The recovery of nano-sized carbon black filler structure and its contribution to stress recovery in rubber nanocomposites.

The hierarchical structural evolution of natural rubber (NR) filled with different contents of nanoscale carbon black (CB) (10 phr-CB10 and 50 phr-CB50) after first loading and recovering for different times was investigated by X-ray nano-CT, wide-angle X-ray scattering (WAXS) and solid state NMR techniques. The CB filler structures as captured by X-ray nano-CT recover gradually with increasing recovering time, but the filler network with different CB contents shows dramatically different structure evolution. For CB10, limited by the filling content, CB particles mainly induces a hydrodynamic effect in spite of deformation or recovering. For CB50, the CB filler forms a 3D connected network, partially destructed during deformation, and the destructed part can be partially recovered during recovery. This suggests that the connected CB filler structure mainly acts as a network reinforcement, whereas the destructed part can induce a hydrodynamic effect. The different effects induced by different CB filling contents are also reflected by the NR matrix, which is reflected by the onset strains εc of strain-induced crystallization (SIC) of NR as captured by WAXS. For CB10, εc remains almost constant, i.e. εc = ca. 1.49, while that of NR with CB50 slightly decreases from initial ca. 1.12 to 0.96 with increasing recovering time up to 50 h. Also, the bound rubber fraction and entangled rubber network remain unchanged after deformation and under different recovery time as detected by the magic sandwich echo (MSE) FID and proton multiple quantum (MQ) NMR. These results demonstrate the key role of the CB filler network in determining the stress-softening behavior of reinforced rubber.

Recent advances in post-stretching processing of polymer films with in situ synchrotron radiation X-ray scattering.

The stretch-induced structural evolution mechanism is a long-standing scientific question in the post-stretching processing of polymer films. X-ray scattering, especially a combination of small- and wide-angle X-ray scattering (SAXS/WAXS), provides a powerful method to study the hierarchical structure of polymer films. Recent advances in synchrotron radiation (SR) light sources and detection techniques allow one to measure the structural evolution of polymer films during post-stretching processing in real time with ultrahigh time resolution, which benefits the understanding on this topic. This review summarizes some recent investigations on post-stretching processing of polymer films, which combine in situ X-ray scattering techniques with purposely designed tensile apparatus in terms of three aspects: uniaxial stretching, biaxial stretching and stretching with chemical reactions. Concerning the polymer bulk, traditional deformation mechanisms like stretch-induced crystallization (SIC), crystal slipping, phase transition and melting-recrystallization are discussed for the uniaxial and biaxial post-stretching of polymer films. New deformation models have been developed to focus on the structural evolution on the length scale of lamellar stacks, which consider the potential microphase separation of the interlamellar amorphous phase and microbuckling. For solution systems, the coupled effects of the mechanical work from external force and the chemical potential from possible chemical reactions are taken into account for the structural evolution during stretching in solution. Roadmaps of structural and morphological evolution in the processing parameter space (i.e., temperature, stress, strain and the concentration of additive in the bath solution) are eventually constructed for precursor films. The accumulation of a structural evolution database for post-stretching processing of polymer films can be expected to provide a helpful guide for industrial processing for high-performance polymers in the near future.

Microstructural Origin of the Double Yield Points of the Metallocene Linear Low-Density Polyethylene (mLLDPE) Precursor Film under Uniaxial Tensile Deformation.

The microstructural origin of the double yield points of metallocene linear low-density polyethylene (mLLDPE) precursor films has been studied with the assistance of the synchrotron radiation small- and wide-angle X-ray scattering (SAXS/WAXS). It has been shown that the microstructural origin of the double yield points is highly related to the initial orientation of the original precursor film. For less oriented mLLDPE precursor films, the rearrangement of lamellae and the appearance of the monoclinic phase are the microstructural origins of the first yield point. In comparison, for the highly-oriented mLLDPE precursor film, only the orthorhombic-monoclinic phase transition appears at the first yield point. The melting-recrystallization and the formation of the fibrillary structure happen beyond the second yield point for all studied mLLDPE precursor films. Finally, the detailed microstructural evolution roadmaps of mLLDPE precursor films under uniaxial tensile deformation have been established, which might serve as a guide for processing high-performance polymer films by post-stretching.

Reconstructing the mechanical response of polybutadiene rubber based on micro-structural evolution in strain-temperature space: entropic elasticity and strain-induced crystallization as the bridges.

Strain-induced crystallization (SIC) in polybutadiene rubber (BR) was studied by in situ synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) over a broad temperature range (-90 °C → 25 °C). 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 micro-structural evolution information, the macroscopic mechanical response of BR, together with poly(isobutylene-isoprene) rubber (IIR) and natural rubber (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 the large strain region, are discussed, and are mainly ascribed to the micro-macro connection approach and the network inhomogeneity.

Strain-induced crystal growth and molecular orientation of poly(isobutylene-isoprene) rubber at low temperatures.

With the combination of a low-temperature extension rheometer and in situ synchrotron radiation wide-angle X-ray diffraction (SR-WAXD), the strain-induced crystallization (SIC) of poly(isobutylene-isoprene) rubber (IIR) was studied in the low-temperature region (-60 °C → 25 °C). The detailed structural evolution of IIR during the SIC is summarized in the strain-temperature space, where three distinct temperature zones are defined. The absence of the SIC in zone I (T > 0 °C) results in the poorest drawability of IIR among all measured temperatures. And with respect to the lowest temperature zone III (-60 °C < T < -50 °C), the SIC still occurs with low ultimate crystallinity (ca. 0.9%). More complicated structural evolution induced by the strain occurs in the intermediate-temperature zone II (-50 °C ≤ T ≤ 0 °C). The orientation ratio of the amorphous part Oa increases monotonically with the increment of the strain, but reaches a platform with Hencky strain ε > 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 (

In situ characterization of strain-induced crystallization of natural rubber by synchrotron radiation wide-angle X-ray diffraction: construction of a crystal network at low temperatures.

Strain-induced crystallization (SIC) of natural rubber (NR) at descending temperatures as low as -60 °C is systematically investigated by in situ synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) measurement. The detailed structural evolution of NR during SIC is studied in the strain-temperature space, where up to four regions are defined depending on the SR-WAXD results. In region I, the molecular chains begin to be oriented under tensile loading. The onset of crystallization happens in the very beginning of region II, and the NR crystal acts as a new physical cross-linking point to form a 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 III. The crystal network is finally accomplished in region IV, where the crystallinity remains almost constant. Interestingly, regions III and IV exist only in the intermediate-temperature zone II (-40 °C to -10 °C), which are missing in zones I (-10 °C to 25 °C) and III (-60 °C to -40 °C). This suggests that sufficient crystallinity (χII-III > 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.

Stretch-induced complexation reaction between poly(vinyl alcohol) and iodine: an in situ synchrotron radiation small- and wide-angle X-ray scattering study.

Fibrillation and the complexation reaction between poly(vinyl alcohol) (PVA)-iodine (i) complexes have been studied with in situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS and WAXS) during the uniaxial stretching of PVA films in KI/I2 aqueous solution. SAXS results show that stretching induces the formation of nanofibrils, which pack periodically in the later stage of stretching with an average inter-fibrillar distance of around 10 nm. The onset strains for fibrillation and the appearance of periodicity of nanofibrils are located at the beginning and the end of the stress plateau, and decrease with increasing iodine concentration. In the stretching process as a whole, the presence of iodine ions reduces the crystallinity of the PVA crystal but favors the formation of a PVA-I complex. The complexation reaction is promoted by the synergistic effect of stretch and iodine ions, during which stretching drives the formation of polyiodine via the effect of entropic reduction while iodine concentration dictates crystallization of PVA-I3- co-crystals through the role of chemical potential. A morphological and structural phase diagram is constructed in the strain-iodine concentration space, which defines the regions for fibrillation and complexation reactions and may serve as a roadmap for the industrial processing of PVA polarizer.

A simple constrained uniaxial tensile apparatus for in situ investigation of film stretching processing.

A simple constrained uniaxial tensile apparatus was designed and constructed to obtain stress-strain curve during stretching and subsequent structural evolution of polymeric films. Stretch is carried out through two motor driven clamps in the machine direction and scissor-like clamps in the transverse direction keeping the sample width constant. The force information during film stretching process is recorded by a tension sensor and structural evolution can be obtained by in situ X-ray scattering technique. All parameters related to film stretching manufacturing, such as temperature, draw ratio, and stretching speed can be set independently, making the apparatus an effective method to explore the relationship between processing parameters and structure.