Jinrong Wu

Large-scale orientation in a vulcanized stretched natural rubber network: proved by in situ synchrotron X-ray diffraction characterization.

In situ studies of strain-induced crystallization in unfilled and multiwalled carbon nanotube (MWCNT)-filled natural rubber (NR) were carried out by using synchrotron wide-angle X-ray diffraction (WAXD). Synchrotron WAXD results indicate that more nuclei appear in the MWCNT-filled NR sample, leading to higher crystallinity, lower onset strain of crystallization, and remarkable enhancement in tensile strength. During deformation, despite the amorphous chains remaining in isotropic orientation, the domains of larger scale (10-100 nm) with high network chain density in the NR matrix are oriented. The MWCNTs induce significant variation of this orientational process, and it is monitored by the stearic acid (SA) crystallites, which are effective nanoprobes of the amorphous phase. The results indicate that a small amount of MWCNTs and SA crystallites can be used as new tools to analyze the microstructural orientation of NR during deformation. The results also yield new insight into the strain-induced crystallization mechanism.

Using Two-Dimensional Correlation Dynamic Mechanical Spectroscopy to Detect Different Modes of Molecular Motions in the Glass-Rubber Transition Region in Polyisobutylene

In this Article, we report the first study of the molecular dynamics in the glass-rubber transition region in polyisobutylene by 2D correlation dynamic mechanical spectroscopy (2DC-DMS). With the help of the high resolution and high sensitivity of the technique, the sub-Rouse modes are independently separated from the Rouse modes and local segmental motion (LSM). According to the positions and widths of autopeaks of three modes of molecular motions, the loss tangent peak is resolved into three peaks by nonlinear fitting method. Moreover, the glass-rubber transition region is divided into three regions. 2DC-DMS has been demonstrated to be a powerful tool for studying the molecular motions with different time/length scales.

Molecular motions in glass-rubber transition region in polyisobutylene investigated by two-dimensional correlation dielectric relaxation spectroscopy

In this letter, we report the first study of the molecular relaxation dynamics in the glass-rubber transition region in polyisobutylene by 2D correlation dielectric relaxation spectroscopy (2DC-DRS). With the help of the high resolution and high sensitivity of 2DC-DRS, it is also the first time to observe and locate the positions of the Rouse modes and sub-Rouse modes in type-B polymers in the dielectric spectrum. 2DC-DRS was also applied to compare the temperature dependences of different molecular motions. Moreover, 2DC-DRS has been demonstrated as a powerful tool for studying the molecular motions with different time/length scales.

Study on liquid-liquid transition of chlorinated butyl rubber by positron annihilation lifetime spectroscopy

The existence of liquid-liquid transition (Tll) above the glass transition (Tg) in amorphous polymers remains a controversial topic. In this letter, positron annihilation lifetime spectroscopy was used to detect Tll in cured and uncured chlorinated butyl rubber. It is found that the temperature spectra of orthopositronium lifetime and free-volume fraction of both samples clearly demonstrate two inflection points around −70 and −10°C, corresponding to Tg and Tll, respectively, while the orthopositronium intensity decreases at the transition regions, which may imply molecular cooperativity. Moreover, two equations should be adopted to describe the development of free-volume fraction after Tg.

Detecting different modes of molecular motion in polyisobutylene and chlorinated butyl rubber by using dielectric probes

To explore the complex molecular dynamics of glass-forming substances in the softening dispersion region from the glassy state to the rubbery state, polyisobutylene (PIB) and chlorinated butyl rubber (CIIR), representives of an amorphous polymer with effective chain packing and low intermolecular cooperativity, were doped with low molecular weight brominated para-tert-octyl phenolic resin (PR) and then tested using dielectric spectroscopy. The results show that the PR molecules, having little effect on the overall molecular dynamics of PIB and CIIR, can be used as the dielectric probes to successfully monitor local segmental motion, sub-Rouse modes and Rouse modes in the dielectric spectra simultaneously. The relaxation times of the local segmental motion and the Rouse modes can be directly obtained from the dielectric spectra. It is found that the Rouse modes have weaker temperature-dependence than the local segmental motion, indicating the thermorheological complexity of PIB and CIIR. Consequently, the local segmental motion superposes the Rouse modes at a temperature near Tg. This study suggests that the dielectric probes have the capability to detect and study different modes of molecular motion around the softening dispersion region.