Understanding oil resistance of nitrile rubber: CN group interactions at interfaces.

摘要

Nitrile rubber (NBR) is copolymer of acrylonitrile and butadiene. It is resistant to swelling by hydrocarbon oils. Swelling and thermal degradation decrease as acrylonitrile content increases. Infrared-visible Sum Frequency Spectroscopy (SFS) is used in the present study to probe the molecular origin of oil resistance. SFS is a surface specific spectroscopic technique and was used to probe an NBR/liquid interface. Oil resistance of the NBR is reflected in changes in the SFS spectra of NBR at the interface. As reference materials, two additional polymers polyacrylonitrile (PAN) and polybutadiene (PBD) were included and analyzed prior to analyzing the nitrile rubber.;SFS analysis of the film revealed a shift of the CN stretching mode at the sapphire/PAN interface compared to that of PAN bulk when the PAN film was annealed above its glass transition temperature. This demonstrates how environment affects nitrile dipole-dipole interaction. The influence of liquid environment on the PAN surface was directly assessed by comparing the SFS spectra of the PAN/air to that of PAN/heptane and PAN/water interfaces. This showed a minor effect of the solvent on the nitrile CN stretching mode of PAN.;The second section was devoted to the analysis of PBD. A positive shift of the methylene stretching mode of PBD is detected in the SFS spectrum. This indicates that the polar sapphire surface influences the specific vibrational mode of PBD.;Finally, NBR rubbers with 40% or 20% acrylonitrile content (ACN) were characterized. In the SFS spectra of NBR (40% and 20% ACN)/air surfaces, a new vibrational band was observed at 2050 cm-1, in addition to the CN stretching mode of nitrile rubber at 2233 cm-1. This band was not detected in bulk NBR (40% and 20% ACN). Additives in bulk NBR were determined using High Performance Liquid Chromatography (HPLC). Results suggested the presence of amines molecules. However, purified NBR also showed the 2050 cm-1 band. There are a limited number of assignments that can be present in this range. This includes nitrile groups interacting with salts. Probable assignments are proposed after considering the chemical compounds present in NBR emulsion polymerization.;The final part of this dissertation is concerned with the interaction between an NBR film and two solvents: heptane and toluene. No change at the sapphire/NBR interface upon heptane exposure indicates the stability of the NBR film in contact with heptane. This is in accord with the oil resistance of NBR to hydrocarbon solvents. Solubilization of the NBR rubber thin film after toluene exposure is revealed by changes in the SFS spectrum at the sapphire/NBR interface. This shows that SFS is capable of accurately detecting molecular changes at polymer/liquid interfaces.;SFS results are consistent with the resistance of NBR to aliphatic solvent and instablity in aromatic solvents. Quantitative interpretation of the oil resistance will require an assignment of the 2050 cm-1 absorption band.