Upcoming efficient air-borne wind energy concepts and communication technologies applying lighter-than-air platforms require high-performance barrier coatings, which concomitantly and nonselectively block permeation not only of helium but also of ozone and water vapor. Similarly, with the emergence of green hydrogen economy, lightweight barrier materials for storage and transport of this highly diffusive gas are very much sought-after, particularly in aviation technology. Here the fabrication of ultraperformance nanocomposite barrier liners by spray coating lamellar liquid crystalline dispersions of high aspect ratio (∼20 000) silicate nanosheets mixed with poly(vinyl alcohol) on a PET substrate foil is presented. Lightweight nanocomposite liners with 50 wt % filler content are obtained showing helium and hydrogen permeabilities as low as 0.8 and 0.6 cm3 μm m–2 day–1 atm–1, respectively. This exhibits an improvement up to a factor of 4 × 103 as compared to high-barrier polymers such as ethylene vinyl alcohol copolymers. Furthermore, ozone resistance, illustrated by oxygen permeability measurements at elevated relative humidity (75% r.h.), and water vapor resistance are demonstrated. Moreover, the technically benign processing by spray coating will render this barrier technology easily transferable to real lighter-than-air technologies or irregular- and concave-shaped hydrogen tanks.
Systematic studies on the influence of crystalline vs disordered nanocomposite structures on barrier properties and water vapor sensitivity are scarce as it is difficult to switch between the two morphologies without changing other critical parameters. By combining water-soluble poly(vinyl alcohol) (PVOH) and ultrahigh aspect ratio synthetic sodium fluorohectorite (Hec) as filler, we were able to fabricate nanocomposites from a single nematic aqueous suspension by slot die coating that, depending on the drying temperature, forms different desired morphologies. Increasing the drying temperature from 20 to 50 °C for the same formulation triggers phase segregation and disordered nanocomposites are obtained, while at room temperature, one-dimensional (1D) crystalline, intercalated hybrid Bragg Stacks form. The onset of swelling of the crystalline morphology is pushed to significantly higher relative humidity (RH). This disorder–order transition renders PVOH/Hec a promising barrier material at RH of up to 65%, which is relevant for food packaging. The oxygen permeability (OP) of the 1D crystalline PVOH/Hec is an order of magnitude lower compared to the OP of the disordered nanocomposite at this elevated RH (OP = 0.007 cm3 μm m–2 day–1 bar–1 cf. OP = 0.047 cm3 μm m–2 day–1 bar–1 at 23 °C and 65% RH).
An alkylborane initiated reversible addition-fragmentation chain transfer (AI-RAFT) process was developed for the synthesis of star-polymers using a onepot approach at room-temperature in the presence of oxygen. Linear poly(tertbutyl acrylate) arms were first polymerized using a latent trialkylborane-amine initiator, which generated trialkylborane, in situ, and subsequently radicals after reaction with oxygen. Polymerizations were optimized to maximize monomer conversion (~70-80%) and minimize arm-dispersity (~1.10) through the oxygen concentration, initiator concentration, and polymerization time.The oxygen concentration was a critical AI-RAFT parameter, providing maximum conversion at a~0.5:1 molar ratio of oxygen-to-initiator. After arm-polymerization, multifunctional acrylates were injected into the reactor to commence crosslinking without intermediate purification. The impact of the crosslinking time and the crosslinker's functionality, concentration, and injection time were investigated to enhance arm incorporation and diminish starpolymer dispersity, quantified by deconvolution of size-exclusion chromatography data. Crosslinker concentration had the largest influence on arm conversion with optimal concentrations at a 20-25-fold excess to chain transfer agent.Under optimal conditions, arm conversions were maximized to~75-85% and star-dispersity minimized to~1.35-1.50. Herein an initial effort is made toward the synthesis of star-polymers with well-defined structures and high-arm conversions, while also striving for oxygen tolerance, minimal purification, lowtemperatures, and metal-free conditions. K E Y W O R D S star polymers, reversible addition fragmentation chain transfer, alkylborane initiation, oxygen tolerant, room temperature, controlled radical polymerization
When suspensions are exposed to shear forces, the particles may form ordered structures depending on their shapes, concentrations, and the material. For some processes, e.g., for wet-film coating, it is important to know how fast these structures form in shear fields and for how long the structures persist when the shear is relaxed. To obtain information on the particle structure formation and the decay time, the effective viscosity of nematic suspensions of Na-hectorite nanosheets was investigated by rheology employing a cone-plate measurement geometry. The necessary time for the formation textured nematic films could be deduced by carrying out effective viscosity measurements at constant time steps. Information could also be obtained on the lifetime of the platelet textures when shear is relaxed. All this information was employed to identify geometrical requirements for slot dies to produce barrier liners with nanosheet layers oriented parallel to PET substrates. Thereby, we obtained green and simple coatings that are in line with state-of-the-art high-performance materials such as metalized plastic foils in terms of oxygen barrier properties.
Conventional biodegradable polymers such as poly(lactic acid) (PLA) are an attractive alternative to replace traditional nondegradable food packaging films which plague the environment. However, PLA has shown to not be degradable in some environmentally relevant conditions, including within the freshwater systems. Additionally, PLA suffers from very poor barrier properties, which could result in food spoilage. Compositing with clay has been used to improve barrier properties according to tortuous path theory. Here a synthetic, large aspect ratio Na-Hectorite is used that may be utterly delaminated in an organic solvent and composited with PLA by modification with 18-crown-6 (18C6Hec), yielding a castable, homogeneous nematic suspension. Upon drying, thermodynamics drive the suspension toward segregation into sublayers of PLA and partially restacked 18C6Hec in situ. This unique self-assembled nanostructure combines the best of two worlds: The aspect ratio remains high and results in a 99.3% reduction in oxygen permeability. Additionally, the film shows surprisingly high resistance to swelling at elevated humidity, but once soaked in water, clay swelling is triggered, which fragments the film and drastically increases the surface area by 2500%. Accelerated degradation is observed under controlled enzymatic conditions and in an environmentally relevant wastewater medium during CO 2 evolution testing.
Cellulose acetate (CA) was partially acrylated, and the resulting cellulose acetate acrylate (acryl-substitution degree of 0.2) underwent quantitative thio-Michael click reactions with various thiols. A toolbox of functional CA polymers was obtained in this way, and their properties were studied. The modification with fatty alkyl thiols led to hydrophobic materials with large water drop contact angles. Octadecylthio-, butoxycarbonylpropylthio-, and furanylthio-modifications formed highly transparent materials. The new derivative CAASFur disintegrated completely under industrial composting conditions. Films of modified CA polymers were cast and investigated in terms of barrier properties. The nanocomposite of CAAS18 compounded with a synthetic layered silicate (hectorite) of a large aspect ratio showed permeabilities as low as 0.09 g mm m −2 day −1 for water vapor and 0.16 cm 3 mm m −2 day −1 atm −1 for oxygen. This portfolio of functional CA polymers opens the door to new applications.
Expensive biodegradable packaging as a preventative measure against continued accumulation of plastic waste in our environment is often in conflict with the need for high performing packaging materials that prevent...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.