8.8 Themoplastics
Basically，a thermoplastic is any material that softens when it is heated. However，the term is commonly used to describe a substance that passes through a definite sequence of property changes as its temperature is raised. In Figure 8-4 the thermoplastic characteristics of amorphous，crystalline and liquidcrystalline polymers are compared. An amorphous polymeric material contains randomly entangled chains. A microcrystalline （usually abbreviated to "crystalline"） material contains domains in which the polymer chains are packed in an ordered array. These "crystalline" domains are embedded in an amorphous polymer matrix.
Figure 8-4 Comparison of the Thermal Behavior of Amorphous，Crystalline，and Liquid Crystalline Polymers
Both amorphous and crystalline thermoplastics are glasses at low temperatures，and both change from a glass to a rubbery elastomer or flexible plastic as the temperature is raised. This change from glass to elastomer usually takes place over a fairly narrow temperature range （2-5℃），and this transition point is known as the glass transition temperature （Tg）. For many polymers，the glass transition temperature is the most important characterization feature. It can be compared to the characteristic melting point of a low-molecular-weight compound，although care should be taken to remember that Tg is definitely not a melting temperature in the accepted sense of the word.② It is more a measure of the ease of torsion of the backbone bonds rather than of the ease of separation of the molecules.③
At temperatures above Tg，amorphous polymers behave in a different manner from crystalline polymers. As the temperature of an amorphous polymer is raised，the hard rubbery phase gradually gives way to a soft，extensible elastomeric phase，then to a gum，and finally to a liquid. No sharp transition occurs from one phase to the other，and only a gradual change in properties is perceptible.
Crystalline polymers，on the other hand，retain their rubbery elastomeric or flexible properties above the glass transition，until the temperature reaches the melting temperature （Tm）. At this point，the material liquefies. At the same time，melting is accompanied by a loss of the optical birefringence and crystalline X-ray diffraction effects that are characteristic of the crystalline state.