Hard-segment phase domain

Note Hard-segment phase domains are typically of 2-15 nm linear size. 

Note Soft-segment phase domains are often larger than hard-segment phase domains and are often continuous. 

Note The types of domain in a segmented copolymer usually comprise hard- and soft-segment phase domains. 

Figure 1. Molecular schematics of some possible segmented polyurethane morphologies. For convenience, good phase separation is assumed, (a) Hard-segment domains dispersed within a soft-segment matrix (b) interconnected hard-segment phase giving rise to continuous phases of both hard and soft segments (c) soft-segment phase is dispersed within a continuous matrix of hard segments.

Sometimes, copolymers with many, but short blocks, are named segmented copolymers (multiblock copolymers). Segmented copolymers contain phase domains of microscopic or smaller size. The domains constituted principally of single types of stmctural unit. Typically, a segmented copolymer comprises hard-and soft-segment phase domains. 

Figure 3 Schematic representation of the structure of the segmented polyurethane chain (a), association of hard segments into domains of globular morphology (b) and co-continuous soft and hard phase morphology (c).

Fig. 1. Schematic representation of the segregation and domain structure of segmented PU elastomers a random hard segment segregation b fractionation after hard segment length c crystalline structure of hard segment phase as proposed by Black-well and Hespe. ...

The pseudocross-links, generated by the hard-segment interactions, are reversed by heating or dissolution. Without the domain crystallinity, thermoplastic polyurethanes would lack elastic character and be more gum-like in nature. In view of the outlined morphology, it is not surprising that many products develop their ultimate properties only on curing at elevated temperature, which allows the soft- and hard-phase segments to separate. 

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. 

PU elastomers contain alternative soft and hard segments, which separate into different phases. Hard domains play a role of cross-links, whereas soft blocks provide extensibility. Therefore, morphology and properties of PU are defined by relative amount of soft and hard segments. For example, at 70% concentration of soft segments, the material is described as a mbbery matrix with... 

Reaction-induced phase separation is certainly also the reason for which an inhomogeneous structure is observed for photocured polyurethane acrylate networks based on polypropylene oxide (Barbeau et al., 1999). TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 nm, mostly constituted by clusters of small hard units (the diacrylated diisocyanate) connected by polyacrylate chains. In addition, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements. Post-reaction increases the crosslink density inside the hard domains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates the presence of a two-phase structure. 

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