CNC Material Characterization

stability, dispersion uniformity, sulfur content, pH and conductivity. More im-portantly however, even with very high zeta potential (ζ), prolonged storage at room temperature leads to particle aggregation, which has shown to have con-siderable impact on the self-assembly properties and consequently the photonic band-gap.[58, 59] It is thus advised to always store CNC suspensions in the fridge at low temperatures (4 °C). Even when stored in the fridge some tip-sonication is usually employed before drop- or dish casting the CNC suspension to break up potential agglomerates.

In the case of the used spray-dried C-CNCs the interparticle separation dis-tance was reduced to less than the Debye screening length. In this case interparti-cle separation reaches distances, where Van-der-Waals forces start to take effect.

From this point on it is in fact possible to separate the particles again, but only through the application of considerable amounts of energy through, for instance, sonication. Therefore, emphasis is given on this specific topic of tip sonication, playing a central role in redispersing sodium neutralized spray-dried C-CNCs in Chapter 3 Section 3.7.4.

Counter-ion exchange either through membranes or direct electrolyte addition has a considerable impact on self-assembly behaviour and may alter the pitch (see Chapter 6). Furthermore, increasing the pH through the addition of, for instance NaOH into HM-CNC suspensions (as will be explored in Chapter 5 Section 5.2.2.1) also strongly impacts the liquid crystalline and self-assembly behaviour. However, the addition of ions into CNC films is important for this work as they improve electrochemical properties of dried HM-CNC films, which will alter their behaviour as solid-state electrolytes in field-effect transistors in Chapter 5 Section 5.2.2.1.

It is important to note that after obtaining CNCs in a stable suspension they can be deposited through various techniques to obtain solid films or membranes (as explained more in detail in Chapter 3). The following section on the character-ization of the CNCs, used in this work, studies CNCs in both forms: suspensions and films.

A complete and very comprehensive work on cellulose nanomaterial characteriza-tion is given by Foster et al. in“Current characterization methods for cellulose nano-materials”. Structural and morphological aspects will be analysed through Atomic Force Microscopy (AFM), Dynamic Light Scattering (DLS), and XRD, while chem-ical composition will be analysed through Elemental Analysis (EA) and FTIR.

For thermal analysis TGA will be used. Further characterizations such as optical (for photonic properties), electrochemical (for electrolytic properties) or electro-optical (for device analysis) will be remitted to each section where necessity arises.

2.4.3.1 Structural, Chemical and Morphological Characterization

X-ray diffractograms were run for HM-CNC and C-CNC membranes in the range of 5° to 70°. The diffractograms as represented in Figure 2.15a show the typical main peaks of the crystallographic planes as already identified before for Avi-cel. The amorphous peak I_min and the crystalline peak I₀₀₂ were reduced and increased, respectively by the sulfuric acid hydrolysis, promoting an overall in-crease in crystallinity. Following Equation 2.1 the obtained crystallinity index for HM-CNC and C-CNC yield 87.5% and 84.1%, respectively. For crystallite sizes (using Equation 2.2) values of 5.8 nm and 6.3 nm were obtained for HM-CNCs and C-CNCs, respectively.

Figure 2.15 – Structural and chemical characterization of HM-CNC and C-CNC.

a) XRD diffractograms and b) FTIR.

As expected, the crystallinity index of CNCs is higher when compared to Avicel, whereas crystallite size is decreased. The usual crystallite size of CNCs extracted using strong sulfuric acid hydrolysis, as the one explained before, falls consistently between 5.8 nm and 8.3 nm.[see 10, p. 79] The obtained results fall within this range. These results based on XRD show how the sulfuric acid

treatment increases the degree of crystalline order when moving from Avicel to CNCs. Also it can be observed that CNCs obtained in a controlled lab environ-ment present higher crystallinity when compared to CNCs obtained on a larger industrial scale.

Considering the FTIR data (obtained from CNC films) in Figure 2.15b the usual peaks for cellulose are obtained. However, an additional peak around 812 cm^-1 is detected, which is connected to vibrations between sulfur and oxygen (S-O) as a consequence of the sulfate surface functionalization.

DLS and AFM were used to investigate the individual sizes (length and thick-ness) of the two types of CNCs. The CNCs’ dimensions together with their sur-face charge (zeta potential) strongly influence interparticle interactions and the excluded volume fraction, which in turn influence liquid crystalline properties and the EISA process. DLS is an excellent technique that provides in a fast and efficient way information about size, size distribution and surface charge (if cou-pled with an electrophoretic cell). Nevertheless, it is important to note that DLS assumes spherical particles and is therefore not the ideal technique for morphol-ogy determination of rod-like CNCs. Despite this controversy, DLS is nonetheless often justified for comparison reasons between samples with similar particle mor-phologies. DLS and Zeta Potential measurements were performed using a SZ-100 nanopartica series (Horiba, Lda) with a laser of 532 nm, at Room Temperature (RT). Suspensions were diluted to 0.01 wt% and prepared with 10 mM NaCl.

Figure 2.16a shows the DLS data obtained for the two CNC types, where a similar average size is predicted with a higher dispersion in size for HM-CNCs.

The measured zeta potential is given in Figure 2.16b, where a more negative charge was verified for HM-CNCs. The higher the zeta potential, the higher will be interparticle distances and agglomeration becomes less probable. This confers higher colloidal stability to the HM-CNC suspension when compared to C-CNC. This behaviour was verified through sample shelf-life assessment.

Suspensions of HM-CNCs showed excellent colloidal stability over time. On the other hand, visually, C-CNC suspensions did not seem to destabilize, however when drop-cast after a few weeks of standing, photonic properties were decreased with lower CPL distinction and the photonic band gap was blue-shifted. Only with tip sonication treatment the suspensions returned to their initial properties.

This can be connected to two different processes. Firstly, lower surface charge leads to agglomeration between particles. Secondly, the presence of Na counter-ions lead to the formation of a gel-state where EISA is hindered. After a short interval of tip sonication agglomerations are broken, the suspension leaves the gel-state and colloidal stability is re-established.

When comparing DLS data with AFM measurements a clear difference be-tween particle morphologies can be observed. Whereas DLS predicted similar particle dimensions, AFM shows the real CNC ones (see Figure 2.16c to f). Over 50 CNC particles from each sample were measured using ImageJ software. The number of investigated particles (N > 50) was chosen in order to obtain statis-tically relevant data that fall out of the experimental noise. Wherever a higher precision (M = ^√1

N) was needed the number of investigated particles were in-creased. The resulting measurement data and Bell curves are given in Figure 2.16e and f for lengths and widths, respectively. From these data the averages and standard deviations were extracted. It can be seen that HM-CNCs are shorter (137± 40 nm) when compared to C-CNCs (181± 62 nm). The widths are also distinct for both CNCs types (3.3±1.0 nm and 3.9±1.2 nm for HM-CNCs and C-CNCs, respectively), giving distinct aspect-ratios of 41.5±17.4 and 46.4±20.8.

The obtained results lie within the expected range of CNC dimensions for sulfuric acid hydrolysed Avicel and Softwood Pulps.[40, 42]

Figure 2.16 – Morphological CNC characterization. a) Particle size DLS data. b) Zeta potential data. c) and d) AFM images of C-CNCs and HM-CNCs dispersed on a atomically flat mica film. e) and f) CNCs lengths and widths (or thicknesses) size distribution as measured from at least 50 individual particles from c) and d).

Some more recent works show that the CNC particles are not cylinders but rather flat ribbons, where the width dominates over the thickness. This can be taken into consideration through the deconvolution of the data obtained by AFM (as explained more in detail below). With this, a corrected aspect-ration can be calculated using ^{√ L}

W×T, where W and T represent the width and the thickness, respectively.

Thicknesses were measured using the average height across the CNC on the AFM image. This type of measurement reduces inaccurate data that might come from the convolution of the AFM tip while being dragged across the width of the CNC. It was seen that there is a considerable discrepancy between the two meth-ods, where the latter (height profile) yields more accurate results in terms of real thickness. Whereas the measured width can be deconvoluted to give information about the non-cylindrical nature of the CNC particle (as schematized in Figure 2.17).

Figure 2.17 – Width measurement of a CNC particle using AFM. The convolution of the tip around the edges of the CNC gives an apparent width that appears on the AFM image. A better approximation to the real width lies in the height profile data.

Comparing DLS with AFM results it becomes evident that DLS data of non-spherical particles should be treated with special care. In fact, DLS gives rather information about the hydrodynamic diameter of the particle instead of its real physical size. Chapter 3 Section 3.7.4 resumes to DLS as a characterization tool to assess the hydrodynamic diameter of C-CNCs in suspension for consecutive tip-sonication cycles, giving insight on the breaking process of agglomerated CNCs to obtain a stable colloid.

2.4.3.2 Thermal Characterization

Thermal degradation of CNCs proceeds in the same way as described for Avicel or native cellulose. However, the presence of inorganic ions (functionalization at C6) leads to thermal degradation at lower temperatures than for untreated cellulose.[see 10, p. 129][60] Furthermore, it was shown that lower pH leads to

earlier onset of degradation when compared to neutral or basic pH.[61] This can be observed in Figure 2.18a where TGA curves of HM-CNCs and C-CNCs are compared. Evidently, acidic HM-CNCs show an onset of degradation around 187

°C, whereas NaOH neutralized C-CNCs present this onset around 224 °C. These temperatures are still much lower when compared to Avicel (270 °C). These ob-servations lead to the conclusion that the presence of the sulfate groups decreases thermal stability as they catalyse carbonization of the cellulose backbone.[62, 63] Elemental Analysis (Thermo Finnigan-CE Instruments Flash EA 1112 CHNS series) evidences a decreased amount of sulfate groups for C-CNCs when com-pared to HM-CNCs (0.35% to 1.02% - see Table 2.4). This decrease in surface sulfate groups is fundamentally at the basis for the increased thermal stability observed for C-CNCs. Furthermore, a transition from a one-step (Avicel) to a two-step degradation (HM-CNCs and C-CNCs) is observed. HCl hydrolyzed CNCs or solvolytically desulfated CNCs for instance show a similar one-step pyrolytic behaviour as Avicel.[58, 62, 64] However, despite decreasing thermal stability, the surface sulfate groups not only confer electrostatic stability in suspension but are also an important factor for chiral nematic self-assembly. Consequently, removing the sulfate groups in order to increase thermal stability is not in the interest of this work, where photonic films are desired.

Figure 2.18 – Thermal analysis of HM-CNCs and C-CNCs. a) TGA curves for both CNC types. b) CNC films on Glass/ITO substrates were submitted to the indicated temperatures for 15 min.

It should be noted that while thermal degradation of the two CNC types sets in at 187 °C and 224 °C, for HM-CNC and C-CNCs, respectively (Figure 2.18a), film degradation in the form of carbonization or film deformation (due to internal stress) may set in at lower temperatures. This is an important aspect, as this behaviour will impair effective thin-film deposition. In general, it was found that HM-CNCs will not sustain any deposition procedures where temperatures above

130 °C are involved, while C-CNCs showed thermal stability for temperatures until 170 °C (see Figure 2.18b). For temperatures above this threshold the films start to degrade, which is accompanied by a loss of their chiral nanostructure.