SYNOPSISTwo series of segmented polyurethane elastomers based on 4,4'-diisocyanato dicyclohexylmethane were investigated with respect to their thermal properties and deformation behavior. We used a crystallizable soft segment, 1,4-poly( tetramethylene glycol), for one series and a noncrystallizable soft segment, 1,2-poly(propylene glycol), for the other. Both systems exhibited mechanical self-reinforcement that depended strongly on the deformation rate. We propose a mechanism for the observed stress/strain behavior in terms of two competing processes: ( 1 ) the buildup of orientation caused by deformation and ( 2 ) the loss of orientation caused by plastic slippage and segmental relaxation during deformation. The kinetics of these processes depend strongly on the deformation rate and overall molecular weight.
X-ray methods have been used to investigate the structure of the hard domains in polyurethane elastomers based on trans, trans-dicyclohexylmethane 4,4'-diisocyanate (HMDI) with butanediol as the chain extender. The crystalline hard segments become oriented on stretching the films at room temperature, although the degree of crystallinity remains relatively low. The ordering is improved considerably by annealing the films for several hours at 130-190°C: the fiber diagram contains 25 sharp Bragg reflections which are indexed bya triclinicunit cell with dimensions a=5.1 A, b= 10.2 A, c =37.5 A, a = 115.2", p = 84.9", and y =94.2". The cell contains dimer units of two chains which are probably staggered along the caxis direction in the bcplane. In the samples annealed for shorter periods of time, a second less abundant crystal structure is observed that coexists with the first. This second structure is also triclinic with a more extended fiber repeat of c= 41.3 A and is slowly converted to the contracted form on further annealing. Molecular models show that in the extended form the butanediol units have the all-trans conformation, whereas in the contracted form they probably have the tg'tg-t or tg-tg+t conformations, which have lower potential energies than the all-trans form.The chains in both the extended and the contracted conformations are able to form a network of intermolecular hydrogen bonds, and in this regard the structures are very similar to those reported previously for the analogous diphenylmethane diisocyanate (MD1)-based elastomers. Thus the higher melting points for the HMDI-based hard segments are most likely due to hydrophobic interactions between the dicyclohexyl methanes that are stronger than those between the diphenyl methanes of the MDI-based elastomers.
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