Synergistic Effect of Hydrogen Bonds and Diffusion on the Î²

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General Research

Synergistic Effect of Hydrogen Bonds and Diffusion on the #-Crystallization of Poly(vinylidene fluoride) on Poly(methyl methacrylate) Interface Ce Mi, Zhongjie Ren, Huihui Li, Shouke Yan, and Xiaoli Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05545 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 14, 2019

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Industrial & Engineering Chemistry Research

Synergistic Effect of Hydrogen Bonds and Diffusion on the -Crystallization of Poly(vinylidene fluoride) on Poly(methyl methacrylate) Interface Ce Mi, Zhongjie Ren, Huihui Li, Shouke Yan,* Xiaoli Sun* † Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China

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Abstract The PVDF thin film was prepared on PMMA sublayer. Compared to the PVDF on Si wafer, the PMMA layer shows great influence on the crystallization of PVDF under quenching thermal treatment. When PMMA layer is thinner than 90 nm, mixture of - and -crystals form. Pure -crystals are obtained when the PMMA layer is around 90 nm. With the further thickening of PMMA layer, pure -crystals always form but with a decreased crystallinity. The nano-domains and needlelike morphology of -crystals is observed. Moreover the formation mechanism of -phase on PMMA interface is clarified. The hydrogen bonds at PVDF and PMMA interface is formed adopting the perpendicular orientation, which serve as the key role to induce -PVDF. The amount of -PVDF is not only determined by the interaction of PMMA and PVDF at interface, but also the diffusion extent of PMMA into PVDF layer.


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1 Introduction Electroactive polymer materials have aroused heated discussion in recent years. Among these materials poly(vinylidene fluoride) (PVDF) has been extensively investigated due to its good electroactive response, including piezoelectricity, pyroelectricity and ferroelectricity.1-5 PVDF is a polymorphic semicrystalline polymer, which has at least four kinds of crystal modification: the nonpolar -phase, and the polar -, -, -phases.6-8 All of these phases have specific structures that contribute to their unique properties, among which -phase has attracted most attention.9-13 It has been well reported that -PVDF possesses an orthorhombic unit cell packed with all trans planar zigzag molecular chains (TTT). The spontaneous polarization of -PVDF allows its application in many high-tech fields, such as sensors, transducers, ferroelectric memories and so on. Tremendous efforts have been devoted to gain as much content as -PVDF, including stretching mechanism, pressure quenching at high temperature, non-isothermal crystallization at ultra-high cooling rate, application of high electric field, copolymerization and making composites by doping the polymer with fillers.2,14-17 Among the various methods to obtain -PVDF, employing poly(methyl methacrylate) (PMMA) to induce the formation of -crystals is the simplest method.18-20 Large amounts of -phase can be obtained after quenching the miscible blends of PVDF/PMMA from the melt to ice water and further annealing. 17,21

The commodity of ferroelectric PVDF/PMMA blends outperform the commonly employed specialty

copolymer poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) at high temperature.4,17 It is generally accepted that the PMMA retards the crystallization of PVDF. For blends comprising < 10 wt％ PMMA, mixture of - and -phase is obtained. Neat -PVDF can be obtained when PMMA content is around 20％. When PMMA content is above 40 wt％, there are no strong characteristic bands of either - or -phase, because of the low crystallinity of PVDF and imperfect crystal structure.17,22-23 The mechanism that PMMA induces the formation of -phase PVDF is not clear now. Song et al explained that 3 ACS Paragon Plus Environment


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high-rate quenching results in nucleation of the -phase.24 Li et al thought the reason might be related to the difference in crystal structures.17 For steric reasons the dipole/dipole interactions favor the transconformation of the highly polar -PVDF polymorph over the trans-gauche conformation of the nonpolar -PVDF. There are many requirements when achieving piezoelectric- or ferroelectric-based devices based on PVDF. Obtaining the polar phase of PVDF is the prerequisite. Polar phase should be also achieved with proper polarization direction. Taking the uniaxial mechanical drawing β-PVDF as an example, c axis is preferentially parallel to the draw direction whereas b axis (parallel to molecular dipole) is randomly perpendicular to the draw direction. When the external electric field is applied perpendicular to the film surface, b axis is able to rotate around drawing direction and adopt the orientation parallel to the direction of external electric field. 25-26 The -phase generally adopt random orientation in the bulk PVDF/PMMA blends film which may cause the fabricated devices with poor properties due to the worse polarization direction. Thus the strategy can be put forward to prepare PMMA and PVDF as a bilayer structure. The planar interface is expected to have better orientation of PVDF molecular chains than that of bulk heterojunction. Moreover blend films suffer from larger interfacial area between two phases hence loss of crystallinity at higher PMMA fraction. By contrast, bilayer structures with planar interface produce a gradient of crystallinity depending on the diffusion extent of PMMA into PVDF layers. Up to now, the crystallization behavior of PVDF in bilayers has not been paid attention. The study on this aspect is of special importance for interesting perspectives of the technological applications. In the present work, the crystallization behavior of PVDF on PMMA sublayer with different thickness is studied. It is found that the crystal modification and morphology of PVDF depend strongly on the thickness of PMMA layer. The nano-grain and needlelike morphologies of -crystals are observed and perpendicularly oriented hydrogen bonds between PVDF and PMMA are clearly verified. According to the obtained results, the role of hydrogen bonds and diffusion on the formation of -crystals are discussed. 4 ACS Paragon Plus Environment

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2 Experimental section 2.1 Material and preparation procedures The PVDF (Mw=530,000 g/mol) and PMMA (Mw=47,550 g/mol) were purchased from Aldrich Co. N, N-dimethylformamide (DMF) was used to dissolve PVDF pellets at 90 oC for 8 h and chloroform (CHCl3) was used to dissolve PMMA powder at 50 oC for 8 h. For sample of PVDF on Si, the uniform PVDF thin film was obtained by spin-coating its DMF solution with a concentration of 5 wt％ onto Si wafer at 90 oC and then dried in a vacuum oven at 90 oC for 12 h. For sample of PVDF on PMMA bilayers (PVDF/PMMA/Si), a uniform PVDF film was firstly spin-coated onto glass substrate and then peeled off from glass by using hydrofluoric acid (HF) solution. The PVDF film was transferred onto water surface to remove residual HF and then transferred onto Si wafer covered by a PMMA layer. Finally the sample was dried in a vacuum oven at 60 oC for 6 h. Samples with PVDF sandwiched between two PMMA layers was obtained by spin-coating PMMA solution with a certain concentration onto the surface of PVDF/PMMA/Si. For all the samples, the PVDF layer thickness is kept at 210 nm. 2.2 Measurement Transmitted infrared (transmitted IR) spectra and reﬂection-absorption IR (RAIR) spectra were obtained from a Perkin-Elmer Spectrum 100 spectrometer with resolution of 4 cm-1. For the RAIR measurement, a reflection attachment (Spectra-Tech, FT80 RAS) was employed at an incident angle of 80o together with a rotatable wire grid polarizer of KRS5-substrate (ST Japan). The crystalline structure of the PVDF films was investigated via grazing incident X-ray diffraction (GIXRD). GIXRD measurements were performed using monochromatic X-ray wavelength 1.54 Å at the Beijing Synchrotron Radiation Facility, Beijing, China. The film thickness was measured with a Dektak profilometer. The surface morphology of the films was characterized by an Agilent Technologies 5500 atomic force microscope (AFM) (Agilent Technologies Co. Ltd, U.S.). The scan rate varied from 0.5 to 1.0 Hz. The XPS



measurements were performed with Thermo Scientific Escalab 250Xi spectrometer using an Al K(alpha) X-ray source.

3 Results and discussion Figure 1a shows the transmitted IR spectra of as cast PVDF thin film on Si wafer, which was firstly heat-treated at 200 oC for 7 min and then quenched immediately into ice water for 30 min and then annealed at 25 oC for 2 h (melt-quenching treatment). The characteristic adsorption peaks for -PVDF crystals at 763, 796, 975 cm−1 and the peaks for -crystals at 840 and 1280 cm−1 are observed in the as cast film suggesting that both - and -PVDF crystals form during the solvent evaporation process.27-28 After melt-quenching treatment, the strong characteristic absorption peaks for -PVDF crystals are evidenced and the intensity of the peaks at 840 and 1280 cm−1 for -PVDF crystals weakens significantly. Thus, for single PVDF layer, -phase is the predominant crystal phase and melt-quenching treatment does not favor the formation of -crystals.

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Figure 1. (a) The transmitted IR spectra of samples: as cast PVDF thin film and PVDF film after meltquenching treatment. (b) The transmitted IR spectra of neat PVDF and PVDF on PMMA sublayer with different thickness of PMMA layer after melt-quenching treatment. (c) The band intensity at 840 and 763 cm-1 as a function of PMMA thickness. (d) GIXRD patterns of neat PVDF and PVDF on PMMA sublayer with different thickness of PMMA layer after melt-quenching treatment. (e) The degree of crystallinity as a function of the thickness of PMMA layer Figure 1b shows the spectra of neat PVDF and PVDF/PMMA/Si layered samples with different thickness of PMMA layer after melt-quenching treatment. The spectra intensity is calibrated by the internal standard peak of PVDF (877 cm−1) to eliminate the intensity-difference caused by change of thickness. On 30 nm-thick PMMA layer, the bands at 840 and 763 cm-1 are both observed indicating that mixtures of - and -crystals. When the PMMA layer thickens to 50 nm, predominant -crystals form and only a 7 ACS Paragon Plus Environment


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small amount of -crystals are observed. The band intensity at 840 and 763 cm-1 as a function of PMMA thickness is shown in Figure 1c. When the PMMA layer thickness increases to 70 nm, nearly pure crystals form. With the further thickening of the PMMA layer, -crystals increase first gradually and then decrease. If the PMMA layer is 192 nm, -crystals are less than half of the maximum value. An optimum PMMA layer thickness value for promoting the formation of pure -crystals with the highest crystallization degree is found to be around 90 nm. The dependence of crystallization behavior of PVDF on PMMA layer can be further proved by GIXRD results (see Figure 1d). On thin PMMA layer, e.g. 30 nm, not only (020) and (110) diffraction peaks of -crystals are observed. The (200) diffraction peak of -crystals showing up as a shoulder is also evidenced.29 Only (200) diffraction peak of -crystals is observed when the PMMA layer is 90 nm indicating the formation of pure -crystals. GIXRD results are consist with IR results implying IR characterization method can provide reliable information on the crystal modification of PVDF. The calculated degree of crystallinity is shown below in Figure 1e. With thickening of PMMA sublayer, the degree of crystallinity of PVDF decreases from 44.1 % for neat PVDF to 3.8% for PVDF on 192nm-PMMA sublayer. As for the sample with optimum thickness of PMMA layer (90 nm), the degree of crystallinity is 10.0 %. To further improve the degree of crystallinity, the annealing temperature is changed from 25 oC to 140 oC and the GIXRD patterns are shown in Figure 2 (a). It proves that the increase of annealing temperature doesn’t affect the crystal form of PVDF when annealing temperature is below the melting point of PVDF. But the degree of crystallinity or PVDF is enhanced obviously from 10.0 % at 25 oC to 24.9 % at 140 oC, as can be seen in Figure 2b. Therefore it is an effective way to improve the degree of crystallinity of double-layer samples by increasing the annealing temperature. But in order to focus on the formation mechanism of -PVDF, the annealing temperature and time is separately fixed at 25 oC and 2 h in the following work. It is noted that 2 h is long enough to complete the crystallization of  crystals.


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Figure 2. (a) GIXRD patterns of PVDF on 90 nm-PMMA sublayer after melt-quenching treatment under different annealing temperature for 2h. (b) The degree of crystallinity as a function of the annealing temperature.

Figure 3. AFM images (1 μm × 1 μm) of samples of PVDF on PMMA sublayer (a) 30 nm; (b) 90 nm; (c) 192 nm after melt-quenching treatment. The morphologies of PVDF crystals on PMMA sublayer with PMMA thicknesses of 30 nm, 90 nm and 192 nm are studied by AFM. As shown in Figure 3a, the sample with thinner PMMA sublayer (30 nm) shows obvious -spherulites with edge-on lamellae as indicated by the dotted circle. Also some nanometer scale crystal domains (ca. 13 nm) are observed. They are -PVDF crystals as proved by a combined technique of IR-AFM characterizations (see Figure S1 in supporting information). For the sample with 9 ACS Paragon Plus Environment


PMMA thickness of 90 nm (Figure 3b), a large amount of nano-crystals are observed with a size of ca. 20 nm, further suggesting the -crystals appear as nano-domains. For the sample with thicker PMMA sublayer (192 nm), AFM image (Figure 3c) displays a characteristic needlelike -crystals with the length and width of ca. 110 and ca. 20 nm, respectively. It is noted here that the needlelike morphology of crystal are similar to the previous work.12

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Figure 4. The transmitted IR spectra (a) and RAIR spectra (b) in the stretching bond region of C=O groups for the bilayer sample of PVDF on PMMA (90 nm-thick). (c) The schematic of RAIR measurement mode. (d) The sketch for the interaction of PVDF and PMMA. The reason that PMMA induces the formation of -crystals may be induced by the hydrogen bonds between C=O groups in PMMA and CH2 groups in PVDF.17, 30 To get a deeper insight, the transmitted IR spectra in the stretching bond region of C=O groups for the bilayer sample of PVDF on PMMA (90


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nm) are obtained (see Figure 4a). If hydrogen bonds formed at the interface after melt-quenching treatment, there should be a clear stretching bond of C=O groups shift at around 1730 cm-1 to lower frequency.31 Unfortunately, no clear shift of this band is observed in the sample after quenching from melt. This result may reflect that no hydrogen bonds are form at all or at least the fraction of C=O groups associated with PVDF is too small to be detected by IR. Another possibility is that the formed hydrogen bonds adopt an orientation perpendicular to the surface, which also does not contribute IR absorption. Therefore, RAIR is employed to check the orientation of hydrogen bonds and the spectra are shown in Figure 4b. In the RAIR mode, the spectra shows remarkable shift of the C=O stretching band from 1739 to 1732 cm−1 after melt-quenching treatment. The difference of wavenumber about the C=O stretching band for as cast films under two kinds of modes can be attributed to the different measurement mode mechanism. Under RAIR mode, p-polarized IR light with grazing incidence angle is required and only the vibrations with the transition moment perpendicular to the substrate will be enhanced in p-polarized RAIR spectra (as can be seen from Figure 4c). The shifted C=O stretching bond to lower frequency for the sample after quenching is only observed under RAIR mode implies that C=O groups interacting with PVDF are perpendicular to the substrate under thermal treatment. The perpendicular molecular interaction between PVDF and PMMA leads to the formation of orientated -crystals. Since the dipoles of crystals corresponding to the three strongest bands at 886 (a), 1284 (b), and 1408 cm-1 (c) are in orthogonal direction, the “relative intensity” method can be used to determine the lattice orientation. The intensity ratio between 1284 and 1408 cm-1 (D b/c) for the grazing incident reflection mode is much larger than that of transmitted mode (as can be seen in Figure S2 in supporting information). This indicates that the molecular dipole (parallel to b-axis) of -crystals prefer to be perpendicular to the substrate than that that of c-axis. The sketch for the interaction of PVDF and PMMA is seen in Figure 4d. If hydrogen bonds at the interface play a key role on the formation of -crystals, the larger contact area between PMMA and PVDF should lead to more hydrogen bonds, thus there should be more -crystals. 11 ACS Paragon Plus Environment


To verify such hypothesis, the PVDF is sandwiched between two PMMA layers with the thickness of 30 nm. Figure 5a and 5b show the transmitted IR spectra and the fraction of -crystals (F) for different samples. The F in a sample containing - and -PVDF can be calculated by Eq. (1): 𝐹 =

𝐴 𝐾 ( )𝐴 +𝐴 K

(1)

where A and A represents the absorbance at 763 and 840 cm−1; K and K are the absorption coefficients at the respective wavenumber, whose values are 6.1 × 104 and 7.7 × 104 cm2 mol−1, respectively.32 F changes from 69.6％ in bilayer film to 100％ in trilayer film along with the increased crystallinity. The comparative experiment further suggests that the non-ignorable -crystals exist on the PVDF free surface in bilayer sample due to the weakening effect of PMMA sublayer when extending into interior of PVDF film. Once the free surface is replaced with top PMMA layer, both boundaries with hydrogen bonds control the crystallization of 210 nm thick PVDF leading to the formation of pure -crystals.

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Figure 5. (a) The transmitted IR spectra for different samples after melt-quenching treatment: neat PVDF; PVDF/PMMA (30 nm, simplifying as F on M) bilayer sample; PMMA (30 nm)/PVDF/PMMA (30 nm) trilayer sample (simplifying as M on F on M). (b) The fraction of  crystals (F) for different samples. The C1s spectrum (c) and F1s spectrum (d) for annealed PVDF on Si and on PMMA sublayer with different thickness. (e) The percentage of PMMA diffused into the surface of PVDF layer for different samples. The contact area between PMMA and PVDF is not the only factor to affect the crystallization of PVDF. The dependence of PVDF crystallization behavior on the thickness of PMMA layer implies that interdiffusion between PVDF and PMMA occurs at the interface which further affect the crystallization of PVDF. The chemical composition of PVDF surface is analyzed by XPS. As we know, the PVDF film is intact and the depth measured by XPS is less than 30 nm, so the results can truly reflect the information 13 ACS Paragon Plus Environment


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of just PVDF surface. It can also be proven by the same XPS results for untreated pure PVDF film and untreated PVDF on PMMA bilayers, which means there is no obvious diffusion between PVDF and PMMA for these untreated samples and we can only get the surface information of top layers. Figure 5c shows the C1s spectrum for annealed PVDF on Si and on PMMA sublayer with different thickness. These C(1s) XPS spectra show five types of carbon: carbon bonded to hydrogen (CHx; 284.3 eV), carbon singly bonded to carboxyl (C–CO2–; 284.9 eV), carbon singly bonded to oxygen (–C–O–C=O; 286.1 eV), carbon of carboxyl (O–C=O; 288.3 eV) and carbon singly bonded to fluorine (C–F2; 290 eV). For sample of PVDF on Si, it is noted that the carbon CH2 of PVDF is significantly shifted by the adjacent CF2 group, resulting in a chemical shift of 2.3 eV.33 For those samples of PVDF on PMMA sublayers, there is an obvious shift for the carbon singly bonded to fluorine compared to the sample of PVDF on Si. Same shift can be observed from Figure 5d (the F1s spectrum), which means there is a strong interaction between PVDF and PMMA. Combined with the discussion before in the RAIR characterization, we can attribute that to strong hydrogen bond interaction. Talking about interdiffusion between PVDF and PMMA, the C1s spectra of the surface of PVDF top layer (Figure 5c) show the C1s peaks characteristic of the carbonyl group (284.9 eV and 288.9 eV), indicating that PMMA diffuses into PVDF surface. The relative intensity of the characteristic carbon atoms (284.9 eV and 288.9 eV) has increased compared to the carbon bonded to fluorine (290.6 eV) with the thickening of PMMA sublayer, which means that more PMMA has diffused into PVDF layer with the thickening of PMMA sublayer (relevant peak-differentiating-imitating curve of C1s XPS spectrum for different samples can be seen in Figure S3 in supporting information). The diffusion extent of PMMA into PVDF layer can be quantitatively estimated from the C /F atom ratio.33 The experimental CPVDF/FPVDF ratio is 1.065 for neat PVDF. Thus, CPMMA can be expressed by Eq. (2): ％CPMMA = 100 − 106.5𝐹total /𝐶total

(2)


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The percentage of PMMA on PVDF layer surface calculated by the Eq. (2) are shown in Fig. 5 (e). It is clear that the percentage of PMMA increases significantly with the thickening of PMMA sublayer. There are more PMMA molecular chains diffusing into PVDF layer which form hydrogen bonds and induce the formation of -crystals.

4 Conclusions Prior work has documented that -crystals of PVDF can be fabricated by blending of PVDF and PMMA. However, the mechanism for the formation of -crystals is still not identified and the -crystals adopt random orientation which go against their application in devices. In this study we prepared double layer of PVDF/PMMA films and studied crystallization of PVDF. We found that the crystal modification of PVDF is highly dependent on PMMA film thickness and pure -crystals with highest crystallinity can be obtained when PMMA thickness is proper. Meanwhile the nano-domains and needlelike morphology of -crystals are observed. Moreover the hydrogen bonds at PVDF and PMMA interface adopting the perpendicular orientation are identified. These findings are helpful for clarifying the formation mechanism of -phase. Our results provide guidance for producing oriented ferroelectric PVDF film thorough layer-by-layer methods.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at… AFM images of different samples after melt-quenching treatment and its corresponding FTIR spectra by a combined technique of IR-AFM characterization; the transmitted IR spectra of PVDF/PMMA blend (80/20) and RAIR spectra of PVDF on PMMA (90 nm) bilayer sample after



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melt-quenching treatment; and the relevant peak-differentiating-imitating curve of C1s XPS spectrum for different samples by XPS characterization.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. * E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This study was financially supported by the National Natural Science Foundations of China (No.21274010 & 51221002). A portion of this work is based on the data obtained at 1W1A, BSRF. The authors gratefully acknowledge the assistance of scientists of Diffuse X-ray Scattering Station during the experiments.

References (1) Lovinger, A. J. Ferroelectric polymers. Science 1983, 220, 1115-1121. (2) Fukada, E. History and recent progress in piezoelectric polymers. IEEE Trans. Ultrason. Ferroelect. Freq. Control. 2000, 47, 1277-1290. (3) Charlot, B.; Gauthier, S.; Garraud, A.; Combette, P.; Giani, A. PVDF/PMMA blend pyroelectric thin films. J. Mater. Sci. Mater. El. 2011, 22, 1766-1771. 16 ACS Paragon Plus Environment

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Synergistic Effect of Hydrogen Bonds and Diffusion on the -Crystallization of Poly(vinylidene fluoride) on Poly(methyl methacrylate) Interface

Ce Mi, Zhongjie Ren, Huihui Li, Shouke Yan,* Xiaoli Sun*


Synergistic Effect of Hydrogen Bonds and Diffusion on the Î²

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