Conformal Ultrathin Coating by scCO₂-Mediated PMMA Deposition: A Facile Approach To Add Moisture Resistance to Lightweight Ordered Nanocellulose Aerogels

Abstract

A facile approach for adding moisture resistance to transparent nematic aerogels composed of individualized cellulose nanofibers (i-CNF) at full preservation of the anisotropic aerogel structure is presented. Sequential nitroxide-mediated oxidation and mechanical cellulose fiber delamination were applied to obtain i-CNF dispersions in water. Nematic ordering caused by repulsive forces between i-CNF surface carboxylate groups was set by acid-induced hydrogen bonding and gelation, respectively. Solvent exchange to acetone, impregnation with the PMMA, scCO 2 -mediated antisolvent precipitation of the secondary polymer, and scCO 2 extraction of interstitial acetone afforded highly hydrophobic nanocomposite aerogels. Birefringence studies utilizing polarized light and the Michel−Levy Chart to evaluate interference patterns revealed that nematic i-CNF ordering is virtually not affected by the respective surface modification and scCO 2 drying steps. Morphological studies provide evidence that the large internal surface (>500 m 2 g −1 ) of the hybrid aerogels consists of a homogeneous ultrathin PMMA (mono)layer that virtually does not affect the high porosity (≥99%) and transparency (>77% transmission, 600 nm, 2.13 mm thickness) inherent to i-CNF aerogels. The obtained materials exhibited excellent resistance toward moisture as apparent from the high water contact angle (119.4°± 7.5). As PMMA imparts the aerogel stiffness and hydrophobicity, aggregation of nanofibrils in moist environment or under vacuum conditions can be avoided even at ultralow densities as low as 9.6 mg cm −3 .

Chemical versus physical grafting of photoluminescent amino-functional carbon dots onto transparent nematic nanocellulose gels and aerogels

“…Fig. 1b) and a carboxyl content of 1.29 mmol g -1 (20.9% DO) as previously reported (Plappert et al 2018).…”

“…At concentration typically above 0.4% w/v the highly anisometric nanofibers can form anisotropic nematic structures (Kobayashi et al 2014;Saito et al 2011) due to entropic effects (Onsager 1949). The obtained degree of ordering can be preserved throughout acid-induced gelation and conversion into aerogels (Plappert et al 2018). Similar materials have recently been prepared also from 2,3-dicarboxyl cellulose (2,3-DCC) (Plappert et al 2017).…”

“…Dispersions of i-CNF were prepared by TEMPOmediated oxidation (Isogai et al 2011) and subsequent nanofibrillation of never-dried beech bisulfite dissolving pulp using high pressure homogenisation equipment as previously described (Plappert et al 2018). In brief, 8 g (dry weight) of the pulp was disintegrated in 800 mL of H 2 O before 128 mg 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), 800 mg NaBr and 12 mL NaClO solution were consecutively added.…”

Preparation and Characterization of Bacterial Cellulose-Carbon Dot Hybrid Nanopaper for Potential Sensing Applications

“…Degree of oxidation: was determined by conductometric titration of the introduced carboxyl groups as described elsewhere [3,42]. In brief 0.1 g of TOBC that was beforehand subjected to freeze-drying (−80 • C, 30 Pa) was thoroughly disintegrated by continued magnetic stirring (1400 rpm, 15 min) in 52.25 mL of millipore-grade water.…”

“…Since the turn of the millennium, a multitude of advanced cellulosic materials has been developed. Their intriguing properties including transparence, low oxygen transmission, high mechanical strength, large specific surface, or tailor-made surface chemical and physical properties [1][2][3] literally invite use in both established and novel applications. Diagnostic or theranostic devices in human medicine typically rely on sensitive biosensors and event-triggered controlled release of active compounds.…”