Gradient composites of alkaline poly(6-hexanelactam) with graphite:: One-step synthesis, structure, and mechanical properties

In a one-step synthesis of gradient composites, molten monomer of 6-hexanelactam was mixed with graphite (5 wt.-%) and alkaline polymerization was performed under quasi-isothermal conditions at about 170 degreesC. The following initiator/activator system insensitive to traces of water and other low-molecular-weight compounds adsorbed on filler surface was used: The sodium salt was prepared through the reaction of sodium dicaprolactamobis(2-methoxyethoxo)aluminate with 6-hexane-lactam; N-acyllactam was formed in situ in molten monomer by solving flexible or rigid polyurethane foam consisting of either toluene diisocyanate or diphenylmethane-4,4'-diisocynate, and a poly(propylene oxide) based polyol. To obtain gradient composites with a compositional variation between plane-parallel surfaces, the incorporated filler underwent sedimentation due to gravity during initial stages of polymerization. The graphite-free surface is suitable for treatment with adhesives, while the graphite-rich surface layer (containing about 11 wt.-% of graphite) possesses improved friction characteristics. Graphite slightly (i) reduces the polymer yield and the mean spherulite diameter; (ii) increases the crystallinity due to its nucleation activity; (iii) decreases the compliance, but does not affect its time dependence given by the matrix and (iv) reduces the yield strength, tensile strength and elongation at break. The friction coefficient of the graphite-rich surface is reduced to almost 50% of that found for the graphite-free surface; composites with cross-linked matrix also show better wear properties.