Compositional analysis of multilayered plastic constituents and constituent mixtures using benchtop 1H NMR spectroscopy.

Multilayered plastics are widely used in food packaging and other commercial applications due to their tailored functional properties. By layering different polymers, the multilayered composite material can have enhanced mechanical, thermal, and barrier properties compared to a single plastic. However, there is a significant need to recycle these multilayer plastics, but their complex structure offers significant challenges to their successful recycling. Ultimately, the use and recycling of these complex materials requires the ability to characterize the composition and purity as a means of quality control for both production and recycling processes. New advances and availability of low-field benchtop 1H NMR spectrometers have led to increasing interest in its use for characterization of multicomponent polymers and polymer mixtures. Here, we demonstrate the capability of low-field benchtop 1H NMR spectroscopy for characterization of three common polymers associated with multilayered packaging systems (low-density polyethylene [LDPE], ethylene vinyl alcohol [EVOH], and Nylon) as well as their blends. Calibration curves are obtained for determining the unknown composition of EVOH and Nylon in multilayered packaging plastics using both the EVOH hydroxyl peak area and an observed peak shift, both yielding results in good agreement with the prepared sample compositions. Additionally, comparison of results extracted for the same samples characterized by our benchtop spectrometer and a 500-MHz spectrometer found results to be consistent and within 2 wt% on average. Overall, low-field benchtop 1H NMR spectroscopy is a reliable and accessible tool for characterization of these polymer systems.

Development of Structure-Property Relationships for Ammonium Transport through Charged Organogels.

Ammonia is a promising carbon-free fuel, but current methods to produce ammonia are energy intensive. New methods are thereby needed, with one promising method being electrochemical nitrogen reduction cells. Efficient cell operation requires robust catalysts but also efficient membrane separators that permit the selective transport of ions while minimizing the transport of the products across the cell. Commercial membranes have an unknown morphology which makes designing improved cells challenging. To address this problem, we synthesized a series of membranes with controlled crosslinking density and chemical composition to understand their impact on ammonium transport. Higher crosslinking density led to lower ammonium permeability. At the highest crosslinking density, similar ammonium permeability was observed independent of the water volume fraction and hydrophobicity of the monomers. These results suggest new directions to develop membranes with reduced ammonium crossover to improve the efficiency of these electrochemical cells.

Self-Folding PCB Kirigami: Rapid Prototyping of 3D Electronics via Laser Cutting and Forming.

This paper demonstrates laser forming, localized heating with a laser to induce plastic deformation, can self-fold 2D printed circuit boards (PCBs) into 3D structures with electronic function. There are many methods for self-folding but few are compatible with electronic materials. We use a low-cost commercial laser writer to both cut and fold a commercial flexible PCB. Laser settings are tuned to select between cutting and folding with higher power resulting in cutting and lower power resulting in localized heating for folding into 3D shapes. Since the thin copper traces used in commercial PCBs are highly reflective and difficult to directly fold, two approaches are explored for enabling folding: plating with a nickel/gold coating or using a single, high-power laser exposure to oxidize the surface and improve laser absorption. We characterized the physical effect of the exposure on the sample as well as the fold angle as a function of laser passes and demonstrate the ability to lift weights comparable with circuit packages and passive components. This technique can form complex, multifold structures with integrated electronics; as a demonstrator, we fold a commercial board with a common timing circuit. Laser forming to add a third dimension to printed circuit boards is an important technology to enable the rapid prototyping of complex 3D electronics.

Metallophobic Coatings to Enable Shape Reconfigurable Liquid Metal Inside 3D Printed Plastics.

Liquid metals adhere to most surfaces despite their high surface tension due to the presence of a native gallium oxide layer. The ability to change the shape of functional fluids within a three-dimensional (3D) printed part with respect to time is a type of four-dimensional printing, yet surface adhesion limits the ability to pump liquid metals in and out of cavities and channels without leaving residue. Rough surfaces prevent adhesion, but most methods to roughen surfaces are difficult or impossible to apply on the interior of parts. Here, we show that silica particles suspended in an appropriate solvent can be injected inside cavities to coat the walls. This technique creates a transparent, nanoscopically rough (10-100 nm scale) coating that prevents adhesion of liquid metals on various 3D printed plastics and commercial polymers. Liquid metals roll and even bounce off treated surfaces (the latter occurs even when dropped from heights as high as 70 cm). Moreover, the coating can be removed locally by laser ablation to create selective wetting regions for metal patterning on the exterior of plastics. To demonstrate the utility of the coating, liquid metals were dynamically actuated inside a 3D printed channel or chamber without pinning the oxide, thereby demonstrating electrical circuits that can be reconfigured repeatably.

Fourier transform infrared spectroscopy investigation of water microenvironments in polyelectrolyte multilayers at varying temperatures.

Polyelectrolyte multilayers (PEMs) are thin films formed by the alternating deposition of oppositely charged polyelectrolytes. Water plays an important role in influencing the physical properties of PEMs, as it can act both as a plasticizer and swelling agent. However, the way in which water molecules distribute around and hydrate ion pairs has not been fully quantified with respect to both temperature and ionic strength. Here, we examine the effects of temperature and ionic strength on the hydration microenvironments of fully immersed poly(diallyldimethylammonium)/polystyrene sulfonate (PDADMA/PSS) PEMs. This is accomplished by tracking the OD stretch peak using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy at 0.25-1.5 M NaCl and 35-70 °C. The OD stretch peak is deconvoluted into three peaks: (1) high frequency water, which represents a tightly bound microenvironment, (2) low frequency water, which represents a loosely bound microenvironment, and (3) bulk water. In general, the majority of water absorbed into the PEM exists in a bound state, with little-to-no bulk water observed. Increasing temperature slightly reduces the amount of absorbed water, while addition of salt increases the amount of absorbed water. Finally, a van't Hoff analysis is applied to estimate the enthalpy (11-22 kJ mol-1) and entropy (48-79 kJ mol-1 K-1) of water exchanging from low to high frequency states.