Effect of Reaction Conditions on the Formation and Thermal Behavior of Cellulose Nanocrystals

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Effect of Reaction Conditions on the Formation
and Thermal Behavior of Cellulose Nanocrystals
Ilari 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
Nanotechnology For the Forest Products
Industry 13 15 June 2007 ? Knoxville,
Tennessee, USA
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Outline

• Brief Introduction/Background
• Objectives
• Production and Thermal Analysis of Cellulose
  Nanocrystals
• Structural Analysis
• Summary

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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

Glucose
Acid hydrolysis of cellulose to form cellulose
nanocrystals
Revol et al., Int. J. Biol. Macromol. 14,
170-172, 1992
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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

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Preparation of Cellulose Nanocrystals

• The cellulose pulp obtained from Whatman no.1
  (98 a-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

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Hydrolysis Reaction
HBr (50ml)
Solution
Cellulose Pulp (1 gram)
Acid Hydrolysis (oil bath, stirring)
Centrifugation
(1,500g)
Ultrasonication
Cellulose Suspension
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Purification Steps
Centrifug.
Supernatant off
5 cycles
contains cellulose nanocrystals
pH 1-2
pH 4-5
Centrifugated Suspension
Turbid Supernatant
Centrifugation (15,000g)
Collected Supernatant

Freeze drying
Remaining Sediment
Cellulose Nanocrystals
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The Effect of Reaction Time and Temperature (2.5M
HBr)
Ultrasonication During the Reaction
Ultrasonication After the Reaction
100C
100C
80C
80C
Yields increases along the reaction time in all
conditions applied.
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The Effect of Ultrasonication (2.5M HBr)
Reaction at 100ºC
Reaction at 80ºC
100
SC During
SC During
80
60
Yield ()
40
SC After
20
SC After
0
0
1
2
3
4
5
Time (hr)
At 80ºC ultrasonication, when applied during,
increased yields but at 100ºC the effect was not
significant (SC Ultrasonication).
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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.
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Optimized HBr Hydrolysis Conditions (2.5M)
Reaction at 100ºC
Optimal Conditions
68
SC During
SC After
Typical yields from hydrolysis with either HCl or
H2SO4 are around 40-45
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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)
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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
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Differential Scanning Calorimetry (DSC)
Loss of absorbed water
Cellulose nanocrystals
Sediment
Starting Cellulose
Tg was observed for cellulose powder and
unreacted cellulose but not for cellulose
nanocrystals
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Cellulose Crystallinity and ?Hvap of H2O

• 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

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5
O (kJ/g)
4
2
3
2
Heat of Evaporation H
1
0
0
10
20
30
40
50
60
70
80
Crystallinity Index ()
Bertran et al. J. Appl. Pol. Sci., 32, 4241-53,
1986
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Cellulose Nanocrystals and ?Hvap of H2O
100C SC during (2.5M HBr)
Cellulose Nanocrystals
Cellulose Sediment
The crystallinity of dispersed nanoparticles seem
to increase during the hydrolysis. Measurements
showed good reproducibility.
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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 2? 22.8 and
• Iam is the intensity of the background scatter
  measured at 2? 18
• The average crystallite size, in nm, was
  determined by the Debye-Scherrer formula
• D k ?Cu/ß cos?
• here k 0.9, ?Cu 0.154056 nm, ß FWHM (full
  width at half maximum, or half-width) in radians,
  ? the position of the maximum of diffraction.

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X-Ray Diffraction (XRD)
80 crystallinity (Cr.I.)
91 crystallinity (Cr.I.)
Count rate (cps) x103
Count rate (cps) x103
2? angle
2? angle
Starting Cellulose Cellulose
Nanocrystals
Acid hydrolysis increased the crystallinity of
cellulose particles
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Transmission Electron Microscopy

• The length distribution of
• cellulose nanocrystals were
• estimated from TEM images.
• Aggregation of cellulose
• whiskers hindered the
• determination of transverse dimensions

200 nm
XRD
3hr, 100ºC, HBr (2.5M) SC during
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Average sizes of Cellulose Nanocrystals
Transverse dimensions are based on XRD analysis.
Lengths were estimated from TEM images. Cr.I.
Crystallinity Index
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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)