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.
The structure of the hard domains of polyurethanes based on 4,4′‐trans,trans‐dicyclohexylmethane diisocyanate (HMDI), with 1,4‐butandiol (BDO) as the chain extender, has been derived by X‐ray diffraction and molecular modeling. X‐ray diffraction patterns of drawn annealed films of HMDI/BDO/poly(tetramethylene adipate) elastomers contain crystalline hard domains: the unit cell is triclinic and contains two chains, each of which has two monomers repeating in c=37.5 Å. A number of chain conformations are compatible with this repeat, but these are reduced to two by the requirement that all the urethane groups should form hydrogen bonds. We have compared packing models for these two options in terms of the agreement between the simulated diffraction patterns for arrays of chain segments. The best agreement is obtained for a model in which the chain extender region has the gaucheplus‐trans‐gaucheminus conformation. Identical chains are linked in sheets by C=O···H−N hydrogen bonds along the α axis of the unit cell. The second chain is rotated by 180° about c relative to the first and positioned at 0,b/2. The crystallographic R‐values for this model are 0.196 (observed data only) and 0.240 (observed plus unobserved data), which were much lower than those for the alternative models, and represent excellent agreement for a structure that is not fully refined. The hydrogen bonds have very similar geometry to those for diphenylmethane (MDI)/diol hard segments. Thus the higher melting point and other improved properties for HMDI‐based polyurethanes are most likely due to stronger interactions between the stacked cyclohexane rings as compared to those between the phenylenes in MDI‐based elastomers.
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