| Currently,elastomers are one of the materials which have been widely used,and their function and service properties are closely related to their structure and characteristics.Silicone rubber is widely used in the aerospace industry as an elastomer material which can maintain its strength and toughness at low temperatures.Since silicone rubber has the lowest temperature of glass transition and crystallization at quiescent.It is generally considered that crystallization will not occur at the temperature higher than its static crystallization temperature.In fact,under the external flow field,stretching can make silicone rubber crystallize in advance in the temperature higher than the static crystallization temperature,resulting in loss of elasticity and function of the material.For example,gaskets used in low-temperature environments can bring failure or even premature aging due to stretch-induced crystallization(SIC).Due to the limitations of realization of the low-temperature environment and the online testing technology,the research on the crystallization behavior induced by the complex external field is still lacking.Therefore,the self-designed and manufactured low-temperature stretching device combined with synchrotron radiation is used to study the molecular chain relaxation behavior and stretch-induced crystallization(SIC)behavior of silicone rubber at low temperatures.Through the in-depth study of the crystallization behavior of silicone rubber,the morphological information of crystals was further collected,and the mechanical model of SIC was established to guide the application of silicone rubber materials.It is necessary to carry out research on the orientation and SIC of silicone rubber materials,and to understand the enhancement mechanism and optimize the nanocomposite system to obtain better performance.By establishing the non-equilibrium phase diagram in the temperature-strain space,it is of great significance to investigate the microstructure of the silicone rubber and control the crystal form to realize the development of new materials.The main results and conclusions are summarized as follows:(1)The dynamic and structural relaxation of the cold crystallization behavior of silica-filled poly(dimethylsiloxane)(PDMS)was investigated in detail using Broadband Dielectric Spectroscopy(BDS)and Differential Scanning Calorimetry(DSC).The structure associated with crystal formation is compared to the evolution and modification of the amorphous phase.In-depth study of the relationship between molecular chain state and multiple relaxation motion was done with the changing of crosslink density by different doses of irradiation.Furthermore,the constrained interface relaxation is attenuated in parallel with the results of the tensile test for mechanical analysis.As the irradiation dose increases,the mobility of the molecular chain decreases and the rapid positional change turns to be the inverse amplitude change of the two peaks of dynamic relaxation.The molecular chain relaxation of silicone rubber during the cold crystallization process has two different forms which contain the changes of movement morphology with the filler content and the crosslinking density.(2)SIC and phase transitions of PDMS have been studied with the in situ synchrotron radiation wide-angle X-ray scattering technique(WAXS)during tensile deformation at temperatures ranging from-45 to-65?.The phase transitions during tensile deformation go through different processes at different temperature regions,where four phases are involved in,namely oriented amorphous(OA),mesophase,a form,and ? form crystals.SIC of a form can proceed via two different multi-stage ordering processes with either mesophase or ? form as the structural intermediate.Further cyclic tensile experiments demonstrate that solid-solid transition from ? to a form is a reversible process controlled by stress,which is attributed to the different helical pitches in ? and ? forms.A non-equilibrium phase diagram of SIC and phase transitions are constructed in strain-temperature space.(3)SIC and phase transitions of PDMS filled with different contents of nano-silica were studied with in-situ synchrotron radiation WAXS technique during uniaxial tensile deformation at temperatures from-40 to-65?.With low filler contents(10 and 25phr),two new transient phases,namely ?' and ?' are first observed during SIC of PDMS at-45 and-50?,which later transform into a and ? forms,respectively,with the increase of stain.The PDMS herein follows two different multi-stage kinetic pathways during uniaxial deformation,namely(i)amorphous-mesophase-?'-a forms and(ii)amorphous-mesophase-?'-?-? forms.Whilst with higher filler content(40 and 55phr)only a and ? forms are observed during SIC of PDMS.The lattices of ?' and ?' forms are proposed,which are low-density and high-entropy crystals.These results demonstrate the complexity of SIC in PDMS,which is influenced not only by temperature and strain but also by the filler contents.The non-equilibrium SIC phase diagrams in strain-temperature space for PDMS with different filler contents are constructed.(4)During the SIC process of the PDMS which has been studied,there is a coupling between the tensile and the temperature field.According to the solid tensile recovery and pre-stretching temperature down and up experiments,the differential effect of the tensile and temperature field on the crystallization behavior during the PDMS crystallization of 40phr filler content can be obtained.The temperature field controls the competitive growth of the crystal during the solids recovery process,and the temperature is the controlling factor of the crystal nucleation during the pre-stretching and cooling process.The contribution of tensile deformation to the crystallization process is controlling the crystal forms.The contribution of the temperature field to the crystal is to promote crystal growth,and there is competition in the crystallization regrowth process at-55?.The growth of a form is dominant,both crystals can grow at-65,-75?,and the growth of ? form is dominant at-85,-95?.It is speculated that there may be a new nucleation during the process. |
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