Check out the latest publications from the members of the Alexander Research Team!
Ultrafast launch of slingshot spiders using conical silk webs
In the Theridiosomatidae spider family, at least three genera (Epeirotypus, Naatlo and Theridiosoma) use their three-dimensional cone-shaped webs as ultrafast slingshots that catapult both the spider and the web towards prey. Also known as slingshot spiders, theridiosomatids build three-dimensional conical webs with a tension line directly attached to the center of the web. In 1932, Hingston hypothesized that the slingshot spider releases the tension line using its front legs, while holding the web with its rear legs. Coddington detailed how female spiders meticulously build their webs line-by-line. But lacking to date has been quantification of spider kinematics, such as displacement, velocity and acceleration. Here we report the first quantification of theridiosomatid motion, revealing that slingshot spiders generate the fastest arachnid full body motion through use of their webs for external latch-mediated spring actuation.
Restricting Molecular Mobility in Polymer Nanocomposites with Self-Assembling Low-Molecular-Weight Gel Additives
Multiscale investigation of molecular gel additives in polymer matrices guides understanding of how solution-state assemblies result in mechanically enhanced, solid-state nanocomposites. Model polymers, poly(ethylene oxide-co-epichlorohydrin) (EO–EPI) and poly(vinyl acetate) (PVAc), were utilized as matrices and reinforced by cholesterol–pyridine (CP) nanofiber networks. The CP nanofillers suppress ethylene oxide segment melting for EO–EPI composites, whereas for PVAc nanocomposites, cause a polymer–gel dissociation transition. Incorporation of crystalline CP fiber networks led to an order of magnitude increase in tensile storage modulus due to restrictions on polymer chain mobility. This decrease in molecular mobility was confirmed by decreased loss moduli for both EO–EPI and PVAc composites. Excitingly, PVAc nanocomposites display an additional relaxation mode caused by release of PVAc chains from the transient molecular gel assembly. For both EO–EPI and PVAc composites, bulk flow can be suppressed to temperatures up to 100 °C by simply increasing the CP concentration.
Nucleation effects of high molecular weight polymer additives on low molecular weight gels
Polymeric species have been introduced to low molecular weight gelators to tailor their nucleation and rheological behavior. This work combines polymers and molecular gels (MGs) in a different manner by using polymers as the major component in a solution. Additionally, using polymers above their entanglement molecular weight is a step towards building polymer–MG composite materials. Specifically, a cholesterol-pyridine (CP) molecular gel was introduced to poly(ethylene oxide-co-epichlorohydrin) (EO-EPI) and poly(vinyl acetate) (PVAc), which have dissimilar chain conformations in anisole. Dynamic light scattering, scanning electron microscopy, and temperature-dependent small- and wide-angle X-ray studies were utilized to investigate the influence of the solution properties of high molecular weight EO-EPI and PVAc on the CP network structure. The collapsed chain conformation and aggregation of EO-EPI led to isolated, branched CP fiber networks, resulting in unexpectedly high dissociation temperatures. In contrast, PVAc gels displayed transient fiber networks, as evidenced by fiber wrapping and bundling. Cooperative interactions between PVAc and CP resulted in gels with dissociation temperatures higher than those of pure CP gels. These structural characteristics significantly influenced the gel mechanics. The collapsed chain conformation of EO-EPI led to weaker, more viscous gels, and the freely extended PVAc chain conformation led to interconnected, elastic gels independent of the molecular gel concentration.
Programming shape and tailoring transport: advancing hygromorphic bilayers with aligned nanofibers
Natural systems utilize nanofiber architectures to guide water transport, tune mechanical properties, and actuate in response to their environment. In order to harness these properties, a hygromorphic bilayer composite comprised of a self-assembled fiber network and an aligned electrospun fiber network was fabricated. Molecular gel self-assembly was utilized to increase hydrophobicity and strength in one layer, while aligned electrospun poly(vinyl alcohol) (PVA) nanofibers increased the rate of hydration and facilitated tunable actuation in the other. Interfacing these two fiber networks in a poly(ethylene oxide-co-epichlorohydrin) (EO–EPI) matrix led to hydration-driven actuation with tunable curvature. Specifically, variations in fiber alignment were achieved by cutting at 0, 90, and 45 degree angles in relation to the length edge of the composite. Along with the ability to program the natural curvature, the utilization of aligned nanofibers increased water transport compared to random nanofiber systems, resulting in a reduction in response time from 20+ minutes to 2–3 minutes.