Structural Evolutions And Mechanical Behaviors Of Typical Elastomer Materials In A Wide Range Of Strain Rate

In recent years,with the rapid development of aerospace,military and defense fields,elastomers with good mechanical properties in a wide range of temperature and strain rate have received more and more attention.For example,elastomers can be found in sounding balloons,large aircraft tires and binder in polymer bonded explosives.An in-depth understanding of the structural evolution of elastomers under severe conditions such as low temperature,high speed and large strain contributes to the design of high-performance elastomers.However,limited by the experimental conditions,the current researches on the relationship between the structures and properties of elastomers are mainly focused on near-equilibrium conditions.High-speed loading has typical non-equilibrium features,thus many structural changes during high-speed loading cannot be explained solely by equilibrium thermodynamics.In the conditions far away 'from equilibrium,the energy is redistributed at different scales of the materials to form new structures and alter the properties of the materials.Moreover,the elastomers are structurally complex,mainly represented by the multiscales in both spatial and temporal dimensions.In the spatial dimensions,thermoset elastomers typically include not only rubber matrices,but also different types of fillers.And thermoplastic elastomers often have microphase-separated structures,which impart multiscale properties to the elastomers.Accordingly,in the temporal dimensions,each structure exhibits multiscale characteristics and has different relaxation kinetics under different temperatures and stress fields.The macroscopic mechanical behaviors of the elastomers are essentially the coupling of their structures and relaxation dynamics with the external field excitation effect.It is precisely because of the multiscale space-time interaction that understanding the relationship between structures and performances of elastomers has become a huge challenge for academic and industrial fields.Therefore,the work of this thesis attempts to reveal the relationship between structural evolutions and mechanical properties of typical elastomers in a wide range of strain rate,with a view to guiding the wider applications of elastomers.With the cormbination of the homemade high-speed stretching device,ultra-fast synchrotron X-ray scattering,thermal analysis and scanning electron microscopy,this thesis studies the structural evolutions of typical thermoplastic elastomers Fluoroelastomer F2314 and typical thermosetting elastomer natural rubber(with or without carbon black fillers)in a wide range of strain rate.The work deepens the understanding of the deformation process of elastomers at conditions far away from equilibrium.The main contents and conclusions of this thesis are as follows:(1)The high-speed stretching device has been improved,and the ultimate strain rate has been increased to 269 s-1 by increasing the radius of the stretching rollers,changing the sample holding mode and increasing the length of the sample.This device provides the hardware foundation for the loading of elastomers in a wide range of strain rate.Based on the synchrotron radiation X-ray scattering techniques,a fast structure detection system is successfully built.The time resolution of the small-angle and wide-angle X-ray scattering(SAXS/WAXS)signals in synchronous acquisition mode is improved to 2 ms,and the time resolution of the individual acquisition mode is improved to 0.5 ms.This is an innovation of experimental technology.(2)The structural evolutions of fluoroelastomer F2314 are studied during uniaxial tensile in a wide range of strain rate(0.1 to 150 s-1)with the combination of a homemade high-speed stretching device and in-situ SAXS/WAXS techniques.Based on the mechanical behaviors and structural evolutions,three strain rate regions(?,?and ?)are defined.The microphase-separated structure plays an important role in the mechanical response of F2314.In region I,deformation of soft domains is the main process before yielding,accompanied by the destruction of lamellar crystals in hard domains.In the stress plateau zone,deformation of hard domains is confirmed as the primary mechanism of energy dissipation.With the orientation parameter of the amorphous phase reaching a critical value,strain hardening is triggered.Recrystallization also takes place in strain hardening zone.In region II,due to the mismatch between the mobility of molecular chains in hard domains and the acting time of stress,large deformation of hard domains is more and more difficult to occur with the disappearance of recrystallization.In region ?,as almost all molecular chains have no time to adjust or relax to fit the stress field,the sample presents a brittle fracture.(3)The strain-induced crystallization(SIC)behaviors of natural rubber(NR)during uniaxial stretching are studied in a wide range of strain rate(from 0.1 to 233 s-1)with the combination of a homemade high-speed stretching device and in-situ ultra-fast X-ray scattering techniques.SIC phenomenon occurs under all experimental conditions within this strain rate range.At a high strain rate(233 s-1),the SIC can be triggered within a very short time(8 ms),which may be the reason for the application of NR under harsh conditions.The onset strain of SIC of NR(?c)is found to be almost independent of strain rate.The orientation of molecular chains has been found to play a crucial role in the SIC of NR.The orientation parameter of amorphous phase reaching a certain value(about 0.24 to 0.27)is considered necessary for SIC.The orientation of amorphous phase is controlled by strain,which explains the independence of ?c on strain rate.The effect of strain rate is mainly reflected in the construction of the crystal network.When the strain rate is small(from 0.1 to s-1),ac crystal network can be formed in the system during stretching,which increases the maxxium stress and fracture strain of the sample.When the strain rate is large(from 5 to 233 s-1),the crystal network cannot be formed in the system during stretching.(4)Combining the homemade high-speed stretching device and in-situ ultra-fast X-ray scattering techniques,the mechanical behaviors and structural evolutions of natural rubber with four different carbon black filler contents(NRO,NR0,NR30 and NR50)during uniaxial stretching are studied in a wide range of strain rate(from 0.1 to 233 s-1).The influence of carbon black filler on the mechanical behaviors and structural evolutions of natural rubber is discussed in depth.In the terms of mechanical behaviors,the addition of carbon black filler significantly increases the maximum stress during the tensile process of the sample and also reduces the fracture strain of the sample to a certain extent.In the terms of structural evolutions,the addition of carbon black filler does not change the independence of the onset strain of crystallization on strain rate in natural rubber,but significantly reduces the ?c.Meanwhile,the crystallinity of the sample is reduced to some extent.In addition,the degree of orientation of the crystals is also significantly weakened.