Polymer soft segment

Figure 11. Schematic diagram of the proposed morphology of polyether ester copolymers A = crystalline domain B = junction area of crystalline lamella C = polymer hard segment that has not crystallized D = polymer soft segment...

The mechanism for in vivo free LCFA deposition onto the polyurethane surface may result from a competitive desorption of albumin-bound LCFA s onto the lipophilic polymer soft segments. Albumin can carry upwards of 36 LCFA molecules (22), and Biomer has been shown to adsorb preferentially albumin during blood contact. 

The hydroxyl groups can be esterified normally the interesting diacrylate monomer (80) and the biologicaky active haloacetates (81) have been prepared in this manner. Reactions with dibasic acids have given polymers capable of being cross-linked (82) or suitable for use as soft segments in polyurethanes (83). Polycarbamic esters are obtained by treatment with a diisocyanate (84) or via the bischloroformate (85). 

One partieular form of thermoplastic polyurethane elastomers is the elastic fibre known as spandex fibre. Like the usual thermoplastic rubbers these materials consist of hard and soft segments but to qualify for the term spandex by the US Federal Trade Commission the polymer used should contain at least 85% of segmented polyurethane. The first commercial material of this type was introduced by Du Pont in 1958 (Lycra). Several other similar materials have since been introduced including Dorlastan (Bayer), Spanzelle (Courtaulds) and Vyrene (US Rubber). 

As previously mentioned, some urethanes can biodegrade easily by hydrolysis, while others are very resistant to hydrolysis. The purpose of this section is to provide some guidelines to aid the scientist in designing the desired hydrolytic stability of the urethane adhesive. For hydrolysis of a urethane to occur, water must diffuse into the bulk polymer, followed by hydrolysis of the weak link within the urethane adhesive. The two most common sites of attack are the urethane soft segment (polyol) and/or the urethane linkages. Urethanes made from PPG polyols, PTMEG, and poly(butadiene) polyols all have a backbone inherently resistant to hydrolysis. They are usually the first choice for adhesives that will be exposed to moisture. Polyester polyols and polycarbonates may be prone to hydrolytic attack, but this problem can be controlled to some degree by the proper choice of polyol. 

Linear chain polymers with repeating sequences of hard and soft segments Possibility of formation of liquid crystal polymers and thermoplastic elastomers... 

The results also indicate that there is a significant descrease in the chemical shift anisotropy in going from the segmented polymer B (which contains very few soft segments, 0(CH2)4 to the polymer C (which contains 6 times more soft segments). The difference also seems to reflect increased molecular motion of the phenyl rings in the softer of the two segmented polymers. A similar conclusion may be drawn from the Tl-values, which for polymer B is 3 s. as oposed to 0.25 s. for the C polymer. 

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