The Structure of the Hard Segments in MDI/diol/PTMA Polyurethane Elastomers

Abstract We are using x-ray methods to determine the structure of the poly(MDI-diol) hard domains in polyurethane elastomers, in order to understand the effect of chemical structure on the crystalline order and extent of phase separation. Oriented films containing approximately 50% hard segments have been obtained for preparations using butandiol (BDO), propandiol (PDO), and ethylene glycol (EDO) as the chain extender; the soft segment is polytetramethylene adipate (M n ≃ 2089). Poly (MDI-BDO) is the most crystalline, and has a triclinic unit cell with a tilted base plane such that there is a staggering of adjacent chains in the crystal structure. In contrast, poly(MDI-PDO) and poly(MDI-EDO) have monoclinic unit cells with adjacent chains in register. In order to develop molecular models for these polymers we have used single crystal x-ray methods to determine the structures of methanol- and butandiol-capped MDI. Based on the data from these structures, conformational analysis has been used to predict the likely chain conformations for the polymers. A model has been developed for poly(MDI-BDO) in which fully extended chains are staggered to give a hydrogen bonding network in two dimensions perpendicular to the chain axis. In contrast poly(MDI-PDO) and poly(MDI-EDO) crystallize in higher energy contracted conformations, which are necessary for the non-staggered packing of the chains. This difference may explain the higher crystalline perfection of the hard segments in the butandiol preparation.