Supporting Information for Poly(vinylidene fluoride-co-hexafluoropropylene)-graft-Poly(dopamine methacrylamide) copolymers: a nonlinear dielectric material for high energy density storage Mojtaba Rahimabady1,2, Li Qun Xu3, Saeid Arabnejad1,2, Kui Yao1, a), Li Lu2, Victor P.W. Shim2, Koon Gee Neoh3, En-Tang Kang3 1 Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), Research Link, 117602 (Singapore) E-mail: ([email protected]) 2 Department of Mechanical Engineering, National University of Singapore, 21 Lower Kent Ridge Road, 119260 (Singapore) 3 Department of Chemical & Biomolecular Engineering, National University of Singapore, 21 Lower Kent Ridge Road, 119260 (Singapore). Thermally-Initiated Graft Copolymerization of DMA from the Ozone-Pretreated P(VDFHFP)- the P(VDF-HFP)-g-PDMA Copolymer. The P(VDF-HFP) powders were dissolved in NMP to form a 7 wt% solution. A continuous stream of O3/O2 mixture at 300 L/h (generated from an Azcozon RMU16-40EM ozone generator) was bubbled through 30 mL of the polymer solution at room temperature for about 15 min. After the ozone preactivation, the solution was cooled in an ice bath and degased with argon for about 60 min to remove the dissolved ozone and oxygen. DMA monomer (1.5 g, 6.8 mmol) and NMP (12 mL) were introduced into the NMP solution of ozone preactivated P(VDF-HFP). After an additional 30 min of argon purging, the reaction mixture was sealed under an argon atmosphere and stirred at 60 °C in an oil bath for 6 h to complete the graft copolymerization. The P(VDF-HFP)-gPDMA copolymers with lower PDMA grafting densities were also synthesized with a a ) Corresponding author. Tel: 65-68745160; E-mail address: [email protected] polymerization time of 1 h and 3 h, but they were not discussed here with corresponding lower energy density than 6 h. The reaction mixture was subsequently cooled in an ice bath and precipitated into ethanol (500 mL). The resulting P(VDF-HFP)-g-PDMA copolymer was removed by filtration and purified by re-dissolving in NMP and re-precipitating in ethanol. The re-dissolving/re-precipitation process was repeated three times to completely remove the unreacted monomers and the oligomers. The P(VDF-HFP)-g-PDMA copolymer was obtained by drying under reduced pressure for 24 h. Preparation of copolymer thin films. The casting solution was prepared by dissolving the P(VDF-HFP)-g-PDMA copolymer (10 wt%) in dimethylformamide (DMF). Thin copolymer film with a thickness of about 5.5 µm was deposited by spin coating the copolymer solution on an Au-coated silicon substrate and subsequently dried at 80 oC. The dried film was subsequently annealed at 140 oC to further improve the crystallinity. In order to prevent the formation of micro cracks, an uniaxial pressure of 100 MPa was applied to the film at elevated temperature. After this hot-pressing process, a patterned platinum or gold electrode array (each 3 mm in diameter and 150 nm in thickness) was deposited on the film by sputtering through a shadow mask to serve as the top electrodes in Pt/copolymer/Au sandwich devices. Material and device characterization.The graft copolymer structure was confirmed by 1H nuclear magnetic resonance (1H NMR) spectroscopy. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos AXIS Ultra HSA spectrometer equipped with a monochromatized Al Kα X-ray source (1468.6 eV photons). Thermogravimetric analysis (TGA) was carried out on a thermogravimetric analyzer (TA Instrument, Model 2050) at a heating rate of 10 oC/min in nitrogen. Differential scanning calorimetry (DSC) measurements were carried out on a Mettler Toledo DSC 822e in a nitrogen atmosphere from 100 to 200 °C, at a heating rate of 10 °C min−1. The crystalline structure analyses were performed at room temperature using an x-ray diffraction system (D8-ADVANCE, Bruker AXS GmbH, Karlsruhe, Germany). Dielectric constant of the polymer film was measured with an impedance analyzer (Agilent 4294 A). Polarization–electric field hysteresis loops were studied with a standard ferroelectric testing unit (Precision Premier II, Radiant Technologies, USA), connected to a high voltage source. A High Voltage Amplifier System (Trek Model 610D) was used for electrical breakdown testing at DC mode and at 0.5 kV/s ramp rate. Figure S1 shows the respective X-ray photoelectron spectroscopy (XPS) wide-scan spectra of the P(VDF-HFP) and P(VDF-HFP)-g-PDMA copolymers. Figure S1. XPS wide-scan spectra of the P(VDF-HFP) and P(VDF-HFP)-g-PDMA copolymers. In the case of P(VDF-HFP) copolymer, only C and F signals are observed in the wide-scan spectrum. For the P(VDF-HFP)-g-PDMA copolymer, the appearance of distinctive N 1s and O 1s signals in the XPS wide-scan spectrum of the P(VDF-HFP)-g-PDMA copolymer indicates that PDMA has been successfully grafted to the P(VDF-HFP) main chains. From the XPS C 1s and N 1s peak component area ratio of the P(VDF-HFP)-g-PDMA copolymer, the content of PDMA segments in the P(VDF-HFP)-g-PDMA copolymer is determined to be about 5.9 wt%. The graft copolymerization of DMA could be terminated after several hours, due to the fast rate of radical polymerization and the exhaustion of free radicals. Although the DMA monomer was fed with a theoretical grafting percentage of 41.7 wt%, the P(VDFHFP)-g-PDMA copolymer only reached a maximum grafting percentage of 5.9 wt%. Figure S2 shows DSC thermograms of PDMA, P(VDF-HFP) and P(VDF-HFP)-g-PDMA. The melting temperatures (Tm) of PDMA, P(VDF-HFP) and P(VDF-HFP)-g-PDMA copolymer occur at about 112 °C, 159 °C and 160 °C, respectively. After graft polymerization of DMA, the P(VDF-HFP)-g-PDMA copolymer did not exhibit a noticeable melting point depression. Thermal analysis, using DSC, was performed not only to observe the change in Tm, but also to determine the degree of crystallinity. The relative degree of crystallinity of a polymer (XC) can be estimated using the equation: Xc H m 100% H o (1) where ∆ is the enthalpy of fusion for the polymer and ∆ (104.7 J g-1) is the enthalpy of fusion for 100% crystalline P(VDF-HFP). The relative degrees of crystallinity for the P(VDF-HFP) and P(VDF-HFP)-g-PDMA copolymer are found to be 29.7% and 26.5%, respectively. Figure S2. DSC curves of PDMA, and the P(VDF-HFP) and P(VDF-HFP)-g-PDMA copolymers. The DSC results indicate that the P(VDF-HFP)-g-PDMA copolymer was slightly more amorphous than P(VDF-HFP), as the structural symmetry of P(VDF-HFP) was partially perturbed by the introduction of PDMA side chains. The theromogravimetric analysis (TGA) curves of P(VDF-HFP) and P(VDF-HFP)-gPDMA copolymers are shown in Figure S3. The weight loss of P(VDF-HFP) commences at about 423°C, which can be attributed to the decomposition of P(VDF-HFP) units by random scission. For the P(VDF-HFP)-g-PDMA copolymer, the onset decomposing temperature is at about 355 °C. The lower thermal stability of P(VDF-HFP)-g-PDMA copolymer is due to degradation and random scission of the PDMA side chains. Intensity (Arb. Unit) Figure S3. TGA curves of the P(VDF-HFP) and P(VDF-HFP)-g-PDMA copolymers. 5 10 15 20 25 30 2Degree) Figure S4. XRD patterns for P(VDF-HFP)-g-PDMA copolymer film. It is known that P(VDF-HFP) used in this study has a random distribution of VDF and HFP group in its linear structure. In this regard, a polymer chain with degree of polymerization (DP) of 1700 is generated with random distribution of the two monomers using "Materials Studio 5.5" (a complete modeling and simulation environment design allowing users to predict and understand the material’s atomic/molecular structure- property/behavior relationships). The resulting chain model was then covalently bonded to eight PDMA side chains, each containing 6 DMA units (Figure S5). Figure S5. Atomic structure of a randomly generated P(VDF-HFP) copolymer chain, grafted with PDMA side chains. The grafting process will finally result in the addition of 96 hydroxyl groups to each copolymer. The grafted chain is introduced into a periodic simulation box subsequently in the random–walk method with energy minimized in each step, resulting in an amorphous structure with a density comparable to its experimental value (1.77 g/cm3). The amount of hydroxyl groups in the periodic cell is calculated and a concentration of 2.6×10-3 (mole OH)/cm3 is obtained. It is believed that the major improvement in dipolar moment of polymer after grafting arises from these dipolar groups. Figure S6. Unipolar hysteresis loops of the P(VDF-HFP) and P(VDF-HFP)-g-PDMA copolymers at 700 MV/m. The dielectric breakdown values were obtained from Weibull analysis. For each polymer film, at least 20 measurements were conducted to ensure consistency and reproducibility. Weibull cumulative distribution functions: x F ( x) 1 exp( ) (2) was used. The term x is the measured electrical breakdown strength. The parameter α, known as the scale parameter, is the electric field at which at least 63.2% of the samples were observed to fail. The parameter β, known as the shape parameter, evaluates the scattering of the data. A large β corresponds to a narrow distribution of the breakdown field. As shown in Figure S7, the graft copolymer film has α and β parameters of 859 and 8.3, respectively. Figure S7. Weibull distribution of the breakdown field of P(VDF-HFP)-g-PDMA films. Figure S8. Contact angles of the DMF solutions of (a) P(VDF-HFP)-g-PDMA and (b) P(VDF-HFP) with Au-coated Si substrate.