Styrene monomer (Sigma Aldrich, 99.9%, CAS 100-42-5), N,N,N′,N′′,N′′ pentamethyldiethylenetriamine (PMDETA, Sigma Aldrich, 99%, CAS 3030-47-5), methyl 2-bromopropionate (2-MBP, Sigma Aldrich, 98%, CAS 5445-17-0) were used as received. Tert-butyl methacrylate (tBMA, Sigma Aldrich, 98%, 200 ppm monomethyl ether hydroquinone as inhibitor, CAS 585-07-9) and glycidyl methacrylate (Sigma Aldrich, 97%, 100 ppm monomethyl ether hydroquinone as inhibitor, CAS 106-91-2) were passed through a basic alumina column and stored under nitrogen before use. Hydrochloric acid (HCl, Sigma Aldrich, 37%, CAS 7647-01-0), toluene (Sigma Aldrich, anhydrous, 99.8%, CAS 108-88-3), methanol (MeOH, Sigma Aldrich, anhydrous, 99.8%, CAS 67-56-1), tetrahydrofuran (THF, Sigma Aldrich, anhydrous, 99.9%, CAS 109-99-9), anisole (Sigma Aldrich, anhydrous, 99.7%, CAS 100-66-3), ethanol (EtOH, Sigma Aldrich, reagent grade, CAS 64-17-5), ethyl acetate (EtOAc, Sigma Aldrich, anhydrous, 99.8%, CAS 141-78-6), hexane (HEX, Sigma Aldrich, anhydrous, 95%, CAS 110-54-3) glacial acetic acid (Sigma Aldrich, natural, 99.5%, CAS 64-19-7), diethyl ether (Sigma Aldrich, anhydrous, 99.7%, CAS 60-29-7), 1,4-dioxane (Sigma Aldrich, anhydrous, 99.8%, CAS 123-91-1), dichloromethane (DCM, Sigma Aldrich, anhydrous, 99.8%), chloroform-d (Sigma Aldrich, 99.8 atom % D, CAS 865-49-6), trifluoroacetic acid (TFA, Sigma Aldrich, 99%), sodium carbonate (Sigma Aldrich, 99%), aluminium oxide (Alumina, Sigma Aldrich, neutral and basic, CAS 1344-28-1), ammonium hydroxide (NH4OH, Sigma Aldrich, CAS 1336-21-6), multi-walled carbon nanotubes C-150-P (Bayern Material Science, diameter 6–9 nm, length 500 nm, 95%) were used as received. CuBr and CuCl catalysts were stirred in glacial acetic acid at room temperature for 6 h. After a given time, the grey-powdery solid was collected on filter paper using a Buchner apparatus, then washed three times with several mL of acetic acid, ethanol and ethyl acetate and dried under vacuum for 15 h and stored at −16 °C. 1-pyrenemethylamine hydrochloride was converted to the primary amine through a neutralization process with NH4(OH) followed by liquid-liquid extraction. Methanol was used as the solvent and toluene as the extractor: a bright yellow solid was recovered after drying in rotary evaporator.
2.3. Nanocomposite and VOC Exposure Setup Preparation
About 20 mg of AMP-TP were added to 1.5 mL of chloroform into a vial. To facilitate polymer dissolution each sample was ultrasonicated for 10 min at 400 W and 24 kHz with a probe sonicator model UP 400 S by Hielscher Ultrasound. An H3 probe with titanium tip of 3 mm diameter and 100 mm length was used. An ice bath was used to prevent solvent evaporation during sonication. Then, the required amount of MWCNT was poured into the vial and ultrasonicated a second time for 10 min, with the same procedure mentioned above. Eventually, the solution was left to stand for 24 hours. The mixture was then centrifuged with an IEC (model CWS 4236, ThermoFisher Scientific, Hillsboro, OR, USA) machine for 1 h, 4500 rpm at room temperature. The supernatant liquid was then recovered with a Pasteur pipette and eventually transferred into a test tube and sealed.
VOC sensors were fabricated by casting the dispersion onto gold electrodes supported on an integrated device provided by Cad Line (Pisa, Italy). The device is composed of glass fibres woven into an epoxide resin, which grants chemical inertia. In each case, the dispersion was left to evaporate under a fume hood and then dried under vacuum in a Schlenk tube for 4–6 h. This step was repeated between each test to ensure complete solvent removal. The solid dispersions were connected to a digital multimeter (KEITHLEY 2010, Tektronix, Beaverton, OR, USA) and the measured resistances were obtained as a mean from one hundred measurements as allowed by the multimeter settings. The percolation threshold of the composite was assessed using dispersions with different wt.% MWCNTs content. The resistance response of the MWCNT-AMP-TP composite to VOCs was tested with an apparatus built in the laboratory. Sealing of the chambers was allowed by rubber stripes glued to the movable door. Two small holes on backside and top of the chamber permitted the connection wires to pass through the wall (backside) and the solvent to be dropped inside (top). The device bearing the dispersion was stuck to the inside back wall, with the circuit exposed to the interior space. The room volume was set to 4.6 L in all measurements. In the kinetic measurement, the deposition was exposed to a large excess of organic solvent, until complete saturation of the chamber and then resistance values were taken every five minutes for 1 h. A control measurement was performed, prior to every experiment, by measuring the resistance without the presence of solvent. For each deposition three solvent were tested: THF, CHCl3 and Hexane. In the first sets of experiments, the desired amount, usually 200 mL, was put in a beaker and placed inside the closed chamber to test the response over time in a saturated environment. To account for the sensitivity of the devices, dispersions were exposed to an increasing amount of solvent. Around 23 μL of solvent (equal to 5 ppm to the chamber volume) was dropped from the top hole using a Gilson pipette and quickly the hole was closed with a rubber septum. Resistance values were taken every minute for a total of twenty and afterwards a new addition of 5 ppm was done until 100 ppm was reached.
2.4. Characterization and Instruments
Proton nuclear magnetic resonance (1H-NMR) spectra were recorded using a Varian Mercury Plus 400 MHz spectrometer (Varian Inc, Palo Alto, CA, USA).
Fourier transform infrared spectroscopy (FT-IR) spectra were recorded with a Perkin Elmer Spectrum 2000, in Attenuated Total Reflection (ATR) mode.
UV-Vis absorbance spectra were recorded with a Perkin Elmer Lambda 650. This analysis was used to estimate the amount of 1-AMP reacted with the terpolymer. Dilute solutions of free 1-AMP were prepared at different molar concentrations ranging from 10−3 to 10−7. A calibration curve was built by plotting the absorbance value at 345 nm versus molarity (mol/L) for each solution; eventually a linear behaviour was fitted. The molar amount of 1-AMP reacted was calculated by putting the registered absorbance value (345 nm) into the calibration curve by extrapolating the related molarity value. The conversion level of the GMA epoxide group was evaluated by subtracting the moles of 1-AMP reacted to the initial moles of GMA present in the polymer.
Fluorescence measurements were collected using Fluorolog Horiba Jobin Yvon spectrophotometer (Horiba Jobin Yvon, Kyoto, Japan) equipped with a 450 W xenon arc lamp and single and double-grating excitation and emission monochromators, respectively.
Thermal degradation of the materials and functionalized amount of MWCNT in the AMP-functionalized polymers were analysed via thermogravimetric analysis (TGA) with a TA Q 5000 instrument (TA Instruments, New Castle, DE, USA) under nitrogen flux. All samples were tested in the temperature range of 25 °C to 700 °C with a scan rate of 10 °C/min.
Differential Scanning Calorimetry (DSC) was used to determine the glass transition temperature of some TP and AMP-TP. A TA-Instruments Q1000 DSC system was used for these measurements under a nitrogen flux. Each sample was firstly heated from 20 °C to 150 °C and backwards in order to remove the thermal history of the polymer, at a rate of 10 °C/min.
The viscoelastic behaviour of some hydrolysed polymers in water solution was evaluated via rheology measurements. Dynamic viscosity response to different shear rate was tested at room temperature. Temperature sweep tests were done at constant stress to determine the viscosity response to temperature in the range of 20 °C to 90 °C. Oscillation frequency sweep tests were done at a constant stress to establish the regime of viscoelastic response. Instrument Haake Mars III rotational rheometer was used to perform the test.
Gel permeation chromatography (GPC) measurements were carried out with a HP1100 machine (Agilent Technologies, Waldbronn, Germany) from Hewlett Packard equipped with three 300 mm × 7.5 mm PLgel 3 μm MIXED-E columns in series equipped with a GBC LC 1240 RI (refractive index) detector (GBC Scientific Equipment Pty Ltd, Victoria, Australia). The samples were eluted with THF at a rate of 1 mL/min, at 140 bar of pressure and 40 °C. Molecular weights and PI were determined using the software PSS WinGPC Unity from Polymer Standard Service. Polystyrene standards were used for calibration.
Scanning electron microscope (SEM) analysis was performed using a SEM with environmental mode FEI Quanta 450 ESEM FEG (ThermoFisher scientific, Hillsboro, OR, USA) with an accelerating voltage of 30 kV. The MWCNTs/polymer samples were ultrasonically dispersed in chloroform for analysis. The suspensions were deposited on a gold-coated silicon wafer and allowed to dry in a vacuum system overnight. The wafer was then mounted onto a stainless steel sample holder using carbon tape.