Water and ion transport in nylon as studied by NMR

Abstract

Nylon-6 in particular is used in the automotive sector and in electric/household devices. Penetration of moisture results in a loss of protective and mechanical properties. Ingress of ions also causes degradation of the material. To understand mechanical properties it is important to understand water and ion transport and distributions in nylon parts. In this thesis the water and ion transport in nylon 6 films is studied using Nuclear Magnetic Resonance imaging (NMR). The GARField approach is used to measure the distribution of mobile hydrogen nuclei in the film with a spatial resolution of 6 ¹m. To control the relative humidity and the temperature, this NMR setup is equipped with a homebuild climate-chamber. To measure moisture distributions, the NMR signal has to be calibrated. Since NMR measures the hydrogen density of mobile hydrogen atoms, the NMR signal will not only depend on the amount of water present but also on the plasticization of the polymer matrix. A non-linear relation between the signal and moisture content is obtained, which underlines the necessity for a proper signal calibration. This calibration converts signal intensity profiles into moisture content distributions or moisture content profiles. From the moisture content profiles the relation between the diffusion coefficient and the moisture content is calculated. The diffusion coefficient increases rapidly with increasing moisture content and the relation between the diffusion coefficient and the moisture content is highly non-linear. This highly non-linear relationship is also expressed in the sharp front of the water entering the nylon film. The sharp front represents the higher moisture contents as the NMR signal is insensitive to small amounts of moisture. This is because the NMR signal is the average over an area in lateral dimension. EIS is very sensitive for small amounts of water at the substrate and is commonly used to evaluate coating performance. EIS water uptake measurements on the same nylon film revealed that small traces of water precede the front as measured by NMR. With respect to detecting moisture contents EIS and NMR are complimentary. After a successful investigation and characterization of the moisture uptake process the plasticization of the polymer is studied. Plasticization highlights the changing mechanical properties as a result of the interaction with water. Polymer plasticization and polymer/water interactions are investigated by NMR relaxometry experiments. As water enters the polymer it breaks and replaces the hydrogen bonds between polymeric chains, thereby increasing the mobility and plasticizing the polymer matrix. In NMR relaxometry a relaxation decay is measured to characterize the mobility of hydrogen nuclei. It is found that only a small fraction (6%) of the amorphous matrix is significantly plasticized by water and a large part (46%) is not at all affected by water. The amorphous matrix is inhomogeneous. Crystalline zones provide geometrical constraints for a large fraction of the chains close to the crystalline zones in the amorphous matrix. At some distance from the crystalline zones, a higher mobility of the polymer chains is possible. Knowledge of the inhomogeneous structure of the amorphous phase is important for understanding the water transport. The structure of the amorphous phase determines the rate and the amount of water uptake. Plasticization is directly studied using heavy water. Using heavy water, only the hydrogen nuclei on the polymer backbone and their mobility is measured. By comparing the experiments with water and heavy water, it is shown that the water front precedes the plasticization front. After studying the water uptake, the effect of ions on the water uptake is studied. Nylon films are exposed to electrolyte solutions containing mono-valent ions such as Na+, Li+ and Cli. The main effect of the ions in the electrolyte solution is to lower the water activity outside the film, thereby lowering the amount of moisture in the film and the rate of uptake. At equal water activity both the signal from the saturated films and the rate of uptake are in agreement with values measured in case of water vapor sorption. Element analysis shows that after 4 weeks a significant amount of ions is able to enter the film, but from a theoretical analysis it can be shown that this amount is not sufficient to influence the water uptake. Mono-valent ions are not detected in our measurements and little is known about the speed of migration and the underlying mechanism that determines the rate of migration into the polymeric matrix. Therefore, the uptake of paramagnetic ions, such as Mn2+ and Cu2+, in electrolyte solutions is studied. Because of their paramagnetic nature, these ions blank the NMR signal and reveal their presence. Ions start to enter the film immediately after the start of the experiment, but they enter the film at a lower rate than water. The nylon should be plasticized and contain a sufficient amount of water in order for the ions to move into the film. It is observed that at higher water activity the ions move faster through the nylon matrix. In a film equilibrated at a higher water activity, the amount of water and the mobility of the water and the polymer matrix is higher. The water uptake process is successfully studied and characterized by calibrating the NMR signal. Measuring the water uptake with EIS revealed that traces of water precede the front as measured by NMR. NMR relaxometry is used to study the effect of water on the polymer matrix and it is shown that only a small part is significantly affected by water. Electrolyte solutions with mono-valent ions mainly control the water activity when put on top of the film. Migration of ions through the film is studied using paramagnetic ions. At higher water activities the mobility of the water and polymer increases which leads to a higher migration rate of the ions.