PEG, triethyl citrate, Tween 40,. Tergitol⢠NP-9. ⢠Glucose (50 wt%) ... Benzyl trimethyl ammonium bromide does not appear to cause nanocrystal aggregation.
Cellulose Nanocrystals: Preparation and Processing Richard S. Reiner USDA-FS-Forest Products Laboratory
2008 International Conference on Nanotechnology for the Forest Products Industry
Cellulose Nanocrystals
H2SO4 -O -O
3SO
OSO3-
OSO3-
-O
OSO3-
3SO -O
3SO
OSO3-
OSO3-
-O
3SO
OSO3-
OSO3OSO3-
3SO -O
3SO
OSO3-
OSO3-
General Synthetic Procedure • Digestion – Reaction Conditions • 64 wt% H2SO4 • 8:1 vol:wt (acid:cellulose) ratio • 45oC for 60 minutes
– Application • • • • • •
Avoid large cellulose pieces Inert atmosphere (N2, Ar) Apply acid under vacuum Rapid mechanical mixing Glycerin for seal/lubricant Alternate: wet pulp (activate cellulose) and adjust acid concentration
General Synthetic Procedure • Quench (10X Dilution) – (Settle overnight and decant)
• Centrifugation and Wash – 1000nm
10-20nm
Cotton
200-350nm
5nm
Wood
100-300nm
3-5nm
Beck-Candanedo, et.al., Biomacromolecules 6, 1048-54 (2005)
• •
Cotton (CF1) and commercial dissolving pulps have produced very cleanlooking products Bleached softwood kraft pulp always had some non-suspendible residue during nanocrystal preparation
Some Chemical Properties of Cellulose Nanocrystals • Charged-colloidal particles in water – Weak acid: pKa ~3 – Ksp: salt-out of suspension • Effect of countercation?
– Solvent precipitation
• Isotripic/Anisotripic phases 5-10 wt% nanocrystal [Dong, et.al., Langmuir 12, 1076-82 (1996); Dong, et.al., Cellulose 5, 19-32 (1998)]
• Surface hydroxyl groups readily available for surface funtionalization/modification • No evidence of conversion from cellulose I to II
Freeze Drying with Nonionic Surfactants • Numerous surfactants (25-200 wt%) allowed me to regenerate stable aqueous colloidal suspensions of cellulose nanocrystals from freezedried specimens – PEG, triethyl citrate, Tween 40, Tergitol™ NP-9
• Glucose (50 wt%) also appeared to work as well
Nanocrystals in Nonaqueous Solvents •
•
Cellulose nanocrystals have been suspended in polar organic solvents (DMSO, DMF, formamide), however, a small amount of water is necessary for a stable dispersion otherwise a gelatinous precipitate forms [Viet, et. al., Cellulose 14, 109-13 (2007)] I have found dehydrating aqueous suspensions containing significant PEG, Tergitol™ over P2O5 forms a thick paste – Successful air- or oven-drying a 1 wt% nanocrystal in PEG, Tergitol™ – Nanocrystals can be resuspended in water
•
Phosphoric esters surfactants – Literature reported freeze-drying cellulose nanocrystals with BNA to suspend in toluene and cyclohexane [Ljungberg, Biomacromolecules 6, 2732-9 (2005)] – I have used Naxonac™, but was less than satisfied in toluene; however seemed to suspend well in pyridine, but not lutidine
Utilizing Ionic Surfactants Ammonium Cations • Cetyl trimethyl ammonium bromide (CTAB) – Neutralizes nanocrystal charge and causing aggregation – CTAB nanocrystals appear to be extractable with organic solvents • Organic phase has a thick, gelatinous form • Suspect some water may have remained with cellulose
– Centrifuge, wash and solvent exchange • Ethanol, acetone, ether or ethanol, acetone, toluene – Acetic anhydride in dichloroethane with pyridine/DMAP – Maleated polyproplylene in toluene with pyridine/DMAP
– Adding non-ionic surfactant first (PEG, Tergitol™) prior to CTAB, the cellulose nanocrystals do not appear to aggregate
• Benzyl trimethyl ammonium bromide does not appear to cause nanocrystal aggregation
