Mild and Versatile Functionalization of Nacre-Mimetic Cellulose Nanofibrils/Clay Nanocomposites by Organocatalytic Surface Engineering

Development of surface-engineering strategies, which are facile, versatile, and mild, are highly desirable in tailor-made functionalization of high-performance bioinspired nanocomposites. We herein disclose for the first time a general organocatalytic strategy for the functionalization and hydrophobization of nacre-mimetic nanocomposites, which includes vide supra key aspects of surface engineering. The merging of metal-free catalysis and the design of nacre-mimetic nanocomposite materials were demonstrated by the organocatalytic surface engineering of cellulose nanofibrils/clay nanocomposites providing the corresponding bioinspired nanocomposites with good mechanical properties, hydrophobicity, and useful thia-, amino, and olefinic functionalities.


General
Chemicals and solvent were used as purchased without any further purification. Tartaric acid and citric acid were dried in a desiccator over phosphorus pentoxide. Infrared spectrum was recorded by Thermo Scientific NICOLET 6700 FT-IR, Smart orbit, Diamond 30000-200 cm -1 . The Atomic force microscopy (AFM) experiments were carried out with Dimension icon and tapping mode with Bruker TAP525A as a cantilever. Resonance frequency for these experiments was 445 KHz with a 0.32 lines/s scanning rate. High-resolution micrographs of the samples surface and crosssection were recorded using a Hitachi S4800 FE-SEM (Japan). Typical accelerating voltage was 1 kV and working distance was 3-4 mm. Additionally, elemental mapping was carried out using an energy dispersive X-ray detector (Oxford Instruments, UK) at 15 kV. The contact angle (CA) was recorded on DAT 1100-FIBRO-system ab-SWEDEN and the measurement method is TAPPI T 558 pm-95. Tensile properties were determined on a Universal Testing Machine, Instron 5944 (USA).

Fabrication of CNF:
The CNF gel (1.5 wt%) was prepared by high-pressure homogenization of an enzymatically pretreated sulphite pulp (Nordic AB, Sweden), as reported by Henriksson et al. 1

Fabrication of MTM suspension:
A stable MTM suspension (approx. 0.35 wt%, Na+ Cloisite, BYK additives, Germany) was prepared by intense mixing and ultrasonication of MTM powder in Milli-Q water. After centrifugation, only the supernatant fraction of the MTM suspension was used for preparation of nanocomposites and aggregates were removed.

Preparation of nanocomposites:
The MTM/CNF nanocomposite films were prepared by filtration and drying, as reported Medina et al. 2 Briefly, well-dispersed aqueous suspensions of MTM and CNF were mixed in adequate proportion, filtered with a microfiltration membrane, and dried. The MTM/CNF nanocomposite contains 80 wt % CNF and 20 wt % MTM.

Typical procedure for catalytic surface modification of nanocomposites:
The silane derivative (3 equiv) and nanocomposite (338 mg), which contains CNF (270 mg, 1.66 mmol, 1 equiv.), were placed in an oven dried reaction vessel containing L-tartaric acid (36 mg, 0.24 mmol, 5mol%) and dry toluene (50 mL). After heating the reaction mixture for 48 hours at 95 °C, the temperature was decreased to room temperature and toluene was decanted. Next, the modified nanocomposite was washed with acetone (4x50 mL) and put in a desiccator and dried under reduced pressure ( Figure 1). Figure S1: Drying of the MTM/CNF nanocomposite film after washing with acetone Typical procedure for catalytic surface modification of CNF: A suspension of CNF (31g (1.6%w, 500 mg dry mass), 3mmol), was homogenized in EtOH (95%, 60 mL) for 1h at 6000 rpm. The solvent was exchanged with EtOH (95%, 3x 60 mL) using centrifugation. The CNF was next transferred to the reaction vessel in EtOH (95%, 30 mL). Next, (3-Mercaptopropyl)trimethoxysilane (1.4 mL, 7.5 mmol, 2.5 equiv.) and tartaric acid (23mg, 0.15 mmol, 5 mol %) were sequentially added. After refluxing the resulting reaction mixture for 24 h, it was cooled down to room temperature and the modified CNF was washed with EtOH (95%, 3x 60 mL) using centrifugation.

Typical procedure for catalytic surface modification of MTM:
Allyltrimethoxysilane (0.34 mL, 2mmol) and MTM (500 mg) were placed in an oven dried reaction vessel containing tartaric acid (25 mg, 5 wt%) and dry toluene (10 mL). After stirring the reaction mixture at 80°C for 6 hours, the temperature was decreased to room temperature and the S5 toluene was decanted. Next, the modified MTM was Soxhlet extracted (acetone, 24h) and dried under reduced pressure.

Tensile testing:
Tensile properties of the modified and unmodified films were determined on a Universal Testing Machine, Instron 5944 (USA). The samples were first conditioned at 50% relative humidity and 22 ± 1 °C for at least 40 h. Rectangular strips of 3-5 mm in width were then cut and tested, using a gauge length of 22 mm and a strain rate of 0.1 min -1 . The instrument was equipped with a noncontact video extensometer for strain measurement, and the load was measured with a 500 N load cell. All reported values are averaged data from at least 5 specimens.   MTM/CNF nanocomposite;FT-IR cm -1 : 3333, 2915, 1637, 1426, 1369, 1315, 1159, 1004 Silane-modified MTM/CNF nanocomposite: