Vocabulary
Vocabulary
Word
Polymer
Cross-link (verb)
Ionic Bond
Gel (noun)
Engineer (noun)
Definition
A long chain molecule made of repeating parts.
To connect parallel chains of polymers together.
A kind of chemical bond in which two atoms or groups of atoms
are electrically attracted to one another.
A semi-rigid, jelly-like material in which a liquid is dispersed in
a solid.
A person who designs, builds, or maintains engines or
machines.
Key Concepts and Interesting Facts
Atoms are the building blocks of matter.
Polymers are long, chain-like molecules made up of many (“poly”) repeating
units of atoms (“mers”).
Polymers are ubiquitous in nature, from rubber in trees to starch in potatoes to
spider webs to DNA strands. Protein is a polymer, which is found in skin, organs,
muscles, hair and fingernails, feathers and fur. The Golden Orb spider’s silk is
stronger than the strongest synthetic polymer Kevlar.
One of the materials used in this activity, Alginate, is a polymer that comes
from seaweed. It is commonly used as a food thickener in foods from ice cream to
Velveeta, and, when cross-linked, is found in pimento strips stuffed into green
olives. It is also used for dental impressions, wound dressings, drug encapsulation
and in the movie industry to make life masks.
Synthetic polymers made by people include plastics, nylon, silicone, polyester,
polyurethane, epoxy resins and the clear plastic polymethyl methacrylate (PMMA).
Kevlar was invented by chemist Stephanie Kwolek and is now used to make airplane
parts, suspension bridge cables, sports gear and bullet-proof vests, among others.
When polymers are put together they form an array of materials with
different properties. Some are tough and hard; some are stretch and bend easily.
The properties depend on the length of the polymer chains and how the polymers
are mixed together.
Polymers that are oriented parallel to one another can slip past one another and
move. This is why you can push a skewer through a balloon without popping it and
why you can stretch silly putty into a long string. Kevlar is also made this way.
Jello is made of natural polymers that are entangled like a plate of spaghetti.
The alginate worms of this activity are cross-linked (held together) through
ionic bonding. A calcium ion has a plus two charge (two missing electrons), which
is attracted to negative charges on the alginate. One calcium ion can cross link two
different alginate chains, holding them together and making the worm-like gel.
The alginate gel is mostly made of water, and it demonstrates alginate’s ability to
absorb liquid 20 times its volume. That is why it has been used for decades as a
wound dressing, particularly for wounds that produce a lot of “exudate” (puss etc.).
Gel worms do not form in the sodium chloride solution because, unlike
calcium, a sodium ion has only a plus one charge, so it can link to one alginate
chain, but not to another. In fact, if a worm is put into the sodium chloride solution,
it will eventually be “unlinked” and disappear.
If an alginate bead colored with food coloring is left in the solution for a while,
students should notice that the color has diffused out into the solution.
Alginate gels can encapsulate medications or other substances that are slowly
released over time, demonstrating their use in drug delivery. If the bead is colored
with tempera paint instead of food coloring, there solution remains clear because
the paint molecules are too big to diffuse through the pores of the gel.
Polymers play a large part in biomedical engineering. Medical devices currently
made of polymers include sutures, lens implants, contact lenses, and heart valves.
At UNM researchers are making polymers that kill bacteria when exposed to light
(for use as countertops and instruments in hospitals, for example); polymers that
pop off sheets of cells that act like “biological sticky notes” to be stuck on damaged
body tissue; and polymeric scaffolds for re-growing bone inside a patient’s body.