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
– <0.5 wt% H2SO4 begin seeing colloidal
cellulose nanocrystals
• Dialysis
– 1 wt% cellulose
– One week against DI water
• Ultrasonic treatment
– 0oC for 30 minutes
• Centrifugation
• (Ion exchange to acid form)
• Yield 30%
Effect of Synthesis Conditions
Hydrolysis
Time
(min)
Sulfur
Content
(%, )
Sulfur
Content
(%)
Particle
Length
(nm)
10
0.53
0.30
390
20
0.50
0.33
332
30
0.58
0.50
276
45
0.62
0.64
226
60
0.69
0.68
197
120
0.74
0.68
179
240
0.75
0.62
177
Dong, et.al., Cellulose 5, 19-32 (1998)
•
•
More extensive hydrolysis produces narrower size distribution
Inceasing liquid to cellulose ratio increases sulfur content and decreases
particle length (less extensively studied).
Cellulose Starting Material
Cellulose type
Length
Cross section
Tunicate
100nm-μm
10-20nm
Bacterial
100nm-μm
5-10x30-50nm
Valonia
>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