3.2. Wide Angle X-ray Scattering and Polarized Optical Microscopy of Crosslinked APC Films under Strain
Cellulosic liquid crystalline networks obtained from the previous studied systems can be at the origin of promising soft matter devices [
24]. In order to characterize these type of networks, Wide Angle X-ray Scattering (WAXS) studies were performed [
48] namely of crosslinked APC films produced from chiral nematic solutions [
29]. The studies were carried on films subjected or not to a uniaxial stress. The results indicate that the films are constituted by a bundle of helicoidal fiber-like structures, where the cellobiose block spins around the axis of the fiber. Without the stretch, these bundles are warped, only with a residual orientation along the casting direction. The stretch orients the bundles along it, increasing the nematic-like ordering of the fibers. Under stress, the network of molecules that connects the cellobiose blocks and forms the cellulosic matrix tends to organize their links in a hexagonal-like structure. The X-ray diffraction patterns of the free standing, unstretched, cellulose films present three diffraction peaks, anisotropically disposed around the z-axis, as shown in
Figure 10. In
Figure 11a,b, the diffracted intensity is plotted as a function of q (scattering vector), along the x and y axes directions. The diffraction peaks in the curves Ixq were fitted with Lorentzian functions to find the characteristic distances (d) and full-widths at half- height (W).
The characteristic distances associated to the peaks are
d1 = (1.13 ± 0.02) nm;
d2 = (0.44 ± 0.01) nm; and
d3 = (0.50 ± 0.01) nm. In a previous work [
30] only peak 1 was observed due to the limited range of scattering wave vectors investigated. The correlation lengths
D = λ
x/(
W·cosθ) calculated are
D1 ≈ 3 nm;
D2 ≈ 2 nm; and
D3 ≈ 14 nm.
Figure 10.
X-ray diffraction pattern of the unstretched cellulose film. The single arrow represents the casting direction. Peaks 1, 2, and 3 are identified. Reprinted with permission from [
29]. Copyright 2011, Springer.
Figure 10.
X-ray diffraction pattern of the unstretched cellulose film. The single arrow represents the casting direction. Peaks 1, 2, and 3 are identified. Reprinted with permission from [
29]. Copyright 2011, Springer.
Figure 11.
X-ray diffracted intensity of unstretched cellulose film. (
a) As a function of q, along the x-axis; (
b) as a function of q, along the y-axis. The insert refers to a zoom around q ≈ 13 nm
−1. Reprinted with permission from [
29]. Copyright 2011, Springer.
Figure 11.
X-ray diffracted intensity of unstretched cellulose film. (
a) As a function of q, along the x-axis; (
b) as a function of q, along the y-axis. The insert refers to a zoom around q ≈ 13 nm
−1. Reprinted with permission from [
29]. Copyright 2011, Springer.
The analysis of the diffraction peaks in the x–y plane indicates that peaks 1 and 2 are more intense along the x direction, which corresponds to the direction perpendicular to the casting. On the other hand, we could not fully verify if peak 3 is also anisotropic with respect to the z-axis since the presence of peak 2, particularly along the x-axis direction, prevents a reliable azimuthal analysis of it. Nevertheless, visual inspection of
Figure 10 seems to indicate that this peak is preferentially located in the direction perpendicular to the casting direction. The azimuthal angle analysis of the diffracted intensity (angle φ, measured in the xy plane, starting from the x-axis, in the counter clockwise direction) of the peaks allowed us to obtain the orientational order parameters (OP) [
29].
Concerning the case where the film is subjected to a uniaxial stress parallel to the casting direction (
Figure 12). We did not observe significant differences in the patterns obtained in the different film locations of the X-ray exposures, indicating, at least, in our experimental conditions, homogeneity of the film.
Figure 12.
Cellulose film at maximum stretch with ɛ
y = 0.40. (
a) Sketch of the cellulose film under stretch and indication from where the X-ray diffraction patterns were taken. The single arrow indicates the casting direction and the double arrow the stretching direction; (
b) diffraction patterns obtained at the different film location. Reprinted with permission from [
29]. Copyright 2011, Springer.
Figure 12.
Cellulose film at maximum stretch with ɛ
y = 0.40. (
a) Sketch of the cellulose film under stretch and indication from where the X-ray diffraction patterns were taken. The single arrow indicates the casting direction and the double arrow the stretching direction; (
b) diffraction patterns obtained at the different film location. Reprinted with permission from [
29]. Copyright 2011, Springer.
As the stress increases, the order parameters of the three peaks increase, reaching values at the maximum stress applied (ɛ
y = 0.40) of OP
1 = (0.85 ± 0.03); OP
2 = (0.43 ± 0.02); and OP
3 = (0.82 ± 0.01). This increase in the orientational order parameter of peak 1 is consistent with the results obtained in [
30].
Figure 13 shows the diffracted intensity as a function of q along the x (
Figure 13a) and y (
Figure 13b) axis directions. As the stretch is increased, peak 3 become more and more defined in the diffracted intensity
versus q curves (
Figure 13b). The positions of the peaks were not modified by the stress. An intriguing result was obtained when we analyzed peak 2 profile as a function of the azimuthal angle φ (
Figure 13c).
A hexagonal distribution of diffraction maxima could be identified in the peak 2 position, which provided a hexagonal lattice parameter dH ≈ 0.5 nm about the same of the characteristic distance d3. The value of dH was obtained assuming that the diffraction peaks of hexagonal symmetry observed in the position of peak 2 (d ≈ 0.44 nm) correspond to the (100) diffraction plane of the two-dimensional hexagonal packing, i.e., dH = d100/(cos(π/6)). The typical dimension of an extended glucose molecule is of the order of 0.5 nm and the cellobiose, composed by two glucose molecules, is the repeating building-block in the cellulosic sample. The structure proposed for the cellulosic film has to take into account the diffraction characteristic distances observed, the dimension of the cellobiose building block, and the information that, before the casting, the system presents a cholesteric structure. We propose that in the film there is a bundle of helicoidal fiber-like structures where the cellobiose block spins around the axis of the fiber. The distance between the fibers should be of the order 1.1 nm, corresponding to peak 1 of the diffraction pattern. Since there is no evidence of chiral activity in the cellulosic films in the macroscopic scale, these bundles should have fibers with both the levogyre and dextrogyre arrangements, with equal probabilities. Without the stretch, these bundles of fibers may be warped, only with a residual orientation along the casting direction. The stretch orients the bundles along it, increasing the nematic-like ordering of the fibers. Under stress, the network of molecules that connects the cellobiose blocs and forms the cellulosic matrix tends to organize their links in a hexagonal- like structure.
Figure 13.
X-ray diffracted intensity of stretched cellulose film at maximum stretch with ɛ
y = 0.40. (
a) As a function of q, along the x-axis; (
b) as a function of q, along the y-axis. The insert refers to a zoom around
q ≈ 13 nm
−1; (
c) as a function of the azimuthal angle φ, referring to peak 2. The sinusoidal curve is only a guide for the eyes. Reprinted with permission from [
29]. Copyright 2011, Springer.
Figure 13.
X-ray diffracted intensity of stretched cellulose film at maximum stretch with ɛ
y = 0.40. (
a) As a function of q, along the x-axis; (
b) as a function of q, along the y-axis. The insert refers to a zoom around
q ≈ 13 nm
−1; (
c) as a function of the azimuthal angle φ, referring to peak 2. The sinusoidal curve is only a guide for the eyes. Reprinted with permission from [
29]. Copyright 2011, Springer.
The same samples used to perform the WAXS essays were used to perform POM essays. The optical microscopy texture of the sample under crossed polarizers (casting direction parallel to the analyser) revealed that the film is birefringent, but with a non-uniform texture (
Figure 14a).
The analysis of the texture under crossed polarizers leads to the macroscopic arrangement of the director sketched in
Figure 14b. There are light and dark regions that form a pattern with stripes, with a periodicity of ≈ 4.4 μm (distance between three light regions), perpendicular to the direction of the casting. This periodic structure is originated by the shear-casting procedure. After the cast the molecular chains have a collective relaxation that results in the formation of that pattern. This morphology has been observed in other cellulose derivative films and was found to be influenced by the precursor solution composition, solvent evaporation rate, film thickness, and rate and duration of shear [
28,
49]. The application of successive stretches in the direction of the casting (
i.e., in this experimental configuration, parallel to the analyser direction) showed that the texture of the film becomes increasingly dark. This is because the direction of the optical axis, previously created by the casting, is parallel to the analyser direction. If the optical axis of the sample is rotated by 45° in the plane of the microscope plate, the texture became bright. This result is consistent with the nanoscopic structure proposed in the previous X-ray scattering section. The fiber-like structure where the cellobiose block spins around the axis of the fiber defines the director direction, parallel to the casting direction. As the X-ray beam probes a large portion of the sample (typical beam diameter of about 1 mm) the diffraction pattern reveals the mean orientation direction of the director. Upon relaxation (initially at ɛ = 0.29 and after
tR = 5 min) the film did not recover the initial texture, in accordance with the X-ray results already discussed.
Figure 14.
Optical microscopic textures under crossed polarizers (OMP) of (
a,e) the film unstretched; (
b) and sketch of the director orientation; (
c) OMP of the film under stretch parallel to the casting direction; and (
d) upon relaxation; (
f) OMP of the film under stretch perpendicular to the casting direction; and (
g) upon relaxation. The single arrow indicates the casting direction and the double arrow the stretching direction. Reprinted with permission from [
29]. Copyright 2011, Springer.
Figure 14.
Optical microscopic textures under crossed polarizers (OMP) of (
a,e) the film unstretched; (
b) and sketch of the director orientation; (
c) OMP of the film under stretch parallel to the casting direction; and (
d) upon relaxation; (
f) OMP of the film under stretch perpendicular to the casting direction; and (
g) upon relaxation. The single arrow indicates the casting direction and the double arrow the stretching direction. Reprinted with permission from [
29]. Copyright 2011, Springer.
When the stretch is applied perpendicular to the casting direction the texture of the sample under crossed polarizers (
Figure 14e,f) becomes increasingly darker with increasing stretch, and then clearer after its relaxation (
Figure 14g). Assuming the structure depicted in
Figure 14b, the stretch along the direction perpendicular to the casting will impose a tendency of reorientation of the fiber-like arrangement parallel to the stretching direction. This reorientation may in principle proceed either in the clockwise or the anticlockwise direction but due to the initial oscillatory form of the director around the casting direction due to the band structure, different regions will rotate in opposite sense depending on the initial director orientation at each point. Depending on the location of the film analyzed and the boundary conditions imposed by the borders of the film, the fibers will tend to reorient to their local environment. This fact can explain the different X-ray patterns obtained in different film positions (
Figure 15). At the border of the film (positions A and B in
Figure 15) the director, that was initially oriented on average parallel to the casting direction (x-axis), is now primarily oriented at ± π/4 with respect to the x-axis due to the boundary conditions that prevent a complete rotation of the director to the stretching direction. In the center of the film (position C in
Figure 15), the director is oriented primarily along the y-axis (stretching direction). Let us look in more detail at the appearance of the film under stress, without the crossed polarizers. The film initially had 2 mm in length (L
0), 5 mm in width and 21 μm in thick and was stretched every 6 min until 16 mm in length, and then relaxed.
Figure 16 shows the sequence of stretches of the film from ɛ = 0.71 (
Figure 16b) until ɛ = 7 (
Figure 16d), with the stress applied perpendicular to the casting direction. It is clearly observed an additional periodicity in the direction perpendicular to the casting direction.
Figure 15.
Sketch of the cellulose film under stretch (ɛ = 7.00) and indication from where the X-ray diffraction patterns were taken. The single arrow indicates the casting direction and the double arrow the stretching direction. Diffraction patterns obtained at the different film locations. Reprinted with permission from [
29]. Copyright 2011, Springer.
Figure 15.
Sketch of the cellulose film under stretch (ɛ = 7.00) and indication from where the X-ray diffraction patterns were taken. The single arrow indicates the casting direction and the double arrow the stretching direction. Diffraction patterns obtained at the different film locations. Reprinted with permission from [
29]. Copyright 2011, Springer.
The distance (along the x-axis) between two successive stripes increases linearly with ɛ (
Figure 17). Under crossed polarizers these stripes are also birefringent (
Figure 18). This result indicates that the effect of the casting in the macroscopic structure of the cellulosic film is not only to impose a periodic bend organization of the local director, as sketched in
Figure 14b in its direction, but also bends this super structure in the direction perpendicular to it in a larger length scale. Upon relaxation, the film takes a long time to recover its original length (
Figure 16e,f). However, its shape is strongly modified, in particular its width (direction perpendicular to the stretch), indicating the plastic behavior of the deformation.
Figure 16.
Textures from optical microscopy without crossed polarizers (OM) of the film unstretched (
a); under successive stretches (
b,c,d); and under relaxation (
e,f). Stretch perpendicular to the casting direction. The single arrow indicates the casting direction and the double arrow the stretching direction. Reprinted with permission from [
29]. Copyright 2011, Springer.
Figure 16.
Textures from optical microscopy without crossed polarizers (OM) of the film unstretched (
a); under successive stretches (
b,c,d); and under relaxation (
e,f). Stretch perpendicular to the casting direction. The single arrow indicates the casting direction and the double arrow the stretching direction. Reprinted with permission from [
29]. Copyright 2011, Springer.
Figure 17.
Distance between two consecutive stripes along the x-axis, as a function of ɛ. Data corresponding to the experimental situation of
Figure 16. Reprinted with permission from [
29]. Copyright 2011, Springer.
Figure 17.
Distance between two consecutive stripes along the x-axis, as a function of ɛ. Data corresponding to the experimental situation of
Figure 16. Reprinted with permission from [
29]. Copyright 2011, Springer.
When the film is stretched in the direction parallel to the casting direction this additional periodicity is not observed.
3.3. Atomic Force Microscopy of HPC Films
To perform the Atomic Force Microscopy (AFM) experiments, solutions of cellulose derivatives at several concentrations, ranging from the isotropic to the anisotropic phase region, were prepared.
Figure 18 shows the 3D topography image (20 × 20 μm
2 scan) of the free surface of a sheared HPC film prepared from a 60% w/w solution at a shear rate
v1 = 5 mm/s. The image shows two different scale ranges: a primary set of “large” bands, perpendicular to the shear direction, and a smoother texture characterized by a secondary periodic structure containing “small” bands.
Figure 19 shows a top view image of the height scan of the surface shown in
Figure 18 and the analysis of the height profile at two cross sections: AA' and BB'. Cross section AA' was taken along the shear direction. The periodicity of the “large” bands, ∆
l1, and the average peak-to-valley height for these bands,
h1, were determined from the AA' height profile plot, as indicated. Cross section BB' was taken along the direction of the secondary periodic “small” bands. The periodicity of the “small” bands, ∆
l2, and their peak-to-valley height,
h2, were measured from the BB′ height profile plot, as indicated. The arrows on the top of the view image along AA' and BB' lines mark the points used for the measurements performed in the height profile plots.
Figure 18.
3D topography image (20 × 20 μm
2 scan) of the free surface of a sheared HPC film prepared from a 60% w/w solution at a shear rate
v1 = 5 mm/s. Reprinted with permission from [
28]. Copyright 2002, American Chemical Society.
Figure 18.
3D topography image (20 × 20 μm
2 scan) of the free surface of a sheared HPC film prepared from a 60% w/w solution at a shear rate
v1 = 5 mm/s. Reprinted with permission from [
28]. Copyright 2002, American Chemical Society.
Figure 19.
Top view image of the height scan of the surface shown in
Figure 20 and the height profile analysis at the two cross sections: AA' and BB'. The arrows on the top of a view image along AA' and BB' lines mark the points used for the measurements of the height profile. Reprinted with permission from [
28]. Copyright 2002, American Chemical Society.
Figure 19.
Top view image of the height scan of the surface shown in
Figure 20 and the height profile analysis at the two cross sections: AA' and BB'. The arrows on the top of a view image along AA' and BB' lines mark the points used for the measurements of the height profile. Reprinted with permission from [
28]. Copyright 2002, American Chemical Society.
Figure 20 shows the top view image of the amplitude scan of the free surface of three sheared HPC films prepared at a shear rate of
v1 = 5 mm/s from solutions of different concentrations: (a) 30% (w/w), (b) 50% (w/w); and (c) 65% (w/w). The surface of the film prepared from 30% (w/w) does not possess any periodicity. A primary and a secondary set of bands were observed only on the films prepared from anisotropic solutions,
i.e., 50%–65% (w/w). Moreover, the films prepared from the solutions of the same concentration, using a higher shear rate v
2 = 10 mm/s), exhibit similar topographies, but they are characterized by different parameters. At a constant concentration, for example at 65% (w/w), the periodicity of the bands (∆
l1) shows a tendency to decrease when the shear rate increases (∆
l1(
v1) = 2.97 μm and ∆
l1(
v2) = 1.97 μm). At a constant shear rate, as the concentration of the polymer increases, the periodicity of the bands decreases (for
v1, at 50% and 65% (w/w), ∆
l1 = 4.68 μm and ∆
l1 = 2.97 μm, respectively).
Figure 20.
Top view image of the amplitude scan of the free surface of three sheared HPC films prepared at a shear rate of
v1 = 5 mm/s from solutions with HPC/water ratios: (
a) 30% (w/w); (
b) 50% (w/w); and (
c) 65% (w/w). Reprinted with permission from [
28]. Copyright 2002, American Chemical Society.
Figure 20.
Top view image of the amplitude scan of the free surface of three sheared HPC films prepared at a shear rate of
v1 = 5 mm/s from solutions with HPC/water ratios: (
a) 30% (w/w); (
b) 50% (w/w); and (
c) 65% (w/w). Reprinted with permission from [
28]. Copyright 2002, American Chemical Society.
A tuneable topographic system may be obtained from HPC aqueous liquid crystalline solutions [
50]. The results point out that two kinds of periodicities may be locked and adjusted in these systems as a function of the processing conditions. The set of “large” bands, which develops perpendicular to the shear direction, can be described as a relaxation process, which occurs immediately after the end of a shear applied to polymer liquid-crystalline solutions and attributed to contraction strains of the sheared sample induced by stress relaxation after cessation of flow. The secondary bands periodicities show a net tendency to decrease with polymer content, which seems an indication that the development of the “small” bands are mainly ruled by the cholesteric liquid crystal characteristics imposed by the initial precursor solutions. The pitch of the precursor chiral nematic solution and the related values of the liquid crystal elastic constants of the material can therefore be mainly responsible for the variation in size of the secondary bands, observed experimentally [
24]. The films are found to be self-affine between 300 nm and 4 μm but not for higher scales. In general, the fractal dimension is found to increase with both polymer concentration and shear rate. This trend reflects the increasing complexity of the surface topography when the films are prepared with higher polymer concentrations or with higher shear rates [
28].
3.4. Mechanical Behavior of Solid Cellulose Derivatives Films
Anisotropic solid films were fabricated by spreading the HPC anisotropic solutions, 60% (w/w), with the help of a calibrated shear casting knife, at room temperature, in an appropriate Teflon mould, this procedure allows the precise control of the shear casting flow speed (
v) (1 mm·s
−1) and enables the ready removal of the films without damaging at room temperature, other methods to obtain sheared HPC solid films involved higher processing temperatures [
51]. After solvent controlled evaporation, the solid cast shear films had an average thickness between 14 and 30 μm. As evaporation continues, the density of rod-like fragments increases near the free top surface, giving rise to increased orientational order. This in turn causes elongation at the top of the film in the direction parallel to the director, and since the dimensions of the bottom surface in contact with the glass substrate are fixed, the top surface buckles, forming a set of grooves shown previously [
52,
53,
54].
Figure 21.
Stress-strain relations. Squares correspond to strain parallel to the shear direction and the nematic director. Circles correspond to strain perpendicular to the shear direction and the nematic director. For this geometry, above a threshold, the stress is nearly independent of strain. This indicates “semi-soft” elasticity, characteristic of nematic elastomers. The negative slope at higher strains corresponds to failure due to tearing of the films. Reprinted with permission from [
55]. Copyright 2013, Nature Publishing Group.
Figure 21.
Stress-strain relations. Squares correspond to strain parallel to the shear direction and the nematic director. Circles correspond to strain perpendicular to the shear direction and the nematic director. For this geometry, above a threshold, the stress is nearly independent of strain. This indicates “semi-soft” elasticity, characteristic of nematic elastomers. The negative slope at higher strains corresponds to failure due to tearing of the films. Reprinted with permission from [
55]. Copyright 2013, Nature Publishing Group.
The modulus of the cellulose network has been measured for strain along the director, as well as perpendicular to it. Results are shown in
Figure 21; for small strains, Young’s modulus is 263 ± 39 MPa for shear parallel and 140 ± 9 MPa perpendicular to the director. Above a threshold of 4 MPa, the stress in the perpendicular direction is nearly independent of the strain. This “semi-soft” elastic response is characteristic of liquid crystal elastomers [
56] and has been associated to a director reorientation. However, as already highlighted [
57], a reset of order can also be the underlying mechanism for this process as observed in sheared anisotropic cellulosic films [
30]. The film rupture is preceded of a plastic regime when stretching is imposed parallel to the casting direction while it is preceded of significant necking during the “semi-soft” elastic response when stretching is imposed perpendicular to the casting direction.
When exposed to water vapor, free standing films prepared from a 60% (w/w) solution bend, as shown in
Figure 22. When water vapor penetrates the free surface of the film, the sample bends around an axis parallel to the shear direction, with the free surface on the outside (
Figure 22a). This is consistent with expansion of the free surface of the film in the direction perpendicular to the director. Such an expansion is expected, since the order parameter is reduced by the presence of the solvent water, and furthermore the presence of water molecules between the cellulose chains in the rigid segments is expected to cause the thickness of the rod-like fragments to increase.
Figure 22.
Bending of freestanding films. (
a) The film free top surface; and (
b) the film glass bottom surface exposed to water vapour. Sheared films were prepared from liquid crystalline HPC/water solution, the arrows indicate shear direction. Reprinted with permission from [
55]. Copyright 2013, Nature Publishing Group.
Figure 22.
Bending of freestanding films. (
a) The film free top surface; and (
b) the film glass bottom surface exposed to water vapour. Sheared films were prepared from liquid crystalline HPC/water solution, the arrows indicate shear direction. Reprinted with permission from [
55]. Copyright 2013, Nature Publishing Group.
The shear stress associated with such bend has been measured, as a function of time (
Figure 23), in a 20 mm × 20 mm × 32 μm planar sample at 24 °C with free surface exposed to humidity. Measurements were taken with Mettler Toledo AG204 load sensor. The maximum stress measured was 383 Pa.
When the film is allowed to dry, either by heating or by being placed in a low-humidity environment, the film unbends, reversibly assuming its original shape. The bending is relatively fast, on the scale of ≈1 s.
Figure 23.
Dynamics of stress evolution. Shear stress in a 20 mm × 20 mm × 32 µm planar sample at 24 °C with free surface exposed to humidity as function of time, measured with a Mettler Toledo AG204 load sensor. The maximum stress measured was 383 Pa. Reprinted with permission from [
55]. Copyright 2013, Nature Publishing Group.
Figure 23.
Dynamics of stress evolution. Shear stress in a 20 mm × 20 mm × 32 µm planar sample at 24 °C with free surface exposed to humidity as function of time, measured with a Mettler Toledo AG204 load sensor. The maximum stress measured was 383 Pa. Reprinted with permission from [
55]. Copyright 2013, Nature Publishing Group.
Interestingly, when the glass side of the film is exposed to water vapor, it bends around an axis perpendicular to the shear direction, with the glass side being convex (
Figure 22b). This is consistent with a nearly isotropic expansion of the glass side surface due to the presence of moisture. Since corrugations on the free surface give rise to a smaller effective modulus for bend in this direction, as has been confirmed by independent measurements, bend occurs around an axis perpendicular to the shear direction. The bend produced by exposing the glass side to water vapor is considerably smaller than that of the free surface. Taking advantage to the bending moisture effect a soft cellulosic motor was built (
Figure 24) [
55].
Figure 24.
Moisture-driven liquid crystal cellulose engine. (
a) Schematic of the motor, showing location of moist air and rotation direction. The alignment direction is parallel to the axes of the wheels. The free surface of the film is on the outer side; (
b) series of video frames showing rotation. The motor is housed in a dry environment. Momentum transfer from the moist air is small, and opposes the observed motion. Belt dimensions are: 1.0 cm × 8.0 cm × 30 µm; wheel diameter is 14 mm. The direction of rotation is indicated by a black arrow, and of moist air flow by a white arrow (
a). In (
b), t = 2 s/frame. Reprinted with permission from [
55]. Copyright 2013, Nature Publishing Group.
Figure 24.
Moisture-driven liquid crystal cellulose engine. (
a) Schematic of the motor, showing location of moist air and rotation direction. The alignment direction is parallel to the axes of the wheels. The free surface of the film is on the outer side; (
b) series of video frames showing rotation. The motor is housed in a dry environment. Momentum transfer from the moist air is small, and opposes the observed motion. Belt dimensions are: 1.0 cm × 8.0 cm × 30 µm; wheel diameter is 14 mm. The direction of rotation is indicated by a black arrow, and of moist air flow by a white arrow (
a). In (
b), t = 2 s/frame. Reprinted with permission from [
55]. Copyright 2013, Nature Publishing Group.
Nano crystalline cellulose rods can be incorporated into composite materials, enhancing their mechanical properties [
58]. NCC filler was used as a probe, which can influence the mechanical properties of the films but does not destroy the liquid crystalline characteristics of the composite material. In fact, adding 0.1% of NCC rods implies that the Young’s modulus of the films, as well as the tensile strength, measured in perpendicular (Per) and parallel (Par) directions to the casting, were increased by a factor of 2.5 and 3.2 for Par and 3.0 and 2.2 for Per, respectively, when compared with films prepared from HPC anisotropic solutions [
24]. Because of the high degree of molecular orientation, the HPC and the HPC/NCC films exhibit high modulus and strength along the shear direction and the mechanical strength in the transverse direction is low. These anisotropic mechanical properties are consistent with the molecular orientation, which results from the flow of the liquid crystalline solution under shear stress. The fact that the Young’s modulus and strength is much higher for HPC/NCC compared with HPC films, along the shear direction, is an indication that NCC rods align along this direction when the films are prepared. NCC rods enhanced the brittle behavior along this direction, as well as in the Per direction and act as a stiffener to the anisotropic cellulose matrix, which is also reflected in the strength deformation values for Par and Per directions.
The features (not the values) of the stress/strain curves obtained were similar to those from other APC and HPC already reported in the literature for isotropic and anisotropic cellulose derivatives [
59,
60,
61].
3.5. Cellulose Derivatives Composites in Electro-Optical Applications
Cellulose derivatives composites for electro-optical (EO) applications were introduced in 1982 by Craighead
et al. [
62], followed a few years later by a different type of cellulose derivative EO cell, named cellulose-based polymer dispersed liquid crystal (CPDLC) [
63,
64]. Due to the good match between the ordinary refractive index of the nematic liquid crystal (NLC) E7 (1.510) and the refractive indexes of HPC (1.49), a very clear ON state is achieved when an electric field
Eon is applied to the devices as can be seen in
Figure 25.
The CPDLC cell was composed of a rough cellulose derivative polymeric film surrounded by two NLC layers and the set placed in between two transparent conducting rigid or flexible substrates. These thin solid films were prepared from cellulose derivatives solutions, casted, and sheared simultaneously by moving a calibrated Gardner knife at 1.25 mm·s
−1. The films were allowed to dry at room temperature and kept in a controlled relative humidity (20%) chamber until further use. To evaluate the EO properties of these devices, the EO characterization was carried out using a laser-equipped optical bench in association with a function generator, a voltage amplifier, and a diode detector. The laser light was perpendicular to each sample and upon crossing it was collected at the diode detector whose output was fed to an amplifier and later recorded with a digital storage scope. All measurements were performed at room temperature. These cells showed very challenging properties, presenting high transmission coefficients values (around 0.8) in the ON state, but exhibiting turn-ON fields around 1.5 V/µm, giving rise to rather high turn-on voltages [
65]. Later on, a light scattering EO device where layers of two different cellulose derivatives were deposited as nonwoven nano and microfiber mats onto the conductive substrates by electrospinning, were presented [
66]. These devices can be used as high efficiency light shutters or as privacy windows since they can be electrically controlled to scatter light (OFF state) or to transmit it (on state) [
67]. These last devices presented an innovative method of preparing cellulose-based light scattering devices, which led to a major improvement in their EO properties and also their production cost. Using cellulose-based nano and microfibers mats as a network, EO light-scattering devices were produced as polymer-stabilized liquid crystal-type devices. In these devices, the cellulose derivatives were deposited as nonwoven nano- and microfiber mats onto the conductive substrates by electrospinning, and the cell was filled up by capillarity with a NLC. In these optical shutters, the LC is embedded with the fibers as a continuous phase, maximizing the LC/polymer surface contact and thus promoting improved EO properties. An increase in the on transparency and a marked decrease in the operating voltage of these devices were observed as the consequences of the improved interaction of the NLC with the nano and microfibers. More recently, a new type of EO device with improved EO properties was presented taking advantage of the high surface area of nanoscale cellulose whiskers [
27].
On this later technology, cellulose-based LC EO devices were prepared by stacking between two transparent conductive oxide–coated glasses, two layers of a nematic LC having in between a thin film composed of nano crystalline cellulose rods. The major step forward in the EO properties of this type of device is the significant reduction of the turn ON electric field and the decrease in the response time to reach the ON state as can be seen in
Figure 26.
Figure 25.
Macroscopic effect of the (
a) ON; and (
b) OFF states. Reprinted with permission from [
66]. Copyright 2009, AIP Publishing.
Figure 25.
Macroscopic effect of the (
a) ON; and (
b) OFF states. Reprinted with permission from [
66]. Copyright 2009, AIP Publishing.
Figure 26.
Curves of the applied electric field dependence of the light transmission coefficient of these devices compared with electrospun cellulose fiber devices (HPC and CA). Reprinted with permission from [
27]. Copyright 2013, Taylor and Francis.
Figure 26.
Curves of the applied electric field dependence of the light transmission coefficient of these devices compared with electrospun cellulose fiber devices (HPC and CA). Reprinted with permission from [
27]. Copyright 2013, Taylor and Francis.