Synergistic effect of organomodification and isocyanate grafting of layered double hydroxide in reinforcing properties of polyurethane nanocomposites

Abstract

The present work offers a novel approach for interlamellar surface modification of the hydroxyl groups of pristine and organomodified (dodecyl sulfate, DS and stearate, St) layered double hydroxides (LDHs) by grafting with isocyanate group of methylene diphenylene diisocyanate (MDI) and isophorone diisocyanate (IPDI). FTIR and ¹H NMR studies confirm the grafting of MDI and IPDI with LDHs. This has been further supported by XRD studies, which show a substantial increase in the interlayer spacings of isocyanate grafted DS and St-LDHs. The extent of grafting (wt%) has also been estimated on the basis of the residue left in air atmosphere at 1000 °C in thermogravimetric analysis (TGA). The synergistic effect of organomodification and isocyanate grafting of layered double hydroxide in reinforcing the properties of the polyurethane (PU) nanocomposites has also been investigated and the findings have been compared with PU nanocomposites obtained by individually grafted as well as organomodified LDHs. TEM and AFM studies establish better homogeneous dispersion and exfoliation of MDI grafted St-LDH in PU matrix as compared to the other isocyanate grafted LDHs. Among all the LDHs/PU nanocomposites, 3 wt% loaded MDI-grafted-St-LDH/PU exhibits the maximum improvements in tensile strength (391%), thermal stability (26 °C) and glass transition temperature (11 °C) and significant synergistic effect of organomodification and isocyanate grafting of LDHs. The enhancement in the properties of all grafted organomodified LDHs/PU nanocomposites is attributed to their nanolevel dispersion and interfacial interaction with PU.