Zhimin Xie

Molecular Dynamics Investigation of the Adhesion Mechanism Acting between Dopamine and the Surface of Dopamine-Processed Aramid Fibers

Dopamine, as a universal material for surface treatment, can effectively improve the surface performance of aramid fibers. However, directly processing the surface of aramid fibers using dopamine currently incurs a high cost. To seek dopamine substitutes, one must first explore the adhesion mechanism responsible for binding the dopamine to the surface of the fiber. In this study, we construct an all-atomic molecular dynamics model of an aramid fiber before and after surface modification using dopamine. A force field based on condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) is used. Using it, we analyze the surface adhesion mechanism of polydopamines aggregated by 21 kinds of molecular structures typically found on the surface of aramid fibers. The results show that a clear and smooth interface is formed between the polydopamine nanofilm layer and the surface of the aramid fiber. The high atomic density of the polydopamine in the small interface region is found to be conducive to noncovalent bonds of polydopamines with the surface of the aramid fiber. In addition, we investigate the works of adhesion of the 21 molecular structures typically found on the surface of aramid fibers. The results suggest that the work of adhesion of 5,6-indolequinone is the highest, followed by annular eumelanin molecules with annular planar structure. Straight-chain shaped dimers proved to be the molecules with the highest adhesion ability of the dihydroxyindole chain oligomers. Therefore, there is reason to suppose that more molecular structures (as above) can be formed by processing the surface of aramid fibers using dopamine by controlling the processing conditions. These molecular structures help improve the adhesion ability of the dopamine on the surface of the aramid fiber. Additionally, if these polydopamine molecules with high adhesion ability can be synthesized on a large scale, then new surface-processing materials are possible.

Dynamic Mechanical Properties of Aged Filled Rubbers

Abstract It is well known that the macromolecular structure and the microstructure of the fillers play an important role in the mechanical properties of filled rubbers. This paper focuses on the dependence of the complex modulus of aged natural rubber vulcanizates on the filler network and polymer structure. Dynamic mechanical analysis (DMA) at 30°C, 50°C, and 70°C on the aged rubbers with/without prestrain showed the Payne effect, i.e., a storage modulus drop with increasing amplitude, and the appearance of a loss tangent maximum at strain of about a few percent. The storage modulus increased with the aging time at 70°C, 24 < 72 < 240 hr, in the case of nonprestrain. When the prestrain was applied, strain‐induced crystallization was generated that enhanced the storage modulus. As time passed, the prestrain relaxed and the crystalline structures began to disappear. After 72 hr, the crystalline structures had almost disappeared, and they had only a weak effect. Consequently, there existed a higher modulus for an aging time of 24 hr than 72 hr at testing temperatures of 30°C and 50°C. It was concluded that the storage modulus was determined by the postvulcanization, strain‐induced crystallization, aging, and relaxing time.

Simulation of Electrical Resistivity of Carbon Black Filled Rubber under Elongation

It has been known that the carbon black (CB) network is responsible for the electrical and mechanical behaviors of filled rubber. Due to the complexity involved in the filled rubber in relation to the conductive mechanism of the CB network, there has been little work concerned with simulation of the electrical behavior at large strains. Based upon an infinite circuit model, the electrical resistivity of CB filled rubber under elongation is simulated. For CB (N330) filled natural rubber with volume fraction of 27.5%, the simulated electrical resistivity increases with elongation at small stains, corresponding to the breakup of the agglomerates. The reduction in resistivity at larger strains corresponds to the decrease of the junction width, which results in a decrease of the contact resistance. Good agreement is found between the simulations and the experimental data available in the literature. The simulated results confirm the effects of the breakdown of the CB network and the alignment of CB aggregates under strain on the electrical resistivity.

Simulation of carbon black network in filled rubber

Carbon black (CB) is one of the most important fillers for rubber and plastics materials. How to describe the CB network is a fundamental problem for establishing relationships between the CB network and the mechanical properties of filled rubber. In view of the electrical conductivity of CB, an infinite circuit consisting of numerous contact resistors, interconnected with each other, is proposed to simulate the CB network in filled rubber; the resistances were determined by considering the tunneling conduction mechanism and a Gaussian distribution for the CB aggregate junction width. As an example, the electrical resistivity of CB (N330) filled natural rubber during uniaxial deformation was studied. It was found that the logarithm of resistivity was an approximately linear function of the extension ratio, and the resistivity increased with the increase of average number of primary particles per aggregates. Additionally, some published experimental points lie between the curves calculated for five primary particles and for seven primary particles per aggregate at extension ratios below 1.2. The calculations suggested that the average number of primary particles per aggregate for CB type N330 might be between five and seven.

Investigation of the carbon-black network in natural rubber under cyclic deformation

Carbon‐black (CB) filled rubbers are often subjected to cyclic deformation in service, particularly in tire applications. To improve resistance to fatigue, better understanding of how the meso‐ or microstructures evolve under cyclic loading is necessary. The presence of a CB network for reinforcement of rubber has been proposed. We describe the study of a CB network in natural rubber by an analysis of the impedance spectrum. The characteristic frequency of the filled rubber is used to characterize the junction width of the CB network. As the junction width of the CB network decreases, theoretically the characteristic frequency will increase. When the CB concentration was less than 10%, the nonfatigued rubbers had normal frequency characteristics, i.e., the measured frequency increased with an increase of the CB concentration. However, rubber with higher CB loading had an abnormal frequency response, whose the origin has been unclear until now. Much work will be done to clarify it. By examining the characteristic frequency, the dependence of the CB network in natural rubbers on the deformation cycle was studied. In general, the higher the number of fatigue cycles, the lower the characteristic frequency that was obtained. Although the rubber specimens showed little change in length in the tension direction before and after the fatigue cycles, the predicted change (different from the recoverable change) in the CB network did occur.

Characteristic Frequency of Carbon Black Filled Rubber

Due to the high electrical conductivity of carbon black (CB) particles, the presence of CB improves the conductivity of filled rubbers. The impedance spectra of CB filled rubber were simulated using an infinite resistor‐capacitor (RC) circuit by considering the tunneling conduction mechanism for the CB contact regions. The calculated results had a similar appearance to the experimental results, i.e., the Cole‐Cole plot was a semi‐circle and a peak appeared in a plot of the imaginary component of complex impedance with respect to the frequency from which the characteristic frequency was obtained. For a simple RC circuit in parallel connection, the logarithm of the characteristic frequency should be a linear function of the 1/3 power of the average number of primary particles per aggregate. A slight deviation from the line found in the simulations was attributed to the network contribution. Additionally, low CB loading had little effect on the characteristic frequency, in accordance with the experimental data, while high loading had a marked effect. Furthermore, the calculations showed that the characteristic frequency was affected not only by the distribution of individual CB aggregates, but also by the percentage of agglomerates at high CB loading.