© Fraunhofer IAP
Characterization of cellulose and nanocellulose via NMR and permeation
measurements
COST Training School
“Characterization of nanocellulose“
17.01.2017
Melanie Bartel, Andreas Ebert, Johannes Ganster
Fraunhofer Institute for Applied Polymer Research IAP
Natural Polymers Division
Department Materials Development and Structure Characterization
Potsdam, Golm
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Content
Principles of NMR characterization
Liquid state NMR characterization of cellulose and derivatives
Determination of the degree of substitution
13C APT NMR spectrum and structure determination with the help of 2D COSY NMR
Comparison between 13C liquid and solid state NMR
Solid state NMR characterization of cellulose and derivatives
Determination of the degree of substitution
Identification of cellulose polymorphs and determination of the crystallinity
Influence of nanocellulose coating on the barrier properties of PLA
Summary
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Non-destructive analysis method for solids and liquids
Strong external magnetic field (i.e. 11.7 T for 500 MHz 1H, 125 MHz 13C)
Existence of nuclear spin J (nuclear spin quantum number I ≠ 0 for odd
proton and/or neutron number – oe/eo, oo nuclei)
Magnetic moment µ = 𝛾 J
Zeeman splitting in magnetic field
Quantization of direction in external magnetic field Jz = m ħ
(m magnetic quantum number)
Precession of the nuclear dipoles with the larmor frequency in external
magnetic field B0 with energy E = - µz B0 = - m 𝛾 ħ B0
Thermal equilibrium according to Boltzmann distribution
Longitudinal and transverse relaxation T1 and T2
Fourier Transformation of FID results in the spectrum
Principles of Nuclear Magnetic Resonance characterization
Literature: http://zerpoii.opentronix.com/?m=201505, date: 05.01.2017
J = I ( I + 1 ) ħ
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Ndown
Nup
= e − ΔE / kBT
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Measured parameter
Most frequently investigated nuclei: 1H, 2D, 13C, 19F, 29Si, 31P
Chemical shift
different electronic environments of the nuclei yield in different larmor frequencies
chemical shift in ppm
Identification of functional groups, chemical environments
Signal integrals
proportionality between signal integrals and the number of the considered nuclei
quantitative composition of the sample
Coupling constants
interaction with neighbored spins results in characteristic signal fine structure
Number of neighbor nuclei, estimation of substituents, molecular structure in the broader sense
Line width and shape
Anisotropies, solubility, molecular weight, interactions
Relaxation times
Molecular mobility, interactions
Principles of Nuclear Magnetic Resonance characterization
𝛾 ( 1 - 𝜎 ) B0 / 2𝜋 = ߥ
δ = ( ߥ - ߥref ) [Hz] / ߥref [MHz]
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Liquid state NMR investigations
INOVA 500 (Varian, 2000) spectrometer
11.7 T Oxford AS superconducting magnet
1H 500 MHz, 13C 125 MHz
2 RF channels, 0.1 Hz resolution
Motorola Acquisition Computer, 12.5 ns Auflösung, 16-bit ADC
5 mm switchable Probe 1H – 19F / 15N – 31P
5 mm PFG indirect detection probe
Solid state NMR investigations
INOVA 400 (Varian, 1991) spectrometer
9.4 T Oxford superconducting magnet
1H 400 MHz, 13C 100 MHz
2 RF channels, 0.1 Hz resolution
Motorola Acquisition Computer, 12.5 ns Auflösung, 16-bit ADC
5 mm CP/MAS Probe 1H / 2H – 31P , up to 10 kHz spinning speed
Requirements to the samples : liquid state NMR: > 5 mg (1H) and > 20 mg (13C), solubility in deuterated solvents
solid state NMR: ~ 500 mg, all solids that can be homogeneously filled in the rotor, grinding
IAPs NMR Laboratory
5 / 23
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Determination of the degree of substitution of methyl -, hydroxypropyl cellulose / s tarch
13C NMR in solution
MHPC in solution results in broad lines
Estimation of DS
Methyl groups of the hydroxypropyl group
Methoxy groups
MHPC hydrolysed in TFA
Quantification of DS
Limit for determination of DS around 0.02
Liquid state NMR characterization of cellulose and derivatives
6 / 23
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13C APT NMR spectrum of methyl cellulose
APT (90°-t-180°-t-at) spectrum to differ quarternary and secondary carbons from primary and tertiary carbons
Liquid state NMR characterization of cellulose and derivatives
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Structure determination with the help of 2D 1H-1H COSY NMR
1H-1H COSY: 2-dimensional homonuclear correlation spectroscopy
Pulse sequence: 90°x – t – 90°x – at
with t as measure variable
No exact correlation between the protons and the signals in the spectrum possible
Solution: Correlation between neighbored protons for structure determination
Liquid state NMR characterization of cellulose and derivatives
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1 3
4
5
6a 6b
1
Acetyl-CH3
2
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Structure determination with the help of 2D 1H-1H COSY NMR
Measured spectrum has two frequency axis (2 FT)
Further 2D NMR experiments
1H-13C HETCOR
13C-13C INADEQUATE, …
Liquid state NMR characterization of cellulose and derivatives
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1
4
2 5
6a 6b
3
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Structure determination with the help of 2D NMR 1H-1H COSY
Correlation peaks form one corner of a square
Cross peaks form the diagonal of the spectrum
starting point: cross peak of known proton
Liquid state NMR characterization of cellulose and derivatives
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1,2
4,5 3,4
2,3
6a,6b
5,6b 5,6a
1 3,4
2 5
6a 6b
1
4
2 5
6a 6b
3
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Comparison between 13C liquid and solid state NMR us ing the example of methyl cellulose
Solid state NMR
Limited mobility broadens signals
Substituents can often be identified
MS difficult or not quantifiable
Signals can overlap with spinning sidebands
Liquid state NMR of the polymer
Broadened signals compared to
lower molecular weight substances
Identification of derivatives
Estimation of DS
Liquid state NMR of the hydrolyzed sample
Quantification of the DS values of cellulose (DS6, DS2, DS3)
Difference in chemical shifts high enough for signal separation
Low line width of the signals
Hydrolysis of the derivatives (ether)
No scission of the substituent required
NMR characterization of cellulose and derivatives
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Characteristics of 13C solid state NMR measurements
MAS Magic Angle Spinning
At an angle of 54.7° concerning the z-axis because of angle dependent weak dipolar coupling and anisotropy
Anisotropic chemical shift goes to its isotropic value with spinning side bands
Spinning simulates the isotropic molecular motion in liquids
Lower line widths
DD Dipolar Decoupling
elimination of strong heteronuclear dipolar couplings (kHz)
CP Cross Polarization
Problems: 13C nuclei with low natural abundance and low 𝛾
Magnetization transfer from protons to carbons
energy conserved
Hartmann-Hahn-condition 𝛾C B1C = 𝛾H B1H
Solid state NMR characterization of cellulose and derivatives
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Determination of the DS of cellulose derivatives
Requirements :
separable signal of the substituent outside of unsubstituted cellulose signals
proportionality between signal and number of carbons
Possible determination of degree often with the help of the end groups of substituents
DS: ratio of 1 carbon of the substituent / normed C1 of cellulose
Methylcellulose:
overlapping signals
possibility:
deconvolution of the spectrum
assuming C6 signal of cellulose
Linter: typically cellulose I spectrum
Solid state NMR characterization of cellulose and derivatives
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Literature: Kunze, Fink Wissenschaft + Technik 1999, 12
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Identification of cellulose polymorphs
Cause: nonequivalent 13C nuclei and extensively frozen molecule conformation
Native cellulose I appear with 2 main crystal modifications I𝛼 and I𝛽
C1 splitting in 3 signals with the central line for I𝛼
Possible determination of the ratio I𝛼 / I𝛽 and the composition cellulose modifications
Experimental spectrum consists of a mixture of cellulose I, II and III
Required: pure spectra of the assumed components
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Solid state NMR characterization of cellulose and derivatives
Cellulose I Cellulose II
Literature: Kunze, Fink Wissenschaft + Technik 1999, 12
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Determination of the “NMR crystallinity”
“NMR crystallinity“ 𝛘c denotes the degree of arranged cellulose to the total amount of cellulose
Crystalline cellulose molecules have a different chemical environment compared to the amorphous part
different chemical shifts for the amorphous and crystalline cellulose chains
Deconvolution of the spectrum particularly with regard to the C4 peak
In good agreement with the experimental data
Solid state NMR characterization of cellulose and derivatives
𝛘c = Ic
Ic + Ia
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𝛘c ~ 34 %
Literature: Kunze, Fink Wissenschaft + Technik 1999, 12
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Influence of the measurement parameters on the spectrum
Mobility selective measurements possible
Partially converted linters into alkali cellulose: swollen solid with liquid-like and solid-like areas
MAS with long relaxation delay d1 : real spectrum (long T1 of rigid molecules)
MAS with short relaxation delay: over determination of disordered molecules
CPMAS: method for rigid systems with strong dipolar couplings
Solid state NMR characterization of cellulose and derivatives
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Literature: Kunze, Fink Wissenschaft + Technik 1999, 12
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Barrier properties of cellulos ic materials
Packaging applications without special requirements
in the range of oxygen transmission rate and water vapor transmission < 10 cm³/(m²dbar) or g/(m²d)
Comparison with common materials show high barrier against oxygen
Hydrophilic cellulose high water vapor transmission
Cellulose with a lot of application possibilities
coating
multi-layer-films
processing of composites
…
Influence of nanocellulose coating on the barrier properties of PLA
Literature: H.-C. Langowski: Anwendung der Nanotechnologie in Materialien für den Lebensmittelkontakt
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18 / 23
Influence of nanocellulose coating on the barrier properties of PLA
Literature: H.-C. Langowski: Anwendung der Nanotechnologie in Materialien für den Lebensmittelkontakt
Barrier properties of cellulos ic materials
Packaging applications without special requirements
in the range of oxygen transmission rate and water vapor transmission < 10 cm³/(m²dbar) or g/(m²d)
Comparison with common materials show high barrier against oxygen
Hydrophilic cellulose high water vapor transmission
Cellulose with a lot of application possibilities
coating
multi-layer-films
processing of composites
…
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19 / 23
Nanocellulose water dispersion with thixotropic behavior
SEM of the dried dispers ions
Coating: 1. Activation of the surface 2. Nanocellulose coating
Corona treatment
Surface oxidation
Increasing polarity of the surface
Generating radicals
Influence of nanocellulose coating on the barrier properties of PLA
Literature: http://www.nature.com/srep/2012/121114/srep00849/images_article/srep00849-f1.jpg
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Composite-layer film composed of a ~ 13 µm thick nanocellulose coating and PLA4043D
Reduction in oxygen transmiss ion rate on 1 %
Increase in water vapor transmiss ion on 115 %
Influence of nanocellulose coating on the barrier properties of PLA
PLA4043D O2 -TR
[cm³mm/m²d]
WDD100
[g/m²d]
Without Nanocellulose coating 16,2 ± 0,7 33 ± 2
With Nanocellulose coating < 0,1 38 ± 4
Nanocellulose coating from top Cross-section
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Influence of nanocellulose coating on the barrier properties of PLA
PLA4043D O2 -TR
[cm³mm/m²d]
WDD100
[g/m²d]
Without Nanocellulose coating 16,2 ± 0,7 33 ± 2
With Nanocellulose coating < 0,1 38 ± 4
Nanocellulose coating from top Cross-section
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Composite-layer film composed of a ~ 13 µm thick nanocellulose coating and PLA4043D
Reduction in oxygen transmiss ion rate on 1 %
Increase in water vapor transmiss ion on 115 %
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Summary
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NMR characterization of cellulose and cellulose derivatives
Very important method for liquids and solids for the determination of
structure – spectrum pattern, 2D
Liquids : degree of substitution – pretreatment
Solids : degree of substitution – chemical shift
crystal modifications
“NMR crystallinity”
motion selective measurements
Barrier properties of nanocellulose coating on PLA
Cellulose high barrier against oxygen
poor barrier against water vapor
Nanocellulose coating on PLA
improves oxygen transmission rate to ~ 1 %
increases water vapor transmission to ~ 115 %
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Thank you for your attention! See you tomorrow to the practical training.
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APT – Attached Proton Test
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Relaxation times
