SAXS study on poly(dimethylsiloxane) networks with controlled distributions of chain lengths between crosslinks

Spatial Distribution of the Network Structure in Epoxy Resin via the MAXS-CT Method

We have succeeded in visualizing the spatial heterogeneity of the reaction ratio in epoxy resins by combining medium-angle X-ray scattering (MAXS) and computed tomography (CT). The reaction ratio is proportional to the degree of cross-linking between epoxy and amine in epoxy resins. The reaction ratio and its spatial inhomogeneity affect the toughness of epoxy resins. However, there has been no non-destructive method to measure the spatial inhomogeneity of the reaction ratio, although we can measure only the spatially averaged reaction ratio by Fourier-transform infrared spectroscopy (FT-IR). We found that the scattering peak reflected the cross-linking structures in the q region of MAXS and that the peak intensity is proportional to the reaction ratio. By reconstructing CT images from this peak intensity, we visualized the spatial heterogeneity of the reaction ratio. The application of this method may not be limited to epoxy resins but may extend to studying the heterogeneity of cross-linked structures in other materials.

A processable, high-performance dielectric elastomer and multilayering process

Dielectric elastomers (DEs) can act as deformable capacitors that generate mechanical work in response to an electric field. DEs are often based on commercial acrylic and silicone elastomers. Acrylics require prestretching to achieve high actuation strains and lack processing flexibility. Silicones allow for processability and rapid response but produce much lower strains. In this work, a processable, high-performance dielectric elastomer (PHDE) with a bimodal network structure is synthesized, and its electromechanical properties are tailored by adjusting cross-linkers and hydrogen bonding within the elastomer network. The PHDE exhibits a maximum areal strain of 190% and maintains strains higher than 110% at 2 hertz without prestretching. A dry stacking process with high efficiency, scalability, and yield enables multilayer actuators that maintain the high actuation performance of single-layer films.

Nanoparticles can modulate network topological defects during multimodal elastomer formation

We experimentally show that nanoparticles (NPs) can significantly regulate the network topological defects during a molecularly controlled elastomeric synthesis. Using positron annihilation lifetime spectroscopy, we demonstrate this on well-defined model systems of poly(dimethyl siloxane) elastomers and layered silicate nanoparticles (NPs). The evolutions of topological defects in elastomeric networks prepared from unimodal, bimodal, and NP dispersed bimodal elastomers are sequentially investigated. The extent of NP induced defect regulation is identified by varying the particle concentration from moderately low to an approximate upper limit. The fraction of free volume hole defects present between packed chains in the network generated by molecular control is significantly reduced. The fraction of smaller interstitial cavities near the cross-link sites shows a moderate increase at the lowest NP concentration. However, this fraction decreases at a high NP concentration and is nearly the same as that of bimodal networks that are devoid of NP infusion. Despite the variations in their fractions with NP infusion, the sizes of both these types of defects that remain in the network are minimally affected. The collective topological defects arising from chain induced heterogeneity also show a qualitative reduction upon NP infusion.

UNCONVENTIONAL RUBBER NETWORKS: CIRCUMVENTING THE COMPROMISE BETWEEN STIFFNESS AND STRENGTH

The failure properties of rubbery networks exhibit a maximum as a function of cross-link density or modulus. To avoid excess creep, elastomers are usually formulated such that their state of cure falls past this maximum, which means there is an inevitable compromise between modulus and failure properties (stiffness and strength). This review describes various approaches to circumventing the problem by the use of unconventional network structures. The obtainable improvements in mechanical properties can be substantial (e.g., threefold increases or greater in strength), although these methods entail additional processing steps or the use of blends and may affect the hysteresis of an elastomer; thus, their practical utility cannot be assessed a priori.