Building new materials using macromolecules

The properties of mixtures of pure substances can be very different from the parent materials. The art of metallic alloys is an important part of industrial practice. While there are a few polymer blends that are truly compatible in a thermod5mamic sense, most pairs of pure homopolymers do not mix. The theoretical reason for the low tendency to mix is the very small entropy of mixing of macromolecules. The entropy of mixing depends on the number density of molecules, which will be small if each molecule occupies a large volume. 

Another approach to creating materials with properhes different from any pure homopolymer is to use copolymers. Since the different mers are covalently attached to the macromolecule, they cannot undergo macroscopic phase segregation. The local structure of a bulk copolymer depends on the sequence distribution of the constituent subunits. Consider a copolymer composed of two t5q es of subunits, a so-called AB copolymer. The total munber of mers, N, is the sum of type A subunits, N, and type B subunits, N = N + Ng. The intramolecular composition is specified by the fractions x = N,j/N and Xg = Ng/N. A random copolymer is characterized by a sequence distribution that follows Bemoullian statistics the probabilities depend only on and Xg. 

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