Infrared Dichroic Studies of Phase Separation and Orientation of Dibenzyl Hard Segments in Polyurethane

Abstract

Polyurethanes (PUs) based on diethylene glycol (DEG) and a new isocyanate of variable geometry, 4,4^'-dibenzyl diisocyanate (DBDI), were synthesized by the standard two-step procedure. A macro-diol, hydroxyl-terminated poly(ethylene adipate) (PEA) M _W = 2000 50 (PEA₂₀₀₀ ) was also used in the PU synthesis. The ability of the two benzene rings in the DBDI unit to rotate in two discrete “sin” and “anti” positions around the —CH₂CH₂— ethylene bridge enables a specific coplanar packing structure of the hard domains dibenzyl-based polymers. Molecular orientation of the DBDI-based hard segments and soft segments was monitored by means of infrared dichroic experiments. The effect of the strain history on the orientation of macromolecular chains was studied under uniaxial extension and in cyclical deformation experiments. The high modulus domains derived from the DBDI unit containing the aromatic urethane blocks served to reinforce the rubbery prepolymer phase. Since the two (soft and hard) phases are interpenetrating, both were deformed by the applied stress. When the load was removed, the soft domains relaxed to a nearly isotropic state, while the rigid phase remained plastically deformed.