What's a Polymer Anyway?
Long molecules, three properties, one trick.
You've already eaten many of them so far. Your bread had gluten, your yogurt had pectin and casein, and remember the honey stick? Well, it was held together by polysaccharides.
Your toothpaste? Cellulose ether — basically HPMC.
A polymer is a long molecule made of many small, similar parts hooked together in a chain. Poly means many. Mermeans part. That's the whole etymology. Plastics are polymers. So is starch. So is your hair. So is silk, and even DNA. The category is huge.
What makes a polymer behave like a polymer, instead of like the small molecules it's made from, is the chain itself. A single sugar dissolves in water and disappears. A chain of ten thousand sugars hooked end to end gets sticky, viscous, sometimes solid. Long things tangle. Long things grip each other. Long things, when they line up right, form sheets.
Two kinds of polymers, very different
In polymers, you'll see two big categories.
The first is synthetic polymers: the plastics. Polyethylene, polystyrene, PET, nylon, etc. These were mostly invented in the 20th century by chemists who chained together small petroleum-derived monomers into long molecules with very specific properties. They're cheap, durable, and largely uncomposable by nature. That last bit is why we're in this mess.
The second is biopolymers (the ones we're working with).
Made by living things. Plants make cellulose to hold themselves upright. Animals make collagen to hold themselves together. Algae make carrageenan and alginate to keep their cell walls in place under saltwater pressure. Bacteria make polyhydroxyalkanoates, weirdly, as energy storage. All of these are polymers, and all of them can, with the right encouragement, be brought out of their original organisms and turned into something else.
A sheet of bioleather. A thread. A film. A skin you can cut a sleeve from.
To get polymers to do the things we want them to do, there's only one trick. You'd think the cool stuff about polymers would be somehow complicated. It is not. The thing is this:
A polymer chain in water can move around freely. A polymer chain that's hooked to other polymer chains can't. The first state is a liquid. The second state is a solid.
Everything in this book, every recipe, every cure schedule, every weird ratio of agar to glycerin to chitosan, is like a choreography for moving the chains from state one to state two.
There are about half a dozen ways to make chains hook to each other. Some happen when you cool things down. Some happen when you heat things. Some happen when you add a specific ion to the water. Some happen when one chain is positively charged, and another is negatively charged, and they grab each other electrostatically.
The three properties that matter for our work
Our aim is that the biopolymers we create have three properties that synthetic plastics don't have, and these are why we will spend our time fishing around in pots of seaweed.
Renewable
You don't mine kelp. You grow it. A kelp blade can elongate up to 60 centimeters a day under good conditions, and there's no oil well in the world that refills that fast.
Biodegradable
When you're done with the material, microbes can eat it. Specific microbes, specific conditions, specific timelines (we'll get into the weeds about industrial vs. home composting in Part V), but the principle holds. The carbon you took out of the air goes back to the air, or to the soil, or to a fish. The loop closes.
Tunable
The same agar can be the soft jelly inside a bubble tea pearl or a stiff translucent sheet you could cut into the panel of a coat. The same gelatin can be a gummy bear or the binder in a flexible bioleather. The properties of the final material depend on how you cook the polymer, what you mix it with, and what conditions you cure it under. Tuning those parameters is the craft.
