Supporting Information for Light-Healable Supramolecular Nanocomposites Based on Modified Cellulose Nanocrystals Mahesh V. Biyani, E. Johan Foster, * and Christoph Weder * Adolphe Merkle Institute, Polymer Chemistry and Materials, University of Fribourg, Rte de l Ancienne Papeterie, CH-1723 Marly 1, Switzerland * To whom correspondence should be addressed. E-mail: johan.foster@unifr.ch, christoph.weder@unifr.ch Experimental Section Materials. 2-Amino-4-hydroxy-6-methylpyrimidine, hexamethylene diisocyanate (HMDI), dibutyltindilaurate (DBTDL), and tetrahydrofuran were obtained from Sigma Aldrich. N,N- Dimethylformamide (DMF) extra dry over molecular sieves was purchased from Acros Organics. Hydroxy-terminated poly(ethylene-co-butylene) (KRASOL LBH-P 3000) with a numberaverage molecular weight of 3100 g/mol and a polydispersity index of 1.25 was received from Cray Valley SA. All chemicals were used without further purification. Analytical Methods. 1 H-NMR (300 MHz, 200 scans) and 13 C-NMR (300 MHz, 2000 scans) spectra were recorded in CDCl 3 at room temperature on a Bruker Avance III 300 MHz. FT-IR spectra were collected using vacuum dried powder/films samples on a Perkin Elmer Spectrum 65 spectrometer in ATR mode between 4000-600 cm -1 with a resolution of 4 cm -1 and 50 scans per sample. UV-visible absorbance spectroscopy was used to monitor the surface functionalization of the CNCs. The spectra were acquired using DMF as a solvent for CNC-UPy and UPy-NCO on a Shimadzu UV-2401 PC spectrophotometer in the wavelength range of 200-400 nm. To study the change in elemental composition upon functionalization of the CNCs elemental analyses were carried out. A Flash 2000 Organic Elemental Analyser (Thermo Scientific, Brookfield, Wisconsin, USA) equipped with CHNS/O columns was used for these experiments. For the CHNS measurements He was used as carrier and O 2 for the oxidation of the samples. The samples were prepared by adding 3-5 mg of neat CNCs or CNC-UPy (both dried at 70 ºC in vacuum oven) in a universal soft tin container (Thermo Scientific, Brookfield, Wisconsin, USA). Prior to each measurement the instrument was calibrated with (2,5-bis(5-tert-butyl-2-1
benzo-oxazol-2-yl) thiophene). Three measurements were performed for each sample and the results were averaged. Transmission Electron Microscopy (TEM). TEM images were acquired to study the dispersion and dimensions of unmodified and modified cellulose nanocrystals (CNCs). Sample preparation involved depositing 3 µl of a dispersions of CNCs in DMF (CNC content = 0.1 mg/ml) onto carbon-coated grids (Electron Microscopy Sciences) and drying the sample in an oven at 70 ºC. A Philips CM100 Bio-microscope operated at an accelerating voltage of 80 kv was used to acquire the images. The CNC and CNC-UPy dimensions were determined by analyzing 5 TEM images individually for both type of CNCs and measuring length and width of more than 100 CNCs. The dimensions thus determined are reported as average values ± standard error. Atomic Force Microscopy (AFM). AFM images were acquired to visualize the healing of deliberately applied defects in nanocomposites films. An AFM Nano Wizard II (JPK Instruments) microscope was used to acquire the images. The scans were performed in tapping mode using silicon cantilever with spring constant of 42 N/m at the resonance frequency 320 khz. Optical Microscopy. To measure the depth of scratch for damaged samples as well as to monitor the healing as function of time optical microscopy images were taken on Olympus BX51 microscope equipped with DP72 digital camera. Sonication. CNC and CNC-UPy were dispersed in different solvents at room temperature using BANDELIN SONOREX TECHNIK RL 70 UH sonicator operating at 40 khz. Isolation of Cellulose Nanocrystals (CNCs) from Tunicates. CNCs were isolated by sulfuric acid hydrolysis of cellulose pulp obtained from tunicates (Styela clava) following the standard protocols 1,2 with minor modifications. In the typical procedure 6 g of bleached tunicate mantles were blended with 600 ml of deionized water in high speed blender yielding a cellulose pulp. Sulfuric acid (95-97%, 500 ml) was slowly added under vigorous stirring maintaining the temperature bellow 20 ºC. After 500 ml of the acid addition, the dispersion was removed from the ice bath and was heated to 40 ºC and the final 100 ml of acid was added to the pulp. After complete acid addition the dispersion was heated to 60 ºC for 1 h under continuous stirring. The mixture was then cooled to room temperature, centrifuged for 20 min at 3600 rpm, and the 2
supernatant solution was decanted. Deionized water was added and the centrifugation step was repeated until the ph of the dispersion reached about 5. After the last centrifugation the resulting CNCs were dialyzed against deionized water for 2 days to remove the last residues of the sulfuric acid. After dialysis the dispersion was sonicated for 12h. The concentration of the CNCs in the final dispersion was determined gravimetrically to be ca. 5 mg/ml. The aqueous CNC dispersion produced was lyophilized using a VirTis BenchTop 2K XL lyophilizer. CNCs isolated by sulfuric acid hydrolysis carry a small amount of surface sulfate groups; their concentration was determined by standard conductometric titration according to a previously described protocol 3 with minor changes. Thus, 50 mg of freeze-dried CNCs were dispersed in 10 ml of 0.01 M HCl by stirring for 10 min and sonication for 30 min before the dispersion was titrated with 0.01 M NaOH. The titration curve (Figure S1) shows an initial sharp decrease in conductivity, which corresponds to the neutralization of the excess of HCl, followed by a plateau region, which corresponds to the neutralization of the sulfate-ester surface groups, and finally a linear increase in conductivity, which is related to the excess of alkali after all acidic groups have been neutralized. From the width of the plateau region, the density of sulfate surface groups was determined to be 75 mmol/kg. The average length and width of CNCs were 1720 ± 514 nm and 25 ± 6 nm respectively with an aspect ratio of 69. Synthesis of UPy-NCO. 2-(6-Isocyanatohexylaminocarbonylamino)-6-methyl-4[1H]pyrimidinone (UPy-NCO) was synthesized according to the procedure reported by Folmer et al. 4 Preparation of CNC-UPy by Surface Modification of CNCs with UPy-NCO. The CNCs were decorated with UPy-groups by reaction with UPy-NCO. Thus, CNCs (0.50 g, 1 equivalent, 3.084 mmol) and anhydrous DMF (200 ml) were charged into a three-necked 250 ml round bottom flask equipped with a magnetic stirring bar, reflux condenser, and nitrogen inlet and the mixture was stirred for 30 min and sonicated for 2 h, all under N 2 atmosphere. To the CNC dispersion a catalytic amount of DBTDL (1 drop) and UPy-NCO (0.99 g, 1.1 equivalents, 3.39 mmol) were added and the reaction mixture was stirred for 16 h at 100 ºC under nitrogen. The product was separated by centrifugation at 8000 rpm for 15 min and the supernatant was discarded and replaced with DMF (100 ml). This process was repeated 4 times. After the last washing step, the modified CNCs were kept suspended in DMF. The yield of CNC-UPy, measured gravimetrically by drying an aliquot of the final dispersion, was 0.61g. The average length (1770 ± 560 nm), 3
width (26 ± 6 nm), and aspect ratio (68) of the CNC-UPy were determined by TEM. The degree of substitution of CNC-UPy calculated from UV-Vis spectroscopy (Figure S3) which is around 0.18. Synthesis of UPy-K-UPy by End Group Functionalization of Hydroxy-Terminated Poly(ethylene-co-butylene) with UPy-NCO. The hydroxy-terminated poly(ethylene-cobutylene) was functionalized with UPy-NCO according to protocol by Folmer et al. 4 UPy-K-UPy was obtained as colourless elastic solid after the end group functionalization. The number-average molecular weight and polydispersity of formed supramolecular polymer was 3800 g/mol and 1.21 respectively, which was determine by gel permeation chromatography. 1 H-NMR (300MHz, CDCl 3 ): δ 13.1 (s, 2H, CH 3 CNH), 11.9 (s, 2H, CH 2 NH (C=O)NH), 10.1(s, 2H, CH 2 NH(C=O)NH ), 5.8 (s, 2H, CH=CCH 3 ), 4.8 (s, 2H, NH(C=O)O), 4.1 (m, 4H CH 2 O(C=O)NH ), 3.3 (m, 4H, CH 2 NH(C=O)NH), 3.1 (m, 4H,CH 2 NH(C=O)O), 2.2(s, 6H, CH 3 C=CH), 1.6-1.1 (m, 405, CH 2 CH 2 CH 2 ), 0.8 (m, 127, CH 2 CH 3 ). 13 C-NMR (300MHz,CDCl 3 ): δ 173.34, 158.14, 157.01, 156.75, 154.88, 148.37, 106.86, 73.95, 39.33, 39.04, 38.53, 38.07, 36.28, 33.67, 33.40, 30.80, 30.36, 29.91, 29.51, 26.94, 26.75, 26.60, 26.44, 26.28, 26.21, 26.03, 19.08, 11.03, 10.84, 10.79. Preparation of UPy-K-UPy/CNC and UPy-K-UPy/CNC-UPy Nanocomposites. As UPy-K- UPy is insoluble DMF, a common organic solvent for the CNCs, it was dissolved in THF at a concentration of 50 mg/ml by stirring for 4 h. In parallel, CNCs or CNC-UPy were dispersed in THF via a solvent exchange procedure. Thus, the lyophilized CNC aerogel was dispersed in DMF at a concentration of 5 mg/ml by stirring and sonication for 2 h; the as-prepared CNC-UPy dispersion in DMF (vide supra) was diluted to a CNC-UPy content of 5 mg/ml and sonicated for 6 h. The DMF was exchanged with THF by centrifugation (8000 rpm, 15min) followed by sonication; the same procedure was repeated twice to make sure complete removal of DMF. The final content of CNC/CNC-UPy in THF was around 4-5 mg/ml. Nanocomposites comprising 10, 15, or 20 % w/w of CNCs or CNC-UPy were prepared by combining appropriate amounts of CNC or CNC-UPy dispersions in THF with the UPy-K-UPy solution create 1g of the respective nanocomposite. The mixtures were stirred for 30 min and sonicated for 1 h, before they were cast into Teflon Petri dishes. The solvent was evaporated in an oven at 60 ºC for 16 h and the resulting films were subsequently dried in a vacuum oven at 50 ºC for 24 h. After the complete removal of 4
the solvent, the nanocomposites films were compression molded in Carver press using spacers between Teflon sheets at 90 ºC, 1000 psi for 5 min to yield films having a thickness in the range of 200-250 µm. Stress-Strain Measurements. Stress-strain measurements of films of the neat supramolecular polymer and the nanocomposites were performed using a TA Instruments Model Q800 dynamic mechanical analyzer. All measurements were carried out at 25 ºC with a strain rate of 5%/min. The samples used for these experiments had been cut into a dog-bone shape having a width of ~2.1 mm; a gap distance between the jaws of initially ~8 mm was applied. All the stress-strain measurements performed with the samples which were 5 days old after the melt pressed. Dynamic Mechanical Analysis (DMA). DMA measurements of films of the neat supramolecular polymer and the nanocomposites were performed using a TA Instruments Model Q800 dynamic mechanical analyzer. All tests were carried out in tensile mode using a temperature sweep method from -70 ºC to 100 ºC and applying an oscillatory deformation with a frequency of 1 Hz and a strain with an amplitude of 15 µm. The samples used for these experiments were of rectangular shape with a width of ~6 mm and a length of ~15 mm. Optical Healing Experiments. Optical healing experiments were performed by exposing the samples to light from a Bluepoint 4 Ecocure from Honle UV Technology using a UV filter with transmission in the range of 320-390 nm. Films of UPy-K-UPy and nanocomposite samples were damaged by applying cuts with a depth of 50-70 % of the sample thickness with a blade attached to a caliper. These samples were exposed to light from the above setup with an intensity 350 mw/cm 2 for 20 s. All the stress-strain measurements for optically healed samples were performed 24 h after healing. Damaging the Nanocomposites Films. Controlled damages to the nanocomposites as well as UPy-K-UPy films were made using a custom-built cutting device. The basic design involves an air-pressured, motorized razor blade that is mounted with its edge parallel to a sample holder and kept at a constant height. The distance between sample holder and blade can be adjusted with a caliper, thus allowing to control the depth of the cuts that are applied upon drawing the blade across the sample. 5
Table S1. Elemental analysis results for CNC and CNC-UPy. Experimental values (Theoretical values) Sample %C %H %N CNC 40.51 (44.44) 5.71 (6.17) - (0) CNC-UPy 56.30 8.48 2.97 Table S2. Young s modulus, maximum stress, and strain at break of nanocomposite films of UPy-K-UPy and 10% w/w unmodified CNCs. The wide variation of the data reflects that the samples are highly inhomogeneous. Young s Maximum Strain Sample Modulus Stress (MPa) at Break (%) (MPa) 1 24 0.80 18.3 2 11 0.35 22.1 3 60 1.80 21.9 4 75 1.76 4.5 5 11 0.74 14.6 6 26 1.95 20.7 6
Table S3. Young s modulus, maximum stress, and strain at break of deliberately damaged and thermally healed films of UPy-K-UPy and nanocomposites of UPy-K-UPy and 10 or 15% w/w CNC-UPy. Sample Young s Modulus (MPa) Maximum Stress (MPa) Strain at Break (%) UPy-K-UPy 12 ± 1 1.4 ± 0.1 38.7 ± 1.8 UPy-K-UPy/CNC-UPy 10% w/w 45 ± 9 1.8 ± 0.3 9.9 ± 0.2 UPy-K-UPy/CNC-UPy 15% w/w 74 ± 10 2.3 ± 0.2 8.9 ± 0.9 Table S4. Maximum temperatures observed during the optical healing, measured by an IR camera. Sample Max. temp. reached at higher intensity/shorter time ( C) Max. temp. reached at lower intensity/longer time ( C) (350 mw/cm 2, 20 s) (250 mw/cm 2, 80 s) Upy-K-UPy 50 75 * UPy-K-UPy/CNC-UPY 10% 70 62 UPy-K-UPy/CNC-UPY 15% 108 71 UPy-K-UPy/CNC-UPY 20% 132 78 * Since UPy-K-UPy does not heal well upon healing at higher intensity/shorter time, the intensity used for this entry at longer healing time was kept at 350 mw/cm 2. 7
Figure S1. Conductometric titration curve of lyophilized, unmodified CNCs. 8
100 Wavenumber (cm -1 ) 4000 3500 3000 2500 2000 1500 1000 90 % Transmittance 80 70 60 50 40 30 CNC UPy-NCO CNC-UPy 1700 cm -1 Figure S2. FT-IR/ATR spectra of CNCs, UPy-NCO, and CNC-UPy. The inset shows an expansion of the characteristic peak of the carbonyl group from the UPy moieties at 1700 cm -1, which confirms the successful surface functionalization. 9
Figure S3. (a) UV-Vis absorption spectra of dispersions of CNCs or CNC-UPy in DMF and solutions of UPy-NCO in DMF (concentration = 3.13-25 µg/ml). As can be seen from the spectra, the CNC-UPy shows an absorbance with maximum around 282 nm that is absent in the spectrum of the neat CNCs and characteristic of the UPy motif. (b) Plot showing the absorbance of UPy-NCO solutions as a function of UPy-NCO content (squares) and of a 0.125 mg/ml CNC- UPy dispersion (circle). The data were used to determine the level of UPy functionalization of CNC-UPy. The calibration curve (b) shows that 0.015 mg UPy is present per 0.125 mg of CNC- UPy (i.e. 12 % w/w). This corresponds to a degree of substitution of 0.18, i.e. 18% of all hydroxyl units are functionalized. 10
Figure S4. (a) Schematic representation of the nanocomposite preparation and (b) images of nanocomposite films made of UPy-K-UPy and 10% w/w CNC-UPy (left) or unmodified CNCs (right) on text as well as on black background. 11
2 Stress (MPa) 1 0 5 10 15 20 25 Strain (%) Figure S5. Stress-strain curves of nanocomposite films of UPy-K-UPy and 10% w/w CNCs. The wide variation of the data reflects that the samples are highly inhomogeneous. 12
5 4 Absorbance 3 2 1 UPy-K-UPy 180 µm film UPy-K-UPy/CNC-UPy 10% 60 µm film UPy-K-UPy/CNC-UPy 15% 40 µm film UPy-K-UPy/CNC 10% 50 µm film 0 275 300 325 350 375 400 Wavelength (nm) Figure S6. UV-Vis absorption spectra of films of UPy-K-UPy and UPy-K-UPy/CNC-UPy nanocomposites. The UPy-K-UPy/CNC-UPy nanocomposite films show a pronounced absorbance at 290 nm, which corresponds to the UPy motif. The pronounced difference in the CNC-UPy containing films is presumably due to the high local concentration of UPy. 13
Figure S7. Optical microscopy image of (a) a damaged and (b) healed film made from UPy-K- UPy/CNC-UPy 10% w/w. The comparison of the images shows that the cut applied to the sample disappears upon UV irradiation. 14
Figure S8. AFM height images and their respective cross sections for UPy-K-UPy/CNC-UPy 10% w/w nanocomposite films in the deliberately damaged (a), partially healed (b), and completely healed (c) state (colour range; black =7µm, white = 0 µm). 15
Stress (MPa) 2 1 UPy-K-UPy/CNC-UPy 15% w/w UPy-K-UPy/CNC-UPy 10% w/w UPy-K-UPy 0 5 10 15 20 25 30 35 40 Strain (%) Figure S9. Stress-strain curves of films of UPy-K-UPy and nanocomposites of UPy-K-UPy and 10 or 15 % w/w CNC-UPy for thermally healed samples. 16
Scheme S1. Synthesis of UPy-K-UPy by end-capping KRASOL LBH-P 3000 with 2-(6- isocyanatohexylamino carbonyl amino)-6-methyl-4[1h]pyrimidinone (UPy-NCO) 17
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