Flexible siloxane segments

PAIs modified with a siloxane linkage have been synthesized under microwave radiation in ionic liquids by the isocyanate method [39]. 4,4 -Methylene-bis(4-phenylisocyanate) was used. The poly(amide-imide-siloxane)s were obtained in high yields and good inherent viscosities. The high softening temperatures and poor solubility of PAIs in organic solvents can be improved via the incorporation of the flexible siloxane segments into the polymer backbone. 

The molar mass of the polyesters depended on the reaction temperature, enzyme activity, enzyme concentration, and to a lower extent on the applied vacuum. For example, when the polyesters were synthesized using 1,4-butanediol, 1,6-hexanediol and 1,8-octanediol in the bulk at 70°C under reduced pressure for 24 hours, the weight average molecular weights obtained were 15,100 g moF, 16,000 g mol, and 16,700 g mof , respectively. Thermal analysis of these copolymers revealed no melting phenomenon, perhaps due to the presence of the bulky and flexible siloxane segments in... 

Once cured, PDMS networks are essentially made of dimethylsiloxane polymeric chains crosslinked with organic linkages. The general and inherent molecular properties of the PDMS polymers are therefore conferred to the silicone network. Low surface energy and flexibility of siloxane segments are two inherent properties very useful in adhesion technology. 

Hydrosilation reactions have been one of the earlier techniques utilized in the preparation of siloxane containing block copolymers 22,23). A major application of this method has been in the synthesis of polysiloxane-poly(alkylene oxide) block copolymers 23), which find extensive applications as emulsifiers and stabilizers, especially in the urethane foam formulations 23-43). These types of reactions are conducted between silane (Si H) terminated siloxane oligomers and olefinically terminated poly-(alkylene oxide) oligomers. Consequently the resulting system contains (Si—C) linkages between different segments. Earlier developments in the field have been reviewed 22, 23,43> Recently hydrosilation reactions have been used effectively by Ringsdorf 255) and Finkelmann 256) for the synthesis of various novel thermoplastic liquid crystalline copolymers where siloxanes have been utilized as flexible spacers. Introduction of flexible siloxanes also improved the processibility of these materials. 

These materials also have high thermal and oxidative stabiHty. Flexible segments such as amide siloxane can be incorporated into the imide-based stmcture for hot melt or injection appHcations. General Electric (GE) and Hoechst-Celanese are suppHers of these high performance plastics. 

Spurred on by discussions on whether the smectic layers of the fluorocarbon-substituted polymers XXIII-m-n are induced by the immiscibility between hydrocarbon and fluorocarbon segments or by the fact that fluorocarbon segments form rod-like mesogenic units by themselves, Pugh et al. synthesized a series of siloxane-terminated monomers XXIV-m-n, with m=l,2 and n=4-S [79]. In these monomers, the rigid fluorocarbon segment was replaced by short polydimethylsiloxane (PDMS) segments that are also immiscible with hydrocarbons but are very flexible (Fig. 15). 

SilOXaneS. Polysiloxanes are used as segments in block copolymers because of their low temperature flexibility, high gas permeabilities, excellent electrical properties, biocompatibility, and surface properties. 

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