Liquid crystalline and gelling propertiesof cellulose nanocrystal suspensions

Liquid crystalline and gelling propertiesof cellulose nanocrystal suspensions

Derek GrayDepartment of Chemistry

McGill UniversityMontreal, Canada

International Liquid Crystal Elastomer Conference, Lisbon, September 7, 2011

Cellulose

• Homopolymer of glucose.• Insoluble, but hydrophilic surface.• Present as long, crystalline microfibrils.• Oxidizing chemicals may convert some hydroxyl groups to

carboxylic acids.

OCH2OH

H H

OHH

H

HO

H

H

O

HH

CH2OH

OH

O

OHO

H

OCH2OH

H H

OH

H

HO

H

O

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Elastic modulus of crystalline cellulose, >110 Gpa.

Cellulose properties

Like most polymers, cellulose is not completely crystalline, but containsdisordered regions. A variety of models seek to explain the dual nature ofcellulose. In the fringed micelle model, the cellulose chains pass throughboth crystalline regions and amorphous (disordered) regions.

Other models attribute the disorder to imperfections, chain ends and surfaceregions of the microfibril.

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Cellulose derivatives, esters and ethers were the first widelyused polymeric materials. An accidental discovery...

• Hydroxypropylcellulose dissolves in water at roomtemperature.

• It shows an upper consolute solution temperature ataround 40C.

• This led to the observation of an unusual liquidcrystalline phase (Werbowyj and Gray, 1976).

base

Propylene oxide

OCH2CH(OH)CH3

O

O

O

OCH2CHCH3

HO

OCH2CH(OH)CH3

OH

HO

OH

O

O

O

One of the first polymeric liquid crystal phases...

• Hydroxypropylcellulose (MS ~4, DS ~2.5) in water atroom temperature forms fluid mesophase at <~40% w/wconcentration.

• The solutions shows the iridescent colours characteristic ofa chiral nematic (cholesteric) liquid crystal.

• Soon, many other cellulose derivatives were found to formliquid crystalline phases.

• Recently, cross-linked HPC derivatives have been shownto form elastomeric thin films (Godinho et al., 2009-)

Gray, Carbohydrate Polymers, 1995, 25, 277-284.

Godinho et al., Cellulose, 2009, 16, 199-205.

~45% HPC/water, white light

© DGG

~45% HPC/water, crossed polars

© DGG

~55% HPC/water, crossed polars, planar texture with oily streaks

© DGG

Meanwhile, biologists had found chiral nematic order in naturalmaterials

• The groups of Bouligand, Neville and Rolland foundstructures with chiral nematic order in a wide range ofanimal and plant tissues.

• However, the ordered structural elements were notmolecular, but consisted of microfibrils of cellulose,chitin, collagen…

• The next step was to try and replicate these materials invitro. "Cellulose crystallites, cellulose nanocrystals, …"

Reviews:

Nanocelluloses:Klemm et al., Angew. Chemie Int.Ed., 2011, 50, 5438-5466.

Nanocrystalline cellulose:Habibi et al., Chem. Rev. 2010, 110,3479–3500.

Cellulosic nanomaterials:Moon et al., Chem. Soc. Rev., 2011,40, 3941–3994.

We consider only the relatively short nanocrystals preparedfrom Cellulose I by acid hydrolysis:-

•Preparation of cellulose nanocrystals.

•Chiral nematic (cholesteric) suspensions of nanocrystals inwater.

•Chiral nematic films cast from nanocrystal suspensions.

• Gelation and liquid crystal order in cellulose nanocrystalsuspensions.

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Fifty years ago, the production of colloidal suspensions ofcellulose by the sulphuric acid hydrolysis of cellulose fibreswas first reported by Rånby.

The hydrolysis produced rod-like particle of cellulose, withdimensions in the 0.5-100 nm range, which formed aqueouscolloidal suspensions stabilized by sulphate groups.

Nanocrystals from cotton(Mukherjee et al., 1952)

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1. Start withcellullose

2. Removecellulose

3. Product:cellulose

Tight control of starting material history and reactionconditions are necessary!

Cellulose nanocrystal preparation

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© DGG

The acid degrades the amorphousregions, leaving small cellulosecrystallites (nanocrystals).

Typical nanocrystal dimensionsfor acid-hydrolysed cotton,200 nm long, 10 nm wide

TEM image of dilute suspension on carbon gridRevol et al., (1992)

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J.-F. Revol, H.Bradford, J. Giasson, R.H. Marchessault andD.G. Gray, Int.J.Biol.Macromol., 14, 170-172 (1992).

Discovery! (J.-F. Revol et al. , 1992)

Cellulose crystallites formchiral nematic (cholesteric) suspensions!

• Cellulose source; wood pulps, ramie, cotton, bacterialcellulose.

• Hydrolysis; H2SO4 (63 wt%), 70°C, 30 min.• Wash/dialyse; water.• Disperse; brief ultrasonic treatment.

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J.-F. Revol, H.Bradford, J. Giasson, R.H. Marchessault and D.G. Gray,Int.J.Biol.Macromol., 14, 170-172 (1992).

A suspension of cellulose nanocrystals inpure water at low concentrations forms aclear stable isotropic fluid.

At higher concentrations, the nanocrystalsself-align to form a chiral nematic liquidcrystalline phase in equilibrium withisotropic phase.

Biphasic cellulosenanocrystal suspension(crossed polars)

Chiral nematic suspensions of cellulosecrystallites

(J.-F. Revol et al., 1992)

© DGG

Chiral nematic (cholesteric) arrangement of cellulosenanocrystal suspensions

P/2

Left-handed Right-handed(not observed)

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The ordered structure of the suspensions can bepreserved on drying

• Films with optical properties characteristic of chiralnematic liquid crystals can be prepared.

• The films reflect circularly polarized light with anarrow wavelength that varies with the angle ofviewing.

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Scale bar, 10 mm.

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© DGG

• The reflection colour of these films can becontrolled by simply adding a little salt to thedeionized suspension!

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© 2002 PapricanCHEMISTRYMcGill University

Planchettes embedded in paper and boardCHEMISTRYMcGill University

© DGG

© DGG

40 m

Image of part of a solid film cast from cellulosenanocrystal suspension

optical microscope, between crossed polars

The aim of the workwas to determine whatstructure could givethis pattern !

It looks like patternsshown by layer-likesmectic or some chiralnematic (cholesteric)liquid crystals.

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Roman and Gray, Langmuir, 21(12), 5555-5561 (2005)

-20 -10 10 20y

-20

-15

-10

-5

z

-20 -10 10 20x

-20

-15

-10

-5

z

-20 -10 10 20y

-10

-5

5

10z

-20 -10 10 20x

-10

-5

5

10z

Selected layers in a parabolic focal conic

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Roman and Gray, Langmuir, 21(12), 5555-5561 (2005)

Model of parabolic focal conic near centre xy plane showing layerorientation (left), and polarizing light microscope image of film

assembled from cellulose nanocrystals in same orientation (right)

Cellulose nanocrystals spontaneously self-assemble in complex, regular pattern!

Roman and Gray, Langmuir, 21(12), 5555-5561 (2005) CHEMISTRYMcGill University

Possible uses for Chiral Cellulose Films:• Anti-counterfeiting components in security papers.• As iridescent components in decorative laminates.• Optically variable non-toxic pigments.

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Possible uses for Cellulose Nanocrystals (NCC):• As reinforcing agent in composite materials.• As rheology modifier in cosmetics and foods.• As carrier or scaffold in biomedical applications.

Availability of NCC):• Research quantities from FPInnovations.• CelluForce pilot plant nearing completion.

Cellulose nanocrystal surfaces are hydrophilic:• For some applications, surface modification required.• Hydrophobicization required for compatibility with

most polymers.

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Gels containing NCC:• Magnetically oriented ordered gels for protein NMR.• NCC reinforcement of poly(vinyl alcohol) gels.

Cellulose nanocrystal surfaces are anionic:• A method of making cationic cellulose nanocrystals is

described. These formed thixotropic aqueous gels.

Cationic Surface Functionalization of CelluloseNanocrystals

EPTMAC HPTMAC-CNC

NONaOH CNC

OOH

NCl ClCNC-OH

• (2,3-Epoxypropyl)trimethylammonium chloride (EPTMAC)

• NaOH 7% w/v at 65oC for 5h

• Dilute to quench, dialyse and sonicateCNC-OH Cellulose nanocrystals

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•AFM images show no change in CNC surface morphology.•Cationic CNC makes better monolayers on anionic mica.

CNC before functionalization13 ± 3 × 176 ± 21 nm

HPTMAC-CNC (after)11 ± 2 × 174 ± 18 nm

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Cationic Surface Functionalization of Cellulose Nanocrystals

0.26 e/nm20.41 e/nm2Surface charge density

11 ± 2 × 174 ± 18 nm13 ± 3 × 176 ± 21 nmNanocrystal dimensions+30 ± 5 mV-39 ± 3 mVzeta-potential

HPTMAC-CNCUn-functionalized CNC

While the functionalized nanocrystals looked the same and thecharge density was similar

(with reversed charge)

there was one significant difference…

Cationic Surface Functionalization of Cellulose Nanocrystals

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• HPTMAC-CNC gelled at low concentrations (3.5%).• No chiral nematic liquid crystal phase separation was observed.• Shear birefringence was seen when suspensions were shaken.• Thixotropy and shear thinning were also evident.

Cationic Surface Functionalization of Cellulose Nanocrystals

CHEMISTRYMcGill UniversityHasani, Cranston, Westman, Gray, Soft Matter, 2008.

Cellulose nanocrystal surfaces are hydrophilic:• For some applications, surface modification required.• Hydrophobicization required for compatibility with

most polymers.

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Gels containing NCC:• Magnetically oriented ordered gels for protein NMR.• NCC reinforcement of poly(vinyl alcohol) gels.

Cellulose nanocrystal surfaces are anionic:• A method of making cationic cellulose nanocrystals is

described. These formed thixotropic aqueous gels.

The nanocellulose suspensions respond to a magnetic field

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Individual cellulose nanocrystals have anegative diamagnetic anisotropy which alignstheir long axes orthogonal relative to an appliedmagnetic field. So chiral nematic axis orientsalong field.

Residual dipolar couplings (RDCs) provide long-range information onthe orientation of inter-nuclear vectors within a molecular frame ofreference and are thus valuable in structural and dynamicalinvestigations of macromolecules.•RDCs result from anisotropic averaging of the orientation-dependentdipolar coupling interaction when molecules are dissolved in an orderedmedium, such as a liquid crystal.•We have worked out a detailed protocol for preparing oriented samplesusing liquid crystalline suspensions of cellulose nanocrystals, andoffer guidelines for when this medium can be employed in biomolecularNMR. The cellulose systems requires gelation of the ordering mediumin the magnetic field.

• Fleming, Gray, Prasannan and Matthews, J. Am. Chem. Soc., 122, 5224-5225 (2000)

• Denisov, Kloser, Gray, Mittermaier, J. Biomolecular NMR, 47, 195–204 (2010).CHEMISTRYMcGill University

Cellulose nanocrystal suspensions can be used as protein orientationsystem for NMR residual dipolar coupling measurements 1,21,2

Polyvinyl Polyvinyl alcohol/cellulose nanocrystal hydrogelsalcohol/cellulose nanocrystal hydrogels

Hydrogels are polymeric networks, held together by crosslinks, whichswell but do not dissolve in water.

Physically cross-linked PVA hydrogels:

Physically cross-linked hydrogel

Freeze Thaw

# of cycles

temperature, time

Important parameters: MW, concentration, #, temp & time of cycles

Yokoyama, Masada, Shimamura, Ikawa & Monobe,Colloid and Polymer Science, 1986, 264, . 595-601

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Abitbol, Johnstone, Quinn and Gray Soft Matter, 7(6), 2373-2379 (2011)

With each freeze, polymer is increasingly rejected from ice.

Resultant gel:

Nature of Nature of physical gel formation in aqueous PVA systemsphysical gel formation in aqueous PVA systems

Water-filled pore

Cross-links

Polymer

CNCs

What happens when cellulose nanocrystals (CNCs) are added?

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Swelling of hydrogels in waterSwelling of hydrogels in water

Swelling results:

Increased swelling&

Structural resilience

1

1.05

1.1

1.15

1.2

1.25

0 50 100 150 200 250 300 350 400

W /

Wf - pure PVA hydrogel

- 0.75 wt. % CNC- 1.5 wt. % CNC- 3.0 wt. % CNC

x CNCs CNCs

Time (minutes)

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Abitbol, Johnstone, Quinn and GraySoft Matter, 7(6), 2373-2379 (2011)

CNC reinforced PVA hydrogelsCNC reinforced PVA hydrogels

CNCs do reinforce gels! 1.5 wt. % CNC loading seems optimal.

0

500

1000

1500

2000

2500

3000

3500

4000

0 0.05 0.1 0.15 0.2 0.25

0

5000

10000

15000

20000

25000

30000

35000

40000

0.05 0.1 0.15 0.2 0.25

Stress versus strain curves:

- pure PVA hydrogel- 0.75 wt. % CNC- 1.5 wt. % CNC- 3.0 wt. % CNC

Strain

Equi

libriu

m st

ress

(Pa

)

Strain

Cho

rd m

odul

us (P

a)

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Abitbol, Johnstone, Quinn and GraySoft Matter, 7(6), 2373-2379 (2011)

• The addition of CNCs into PVA hydrogels resulted in gels which were lesscrystalline and swelled to a greater extent, but had improved elasticproperties.

• The cellulose nanocrystals also induce order in the PVA hydrogels formedby freeze-thaw processing.

Abitbol, Johnstone, Quinn and Gray Soft Matter, 7(6), 2373-2379 (2011)

PVA, no cellulosenanocrystals

PVA with 1.5 wt%cellulose nanocrystals

53 freeze-thaw cycles

1

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Abitbol, Johnstone, Quinn and GraySoft Matter, 7(6), 2373-2379 (2011)

Conclusions

•Cellulose derivatives form lyotropic and thermotropic chiralnematics. Left and right-handed phases are observed.

• Cellulose nanocrystals also form chiral nematic colloidcrystals. To date only left-handed phases have beenobserved.

•Solid cellulose films with chiral nematic structure readilyform on evaporation of water.

•Cellulose nanocrystals can also form gels, and can reinforceaqueous polymer gels.

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Acknowledgements

• Revitalizing liquid crystalline cellulosics: M.-H. Godinho et al.• Preparation and EM of cellulose nanocrystals: J.-F. Revol.• Phase separation of suspensions: Xue-Min Dong• Structure of cellulose nanocrystal films: Dr. Maren Roman• Cationic nanocrystals: Merima Hasani and Emily Cranston• Protein NMR: Steve Matthews group, Imperial College London• PvOH gels: Tiffany Abitbol

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Support from NSERC, FPInnovations/Paprican*, FQRNT