Cellulose Nanofibrils Films: Molecular Diffusion through Elongated

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Cellulose Nanofibrils Films: Molecular Diffusion through Elongated Sub-Nano Cavities David Roilo, Cecilia Ada Maestri, Marina Scarpa, Paolo Bettotti, Werner Egger, Tönjes Koschine, Roberto Sennen Brusa, and Riccardo Checchetto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02895 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017

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The Journal of Physical Chemistry

Cellulose Nanofibrils Films: Molecular Diffusion through Elongated SubNano Cavities

David Roilo,† Cecilia Ada Maestri,‡,◊ Marina Scarpa,‡ Paolo Bettotti,‡ Werner Egger,§ Tönjes Koschine,§ Roberto Sennen Brusa# and Riccardo Checchetto†,*

†

IdEA Laboratory, Department of Physics, University of Trento, Via Sommarive 14, I-

38123 Povo (TN), Italy ‡

Nanoscience Laboratory, Department of Physics, University of Trento, Via

Sommarive 14, I-38123 Povo (TN), Italy ◊

Centre for Integrative Biology (CIBIO), University of Trento, Via Sommarive 9, I-

38123 Povo (TN), Italy §

Universität der Bundeswehr (München) und Institut für Angewandte Physik und

Messtechnik, LTR 2 Werner Heinsenberg Weg 39, 85577 Neubiberg, Germany #

Department of Physics, University of Trento and INFN-TIFPA, Via Sommarive 14, I-

38123 Povo (TN), Italy *

Email: [email protected]; telephone: +390461281650

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ABSTRACT: We have studied the kinetics of gas transport through films made of self-assembled cellulose nano-fibrils (CNF) by a time- resolved mass spectroscopy technique. Few µm thick films deposited on polylactic acid (PLA) substrates act as impermeable barriers for CO2, O2 and N2 and reduce the 2H2 (deuterium) and He permeation flux by a factor ∼ 103 with respect to the uncoated substrate. Penetrant transport is controlled by the solution-diffusion mechanism and the coating acts as a diffusive barrier. 2H2 and He diffusivity values are in the 10-10 and 10-9 cm2 s-1 range, respectively, and their migration occurs by thermally activated process with 39 ± 1 kJ mol-1 and 33 ± 2 kJ mol-1 activation energy. Positron Annihilation Lifetime Spectroscopy analysis indicates that the diffusive path between the packed nanofibrils consists of elongated cavities with cross sectional size ∼ 0.31 nm. Results evidence that the selective transport of the small size penetrants is due to sieving effects and that small penetrant transport occurs in configurational diffusion regime.


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1. INTRODUCTION Packaging technology is currently based on the use of petroleum- derived polymer materials such as polyvinyl alcohol (PVA), polyvinylidene chloride (PVC) and polyethylene terephthalate (PET).1 Environmental concerns, production costs and end-of-life disposal challenges require the introduction of innovative packaging materials produced using green technologies and able to meet requirements such as biodegradability and carbon neutrality.2 Cellulose nanofibrils (CNF) are highly stable nanostructures obtained from cellulose pulp that show excellent optical and mechanical properties.3 CNF are obtained by mechanical treatment of cellulose fibers breaking them into fibrils: these fibrils contain amorphous and crystalline regions of cellulose, have diameter ∼ 5 nm and length up to several micrometers.4 CNFs can also be prepared by acid hydrolysis of the cellulose fibers. Acids attack the non-crystalline fraction of the fibers and produce nanoparticles with similar diameter but length in the 100 nm range, often called cellulose nanocrystals.5 The large aspect ratio of the cellulose nanofibrils and their ability to form intra- and inter-fibrillar hydrogen bonds permit them to assemble in dense films exhibiting high optical transparency and low gas permeability.6 Measurements carried out by Rodionova et al.,7 Aulin et al.8 and Syverud and Stenius9 on nanocellulose self-supporting films with thickness in the µm range have evidenced, in fact, O2 permeability values competitive with those of synthetic polymers for packaging applications such as ethylene vinyl alcohol (EVOH) or polyvinyl alcohol (PVOH).10 Fukuzumi et al. demonstrated that sub-micrometer thick CNF layers act as effective gas permeation barriers also when deposited as a surface coating on commercial polymers.11 CNF films and CNF surface coatings are thus attracting great technological interest in order to replace the petroleum- based commercial packaging materials. A literature survey indicates that the gas barrier performances of CNF films depend on the nature of penetrant molecules

12

and sample temperature

13

and also on CNF structural factors 3

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such as the fibril entanglement, 14 the surface functionalization of the cellulose nanofibrils

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15

and

their length, 16 which are supposed to determine the tortuosity of the penetrant migration path. In this paper, we present a study on the gas transport through bilayer membranes consisting of μm thick CNF layers deposited on polylactic acid (PLA) films. The bilayer membrane samples were prepared by a liquid casting technique using a procedure described in Ref. 17 and structurally characterized by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The main aim of this study is to investigate in detail the mechanism of penetrant molecules transport through well characterized CNF layers and evidence the mechanism behind the excellent barrier performances of the nanocellulose coatings. To this task, we used test gases with different molecular sizes and condensation properties and studied the gas transport kinetics at different temperatures. Measurements were carried out by monitoring the gas permeation flux as function of time by a mass spectroscopy technique: this procedure permitted the detection of the gas permeation signal with a signal-to-noise ratio large enough to separately determine the gas diffusivity and solubility at each of the examined temperatures. Experimental results on the gas diffusivity are discussed using experimental information on their void structure obtained by depth- profiling the CNF layers by Positron Annihilation Lifetime Spectroscopy analysis. 18 The reason behind the use of PLA as substrate is double. First, this polymer has a hydrophilic character and thus permits the deposition of the CNF coatings without necessity of plasma treatments or surface functionalization to ensure coating adhesion. These procedures will affect, in fact, the gas transport kinetics. Second, PLA is a highly transparent bio-plastic with potential applications in packaging technology once its gas barrier properties are enhanced.

19

Here we also demonstrate that the liquid casting deposition method permits to deposit thin CNF coatings, which act as nearly impermeable gas barriers without remarkable variations of the


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substrate optical transparency. We also discuss the fine details of our CNF coating preparation in relation to the observed gas barrier properties.

2. EXPERIMENTAL SECTION 2.1 Sample Preparation. 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)- mediated oxidation of cellulose fibers at alkaline conditions and room temperature was used to convert primary hydroxyls into carboxylate groups and obtain carboxylated cellulose nanofibrils (CNF).17 1 g of cellulose pulp (SCA-Ostrand, Sweden) was suspended in 100 mL water under stirring for 1 h. Catalytic amounts of TEMPO (0.16 mg) and NaBr (0.1 g) were added to the slurry. Then, NaClO (3.1 g) was added to the mixture, which was stirred at pH in the 10.5 to 11.0 interval by adding NaOH (1 M) until no further pH decrease was observed. The pH was then lowered to 7.0 by washing the slurry with 400 mL high-purity water for at least 10 times. A 13 mm diameter ultrasonic tip (Bandelin Sonopuls HD2200) was used to sonicate 45 mL of the slurry for 4 min at 20 kHz and 100 Weff of output power delivered in the sample volume. A transparent suspension of TEMPOoxidized CNF (TO-CNF) with a density of 2.7 mg mL-1 was obtained.17 TO-CNF coatings were prepared by casting the TO-CNF suspension on 6.2 cm2 PLA disks and drying at 60 °C for 24 hours in an oven. The TO-CNF thickness of the TO-CNF / PLA bilayer was changed by casting different amounts of TO-CNF solution. Given the hydrophilic character of this polymer, no surface treatment was performed on the PLA substrate before the TO-CNF coating deposition. Selfsupporting TO-CNF films were prepared by the same procedure but casting the suspension on a Petri dish and peeling the films off from the dish after annealing. The thickness of these selfsupporting films was measured analyzing their UV-Vis spectra to obtain a calibration between the amount of TO-CNF solution and TO-CNF coating thickness: this procedure was then validated by measuring the film thickness of selected samples by Scanning Electron Microscopy (SEM). Using 5 ACS Paragon Plus Environment


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the 2.7 mg mL-1 TO-CNF suspension, TO-CNF coating thicknesses in the 2 to 12 μm range were obtained by changing the TO-CNF mass / PLA surface ratio in the 0.4 to 2.0 mg cm-2 interval. 2.2 Sample Characterization 2.2.1 Structural Characterization. The morphology and size of isolated TO-CNFs and of their assemblies in the TO-CNF layers was studied by digital AFM microscope (Solver NT-MDT) operating in semi-contact mode. To obtain isolated fibers, the TO-CNF solutions were suitably diluted with water: a drop of this solution was then deposited on a silicon support and dried at 60 °C in an oven.17 The Gwyddion package was used for data visualization and analysis.20 The morphology of the bilayer membrane surface was observed by Field Emission Scanning Electron Microscopy using a JEOL JSM-7001F microscope operating at 2 kV after sample metallization. To analyze the bilayer membrane cross section, samples were previously freeze-cut in liquid nitrogen. 2.2.2 Optical Characterization. UV-Vis spectra of TO-CNF / PLA bilayers and self-supporting TO-CNF films were acquired with a Varian Cary 5000 UV-Vis-NIR spectrophotometer operating in transmittance mode in the 190 to 900 nm wavelength range with a 1 nm resolution. The thickness of the self-supporting TO-CNF films was evaluated from the UV-Vis spectrum using the following equation

=

1 1 4 − − sin

where and are the maximum and minimum wavenumbers considered, the number of

fringes in the range − , the angle of incidence and the refractive index of the material.21 In the present experimental setup, the UV-Vis beam was normally incident to the sample surface ( = 0). The refractive index of TO-CNF fibers, = 1.58, was assumed as constant in the examined wavelength range.22 6 ACS Paragon Plus Environment

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2.2.3 Positron Annihilation Lifetime Spectroscopy. Depth-profiled Positron Annihilation Lifetime Spectroscopy (PALS) measurements in the 1 to 12 keV positron implantation energy range were carried out with PLEPS (Pulsed Low Energy Positron System) apparatus

23,24

intensity positron source NEPOMUC (NEutron induced POsitron source MUniCh).

25,26

at the high The overall

time resolution of the apparatus (pulsing and detector) was 230-240 ps and the beam diameter < 1 mm at all energies. Lifetime spectra containing 4 × 106 counts were acquired. Positrons injected in a material thermalize and then annihilate with an electron in a free or trapped states with lifetime ranging from ∼ 100 to 500 ps. In some matrices, positrons can also form positronium (Ps), the electron-positron bound state. Ps exist in two states: para-positronium (p-Ps) (singlet state with a vacuum mean lifetime of 125 ps) and ortho-positronium (o-Ps) (triplet state with a vacuum mean lifetime of 142 ns). If nanometer- sized regions of lower electron density, such as voids, exist in the material, o-Ps can be tapped and here it annihilates with a surrounding electron having opposite spin. This process is called “pick-off” and reduces the o-Ps vacuum lifetime up to few ns. The reduced o-Ps lifetime can be related through quantum models

27,28

to the size of the

cavities in which the o-Ps annihilate. The positron and o-Ps lifetimes ( ) and their intensities ( )

are obtained by deconvolution and decomposition of the measured spectra into a sum of

exponential decay functions. Spectra were analyzed by the PATFIT package

29

and well fitted (chi

square around 1) with three lifetimes: a short lifetime due to annihilation of p-Ps and free positrons ( = 121 ps, intensity ∼ 10 %), an intermediate lifetime ( = 380 ps, intensity ∼ 72

%) and a longer one ( with intensity ) being the last one pertinent to the o-Ps annihilation process in nano- sized voids. The o-Ps annihilation parameters and will be discussed in detail

in the following sections. 2.3 Gas Transport Analysis. The gas transport analysis was carried out by gas phase permeation tests using penetrant gases with different molecular sizes and condensation 7 ACS Paragon Plus Environment


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properties. We studied the transport of high purity, dry nitrogen (N2), carbon dioxide (CO2) and deuterium (2H2). Tests were also carried out using ambient air at relative humidity (RH) values ranging from 20 to 50%, as measured using a Delta Ohm HD2301 thermo-hygrometer. The kinetic diameter and critical temperature

!

of the test gases are listed in Tab. 1.30

Table 1: Critical Temperature ("# ) and Kinetic Diameter ($% ) of Test Gases 30 Gas Å ! (K) He 5.19 2.60 2 H2 38.2 2.89 N2 126.2 3.64 CO2 304.2 3.3 O2 164.6 3.46

Permeation tests were carried out using membrane samples shaped in form of thin discs having 1.3 cm diameter at sample temperature between 293 and 324 K. We used the following experimental procedure. At time = 0 one side of the membrane sample (high pressure side:

HPS) was exposed to the permeating gas kept at fixed pressure ()*+ and temperature . In the permeation process, gas molecules are absorbed by the membrane surface layers, diffuse through the membrane, reach the opposite side (low pressure side: LPS) and desorb in a high vacuum chamber (background pressure in the low 10-6 Pa order, volume , = 2 × 10/ m3). In our experimental procedure, the high vacuum chamber (analysis chamber) is continuously pumped (pumping speed of the vacuum system in the 10/ m3 s-1 range, depending on the permeant gas). The partial pressure of the permeating gas (0*+ in the analysis chamber is measured as a

function of time by a calibrated Quadrupole Mass Spectrometer (QMS: Balzers QMG 420). The following equation describes the relation between (0*+ and permeation flux 12345 : 6 12345 = ,

7(0*+ + 2 (0*+ 2 7


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where 6 is the membrane surface area.31 In the present experimental setup, the condition +9 :

≫

*

?*= ?@

holds both in transient and stationary conditions, thus the flux can be obtained

from pressure measurements according to the following relation 31

12345 = 2 (0*+ 3 . A

CDE is always present in the With each test gas, a background partial pressure signal (0*+

analysis chamber mostly due to outgassing effects from the chamber walls or small leaks.31 We measured this signal before each experimental run setting ()*+ = 0. The permeation signal (0*+ CDE was then evaluated by subtracting the background signal (0*+ from the measured signal 53DF . In the present experimental procedure, the signal-to-noise ratio can be quantified by (0*+

the relation

GHIJ *

LIMN K*

CDE CDE where O(0*+ are the fluctuations of background partial pressure signal (0*+ .

CDE The quantity O1 = A 2 O(0*+ was thus assumed as a detection limit of the present apparatus: in

the present experimental setup O1 ∼ 10-2 mL m-2 day-1 for the examined test gases. Before gas

transport tests, the membrane samples were outgassed by keeping them in high vacuum conditions for 12 hours inside the experimental apparatus.

3. RESULTS 3.1 Structure. Fig. 1a shows an AFM image of isolated CNFs deposited from a diluted aqueous suspension on a silicon wafer support. Statistical analysis on several images was published in a previous paper and indicated a mean TO-CNF width of 4 nm and CNF length distributed in the 50 to 300 nm interval.17 Fig. 1b shows an AFM image of the TO-CNF coating surface: we note that fibrils are oriented with their long axis parallel to the PLA surface and form a dense random network structure.



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Fig. 1: (a) AFM image of isolated cellulose nanofibers deposited on a silicon substrate (scale bar = 1 μm); (b) AFM image of the TO-CNF coating surface deposited on the PLA surface (scale bar = 1 μm).

Fig. 2a shows a SEM micrograph of the surface of a TO-CNF coating deposited on the PLA substrate: the coating presents a flat surface morphology, it appears homogeneous and does not show any specific structure. Detailed analyses on different parts of the surface carried out on samples with different TO-CNF coating thickness have evidenced the lack of surface defects such as cracks or pinholes. Fig. 2b shows a SEM micrograph of the TO-CNF bilayer membrane crosssection: the adopted procedure permits the deposition of TO-CNF coating layers with uniform thickness over the entire surface of the PLA substrate. The inset of Fig. 2b shows a representative cross -sectional image of the TO-CNF coating evidencing the individual cellulose nanofibrils: it can be seen that these nanofibrils are aligned to each other and that their main axis is parallel to the sample surface.


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Fig. 2: (a) SEM micrographs of the surface of a TO-CNF coating deposited on the PLA substrate (scale bar= 5 μm). (b) SEM micrograph of the cross section of a TO-CNF / PLA bilayer membrane

(scale bar = 50 μm). The inset of panel (b) shows a SEM micrograph of the TO-CNF cross-section at

higher magnification (scale bar = 200 nm).

3.2 Optical Properties. Fig. 3 shows the UV-Vis spectrum of a 23 ± 1 μm thick PLA film and of a TO-CNF / PLA bilayer membrane consisting of the same PLA film coated with a 4.9 ± 0.3 μm thick TO-CNF layer. Fig. 3 shows that the PLA film optical transmittance (O.T.) is almost zero in the low UV range and gradually increases starting from 225 nm: at 300 nm 86% of the UV light is transmitted while transmittance at 600 nm is close to 95%, in agreement with literature data.32 The presence of the TO-CNF coating layer only slightly reduces the PLA films transmittance in the visible region which is ∼ 90% at 600 nm. 11 ACS Paragon Plus Environment


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Fig. 3: UV-Vis spectra of a 23 ± 1 μm thick PLA film (black trace) and of the TO-CNF bilayer with a 4.9 ± 0.3 μm thick TO-CNF coating (red trace). Fig. 4 shows the optical transmittance values of self-supporting TO-CNF films and of a TOCNF / PLA bilayer at representative wavelengths P of 300 nm (UV) and 600 nm (Vis) for different TO-CNF layer thicknesses. Bilayer samples maintain visible light transmittance values larger than 85% even for TO-CNF thicknesses larger than 10 μm while transmittance in the UV region slightly decreases with increasing thickness of the TO-CNF layer from 86 ± 1% (uncoated PLA film) to 72 ± 2% with a 12 μm thick TO-CNF coating layer.


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Fig. 4: Optical transmittance (O.T.) at 600 nm and 300 nm of self-supporting TO-CNF films and of TO-CNF / PLA bilayer samples as function of TO-CNF layer thickness. Experimental value and indetermination are the average value and standard deviation of at least four measurements. Assuming that the TO-CNF film-air interface causes negligible light scattering, optical transmission spectra obtained with TO-CNF self-supporting films with different thicknesses

permit us to evaluate the optical absorption coefficient Q by the Lambert's law: 33 ≃ S 1 − ℛ exp−Q 4 X. . % =

100 5 S

In the previous equations is the intensity of the light beam transmitted through a TO-CNF

film of thickness , S is the intensity of the incident beam and ℛ the TO-CNF film reflectivity (for

normal incidence of the light beam). The Q and ℛ parameters were obtained from the slope and SS

intercept, respectively, of the straight line which best fits the %Z values plotted as a function of the film thickness.

The following values were obtained for the TO-CNF film absorption coefficients: Q[SS\5 =

(1.0 ± 0.4) × 10-3 μm-1 and QSS\5 = (25 ± 1) × 10-3 μm-1. Reflectivity values are ℛ[SS\5 = (5.0 ±

0.2) × 10-2 and ℛSS\5 = (4.2 ± 0.2) × 10-2. These data are in agreement with the assumption



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that the refractive index of the present TO-CNF samples is equal to that of pure cellulose thus confirming the very low porosity and the lack of specific structures such as fibrils agglomerates which would have, in fact, generate light scattering effects and reduce the film transparency. 3.3 PALS Analysis. Depth- profiled PALS analysis was carried out using positrons accelerated to energies between 4 and 12 keV. Such energies correspond to mean implantation depth values ^̅ between 27 and 1.4 × 10 nm, as indicated by the relation 34 ^̅ a =

40 .[ c . b

In this relation b is the cellulose density (1.5 g cm-3)

35

and c is the positron implantation

energy (keV). In the following we will only discuss the information obtained by the o-Ps annihilation process because the value permits to evaluate the average size of the voids where o-Ps annihilates, while the intensity of the o-Ps annihilation signal is proportional to the cavity

number density.36,37 Fig. 5 shows the values of these parameters, as a function of the positron implantation depths. We can see that both the long-lived component and the signal are constant and maintain average values of 1.37 ± 0.02 ns and 17.7 ± 0.3 %, respectively. This result

indicates that voids with uniform average size are distributed with constant concentration in the TO-CNF film. Fig. 6 reports the and values measured in the 290 to 330 K temperature interval, as obtained from the annihilation spectra of positrons implanted at 4 keV energy: This figure clearly shows that in the temperature range where penetrant transport was studied, neither the average void size nor the cavity concentration change.


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Fig. 5: o-Ps lifetime ( , left vertical axis, full circles) and intensity of the o-Ps annihilation signal ( , right vertical axis, open squares) as functions of the positron mean implantation depths.

Fig. 6: o-Ps lifetime ( , left vertical axis, full circles) and intensity of the o-Ps annihilation signal ( , right vertical axis, open squares) as functions of temperature. In the inset we report the positron annihilation spectrum obtained at T = 290 K and E = 4 keV to evidence the very high signal-to-noise ratio of the present measurements. 3.4 Gas Transport Kinetics. Fig. 7 shows the CO2 and N2 permeation flux, 12345 , as a function of time (in the following: permeation curves) measured through a 23 ± 1 μm thick PLA 15 ACS Paragon Plus Environment


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film at

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= 293 ± 1 K and ()*+ = (3.5 ± 0.1) × 104 Pa. The inset of this figure shows the

permeation curves for N2 and O2 obtained using ambient air at RH = 37% and same temperature and total pressure value in the HPS.

Fig. 7: CO2 and N2 permeation curves obtained with the PLA membrane at 293 ± 1 K and ()*+ = (3.5 ± 0.1) × 104 Pa. The inset shows the permeation curves of nitrogen and oxygen obtained using ambient air at RH = 37 % with the same

and ()*+ values. Experimental data are reported as

points while lines are numerical fits by Eq. 7. For each test gas, the curves show an initial transient time interval where 12345 increases with time until the permeation flux becomes constant, indicating that stationary transport conditions are established. From the permeation curves, the gas transport parameters pertinent to PLA samples can be evaluated. In fact, in polymers the gas transport process is controlled by the solution-diffusion mechanism.38 The gas permeability e = is given by the

product of the gas solubility and the gas diffusivity in the membrane layers (()*+ and (0*+

are the concentrations of the gas molecules dissolved in the HPS and LPS membrane layers, respectively). The e value can be determined from the measure of the permeation flux in stationary transport conditions by the relation


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12345 f. . g. =

()*+ ()*+ − (0*+ ≅ 6

where is the membrane thickness. The assumption that ()*+ ≫ (0*+ is justified by the fact that in the present experiment the analysis chamber is continuously pumped and therefore (0*+ is

always negligible compared to ()*+ . The gas diffusivity can be independently determined by the analysis of the permeation curves in transient time conditions fitting the permeation flux data with the equation s

12345 = 12345 f. . g. j1 + 2 k −1 \ l \t

/

m no po q ro

u 7

which holds when the membrane has planar geometry and thickness much smaller than its lateral size, as in the present experiments.39 The transport parameters pertinent to the test gases in the PLA films, as obtained by this procedure, are reported in Tab. 2. These values are in agreement with the ones obtained by Komatsuka et al.40 and Bao et al.41 We carried out gas permeation tests on composite TO-CNF / PLA bilayer membranes with TO-CNF coating thickness Zv/!wx from 2.6 to 6.6 ya using a PLA film 23 ± 1 ya thick as a substrate and dry CO2 and N2 as test gases. Supplementary tests were carried out using ambient air with different relative humidity (RH) values. Using these gases, no permeation signal was detected in hours- lasting measurements performed at = 293 K and ()*+ values up to 105 Pa: this evidence indicates that the permeation flux for the present test gases was under the detection limit of our experimental apparatus, ∼ 10-2 mL m-2 day-1. Table 2: Permeability and Diffusivity of CO2, N2 and O2 in 23 ± 1 µm thick PLA Film at 293 ± 1 K Temperature CO2 N2 O2 /| /~ ga 6.5 ± 0.6 × 10 1.26 ± 0.09 × 10 2.8 ± 0.2 × 10/~ z { f a ya 5.2 ± 0.2 × 10 21 ± 1 90 ± 5 Φ a 7 ( 17 ACS Paragon Plus Environment


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Assuming that the gas barrier properties of the composite membrane can be attributed only to the TO-CNF coating layer, an upper limit for the gas permeability Φ5D can be evaluated by the relation Φ5D = 12345 f. . g.

Zv/!wx ( . Using ()*+ = 105 Pa and Zv/!wx ∼ 1 ya )*+

we obtain Φ5D ∼ 10-4 mL μm m-2 day-1 kPa-1. In order to get a deeper insight into the penetrant transport process through the TO-CNF coating layer, we carried out permeation experiments using as test gases 2H2 and He, which have kinetic diameter of 2.89 and 2.60 Å, respectively. Deuterium was preferred over hydrogen as a test gas given the much better signal to noise ratio in the detection of corresponding QMS signal. Permeation tests carried out on the uncoated PLA substrate have shown that the room temperature permeability and diffusivity values are (2.01 ± 0.06) × 103 mL μm m-2 day-1 kPa-1 and (7 ± 3) × 10-7 cm2 s-1 for 2H2. The He permeability is of (2.6 ± 0.1) × 103 mL μm m-2 day-1 kPa-1 while its diffusivity is ∼ 10-6 cm2 s-1. The measured values for 2H2 are well compatible with those reported by Komatsuka et al.40 but the permeability measured for He is somewhat smaller than that detected by Guinault et al.42 In Fig. 8 (upper panel) and 8 (middle panel) we report the 2H2 and He permeation curves, respectively, detected using a TO-CNF / PLA bilayer membranes with Zv/!wx = 6.5 ± 0.1 μm at ()*+ = (3.5 ± 0.1) × 104 Pa; here we also report the permeation curves pertinent to the uncoated PLA membrane. We can observe that a few μm thick TO-CNF coating layer reduces the permeation flux in stationary transport conditions by at least three orders of magnitude with both penetrant molecules. Fig. 8 (upper and middle panels) also show that the nanocellulose coating gives rise to slower gas transport kinetics. In fact, stationary transport conditions through the 23 ± 1 μm thick PLA film are set in 1 to 2 s while transient transport conditions through the bilayer membrane last for much longer time intervals with both penetrants. Permeation tests have shown that, by increasing the coating thickness, the transient time interval systematically increases and that the value of the permeation flux in stationary 18 ACS Paragon Plus Environment

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transport conditions decreases. As an example, in Fig. 8 (lower panel) we show the 2H2 permeation curves obtained

= 301 ± 2 K with TO-CNF coatings thickness ranging from 2.6 to 6.5 μm. Fig. 8

permits us to conclude that the transport properties of the TO-CNF / PLA bilayer membranes are effectively determined by the TO-CNF coating layer both in transient and stationary transport conditions. The PLA film merely acts as a gas permeable mechanical support and consequently the numerical analysis of the permeation curves pertinent to the TO-CNF / PLA bilayer membranes by Eqs. 6 and 7 provides the values of the penetrant transport parameters pertinent the TO-CNF layer.

Fig. 8: Permeation curves of uncoated PLA film and TO-CNF / PLA bilayer membranes pertinent to 2

H2 (upper panel) and He (middle panel). In the lower panel, we show the 2H2 permeation curves

of TO-CNF / PLA membranes with different TO-CNF coating thickness. Measurements were carried out at 301 ± 2 K and ()*+ = (3.5 ± 0.1) × 104 Pa. 19 ACS Paragon Plus Environment


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The penetrant transport parameters were obtained using TO-CNF films with different thicknesses. The average room temperature 2H2 permeability and diffusivity values are eZv/!wx =

0.10 ± 0.03 mL μm m-2 day-1 kPa-1 and Zv/!wx = (2.2 ± 0.4) × 10-10 cm2 s-1; the average values for

He are eZv/!wx = 0.4 ± 0.1 mL μm m-2 day-1 kPa-1 and Zv/!wx = (4.3 ± 0.8) × 10-9 cm2 s-1. No

report exists in the scientific literature on the 2H2 and He diffusivity in nanocellulose- based materials. More detailed and original information on the gas transport process can be obtained: i) by measuring the value of the penetrant permeation flux as function of the gas pressure in the HPS of the bilayer membrane and ii) by studying the permeation process at different temperatures. Point i) was studied by changing the ()*+ value in the 10 to 100 kPa pressure range. Fig. 9 shows the value of the penetrant permeation flux in stationary transport conditions, 12345 f. . g. , as a function of ()*+ through a TO-CNF / PLA bilayer membrane at

= 295 ± 1 K.

We can observe a linear relationship between 12345 f. . g. and ()*+ providing an experimental evidence of the fact that gas transport through the TO-CNF layers is controlled by the solutiondiffusion mechanism, see Eq. 6.38


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Fig. 9: 2H2 (a) and He (b) permeation flux in stationary transport conditions, 12345 f. . g. , as function of pressure in the HPS (()*+ ) at 295 ± 1 K. Experimental indeterminations are inside the size of the symbol. The straight line is a guide for the eye. Point ii) was analyzed by studying the penetrant permeation process in the 295 to 324 K temperature interval using a TO-CNF / PLA bilayer membrane with ()*+ = (3.5 ± 0.1) × 104 kPa. The nanocellulose film thickness Zv/!wx was 2.6 ± 0.1 μm for 2H2 and 6.5 ± 0.3 µm for He. In Fig. 10

we present the 2H2 (upper panel) and He (lower panel) permeation curves obtained at the examined temperatures: it can be seen that by increasing , the transport of penetrant molecules becomes faster as shown by the increase of 12345 f. . g. and the decrease of the transient time interval.

Fig. 10: 2H2 (upper panel) and He (lower panel) permeation curves pertinent to the TO-CNF / PLA bilayer membrane obtained at different sample temperatures.



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In Fig. 11 we present the Arrhenius plot of the eZv/!wx and Zv/!wx parameters for 2H2 (upper panel) and He (lower panel) as obtained by the analysis of curves in Figs. 10. Fitting of the permeability data with the equation eZv/!wx = eS exp −

Z

(8a)

provides the following values for the activation energy of permeation: cΦ = 38 ± 3 kJ mol-1 for 2

H2 and cΦ = 39 ± 1 kJ mol-1 for He. The fitting of the diffusivity data by the equation

m Zv/!wx = S exp − Z

(8b)

provides the following values for the activation energy for diffusion: c = 39 ± 1 kJ mol-1 for 2H2

and c = 33 ± 2 kJ mol-1 for He. Pre-exponential factors S are ∼ 1 × 10/ cm2 s-1 for 2H2 and ∼ 4 × 10/ cm2 s-1 for He. Note that in glassy polymers as, for example, polyimides c values for small size penetrant such as H2 ranging between 5 and 20 kJ mol-1 43 that is at least a factor 2 lower than shown in the present TO-CNF films.


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Fig. 11: Arrhenius plot of the 2H2 (upper panel) and He (lower panel) transport parameters in the TO-CNF films. Diffusivity values are reported as solid circles in the left vertical axis, permeability values are reported as open squares in the right vertical axis. The 2H2 and He solubility values, as a function of temperature, were evaluated by the relation Zv/!wx = eZv/!wx ⁄Zv/!wx : as often observed in size sieving polymers 38 the obtained values are weakly temperature- dependent. The average 2H2 and He solubility values of the present TO-CNF samples are Zv/!wx ~ 5 × 10-3 mL cm-3 atm-1 and Zv/!wx ~ 1 × 10-3 mL cm-3 atm-1, respectively. Notice that for a perfect gas at standard temperature, the vacuum solubility is \

DE = * : = Z = 1 mL cm-3 atm-1.

4. DISCUSSION Experimental results on the gas transport through the present TO-CNF films indicate that they act as effective gas permeation barriers and fully satisfy both requirements for food packaging applications

44

and OLED devices.45 Moreover, the 2H2 and He permeability values are orders of

magnitude lower than that of commercial polymers.38 Fig. 12 summarizes the room temperature gas barrier performances of the present TO-CNF films deposited on the PLA support.

Fig. 12: Room temperature gas permeability values of the uncoated and TO-CNF coated PLA film. The detection limit of the present apparatus is ∼ 10-4 mL ym m-2 day-1 kPa-1. 23 ACS Paragon Plus Environment


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The present 2H2 permeability values are two orders of magnitude lower than those obtained by Bayer et al. in nanocellulose membranes prepared by hydrolysis of cellulose nanofibers in hydrochloric or sulfuric acids

13

and result comparable to those obtained by

Fukuzumi et al.15 studying CNF films produced with a procedure similar that described in Sect. 2.1. Their samples presented O2, CO2 and N2 permeability values slightly larger than those reported here 15, reasonably as consequence of different CNF packing conditions due to peculiarities in the CNF preparation procedure. In fact, it is known that the drying of CNF heavily modifies the structure of the material and that the fibril coalescence reduces the swelling properties.

46-48

A

recent review of the coalescence of cellulose fiber phenomena lists all the possible reactions that might occur during drying.49 The process of the coalescence of cellulose fibers is not fully understood and a number of different reactions contribute to it (such as: lactone bridge formation and crosslinking between adjacent nanocrystals, also called cross-crystallization). The main difference between our TO-CNF films and those reported in literature is that we thoroughly washed the oxidized CNF solution with distilled water, instead of adding HCl to neutralize the pH. This fact results in a reduced presence of ions between the polysaccharide chains and in a decreased shielding of the partial charges borne by the nanofibers: this situation eases the fiber coalescence. It also avoids the formation of salt nanocrystals between the TO-CNFs during film drying. Furthermore, these conditions might favor the planar and parallel arrangement of the short fibrils: the resulting denser packing clearly enhances the gas barrier properties. The separate evaluations of the and gas transport parameters pertinent to the TOCNF coating permits us to infer that the nanocellulose film acts as a diffusive barrier for penetrant gas molecules. Evidence of this comes from a comparison of the measured gas transport values with literature data on the commercial polymers used for gas barrier applications. In fact, the 2H2 solubility in the present TO-CNF layers is close to that of, for example, PET, PS or PVA (∼ 10-2 mL 24 ACS Paragon Plus Environment

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cm-3 atm-1).38,50 The 2H2 diffusivity is, on the contrary, orders of magnitude lower, being ∼ 10/ cm2 s-1 in PET and ∼ 10/ cm2 s-1 in Teflon. 51

The selective 2H2 and He transport and the barrier properties for CO2, N2 and O2 will be discussed considering the diffusive mechanism and path for penetrant molecules: to this task, we use information on the void structure of the present CNF film samples obtained by PALS analysis. The building blocks of the TO-CNF films are rigid cellulose nanocrystals having shape of stick with 4 nm cross-section and length between 50 and 300 nm. CNFs form a layered structure consisting of a highly packed TO-CNF assembly, see Figs. 1 and 2.5,52 Owing to the gas- impermeable nature of the nanocrystals,52 penetrant molecules are hosted and migrate through empty regions between the nano-fibrils. The selective transport properties of the TO-CNF membranes can be explained describing the penetrant diffusive paths as formed by interconnected elongated cavities having the packed cellulose nano-fibrils as walls. A geometrical description of such cavity, permitting a simple quantification of the cavity size, is that of a prism having square cross section of size 72 and

length ED@ = a 72 . The o-Ps annihilation process provides information on the average size of

this inter-fibrillar regions. Assuming the previous geometry, the measured value permits, in fact, to evaluate the 72 parameter by the following equation: / = PS 1 − ?

9

?9

+ sin ?

?9

9

5?9

+ 5?

9

5?9

+ sin 5?

9

.

here PS ≃ 0.5 ns-1 is the o-Ps annihilation rate in the bulk state and Δ = 0.166 nm is the

empirical electron layer thickness.53 Values for the 72 size as function of a = ED@ ⁄ 72 are

reported in Fig. 13 with = 1.37 ns, see Figs 5 and 6. Fig. 13 shows that as soon as the cavity

length ED@ is a factor 5 larger than its cross-sectional size 72 , a saturation value 72 = 0.31 nm

is obtained. This condition is fully satisfied because in the present model the cavity length ED@

is of the same order as the nano-fibril length ∼ 100 nm. We thus assume 0.31 nm as average 25 ACS Paragon Plus Environment


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cross-sectional size of the cavities forming the diffusive paths for penetrant molecules. This value suggests that the selective transport properties of the present TO-CNF films are due to a sizesieving effect. In fact, 72 = 0.31 nm indicates that the cavity size is comparable with the kinetic diameter of the 2H2 and He penetrant molecules and smaller than those of the other penetrants. Penetrant diffusivity in the semi-crystalline PET and Teflon is larger than that measured in the present TO-CNF coatings, because these commercial polymers present a more “open” free volume structure which is formed by cavities with larger average size that thermally redistribute in their amorphous fraction. 54-57 When the cavity size 72 is comparable to the kinetic diameter of the diffusing molecules, as in the present CNF film samples, diffusion occurs in Knudsen or in configurational regime depending on the value of the ⁄72 ratio.58-60 When ⁄72 < 0.5 , diffusion occurs in Knudsen regime. Penetrant molecules maintain their gaseous characteristics and travel at thermal velocity =

~ Z

, where ¡ is the penetrant molecular weight. The pore diameter 72 provides

a measure of the diffusional length and the diffusivity \¢?F3\ is given by the relation

\¢?F3\ = 72 . Diffusivity is not thermally activated and the weak temperature dependence, \¢?F3\ ∝

S.

, is only due to the velocity term.

When ⁄72 > 0.8 diffusion occurs in configurational regime and the penetrant diffusion

process shows similarities with the surface diffusion process. Modelling of configurational regime assumes that penetrant molecules in empty regions interact with the host matrix and lose their gaseous state residing in a periodic potential at thermal equilibrium with the host matrix.58-60 In the case of strongly corrugated potential with respect to ¥¦ , the particles vibrate at frequency §3 within a bound state. Diffusion occurs by jumps between these sites and has thermally activated character because the jump occurs after the penetrant has accumulated enough thermal energy 26 ACS Paragon Plus Environment

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to overcome a potential barrier c . The diffusivity E¨\© of a molecule can be expressed by the relation:

E¨\© = §3 F l /m⁄ Z ª

where 1/¬ is the probability that a molecule jumps into one of the ¬ adjacent diffusional sites and

F the average distance between these sites.58-60

Our measurements reveal that penetrant diffusion occurs in configurational regime because: i) ⁄72 > 0.8 with both penetrants and ii) it has thermally activated character. It is worthwhile to compare the measured values of pre-exponential factor, ∼ 4×10-3 cm2/s,

with the pre-exponential term suggested by the previous model, S = ª §3 F . Assuming §3 ∼ 1012 s1 61

and ¬ = 2, which implicitly assumes that in the small size cavity a penetrant molecule can jump

only in sites at its right or left and that penetrant concentration is not large enough to set singlefile diffusion conditions, then F ∼ 1 nm indicating that 72 < F ≪ ED@ . This estimate points out that the transport of penetrant molecules is controlled by jumps between diffusional sites inside the cavities rather than by jumps transferring molecules from a cavity to an adjacent one. Note that the estimated distance between diffusional sites, F ∼ 1 nm, well compares with

the average distance between the carboxyl groups (-COOH) at the CNF surface, ∼ 2 nm: there is, in

fact, a -COOH group every 2.5 cellobioses and the distance between neighbor cellobioses is ∼ 1 nm.

62

To conclude this discussion, we suggest that the major contribution to the measured

activation energy for diffusion is related to structural constraints to the nanochannel walls upon penetrant jumps.61 Reasonably, the term due to the interaction between 2H2 and He and chemical groups at the cavity walls, such as carboxylic or hydroxyl groups,6 is of minor importance having weak dispersive character.



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Fig. 13: Relation between cavity size 72 and a = ED@ ⁄72 parameter for the measured average value of the o-Ps annihilation time = 1.37 ns. The dashed line is a guide for the eyes. The inset reports a graphical representation of the structure of the interfibrillar cavities, see Fig. 2.

5. CONCLUSIONS Gas- phase permeation measurements show that few µm thick nanocellulose films are impermeable barriers for CO2, O2 and N2 but permit the selective transport of 2H2 and He. Diffusive paths consist of interconnected elongated cavities between tightly packed cellulose nanofibrils. Positron Annihilation Lifetime Spectroscopy analysis indicate the cavity size is ∼ 0.31 nm suggesting that the selective transport of small penetrants is due to sieving effects. Diffusion has configurational character and occurs by thermally activated process with 39 ± 1 kJ mol-1 and 33 ± 2 kJ mol-1 activation energy for 2H2 and He, respectively.

Acknowledgments The authors gratefully acknowledge SCA-Ostrand (Sweden) for the supply of the cellulose material. The Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) facility is gratefully acknowledged 28 ACS Paragon Plus Environment

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for allocation of beam time at the NEPOMUC facility. We also thank G. Carotenuto (ICBM-CNR, Napoli) for providing us the PLA films.



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TOC GRAPHIC


Cellulose Nanofibrils Films: Molecular Diffusion through Elongated

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