For any given polyurethane, soft segments and isocyanates may make up more than 95% of the polymer volume. Very often, a low-molecular-weight component is added that primarily serves as a spacer between neighboring poly isocyanates. Thus, the polymer structure will be, for instance, soft segments of 1000 or 2000
FIGURE 2.83 Ethylene glycol chain extender (in boxes) linking isocyanates through urethane bonds.
molecular weight and hard segment consisting of diisocyanates of 250g/mol (as in the case of 4,4' MDI) strung together with a short-chain diol or diamine of about l00g/mol. The low-molecular-weight diol or diamine stringing together diisocyanates is called the "chain extender." Figure 2.83 is a representation of this concept with an ethylene glycol chain extender. These chains of polyisocyanate and chain extender (ethylene glycol here) are termed the "hard segment." The end isocyanate units of the hard segments are implicitly connected to high-molecular-weight polyether or polyester soft segment polyols. The importance of this structural detail is that as the addition polymerization occurs, these hard segments, and their immiscibility with the soft segment, drive the phase structure of the resulting polyurethane (see Chapter 4) . The chain extender then has a significant effect on the kinetics and extent of phase separation in these systems. The chain extender can also have a significant effect on the strength of the hard segment phase (similar to a crystal-phase melting point) by virtue of the interphase hard segment hydrogen bonding (see Fig. 2.84) .
Thus, the chain extender structure can have a significant effect on polyurethane properties by its ability to drive phase separation, its ability to complement or interfere with a regular hard segment structure, and its ability to promote interhard segment hydrogen bonding [185-189]. Virtually any low-molecular-weight polyfunctional molecule capable of reacting with isocyanates has been explored for its influence on polyurethane properties. The most prevalent and industrially important chain extender in foams is water, which makes urea hard segment and 1,4-butanediol used commonly for elastomer hard segments. The urea units can be particularly robust due to the presence of the bidentate hydrogen bonding available to the interchain structure as seen in Figure 2.85. While the most common source of urea hard segment is obtained by the reaction of isocyanate and water, an alternative source is from the reaction of isocyanates and amines. Hydroxyl functional chain extenders can also serve as cross-linkers. In contrast, secondary amines offer a single isocyanate reaction site and the possibility of maintaining melt processability.
There are a huge number of potential chain extenders that can be employed to alter hard segment structure, polymerization kinetics, and polymer properties. Table 2.13 is a limited list sampling the numerous types and structural parameters of each. The potential effects on properties is covered in Chapter 4 and chapters associated with polymer properties.
FIGURE 2.84 Hydrogen bonding between chain extended urethane hard segments. Cohesive energy between hard segment chains is influenced by the chain extender, by the spacing between H-bonding interactions, and the regularity of the assembled chains into quasi-crystalline structures. Irregularities in hard segment structure can lead to a more nematic-like liquid-crystal structure.
FIGURE 2.85 Illustration of the origin of the greater stabilization of hard segments afforded by urea chain extension versus the urethane chain extension shown in Figure 2.83.
TABLE 2.13 Structure and properties of common chain extenders used in urethane chemistry