Effects of Water and Temperature on Conformational Order in Model Nylon Thin Films

Abstract

Nylon 6,6 thin films have been examined by Raman spectroscopy to determine how chain conformation is influenced by environmental parameters such as exposure to water and temperature variations. The motivation for this work is to elucidate how interactions between water and the model polymers mediate polymer structures in applications such as the removal of salt from water in reverse osmosis membranes. Raman spectra show that model self-assembled monolayers containing Nylon 6,6 chains are semicrystalline under ambient conditions. The native chains adopt an unusual kinked and folded conformation related to that found in γ-Nylon 6,6. The regular chain deformations allow adjacent tethered chains to maximize hydrogen-bonding between neighboring amide groups under the constraints imposed by surface tethering. With increasing temperature, the films undergo a phase transition associated with the disruption of hydrogen bonds leading to structures containing more linear regions that are closer to the “all trans” case. Similar structural changes are observed on exposing the Nylon films to water. The salt content of the water does not appear to have a significant impact on the structure or phase transition in the Nylon 6,6. These results suggest that while inclusion of water has a profound effect on the polymer structure, either salt is excluded from the polymer or there is sufficient free volume within the films to accommodate hydrated ions without inducing further structural changes.