Hybrid Nanocomposites for 3D Optics: Using Interpolymer Complexes with Cellulose Nanocrystals.

Manipulation of optical paths by three-dimensional (3D) integrated optics with customized stacked building blocks has gained considerable attention. Herein, we present functional thin films with assembly ability for 3D integrated optics based on nanocomposites made of cellulose nanocrystals (CNCs) embedded in hydrogen-bonded (H-bonded) interpolymer complexes (IPCs). We selected H-bonded IPC poly(ethylene oxide) and neutralized poly(acrylic acid) to render films assembly ability without undesired interplay with charge distribution in CNCs. The CNCs can form a stable chiral nematic liquid crystalline phase with long-range orientational order and helical organization. The resulting nanocomposites are characterized with a high elastic modulus of 8.8 GPa and an adhesion strength of 1.35 MPa through reversible intermolecular interactions at the contact interface upon exposure to acidic vapor. Instead, simply stacked into 3D optics, these functional thin films serve as a facile material for providing a conceptually simple approach to assemble 3D integrated optics with different liquid crystalline orderings to manipulate the light polarization state.

Introduction

An optical path is typically determined by a set of elements selected to achieve certain functions, such as imaging and wave front manipulation. Although conventional free-space components can manipulate the optical path, they are highly demanding in terms of control of the index of refraction interface, precise assembly, and stable joining of dissimilar devices. Alternatively, three-dimensional (3D) integrated optical materials and components consisting of vertically stacked layers of horizontal two-dimensional integrated optics with properly placed vertical waveguide interconnects between the layers can be used.[1] Recently, 3D integrated directional couplers, polarization splitters, and electro-optical devices have been realized in polymer material systems.[2−4] A highly effective assembly approach is employed to replace the free-space optical elements with functional thin films,[5] such as liquid-crystal polymers,[6−8] and stack them to obtain the desired optical paths. Progress in the fabrication of optical structural materials and devices with assembly ability remains a great challenge, particularly in devices whose performance is highly sensitive to the material microstructure and the interface between the components. Herein, we report on functional thin films with a precise angle-dependent structural color by the assembly of cellulose nanocrystals (CNCs) and hydrogen-bonded (H-bonded) interpolymer complexes (IPCs), which can be exploited as building blocks for 3D customized nanocomposites and integrated optics. Unlike nanostructured optical films reported recently,[9−11] the proposed 3D optics has an integrated structure instead of a simply stacked sandwiched structure assembled with various nematic and chiral nematic phase, exhibiting distinctive optical feature.

H-bonded IPC is composed of two initially water-soluble polymers which are bound via a temporary and reversible H bonding network along the polymer backbone. When the molar ratio of monomeric units is not very different from unity, an insoluble product can be formed.[12,13] The properties of such self-assembled materials respond strongly to environmental stimuli such as pH, temperature, and solvents. An example for IPC pair is poly(acrylic acid) (PAA), a weak polyelectrolyte in an aqueous solution, and a nonionic polymer such as poly(ethylene oxide) (PEO).[14−16]

CNCs are rodlike particles with a typical aspect ratio of 1:20 characterized by low density (1.6 g·cm–3), high mechanical strength (2–3 GPa), large surface area (up to 700 m2·g–1), and high elastic modulus (110–140 GPa). When processing the CNCs using sulfuric acid as the hydrolyzing agent, it reacts with the surface hydroxyl groups of cellulose to yield charged surface sulfate esters.[17] CNCs self-assemble into a long-range helical arrangement, known as chiral nematic ordering.[18,19] In CNC aqueous suspensions, phase transition from isotropic to anisotropic state occurs above a critical CNC concentration.[20] Furthermore, chiral nematic ordering can be preserved during solvent evaporation, leading to iridescent colors when viewed from different angles.[21,22] Unlike a previous work suggesting that CNCs can assemble with water-soluble polymers to form vivid iridescent films,[23−25] the assembly of IPC with CNC involved complex interactions between interpolymer complex chains and CNC, which can provide a CNC-based IPC (IPC-CNC) film with distinct adhesive behavior.

The breadth of applications of H-bonded IPCs was expanded here by the development of an IPC-CNC optical building block. In this work, we combined PEO with PAA and characterized the effect of PAA concentration on complexation and adhesive properties of the resultant IPC-CNC. In addition, the phase ordering and the microstructure evolution of IPC-CNC were studied in both suspensions and cast films, as well as the pitch of the assembled CNCs, when integrated with the PEO–PAA films, was evaluated. As a proof of concept, the assembly of stacked functional three-layer IPC-CNC films with different liquid crystalline orderings is demonstrated.

Experimental Section

Materials

CNC powder was obtained from ScienceK Company (China). The rodlike CNCs prepared by sulfuric acid hydrolysis have dimensions of 120 ± 35 nm in length, L, and 6.5 ± 0.4 nm in diameter, D (Figure S1). Cellulose-type Iβ can be observed by characteristic peaks in wide-angle X-ray diffraction (WAXD, Figure S2) as well as a featured peak at 710 cm–1 assigned to O–H out-of-plane bending in Fourier transform infrared (FTIR) spectroscopy (Figure S3).[26,27] PAA, with a molecular weight of 450 kDa, and PEO, with a molecular weight of 600 kDa, were purchased from Sigma-Aldrich. All materials were used as obtained without further purification.

Film Preparation

The CNC water dispersions were stirred overnight and sonicated for 5 min (sonication power: 160 W) before use. The CNC solution was cast into a Teflon mold, followed by evaporation under ambient conditions for 3 days, giving rise to a freestanding iridescent chiral nematic CNC film, labeled herein as Film-CNC. PEO-CNC (PEO concentration: 3.38 wt %) suspensions with varying CNC concentrations (1.4, 2.1, 3.5, 5.5, and 8%) were prepared in a water:tetrahydrofuran (THF) mixed solvent (7:3 v/v). The solvent mixture is able to dissolve the IPC, whereas precipitation occurs if only water is used. PAA-CNC suspensions (PAA concentration: 5.42 wt %) were prepared with the same protocol as PEO-CNC suspensions. IPC-CNC was then prepared by mixing these two suspensions of equal weight (molar ratio PEO to PAA is 1:1) under ambient conditions. IPC-CNC films were cast from suspensions with different CNC concentrations (1.4, 2.1, 3.5, 5.5, and 8%) into a Teflon mold, followed by evaporation in a saturated water/THF atmosphere for 7 days. Film thicknesses varied from 100 to 140 μm. An IPC neat film and IPC-CNC films were labeled as Film-0, Film-24, Film-32, Film-45, Film-55, and Film-65 (see Table S1, Supporting Information).

Characterization

The rheological properties were studied by using a rotational rheometer Discovery DHR-2 (TA Instruments, USA) to characterize solutions under steady-state shear flow. All rheological measurements were performed using a parallel plate configuration (diameter 40 mm, gap 0.5 mm) at ambient temperature (25 °C). A Discovery DSC-2500 calorimeter (TA Instruments, USA) was used to investigate the thermal properties of the films. Heating under modulated mode from −60 °C up to 160 °C with the heating rate of 5 °C min–1 and modulation at ±1 °C for every 60 s was applied. Images of neat and IPC-CNC films were obtained by a high-resolution scanning electron microscope (Carl Zeiss Ultra Plus), with an acceleration voltage of 1 kV. The IPC-CNC films were carbon coated prior to observation. Circular dichroism (CD) spectra were measured by a spectrometer (Bio-Logic MOS 450) and the solid samples were fixed perpendicular to the beam. Linear polarized optical microscopy (POM) of IPC-CNC films and suspensions was carried out with an Olympus BX51 microscope in transmission mode. Circularly polarized light (reflection mode) was generated with a polarizer combined with a quarter waveplate (U-TP137, Olympus), which detected the reflection from a left-handed circularly polarized (LCP) and a right-handed circularly polarized (RCP) light. Optical density (OD) was calculated by ImageJ. Polarization rotatory: the polarization rotation was measured by using linearly polarized laser sources with wavelengths in the range 400–800 nm. The polarization direction of the incident and transmitted light was determined by utilizing a broadband polarization analyzer mounted on a rotation stage (Thorlabs). The intensity of the transmitted light was measured by a power meter for UV, visible, and near-infrared light (Spectra-Physics). For spectral analysis, a broadband light source was used, which was first linearly polarized by a polarization filter. The incident and transmitted intensity at the various wavelengths were measured by utilizing a fiber-coupled spectrometer (Flame-S-XR1-ES, Ocean Optics).

Results and Discussion

IPC-CNC films were prepared by casting suspensions of PEO–PAA with 8% w/w CNC (Figure , Table S1). As no depletion flocculation was observed (Figure a),[28] it was assumed that polymer chains were adsorbed onto the rodlike CNCs. An indication of such interactions was obtained from the rheological measurements of the IPC-CNC suspensions (Figures b and S4), more specifically both the storage modulus (G′) and the loss modulus (G″) gradually increased upon CNC incorporation. With the introduction of 1.4% CNC, the rheological behavior of IPC-CNC was still liquid-like (G″ > G′)[29] and then became solid-like with increasing CNC content, which indicated densening and strengthening of the network formed in the system. The IPC-CNC suspensions with high concentration of CNCs exhibited an anisotropic texture (Figure S5). Particularly, a distinct fingerprint pattern which corresponds to chiral nematic ordering with a helical pitch of 2 μm was observed for IPC suspension with 8 wt % of CNCs (Figure c). Such observations indicated that the CNC self-assembly was not disrupted by interactions with the IPC chains. After solvent evaporation, a freestanding film was formed without fingerprint textures, which is due to the shrinking of the helical pitch (Figure d). Below the film-free surface where the helix has developed, the CNC rod orientation is generally parallel to the surface interface to minimize the free energy dominated by the surface energy.[30,31] Note that the colors in the film were derived from the birefringence color which resulted from the anisotropic state of the CNC orientation.[32]

Figure 1

(a) Images of homogeneous IPC solution and an IPC-CNC suspension with 8 wt % CNC. (b) Storage (filled symbols) and loss moduli (open symbols) of IPC-CNC suspensions with different CNC concentrations. (c) POM image of IPC-CNC suspension containing 8 wt % of CNCs. The inset is a schematic presentation of a possible organization of CNCs and IPC. (d) POM image of freestanding film prepared from the suspension shown on (c).

The cast IPC-CNC films were transparent, and their appearance varied from colorless to iridescent when increasing the CNC concentration (Figure a). Scanning electron microscopy (SEM) micrographs of the cross section of Film-65 (65 wt % CNC) displayed distinct periodic layered structures corresponding to a chiral nematic texture (Figure b,c). The CNCs were organized in mesoscopic layers, where the distance between adjacent layers corresponded to a half a pitch of the chiral nematic helix (Figure e).

Figure 2

(a) Images of IPC-CNC films with increasing CNC concentrations viewed on white and black backgrounds. (b,c) SEM images of a cross section of film-65 with a well-defined chiral nematic structure. (d) Periodic pitch length for Film-CNC and film-65. (e) Schematic picture of the chiral nematic ordering in an IPC-CNC film. (f) SEM image of a cross section in film-65 with nematic ordering after addition of NaCl. (g,h) POM images of the nematic-ordered film rotated at 0° and 45°, respectively. (i) Schematic picture of the nematic ordering in an IPC-CNC film.

SEM images of the well-defined chiral nematic structure confirmed the preservation of the IPC-CNC assembly after solvent evaporation. At the same time, the successful intercalation of IPC chains between CNCs was quantitatively measurable by the increase of helical pitch length. The pitch length varied from 370 nm for the neat CNCs, in Film-CNC, to 460 nm in the IPC-CNC Film-65 (Figures d and S6), a shift which is ascribed to the adsorption of IPC chains onto CNCs.[28] When the IPC concentration exceeded 55 wt %, the periodic layered structures became perturbed by the intercalation of IPC macromolecules into CNC particles (Figure S7). Similar findings were obtained upon the assembly of CNCs with poly(vinyl alcohol) (60/40 w/w), confirming the homogeneous integration of polymer chains into the helical structure and absence of macroscopic phase segregation.[33]

The following characterizations are focused on the film-65 because of the fascinating CNC chiral nematic texture. Images of Film-65 taken with an LCP light were brighter and more colorful than those collected by an RCP light, which is ascribed to the left-handed chiral nematic nature of CNC (Figure S8). In order to generate an IPC-CNC film with nematic ordering, NaCl (17 mM) was added to the neat IPC-CNC suspension to screen the electrostatic repulsion between CNCs, resulting in an ordering transition from chiral nematic to nematic phase in the final composite.[30,34] The typical nematic feature of highly aligned CNCs (Figure i) was confirmed by SEM (Figure f). Furthermore, when the sample was rotated by 45°, the corresponding POM images demonstrated the anisotropic nature of the film with alternating variation of bright (Figure g) and dark (Figure h).

The IPC-CNC films demonstrated an increase of mechanical properties in comparison with neat IPC film, Film-0 (Figure a). The elastic modulus, E, increased by 3 orders of magnitude from 0.0038 GPa of Film-0 to 8.8 GPa for Film-65, where the elongation at break decreased upon increasing the CNC concentration. Moreover, the disappearance of plastic deformation (see fracture surface, Figure b) coincides with the structural transition from disordered to chiral nematic structure. This may affect the stability limit of the chiral nematic order that remains until prolonged stretching eventually transforms the structure into a fully unidirectional nematic orientation.[33] In addition, incorporation of CNCs and formation of ordered structure hinders the mobility of IPC chains. As a result, glass-transition temperature significantly increased from 2.8 °C for Film-0 up to 19.4 °C for Film-65 (Figure S9).

Figure 3

Mechanical properties of IPC-CNC films. (a) Stress–strain curves of IPC-CNC films with increasing concentrations of CNCs. (b) SEM image of cross section after fracturing of film-65. (c) Adhesion stress vs strain of two bonded films (film-65), one with nematic ordering and the other one with chiral nematic ordering. (d) Storage modulus (E′) of film-65 under wet–dry cyclic conditions.

Note that the adhesive strength between the IPC-CNC films reflects the assemble ability for 3D optics, which can be determined quantitatively by lap shear tests (Figures S10 and S11). A typical shear flow curve of two bonded films, one with nematic ordering and the other with chiral nematic ordering, revealed a maximal stress of 1.35 MPa and an elongation at break of 11.5% (Figure c). Although difficult to directly compare these films with other adhesives owing to the many variables involved in each adhesive system, the adhesion strength of IPC-CNC films was on par with other biomimetic adhesives and supramolecular polymers. For example, mussel-mimetic adhesives such as water-soluble copolypeptides[35] showed an adhesion strength of 1.5 MPa, while the supramolecular velcro-like[36] withstood a maximal adhesion stress of 1.2 MPa, both comparable to the strength of the present adhesive pair.

To understand the adhesive mechanism of IPC-CNC film, we measured the mechanical behavior in alternate dry and wet conditions (Figure d). Film-65 exhibited dynamic mechanical behavior with high reversibility. In dry conditions, the storage modulus was 7000 MPa. When exposed to acidic vapor (hydrochloric acid, pH 2.5 for 30 min), the modulus decreased by 2 orders of magnitude to 10 MPa. Upon exposure to acidic vapor, the H-bonds between PEO and PAA decreased comparatively because of the newly formed H-bonding between polymer and water.[37] Moreover, the strong H-bond interactions between CNCs (as well as CNC-IPC) could be “switched off” because of competitive H-bonding with water.[38−40] Without the topological constraints imposed by the H-bonding networks, the plasticizing effect of water additionally enhanced the mobility of macromolecular chains, giving rise to the soft nature of the IPC-CNC film. This bonding property provides IPC-CNC films with adhesive behavior to assemble into a functional film with a multilayered structure. It may serve as a facile material for configuration of photonic devices with various layers, for example, double layers with Janus structure, or a stacked design (Figure a) consisting of three layers: a nematic structure incorporated between two layers with chiral nematic phase.[41,42] An example for three-layer structure is presented here (∼330 μm in thickness), displaying iridescent color when viewed under reflection (Figure b,c). Note that there was no difference in the thickness of each layer after multilayered film assembly (Figure S12), namely, the interface between the different building blocks was stable and the layers did not merge.

Figure 4

(a) Schematics of the assembly consisting of a nematic layer between two chiral nematic layers. (b) SEM image of the film cross section with the three layers. (c) Optical image of the three-layer assembly showing distinct structural color. POM images of a Film-CNC obtained by (d) LCP and (e) RCP, respectively. POM images of the three-layer assembly obtained by (f) LCP and (g) RCP, showing colorful reflection. (h) OD of the above images. (i) CD spectra for films with chiral nematic or nematic phase and for the three-layer assembly.

To study the optical features of the three-layer assembly, reflected light patterns of RCP and LCP were studied (Figure d–g). In addition, the CD spectra of a single Film-CNC with a chiral nematic texture and of the three-layer assembly were measured (Figure i). The LCP of a chiral nematic film displayed a colorful image with an OD of 0.22, while RCP exhibited a colorless image with OD of only 0.08, attributed to the left-handed nature of CNC chiral nematic structure. Note that colorful images (OD > 0.15) from both LCP and RCP were clearly detected for the three-layer assembly (Figure f,g), as was previously observed for the exocuticles of Plusiotis resplendens,[43] which can be attributed to the nematic structure acting as a half-wave retardation plate to manipulate polarized light.[41,42] The CD peak showed a red shift from 550 nm for Film-CNC to 700 nm for the three-layer assembly because of the higher pitch, induced by IPC intercalation. This shift could be predicted by Bragg’s law λ = nP sin(θ), where the reflected wavenumber λ depends on the pitch P, average refraction index (n = 1.54),[44] and incident light angle θ.[45] The small peak at 240 nm in the nematic-ordered film corresponding to d-glucose molecular chirality was observed[46,47] and inhibited by the stacked design because of the blockage by the layer with chiral nematic ordering (Figures i and S13).

Finally, the functionality of the developed IPC-CNC films as an optical component for manipulation of light polarization was assessed. LCP and RCP light beams passed through a medium with optical activity, experiencing different phase shifts, and the process is also known as circular birefringence.[48] A linearly polarized light beam with wavelength λ0 can be decomposed in LCP and RCP components; upon passing through a medium with optical activity with thickness d, the polarization direction will rotate by an angle ϑ, as a consequence of the different phase shifts experienced by the LCP and RCP components:[48] ϑ = (nR – nL)πd/λ0. This effect is summarized in Figure , where the polarization state of linearly polarized laser beams with various wavelengths is analyzed after passing through chiral nematic Film-65 (Figure a–e) and a neat IPC film, Film-0 (Figure f). Although for film-0 no difference was measured compared to the incident laser beam, an optical rotation was clearly observed for all the investigated wavelengths for the sample embedding CNCs. This was also valid for polarized broadband light as evidenced in the maps shown in Figure g,h. A maximum polarization rotation of 65° was observed at 500 nm wavelength (Figure S14). This measurement allows the wavelength dispersion of the polarization rotation to be quantified, resulting in variation of the rotary polarization by 15° in a spectral interval of 400 nm and demonstrating the possibility of utilizing the developed IPC-CNC films as optical building blocks for broadband manipulation of the polarized light.

Figure 5

Rotation of linearly polarized light. Plot of the intensity of the light transmitted by an IPC-CNC film (film-65) as a function of the angle, ϑ, formed by the direction of the linear polarization of the incident laser beam and the axis of the polarization analyzer and measured at various wavelengths, λin, of the incident laser beam: (a) λin = 405, (b) 445, (c) 561, (d) 638, and (e) 785 nm. Black filled squares show data measured for an IPC-CNC film, whereas open circles refer to the intensity of the incident laser. (f) Light intensity transmitted by a neat IPC film (film-0) vs ϑ at λin = 785 nm (filled triangles). Continuous and dashed lines in (a–f) are fit to the data by a cos2 law. Maps of the dependence of the intensity transmitted by (g) film-65 and (h) film-0 as a function of ϑ and wavelength. Intensities are normalized to the values measured at ϑ = 0°.

Conclusions

In conclusion, we successfully prepared nanocomposites and optical building blocks from assembled H-bonded IPCs with active CNC orderings, which were stacked into a three-layer assembly with a controllable interface. The ordering of the CNCs was manipulated and tuned from chiral nematic to nematic phase. In principle, 3D integrated optics could be realized by stacking these assemblable IPC-CNC films without undesired interplay with charge distribution in CNCs. Introduction of CNCs into the IPC matrix led to a remarkable reinforcement of the matrix by 3 orders of magnitude; however, when exposed to acidic vapor, the nanocomposite demonstrated a soft nature (10 MPa) enabling efficient and stiff bonding at the interface. Utility of the developed stacked optical blocks was demonstrated for broadband manipulation of polarized light.