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Effect of Reaction Conditions on the Formation and Thermal Behavior of
Cellulose NanocrystalsIlari Filpponen, Xingwu Wang, Lucian A. Lucia
Dimitris S. Argyropoulos
Organic Chemistry of Wood Components Laboratory
Department of Forest Biomaterials Science & Engineering
North Carolina State University
Raleigh, North Carolina, USA
2007 International Conference on NanotechnologyFor the Forest Products Industry
13 – 15 June 2007 ● Knoxville, Tennessee, USA
Outline
• Brief Introduction/Background
• Objectives
• Production and Thermal Analysis of Cellulose Nanocrystals
• Structural Analysis
• Summary
From Bulk Cellulose to Cell-Nanocrystals
• Cellulose is one of the most abundant natural biopolymers which upon acid hydrolysis yields highly crystalline rod-like rigid hydrophilic particles having nanoscale dimensions
Acid hydrolysis of cellulose to form cellulose nanocrystals
+ Glucose
Revol et al., Int. J. Biol. Macromol. 14, 170-172, 1992
Experimental-Overall Objectives
• Optimization of the manufacturing process and utilization of thermal analysis for the characterization of cellulose nanocrystals
• Understanding the size and uniformity of nanocrystals in relation to the manufacturing process
Preparation of Cellulose Nanocrystals
The cellulose pulp obtained from Whatman no.1 (98% α-cellulose, 80% crystallinity) filter paper was used as starting material
In this study hydrobromic acid was used in different concentrations (1.5M, 2.5M and 4.0M), respectively
The effect of reaction times, temperatures and applied external energy (ultrasonication during or after the hydrolysis) to the yields were investigated
Cellulose Pulp (1 gram) Acid Hydrolysis (oil bath, stirring)
Ultrasonication Cellulose Suspension
Hydrolysis Reaction
Centrifugation
HBr (50ml)
(1,500g)
Solution
Centrifugated Suspension Turbid Supernatant
Remaining Sediment Cellulose Nanocrystals
Purification Steps
pH 1-2
Supernatant off
pH 4-5
Centrifugation (15,000g)
+ Freeze drying
+
5 cycles
Centrifug.
contains cellulose nanocrystals
CollectedSupernatant
The Effect of Reaction Time and Temperature (2.5M HBr)
Yields increases along the reaction time in all conditions applied.
Ultrasonication After the Reaction Ultrasonication During the Reaction
0
20
40
60
80
100
0 1 2 3 4 5
Time (hr)
Yie
ld (
%)
0
20
40
60
80
100
0 1 2 3 4 5
Time (hr)
Yie
ld (
%)
100°C
80°C
100°C
80°C
The Effect of Ultrasonication (2.5M HBr)
Reaction at 100ºCReaction at 80ºC
At 80ºC ultrasonication, when applied during, increased yields but at 100ºC the effect was not significant (SC = Ultrasonication).
0
20
40
60
80
100
0 1 2 3 4 5
Time (hr)
Yie
ld (
%)
0
20
40
60
80
100
0 1 2 3 4 5
Time (hr)
Yie
ld (
%)
SC During
SC After
SC During
SC After
Yields with Different HBr Concentrations
The yields were seen to increase significantly when acid concentration was Increased from 1.5M to 2.5M. With 4.0 M HBr unwanted reactions were observed.
1.5 M
2.5 M 4.0 M
0
10
20
30
40
50
60
70
80
2hr, 100ºC SCDuring
2hr, 100ºC SCAfter
4hr, 100ºC SCDuring
4hr, 100ºC SCAfter
Hydrolysis Conditions
Yie
lds
(%)
Optimized HBr Hydrolysis Conditions (2.5M)
Reaction at 100ºCOptimal Conditions
Typical yields from hydrolysis with either HCl or H2SO4 are around 40-45%
0.00
20.00
40.00
60.00
80.00
100.00
0 1 2 3 4 5
Time (hr)
Yie
ld (
%)
SC During
SC After
68%
Thermal Analysis
Hypothesis:
Thermal analysis may provide a convenient and rapid tool for the determination and correlation of various physicochemical properties of cellulose nanocrystals (crystallinity, crystal dimensions)
Thermal Analysis
Thermogravimetric analyses (TGA):
Information provided: Thermal degradation, total amount of water (%).
Differential Scanning Calorimetry (DSC):
Information provided: endothermic water evaporation peak (J/g), apparent maximum at around 120-130ºC
Samples were kept in constant humidity (69%) before analysis and measurements were duplicated
Differential Scanning Calorimetry (DSC)
-20 30 80 130 180 230 280 330
Temperature (oC)
Heat
Flo
w (
W/g
)
Loss of absorbed water
Tg was observed for cellulose powder and unreacted cellulose but not for cellulose nanocrystals
Cellulose nanocrystals
Starting Cellulose
Sediment
• Bertran et al. studied the correlation between the cellulose crystallinity and enthalpy of evaporation of absorbed water by using DSC
• Higher crystallinity decreased the energy needed for water removal. Results were in good agreement with X-ray diffraction measurements
Cellulose Crystallinity and ΔHvap of H2O
Bertran et al. J. Appl. Pol. Sci., 32, 4241-53, 1986
0
1
2
3
4
5
6
0 10 20 30 40 50 60 70 80
Crystallinity Index (%)
Hea
t o
f E
vap
ora
tio
n H
2O (
kJ/g
)
Cellulose Nanocrystals and ΔHvap of H2O
0
1
2
3
4
5
6
7
8
0 1 2 3 4 5
Time (hr)
Hea
t o
f E
vap
ora
tio
n H
2O (
kJ/g
)
100°C SC during (2.5M HBr)
The crystallinity of dispersed nanoparticles seem to increase during the hydrolysis. Measurements showed good reproducibility.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 1 2 3 4 5
Time (hr)
Hea
t o
f E
vap
ora
tio
n H
2O (
kJ/g
)Cellulose Nanocrystals Cellulose Sediment
X-ray Diffraction of Cell-Nanocrystals
• Crystallinities were calculated according to Segal et al.
Cr.I. (%) = ((I002 –Iam) / I002) x 100
where I002 is the maximum intensity from (002) plane at 2θ = 22.8° and Iam is the intensity of the background scatter measured at 2θ = 18°
• The average crystallite size, in nm, was determined by the Debye-Scherrer formula:
D = k λCu/β cosθ
here k = 0.9, λCu = 0.154056 nm, β = FWHM (full width at half maximum, or half-width) in radians, θ = the position of the maximum of diffraction.
X-Ray Diffraction (XRD)
Starting Cellulose Cellulose Nanocrystals
Acid hydrolysis increased the crystallinity of cellulose particles
2θ angle
Co
un
t ra
te (
cps)
x10
3
Co
un
t ra
te (
cps)
x10
32θ angle
80% crystallinity (Cr.I.) 91% crystallinity (Cr.I.)
Transmission Electron Microscopy
3hr, 100ºC, HBr (2.5M) SC during
• The length distribution of cellulose nanocrystals were estimated from TEM images.
• Aggregation of cellulose whiskers hindered the determination of transverse dimensions
XRD200 nm
n.d.8.38.291100°C, SC during, 3hr
(sediment)
100-4007.78.691100°C, SC during, 3hr
100-4007.77.689100°C, SC during, 2hr
100-4007.67.088100°C, SC during, 1hr
Length
(nm)
Transverse 2
(nm)
Transverse 1
(nm)
Cr.I.Sample
Average sizes of Cellulose Nanocrystals
Transverse dimensions are based on XRD analysis. Lengths were estimated from TEM images. Cr.I. = Crystallinity Index
Summary• Reaction conditions play a significant role in
determining the yield of cellulose nanocrystals
• Ultrasonication during the hydrolysis reaction improved the yields of cellulose nanocrystals and allowed lower reaction temperatures
• Thermal analysis is seen to provide information that currently is attempted to be correlated with various physicochemical properties of the cellulose nanocrystals (work in progress)