Thermoreversible Cross-Linking of Maleated Ethylene/Propylene Copolymers Using Hydrogen-Bonding and Ionic Interactions

Abstract

Maleated ethylene/propylene copolymers (MAn-g-EPM) were thermoreversibly cross-linked via a reaction with primary alkylamines of different length, either with an equimolar amount to obtain the amide−acid or with an excess to obtain the amide−salt, which was confirmed using Fourier transform infrared (FTIR) spectroscopy. Small-angle X-ray scattering (SAXS) experiments showed the presence of microphase-separated aggregates for both the starting MAn-g-EPM and all alkylamide−acids and −salts, which act as physical cross-links. The materials could easily be remolded at 80 °C into homogeneous films for several times without changes in the FTIR spectra, indicating that the (physical) cross-links are truly reversible. The tensile properties and elasticity were improved by converting MAn-g-EPM to the amide−acids due to hydrogen bonding and even further by converting the amide−acids to the amide−salts due to additional ionic interactions besides hydrogen bonding. Better tensile properties and elasticity were observed for the octadecylamine, which was explained by packing of the long alkyl tails in a crystalline-like order. Apparently, this packing is rather ill-defined, since a scattering peak could only be observed in the SAXS patterns of material modified with a large excess of octadecylamine after processing above 140 °C. Irreversible imide formation occurred for all amide−acids and amide−salts at high temperatures, resulting in disappearance of the aggregates and, hence, a dramatic decrease in mechanical properties. Therefore, these temperatures should be avoided during the (re)processing. Replacement of the excess of alkylamine with a different base, viz. potassium hydroxide, resulted in prevention of imide formation at high temperatures and a further improvement in mechanical properties.