Viscoelasticity and stability of nylon 66 Langmuir films

The viscoelastic behavior and stability of poly(hexamethylene adipamide) (nylon 66) at the air-water interface is discussed. A recoverable isotherm was observed in the pressure region between 0.39 and 0.71 mN/m by a re-compression hysteresis test. In the specific pressure region, the film, tested by a constant-rate compression method and another method based on time dependence of pressure decay at constant area, was stable and showed viscoelastic deformation. The viscoelasticity could be satisfactorily described by a three-parameter standard linear solid model. As the film was compressed near the transition point of surface pressure-surface area isotherm and below the pressure of about 3 mN/m, the behavior of pressure decay against time revealed film instability, which was associated with the aggregation of polymer segments and the formation of microfibrils. In the low compressibility region with the pressure larger than 3 mN/m, the instability was associated with the formation and aggregation of microfibrils.