Alternating block PUs

PUs have been developed and thoroughly investigated for various industrial and biomedical applications [1-3], By alternately connecting soft and hard segments together through urethane bonds, assorted PUs, such as poly(E-caprolactone) (PCL) containing block PUs [21], polylactide (PLA)-based PUs [22], and poly(E-caprolactone-co-lactide acid) (PCLA)-based PUs [23], were prepared with useful shape-memory properties. 

This phenomenon was also observed in the random block PUs. The lower contact angle in the case of PU-a/i indicated a more hydrophilic surface and higher surface energy than the random series of PUs (Table 21.2). A possible mechanism may be that alternating structures enhance phase separation and allow more PEG to be located on the surface [21,34],... 

From the above discussion, it can be concluded that alternating block PU PUCL-flZZ-PEG displays the highest cell viability of rat glial cells. This suggests that alternating block PU (such as sample PUC20-a-E4) would be an ideal class of PUs with... 

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]. 

Structurally, transparent PUs are —(A—B) — block copolymers containing alternating soft segments (A), with flexible chains of low glass transition temperatures, and hard, polar blocks (B). Linear polyols from the soft segments, diisocyanates the hard blocks and the two are linked or chain-extended, preferably with a linear diol. Diamine chain extenders usually lead to coloured urethanes which darken further on exposure to light. 

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