Effects of pseudo-network on physical properties of polyurethane

Summary

Temperature dependent properties of some segmented copolyurethanes were compared with those of soft- and hard-segment polyurethanes.

To understand the chemical nature of associated phase more explicitly, far-infrared spectral behaviors were precisely analyzed.

Mid-infrared frequency shift of NH-stretching vibration of associated carbonamide group was taken as a measure of the strength of hydrogen bond.

Dynamic viscoelastic response as well as flow property estimated from viscoelastic shift factor were compared with the infrared spectral behaviors.

A strong intersegmental carbonamide association was found through a preferential “solvation” of —(CH₂)₆NHCOO— group in the soft segment derived from poly-butyleneglycoladipate (PBA) and hexamethylene diisocyanate (HMDI), to the same group in the hard segment derived from butanediol (BD) and HMDI, yielding a thermoplastic copolyurethane, (PBA + BD) —HMDI, with high activation energy of segmental flow.

While, a unique aromatic association was observed in copolyurethane derived from diphenylmethane diisocyanate (MDI). A weakening of hydrogen bond in hard segment by aromatic cohesion of soft segment was observed from mid-infrared measurements, which gave a typical thermoelastic copolyurethane with moderate activation energy of flow. The same effect of weakening of hydrogen bond was observed by the replacement of polyester soft segment by less polar polyether, which gave also thermoelasticity.

Polyurethanes derived from toluene diisocyanate (TDI) and xylylene diisocyanate (XDI) did not show the aromatic association of MDI type.

The mid-infrared stretching vibration and farinfrared out-of-plane deformation vibration of associated NH-group were found to correlate with hydrogen bond- andvan der Waals-crystals having been assigned from (200) and (002) interferences, respectively, in X-ray scattering measurements byKilian et al.