TABLE OF CONTENTS Polymers come in many forms, including plastics, rubber, and fibers. Plastics are stiffer than rubber yet have reduced low-temperature properties. Generally, a plastic differs from a rubbery material due to the location of its glass transition temperature (T g ), which is the temperature at which the polymer behavior changes from glassy to leathery. A plastic has a T g above room temperature, whereas a rubber has a T g below room temperature. T g is most clearly defined by evaluating the classic relationship of elastic modulus to temperature for polymers as presented in Fig. 1.5.
At low temperatures, the material can best be described as a glassy solid. It has a high modulus, and behavior in this state is characterized ideally as a purely elastic solid. In this temperature regime, materials most closely obey Hooke s law:
where ? is the stress being applied, and ? is the strain. Young s modulus, E, is the proportionality constant relating stress and strain.
In the leathery region, the modulus is reduced by up to three orders of magnitude from the glassy modulus for amorphous polymers. The rubbery plateau has a relatively stable modulus until further temperature increases induce rubbery flow. Motion at this point does not involve entire molecules, but, in this region, deformations begin to become nonrecoverable as permanent set takes place. As temperature is...
