Melting Transitions of Regular Copolymers

The discussion of block copolymers in the last section included already a number of regular copolymers. An alternating block copolymer of type A By may well be considered a homopolymer, as illustrated by the example of nylon 6,6 [A is 

The almost 30 examples of phase diagrams of this section show that equilibrium is rare in copolymers. Nanophase separation is found frequently when partial crystallization occurs. Dissolutions are possible by randomizing the conformations and by chemical reaction. 

Copolymers combine multiple components within one molecule, i.e., the mixing widiin the molecules is set by the chemical reaction (see Sect. 3.4). Solutions, in contrast, consist of separate molecules, mixed only physically (see Sect. 7.1). Although the effects of these two mixing types may yield different disttibutions, the same expressions are usually applied erroneously to describe glass transitions of copolymers and solutions as a function of composition. 

Copolymers are often sufficiently irregular not to be able to crystallize. Random copolymers of short repeating units, such as vinyl polymers, and mass fractions of 0.3-0.7 are normally amorphous since sequences of identical repeating units must commonly be two to five nanometers long to crystallize, and cocrystallization of different repeating units is not very frequent (see Sect. 5.1.10). 

The glass transitions of amorphous copolymers change smoothly with concentration from one pure component to the other, as is shown in Fig. 7.67 for poly(acrylamide-co-styrene), poly(methyl acrylate-co-styrene), and poly(styrene-co- 

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