You might want to check out the section on Polymer Properties first if you haven't seen it already. There, we talk about different kinds of strength, exactly what we mean by strength, and the difference between strength and toughness.

Some polymers are tough, while others are strong. When designing new materials, one often has to make trade-offs between strength and toughness. One may have to sacrifice a little strength so a product will be tougher, for example.

But sometimes we can combine two polymers with different properties to get a new material with some of the properties of both. There are three main ways of doing this, and they are making copolymers, blending, and making composite materials.

An example of a copolymer that combines the properties of two materials is spandex. It's a copolymer containing blocks of elastomeric polyoxyethylene and blocks of a rigid fiber-forming polyurethane. The result is a fiber that stretches. Spandex is used to make stretchy clothing like bicycle pants.

High-impact polystyrene, or HIPS for short, is an immiscible blend that combines the properties of two polymers, styrene and polybutadiene. Polystyrene is a rigid plastic. When mixed with polybutadiene, an elastomer, it forms a phase-separated mixture that has the strength of polystyrene and the toughness of polybutadiene. For this reason, HIPS is far less brittle than regular polystyrene.

In the case of a composite material, we're usually using a fiber to reinforce a thermoset. Thermosets are crosslinked materials whose stress-strain behavior is often similar to plastics. The fiber increases the tensile strength of the composite, while the thermoset gives it compressional strength and toughness.

Click here to learn how to make fibers stronger.

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