Is Ortho-Terphenyl a Rigid Glass Former?

Ortho-terphenyl (OTP) has long been used as a model system to study the glass transition due to its apparent simplicity and a widespread assumption that it is a rigid molecule. Here, we employ terahertz time-domain spectroscopy and low-frequency Raman spectroscopy to investigate the rigidity of OTP by direct observation of the low-frequency vibrational dynamics. These terahertz phonons involve complex large-amplitude atomic motions where intramolecular and intermolecular displacements are often mixed. Comparison of experimental results with density functional theory and ab initio molecular dynamics simulations shows that the assumption of rigidity neglects important implications for the glass transition and must be revisited. These results highlight the significance of terahertz modes on elasticity, which will be even more critical in more complex systems such as biomolecules.

Submillimeter-Long WS2 Nanotubes: The Pathway to Inorganic Buckypaper.

WS2 nanotubes present many new technologies under development, including reinforced biocompatible polymers, membranes, photovoltaic-based memories, ferroelectric devices, etc. These technologies depend on the aspect ratio (length/diameter) of the nanotubes, which was limited to 100 or so. A new synthetic technique is presented, resulting in WS2 nanotubes a few hundred micrometers long and diameters below 50 nm (aspect ratios of 2000-5000) in high yields. Preliminary investigation into the mechanistic aspects of the two-step synthesis reveals that W5O14 nanowhisker intermediates are formed in the first step of the reaction instead of the ubiquitous W18O49 nanowhiskers used in the previous syntheses. The electrical and photoluminescence properties of the long nanotubes were studied. WS2 nanotube-based paper-like material was prepared via a wet-laying process, which could not be realized with the 10 µm long WS2 nanotubes. Ultrafiltration of gold nanoparticles using the nanotube-paper membrane was demonstrated.

Mechanical Behavior of Hybrid Thin Films Fabricated by Sequential Infiltration Synthesis in Water-Rich Environment.

Sequential infiltration synthesis (SIS) is an emerging technique for fabricating hybrid organic-inorganic materials with nanoscale precision and controlled properties. Central to SIS implementation in applications such as membranes, sensors, and functional coatings is the mechanical properties of hybrid materials in water-rich environments. This work studies the nanocomposite morphology and its effect on the mechanical behavior of SIS-based hybrid thin films of AlOx-PMMA under aqueous environments. Water-supported tensile measurements reveal an unfamiliar behavior dependent on the AlOx content, where the modulus decreases after a single SIS cycle and increases with additional cycles. In contrast, the yield stress constantly decreases as the AlOx content increases. A comparison between water uptake measurements indicates that AlOx induces water uptake from the aqueous environment, implying a "nanoeffect" stemming from AlOx-water interactions. We discuss the two mechanisms that govern the modulus of the hybrid films: softening due to increased water absorption and stiffening as the AlOx volume fraction increases. The decrease in the yield stress with SIS cycles is associated with the limited mobility and extensibility of polymer chains caused by the growth of AlOx clusters. Our study highlights the significance of developing hybrid materials to withstand aqueous or humid conditions which are crucial to their performance and durability.