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)