Infrared thermal analysis of polyurethane block polymers

Abstract

Infrared (IR) thermal analysis was used to study the extent, distribution, and the thermal behavior of hydorgen bonding in MDI-based segmented polyurethane elastomers. Two series of polyurethane elastomers were studied; one was based on a polytetrahydrofuran soft segment (ET series), while the second was based on a poly(tetramethylene adipate) soft segment (ES series). The hard segment in these materials was formed form p, p′-diphenylmethane diisocyanate (MDI) extended with butanediol. Bonded and free NH and C=O infrared absorbances were resolved as a function of temperature using a nonlinear least squares analysis for the fitting of Gaussian curve shapes. Results of this analysis revealed that at room temperature, about 80% of the NH groups in the ET and ES polyurethanes formed hydrogen bonds. In the ET polyurethanes, about 65% of the urethane carbonyls were hydrogen-bonded at room temperature. The extent of interurethane hydrogen bonding was found to be higher in the materials having better phase separation. Hard to soft segment hydrogen bonding was found to be weaker and was shown to dissociate first when the polyurethanes were heated from room temperature. A measurable amount of hydrogen bonding (∼35–40%) was found to remain at temperatures as high as 200°. The ΔH of hydrogen bond dissociation calculated from the Van't Hoff equation was used as an indication of hard domain perfection. Hard domain perfection was found to increase with increasing hard and soft segment length in both the ET and ES polyurethanes. Trends of soft segment T_g and ΔH suggest a dispersion of hard segments in the soft segment phase as well as a dispersion of soft segment material in the hard segment domain.