Influence of Strain on the Microstructure and Transverse Relaxation Characteristics of HTPB Coating Using Low-Field <sup>1</sup>H NMR Spectroscopy

Abstract

During the storage process, the HTPB (hydroxyl-terminated polybutadiene) coating is continuously affected by the strain, and the microstructure and mechanical properties will be degraded, which will seriously affect the performance of composite solid propellant and solid rocket motor and cause great harm. In order to analyze the microstructure and transverse relaxation characteristics of HTPB coating under different strains, low-field 1H NMR tests was carried out under 0%, 5%, 10% and 15% strain conditions, and the crosslinking density and transverse relaxation parameters of HTPB coating were analyzed. The results show that, the transverse relaxation decay can be divided into two segmental mobilities corresponding to two distinct transverse relaxation times. With the increase of strain, the crosslinking density shows a decline tendency, the transverse relaxation decay amplitude slows down, and the inversion curve has a tendency to move to the right. The ratio of the fast transverse relaxation time and the peak area are much larger than the slow transverse relaxation time, and the proportion of the fast relaxation time and the peak area enlarge with the increase of the strain, while the proportion of slow transverse relaxation time is reduced. With the increase of strain, there is a transition from slow transverse relaxation to fast transverse relaxation, and there is an inverse linear relationship between crosslinking density and transverse relaxation time.