Composite Cholesteric Nanocellulose Films with Enhanced

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Composite Cholesteric Nanocellulose Films with Enhanced Mechanical Properties Brandon Vollick, Pei-Yu Kuo, Héloïse Thérien-Aubin, Ning Yan, and Eugenia Kumacheva Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04780 • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on December 28, 2016

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Chemistry of Materials

Composite Cholesteric Nanocellulose Films with Enhanced Mechanical Properties Brandon Vollicka, Pei-Yu Kuob, Héloïse Thérien-Aubina, Ning Yanb,d, Eugenia Kumachevaa,c,d a

Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada

b

Faculty of Forestry, University of Toronto, Toronto, Ontario M5S 3B3, Canada

c

Institute of Biomaterials and Biomedical Engineering, University of Toronto, 4 Taddle Creek Road, Toronto, Ontario

M5S 3G9, Canada; d

Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada.

ABSTRACT: Cellulose nanocrystals (CNCs) form cholesteric films that exhibit birefringence, iridescence, and circular dichroism, however these films are brittle and prone to cracking. Here we report composite cholesteric films formed from CNCs and soft reactive latex nanoparticles (NPs) (the latter underwent in-situ crosslinking during film formation). Composite films exhibited a selfstratified morphology, with lateral cholesteric CNC-rich layers and isotropic latex NP-rich layers. The films retained their photonic properties and exhibited significantly enhanced mechanical properties. In comparison with latex-free CNC films, composite films had ∼60 % higher toughness but did not compromise their tensile strength. The combination of photonic performance and improved mechanical properties of the composite nanocellulose films expands the range of applications of these materials for the fabrication of optical devices.

1. INTRODUCTION Cellulose nanocrystals (CNCs) have drawn significant attention in recent years, due to their natural abundance, relatively low cost, high mechanical strength and ease of surface functionalization.1–3 These properties lend themselves to applications in drug delivery,4 membranes,5 recyclable substrates,6 and the reinforcement of thermoplastics.7,8 One of the truly unique features of CNCs is their ability to form cholesteric (Ch) liquid crystalline phases in aqueous suspensions, when the CNC volume fraction reaches a threshold value.9 In the Ch phase, the CNCs exhibit long-range orientational order combined with a left-handed helical alignment.10 Importantly, upon complete water evaporation, the Ch arrangement of CNCs is, to a large degree, preserved in solid films.2 As a result, CNC films selectively interact with circularly polarized light, and are birefringent and iridescent, that is, exhibit a photonic band gap. Owing to these properties, Ch-CNC films show promising applications as humidity sensors,11,12 optical encryptors,13,14 structural pigments,14 light shutters15 and templates for the synthesis of Ch inorganic materials such as mesoporous silica,16 carbon17 and titanium dioxide.18 The limitation of Ch-CNC films is their poor mechanical properties: the films are brittle and prone to cracking, which limits their applications as advanced optical materials.

To overcome this drawback, water-soluble polymers such as polyethylene glycol19 and polyvinyl alcohol20 have been added to aqueous CNC suspensions to form composite CNC-polymer films with enhanced flexibility and preserved Ch structure, however the use of hydrophilic polymers narrowed the range of applications of the composite films. Functionalization of CNCs with polystyrene or the use of surfactants enabled the formation of Ch-CNC phases in non-polar organic solvents,21,22 thus making possible the addition of hydrophobic polymers; yet, the formation of Ch films from such suspensions has not been reported. In another approach, Ch films were formed CNCs carrying neutralized acidic groups formed Ch films from a suspension in dimethylformamide, a polar organic solvent.23 Subsequent introduction of polystyrene into the CNC suspension yielded composite Ch-CNC films, however their mechanical properties have not been reported. An alternative approach to composite CNC-polymer materials relies on the use of CNCs and latex nanoparticles (NPs). Latex NPs can be synthesized with a broad range of compositions, dimensions, morphologies, and surface chemistries,24-26 which would broaden the range of applications of CNC-polymer composite materials. Importantly, both hydrophilic and hydrophobic latex NPs can be introduced in aqueous CNC suspensions. Composite materials based on CNCs and styrene-butadiene rubber latex,27 polyvinyl acetate latex,28 and isoprene rubber29 have been reported. In these materials, latex films were

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reinforced with CNCs, which implied that a relatively low fraction of CNCs (not exceeding 20 wt%) in the films. As a result, these composite materials did not exhibit a Ch structure. Recently, our group has developed composite CNClatex films at CNC volume fraction up to 60 vol%.30 These films had a Ch structure and exhibited photonic crystal properties and circular dichroism. The formation of Ch films was favored when latex NPs had a small size, negative charge and a low glass transition temperature, Tg. Importantly, at the temperature of film formation the NPs deformed and spread throughout the Ch phase. This promising approach to composite CNC-latex films was however limited by the low Tg of latex NPs, which resulted in film tackiness (or stickiness), due to a strong viscoelastic energy dissipation, and low structural integrity.30 The mechanical properties of the films have not been explored. In the present work, we report a comprehensive study of the optical, structural and mechanical properties of composite films formed from CNCs and reactive low-Tg latex NPs. Hexanediamine was used to crosslink poly(butylacrylate-co-2-(methacryloyloxy)ethyl acetoacetate)) NPs. The copolymer was covalently labeled with the fluorescent marker 9-vinylanthracene, thus enabling the characterization of the NP distribution in the films using fluorescence microscopy, along with independent characterization of the Ch phase by polarized optical microscopy. Notably, in previous works for the preparation of composite NP-CNC films, the NPs were mixed with the CNC suspension prior to its phase separation into an isotropic and Ch phase and the mixed suspension was immediately cast to form a film.30–32 In the suspension, a large fraction of latex NPs partitioned in the isotropic phase. In the present work, a different approach to film preparation was undertaken. An isotropic mixed suspension of CNC, latex NPs and a crosslinker was allowed to equilibrate for 1 week, thus enabling NP and crosslinker partition in the Ch phase under close-to-equilibrium conditions. Subsequently, composite films were prepared from the Ch-CNC phase containing latex NPs and the crosslinking agent. The resultant films self-stratified into layers of Ch CNCrich phase and isotropic latex NP rich phases and were iridescent and birefringent. These films exhibited a marked increase in toughness without a significant decrease in tensile strength, in comparison with latex-free CNC films. 2. EXPERIMENTAL SECTION 2.1. Materials. Butyl acrylate (BuA, ≥99%), 9-vinyl anthracene (VA, 97%), 2-(methacryloyloxy)ethyl acetoacetate (MAEA, 95%), potassium persulphate (KPS, ≥99%), hexamethlene diamine (HDA, 98%), sodium dodecyl sulfate (SDS, 92.5-100.5%), dialysis tubing (cellulose, 12-14 kD) and activated aluminum oxide were purchased from Sigma-Aldrich Canada. BuA was distilled before use and MAEA was filtered using an aluminum oxide column. An aqueous 12.2 wt% suspension of CNCs was purchased from the University of Maine Process Development Centre. The CNC dimensions were determined by image analysis of their transmission electron microsco-

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py images using ImageJ software (Figure S1, Supporting Information). 2.2. Latex Synthesis. Latex NPs were synthesized by emulsion polymerization by copolymerizing BuA (12.83 mL, 89.5 mol %), MAEA (1.91 mL, 10 mol %) and VA (0.10 g, 0.05 mol %) under nitrogen atmosphere in 55 mL of water at 80 oC using SDS (0.11g) as a surfactant and KPS (0.14g) as the initiator (Figure S2, Supporting Information)).31 The relationship between the total molar ratio of monomer -to-surfactant and the average diameter of latex NPs synthesized in emulsion polymerization is shown in Figure S4, Supporting Information). Following synthesis, the latex dispersion was purified by dialysis against deionized water, with water change daily for 7 days. 2.3. Latex Characterization. The hydrodynamic radius and the electrophoretic mobility of latex NPs were characterized by dynamic light scattering using Malvern Zetasizer Nano-ZS (Figure S3, Supporting Information). Aggregation of latex NPs in the presence of 0.9 wt% HDA was examined by measuring the hydrodynamic radius of the NPs in their 4.2 wt% suspension. The glass transition temperature, Tg, of latex NPs in non-crosslinked and crosslinked films was determined under a nitrogen atmosphere using a differential scanning calorimeter (DSC) model Q 100 from TA Instruments. Dynamic DSC measurements were carried out at a ramp rate of 10 oC/min−1 in the temperature range of -75 to 100 °C to obtain heat-flow curves of the samples. 2.4. Film Preparation. A thin layer of Sylgard 184 Silicone Elastomer mixed with a curing agent was cast on the bottom of a 60×15 mm polystyrene Petri dish and cured at 70 oC for 4 h. The resultant elastomeric film was exposed to air plasma (500 mTorr, 45W, 5 min) in a Harrick Expanded Plasma Cleaner. Mixed latex-CNC suspensions were prepared at the CNC concentration of 5 wt% and latex NP concentrations, CNPaq, of 1.25, 1.67, 2.5 and 5 wt% (Table S1). A crosslinker, HDA, was added to the mixed suspension at a concentration, CHDAaq, varying from 0 to 7.6 mM. The mixed suspension with a total mass of 20 g was equilibrated for 7 days, which resulted in its phase separation into an isotropic (top) phase and an anisotropic (bottom) Ch-CNC phase. A portion (3.1 g) of the Ch-CNC phase was extracted with a micropipette and cast onto a plasma-treated elastomeric film in a Petri dish. Composite films formed at 28 oC and ∼90% relative humidity for 7 days were subsequently removed from the silicone substrate for further characterization. Crosslinked CNC-free latex films were prepared by solution-casting a 1.67 wt% latex dispersion mixed with 0.38, 0.76, 3.8, or 7.6 mM of HDA onto an elastomer substrate and drying it overnight under ambient conditions. 2.5. Fluorescence Spectroscopy. The concentration of latex NPs in the Ch phase was determined using fluorescence spectroscopy (Cary Eclipse Fluorescence Spectrophotometer, Varian) at the excitation wavelength of 360 nm and emission wavelength of 410 nm. Calibration standards were prepared by diluting 100-fold the mixed suspensions at CNC concentration of 5 wt% and latex NP concentration, CNPaq, of 0.2, 0.4, 1, 2, and 5 wt% (Figure S6, Supporting Information)


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2.6. Attenuated Total Reflectance Fourier-Transform Infrared Spectroscopy (ATR-FTIR). Characterization of the crosslinking reaction between the acetoacetate groups in the latex NPs and HDA was carried out for a crosslinked latex NP film and a crosslinked CNC-latex film using ATRFTIR experiments on a Bruker Vertex 70 spectrometer with a 1.85 mm diameter diamond crystal. A non-crosslinked latex NP film was used as a control system.

2.7. Characterization of extinction of the composite films. Extinction spectra of the composite films were acquired using Varian Cary 5000 UV-Vis-NIR Spectrophotometer. The films were placed orthogonally to the incident beam path with a transmission area of 2 cm2. The measurements were taken at five different locations within the central 1 cm-diameter region of the film at wavelengths in the spectral range from 500 to 900 nm. 2.8. Circular Dichroism Spectroscopy. A Jasco-810 Spectropolarimeter was used to characterize the circular dichroism (CD) of the composite films. A film area of 6 mm2 cut from the central region of the composite film was placed orthogonally to the beam path. 2.9 Confocal Fluorescence Microscopy. A Zeiss LSM 710 NLO Confocal Microscope was used to examine the distribution of fluorescent latex NPs in the composite films by placing the film perpendicular to the beam path and imaging it at a depth of ~30 µm in the central film region with a diameter of 1 cm at excitation wavelength of 420 nm. Image analysis was conducted using ImageJ software. 2.10. Electron Microscopy Characterization of film structure. A scanning electron microscope equipped with a QUANTA field emission gun 250 and a bright field/dark field scanning transmission electron microscope detector was used to image the cross-sectional area of the composite films. Prior to imaging, the films were freezefractured under liquid nitrogen and subsequently, coated with 10 nm-thick gold film at a sample temperature of 22oC. 2.11. Mechanical Analysis. Static-force tensile strength tests were conducted in using a TA Q800 Dynamic Mechanical Analysis instrument (TA Instruments). Composite films were prepared with the concentration of latex NPs in the films, CNPs, of 0, 8, 10, 15, or 24 wt% and the concentration of HDA in the films, CHDAs, of 0.7 wt%. Films were cut into 30 × 2 mm strips for analysis and clamped lengthwise for testing. Prior to the measurements, the films were maintained at a temperature of 20 °C and a relative humidity of 70% for 7 days. The films were tested at 25 °C with 3 N/min ramp. The tensile stress was calculated as force per unit area, where the dimensions of the film were measured with a caliper with a precision of ±0.02 mm. For each composition, at least, three films were tested. The tensile strength was determined as the ultimate strength of the film before failure and the modulus of toughness (the area under the corresponding stress/strain curve)33 was used to quantify the ability of the material to deform without fracture. 3. RESULTS 3.1 Latex nanoparticles. The latex NPs had the average hydrodynamic diameter and electrokinetic potential (ζpotential) of 47 ± 6 nm and -48 ± 8mV, respectively. The

effect of HDA addition on latex NP size and electrokinetic potential was examined in the latex dispersion at CNPaq = 2.5 wt% and CHDAaq = 3.8 mM. No change in NP size was observed after 3 days, however a 30 mV increase in electrokinetic potential was observed (that is, it became more positive). Despite this change, no latex NP aggregation was observed. The crosslinking of latex NPs with HDA could occur as a result of the reaction of acetoacetate groups within the NPs with HDA, to form an enamino-ester (Figure 1, top) and was characterized by ATR-FTIR spectroscopy by acquiring the spectra of three types of films, namely, films prepared from an HDA-free latex NP dispersion, from a mixture of HDA and latex NPs, and from a mixture of HDA, latex NPs and CNCs. In Figure 1b, new peaks emerged at 1605 and 1653 cm-1 in the spectrum of the film formed from latex NPs crosslinked with HDA (red spectrum), in comparison with the non-crosslinked latex film (blue spectrum). The peak at 1605 cm-1 was attributed to an N-H bend of HDA, and thus could not be used for the characterization of the crosslinking reaction, however the peak at 1653 cm-1 corresponded to the C=C stretch of the enamine, confirming that HDA has reacted with the acetoacetate comonomer of the NPs. Figure 1c shows a zoomed in fragment of the spectrum in the range of 1500–1800 cm-1, which was acquired for the CNC-latex film obtained in the presence of HDA. The appearance of the peak at 1653 cm-1 in the spectrum of this film (Figure 1c) suggested that the crosslinking reaction between HDA and the acetoacetate comonomer took place in the CNC-latex mixture. Note that a peak at 1640 cm-1 corresponded to the OH bend of water absorbed by CNCs.

Figure 1. Latex crosslinking. (a) A crosslinking reaction between the acetoacetate groups of the latex NPs and HDA. (b) ATR-FTIR absorbance spectra of films formed from HDA-free dispersion of latex NPs (blue), latex NPs crosslinked with HDA (red), and from the mixture of latex NPs, CNCs and HDA (green). (c) zoomed in segment of the spectrum of films formed from the mixture of latex NPs, CNCs and HDA. Since HDA can interact with both CNCs and latex NPs, we examined the change in the the ζ-potential of the CNCs in the presence of HDA in the latex-free suspension. The value of the ζ-potential of the CNCs decreased with increasing CHDAaq until the pH of the suspension surpassed the value of ~11, the pKa of HDA34 (Figure S5, Supporting Information). Thus neutralization of the negatively charged CNCs by protonated HDA occurred in the CNC suspension. The values of Tg for non-crosslinked and crosslinked latex NPs in the films were -35oC and -8oC, respectively



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(measured by DSC, Figure S7). Since in the presence of CNCs, the amount of HDA available for latex crosslinking reduced, for composite crosslinked latex-CNC films we expected the range of -35≤Tg≤-8oC , that is, below the temperature of film formation. 3.2. Suspensions of latex NPs and CNCs. The recipes for the formulations used for film preparation are summarized in Table S1 in Supporting Information Following 7 day equilibration, phase separation took place in the mixed suspension into an isotropic top phase and a Ch-CNC bottom phase (Figure 3a,c). Fluorescence of the Ch-CNC phase at λexc=365 nm, corresponding to anthracene-labeled NPs, confirmed the partition of the latex into the Ch phase. All suspensions had a pH ≈7. This bottom Ch-CNC phase was subsequently used for film preparation. With an increasing HDA concentration, CHDAaq, in the original suspension, the volume fraction, φCh, of the Ch phase reduced until complete disappearance (Figure 3b). This effect was attributed to the shielding of electrostatic interactions between CNCs (essential for the formation of the Ch phase1,2) by protonated HDA. With an increasing concentration of latex NPs, CNPaq, in the initial suspension prior to its phase separation from 0 to 2.5 wt% (corresponding to the NP concentration in the Ch phase from 0 to 1.1 wt %, respectively), an increase in φCh was observed (Figure 3c and d). This effect was ascribed to the incorporation of the negatively charged NPs within the Ch phase, thereby increasing the inter-CNC distance, in agreement with earlier report.30

Figure 2. Phase separation of latex-CNC NP suspensions. (a) Photographs of suspensions containing 5 wt% of CNCs and 1.67 wt% of latex NPs at 0.004≤CHDAaq≤0.9 wt% (increasing right-to-left), following 1 week equilibration. (b) Variation in the fraction of Ch phase, plotted as a function of CHDAaq for CNP aq of 1.25 and 1.67 wt%. (c) Photographs of mixed suspensions containing 5 wt% of CNCs, CHDAaq=0.04 wt% and 0≤CNPaq≤2.5 wt% (increasing from left-to-right), following their 1 week equilibration. (d) Variation in the fraction of Ch latex-CNC phase, plotted as a function of CNPaq. 3.3. Optical properties of composite CNC-latex films. Composite films prepared from the mixed suspension of CNCs, latex NPs and HDA were iridescent and did not exhibit the “tackiness” observed in previous work.30 The concentration of latex NPs and HDA in the composite latex-CNC films were determined using a calibration graph and elemental analysis, respectively (see Supporting Information). The concentrations of CNPaq of 1.25, 1.67, 2.5, and 5 wt% in the initial composite suspension corresponded to CNPs of 8, 10, 15, and 24 wt%, respectively, in the composite film, while

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CHDAaq of 0.38, 0.76, 3.8, and 7.6 mM corresponded to CHDAs of 0.07, 0.14, 0.7 and 1.4 wt%, respectively, in the film. Figure 3a shows photographs (left-to-right) of a pure CNC film, a latex-free CNC film containing HDA and a composite crosslinked CNC-latex film. With addition of HDA and crosslinked latex NPs to CNPs=24 wt%, film iridescence in the visible range decreased, which together with spectroscopic and structural characterization of the film (see below), reflected a disruption of the Ch structure. The optical properties of the composite films were characterized by UVvisible spectroscopy. Figure 3b and c shows that composite CNC-latex films exhibited an extinction peak in the spectral range from ~650-800 nm, corresponding to the stop band of the transmitted light. The wavelength of the maximum extinction, λmax, consistently decreased with increasing CHDAs, due to reduced inter-CNC repulsion during film formation, thereby leading to a smaller Ch pitch (Figure 3b). Conversely, λmax increased at a higher CNPs (Figure 3c), due to entrapping of the latex NPs between the CNCs in the Ch phase, thus increasing the pitch of the CNCs in the film.31 At the highest CNPs=24 wt%, the stop band was not observed. These trends remained consistent for the films prepared at varying CNPs and CHDAs (Figure 3d).

Figure 3. (a) Left: latex-free CNC film, middle: latex-free CNC film at CHDAs=0.7 wt%, right: crosslinked-latex CNC film at CNPs=24 wt% and CHDAs=0.7 wt% and (b) Extinction spectra of composite CNC at CNPs=10 wt% and varying CHDAs; (c) Extinction spectra of composite CNC films at CHDAs=0.7 wt% and varying CNPs; (d) Variation in λmax of the composite CNC films at varying concentrations of HDA and latex NPs. Composite films exhibited a positive CD peak in the range of ~650-850 nm, characteristic of the left-handed Ch structure. Figure 4a shows representative CD spectra of the films with CNPs=15 wt%, a weak red-shift in λmax was observed, however the CD intensity did not significantly change.At CNPs=24 wt%, however, a notable decrease in CD signal was observed, correlating with the disappearance of a stop band in the visible region in Figure 3c. Figure 4b shows that with increasing CHDAs, the CD peak consistently blue-shift and decreased in intensity. Figure 4c shows the general relationship for the composite films, with a weak variation in CD intensity with increasing CNPs (except for CNPs=24 wt%) and an overall decrease in CD with increasing CHDAs.


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Chemistry of Materials composite CNC-latex films at CHDAs=0.7 wt% and CNPs of 10 (a, c) and 24 wt% (b, d).

Figure 4. (a) CD CHDAs=0.7 wt% and posite CNC films (c) Variation in CD 24 wt%.

spectra of composite CNC films at varying CNPs; (b) CD spectrum of comat CNPs=10 wt% and varying CHDAs. intensity, plotted vs. CHDAs for 8≤CNPs≤

3.4. Microstructure of the composite films. Confocal fluorescence microscopy and polarized optical microscopy imaging were performed on the composite films with varying latex NP content (at CHDAs=0.7 wt%.). Dual-mode imaging enabled the characterization of the distribution of the Ch CNC-rich regions and highly fluorescent latex NP-rich regions. Figure 5a and b, shows representative confocal fluorescence microscopy images with an increasing average size of NP-rich domains at higher CNPs, which implied a stronger phase separation between the CNC-rich and a latex NP-rich phases. Polarized optical microscopy images showed bright birefringent CNC-rich regions at CNPs=10 wt%, which became longer and larger in films with CNPs=24 wt% (Figure 5c and d, respectively). Importantly, latex-rich regions in Figure 5c and d exhibited birefringence, while CNC-rich regions in Figure 5a and b exhibited fluorescence, indicating that phase separation between the CNCs and latex NPs was not complete and each phase contained the counterpart component. As we show below, this feature was beneficial for the mechanical properties of the composite films.

Scanning electron microscopy was used to image the crosssectional area of the composite films on a smaller length scale. At CNPs=10 wt%, a layered structure of the film crosssection, characteristic of the Ch order, was observed (Figure 6a). The Ch structure was interrupted with “islands” of NPrich domains. At CNPs=24%, the NP-rich domains became larger and more abundant, due to the higher content of latex NPs in the film and stronger phase separation between the CNCs and NPs. The average half-pitch for the films shown in Figure 6a and b was 239 ± 27 and 290 ± 15 nm, respectively. A larger half-pitch at higher CNP was expected due to a larger NP fraction partitioned in the Ch-CNC phase, thus leading to an increased inter-CNC distance. This effect was in agreement with red-shift of extinction and CD peaks in Figures 3 and 4. We note that the composite latex-CNC films had a stratified structure with planar Ch-CNC layers and close-packed latex NP layers, as opposed to a uniform distribution of hydrophilic polymers added as a solution to the precursor CNC suspension prior to film preparation.20

Figure 6. Scanning electron microscopy images of the crosssectional area of composite CNC films at CNPs of (a) 10 wt% and (b) 24 wt% and CHDAs = 0.7 wt%.

Figure 5. Confocal fluorescence microscopy images (top), and polarized optical microscopy images (bottom) of

3.5. Mechanical properties of the composite films. The effect of latex NPs on tensile strength and toughness of the composite latex-CNC films was investigated by comparing stress-strain behavior of films with varying CNPs (Figure 7). In this study, the toughness was characterized as the total area under the stress-strain curve in tensile tests. The addition of NPs to the CNC films at the concentration of up to CNPs= 15 wt% increased the modulus of toughness (MT) from ∼74 to ∼120 kJ·m-3, with a decrease in tensile strength (TS) from 33 to 29 MPa. At CNPs=24 wt%, the structural integrity of the composite film



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greatly diminished and the modulus of toughness, MT, was only ~58 kJ·m-3.

Figure 7. Representative tensile stress-strain curves for composite CNC/latex NP films with varying CNPs and CHDAs=0.7 wt%, with the corresponding tensile strength (TS) and the modulus of toughness (MT). The color of the labels corresponds to the color of the curves. Tukey's range test35 was used to determine whether the change in mechanical film properties was significant , that is, p≤0.05. The test confirmed that the change in tensile strength for the composite films at 0≤CNPs≤15 wt% was not significant (p>0.05), which implied that in this concentration range of CNPs, its tensile strength of the films was not compromised. More importantly, the addition of latex NPs to the CNC films resulted in a significant (p=0.02) increase in film modulus of toughness. Thus, the latex NPs acted as a toughening agent. Under stress, the stress was concentrated close to the soft latex inclusions, resulting in their deformation and expansion, and thus energy dissipation.36 At CNPs=24 wt%, the film became too soft and both the toughness and tensile strength decreased significantly. 4. DISCUSSION The preparation of the composite latex-CNC films was carried out from the separated Ch phase of the CNC suspension containing latex NPs and HDA. This mixture did not show evidence of phase separation after 7 day equilibration, however upon evaporation of water during film formation, the concentration of the latex NPs and CNCs increased and the mixture underwent further phase separation into an isotropic and Ch phases.30,32 As a result, the resulting film contained laterally aligned CNC-rich Ch regions and isotropic latex-rich regions, in agreement with our earlier work.30 The introduction of latex NPs in the films did not significantly disrupt the extent of order in the CNC-rich Ch regions (up to CNPs =15 wt%) and led to a red-shift in the stop-band and CD signal, in agreement with Bragg's law, due to partition of the NPs between the CNC layers. Addition of HDA to the latex-CNC mixture resulted in interparticle and intraparticle latex crosslinking, although the contribution of each effect could not be determined. As the result of crosslinking, non-tacky composite films amenable to mechanical characterization were obtained. Based on the results of SEM imaging, in the films, latex NPs were sufficiently soft to coalesce. With HDA addition the ability of CNCs to form Ch films reduced, most probably, due to the neutralization of the negatively charged CNCs by protonated HDA.34 Nevertheless, for 8≤CNPs≤15 wt% up to CHDAs=0.7 wt%, the composite films retained their iridiscence, birefringence, and CD properties.

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The addition of crosslinked low-Tg latex NPs resulted in a significant - by ∼60% - increase in toughness of the composite films, in comparison with latex-free CNC films. At the same time the tensile strength of the composite films was not compromised. Such a combination of tensile strength and toughness is a very favorable feature, as generally, improving one property comes at the expense of another. The contribution of the layered structure37 of the latex-CNC films, as opposed to the uniform distribution of water-soluble polymers in composite CNC-based films12,20 is yet to explored. Finally, the loss of the mechanical integrity of the composite films at CNPs=24 wt% coincided with a diminution of its optical properties. Thus at the optimized 8≤CNPs≤15 wt% the mechanical properties of the composite films were significantly improved without compromising in their optical properties. 5. CONCLUSIONS We report the preparation of crosslinked composite latexCNC films that possess birefringence, iridiscence, and CD properties and enhanced mechanical properties. Addition of latex NPs up to CNPs=15 wt% resulted in a red-shift in extinction and CD spectra of the films and did not significantly affect the structure of the Ch phase of the composite films. In contrast, the amount of crosslinker introduced in the films (up to 1.4 wt%) was limited by the disruption of the Ch structure of the films. Film toughness was increased by ∼60 % due to the addition of latex NPs, without compromising film tensile strength, in comparison with latex-free CNC films. The combination of photonic performance and improved mechanical properties of the composite latex-CNC films expands the range of applications of these materials, in particular, for the fabrication of optical devices.

ASSOCIATED CONTENT Supporting Information. Determination of CNC size, NP diameter and electrokinetic potential, synthetic formulations for fine-tuning latex NP diameter, effect of HDA on CNC surface charge, compositions of latex-CNC suspensions, determination of latex and HDA concentration in the Ch-CNC phase. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: Eugenia Kumacheva at [email protected]

Present Addresses Héloïse Thérien-Aubin: Max Plank Institute for Polymer Research, Mainz, 55128, Germany

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

ACKNOWLEDGMENT The authors are grateful for financial support of this work by NSERC Canada (Strategic and Discovery grants). EK thanks Canada Research Chair program. BV thanks Ilya Gourevich for support in electron microscopy imaging.


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