Dually cross-linked single networks: structures and applications

Cross-linked polymers have attracted an immense attention over the years, however, there are many flaws of these systems, e.g. softness and brittleness; such materials possess non-adjustable properties and cannot recover from damage and thus are limited in their practical applications. Supramolecular chemistry offers a variety of dynamic interactions that when integrated into polymeric gels endow the systems with reversibility and responsiveness to external stimuli. A combination of different cross-links in a single gel could be the key to tackle these drawbacks, since covalent or chemical cross-linking serve to maintain the permanent shape of the material and to improve overall mechanical performance, whereas non-covalent cross-links impart dynamicity, reversibility, stimuli-responsiveness and often toughness to the material. In the present review we sought to give a comprehensive overview of the progress in design strategies of different types of dually cross-linked single gels made by researchers over the past decade as well as the successful implementations of these advances in many demanding fields where versatile multifunctional materials are required, such as tissue engineering, drug delivery, self-healing and adhesive systems, sensors as well as shape memory materials and actuators.

Effect of Metal-Ligand Coordination Complexes on Molecular Dynamics and Structure of Cross-Linked Poly(dimethylosiloxane)

Poly(dimethylosiloxane) (PDMS) cross-linked by metal-ligand coordination has a potential functionality for electronic devices applications. In this work, the molecular dynamics of bipyridine (bpy)–PDMS-MeCl₂ (Me: Mn²⁺, Fe²⁺, Ni²⁺, and Zn²⁺) are investigated by means of broadband dielectric spectroscopy and supported by differential scanning calorimetry and density functional theory calculations. The study of molecular motions covered a broad range of temperatures and frequencies and was performed for the first time for metal-ligand cross-linked PDMS. It was found that the incorporation of bpy moieties into PDMS chain prevents its crystallization. The dielectric permittivity of studied organometallic systems was elevated and almost two times higher (ε′ ~4 at 1 MHz) than in neat PDMS. BpyPDMS-MeCl₂ complexes exhibit slightly higher glass transition temperature and fragility as compared to a neat PDMS. Two segmental type relaxations (α and α_ac) were observed in dielectric studies, and their origin was discussed in relation to the molecular structure of investigated complexes. The α_ac relaxation was observed for the first time in amorphous metal-ligand complexes. It originates from the lower mobility of PDMS polymer chains, which are immobilized by metal-ligand coordination centers via bipyridine moieties.

Self-healing and reprocessable bromo butylrubber based on combined ionic cluster formation and hydrogen bonding

The properties of modified bromobutyl rubber are strongly influenced by competing interactions via hydrogen bridges and ionic cluster formation. Dynamic network formation enables self-healing and reprocessability of the material.

Elastomer Reinforced with Innate Sulfur-Based Cross-Links as Ligands

Although the incorporation of sacrificial bonds into an elastomer is an effective way to provide a combination of high strength and high fracture toughness, this method normally involves complicated chemical processes. The coordination between metal ions and polysulfides has been documented. However, the potential of polysulfide structures in vulcanizates as ligands has long been neglected. Using innate sulfur-based cross-links, we show how weak and nonpolar elastomers achieve significant reinforcement without modification of the backbone. By simply soaking vulcanizates into solutions containing metal ions, dual ions are simultaneously introduced into the vulcanizate to generate coordinations with different bond strengths, resulting in an unprecedented high modulus. Overall, this work presents a universal yet high-efficiency reinforcing strategy to prepare high-performance elastomers without additional chemical modifications, which should promote comprehensive research and industrial application of sacrificial bond strategies for elastomers.

Effects of MAH/St grafted nanocellulose on the properties of carbon reinforced styrene butadiene rubber

Nanocellulose was extracted from bagasse (bagasse nanocellulose, BNC) by hydrolysis with alkali and acid, then grafted by maleic anhydride (MAH) and styrene (St). The modified nanocellulose (BMS) was further investigated to partially replace carbon black (CB) in St butadiene rubber (SBR) composites via coagulation to prepare SBR/BMS/CB composites. Through the comparison of the vulcanization characteristics, processing properties, mechanical properties, and dynamic mechanical performance of them, BMS proved to be more efficient than BNC. The results showed that BMS could activate the vulcanization process, suppress the Payne effect, and increase the modulus and hardness. Moreover, a fine BMS dispersion and strong interfacial interaction were achieved in SBR/BMS/CB composites. The observed reinforcement effects were evaluated based on the results of G′, tanδ and scanning electron microscopy (SEM) analyses of SBR/BMS/CB in comparison with SBR/BNC/CB composites.

A stretchable and rapidly self-healable polysiloxane elastomer based on reversible aluminum–amino coordination

Dynamic coordination between aluminum and amines promotes rapid self-healing for polysiloxane elastomers at lower temperature.

A novel polysiloxane elastomer based on reversible aluminum-carboxylate coordination

A novel crosslinked polysiloxane based on aluminium-carboxylate coordination was designed and studied.

Viscoelastic and self-healing behavior of silica filled ionically modified poly(isobutylene-co-isoprene) rubber

Rubber composites were prepared by mixing bromobutyl rubber (BIIR) with silica particles in the presence of 1-butylimidazole. In addition to pristine (precipitated) silica, silanized particles with aliphatic or imidazolium functional groups, respectively, were used as filler. The silanization was carried out either separately or in situ during compounding. The silanized particles were characterized by TGA, ¹H–²⁹Si cross polarization (CP)/MAS NMR, and Zeta potential measurements. During compounding, the bromine groups of BIIR were converted with 1-butylimidazole to ionic imidazolium groups which formed a dynamic network by ionic association. Based on DMA temperature and strain sweep measurements as well as cyclic tensile tests and stress–strain measurements it could be concluded that interactions between the ionic groups and interactions with the functional groups of the silica particles strongly influence the mechanical and viscoelastic behavior of the composites. A particularly pronounced reinforcing effect was observed for the composite with pristine silica, which was attributed to acid–base interactions between the silanol and imidazolium groups. In composites with alkyl or imidazolium functionalized silica particles, the interactions between the filler and the rubber matrix form dynamic networks with pronounced self-healing behavior and excellent tensile strength values of up to 19 MPa. This new approach in utilizing filler–matrix interactions in the formation of dynamic networks opens up new avenues in designing new kinds of particle-reinforced self-healing elastomeric materials with high technological relevance.

A self-healing rubber/silica composite with superior mechanical performance was obtained by tuning specific noncovalent filler–matrix interactions.