In-situ Studying The Structure For Natural Rubber Composites With Synchrotron Radiation Diffraction And Imaging Technology

The structure of rubber composites is very important for academic research as well as the industrial manufacture. Though the theory of flow-induced crystallization, the self-enhancement mechanism and structure for filled rubber composites have been widely researched, the detailed structural information for rubber composites are still unresolved yet, which are the key information in understanding the flow-induced crystallization, self-enhancement mechanism and the relation between structure and properties. The stretching or compression is always occurs during the service life of rubber production, resulting in the flow-induced crystallization (FIC). The most widely recognized mechanism for FIC is the entropic reduction model (ERM), which involves the effect of the flow on entropic reduction of melt only without changing the free energy of final crystal. The classic self-enhancement mechanism for natural rubber proposes that these lamellae generated by strain-induced crystallization act as the role of in-situ hard fillers embedded in soft amorphous matrix and physical cross-link, increasing the Young’s moduli and fracture toughness, but ignore the local structure generated by stain-induced crystallization around the crack tip; inorganic nanofillers like carbon black (CB), silica and CaCO3are almost inevitably added in commercial rubber products, where filler network provides an important contribution to the mechanical performance of rubber composites. Recent studies focus on the influence of properties, but ignore the structural information of fillers, which may be the essential information in building theoretical enhancement models.In this thesis, uniaxial free stretch (FS) and constrained stretch (CS) are used to acquire the different orientation of final crystal induced by strain, in order to study the influence of different final crystal orientation on the flow-induced crystallization. To study the toughening mechanism of natural rubber with the aid of in-situ infrared microspectroscope imaging (FTIRI) and synchrotron radiation micro-beam scanning X-ray diffraction (SR-μSXRD). Through studying the fillers network change during the stretch process to analyze the relation between structural information of fillers and mechanical properties with the aid of synchrotron radiation full field transmission X-rays microscopy (TXM). The main researches work and conclusions are introduced as follows: (1) The classical ERM only involves the effect of the strain on entropic reduction of polymer melt only without changing the free energy of final crystal. Aiming to demonstrate the effect of final crystal deformation on FIC, stretch-induced crystallization of a cross-linked natural rubber is studied with in-situ synchrotron radiation wide angle X-ray diffraction (WAXD) measurements, during which both FS and CS are imposed on to obtain the structure evolution during the stretch process. It is found that strain induced crystals during the FS show uniaxial orientation, while poly-crystals possessing single-crystal like orientation is found during the CS of natural rubber, whilst the c-axis and a-axis align in the stretch direction (SD) and constrained direction (CD), respectively. Indicating the final crystal during the FIC can be change with the external field parameters. With the aid of molecular dynamic software Materials Studio6.0, we calculated the free energy increased△Gcf when the CD along a-and b-aixs, found that the△Gcf for CD along b-axis is about0.086kJ/mol larger than that along a-axis, indicating that a lower free energy increased△Gcf and favoring for nucleation is achieved when CD along a-axis. Through coupling the final free energy of crystal into the classic ERM, our experimental results can be quantities understood.(2) The2D crystallinity distributions around crack tip for stretched natural rubber are obtained through FTIRI with mapping technology for different strains. Finding the crystallinity show a decay trend with the distance away from the crack tip increasing coupled with weak oscillation of crystallinity superposed on the decay trend. The2D crystallinity distributions around crack tip show there exists a hierarchical structure composing of regions with high and low crystallinity, respectively. As high crystallinity corresponds to high modulus and vice versa, it represents a soft-hard hierarchical structure. Based on the shear lag model, the hierarchical structure can avoid the stress concentration and dissipate energy, significantly enhance the ability of crack resistance.(3) With the aid of SR-μSXRD, the crystallinity distribution around the crack for stretched natural rubber is obtained. Results indicating that the crystallinity show a decay trend with increasing distance from the crack tip, coupled with weak oscillation of crystallinity superposed on the decay trend.2D crystallinity distributions around crack tip show there exist a deformed double network structure composing of regions with high and low crystallinity, respectively. As high crystallinity corresponds to high modulus and vice versa, it represents a soft-hard double network structure. Theoretical analysis indicates the soft-hard double network structure can avoid the stress concentration and enhance the ability of crack resistance. The fracture energy enhancement factor obtained by utilizing the double network model indicates a theoretical enhancement of toughness by3orders at the strain of5.5. This work proposed that upon stretching the micro-scale double network structures concomitant with conformation at molecular level and crystallization in nano-scale spontaneously developed in natural rubber construct multi-scale hierarchical structure, which accounts for the remarkable mechanical performance.(4) The three-dimensional (3D) distributions of CB at the strain of0and3are obtained with the aid of in-situ full field TXM, and found the3D dispersion of CB aggregates in rubber matrix is not homogeneous with some spots filled by highly concentrated aggregates. Furthermore, coincidence of large aggregates and large pores concomitant with inhomogeneous3D dispersion of nano pores in the rubber matrix indicates that CB aggregates lead to stress concentration due to the exist of large pores. Statistical analysis shows that the frequency of similar-sized aggregates (inter-aggregates distance) decreases with the increase of aggregate size (inter-aggregates distance) monotonously without strain. While an oscillation of the frequency-size plot is induced by strain3on top of the damping trend, which is interpreted as stretch-induced breakage of CB aggregates. Under strain of3, a most suitable inter-aggregate distance exists after stretched to strain3, indicating the fillers network becomes homogeneous after stretch to strain3. Based on the double-network mode, we found that stretch-induced breakage of CB aggregates depress the contribution of CB network to the mechanical property of rubber composite parallel and perpendicular to the stretching direction. The experimental results directly prove the structural origin of Payne effect.