Synthesis of photoactuating acrylic thermoplastic elastomers containing diblock copolymer-grafted carbon nanotubes
Markéta Ilčíková,1,‡ Miroslav Mrlík,2, ‡ Tomáš Sedláček,2 Miroslav Šlouf,3 Alexander Zhigunov, 3 Kaloian Koynov, 4 Jaroslav Mosnáček*, 1
1
Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 845 41, Bratislava, Slovakia *E-mail:
[email protected]
2
Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Nad Ovcirnou 3685, 760 01 Zlin, Czech Republic
3
Institute of Macromolecular Chemistry AS CR, Heyrovsky sq. 2, 16206 Prague 6, Czech Republic 4
Max Planck Institute for Polymer Research, Ackermanweg 1, D-55128 Mainz, Germany
SUPPORTING INFORMATION
Experimental details A. Materials The multi-walled carbon nanotubes (CNT) were obtained from Nanostructured & Amorphous Materials, Inc.; Houston, TX 77084, USA, with purity > 95 %, the outer diameter in the range of 60 to 100 nm, the length in the range of 5 to 15 µm and the specific surface area of 64 m2/g. The monomer nbutyl acrylate (BA) was purchased from Aldrich and it was purified from stabilizer by passing through a basic alumina before use. The 4-aminophenethyl alcohol and isoamyl nitrite were purchased from Across Organics; acetone, diethyl ether and chloroform were obtained from Centralchem (Slovakia). S1
Tetrahydrofuran (THF, POCH S.A., Poland) was dried by sodium wires and freshly distilled before the reaction. Triethyl amine (Et3N; Fluka, Switzerland), 2-bromopropionyl bromide, N,N,N’,N’’,N’’pentamethyldiethylenetriamine (PMDETA), copper(II) bromide (CuBr2), tin(II) 2-ethylhexanoate (Sn(Oct)2), N,N’-dimethyl formamide (DMF) and anisole (all from Aldrich, USA) were used as received. CNT covalently modified with ATRP initiator, 4-(2-bromopropionyloxy)ethylphenyl-modified CNT (CNT-Br), were synthesized as described previously.1 Synthesis of poly(butyl acrylate)-modified CNTs / poly(butyl acrylate) nanocomposite (CNT-g-PBA-Br / Br-PBA-Br) 100 mg of CNT-Br (7x10-6 mol of Br), n-butyl acrylate (12.5 mL, 1.1x10-4 mol) anisole (6.3 ml), PMDETA (0.023 mL, 1.1x10-4 mol), were placed into 50 mL Schlenk flask and sonicated for 6 minutes. A difunctional initiator dimethyl 2,6-dibromoheptanedionate (0.024 mL, 1.1 x10-4 mol) was added. Four freeze-thaw cycles were performed and CuBr2 (0.0049 g, 2.2x10-5 mol) was added into frozen reaction mixture. The argon-purged mixture of reducing agent (SnOct2, 0.035 g, 0.86x10-4 mol) dissolved in 1 mL of anisole was then added. The reaction flask was placed into oil bath preheated to 80 °C. The reaction was stopped after 21 hours at 80 % monomer conversion (1H NMR). The molar mass and dispersity of the Br-PBA-Br were 65 600 g/mol and 1.3, respectively (GPC; THF, PS standards). 4 mL of the reaction mixture was taken from to reaction flask to separate the CNT-g-PBA-Br from the nanocomposite for further analysis and was purified as described below. The rest of the reaction mixture was evacuated and about 7 mL of anisole was removed. Then the reaction mixture was dissolved in 20 mL anisole and evacuated again. After 24 hours of evacuation process, almost 22 mL was removed. Approximately 20.3 mL corresponded to anisole, and 1.7 mL corresponded to residual monomer (as determined by 1
HNMR). The amount of Nanoamor-PBA-Br/ Br-PBA-Br nanocomposite was 9.84g.
Synthesis of CNT-g-poly(n-butyl acrylate)-b-poly(methyl methacrylate) / poly(methyl methacrylate)-bpoly(n-butyl acrylate)-b-poly(methyl methacrylate) nanocomposite (CNT-g-PBA-b-PMMA / PMMA-bPBA-b-PMMA) 9 g of CNT-g-PBA-Br / Br-PBA-Br was dissolved in mixture of argon-purged anisole (7 mL) and methyl methacrylate (4.8 mL, 0.045 mol). The argon-purged mixture of CuCl (0.022 g, 2.2x10-4 mol), PMDETA (0.055 mL, 3.2x10-4 mol) and SnOct2 (0.0044 g, 1.09x10-5 mol) dissolved in 3 mL of anisole was added to Schlenk flask under argon flow. The polymerization was performed at 40 °C and stopped after 7 hours at monomer conversion of 57 % (1HNMR). The polymer composite was precipitated to methanol and dried under vacuum overnight. According to GPC (THF, PS standard) the molar mass and dispersity of PMMA-b-PBA-b-PMMA were 78 537 g/mol and 1.38, respectively. From 1HNMR, the ratio PBA:PMMA was 100:33, what stands for 512 units of PBA ( 65 600g/mol from GPC) and 153 units of PMMA and Mn of 80 980 g/mol. S2
Separation of modified CNT and pure PMMA-b-PBA-b-PMMA The CNT-g-PBA / PBA or CNT-g-PBA-b-PMMA / PMMA-b-PBA-b-PMMA (0.575g) nanocomposites were dissolved in 70 mL THF. The modified CNT were separated from the dissolved polymer by centrifugation (10 000 rpm, 10 min). The supernatant was separated and the CNT sediment was dissolved in 70 mL THF and the centrifugation was repeated. This procedure was repeated four times. The modified CNT were dried in vacuum oven at 60 °C and 30 mbar overnight. The pure PMMA-b-PBA-bPMMA matrix was obtained from the polymer solution after two filtrations over 0.22 μm PTFE filer and the subsequent solvent removal using rotatory vacuum evaporator. Preparation of polymer films Synthesized composite samples or separated pure PMMA-b-PBA-b-PMMA matrix was dissolved in the THF (12 wt% solution) under stirring at room temperature overnight and casted in to the teflon chambers. Then the chambers were put into the vacuum oven at 60 °C overnight in order to evaporate the residual solvent and the polymer films of 1 mm thickness were obtained. B. Methods Gel Permeation Chromatography The molar mass and dispersity of the polymers were estimated by gel permeation chromatography (GPC), consisted of a Waters 515 pump, two PPS SDV 5 μm columns (d = 8 mm, l = 300 mm; 500 Å + 105 Å) and a Waters 410 differential refractive index detector, with THF as an eluent at flow rate of 1.0 mL/min. Polystyrene and poly(methyl methacrylate) standards were used for calibration. Anisole was used as the internal standard to correct for any fluctuation in THF flow rate. Nuclear Magnetic Resonance Monomer conversions were determined by 1H NMR on a 400 MHz VNMRS Varian NMR spectrometer equipped with 5mm 1H-19F/15N-31P PFG AutoX DB NB probe at 25°C in deuterated chloroform as a solvent. Scanning Force Microscopy The visualization of homo and diblock copolymer was performed by scanning force microscope (D3100, Bruker) in tapping mode (Olympus OMCL-AC160TS cantilevers with a nominal spring constant of 42 N/m). The sample was prepared by drop coating of 5 mg/mL toluene solution of modified carbon nanotubes on freshly cleaved mica surface. For all samples we recorded the topography and the phase contrast signal. S3
Thermogravimetric Analysis The amount of attached organics substances was determined by thermogravimetric analysis. The measurement was performed at TGA/STDA851e( Mettler Toledo, Switzerland). All the measurements were performed at heating rate 10 °C/min in nitrogen atmosphere. Transmission electron microscopy The dispersion of carbon nanotubes within the polymer matrix was observed by transmission electron microscopy (microscope Tecnai G2 Spirit Twin 12, FEI company; accelarating voltage 120 kV). The samples were prepared by ultramicrotome (Ultracut UCT, Leica) under cryo-conditions (the sample and knife temperatures were -70 and -45 °C, respectively). Small X-ray Scattering The morphology of polymer matrix as well as composite was determined via small angle X-ray scattering (SAXS). The samples were characterized before and after annealing (150 °C, 60 min). Dynamic Mechanical Analysis The DMA was performed at DMA device Mettler Toledo (Switzerland) in shear mode at frequencies 0.5 Hz, 1 Hz, 2.5 Hz and 5 Hz in temperature range from -80 °C to 150 °C, under nitrogen atmosphere. The samples of circular shape with 4 mm in diameter and 1 mm thickness for DMA were cut from the cast polymer films prepared as described above. Rheology The rheological properties were analyzed using rotational rheometer PHYSICA MCR-502 (Anton Paar, Austria) with parallel plate configuration. Samples of circular shape with 10 mm in diameter and 1 mm thickness were cut from the cast polymer film prepared as described above. The gap was adjusted at 1 mm. In order to avoid degradation of the samples using elevated temperatures all measurements were performed under nitrogen atmosphere. The linear viscoelastic region was established at 1 % strain () value. The storage and loss moduli were measured as a function of the frequency from 0.1 to 10 Hz starting at lowest frequency.
Photoactuation measurements S4
The photoactuation of materials was tested by thermomechanical analyser TMA (Mettler Toledo, Switzerland) at room temperature. Samples in the form of the stripes with dimensions of 1331 mm were cut from the cast polymer films prepared as described above. The measuring load was established at 0.05N. The samples within 2 hours reach equilibrium corresponding to the pre-strain of 7.4 %, then they were irradiated by red light emitting diode (LED; Luxeon Rebell, Philips) with emitting wavelength of 627 nm at various light power (1.5 mW, 2.4 mW; 6.6 mW, 18.2 mW), whereas the distance between the sample and the light source was settled to 8 mm. However, for the measurement performed at 1.5 mW, there can be seen a decrease of the baseline during the first 10 on-off cycles (Figure 4) due to the residual unrelaxed polymer chains which relax after addition of the additional energy from the diode. Furthermore, baseline was negligibly changed within all other measurements in comparison to absolute measured values of actuation. All measurements were performed at ambient temperature.
Figure S1 TEM image of 1 wt. % MWCNT-g-PBA-b-PMMA/PMMA-b-PBA-b-PMMA nanocomposite
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(a)
(b) Before annealing After annealing
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before annealing after annealing
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10-1
10-1 -2
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10-2 10-3
10-3 0,01
0,1
(c)
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102 10
Intensity
q, A
-1
0,0196 A
0,0518 A-1
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10-2 10-3
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0,0343 A-1
100 10
q, A-1
(a) matrix (b) composite
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0.0188 A-1 0.0312 A-1 0.0539 A-1
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q, A
Figure S2 SAXS diffractograms of a) PMMA-b-PBA-b-PMMA copolymer b) 1 wt. % MWCNT-gPBA-b-PMMA/PMMA-b-PBA-b-PMMA nanocomposite measured before and after annealing at 150 °C for 60 min. Figure c) shows overlapped spectra of a) and b) with marked peak maxima (the corresponding periodic distances can be calculated as d = 2π/q); the curves corresponding to samples after annealing are denoted by lighter colors (light gray, light red).
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Table S1. Theoretical vs. experimental peak positions in SAXS diffractogram of the nanocomposite (1 wt. % MWCNT-g-PBA-b-PMMA/PMMA-b-PBA-b-PMMA; Fig. S2c, red line). The peak positions suggest hexagonally packed cylindrical structure. The second and third peaks were broad, low and partially merged, which resulted in slight shift in the 2nd peak position and indefinite 3rd peak position. The 4th peak position was in perfect agreement with theoretical prediction. The peak positions are given in 1/Å; q* denotes the position of the first peak. HPC peaks Theoretical peak position ratios Experimentally observed peak positions (q, 1/Å) Experimentally observed peak ratios (q/q*)
Peak 1
Peak 2
Peak 3
Peak 4
1
3 = 1.73
4 = 2.00
7 = 2.64
0.0196
0.0343
low
0.0518
1
1.75
low
2.64
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500
Matrix 10s Matrix 30s Nanocomposite 10s Nanocomposite 30s
L (m)
400 300 200 100 0
0
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Light power (mW)
Figure S3 Comparison of actuation length change for matrix and nanocomposite irradiated with red diode at various diode power (1.5 mW, 2.4 mW, 6.6 mW, 18.5 mW) for 10s and 30s at 0.05 N measuring load.
REFERENCES 1. Ilčíková, M.; Mrlík, M.; Sedláček, T.; Chorvát, D.; Krupa, I.; Šlouf, M.; Koynov, K.; Mosnáček, J., Polymer 2013, 55, 211.
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