Effect of montmorillonite content in nanocomposites of segmented polyurethanes with poly(2-alkyl-2-oxazoline) sequences

A series of polyurethane/montmorillonite (PU/MMT) nanocomposites, based on poly(tetramethyleneglycol) (PTMG), alpha,omega-dihydroxylated poly(2-alkyl-2-oxazoline) (PROZO), [ 4,4'-methylenebis-(phenyl isocyanate)] (MDI) or [4,4'-methylenebis-(cyclohexyl isocyanate)] (H12MDI) and montmorillonite (MMT), were investigated. The amount of added clay ranged from 3 up to 10% by weight. The influence of the MMT weight fraction in the polymer matrix on the morphology and thermal, dynamic mechanical and tensile properties was characterized by scanning electron microscopy, X-ray diffraction analysis, Fourier-transform infrared spectrophotometry, differential scanning callorimetry, thermogravimetric analysis and dynamic-mechanical analysis. The results of the investigations were used to rank the nanocomposites according to their properties in relation to the polyurethane matrix structure and MMT content. It was evidenced that the silicate layers were preferentially, but not exclusively, attracted to the hydrophilic polar PROZO domains, their level of dispersion being controlled through specific intermolecular interactions and the filler loading amount. Introducing clay in the polyurethane matrix resulted in an increase of both tensile strength and thermal stability. Their non-monotonic dependence on clay content was explained by considering that concomitant and contrasting effects develop as the clay amount increases: an improvement of the filler-matrix interactions and a reduction of the matrix labile physical crosslinking degree by hydrogen bonding coupled with a lower dispersion homogeneity. For high inorganic filler amount aggregated clay particles together with regions with mixture of exfoliated and intercalated morphologies were revealed.