SOLAR ENGINES AND POLYMER
experiment, a solar-powered engine will be built from lightweight polymeric
materials. It will utilize oriented polyethylene strips to convert
radiant energy from the sun into mechanical work. This experiment
is based on a previously published procedure appearing in Popular Science.
Theory Polymer are very large molecules made of many chemically joined
repeat units much like a bicycle chain is made of many individual links.
These molecules can pack in crystalline regions which are highly ordered,
or in amorphous regions which are not ordered. Polyethylene is a
semi-crystalline polymer. This means that some areas of the polymer
consist of crystals of neatly packed carbon chains held rigidly together
(called crystallites because they are very small) while other areas consist
of chains tangled randomly together like a bowl of spaghetti noodles. The
dark circles below represent the crystallites.
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:
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."
1 - 1/4" diameter dowel
2 - 16 oz. polystyrene
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
1 - 6 or 7" diameter plastic
disk (such as the lid of a tub of margarine
2 - pint cans
32 - paper clips
Obtain a black polyethylene
trash bag. Using a metal-edged ruler, a single-edge razor, and a
flat cutting board, cut one inch wide, three or more inch long strips parallel
to the bag's top. (Strips cut perpendicular to the bag's top will not cold-draw
well due to orientation during manufacturing process.) Grasp the ends of
a strip between thumb and forefinger and pull the ends apart with a slow,
steady motion. The plastic will "neck" sometimes in several places and
this neck will finally extend from thumb to thumb.
single edge razor blade
flat file compass
one sheet medium sandpaper
1/4" drill bit
A SOLAR ENGINE
TOP OF PAGE
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.
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.
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.
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.
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?).
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.
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.
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.
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.
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.
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.
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."
E. D. Ray, Popular Science,
February Issue, 126-128 (1981).
E. Pines, K. L. Wun and
W. Prins, J. Chem. Educ. 50, 753-756 (1973).