Hard segment effect

With a typical of 25 000-30000 the molecular size is low compared wjth most conventional covalently cross-linked elastomers. With such rubbers values of about 100000 are desirable so that the effects of a significant amount of non-load-bearing chain ends do not occur. Such a problem does not arise in block copolymers terminated by hard segments. 

The structural effect of alkyl groups such as methyl, ethyl, and -butyl on the Rp is small. Alkyl 4-methyl-phenylcarbamate can be chosen as a model compound for the hard segment of poly(ether-urethane) (PEU). This group can initiate grafting reaction with Ce(IV) ion and the grafting site was proposed at the hard segment of PEU [3,15] as shown in Scheme (1). 

Miller J.A., Lin S.B., Hwang K.K.S., Wu K.S., Gibson P.E., and Cooper S.L. Properties of polyether-polyurethane block copolymers Effect of hard segment length distribution. Macromolecules, 18, 32, 1985. 

It is important to be aware of the chemical effects of isocyanates. The polynre-thanes you will develop will be combinations of polyols and isocyanates. The ratio of the two compounds will in pait dictate both the physical and chemical properties of the product. As a general rule, the isocyanates are hard segments that impart rigidity to the polymer. The polyol is the so-called soft segment. The various molecular weights (more correctly equivalent weights available in the form of polymeric MDIs) provide certain advantages. Table 2.2 lists a few commercially available polyisocyanates and their physical properties. 

While Table 3.3 deals with elastomers, it is important to mention that the effect of MDI illustrated in the table applies to MDI foams as well. Figure 3.7 shows the effect on the tensile strength of the polyurethane of increasing amounts of MDI. The increase in hard segments increases the brittleness but does not improve the strength of the polymer, as reflected in the elongation figures. The increase also exerts positive effects on the compression of the foam as noted in the next section. 

FIGURE 3.7 Effect of increase in hard segments on tensile strength of a polyurethane. 

Bulky side chains in the hard segment will tend to make the hydrogen bonding more difficult. The bulky side chains of Ethacure 300 will lower the hardness of a material by several points compared to that of a material cured with MOCA. The use of triols or macro diols in the curing phase will have a softening effect. If a trifuctional isocyanate or hydroxyl is used in the initial preparation of the prepolymers, this softening does not happen. 

At this point, the chains have not fully developed and the hard segments will have the joined chain at random lengths in the structure. This has the effect of increasing the viscosity of the mix as well as improving the compression set and swelling in solvents. This is important in very hard compounds. A trifuctional isocyanate such as Tolonate HDT (Rhone Poulenc) will do the same as TMP but with less decrease in dynamic properties. 

Yoshino M, Ito K, Kita H et al (2000) Effects of hard-segment polymers on C02/N2 gas-separation properties of polyethylene oxide)-segmented copolymers. J Polym Sci Part B Polym Phys 38(13) 1707-1715... 

Figure 3. Small-angle x-ray scattering intensity curves demonstrating the effect of soft-segment molecular weight for three polycaprolactone polyurethanes (MDI/BD) of approximately equal hard-segment content. (A) 57% by wt, FCL 2000-178 (S) (B) 53% by wt FCL 830-123 (S) and (C) 61% by wt, FCL 830-134 (S).

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