MODELING THE STRUCTURE OF THE HARD DOMAINS IN HMDI-BASED POLYURETHANEELASTOMERS

The structure of the hard domains of polyurethanes based on 4,4'-trans ,trans-dicyclohexylmethane diisocyanate (HMDI), with 1,4-butandiol (BD O) as the chain extender, has been derived by X-ray diffraction and mo lecular modeling. X-ray diffraction patterns of drawn annealed films o f HMDI/BDO/poly(tetramethylene adipate) elastomers contain crystalline hard domains: the unit cell is triclinic and contains two chains, eac h of which has two monomers repeating in c = 37.5 Angstrom. A number o f chain conformations are compatible with this repeat, but these are r educed to two by the requirement that all the urethane groups should f orm hydrogen bonds. We have compared packing models for these two opti ons in terms of the agreement between the simulated diffraction patter ns for arrays of chain segments. The best agreement is obtained for a model in which the chain extender region has the gauche(+)-trans-gauch e(-) conformation. Identical chains are linked in sheets by C=O ... H- N hydrogen bonds along the a axis of the unit cell. The second chain i s rotated by 180 degrees about c relative to the first and positioned at 0,b/2. The crystallographic R-values for this model are 0.196 (obse rved data only) and 0.240 (observed plus unobserved data), which were much lower than those for the alternative models, and represent excell ent agreement for a structure that is not fully refined. The hydrogen bonds have very similar geometry to those for diphenylmethane (MDI)/di ol hard segments. Thus the higher melting point and other improved pro perties for HMDI-based polyurethanes are most likely due to stronger i nteractions between the stacked cyclohexane rings as compared to those between the phenylenes in MDI-based elastomers.