1.4 Chemical Structure and Thermal Transitions
As the previous discussion has shown, many important synthetic polymers such as polystyrene and poly(methyl methacrylate) consist of long, flexible chains of very high molecular weight. In many cases, individual chains are randomly coiled and intertwined with no molecular order or structure. Such a physical state is termed amorphous. Commercial-grade (atactic) polystyrene and poly(methyl methacrylate) are examples of polymers that are amorphous in the solid state. Below a certain temperature called the glass-transition temperature (Tg), long-range, cooperative motions of individual chains cannot occur; however, short-range motions involving several contiguous groups along the chain backbone or substituent group are possible. Such motions are called secondary-relaxation processes and can occur at temperatures as low as 70 K. By comparison, glass-transition temperatures vary from 150 K for polymers with very flexible chains such as polydimethylsiloxane
to well over 600 K for those with highly rigid aromatic backbones such as the high-modulus fiber poly[2,2’-(m-phenylene)-5,5’-bibenzimidazole] (PBI) (see Section 10.2.1)
with a Tg reported in the range from 700 to 773 K.
Polymer chains with very regular structures, such as linear polyethylene and isotactic polypropylene, can be arranged in highly regular structures called crystallites. Each crystallite consists of rows of folded chains. Since sufficient thermal energy is needed to provide the necessary molecular mobility for the chain-folding process, crystallization can occur only at temperatures above Tg. If the temperature is too high, chain folds become unstable and high thermal energy disorders the crystallites—a crystalline–amorphous transition then occurs. The temperature that marks this transition is called the crystalline-melting temperature or Tm. Crystalline melting temperatures can vary from 334 K for simple, flexible-chain polyesters such as polycaprolactone
to over 675 K for aromatic polyamides such as poly(m-phenylene isophthalamide) (Nomex)
As an approximate rule of thumb, Tg is one-half to two-thirds of Tm expressed in absolute temperature (Kelvins). The glass-transition and crystalline-melting temperatures can be determined by a wide range of techniques including measurement of volume (dilatometry), specific heat (calorimetry), and mechanical properties, particularly modulus (e.g., dynamic mechanical analysis), as discussed in Chapter 4.