The physical crosslinking of polyurethane elastomers studied by X-ray investigation of model urethanes

Polyurethane elastomers extended with aliphatic glycols or diamines, show a characteristic fluctuation in their thermal and hydrothermal properties which is dependent on the number of CH₂ groups in the chain extender (even or odd).The causes of this behavior are examined in the following report using X-ray crystallographic analyses of single crystals of model urethanes. These model compounds were prepared by reaction between diphenylmethane -4-mono-isocyanate and glycoles of the HO-(CH₂)_n-OH structure withn=2 to 6.Whilst strain-free hydrogen bonds can form between neighboring molecules in urethanes with even chain extenders, significant strains occur in urethanes containing odd chain extenders which result in reduced stability of the physical crosslinking system.In memoriam to Prof. Dr. Dr. h. c. mult. Otto Bayer.