By adopting coarse-grained molecular dynamics simulations, the effect of polymer functionalization on the relationship between the microstructure and the electric percolation probability of nanorod filled polymer nanocomposites has been investigated. At a low chain functionalization degree, the nanorods in the polymer matrix form isolated aggregates with a local order structure. At a moderate chain functionalization degree, the local order structure of the nanorod aggregate is gradually broken up. Meanwhile, excessive functionalization chain beads can connect the isolated aggregates together, which leads to the maximum size of nanorod aggregation. At a high chain functionalization degree, it forms a single nanorod structure in the matrix. As a result, the highest percolation probability of the materials appears at the moderate chain functionalization degree, which is attributed to the formation of the tightly connected nanorod network by analyzing the main cluster. In addition, this optimum chain functionalization degree exists at two chain functionalization modes (random and diblock). Lastly, under the tensile field, even though the contact distance between nanorods nearly remains unchanged, the topological structure of the percolation network is broken down. While under the shear field, the contact distance between nanorods increases and the topological structure of the percolation network is broken down, which leads to a decrease in the percolation probability. In total, the topological structure of the percolation network dominates the percolation probability, which is not a necessary connection with the contact distance between nanorods. In summary, this work presents further understanding of the electric conductive properties of nanorod-filled nanocomposites with functionalized polymers.
ABSTRACT Heat buildup, wet skid resistance (WSR), wear resistance (WR), and cutting and chipping resistance (CCR) of carbon black (CB), carbon–silica dual-phase filler (CSDPF), and silica-filled two kinds of styrene–butadiene rubber (SBR) were investigated. For the same SBR systems, the composite filled with silica exhibited the lowest heat generation and highest WSR performance, whereas it showed the worst WR and CCR among the three composites. The CSDPF-filled composite obtained a balanced overall performance. Rubber processing analyzer (RPA) strain sweep results showed that the CSDPF-filled composite exhibits the lowest Payne effect, which is related to filler networking in the rubber matrix. Solid-state 1H low-field NMR demonstrated that the sequence of the filler–rubber interaction of the composites was CB > CSDPF > silica. Bis-(3-(triethoxysilyl)-propyl)-tetrasulfide increased the cross-link density of the silica-filled composite. For the composites with different fillers, the lower filler network structure and higher cross-link density result in the lowest heat generation of silica-filled composite, and the strongest filler–rubber interaction leads to the best WR and CCR performances of the CB-filled composite. Filled SBR5025 composites exhibited better WR, lower heat buildup, and worse CCR than filled SBR1712 composites with the same filler.
For styrene-butadiene rubber (SBR) compounds filled with the same volume fraction of carbon black (CB), precipitated silica and carbon–silica dual phase filler (CSDPF), filler-rubber interactions were investigated thru bound rubber content (BRC) of the compounds and solid-state 1H low-field nuclear magnetic resonance (NMR) spectroscopy. The results indicated that the BRC of the compound was highly related to the amount of surface area for interaction between filler and rubber, while the solid-state 1H low-field NMR spectroscopy was an effective method to evaluate the intensity of filler-rubber interaction. The silica-filled compound showed the highest BRC, whereas the CB-filled compound had the strongest filler-rubber interfacial interaction, verified by NMR transverse relaxation. The strain sweep measurements of the compounds were conducted thru a rubber process analyzer; the results showed that the CSDPF-filled compound presented the lowest Payne effect, which is mainly related to the weakened filler network structure in polymer matrix. The temperature sweep measurement, tested by dynamic mechanical thermal analysis, indicated that the glass transition temperature did not change when SBR was filled with different fillers, whereas the storage modulus in rubbery state and the tanδ peak height were greatly affected by the filler network structure of composites.
Although a large number of studies have been performed to study the dispersion behavior of spherical nanoparticles (NPs) in the polymer matrix, little effort has been directed to anisotropic NPs via simulation, which is convenient for controlling the physical parameters compared to experiment. In this work we adopt molecular dynamics simulation to study polymer nanocomposites filled with anisotropic NPs such as graphene and carbon nanotubes (CNTs). We investigate the effects of the grafting position, grafting density, the length and flexibility of the grafted chains on the dispersion of graphene and CNTs. In particular, we find that when the grafting position is located on the surface center of the graphene or the middle of the CNT, the dispersion state is the best, leading to the greatest stress-strain behavior. Meanwhile, the mechanical property can be further strengthened by introducing chemical couplings in the interfacial region, by chemically tethering the grafted chains to the matrix chains. To monitor the processing effect, we exert a dynamic periodic shear deformation in the x direction with its gradient in the y direction. Polymer chains are found to align in the x direction, graphene sheets align in the xoz plane and CNTs orientate in the z direction. We study the effects of the shear amplitude, the shear frequency, polymer-NP interaction strength and volume fraction of NPs on the stress-strain behavior. We also observe that the relaxation process following the shear deformation deteriorates the mechanical performance, resulting from the disorientation of polymer chains and NPs. In general, this work could provide valuable guidance in manipulating the distribution and alignment of graphene and CNTs in the polymer matrix.
ABSTRACT Silane coupling agents can effectively improve the silica dispersion in rubber matrix and strengthen the interfacial interaction, and they have been widely used in tire treads to achieve low rolling resistance. 3-mercaptopropyl-ethoxy-bis(tridecyl-pentaethoxy-siloxane) (Si747) is a new coupling agent, and the temperature effects on the reactions between Si747 and silica and between Si747 and solution-polymerized styrene–butadiene rubber (SSBR) were investigated via Fourier transform infrared spectroscopy, thermogravimetric analysis, and solid-state 13C nuclear magnetic resonance in the present study. The results show that the Si747 grafting degree on the silica surface increases with increasing temperature, the cross-linking reaction between Si747 and SSBR can occur at 130 °C, and the reaction degree gradually increases with enhancing temperature. The silane–silica/SSBR composites were prepared at different in situ modification temperatures, and the temperature effects on the bound rubber content, filler dispersion, mechanical properties, and viscoelastic properties were investigated. It reveals that slightly pre-cross-linking between Si747 and SSBR lowers the tanδ at 60 °C of the SSBR/silane–silica composites, and in situ modification at 150 °C achieves a combination of low rolling resistance and high wet grip for silane–silica/SSBR composites.
Chiral and circularly polarized luminescence (CPL) materials with multiple stimuli responses have become a focus of attention. Meanwhile, elastomers have found substantial applications in a wide variety of fields. However, how to design and construct chiral elastomers, in particular CPL-active elastomers, still remains an academic challenge. In the present study, chiral helical substituted polyacetylene is chemically bonded with polydimethylsiloxane (PDMS) by hydrosilylation to form a chiroptically active elastomer. A CPL-active film was further fabricated by adding achiral fluorophores. Compared with the corresponding chiral helical polymer, the chiral films show much enhanced thermal stability in terms of chiroptical properties. The films also demonstrate reversible tunability in optical activity and CPL property when being subjected to a stretching-restoring process and exposed to a solvent like toluene. Further, noticeable chiral amplification is observed when the chiral PDMS film is superimposed with a pure PDMS film. This interesting finding is proposed to be due to the photoreflectivity of PDMS. This study provides an alternative strategy to exploit novel CPL-active elastomer materials with multiple stimuli responsivity and tunability, which may open up new opportunities for developing novel chiroptical devices.
A new strategy for preparation of isobutylene–isoprene rubber (IIR)/clay nanocomposites is reported based on two steps, i.e., preparation of swollen orgnomontmorillonite, followed by shear mixing on a two-roll mill with IIR. The dispersion of clay was investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). TEM images demonstrate that both exfoliated and intercalated nanoclay layers co-exist in these nanocomposites. XRD patterns reveal that the basal spacing of clay increases from 4.2 nm for swollen organic modified silicates to 6.2 nm for those dispersed in nanocompounds, and the dispersion structure is extremely disordered and close to an exfoliated structure. The experimental results show that the mechanical and gas barrier properties of nanocomposites increased with increasing amount of clay. The properties of nanocomposites prepared by the new method, such as shore A hardness, tensile strength, air-tightness and so on, were superior to those of nanocomposites by the solution intercalation and the traditional melt intercalation methods with non-swollen organic clay. The IIR/clay nanocomposites which were prepared by the novel method could be used in rubber products which require a high barrier to gas, such as tire inner-tube and inner-liner.
Silicone rubber (VMQ) possesses a saturated −Si–O– main chain and natural rubber (NR) contains a large amount of C═C bonds; thus, due to the large difference in saturating degree and main chain characteristics, it is difficult to compound them and obtain homogeneous composites. In this study, a trimercapto modifier [trimethylolpropane tris(3-mercaptopropionate)] (TMPMP) was chosen to enhance the interfacial compatibility of NR/VMQ composites via a thiol-ene click reaction. Due to the reactivity discrepancy of TMPMP with NR and VMQ, a two-step strategy of compatibilization was adopted. After modification, VMQ exhibited a smaller domain size and a more even distribution in the NR matrix, and the corresponding static mechanical properties and dynamic fatigue crack propagation resistance of NR/VMQ composites were improved. Furthermore, it is pointed out that the crack growth rate (dc/dN) shows a positive relevance with the viscoelastic parameter loss compliance modulus (J”): a less J” value means that more energy dissipation occurred in the linear viscoelastic region in front of the crack tip, resulting in less dc/dN and a better crack propagation resistance.
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