Entanglements, Defects, and Inhomogeneities in Nitrile Butadiene Rubbers: Macroscopic versus Microscopic Properties

Soft, Tough, Antifatigue Fracture Elastomer Composites with Low Thermal Resistance through Synergistic Crack Pinning and Interfacial Slippage

Soft elastomer composites are promising functional materials for engineer interfaces, where the miniaturized electronic devices have triggered increasing demand for effective heat dissipation, high fracture energy, and antifatigue fracture. However, such a combination of these properties can be rarely met in the same elastomer composites simultaneously. Here a strategy is presented to fabricate a soft, extreme fracture tough (3316 J m-2) and antifatigue fracture (1052.56 J m⁻2) polydimethylsiloxane/aluminum elastomer composite. These outstanding properties are achieved by optimizing the dangling chains and spherical aluminum fillers, resulting in the combined effects of crack pinning and interfacial slippage. The dangling chains that lengthen the polymer chains between cross-linked points pin the cracks and the rigid fillers obstruct the cracks, enhancing the energy per unit area needed for fatigue failure. The dangling chains also promote polymer/filler interfacial slippage, enabling effective deflection and blunting of an advancing crack tip, thus enhancing mechanical energy dissipation. Moreover, the elastomer composite exhibits low thermal resistance (≈0.12 K cm2 W-1), due to the formation of a thermally conductive network. These remarkable characteristics render this elastomer composite promising for application as a thermal interface material in electronic devices.

Flexible, sensitive and rapid humidity-responsive sensor based on rubber/aldehyde-modified sodium carboxymethyl starch for human respiratory detection

Natural polymers with abundant hydrophilic groups are potential candidates for humidity sensor designing. Unfortunately, most of natural polymers lack essential stretchability and high conductivity, which hinder their development in the field of flexible humidity sensor. Cooperation with rubbers and conductive nanometer materials is an effective method to make the best use of natural polymers in flexible humidity sensor. In this paper, a flexible and sensitive sensor with rapid response to humidity change is fabricated based on aldehyde-modified sodium carboxymethyl starch (ACMS), carboxylated styrene-butadiene rubber (XSBR) and Ag nanoflakes through film-forming method. The pre-prepared ACMS owns a better dispersibility in the aqueous phase and serves as reducing agent for formation of Ag nanoflakes. After the film-forming process, the composite film shows a strength of 5.66 MPa and a high stretchability with strain of 367 %. Besides, our sensor shows a rapider response to humidity change than the commercial electronic hygrometer that it takes only 1 s to respond to the humidity change from 25 % RH to 27 % RH. Therefore, the XSBR/ACMS/Ag sensor possesses an impressive sensitive response to slight sweat on human skin and breath, which could find applications in monitoring people's health and distinguish their physical condition.

Structure-Property Relationships of Shape Memory, Semicrystalline Polymers Fabricated by In Situ Polymerization and Crosslinking of Octadecyl Acrylate/Polybutadiene Blends

The current work presents the study of a semicrystalline, shape memory polymer synthesized by simultaneous free radical polymerization and crosslinking in a blend of polybutadiene (PB) and octadecyl acrylate. Blending elastomers and phase change materials provide a modular method for new smart materials, such as shape memory polymers. In this system, grafted, side-chain crystalline poly(octadecyl acrylate) (PODA) fixes a programmed shape in the shape memory cycle, while crosslinked polybutadiene drives shape recovery. This work focuses on improving material parameters important for shape memory (crystallinity, gel fraction, melting temperature) by tuning the processing and formulation parameters (amount of crosslinker and PB weight fraction). The result is a shape memory PB-PODA copolymer that can be fabricated by melt processing and programmed without cooling below ambient temperature. It is found that good shape memory (i.e., high shape fixity and recovery) is obtained at a low PB weight fraction where a percolating PODA crystal network is formed at room temperature. The optimized sample shows excellent shape memory properties (fixity > 99%, recovery = 96%). It is shown that it is possible to mold this material into complex 3D shapes or topography with potential use in anticounterfeiting and antitampering applications.

Study of Relaxations in Epoxy/Rubber Composites by Thermally Stimulated Depolarization Current and Dielectric Spectroscopy

Liquid rubber toughened epoxy resins are widely used in electrical equipment and electronic packaging. Previous studies have only investigated the relaxation process of epoxy resins through dielectric spectroscopy. The trap characteristics of the relaxation process by thermally stimulated depolarization current (TSDC) analysis are less studied. In this work, TSDC and broadband dielectric spectroscopy techniques were used to complementarily characterize the dielectric relaxation process of hydroxyl-terminated liquid nitrile-butadiene rubber (HTBN) toughened epoxy resin polymers. The experimental results show that HTBN introduces two new relaxation processes in the epoxy matrix, which are attributed to the α polarization of the rubber molecule and the interfacial polarization based on the correlation between the TSDC and the dielectric spectroscopy data, respectively. The trap parameters of each TSDC current peak were obtained using the multi-peak fitting method. The addition of rubber increases the trap density in epoxy composites significantly, especially for traps with energy levels in the range of 0.5–0.9 eV. The trap energy level of the DC conductivity process increases with increasing rubber concentration. The above results provide analytical ideas for rubber-toughened epoxy resins’ polarization and trap characteristics and theoretical guidance for formulation improvement.

Network topology and stability of homologous multiblock copolymer physical gels

The mechanical properties of physical gels generated by selectively swelling a homologous series of linear multiblock copolymers are investigated by quasistatic uniaxial tensile tests. We use the slip-tube network model to extract the contributions arising from network crosslinks and chain entanglements. The composition dependence of these contributions is established and considered in terms of simulations that identify the probabilities associated with chain conformations. Dynamic rheology provides additional insight into the characteristics and thermal stability of the molecular networks.

Poly(acrylonitrile-co-butadiene) as polymeric crosslinking accelerator for sulphur network formation

The major controlling factors that determine the various mechanical properties of an elastomer system are type of chemical crosslinking and crosslink density of the polymer network. In this study, a catalytic amount of acrylonitrile butadiene copolymer (NBR) was used as a co-accelerator for the curing of polybutadiene (BR) elastomer. After the addition of this copolymer along with other conventional sulphur ingredients in polybutadiene compounds, a clear and distinct effect on the curing and other physical characteristics was noticed. The crosslinking density of BR was increased, as evidenced by rheometric properties, solid-state NMR and swelling studies. The vulcanization kinetics study revealed a substantial lowering of the activation energy of the sulphur crosslinking process when acrylonitrile butadiene copolymer was used in the formulation. The compounds were also prepared in the presence of carbon black and silica, and it was found that in the carbon black filled system the catalytic effect of the NBR was eminent. The effect was not only reflected in the mechanical performance but also the low-temperature crystallization behavior of BR systems was altered.

Counting Secondary Loops Is Required for Accurate Prediction of End-Linked Polymer Network Elasticity

To predict and understand the properties of polymer networks, it is necessary to quantify network defects. Of the various possible network defects, loops are perhaps the most pervasive and yet difficult to directly measure. Network disassembly spectrometry (NDS) has previously enabled counting of the simplest loops-primary loops-but higher-order loops, e.g., secondary loops, have remained elusive. Here, we report that the introduction of a nondegradable tracer within the NDS framework enables the simultaneous measurement of primary and secondary loops in end-linked polymer networks for the first time. With this new "NDS2.0" method, the concentration dependences of the primary and secondary loop fractions are measured; the results agree well with a purely topological theory for network formation from phantom chains. In addition, semibatch monomer addition is shown to decrease both primary and secondary loops, though the latter to a much smaller extent. Finally, using the measured primary and secondary loop fractions, we were able to predict the shear storage modulus of end-linked polymer gels via real elastic network theory (RENT).

Desktop NMR and Its Applications From Materials Science To Organic Chemistry

NMR spectroscopy is an indispensable method of analysis in chemistry, which until recently suffered from high demands for space, high costs for acquisition and maintenance, and operational complexity. This has changed with the introduction of compact NMR spectrometers suitable for small-molecule analysis on the chemical workbench. These spectrometers contain permanent magnets giving rise to proton NMR frequencies between 40 and 80 MHz. The enabling technology is to make small permanent magnets with homogeneous fields. Tabletop instruments with inhomogeneous fields have been in use for over 40 years for characterizing food and hydrogen-containing materials by relaxation and diffusion measurements. Related NMR instruments measure these parameters in the stray field outside the magnet. They are used to inspect the borehole walls of oil wells and to test objects nondestructively. The state-of-the-art of NMR spectroscopy, imaging and relaxometry with compact instruments is reviewed.