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Structure and Properties of Composite Films Formed by Cellulose Nanocrystals and Charged Latex Nanoparticles Héloïse Thérien-Aubina, Ariella Lukach a, Natalie Pitch a, Eugenia Kumacheva a,b,c*
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x
We report the structural and optical properties of composite film s formed from mixed suspensions of cellulose nanocrystals (CNCs) and fluorescent latex nanoparticles (NPs). We explored the effect of the
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variation in NP concentration, size, surface charge, and glass transition temperature on film structure and properties. The chiral nematic order, typical of CNC films, was preserved in films with up to 50 wt.% of negatively-charged latex NPs. Composite films were characterized by macroscopically close-to-uniform fluorescence, birefringence, and circular dichroism properties. In contrast, addition of positively charged latex NPs led to gelation of CNC-latex suspensions and disruption of the chiral nematic order in the composite films. Large latex NPs disrupted the chiral nematic order to a larger extend than small NPs. Furthermore, the glass transition of latex NPs had a dramatic effect on the structure of CNC-latex films. Latex particles in the rubbery state were easily incorporated in the ordered CNC matrix and improved the structural integrity of its chiral nematic phase.
Introduction Cellulose nanocrystals (CNCs) are a class of high-aspect ratio, rod-like, highly crystalline nanoparticles (NPs).1-3 Due to their excellent mechanical properties, relatively low cost, biocompatibility and rich surface chemistry, CNCs represent a promising, green chemistry alternative to conventional NP reinforcement additives, e.g., glass fibres or carbon black, used in composite materials.3-6 At sufficiently high concentrations in aqueous suspensions, CNCs organize into a left-handed chiral nematic (N*) structure.1,2,7 The N* order can be largely preserved in the solid state by slowly drying CNC suspensions.7-10 The resulting CNC films are birefringent, iridescent and exhibit circular dichroism (CD). It is currently established that CNCs retain their ability to form N* structures in the presence of a small (1 mm 2, indicative of the absence of macroscopic phase separation of latex NPs in the film (Fig. 3eh). In the films formed from mixtures of CNCs with EtMA50(+) NPs, the absence in the characteristic POM marble texture and uniform fluorescence throughout the films (Fig. S8, S9) originated from the formation of the colloidal gel. Thus we conclude that loading of anionic EtMA50(−) latex NPs in the CNC films gradually reduced the N* order in the composite film with increasing latex content, which completely disappeared at Clatex>50 wt%. The addition of cationic EtMA50(+) latex NPs had a disruptive effect on the N* order at concentrations as low as 25 wt%. Effect of latex NP size on film structure and properties The effect of the size of latex NPs on the structure of CNClatex films was examined for anionic latex NPs with a diameter of 50 or 150 nm. Figure 4a shows the extinction spectra of the composite films of CNCs and EtMA50(−) or EtMA150(−) NPs
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Nanoscale (red and blue curves, respectively) at Clatex = 25 wt%. The addition of the latex NPs had a limited effect on the position of the photonic band gap, which shifted from 380 ± 20 nm for the latex free CNC film to 410 ± 30 nm or 390 ± 30 with the addition of EtMA50(−) or EtMA150(−), respectively. We consider the apparent variation in the position of the photonic band gap between EtMA50(−) and EtMA150(−) films to be statistically non-significant, and caused by the inherent variability of the films, 36 as discussed above. Importantly, the area under the photonic band gap decreased by 15 and 40% upon the addition of 50 and 150 nm-diameter NPs, respectively, indicative of a stronger disruption of the N*
Figure 3. Polarized optical microscopy images (a-d) and fluorescence microscopy images (e-h) of the composite films formed by CNCs and EtMA50(−) NPs at Clatex of (a,e) 75, (b,f) 50, (c,g) 25, and (d,h) 0 wt%. Scale bars are 500 µm.
The structure of the composite films containing EtMA50(−) or EtMA150(−) NPs was studied using cross-sectional SEM imaging. Low-magnification images (Fig. 4b and c) showed the coexistence of ordered CNC-rich and disordered latex-rich domains aligned in the plane of the film. The fraction of the area occupied by the disordered regions in the cross-section of the films was 12 ± 10 and 19 ± 10 % for the films containing EtMA50(−) and EtMA150(−) NPs, respectively. The high magnification images (Fig. S10a and c) revealed that the CNC-rich regions had an ordered layered structure characteristic of the N* order.44 The average pitch of the N* phase (the distance over which the nematic order undergoes a full turn) was determined as the spacing between the layers based on >25 measurements for each film composition and was found to be 220 ± 60, 180 ± 80 and 220 ± 80 nm for the latexfree films (Fig. 2c), films containing EtMA50(−) NPs (Fig. 4b) and films containing EtMA150(−) NPs (Fig. 4c), respectively. Moreover, a fraction of EtMA50(−) and EtMA150(−) NPs was embedded within the N* phase (Fig. S7 and S12). The inclusion of smaller EtMA50(−) NPs within the N* domains was favoured and perturbed the local order to a lesser extent, compared to the EtMA150(−) NPs (Fig. S7). Overall, the effect of the dimensions of latex NPs on the structure of the composite films and on the extent of the latex NPs demixing was ascribed to the stronger disruption of the N* order by larger NPs, although their number density was 25-fold lower than that of smaller NPs. The formation of a nematic phase in suspensions of rods is usually ascribed to the gain in translational entropy in the ordered phase, compared to the
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ARTICLE morphology of CNCs (where a certain number of layers adds to a full turn around the local director, or the pitch), the space accommodating a 50 nm- and 150 nm-diameter spherical latex NP would be equivalent to 3-5 layers and 9-15 CNC layers, respectively. The inclusion of one large NP in the N* phase would, consequently, disrupt the order of a larger number of CNC layers and decrease the translational entropy of a larger number of CNCs than small latex NPs. Effect of thermal treatment
Figure 4. Effect of dimensions of negatively-charged poly(ethyl methacrylate) latex NPs on extinction and structure of composite CNC-latex films. (a) UV-Vis spectra of latex -free CNC films (black line); composite films formed with addition of 50 nm-diameter (EtMA50(−)NPs (dashed red line previously seen in fig. 2a); and 150 nm-diameter (EtMA150(−)) NPs (blue line) at Clatex = 25 wt%. Representative SEM images of films containing (b) 50 nm-diameter NPs, and (c) 150 nm-diameter NPs. Scale bars are 10 µm. The inset in c show high magnification of the border between ordered and disordered domains the scale bar is 1 µm.
The effect of thermal treatment on the properties and structure of the composite films containing 25 wt% EtMA150(−) NPs with a Tg = 67 °C was examined for the films prepared in three different ways: (i) by drying CNC-latex suspension at 25 °C, (ii) by drying CNC-latex suspension at 25 °C and subsequently, annealing the film at 75 °C (above the Tg of the latex NPs), and (iii) by drying CNC-latex suspension at 75 °C. Composite films formed at 25 °C displayed a photonic band gap at 390 ± 30 nm (Fig. 5a, dashed blue line). Films prepared at 75 °C exhibited a red-shifted, broader and weaker band gap (Fig. 5a, green line), compared to the films dried at 25 °C, which was attributed to a disrupted N* order in the film (Fig. 5d). A similar trend was observed for the latex-free CNC films (Fig. S14) prepared at 75 °C, due to the fast water evaporation from the CNC suspension, which reduced the ability of the CNCs to organize into an N* structure. 46 Composite films obtained by drying mixed suspensions at 25 °C and subsequent annealing at 75 °C, that is, above the Tg of EtMA NPs), displayed a photonic band gap at 400 ± 20 nm (Fig. 5a, red line) and had a more regular N* morphology (Fig. 5d) than the non-annealed films (Fig. 5b). The difference originated from the partial deformation of the latex NPs upon heating above Tg47,48 thereby leading to an improvement in the CNC packing. The deformation of the latex NPs decreased the number of voids between the latex particles in the films, which allowed for the reorganization of the CNCs into a more ordered structure. Consequently, films annealed at 75 °C had a more ordered structure, since the polymer in the rubbery state could infiltrate the CNC matrix. However, films prepared directly at 75 °C had a less ordered structure, due to the rapid evaporation of water, preventing the formation of a well-ordered N* CNC matrix.
The difference in the effect of smaller and larger NPs on the N* structure can be explained geometrically: in the layered
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disordered phase.45 The addition of spherical latex NPs with a diameter larger than the diameter of CNCs led to a reduction in translational entropy, thereby decreasing the driving force for the formation of ordered domains, and leading to the demixing of the system into the latex-rich disordered and CNC-rich N* ordered domains.
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Figure 5. Effect of thermal treatment on the extinction and structure of the CNC-latex films. (a) UV-Vis spectra of the films formed from the aqueous suspension of CNCs and EtMA150(−) NPs dried at 25 °C (dashed blue line); dried at 25 °C and subsequently, annealed at 75 °C (red line) and dried at 75 °C (green). The dashed blue spectrum is reproduced from Fig. 4 for comparison (b-d) Representative SEM images of the cross-sections of the composite films formed from CNC mixtures with EtMA150(−) NPs by (b) drying a mixed suspension at 25 °C, (c), drying a mixed suspension at 75 °C and (d) drying a mixed suspension at 25 °C and subsequently, annealing the fi lm at 75 °C. Scale bars are 2 µm. In all the films Clatex = 25 wt%.
In light of these results, we conclude that the use of latex NPs with a Tg significantly below the film formation temperature (e.g. room temperature of 25 °C) may benefit the co-assembly of CNCs and latex NPs into a N* structures in the composite films. Therefore, in the next step, we prepared composite films with BuA150(−) NPs with a Tg of −48 °C (Table 1). Composite films containing BuA150(−) NPs (Clatex = 25 wt%) exhibited an extinction spectrum with a narrower photonic band gap than the CNC films with EtMA150(−) NPs (Fig. 6a, red line and dashed blue line, respectively). Furthermore, these films had a well-ordered N* morphology (Fig. 6b). The N* order of the CNCs persisted up to Clatex=50 wt%. (Fig. 6a, green line; Fig. 6c) and disappeared only at higher latex content (Fig. 6d). Since better control of the structure of the composite films was achieved for low-Tg latex NPs, we explored whether this
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effect can compensate for the disruptive influence of cationic latex NPs on film structure. Figure 6e-g shows the structure of composite films prepared with BuA150(+) NPs at increasing Clatex. In contrast to films containing high-Tg cationic latex NPs, an N* order was partly preserved in the films containing BuA150(+) NPs at Clatex of 25 and 50 wt.% (Fig. 6e and f, respectively). These results suggest that low-Tg latex NPs are more efficiently incorporated into the CNC matrix than high-Tg latex NPs with similar dimensions and charge We speculate that when latex NPs are in their rubbery state, they do not act as large defects preventing N* order, but likely, engulf the CNCs, thereby partitioning more easily in the N* regions
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ARTICLE In contrast, the addition of high-Tg positively-charged latex NPs severely disrupted the N* order of the composite films. In this case, the latex NPs acted as crosslinkers, leading to CNC gelation. Importantly, post-preparation thermal treatment of the composite films above the Tg of the latex NPs improved CNC ordering in the composite films, implying that latex NPs in the rubbery state were less disruptive for the N* order in the films than rigid NPs. This result provided guidance in the preparation of the composite films from the mixture of CNCs with low-Tg latex NPs. We conclude that to obtain composite films with the highest extent of N* order it is imperative to use negativelycharged, small and low-Tg latex NPs. Studies of optical, physical and mechanical properties of such films, are crucial in the design of new CNC based optical devices, and are the subject of further work.
Acknowledgements The authors acknowledge funding from NSERC Canada. The authors thank Prof. Ron Kluger for the use of CD spectropolarimeter and Ilya Gourevich of the Center of Nanostructure Imaging for his assistance in electron microscopy experiments.
Notes and references a
Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario, M5S 3H6, Canada b Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada. c The Institute of Biomaterials and Biomedical Engineering, University of Toronto, 4 Taddle Creek Road, Toronto, Ontario M5S 3G9, Canada *Email: [email protected] Figure 6. (a) Extinction spectra of the films formed from mixed aqueous suspension of CNCs and BuA150(−) NPs at Clatex = 25 wt% (red line) and Clatex = 50 wt% (green line). The dashed blue spectrum is the sectrum of a composite films made of EtMA150(−) NPs at Clatex = 25 wt% reproduced from Fig. 4 for comparison. (b-g) Representative images of the cross-section of the composite CNC-latex films formed with addition of BuA150(−) NPs (b-d) and BuA150(+) NPs (e-g) at Clatex of (b-e) 25 wt% (c,f) 50 wt% and (d,g) 75 wt%. Scale bars are 2 µm.
† The concentration of anthracene in 50 nm-diameter positively- and negatively-charged latex NPs differs due to different their preparation. Electronic Supplementary Information (ESI) available: detailed latex synthesis. Additional characterization of the nanoparticles and films. See DOI: 10.1039/b000000x/
Conclusions
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In conclusion, we examined the effect of the surface-charge, concentration, size and Tg of latex NPs and processing conditions on the structure of CNC-latex films. The composite films containing up to Clatex = 50 wt% of small high-Tg latex NPs exhibited a preserved N* order, typical of CNC films. In suspensions and in films, partial demixing of negativelycharged latex NPs resulted in the formation of N* ordered CNCs-rich domains and disordered latex-rich regions aligned in the plane of the film. The large NPs disrupted the N* order and demixed from the CNCs-rich domains to a larger extent than smaller NPs. A fraction of latex NPs was included in the N* regions of the composite films. This effect was favoured for smaller latex NPs. Macroscopically, these stratified films exhibited close-to-uniform fluorescence and birefringence.
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The co-assembly of cellulose nanocrystals and latex nanoparticles results in composite films with different structure and properties.
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This article can be cited before page numbers have been issued, to do this please use: H. TherienAubin, A. Lukach, N. Pitch and E. Kumacheva, Nanoscale, 2015, DOI: 10.1039/C5NR00660K.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Structure and Properties of Composite Films Formed by Cellulose Nanocrystals and Charged Latex Nanoparticles Héloïse Thérien-Aubina, Ariella Lukach a, Natalie Pitch a, Eugenia Kumacheva a,b,c*
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x
We report the structural and optical properties of composite film s formed from mixed suspensions of cellulose nanocrystals (CNCs) and fluorescent latex nanoparticles (NPs). We explored the effect of the
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variation in NP concentration, size, surface charge, and glass transition temperature on film structure and properties. The chiral nematic order, typical of CNC films, was preserved in films with up to 50 wt.% of negatively-charged latex NPs. Composite films were characterized by macroscopically close-to-uniform fluorescence, birefringence, and circular dichroism properties. In contrast, addition of positively charged latex NPs led to gelation of CNC-latex suspensions and disruption of the chiral nematic order in the composite films. Large latex NPs disrupted the chiral nematic order to a larger extend than small NPs. Furthermore, the glass transition of latex NPs had a dramatic effect on the structure of CNC-latex films. Latex particles in the rubbery state were easily incorporated in the ordered CNC matrix and improved the structural integrity of its chiral nematic phase.
Introduction Cellulose nanocrystals (CNCs) are a class of high-aspect ratio, rod-like, highly crystalline nanoparticles (NPs).1-3 Due to their excellent mechanical properties, relatively low cost, biocompatibility and rich surface chemistry, CNCs represent a promising, green chemistry alternative to conventional NP reinforcement additives, e.g., glass fibres or carbon black, used in composite materials.3-6 At sufficiently high concentrations in aqueous suspensions, CNCs organize into a left-handed chiral nematic (N*) structure.1,2,7 The N* order can be largely preserved in the solid state by slowly drying CNC suspensions.7-10 The resulting CNC films are birefringent, iridescent and exhibit circular dichroism (CD). It is currently established that CNCs retain their ability to form N* structures in the presence of a small (1 mm 2, indicative of the absence of macroscopic phase separation of latex NPs in the film (Fig. 3eh). In the films formed from mixtures of CNCs with EtMA50(+) NPs, the absence in the characteristic POM marble texture and uniform fluorescence throughout the films (Fig. S8, S9) originated from the formation of the colloidal gel. Thus we conclude that loading of anionic EtMA50(−) latex NPs in the CNC films gradually reduced the N* order in the composite film with increasing latex content, which completely disappeared at Clatex>50 wt%. The addition of cationic EtMA50(+) latex NPs had a disruptive effect on the N* order at concentrations as low as 25 wt%. Effect of latex NP size on film structure and properties The effect of the size of latex NPs on the structure of CNClatex films was examined for anionic latex NPs with a diameter of 50 or 150 nm. Figure 4a shows the extinction spectra of the composite films of CNCs and EtMA50(−) or EtMA150(−) NPs
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Nanoscale (red and blue curves, respectively) at Clatex = 25 wt%. The addition of the latex NPs had a limited effect on the position of the photonic band gap, which shifted from 380 ± 20 nm for the latex free CNC film to 410 ± 30 nm or 390 ± 30 with the addition of EtMA50(−) or EtMA150(−), respectively. We consider the apparent variation in the position of the photonic band gap between EtMA50(−) and EtMA150(−) films to be statistically non-significant, and caused by the inherent variability of the films, 36 as discussed above. Importantly, the area under the photonic band gap decreased by 15 and 40% upon the addition of 50 and 150 nm-diameter NPs, respectively, indicative of a stronger disruption of the N*
Figure 3. Polarized optical microscopy images (a-d) and fluorescence microscopy images (e-h) of the composite films formed by CNCs and EtMA50(−) NPs at Clatex of (a,e) 75, (b,f) 50, (c,g) 25, and (d,h) 0 wt%. Scale bars are 500 µm.
The structure of the composite films containing EtMA50(−) or EtMA150(−) NPs was studied using cross-sectional SEM imaging. Low-magnification images (Fig. 4b and c) showed the coexistence of ordered CNC-rich and disordered latex-rich domains aligned in the plane of the film. The fraction of the area occupied by the disordered regions in the cross-section of the films was 12 ± 10 and 19 ± 10 % for the films containing EtMA50(−) and EtMA150(−) NPs, respectively. The high magnification images (Fig. S10a and c) revealed that the CNC-rich regions had an ordered layered structure characteristic of the N* order.44 The average pitch of the N* phase (the distance over which the nematic order undergoes a full turn) was determined as the spacing between the layers based on >25 measurements for each film composition and was found to be 220 ± 60, 180 ± 80 and 220 ± 80 nm for the latexfree films (Fig. 2c), films containing EtMA50(−) NPs (Fig. 4b) and films containing EtMA150(−) NPs (Fig. 4c), respectively. Moreover, a fraction of EtMA50(−) and EtMA150(−) NPs was embedded within the N* phase (Fig. S7 and S12). The inclusion of smaller EtMA50(−) NPs within the N* domains was favoured and perturbed the local order to a lesser extent, compared to the EtMA150(−) NPs (Fig. S7). Overall, the effect of the dimensions of latex NPs on the structure of the composite films and on the extent of the latex NPs demixing was ascribed to the stronger disruption of the N* order by larger NPs, although their number density was 25-fold lower than that of smaller NPs. The formation of a nematic phase in suspensions of rods is usually ascribed to the gain in translational entropy in the ordered phase, compared to the
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ARTICLE morphology of CNCs (where a certain number of layers adds to a full turn around the local director, or the pitch), the space accommodating a 50 nm- and 150 nm-diameter spherical latex NP would be equivalent to 3-5 layers and 9-15 CNC layers, respectively. The inclusion of one large NP in the N* phase would, consequently, disrupt the order of a larger number of CNC layers and decrease the translational entropy of a larger number of CNCs than small latex NPs. Effect of thermal treatment
Figure 4. Effect of dimensions of negatively-charged poly(ethyl methacrylate) latex NPs on extinction and structure of composite CNC-latex films. (a) UV-Vis spectra of latex -free CNC films (black line); composite films formed with addition of 50 nm-diameter (EtMA50(−)NPs (dashed red line previously seen in fig. 2a); and 150 nm-diameter (EtMA150(−)) NPs (blue line) at Clatex = 25 wt%. Representative SEM images of films containing (b) 50 nm-diameter NPs, and (c) 150 nm-diameter NPs. Scale bars are 10 µm. The inset in c show high magnification of the border between ordered and disordered domains the scale bar is 1 µm.
The effect of thermal treatment on the properties and structure of the composite films containing 25 wt% EtMA150(−) NPs with a Tg = 67 °C was examined for the films prepared in three different ways: (i) by drying CNC-latex suspension at 25 °C, (ii) by drying CNC-latex suspension at 25 °C and subsequently, annealing the film at 75 °C (above the Tg of the latex NPs), and (iii) by drying CNC-latex suspension at 75 °C. Composite films formed at 25 °C displayed a photonic band gap at 390 ± 30 nm (Fig. 5a, dashed blue line). Films prepared at 75 °C exhibited a red-shifted, broader and weaker band gap (Fig. 5a, green line), compared to the films dried at 25 °C, which was attributed to a disrupted N* order in the film (Fig. 5d). A similar trend was observed for the latex-free CNC films (Fig. S14) prepared at 75 °C, due to the fast water evaporation from the CNC suspension, which reduced the ability of the CNCs to organize into an N* structure. 46 Composite films obtained by drying mixed suspensions at 25 °C and subsequent annealing at 75 °C, that is, above the Tg of EtMA NPs), displayed a photonic band gap at 400 ± 20 nm (Fig. 5a, red line) and had a more regular N* morphology (Fig. 5d) than the non-annealed films (Fig. 5b). The difference originated from the partial deformation of the latex NPs upon heating above Tg47,48 thereby leading to an improvement in the CNC packing. The deformation of the latex NPs decreased the number of voids between the latex particles in the films, which allowed for the reorganization of the CNCs into a more ordered structure. Consequently, films annealed at 75 °C had a more ordered structure, since the polymer in the rubbery state could infiltrate the CNC matrix. However, films prepared directly at 75 °C had a less ordered structure, due to the rapid evaporation of water, preventing the formation of a well-ordered N* CNC matrix.
The difference in the effect of smaller and larger NPs on the N* structure can be explained geometrically: in the layered
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disordered phase.45 The addition of spherical latex NPs with a diameter larger than the diameter of CNCs led to a reduction in translational entropy, thereby decreasing the driving force for the formation of ordered domains, and leading to the demixing of the system into the latex-rich disordered and CNC-rich N* ordered domains.
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Figure 5. Effect of thermal treatment on the extinction and structure of the CNC-latex films. (a) UV-Vis spectra of the films formed from the aqueous suspension of CNCs and EtMA150(−) NPs dried at 25 °C (dashed blue line); dried at 25 °C and subsequently, annealed at 75 °C (red line) and dried at 75 °C (green). The dashed blue spectrum is reproduced from Fig. 4 for comparison (b-d) Representative SEM images of the cross-sections of the composite films formed from CNC mixtures with EtMA150(−) NPs by (b) drying a mixed suspension at 25 °C, (c), drying a mixed suspension at 75 °C and (d) drying a mixed suspension at 25 °C and subsequently, annealing the fi lm at 75 °C. Scale bars are 2 µm. In all the films Clatex = 25 wt%.
In light of these results, we conclude that the use of latex NPs with a Tg significantly below the film formation temperature (e.g. room temperature of 25 °C) may benefit the co-assembly of CNCs and latex NPs into a N* structures in the composite films. Therefore, in the next step, we prepared composite films with BuA150(−) NPs with a Tg of −48 °C (Table 1). Composite films containing BuA150(−) NPs (Clatex = 25 wt%) exhibited an extinction spectrum with a narrower photonic band gap than the CNC films with EtMA150(−) NPs (Fig. 6a, red line and dashed blue line, respectively). Furthermore, these films had a well-ordered N* morphology (Fig. 6b). The N* order of the CNCs persisted up to Clatex=50 wt%. (Fig. 6a, green line; Fig. 6c) and disappeared only at higher latex content (Fig. 6d). Since better control of the structure of the composite films was achieved for low-Tg latex NPs, we explored whether this
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effect can compensate for the disruptive influence of cationic latex NPs on film structure. Figure 6e-g shows the structure of composite films prepared with BuA150(+) NPs at increasing Clatex. In contrast to films containing high-Tg cationic latex NPs, an N* order was partly preserved in the films containing BuA150(+) NPs at Clatex of 25 and 50 wt.% (Fig. 6e and f, respectively). These results suggest that low-Tg latex NPs are more efficiently incorporated into the CNC matrix than high-Tg latex NPs with similar dimensions and charge We speculate that when latex NPs are in their rubbery state, they do not act as large defects preventing N* order, but likely, engulf the CNCs, thereby partitioning more easily in the N* regions
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ARTICLE In contrast, the addition of high-Tg positively-charged latex NPs severely disrupted the N* order of the composite films. In this case, the latex NPs acted as crosslinkers, leading to CNC gelation. Importantly, post-preparation thermal treatment of the composite films above the Tg of the latex NPs improved CNC ordering in the composite films, implying that latex NPs in the rubbery state were less disruptive for the N* order in the films than rigid NPs. This result provided guidance in the preparation of the composite films from the mixture of CNCs with low-Tg latex NPs. We conclude that to obtain composite films with the highest extent of N* order it is imperative to use negativelycharged, small and low-Tg latex NPs. Studies of optical, physical and mechanical properties of such films, are crucial in the design of new CNC based optical devices, and are the subject of further work.
Acknowledgements The authors acknowledge funding from NSERC Canada. The authors thank Prof. Ron Kluger for the use of CD spectropolarimeter and Ilya Gourevich of the Center of Nanostructure Imaging for his assistance in electron microscopy experiments.
Notes and references a
Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario, M5S 3H6, Canada b Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada. c The Institute of Biomaterials and Biomedical Engineering, University of Toronto, 4 Taddle Creek Road, Toronto, Ontario M5S 3G9, Canada *Email: [email protected] Figure 6. (a) Extinction spectra of the films formed from mixed aqueous suspension of CNCs and BuA150(−) NPs at Clatex = 25 wt% (red line) and Clatex = 50 wt% (green line). The dashed blue spectrum is the sectrum of a composite films made of EtMA150(−) NPs at Clatex = 25 wt% reproduced from Fig. 4 for comparison. (b-g) Representative images of the cross-section of the composite CNC-latex films formed with addition of BuA150(−) NPs (b-d) and BuA150(+) NPs (e-g) at Clatex of (b-e) 25 wt% (c,f) 50 wt% and (d,g) 75 wt%. Scale bars are 2 µm.
† The concentration of anthracene in 50 nm-diameter positively- and negatively-charged latex NPs differs due to different their preparation. Electronic Supplementary Information (ESI) available: detailed latex synthesis. Additional characterization of the nanoparticles and films. See DOI: 10.1039/b000000x/
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In conclusion, we examined the effect of the surface-charge, concentration, size and Tg of latex NPs and processing conditions on the structure of CNC-latex films. The composite films containing up to Clatex = 50 wt% of small high-Tg latex NPs exhibited a preserved N* order, typical of CNC films. In suspensions and in films, partial demixing of negativelycharged latex NPs resulted in the formation of N* ordered CNCs-rich domains and disordered latex-rich regions aligned in the plane of the film. The large NPs disrupted the N* order and demixed from the CNCs-rich domains to a larger extent than smaller NPs. A fraction of latex NPs was included in the N* regions of the composite films. This effect was favoured for smaller latex NPs. Macroscopically, these stratified films exhibited close-to-uniform fluorescence and birefringence.
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The co-assembly of cellulose nanocrystals and latex nanoparticles results in composite films with different structure and properties.
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