Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Stimuli-Responsive Anisotropic Materials Based on Unidirectional Organization of Cellulose Nanocrystals in an Elastomer Osamu Kose,† Charlotte E. Boott,† Wadood Y. Hamad,‡ and Mark J. MacLachlan*,†,§,∥ †
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada FPInnovations, 2665 East Mall, Vancouver, British Columbia V6T 1Z4, Canada § Stewart Blusson Quantum Matter Institute, University of British Columbia, 2355 East Mall, Vancouver, British Columbia V6T 1Z4, Canada ∥ WPI Nano Life Science Institute, Kanazawa University, Kanazawa 920-1192, Japan Downloaded via NOTTINGHAM TRENT UNIV on July 18, 2019 at 07:20:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Cellulose nanocrystals (CNCs) derived from biomass have unique properties, which have inspired their incorporation into a wide variety of materials. However, the number of highly stretchable elastomers that have been prepared with CNCs has been limited. Here, we report shear-aligned pseudonematic CNCs embedded in a poly(ethyl acrylate) elastomer, a homogeneous composite that exhibits reversible optical properties in response to mechanical stimuli. Due to the long-range anisotropy of CNCs, the relaxed composite shows vivid interference color as it is viewed between crossed or parallel polarizers. When the pseudonematic CNC elastomer is stretched parallel to the CNC alignment direction, the CNCs become further aligned and the birefringence of the materials increases. In contrast, when the composite is stretched perpendicular to the CNC alignment direction, the CNCs become more disordered and the birefringence decreases. The extent of the CNC reorientation when the composite was stretched was determined by calculation of the birefringence of the material and two-dimensional X-ray diffraction analysis. Furthermore, the aligned CNCs act as nanoreinforcement in the elastomer, which resulted in the pseudonematic CNC−poly(ethyl acrylate) elastomer having a tensile modulus up to 120 times higher than that of pure poly(ethyl acrylate). is changed.25 Akin to molecular liquid crystals, the alignment of CNCs can change the polarization of transmitted light due to a phase difference, which can result in significant changes in interference color when viewed between crossed or parallel polarizers.15,26 The birefringence of CNCs is attributed to the alignment of cellulose chains inside each CNC spindle, which are bundled in parallel to form a highly crystalline structure.27,28 This morphology creates an optical axis in a CNC, which corresponds to its rod axis and, therefore, results in polarization of transmitted light.29 When observed on the macroscopic level, the chiral nematic arrangement of CNC films results in a structure with relatively low birefringence when viewed along the stacking axis (i.e., CNC spindles align parallel to the film but point in every direction with equal probability); thus, when viewed along the helical axis, this structure is optically isotropic. On the other hand, CNCs that have been aligned by either an electric field, magnetic field, or shear force show significantly larger birefringence due to the alignment of the optical axes of the CNCs.28−30
1. INTRODUCTION Cellulose nanocrystals (CNCs) derived from wood, cotton, or other cellulosic biomass are of great interest to both academia and industry, and have been explored for a range of applications such as sensors, tissue engineering, drug delivery, reinforced plastics, and optics.1−9 One of the most notable properties of CNCs is their ability to self-assemble into a helical architecture referred to as a chiral nematic organization, which is similar to the Bouligand structure found in natural systems.10−15 When CNCs adopt a chiral nematic organization, the orientation of the CNCs rotates in layers with a characteristic repeating distance. If the periodicity (pitch) of the structure is in the range of the wavelength of visible light, the thin films appear iridescent (so called structural color) due to diffraction of the incident light.16−18 Notably, the reflected light is left-handed circularly polarized by the helical structure.9,19 Ordered structures of CNCs can be constructed through evaporation-induced self-assembly of aqueous CNC suspensions under appropriate conditions. In addition, the chiral nematic structure of CNCs can be transferred to materials such as glass, resin, hydrogels, and plastics, and these exhibit optical properties based on the periodic structure.20−24 Another key property of CNCs is that they exhibit a significant change in their birefringence when their orientation © XXXX American Chemical Society
Received: April 26, 2019 Revised: June 25, 2019
A
DOI: 10.1021/acs.macromol.9b00863 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules The preparation of aligned, free-standing CNC films has previously been reported in order to measure their coefficient of thermal expansion.29 The aligned CNC films clearly demonstrated direction-dependent properties, as well as strong birefringence and interference color. Unfortunately, CNC films are brittle and thus application of a large structural deformation to alter the unidirectional organization of the CNCs to influence the optical properties is not feasible. Therefore, to obtain highly stretchable composites with aligned CNCs, it is necessary to incorporate CNCs into stretchable materials. Elastomers are ideal matrices to combine with CNCs to prepare stretchable composites, since they can undergo large deformations when a mechanical force is applied and return to their original shape when the stress is removed.31 However, most elastomeric materials such as polyacrylates, polybutadienes, polydimethylsiloxanes, and their respective monomers are highly hydrophobic. These materials are therefore incompatible with aqueous CNC suspensions or the hydrophilic CNC films. Furthermore, while CNCs in elastomers are mostly regarded as mechanically reinforcing materials, little attention has been paid to how the optical properties depend on the anisotropic features of CNCs in the composite.32−35 We recently reported CNC−elastomer composites with a chiral nematic organization of CNCs;36 when stretched, the helical arrangement of the CNCs unwound to yield an optically anisotropic composite. To the best of our knowledge, CNC− elastomer composites with unidirectional organization are unknown. In this study, we report unidirectionally aligned CNCs in an elastomer as a stretchable composite that exhibits anisotropic optical and mechanical properties when it elongates. For the composite preparation, we applied a modified version of the method that we previously reported to capture the chiral nematic structure of CNCs in hydrophobic poly(ethyl acrylate) (PEA).36 The prepared pseudonematic CNC elastomer (NC-E) is transparent and is able to undergo large deformation when mechanical stress is applied. In addition, relaxed NC-E showed vivid interference colors resulting from the anisotropic organization of CNCs when the composite is viewed between polarizers, and the color reversibly changes as it is stretched and relaxed. Interestingly, this stimuli-responsive optical property varies depending on the stretching direction relative to CNC alignment. Furthermore, the CNCs have a significant reinforcing effect on the poly(ethyl acrylate) and this effect is enhanced when the CNCs are aligned within the NC-E.
mA, 45 kV for 480 s at 60 mm from the detector in transmission mode. Tensile testing was performed on an Instron 5566Q at a rate of 50 mm/min. 2.1. Polarized UV−Vis Spectroscopy. The sample was placed between the linear polarizers, which were oriented parallel or crossed as indicated in the text and figure captions. The shear-aligned direction was fixed at 45° relative to the polarization axis of the polarizers unless otherwise stated. Transmitting intensities of both the crossed I⊥ and parallel I∥ arrangements were recorded on a Cary 5000 UV−vis/near-infrared spectrometer between 400 and 700 nm. 2.2. Theoretical Spectral Fitting to Calculate Birefringence.38,39 When light propagates through a uniaxial medium, light polarized parallel and perpendicular to the optical axis experiences different refractive indices. When they emerge from the sample, they have a phase difference, δ, which can be described by eq 1
δ=
2πdΔn λ
(1)
where λ is the wavelength, d is the sample thickness, and Δn is the birefringence, which is the difference between the refractive indices experienced by rays polarized in two orthogonal directions. If an incident beam of light strikes the sample while it is placed between two crossed polarizers, then the intensity of the transmitted light when the polarizer and analyzer are oriented perpendicular (I⊥) and parallel (I∥) to one another can be described theoretically with eqs 2 and 3, respectively38 I⊥ = I0 sin 2θ sin 2
δ 2
(2)
I = I0 sin 2θ cos2
δ 2
(3)
where θ is the angle of the optical axis (alignment direction of CNCs in this study) relative to the polarization axis and I⊥ and I∥ are transmitting intensities of incident light I0 with polarizers in cross and parallel configurations, respectively. When the alignment direction of relaxed NC-E is fixed at 45° (θ = 45°) relative to the polarization axis of the polarizers, absorbance and transmittance contrast can be measured and then δ (and thus the birefringence) can be determined from nonlinear least-squares fitting. The relations shown in eqs 2 and 3 can be inverted to give the birefringence Δn with respect to I⊥ and I∥ (eqs 4 and 5)
I yzz λ ijjj jjkπ + 2 tan−1 ⊥ zzz, k = 0, 2, 4 ... 2πd j I z k { ÄÅ ÉÑ Å I⊥ ÑÑÑÑ λ ÅÅÅ − 1 ÑÑ, k = 1, 3, 5 ... Δn = ÅÅ(k + 1)π − 2 tan 2πd ÅÅÅÅ I ÑÑÑÑ Ç Ö Δn =
(4)
(5)
where k is a non-negative integer. By measuring the intensities I⊥ and I∥ and the sample thickness d, the birefringence Δn can be extracted from eqs 4 and 5. (Note that this derivation assumes that absorption is negligible, which is valid since our materials appear colorless.38) To account for dispersion effects in Δn, a simplified form of the Sellmeier dispersion equation called the Cauchy formula (eq 6) was used by applying a nonlinear least-squares fitting method with A and B as our fitting parameters. The results are shown in Figure S9 in the Supporting Information, where black dotted lines indicate Δn directly obtained from the measured intensities I⊥ and I∥ using eqs 4 and 5 and colored lines indicate calculated Δn where the dispersion effect is incorporated in a wavelength range of 400−700 nm. Only nearly horizontal parts of each plot from eqs 4 and 5 are relevant, and those horizontal traces with different k were used for the fitting with eq 6.
2. EXPERIMENTAL SECTION All chemicals were purchased from standard suppliers and used without further purification. The aqueous suspension of CNCs was obtained from FPInnovations (6.0 wt %, pH = 2.1, mean particle size (dynamic light scattering) = 133 ± 4 nm, ζ-potential = −50 mV, sulfur content = 0.7 wt %). CNC aqueous suspensions were obtained using sulfuric acid hydrolysis, as described in detail in Hamad and Hu.37 To obtain the protonated form, H-CNCs, the purified suspension was treated with Dowex Marathon C cation-exchange resin to convert sodium sulfate ester groups into their protonated acid form having a pH around 2.1. Polarized optical microscopy (POM) images were recorded on an Olympus BX41 microscope with linear polarizers. Scanning electron microscopy (SEM) was performed using a Hitachi S4700 electron microscope on gold sputter-coated samples. Two-dimensional X-ray diffraction (2D-XRD) images were recorded with a Bruker APEX DUO equipped with an APEX II CCD detector using a Cu Kα1 X-ray beam with a wavelength (λ) of 0.154 nm at 0.6
Δn = A +
B λ2
(6)
2.3. Calculation of Hermans Order Parameter (S). Diffraction intensities I(ϕ) were recorded with respect to the azimuthal angle ϕ B
DOI: 10.1021/acs.macromol.9b00863 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. (a) Scheme illustrating the process for preparing the unidirectionally aligned CNC elastomer. (b, c) Cross-sectional SEM images of a shear-aligned CNCs dry film parallel (b) and perpendicular (c) to the CNC alignment direction. (d, e) Photographs showing NC-E in relaxed (c) and stretched (d) conditions under natural light. (f) Polarized light observation of relaxed NC-E. Samples are back-lit and placed between the polarizer and analyzer. Red arrows in the image indicate polarization axes of both the linear polarizer and analyzer. θ is the angle between the polarization axis of a polarizer and the direction of CNC alignment within the elastomer. (g) Transmitting polarized light intensities of relaxed NCE. The CNC alignment direction was oriented at θ = 0, 15, 30, 45° relative to the polarization axis of the polarizer. (h) Transmitting polarized UV− vis spectra of relaxed NC-E in crossed and parallel arrangements of polarizers at θ = 45°. at 2θ = 22.9°, which corresponds to the (200) diffraction peak of the cellulose Iβ crystal. The plots were then fitted as a Lorentzian distribution by the least-squares fitting method to obtain the profile of I(ϕ). The order parameter S (0 ≤ S ≤ 1) was calculated from I(ϕ) following eqs 7−9 by a procedure previously reported.29,40 Here, γ represents the angle between the (200) lattice planes and the direction of CNC alignment (which corresponds to ϕ = 0° in this measurement). For reference, S = 0 represents an isotropic system and S = 1 represents a perfectly aligned (anisotropic) system. 3⟨cos2 γ ⟩ − 1 2
(7)
⟨cos2 γ ⟩ = 1 − 2⟨cos2 ϕ⟩
(8)
S=
⟨cos2 ϕ⟩ =
∫ I(ϕ) cos2 ϕ sin ϕ dϕ ∫ I(ϕ)sin ϕ dϕ
suspension (approximately 2 cm/s, 40 times, Figure S1). After removal of the top glass slide, the sheared CNC suspension was dried at ambient temperature overnight to obtain a pseudonematic CNC film on the glass slide. The CNC film was then soaked in dimethylsulfoxide (DMSO) in a nitrogen-purged cell for 10 min to swell the CNC film, and the residual DMSO was removed. Ethyl acrylate (EA, 1 mL) with 2,2′-azobis(2-methylpropionitrile) (AIBN, 2.3 mg) as the radical initiator was added to the cell, and the sample was kept still for 4 h at ambient temperature. Finally, a glass slide was placed on the monomer-infused film followed by nitrogen-purging, and then, polymerization was initiated by heating the cell at 60 °C for 18 h to obtain approximately 9 wt % CNC/poly(ethyl acrylate) composite (calculated from the initial weight of the CNC film and the weight of the final composite) that has pseudonematic CNC organization and 210 (± 10) μm thickness. The optical properties of the NC-E are determined by the orientation of the CNCs; if we were to modify the polymerization conditions, only the mechanical properties of NC-E would change. 2.6. Preparation of Random CNC Elastomer. A few drops of water were added to the surface of the dry film of CNCs with pseudonematic order. After the films were dried at ambient temperature overnight, a film with random CNC orientations was obtained. The prepared random CNC dry film was soaked in DMSO in a nitrogen-purged cell for 10 min to swell the film, and then, residual DMSO was removed. EA (1 mL) with AIBN (2.3 mg) as a radical initiator was added to the cell, and the sample was kept still for 4 h at ambient temperature. Finally, a glass slide was placed on the monomer-infused film followed by nitrogen-purging, and then polymerization was initiated by heating the cell at 60 °C for 18 h to capture the random CNCs in an elastomer.
(9)
2.4. Measurement of Tensile Strength. Rectangular specimens (7 mm × 15 mm) were cut from one piece of NC-E film. For parallel stretch, samples were stretched along the direction in which CNCs are aligned; samples were stretched perpendicular to the alignment direction for perpendicular stretch. Young’s modulus of NC-E was calculated from change in stress (Δσ) divided by the change strain (Δε). 2.5. Preparation of Pseudonematic CNC Elastomer (NC-E). D-Glucose (180 mg) was added to the aqueous CNC suspension (6.0 wt %, pH = 2.1, 6.0 g) and then stirred at 80 °C under air until enough water had evaporated from the suspension that the CNC concentration reached 12 wt %. The D-glucose readily dissolved in the CNC suspension. The obtained CNC suspension was cast between two rectangular spacers (50−400 μm thickness) aligned in parallel that were placed on a glass slide. The CNC suspension was then covered by another glass slide, and shear force was applied to the suspension by moving the cover glass slide back and forth over the
3. RESULTS AND DISCUSSION 3.1. Characterization of Unidirectional Organization of CNCs. Due to the high hydrophobicity of elastomeric monomers, significant phase separation and aggregation occur C
DOI: 10.1021/acs.macromol.9b00863 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. (a) Two-dimensional XRD pattern (left) and diffraction intensity at 2θ = 22.9° (right) of the shear-aligned CNC dry film. (b) Twodimensional XRD pattern NC-E in relaxed conditions. (c) Two-dimensional XRD pattern of pure poly(ethyl acrylate). For (a) and (b), the value S is the calculated Hermans order parameter and the arrows in the diffraction images represent the alignment direction of the CNCs. (d) Schematic illustration of polarized light observation for elongated samples. (e) Interference colors observed for NC-E as it is elongated perpendicular to the CNC alignment direction. The samples were viewed between crossed polarizers, and the arrows indicate the polarization axes of the polarizers.
when they are mixed directly with an aqueous CNC suspension. To address this issue, we first prepared shearforce-aligned CNC films, followed by DMSO-mediated elastomeric monomer incorporation and subsequent polymerization to prepare a pseudonematic CNC elastomer (Figures 1a and S1 in the Supporting Information). (We refer to these materials as “pseudonematic” rather than “nematic” since they have similar organization to a nematic phase but they are not obtained from a nematic liquid crystal.) First, a 12 wt % aqueous CNC suspension including 6 wt % D-glucose was cast on a glass slide with 400 μm gap spacers, and then another glass slide was placed over the CNC suspension. Subsequently, a shear force was applied to the suspension by moving the top glass slide back and forth over the suspension 40 times to align the CNC rods. The film was then allowed to dry to yield a pseudonematic CNC film (Figure S2 in the Supporting Information). The alignment of CNCs in the film was confirmed by SEM and POM (Figures 1b,c and S3 in the Supporting Information). The pseudonematic CNC film was swollen with DMSO, and then ethyl acrylate and the radical initiator AIBN were added to the film. The solution was then polymerized at 60 °C to give a homogeneous CNC/elastomer composite (NC-E). In this procedure, glucose and DMSO play critical roles in allowing the CNC films to be compatible with the hydrophobic elastomer precursor and to prevent the film from cracking (see the Supporting Information Figure S4 for more details). Furthermore, the presence of glucose facilitates swelling of the CNC film with DMSO, which improves penetration of the ethyl acrylate monomers into the film. The prepared NC-E is able to undergo a large deformation when mechanical stress is applied, and it returns to its original shape when the stress is removed (Figure 1d,e). The NC-E is
transparent under natural light but shows vivid interference colors when it is viewed between polarizers (polarizer and analyzer) in both crossed and parallel configurations. The alignment direction of CNCs was fixed at 45° with respect to the polarization axis (light passes sequentially through the polarizer, specimen, and analyzer before reaching the detector). Moreover, the complementary color was observed when the polarizers were placed parallel to one another. In sharp contrast, the light was completely blocked when the CNC alignment direction was set parallel (0°) to the polarizer, implying that the organization of CNCs in NC-E is anisotropic, as observed for the precursor CNC film (Figure 1f, Movies S1 and S2 in the Supporting Information). The optical properties of the relaxed NC-E were also characterized by polarized UV−vis spectroscopy. When the sample was rotated between crossed polarizers, the maximum transmittance was recorded when the alignment direction was 45° relative to the polarization axis and the minimum was recorded at 0°. In addition, the spectral contrast was completely reversed when the polarizers were in the parallel configuration (Figure 1g,h). It was found that the UV−vis spectra obtained from the relaxed NC-E were in good agreement with the theoretical spectra produced from the equations that describe transmitting polarized light intensities and the absorbance contrast of a uniaxial birefringent material between crossed and parallel configurations (see Section 2 and Figure S5 in the Supporting Information).38 3.2. Stimuli-Responsive Optical Properties. Twodimensional X-ray diffraction (2D-XRD) measurements were carried out to further understand the orientation of the CNCs in the relaxed NC-E. We looked at the azimuthal angle dependence on the diffraction at 2θ = 22.9°, which D
DOI: 10.1021/acs.macromol.9b00863 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. (a) Photographs of the NC-E samples showing the interference color change as the samples are stretched parallel (left sample in clips) and perpendicular (right) to the CNC alignment direction (viewed between crossed polarizers). Pink and green arrows in the image indicate the stretching directions for parallel and perpendicular stretching, respectively. (b) Transmitting polarized UV−vis spectra of perpendicular-stretch and (c) parallel-stretch NC-E samples in crossed configuration of polarizers. (d) Calculated birefringence Δn of perpendicular-stretched and (e) parallel-stretched NC-E samples in each elongation state.
gradually lost its birefringence and, at a certain extent of elongation, it completely blocked the incident light from passing through the crossed polarizers (i.e., it was no longer birefringent). However, the back light started to transmit again when the sample was elongated further (Figure 2e, second sample from the right and Movie S5 in the Supporting Information). This result can be explained by a decrease in birefringence of disordered CNCs in the NC-E that corresponded with the increasing birefringence arising from the stretched poly(ethyl acrylate) chains. 3.3. Birefringence Change is Associated with CNC Orientation. In contrast to the perpendicular stretching, less dramatic color changes were observed when the NC-E was elongated parallel to the direction of CNC alignment, consistent with the alignment being maintained as the sample is stretched (Figure 3a and Movie S6 in the Supporting Information). In this regard, we investigated the optical property variation of NC-E associated with the stretching direction by polarized UV−vis spectroscopy and quantified the birefringence. Whereas the parallel-stretched NC-E barely changed its absorbance contrast within 40% elongation, NC-E stretched perpendicular to the CNC alignment gave a substantial change in the absorbance spectrum for the same elongation in both the crossed and parallel configurations of polarizers (Figure 3b,c). Despite the dramatic color change, the spectra of perpendicular-stretched NC-E were still complementary between the two polarizer configurations, meaning the material is still anisotropic at that extent of elongation (Figure S7 in the Supporting Information). Based on the transmittance intensities, the birefringence of NC-E in each stretching state was calculated (see Section 2 for the calculation method).38 As we assumed, the birefringence of the NC-E stretched perpendicular to the CNC alignment direction rapidly decreased as it elongated; in contrast, increased birefringence was found in NC-E stretched parallel to the
corresponds to the (200) diffraction of aligned cellulose polymer chains in a single CNC that are parallel to the CNC rod length.27 As expected, both the shear-aligned CNC film and the relaxed NC-E showed a strong angle dependence in their diffraction patterns and intensities (Figure 2a,b). We also confirmed that the pure poly(ethyl acrylate) elastomer matrix does not show angle-dependent diffraction (Figure 2c) and therefore that the angle dependence observed is due to the aligned CNCs. To quantify the extent of alignment, the Hermans order parameter of the materials (0 ≤ S ≤ 1) was calculated based on the diffraction intensities at 2θ = 22.9°.29 We determined that S = 0.73 and 0.72 (± 0.01) for the shearaligned CNC film and relaxed NC-E, respectively, indicating that the aligned structure of CNCs is preserved in the elastomer matrix and the materials are highly anisotropic (see Section 2 for the calculation method). We next conducted a set of experiments to investigate the optical properties of NC-E in response to elongation. As depicted in Figure 2d, the film was oriented such that the CNC alignment direction was fixed at 45° (θ = 45°) relative to both the polarizer and the analyzer, and the sample was elongated perpendicular or parallel to the alignment direction of the CNCs. Interestingly, the color observed changed dramatically as the composite was stretched perpendicular to the CNC alignment direction. The order of the color change matched the Michel-Lévy color chart for decreasing birefringence. After the stress was removed, the NC-E returned to its original shape and birefringence color (Figure 2e, Movies S3 and S4 in the Supporting Information).41 Considering that the pure poly(ethyl acrylate) polymer matrix exhibits birefringence as it elongates due to the alignment of the polymer chains, it is apparent that pseudonematic organization of CNCs in NC-E is becoming more disordered as the sample is stretched perpendicular to the CNC alignment direction (Figure S6 in the Supporting Information).42 It should be noted that NC-E E
DOI: 10.1021/acs.macromol.9b00863 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. (a) Two-dimensional XRD image and diffraction intensity at 2θ = 22.9° of 200% perpendicular-stretched NC-E. The white arrow in the diffraction image indicates CNC alignment, and the red arrow indicates the stretching direction. (b) Birefringence and order parameter plots of perpendicular-stretched NC-E in each elongation state. (c) Two-dimensional XRD image and diffraction intensity at 2θ = 22.9° of 200% parallelstretched NC-E. (d) Birefringence and order parameter plots of parallel-stretched NC-E in each elongation state. (e) Schematic illustration of CNC reorientation in the composite. (f) Strain−stress curve and (g) tensile modulus of NC-E, random CNC−elastomer composite, and pure poly(ethyl acrylate).
S9 in the Supporting Information). Based on these results, we propose that the orientation of the CNCs in the NC-E is slightly disordered in the NC-E material but becomes better aligned as the composite is stretched parallel to the shear direction. On the other hand, the CNCs become completely disordered when the elastomer matrix is stretched in the perpendicular direction (Figure 4e). These effects are reversible. 3.4. Reinforcement Effects of CNCs. It is worth mentioning that CNCs have been shown to exhibit a reinforcing effect on plastics, resins, and elastomers due to the high aspect ratio (ca. 10 for CNCs extracted from woodbased biomass or ∼40 for those from tunicate) and outstanding mechanical strength (elastic modulus = 145−150 GPa) of CNCs.43 However, how the anisotropic order of CNCs affects the reinforcement of elastomers has not been well explored.44,45 To this end, we conducted tensile tests on NC-E relative to CNC alignment as well as pure poly(ethyl acrylate) (PEA) and a randomly aligned CNC composite to further understand the effect of the anisotropic alignment on the mechanical properties. The random CNC composite was confirmed to be isotropic by 2D-XRD analysis (Figure S10). While pure PEA could be elongated over 2500%, all CNCcontaining composites showed higher tensile stress (Figure 4f). The tensile modulus of NC-E stretched parallel to the alignment axis reached 120 times that of pure PEA (Figure 4g). Both of these films have direction-dependent photonic and mechanical behavior. Self-assembled CNCs from aqueous suspensions formed a hydrogen-bonded percolated network upon soaking in DMSO, which was maintained throughout EA polymerization without interference from the polymer resin.
CNC alignment direction (Figure 3d,e). The interference color of the stretched NC-E can be determined by the retardation (R, nm), which is a product of birefringence (Δn) and sample thickness (d, nm), as described by eq 10 R = dΔn
(10)
When the NC-E is stretched either parallel or perpendicular to the CNC alignment direction, it exhibits an interference color that corresponds to decreasing retardation (i.e., the color change matches the order of the Michel-Lévy color chart along the direction of decreasing retardation). However, the degree of retardation is different between the two stretch directions as the parallel-stretched NC-E has a larger birefringence than the perpendicular-stretched NC-E (Figure 3d,e).38 To further determine how the pseudonematic organization of the CNCs is oriented in the NC-E when it undergoes both parallel and perpendicular elongations, we conducted 2D-XRD analysis and calculated the order parameters. When the NC-E was stretched 200% perpendicular to the alignment direction of the CNCs, it showed almost no azimuthal angle dependence in 2D-XRD (Figure 4a); the calculated Hermans parameter (S = 0.01) indicates that the elastomer was nearly anisotropic. This is consistent with the decreased birefringence we observed when stretching the NC-E perpendicular to the CNC alignment direction. It should be noted that when the sample was stretched perpendicular to the CNC orientation, birefringence could not be calculated past 100% elongation since it was effectively isotropic (Figure 4b). In sharp contrast, the birefringence of NC-E increased slightly and the CNCs became slightly better aligned when the sample was stretched parallel to the orientation of the CNCs (Figures 4c,d, S8 and F
DOI: 10.1021/acs.macromol.9b00863 Macromolecules XXXX, XXX, XXX−XXX
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This is the reason for the significant increase in stiffness. As we anticipated, when NC-E is stretched perpendicular to the alignment direction, it has a relatively lower tensile modulus due to the disordering of the CNCs as it is stretched. The isotropic arrangement of the CNCs in the elastomer accounts for the medium tensile modulus of random CNC−elastomer since CNC spindles point in every direction relative to the film plane.
Mark J. MacLachlan: 0000-0002-3546-7132 Notes
The authors declare no competing financial interest.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00863. Experimental procedures including transmission electron microscopy image of CNCs, POM images, glucose effect for DMSO swelling of aligned CNC dry film, theoretical spectrum fitting, polarized UV−vis spectra, birefringence calculation of NC-E, and 2D-XRD analysis of NC-E (PDF) Transmitting polarized light intensities of relaxed NC-E (MOV) Inversion of interference color of NC-E (MOV) Reversible interference color change of NC-E responding to perpendicular-stretch in cross configuration of polarizers (MOV) Reversible interference color change of NC-E responding to perpendicular stretch in parallel configuration of polarizers (MOV) Interference color change of NC-E responding to further perpendicular stretch in crossed configuration of polarizers (MOV) Interference color change of NC-E responding to further stretch toward perpendicular and parallel direction in crossed configuration of polarizers (MOV)
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ACKNOWLEDGMENTS
This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada (Discovery Grant, CREATE grant), the Canadian Foundation for Innovation (JELF grant) and JSR Corporation. C.E.B. thanks the Banting Postdoctoral Fellowships and Killam Postdoctoral Fellowships for funding. The authors gratefully acknowledge the valuable assistance of Anita Lam in 2D-XRD analysis and James Drummond in tensile testing.
4. CONCLUSIONS In summary, we report a highly stretchable, homogeneous elastomeric nanocomposite material containing an anisotropic organization of CNCs. The stretchable nanocomposite can undergo 300% elongation, returning to its original shape when external stress is removed for at least five cycles without property loss (deforms when stretched more than 800% with hysteresis). Interestingly, the optical properties of CNCs are preserved in the elastomer matrix to give vivid interference color that corresponds with a uniaxial birefringent material. Furthermore, the alignment directions of CNCs dominate the optical and mechanical properties of the NC-E when it is exposed to external stimuli. Perpendicular-stretched NC-E shows sensitive color changes due to a dramatic decrease in the birefringence, which is associated with the CNCs becoming more disordered. The elastic modulus of parallel-stretched NC-E was found to be 120 times higher than that for pure poly(ethyl acrylate). In this study, we have proved that manipulation of the CNC orientation in an elastomeric composite significantly enhances its functions and expands the number of possible applications of CNCs.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. G
DOI: 10.1021/acs.macromol.9b00863 Macromolecules XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.macromol.9b00863 Macromolecules XXXX, XXX, XXX−XXX
