In-Situ Synchrotron Radiation Study On The Structural Evolution Of Rubber Elastomer Under Low-Temperature Harsh Conditions

Rubber is polymeric or macromolecular,which is characterized by high elasticity and reversible deformation.Rubber science is also one important branch of polymer or macromolecular science.The main use of rubber is tire material.The cover tire materials,especially high-end tire products,are almost all composed of natural rubber(NR).However,the natural rubber vegetation in China is rare.Only the climate of Guangdong,Guangxi,and Hainan Provinces are suitable for planting rubber trees,which means China relies on imports heavily.Structural research on natural rubber and its composite system is of great value.Since Katz firstly discovered the tensile-induced crystallization behavior of NR in 1925,strain-induced crystallization(SIC)has been considered as the self-reinforced mechanism of rubber,which makes rubber have high mechanical properties under large extension.With the development of analytical science,researchers have gotten more and more detailed microstructural information of rubber.Synchrotron radiation X-ray scattering technology is one of the most popular characterization methods in this century.In-situ wide-angle synchrotron radiation X-ray diffraction(SR-WAXD)is one of the most effective characterization methods to study the strain-induced crystallization behavior of rubber.The glass-transition temperature of the rubber material is very low.In real service,it is often necessary to experience this kind of low temperature and harsh external conditions.However,up till now,this research field is almost blank.Based on the above background,a low-temperature extensional rheometer which can be combined with synchrotron radiation X-ray was designed and constructed,and a series of researches on different kinds of self-reinforced rubber are carried out.The main researches and conclusions have been shown in follows:(1)Using liquid nitrogen as the cooling medium,a low-temperature extensional rheometer which can be combined with synchrotron radiation X-ray was designed and constructed.The low-temperature extensional rheometer can be used in in-situ stretching as low as-155 ?,and the opposite two servo-motor-controlling systems can remain a constant strain rate of 0.0025?30 s-1.(2)Strain-induced crystallization of natural rubber(NR)at low-temperatures(-60?25 0C)was systematically investigated by in-situ SR-WAXD measurement.The detailed structural evolution of NR during SIC is constructed in strain-temperature space,where up to four regions are defined depending on the microstructural information.In region I,the molecular chains begin to be oriented under tensile loading.The onset of SIC occurs at the very beginning of region ?,and the NR crystal acts as a new physical cross-linking point to form the crystal network,namely the series model.The further increment of crystallinity(>ca.8%)leads to the transition of the crystal network from the series model to the parallel model in region ?.The crystal network is finally accomplished in region ?,where the crystallinity remains almost constant.Interestingly,region ? and ? only exist in the intermediate-temperature zone ?(-40?-10 ?),which are missing in the zone ?(-10?25 ?)and ?(-60?-40?).This suggests sufficient crystallinity(?c,?-?>ca.8%)is required to form the parallel model.The new crystal network provides a deep understanding of SIC of NR considering the microscopic features,i.e.oriented amorphous component,the onset of crystallization and crystallinity evolution,and its correlation with the macroscopic stress-strain curve.(3)With the combination of low-temperature extension rheometer and in-situ SR-WAXD,the strain-induced crystallization(SIC)of poly(isobutylene-isoprene)rubber(?R)was studied in the low-temperature region(-60?25?).The detailed structural evolution of IIR during SIC is summarized in the strain-temperature space,where three distinct temperature zones are defined.The absence of SIC in zone ?(T>0?)results in the poorest drawability of ?R among all measured temperatures.And with respect to the lowest temperature zone ?(-60?<T<-50?),SIC still occurs with low ultimate crystallinity(ca.0.9%).More complicated structural evolution induced by strain occurs in the intermediate-temperature zone ?(-50??T?0?).The orientation ratio of the amorphous part Oa increases monotonically with the increment of strain,but reaches a platform with Hencky strain ?>ca.1.8.Meanwhile,the strain-induced crystal growth of IIR is evidenced by the dramatic increment of the lateral crystallite size of(110)and(113)planes.Moreover,the retraction experiment further reveals the network evolutions of IIR:suffering from low ultimate crystallinity(<ca.9%),the network chain of ?R remains in serial upon fracture.Current study clarifies the contribution of SIC and molecular orientation to the self-enhanced mechanical properties of ?R at low temperatures.(4)Strain-induced crystallization in polybutadiene rubber(BR)was studied by in-situ SR-WAXD over a broad temperature range(-90?25?).Depending on the presence or absence of SIC and quiescent crystallization temperature,three temperature regions are divided.Detailed structural evolution is summarized in the strain-temperature space.Based on this microstructural evolution information,the macroscopic mechanical response of BR,together with ?R and NR,is reproduced based on Flory's and Plagge's theories.The origins of the mismatch of calculated and experimental stress-strain curves,especially in large strain region,are discussed,which are mainly due to the lack of micro-macro connectivity and network heterogeneity.(5)SIC of NR,DPNR,and IR during the tensile-retraction process was studied at-60?25 ? by using in-situ SR-WAXD.It was found that the crystallinity of DPNR and IR was much lower than that of NR under the same strain,and the crystallinity of all three rubbers increased at the beginning of retraction.The function of protein is similar to that of a cross-linking point,which connects the polyisoprene molecular chains and affects the onset of SIC.The main innovations are summarized as follows:(1)A homemade low-temperature extensional rheometer,which can be used with synchrotron radiation X-ray,was designed and constructed.In-situ experiments can be carried out at temperatures as low as-155?.(2)The in-situ characterization of NR at-60?25? was carried out by combining the low-temperature extensional rheometer with SR-WAXD.The information of amorphous orientation,crystallization starting point and crystallinity evolution at different strain and temperature were obtained.The series-parallel model was introduced into the SIC process of rubber for the first time,which provided data support for the construction of crystal network.(3)Combining with SR-WAXD,the microstructure evolution of vulcanized IIR at-60?25? was first in-situ tracked by low-temperature extensional rheometer.The crystallinity,amorphous proportion of orientation and grain size of the crystal surface of IIR in strain temperature space were obtained.The differences between IIR and NR were compared in detail.It was proved that there was tensile induced crystal growth in IIR.(4)The microstructure evolution of vulcanized BR at-90?25? was first in-situ tracked by the combination of low-temperature extensional rheometer and SR-WAXD.Based on the obtained microstructure information and Flory's and Plagge's theories,the mechanical response of BR was reconstructed.Through the comparison of NR,BR,and IIR,the previously reported series-parallel network model proposed before is further extended and generalized.(5)Based on the study of Tanaka,Amnuaypornsri and Tosaka on the natural components and chemical crosslinking in NR,the low-temperature extensional rheometer combined with SR-WAXD combined with in-situ stretching recovery at-60?25?,compared NR,DPNR,and IR,explored the role of protein during the stretching process.