Polymer, Vol.44, No.3, 757-768, 2003
Influence of lithium chloride on the morphology of flexible slabstock polyurethane foams and their plaque counterparts
In continuing efforts to understand urea phase connectivity in flexible polyurethane foams and its implications on physical properties, LiCl is used to alter the phase-separation behavior of slabstock foams. Comparisons are also drawn with plaque counterparts, which are prepared using the same polyol, isocyanate, and chain extender (water). LiCl is shown to alter the solid-state phase separation behavior of the foams and the plaques in a similar manner. This is confirmed using multiple characterization techniques, which provide information at different scale lengths. The foams and plaques with and without LiCl are shown to possess a microphase separated morphology with interdomain spacings of ca. 100 Angstrom. SAXS and TEM reveal that addition of LiCl reduces the urea aggregation behavior, typical in slabstock polyurethane foams, leading to a loss in the urea phase macro connectivity. Hard segment ordering, as studied by WAXS and FTIR, is shown to be of a similar nature in the plaque and foam, which do not incorporate LiCl. Addition of LiCl leads to a loss in the segmental packing behavior, or micro level connectivity of the urea phase, in both the plaques and corresponding foams, as inferred from WAXS and FTIR. The LiCl additive interacts with the polyol soft segments in an insignificant manner as shown from FTIR and DMA. In addition, foams containing LiCl are found to possess more intact cell windows due to the influence of LiCl on reaction kinetics as well as its effect on the precipitation of the urea phase. The experimental observations are supported by quantum mechanical calculations using a density functional theory approach, where molecular interactions between LiCl and model ether, urethane, and urea compounds are investigated. Interaction geometries of most stable complexes and their stability energies are calculated. Stability energies of ether/LiCl, urethane/LiCl, and urea/LiCl were determined to be - 189, - 617, and - 687 kJ/mol, respectively, reinforcing that LiCl interacts predominantly with urea hard segments and in a minimal manner with the polyol soft segments.