effects of these factors on strength and extensibility of polyurethanes will be
based on available literature data.
The effect of soft segment concentration on tensile strength is not straightforward.
Usually a maximum is observed at a SSC between 40% and 60%,
depending on the structure and morphology. Figure 36 shows the dependence
of tensile strength on SSC of three series of MDI/BD/PTMO based polyurethanes.
98 Series A is based on PTMO having/Qn = 650, series B on PTMO
having Mn = 1000 and series C on PTMO with )l~r = 2000. The soft segments
based on PTMO 650 and PTMO 1000 do not crystallize while that based on
PTMO 2000 does. The maximum of the tensile strength vs SSC curve for the A
series was obtained at 50% SSC while for the B and C series the maximum lies
at about 40% SSC. This suggests that lamellar morphology may be responsible
for high strength. Soft segment length appears to be important only at high SSC,
where the hard phase is no longer continuous. In polymers with longer soft
segments, hard segments are longer at the same SSC. This is the reason for the
shift of the phase inversion to higher SSC with the C series compared with the
other two. This fact, combined with the crystallizability of the soft segment,
makes polyurethanes at high SSC in the C series superior to polyurethanes
based on shorter soft segments. In fact, at 60°/0 and 70% SSC, polyurethanes