Rheology of Surface Films. IV. Viscoelastic Properties of 6-Nylon Films at Air/Water Interface

Abstract (1) Using a surface-rheometer, the straintime curves at constant shear stress, the stress-relaxation curves at constant strain, and the stress-strain curves at constant rate of strain were determined with the spread films of 6-nylon at air/water interface, and the rheological data were measured in relation to their dependence upon stress, temperature, and molecular weight of the sample under varying compressions, and were arrayed in the light of the mechanical models. (2) It was found that the rheological characteristics of the film were classified into two types according to the degree of compression, i.e., a simple elastic type at large areas which involves a Hookean elasticity alone, and a viscoelastic type at considerably small areas, which is characterized by an instantaneous elasticity, the delayed. elasticity, and stationary flow under constant stress. (3) The stress-strain curves at constant rate of strain revealed that the film, which was simply brittle at large areas transformed to tough as the compression proceeded, the mechanical properties resembling to that in bulk. (4) It was also observed that the film strength, which was readily set up after compression, was considerably improved by the rhythmic strains applied to it, indicating the two-dimensional rheopexy. (5) Instantaneous elastic modulus versus-area curve showed a conspicuous plateau at the areas from about 33 to 24 Å2/residue, which suggests that at least two different phases are experienced during the process of compression. (6) Temperature dependence of the instantaneous elasticity suggests that the elasticity is mainly due to the energy effect rather than entropy effect. (7) Molecular weight variations of the film elasticity and of the gelation area were examined together with their pH dependence of the substratum, and discussions were made in molecular terms. (8) The stationary flow under constant stress behaved as a plastic flow, which is characterized by a yield stress above which flow practically begins to occur. Apparent activation energy of flow was estimated as about 6 kcal./mol. (9) Overall forms of the strain-time curves and of the stress-relaxation ones were found to be well fitted by the mechanical models of six parameters. (10) From the temperature dependence of the relaxation times, it was suggested that at least two different modes of relaxation mechanism do exist within the films of high compression.