Strain-induced uniaxial alignment of cellulose nanocrystals in polymer nanocomposites

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Strain-induced uniaxial alignment of cellulose nanocrystals in polymer nanocomposites

Gumfekar, Sarang P.; Wadood, Mohamed; Cho, Jae-Young; Elias, Anastasia L.; Harris, Kenneth D.

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Strain-induced Uniaxial Alignment of Cellulose

Nanocrystals (CNCs) in Polymer Nanocomposites

Sarang Gumfekar

National Research Council of Canada –

Nanotechnology Research Center Edmonton, Canada

Department of Mechanical Engineering University of Alberta, Edmonton, Canada

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Cellulose Nanocrystals (CNCs)

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• Intermolecular and intramolecular hydrogen bonding • Opportunities for various surface functionalization

• Crystalline structure→ suitable as reinforcement material • Our CNC source: Innotech Alberta

500 1000 1500 2000 2500 3000 3500 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Abs orba nc e ( A.U.) Wavenumber (cm-1₎ Intramolecular bond Intermolecular bond 30 nm Scale: 1.7μm

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Alignment: Benefits and Methods

Benefits

• Manipulation of optical properties (birefringence)

• Unidirectional amplification of mechanical and rheological properties

• Anisotropic electrical properties (in case of conductive fillers)

Methods reported in literature

• Using electric field (applicable for only electrically conducting fillers) • Using magnetic field (applicable for only magnetic fillers)

• Dry spinning process (applying force in various directions)

Why strain-induced technique?

• Simple and robust technique

• Involves stretching the film at controlled strain and temperature • Can be used for non-conducting and non-magnetic materials

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Strain-induced Alignment

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Polyvinyl alcohol (PVA)-CNC Film: 90mm  20mm  300m Film formation: mold casting

Stretching 5 mm/min

23

o

_{C or 70}

o

_C

Heating inside the chamber

Initial composition

2 mg CNC 5 mg PVA 93 mg Water

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Mechanical Properties-Tensile Strength

Tests performed at 23 o_C 0 0.5 2 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Te ns ile st reng th (M Pa) CNC concentration (wt. %) Control @23C Control @70C 0 0.5 2 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Te ns ile st reng th (M Pa) CNC concentration (wt. %) Previously stretched @23C Previously stretched @70C

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Mechanical Properties-Elastic Modulus

6 0 0.5 2 0 3 6 9 12 15 18 Elas tic mod ul us (M Pa) CNC concentration (wt. %) Control @23C Control @70C 0 0.5 2 0 5 10 15 20 25 30 Elas tic mod ul us (M Pa) CNC concentration (wt. %) Previously stretched @23C Previously stretched @70C Tests performed at 23 o_C

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Rheological Properties:

Dynamic Mechanical Analysis (DMA)

• Non-aligned CNCs: Axial modulus = 0.55 x Transverse modulus

• Aligned CNCs: Axial modulus = 3.10 x Transverse modulus

20 40 60 80 100 120 140 0.0 5.0x108 1.0x109 1.5x109 2.0x109 2.5x109 3.0x109 3.5x109 20 40 60 80 100 120 140 0 1x109 2x109 3x109 4x109 5x109 M o d u lu s ( P a ) Temperature (o_C) 2%CNC Unstretched Transverse 2%CNC Unstretched Axial M o d u lu s ( P a ) Temperature (o_C) 2%CNC Stretched Transverse 2%CNC Stretched Axial

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Structural Properties: X-Ray Diffraction

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X-Ray Detector

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Structural Properties: X-Ray Diffraction

• No change in crystallinity of polymer after stretching

• There is limited natural alignment of CNC in polymer without stretching

10 15 20 25 30 35 40 0 1 2 3 4 5 6 7 8 In te ns ity (AU) 2 (deg.) 0%CNC_Stretched_Phi 0 0%CNC_Stretched_Phi 90 10 15 20 25 30 35 40 0 1 2 3 4 5 6 7 8 In te ns ity (AU) 2 (deg.) 2%CNC_Unstretched_Phi 0 2%CNC_Unstretched_Phi 90

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Structural Properties: X-Ray Diffraction

10 10 15 20 25 30 35 40 0 2 4 6 8 10 12 In te ns ity (AU) 2 (deg.) 2%CNC_Stretched_Phi 0 2%CNC_Stretched_Phi 45 2%CNC_Stretched_Phi 90 10 15 20 25 30 35 40 0 2 4 6 8 10 12 In te ns ity (AU) 2 (deg.) 2%CNC_Stretched_Phi 0 2%CNC_Unstretched_Phi 0

• As film orientation changes, crystallinity ‘seen’ by X-Rays changes

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Optical properties: Absorbance

• Enhanced scattering after CNC alignment

• Overall increase in scattering with increase in CNC content of the composite

200 300 400 500 600 700 800 0.2 0.4 0.6 0.8 1.0 1.2 1.4 200 300 400 500 600 700 800 0.2 0.4 0.6 0.8 1.0 1.2 1.4 200 300 400 500 600 700 800 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Ab s o rb a n c e ( AU ) Wavelength (nm) 0% CNC_Stretched 0% CNC_Unstretched Ab s o rb a n c e ( AU ) Wavelength (nm) 0.5% CNC_Stretched 0.5% CNC_Unstretched Ab s o rb a n c e ( AU ) Wavelength (nm) 2% CNC_Stretched 2% CNC_Unstretched

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Alignment Morphology- CNC 0%

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Before stretching After stretching

Surface

Cross-section

Surface

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Alignment Morphology 2% CNC

CNCs not aligned Aligned CNCs

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Conclusions

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• Strain-induced alignment is an effective, easy, and robust technique to

align CNCs

• Alignment of CNCs is facilitated above the glass transition temperature of

the film

• Mechanical, rheological, and structural properties become anisotropic

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