Structure–thermomechanical property correlation of moisture cured poly(urethane-urea)/clay nanocomposite coatings

Abstract

A series of poly(urethane-urea)/clay nanocomposite coatings were prepared by moisture curing of isophorone diisocyanate (IPDI) capped hydroxyl terminated polybutadiene (HTPB)/clay dispersions in a relative humidity (RH) of 50% at 25°C. The curing progress was studied by periodic measurement of gel fraction of the coating samples. The studies revealed tortuosity effects of clay toward moisture diffusion, thus delaying the induction period of gelation, time for complete cure and rate of gel formation of the nanocomposite coatings. The clay platelets were found to be intercalated in the poly(urethane-urea) matrix, evidenced from wide angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM). Effects of nanoclay on state of the hard and soft segments were investigated by WAXD, differential scanning calorimetry (DSC), temperature modulated DSC (MDSC) and solid-state nuclear magnetic resonance spectroscopy (NMR). WAXD studies revealed unusually ordered hard segment morphology of the moisture cured poly(urethane-urea) and its nanocomposites. Slower soft segment dynamics upon clay addition was evident from concentration dependant broadening of the line widths of the NMR peaks, and decreasing reversible heat capacity changes at soft segment glass transition. The volume fraction of immobilized soft segments of the nanocomposites was determined from MDSC and was found to increase linearly with clay loading. The mechanical property analysis showed simultaneous reinforcement and toughening effect of nanoclay on the MCPU matrix. The increment in mechanical property of the nanocomposites varied proportionately with the volume fraction of immobilized soft segments.