Structure Development and Interfacial Interactions in Flexible Polyurethane Foam-Layered Silicate Nanocomposites

Unmodified Na-montmorillonite (MMT) was swollen in a polyol/water mixture using an ultrasound technique. Polyurethane (PU) foam nanocomposites were formed via reaction of these polyol/water/Na-MMT mixtures with toluene diisocyanate (TDI). Forced-adiabatic attenuated total reflectance FTIR spectroscopy was used to determine the kinetics of both the PU copolymerisation and of the microphase separation between poly(ether-urethane) soft segments and polyurea hard segments. Consumption of TDI during the initial stages of the copolymerisation was accelerated significantly by the addition of ≤10 wt% Na-MMT. The initial rate of formation of urea groups also increased significantly upon addition of Na-MMT, but at reaction times >100 s a significant retardation occurred in the development of hydrogen bonding within the urea groups of the hard-segment phase that was recovered only after 1000 s. The reasons for this extensive disruption in structure development were investigated using flow microcalorimetry (FMC), diffuse reflectance Fourier-transform infrared spectroscopy (DRIFTS), and wide angle X-ray scattering (WAXS) to monitor the adsorption process and any chemical reaction between hydrated Na-MMT and a model monoisocyanate; 4-ethylphenyl isocyanate (4-EPI). DRIFTS spectra of 4-EPI adsorbed on Na-MMT revealed urea groups, indicating formation of N,N′-bis(4-ethylphenyl) urea. FMC indicated that a significant quantity of this urea formed at the surface and then desorbed. In addition, DRIFTS spectra indicated that the 4-EPI reacted with hydroxyl groups present at the edges of the silicate platelets to form urethane linkages. Thus, in a PU-foam reaction mixture, the water will tend to associate with the Na-MMT, either within the galleries or on the surfaces of silicate lamellae. Upon reaction with isocyanate, the presence of the Na-MMT both promotes the formation of urea and generates urethane linkages between silicate lamellae and the polyurethane.