Hard segments

Chain extenders are usually low molecular weight symmetrical diols or diamines. Chain extenders react with isocyanates in the same way as polyols do, but because they are low molecular weight, a high concentration of hydrogen-bonded molecules can associate and phase out of the polyol to form plastic-like domains called hard segments . Hard segments will be discussed in Section 4. Some of the more common diol and diamine chain extenders are shown in Table 3. 

Soft segment Hard segment Hard segment... 

Again the relative concentration of soft segment/hard segment and silicone were calculated from Figures lb and 2b, and the results are summarized in Table I. Now the concentration of the soft segment is increased approximately 50% in the air-facing side when compared with the case where the depth of the penetration was deeper. More significant... 

C0NHC6H5CH2C6H5NHC00(C4H80)m-]n-[C0NHC6H5CH2C6H5NHC00C4H80]p Soft segment Hard segment... 

Soft polyol segment—Hard PUR segment—Soft polyol segment... 

Earlier, we reported that complexes of lithium bromide (LiBr) with copolyether-urethane-ureas led to the formation of hydrogels (6). The water absorption curves for these hydrogels indicated the existence of two modes of absorption an initial water absorption that depended on the concentration ratio of LiBr to the urethane-urea segment (hard segment) of the block copolymer, and the water uptake at higher salt concentration which was attributed to the formation of voids in the film. 

The different terms represent segment-segment hard spheres interactions, a, mean field contribution, permanent bond energy between segments in the chain, a "", and free energy of specific hydrogen bond interactions between associating sites, a ° , if any. 

The reaction between these three components leads to the formation of segmented copolymers characterised by the alternation of hard and soft segments. Hard segments are based on blocks formed by reaction of the di- or multi-functional isocyanate with the chain extender, while the polyol-based units form the soft segments. As a result of the thermodynamic incompatibility between hard and soft segments, PURs are characterised by a biphasic morphology (GunatiUake et al., 2011) (Fig. 6.2). 

For a binary blend of thread homopolymers the structure of each chain is specified by its statistical segment length, ctm, segmental hard core diameter, Am = d, degree of polymerization, Nm, and the attractive tail potentials, VMM (r) The structural asymmetry on an equal volume basis is given by the ratio... 

Sample Soft Segment Hard Segm. Content Modulus Uftimate Strength Elong. at Break... 

Soft segments Hard segments Soft segments Hard segments ... 

Figure 2.4. Schematic representation of segmented PU formation as a result of polymerization reaction. Soft segment-hard segment linkages occur via urethane reaction (bottom left). Linkages within the hard segment occur via urethane reaction in the case of EDO and urea reaction (bottom right) in the case of DETDA and water chain extenders. In the case of water chain extension, urea formation also involves the release of one CO2 molecule for each urea group formed [33]...

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