 rpm.  Shade the engine from the sun's light and observe that it stops; remove the shade and observe again the rotation.

As you've probably guessed, the engine turns by the SM contracting on the hot side and relaxing on the shaded side, thereby constantly lifting the flywheel above the rotor's center of gravity and allowing it to continuously "fall around."

REFERENCES                                                           TOP OF PAGE

1. E. D. Ray, Popular Science, February Issue, 126-128 (1981).
2. E. Pines, K. L. Wun and W. Prins, J. Chem. Educ. 50, 753-756 (1973).