Soft segment polyester urethanes, molecular

Figure 7. Molecular structure of soft and hard segment of photochromic polyester urethane with partly photochromic soft segments.

Urethanes are a reaction product of a diisocyanate and long- and short-chain polyether, polyester, or caprolactone glycols [2]. The polyols and the short-chain diols react with the diisocyanates to form linear polyurethane molecules. This combination of diisocyanate and short-chain diol produces the rigid or hard segment. The polyols form the flexible or soft segment of the final molecule. Figure 8.1 shows the molecular structure in schematic form. 

Segmented polyurethanes (PUs) are typical representatives of linear block copolymers of the type (A-B)jj and an important class of thermoplastic elastomers. They are composed of short alternating blocks of soft (SS) and hard segments (HS). The SS impart elastomeric character to the copolymer, whereas the HS form a solid phase (HS microdomains) through intermolecular association and impart dimensional stability to the array of macromolecules. Low molecular mass polyethers and polyesters are typically used as SS, while HS normally consist of an aromatic diisocyanate that has been chain extended with a low molecular weight diol or diamine to form an oligomeric aromatic urethane or urethane urea segment [1-3]. 

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