Hybrid Nanocomposites for 3D Optics: Using Interpolymer Complexes

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Functional Nanostructured Materials (including low-D carbon)

Hybrid Nanocomposites for 3D Optics: Using Interpolymer Complexes with Cellulose Nanocrystals Rui-Yan Zhang, Guang Chu, Gleb Vasilyev, Patrick Martin, Andrea Camposeo, Luana Persano, Dario Pisignano, and Eyal Zussman ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Applied Materials & Interfaces

Hybrid Nanocomposites for 3D Optics: Using Interpolymer Complexes with Cellulose Nanocrystals Ruiyan Zhang,† Guang Chu,† Gleb Vasilyev,† Patrick Martin,† Andrea Camposeo,‡ Luana Persano,‡ Dario Pisignano*,§ and Eyal Zussman*,†

†NanoEngineering

Group, Faculty of Mechanical Engineering, Technion-Israel Institute

of Technology, Haifa 32000, Israel ‡NEST,

Instituto Nanoscienze-CNR and Scuola Normale Superiore, Piazza S. Silvestro

12, I-56127 Pisa, Italy §Dipartimento

di Fisica “Enrico Fermi”, Università di Pisa, Largo Bruno Pontecorvo 3, I-

56127 Pisa, Italy and NEST, Istituto Nanoscience- CNR, Piazza S. Silvestro 12, I-56127 Pisa, Italy

ACS Paragon Plus Environment

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KEYWORDS Interpolymer Complex, Cellulose Nanocrystals, Nanocomposite, Adhesion, Liquid Crystals

ABSTRACT

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) (PEO) and neutralized poly(acrylic acid) (PAA) to render films assemblability 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 high elastic modulus of 8.8 GPa and 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,


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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. While 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 2D 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 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


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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 precise angle-dependent structural color by the assembly of cellulose nanocrystals (CNCs) and hydrogen-bonded interpolymer complexes (IPCs), which can be exploited as building blocks for 3D customized nanocomposites and integrated optics. Unlike nano-structured optical films reported recently,9-11 the proposed 3D optics has an integrated structure instead of simply stacked sandwiched structure assembled with various ordering of nematic and chiral nematic, exhibiting distinctive optical feature.

Hydrogen-bonded IPC is comprised 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


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IPC pair is poly(acrylic acid) (PAA), a weak polyelectrolyte in an aqueous solution, and a non-ionic polymer such as poly(ethylene oxide) (PEO).14-16

CNCs are rod-like particles with typical aspect ratio of 1:20 are 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 a hydrolysing 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 CNCs aqueous suspensions, phase transition from isotropic to anisotropic occurs when above a critical CNCs concentration.20 Furthermore, chiral nematic ordering can be preserved during solvent evaporation, leading to iridescent colors viewed from different angles.21-22 Unlike previous work 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 IPC-CNC film with distinct adhesive behavior.


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The breadth of applications of hydrogen-bonded IPCs was expanded here by the development of a CNC-based IPC (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 resulted 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 rod-like CNCs prepared by sulphuric 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


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infrared (FTIR) spectroscopy (Figure S3).26-27 Poly(acrylic acid), with a molecular weight of 450 kDa and Poly(ethylene oxide), 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: 160W) before usage. 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 CNCs film, labelled 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: 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


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saturated water/THF atmosphere for 7 days. Film thicknesses varied from 100 to 140 μm. 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 oC). Discovery DSC-2500 calorimeter (TA Instruments, USA) was used to investigate thermal properties of the films. Heating under modulated mode from -60 °C up to 160°C with the heating rate of 5 °C∙min1

and modulation at ± 1°C for every 60 seconds 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. CD spectra were measured by a spectrometer (BioLogic 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


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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 left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light. Optical density 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


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IPC-CNC films were prepared by casting suspensions of PEO-PAA with 8% w/w CNC (Figure 1, Table S1). Since no depletion flocculation was observed (Figure 1a),28 it was assumed that polymer chains were adsorbed onto the rod-like CNCs. An indication of such interactions was obtained from the rheological measurements of the IPC-CNC suspensions (Figures 1b, S4), more specifically both the storage modulus (G’) and loss modulus (G’’) gradually increased upon CNCs incorporation. With the introduction of 1.4% CNC, the rheological behavior of IPC-CNC was still liquid-like (G’’>G’),29 then became solid-like with increasing CNC content, which indicated densening and strengthening of the network formed in the system. The IPC-CNCs suspensions with high concentration of CNCs exhibited 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 1c). Such observations indicated that the CNCs self-assembly was not disrupted by interactions with the IPC chains. After solvent evaporation, a free-standing film was formed without fingerprint textures, which is due to the shrinking of the helical pitch (Figure 1d). Below the film free-surface where the helix has developed, the CNC rod orientation is generally parallel to the surface interface, to


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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).


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The cast IPC-CNC films were transparent, and their appearance varied from colorless to iridescent when increasing the CNC concentration (Figure 2a). SEM micrographs of the cross-section of Film-65 (65 wt % CNC) displayed distinct periodic layered structures corresponding to chiral nematic texture (Figure 2b, 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 2e).


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Figure 2. (a) Images of IPC-CNC films with increasing CNC concentrations viewed on white and black backgrounds respectively. (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 FilmCNC, to 460 nm in the IPC-CNC Film-65 (Figure 2d and Figure 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 wt/wt), confirming the


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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 due to the fascinating CNC chiral nematic texture. Images of Film-65 taken with left circularly polarized (LCP) light were brighter and more colorful than those collected by a right-handed circularly polarized (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 in the final composite.30,

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The typical nematic feature of highly aligned CNCs (Figure 2i) was

confirmed by SEM (Figure 2f). Furthermore, when the sample was rotated by 45o, the corresponding POM images demonstrated the anisotropic nature of the film with alternating variation of bright (Figure 2g) and dark optical density (Figure 2h). The IPC-CNC films demonstrated increase of mechanical properties in comparison with neat IPC film, Film-0 (Figure 3a). The elastic modulus, E, increased by three orders of magnitude from 0.0038 GPa of Film-0 to 8.8 GPa for Film-65, where the elongation at


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break decreased upon increasing the CNC concentration. Moreover, the disappearance of plastic deformation (see fracture surface, Figure 3b) 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 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).


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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 (Figure S10, S11). A typical shear flow curve of two bonded films, one with nematic ordering and


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the other one with chiral nematic ordering, revealed a maximal stress of 1.35 MPa and an elongation at break of 11.5 % (Figure 3c). 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 co-polypeptides,35 showed adhesion strength of 1.5 MPa, while the supramolecular velcro-like36 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 3d). 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 two 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


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interactions between CNCs (as well as CNC-IPC) could be “switched off” due to 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 IPC-CNC film. This bonding property provides IPC-CNC films with adhesive behavior to assemble into functional film with a multi-layered structure. It may serve as a facile material for configuration of photonic devices with various layers, e.g., double layers with Janus structure, or a stacked design (Figure 4a) 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 4b, c). Note that there was no difference in the thickness of each layer after multi-layered film assembly (Figure S12), namely, the interface between the different building blocks was stable and the layers did not merge.


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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 threelayer 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) Optical density (OD) of the above images. (i) CD spectra for films with chiral nematic or nematic phase, and for the three-layer assembly.


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To study the optical features of the three-layer assembly, reflected light patterns of RCP and LCP were studied (Figure 4d-g). In addition, the circular dichroism (CD) spectra of a single Film-CNC with chiral nematic texture and of the three-layer assembly were measured (Figure 4i). LCP of a chiral nematic film displayed a colorful image with optical density (OD) of 0.22, while RCP exhibited a colorless image with OD of only 0.08, attributed to the left-hand nature of CNC chiral nematic structure. Note that colorful images (OD > 0.15) from both LCP and RCP were clearly detected for the three-layers assembly (Figure 4f, g), as was previously observed for the exocuticles of Plusiotis

Resplendens,43 which can be attributed to the nematic structure acting as 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-layers assembly due to the higher pitch, induced by IPC intercalation. This shift could be predicted by Bragg law λ=nPsin(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 220 nm in the nematic-ordered film corresponding to D-glucose molecular chirality,46-47 and was inhibited by the stacked


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design due to blockage by the layer with chiral nematic ordering (Figures 4i and Figure 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 , as a consequence of the different phase shifts experienced by the LCP and RCP components:48 =(nR−nL)d/λ0. This effect was summarized in Figure 5, where the polarization state of linearly polarized laser beams with various wavelengths is analyzed after passing through chiral nematic Film-65 (Figure 5a-e) and a neat IPC film, Film-0 (Figure 5f). While 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


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in the maps shown in Figure 5g, 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.


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Figure 5. Rotation of linearly polarized light. Plot of the intensity of the light transmitted by a IPC-CNC film (Film-65) as a function of the angle, , formed by the direction of the linear polarization of the incident laser beam and the axis of the polarization analyzer, and measured at various wavelength, λin, of the incident laser beam: (a) λin=405 nm, (b) 445 nm, (c) 561 nm, (d) 638 nm, 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.  at λin=785


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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  and wavelength. Intensities are normalized to the values measured at =0°.

CONCLUSION

In conclusion, we successfully prepared nanocomposite and optical building blocks from assembled H-bonded IPCs with active CNC orderings, which were stacked into threelayers 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 remarkable reinforcement of the matrix by three 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.

ASSOCIATED CONTENT


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Supporting information The supporting material is available on website. The details of TEM of CNC nanoparticles, WAXD, FTIR of CNC film, rheological properties, POM images of suspension, SEM images for Film-CNC, Film-0, Film-45 and Film-55, optical images for Film-65, table for IPC-CNC films with detailed preparation and mechanical properties, DSC curves, adhesion test experiment, polarization rotation angle, and CD spectra.

AUTHOR INFORMATION Corresponding Authors Prof. Eyal Zussman, Email: [email protected] Prof. Dario Pisignano, Email: [email protected]

ORCID Eyal Zussman: 0000-0002-4310-6548 Dario Pisignano: 0000-0003-3758-5199 Andrea Camposeo: 0000-0002-3533-7389


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Luana Persano: 0000-0002-3533-7389 Ruiyan Zhang: 0000-0002-6739-2075 Guang Chu: 0000-0003-1538-5276

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources The research leading to these results has also received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 682157, “xPRINT”).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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The authors thank Dr. Wang Bo for his help with the POM analysis and platform setup. The support of the Russell Berrie Nanotechnology Institute (RBNI) at the Technion is gratefully acknowledged. E.Z. acknowledges the support of the Winograd Chair of Fluid Mechanics and Heat Transfer at Technion. The research leading to these results has also received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 682157, “xPRINT”).

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