ABSTRACT

The applications of polymers are enormous, ranging from an almost endless list of consumer uses, to high-strength and temperature uses, to high-tech electronics and space exploration uses.

Polymers are generally classified as those being thermoplastic, thermosetting or elastomers. Thermoplastic polymers consist of long-chain linear molecules that permit the plastic to be easily formed at temperatures above a critical temperature called the glass transition temperature (T8). For some thermoplastic polymers (thermoplasts) this temperature is below room temperature; hence they are brittle at ambient temperatures. Thermosets have a three-dimensional network of atoms. They decompose on heating and cannot be reformed or recycled. Certain polymer materials fit into both the thermoplastic and the thermosetting categories. For example the unsaturated polyesters are thermosetting polymers while other polyesters are thermoplastic ones. Polyimides and polyurethanes can also fit into either category depending on their structure. Elastomers, better known as rubbers, are polymeric materials whose dimensions can be changed drastically by applying a relative modest force, but return to their original value when the force is released. The molecules are extensively kinked such that when a force is applied, they unkink or uncoil and can be extended in length up to about 1000%. In general, they must be cooled to below room temperature to be made brittle. Thermoplastic elastomers (TPE), which are block copolymers in which one component of the block is flexible and the other rigid, is a special class of polymers now considered by many to be a fourth class of polymers. One must now also consider the polymer alloys or blends. The term .. alloy" is used here to describe miscible or immiscible blends which are usually blended as melts. Miscible blends are characterized by properties, such as glass temperature values, which lie somewhere between the two compo-

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nents. Immiscible blends are characterized by two or more phases. The most widely used blend is the blend of PPO and PS which is one of the "big five" engineering thermoplastics. Many of these blended polymers fall into the thermoplastic category.