Hard blocks domains

The representation of hard-block domain structure shown in Scheme 4.8, implying rigid, crystallike molecular order, can be misleading because hard blocks are, at best, microcrystalline (as are soft blocks). Although microcrystallinity can be readily obtained, it requires careful selection of raw materials,... 

Halogenonitroaromatic compound, 295 Hard-block domain structure, 218-219 Hard-block microdomain, 215 Hard-block/soft-block phase separation, 220... 

Figure 10.83. Dependence of the softening temperature of hard block domains in SPU-2 vs. volume fraction of DBF (1) and TBP (2).

TBP corrrpletely ruptures hard block domains of rrrethane urea re rdless of their initial concerrtration of elastomer. Orrly SPU chemical network resists swelling by TBP. The eqrrilibrirrm swelling of SPU in DBP decreases with increase in concerrtration of hard blocks. As a result tensile strength of a material is decreased to a lesser degree. The lowest decrease in terrsile strength is observed in SPU-1 swollen to equilibrium in DBP. SPU-1 has 37% hard blocks (Figirre 10.80). 

Aliphatic isocyanates, as well as possessing superior light stability have also been seen to show increased phase separation behaviour over aromatic isocyanates. The T s of systems based upon HDI, H12MDI and IPDI are lower than that of TDI and MDI systems. This is attributed to stronger hydrogen bonding being obtained in the hard block domains. 

Only a low level of crosslinking is beneficial in polyurethane elastomers higher levels result in lowered physical properties due to disruption of the hard-block domain structure. Figure 3.6 shows the general influence of an increasing number of crosslinks on properties. 

Use of a copolymer polyol prevents this crystallization, but at the expense of the final physical properties of the PU. Alternatively, triol chain extenders may be used as they disrupt the hard block domain structure and so increase transparency, though again at the expense of physical properties. 

Figure 10.4 shows TM-AFM phase images for (a) MDFBD(27)/P[3F-r-ME3-1 1-(3.1)1 and (b) MDI/BD(32)/P[3F-(>-ME3-3 2-(4.2)]. The phase image of the polyurethane containing the random soft block is featureless (Figure 10.4a). This is consistent with a surface morphology where the random soft block predominates. With increased tapping force (AMq = 0.5-0.6), a phase-separated structure emerges (data not shown), reflecting the presence of near-surface hard block domains.

The elasticity of thermoplastic polyurethane rubbers (which are also known as thermoplastic urethanes or TPUs) is a function of their morphology which comprises hard and soft phases. The hard phases consist of hydrogen bonded clusters of chain segments, which are linked by flexible chain segments that make up the soft phase. The hard blocks, which are the minor phase, exist as separate domains within a continuous matrix of the majority soft phase, as shown schematically in Fig. 25.9. 

Thermoplastic tri-block copolymers are interesting since they possess novel properties different from those of the homo- or copolymers. The thermoplastic elastomers have many of the physical properties of rubbers, i.e., softness, resilience, and flexibility. The unique properties of this kind of copolymer are due to the microphase separation of the hard crystalline domains dispersed in a continuous amorphous matrix (Fig. 6). Such phase morphology provides a physical network of flexible chains cross-linked by crystalline microdomains. The advantages over natural vulcanized rubbers are that thermoplastic elastomers are readily soluble in an appropriate solvent and can be processed as thermoplastics [109],... 

If the hard blocks are longer than the soft ones, such as in SBS with a high styrene content, the hard phase will be continuous, and the rubbery phase is present as domains (see Figure 9.6). In such a case SBS behaves as a high-impact PS. Another example of this type is a PP/EP block copolymer tails of EP (random copolymer of ethylene and propylene) on the PP chains segregate into rubbery domains in the PP matrix, which improve the impact strength. 

Figure 13.2 Illustration of the network morphology of a microphase-separated triblock copolymer with the glassy end blocks in hard spherical domains bridged by the rubbery center blocks.

In many phase-separating block copolymers (especially segmented multiblock copolymers such as polyurethanes where the blocks are usually short), lowering the soft block Mn increases the Tg of the soft phase because of the crosslink-like topological constraints imposed by hard phase domains. 

---