Exploring Urea Phase Connectivity in Molded Flexible Polyurethane Foam Formulations Using LiBr as a Probe

Abstract

Lithium bromide (LiBr) was incorporated in formulations based on molded flexible polyurethane foams in order to alter systematically the phase separation behavior and thus give insight into urea phase connectivity. The formulations of the materials generated were similar to those of molded flexible polyurethane foams except that a surfactant and a low molecular weight cross-linking agent (such as diethanol amine) were not used. The resulting materials were evaluated by using the techniques of atomic force microscopy (AFM), SAXS, and DSC. Atomic force microscopy and SAXS were used to demonstrate that the materials with and without LiBr were microphase separated and possessed average interdomain spacings of ca. 90 Å, typical of flexible polyurethane foams. AFM phase images also showed that incorporation of LiBr reduced the urea phase aggregation, which is known to take place in flexible polyurethane foams, and led to a more homogeneous distribution of the urea microdomains in the soft polyol phase. Addition of LiBr also decreased the regularity in segmental packing of the hard segments, as was noted by using WAXS. DSC scans revealed that inspite of the significantly different morphologies noted for the samples with different LiBr contents, soft-segment mobility, as reflected by its T_g, remained unaffected on including this additive at the concentrations investigated. Interaction of LiBr with diethyl ether (DEE), 1,3-dimethylurea (DMU) and a model urethane (1,3-dimethylcarbamate) (URET) also was estimated by quantum mechanical calculations (QMC) by using density functional theory (DFT). For DMU and URET, QMC indicated a strong interaction of the Li⁺ ion with ether and carbonyl oxygens, and of the Br⁻ with the (N–H) protons. Interaction energies of Li⁺ with DEE (O… · ·Li⁺), URET (C = O… · ·Li⁺), and DMU (C = O… · ·Li⁺) complexes were calculated to be –201, –239 and –272 kJ/mole, respectively. Interaction energies of DEE, URET, and DMU complexes with LiBr were calculated to be –198, –620, and –691 kJ/mole, respectively, suggesting that the interaction of the salt was predominantly with the hard segments (urea and urethane) and not with the polyether soft segments.