朱 慧娥

English

博士学位論文の要旨及び審査結果の要旨 (Summary of Thesis(DR))othe

朱, 慧娥

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Studies on Ferroelectric Poly(vinylidenefluoride) Langmuir-Blodgett Nanofilms Assistedby Polymer Nanosheets著者 朱 慧娥号 59学位授与機関 Tohoku University学位授与番号 工博第5013号URL http://hdl.handle.net/10097/62802           しゅ けいが 氏 名          朱 慧娥 授 与 学 位          博士（工学） 学位授与年月日          平成２６年９月２４日 学位授与の根拠法規 学位規則第4条第1項 研究科，専攻の名称 東北大学大学院工学研究科（博士課程）応用化学 専攻 学 位 論 文 題 目          Studies on Ferroelectric Poly(vinylidene fluoride) Langmuir–Blodgett Nanofilms Assisted by Polymer Nanosheets 指 導 教 員  東北大学教授 三ツ石 方也 論 文 審 査 委 員          主査 東北大学教授 三ツ石 方也 東北大学教授 芥川 智行              東北大学教授 中川 勝  論 文 内 容 要 旨          Poly(vinylidene fluoride) (PVDF) and its copolymers have attracted much attention because of their excellent mechanical and chemical properties, especially their interesting ferroelectric properties. Because of such special properties, their applications to superhydrophobic materials, nonvolatile low-voltage memories, nanogenerators, and even polymer bulk-heterojunction solar cells have been studied extensively. PVDF and its copolymers are semi-crystalline polymers, which have at least five phases: α, β, γ, δ, and ε phases. Of all the phases, the all-trans polar β phase with the largest spontaneous polarization is responsible for its ferroelectricity. For practical applications, PVDF films at nanometer-scale-thick with high-content β phase are expected for low-voltage operation and high-density data storage. However, methods to obtain high content β phase in PVDF nanofilms are rarely reported to date because the crystallinity of PVDF films decreases markedly with the reduced film thickness. In this thesis, a facile method was described to versatile tuning of PVDF crystal forms from complete α phase to complete β phase in PVDF Langmuir monolayers using different spreading solvents (Chapter 2). Preparation and properties of PVDF Langmuir–Blodgett (LB) nanofilms by assistance of a tiny amount of poly(N-dodecylacrylamide) (pDDA) (Fig. 1) were also demonstrated which show ultrahigh content of β crystals and adjustable film thickness by 2.3 nm per monolayer (Chapter 3). Finally, the application properties of as-prepared PVDF LB nanofilms were characterized with high remanent polarization (Pr) values and a low fatigue rate (Chapter 4). In our group, a polymer nanosheet of poly(N-dodecylacrylamide) (pDDA) (Fig. 1), has been built since 1987, which is an excellent amphiphilic polymer. The amide groups in pDDA backbone form well-organized hydrogen bonding network at the air–water interface, which makes the monolayer have unity transfer ratios onto various Fig. 1 Chemical structures of PVDF and pDDA. ? 106 ?substrates and available for preparing polymer nanosheets with monolayer thickness of about 1.7 nm. Taking advantage of the good film properties of pDDA monolayers, a plenty of functional polymer nanosheets have been constructed, which are either copolymers of DDA and functional monomers or mixed pDDA/functional polymers with no amphiphilicity. Those nanosheets exert not only good film properties but also a variety of functionalities, which are applicable for photodevices, electrodevices, optical logic gates, or pH sensors. In Chapter 2, mixtures of ferroelectric poly(vinylidene fluoride) (PVDF) and amphiphilic pDDA were studied at the air–water interface for their monolayer behaviors. Using surface pressure (π)–area (A) isotherms and Brewster angle microscopy (BAM) images, this study examined the formation of various pure PVDF and PVDF/pDDA Langmuir films. The spreading solvents play a key role in the properties of PVDF Langmuir films at the air–water interface, for instance N-methyl pyrrolidone (NMP) and methylethyl ketone (MEK). The PVDF(NMP) molecules at the air–water interface form scalable domains because of the high solubility of NMP in water. A miniscule amount of pDDA is effective in enhancing the properties of PVDF(NMP)/pDDA Langmuir films. The π–A isotherms of PVDF(NMP) and pDDA mixtures at different molar ratios from 1:4 to 50:1 show a series of consecutive and steep curves with much higher collapse surface pressures (> 40 mN/m) than that of pure PVDF (24 mN/m). This result is originated from the ordered hydrogen bonding network formed among pDDA molecules. Such a hydrogen bonding network is also the very reason that the transfer ability was enhanced tremendously in the PVDF(NMP)/pDDA Langmuir films. However, phase separation in PVDF(MEK)/pDDA Langmuir films was found from the two-plateau π–A isotherms as well as the BAM images. This study provides valuable information related to the morphological evolution of semicrystalline PVDF confined in a pDDA two-dimensional geometry at the air–water interface and gives fundamental insight into the influence of solvents on the Langmuir film properties. AFM images of PVDF(NMP)/pDDA and PVDF(MEK)/pDDA LB monolayers are respectively depicted in Figs. 2a and b. The PVDF(NMP)/pDDA LB monolayer has very smooth surface, however, the PVDF(MEK)/pDDA LB monolayer form regular diamond microstructures, analogous to the PVDF α-single crystals. Well-known peaks at 763, 795, and 976 cm-1 in FT-IR spectra are assigned to non-polar α phase, and the β phase is associated with absorptions at 840, 1276, and 1402 cm-1. Then the peak at 1233 cm-1 is characteristic for the γ phase. In the PVDF(NMP)/pDDA LB nanofilms(Fig. 2c), the peaks for β phase at 840, 1276, and 1402 cm-1 are rather sharp, which are absent in the FTIR spectrum of PVDF(MEK)/pDDA LB nanofilms. In contrast, other peaks at 763 and 976 cm-1 for α phase and 1233 cm-1 for γ phase are negligible in the PVDF(NMP)/pDDA LB nanofilms. Precise controlling of PVDF crystal structures in the nanofilms was realized. This study presents very interesting and controllable crystal structures in mixtures of nonamphiphilic PVDF and amphiphilic pDDA at the air–water Fig. 2 (a) FT-IR spectra of PVDF/pDDA LB nanofilms from NMP (black line) and MEK (red line), and AFM images of their LB monolayers (b) from NMP and (c) from MEK. ? 107 ?interface, which are expected to afford much useful information for additional interface manipulation of PVDF crystals.  The excellent property of monolayer formation was observed in PVDF(NMP)/pDDA Langmuir film (Chapter 2). In Chapter 3, studies were focused on the properties of PVDF(NMP)/pDDA LB multilayers vertically deposited from the water surface to substrates, which are promising for good organic ferroelectrics. This is the first report of the use of a vertical-dipping LB technique to acquire regular PVDF nanofilms. In the π–A isotherms of the mixed Langmuir films at the air–water interface, the rather high collapse surface pressure of PVDF quasi-monolayer at the mixing ratio of 50:1 is an important breakthrough in comparison with the reported values, which makes it successful to prepare highly dense PVDF LB nanofilms with a high film density, about 1.82 g/cm3 and a high crystallinity comparable to PVDF bulk, 52%. The absorption intensity at 196 nm from amide groups in pDDA molecules increases linearly along with the number of deposition layers (Figs. 3a and b), which indicates that these nanofilms have regular layer structures. The monolayer thickness was determined as 2.3 nm.The PVDF(NMP)/pDDA Langmuir film was deposited on one hydrophobic silicon substrate from 0 to 200 layers according to an arithmetic sequence with a common difference of 20 layers (Fig. 3c). The beautiful interference colors also prove the regular layer structure of the PVDF(NMP)/pDDA LB nanofilms, as well as the smooth film surface. In addition, the PVDF/pDDA LB nanofilms obtained with no post-treatment were demonstrated with dominant ferroelectric β phase (95 %) and negligible paraelectric α phase by FT-IR spectra, as well as the XRD patterns. Furthermore, the high transfer properties, controllable β-crystal morphologies and film thickness and uniform film surface must endow the PVDF LB nanofilms with many applications as nanoelectronics such as high-performance memories, capacitors, transistors, and so on.  In Chapter 4, ferroelectric nanocapacitors (FeNCs) made of as-deposited PVDF LB nanofilms (PVDF:pDDA = 50:1, from NMP) were fabricated with different film thicknesses of 12-81 nm by sandwiching the LB nanofilms between Al bottom and top electrodes. Ferroelectric polarization switching was investigated in the as-prepared PVDF FeNCs which show gradually saturated hysteresis loops with increasing film thickness (Fig. 4a). A remanent polarization (Pr, E = 0) value of 6.6 μC/cm2 was obtained in 81-nm-thick PVDF LB nanofilms with no post-treatment such as thermal annealing. That value is one of the highest values ever reported for PVDF micro-/nanofilms. Such a high value must be attributed to the ultra-high content of β phase in these nanofilms. In addition, ferroelectricity was detected first in a PVDF homopolymer LB nanofilm as thin as 12 nm, indicating a potential low-voltage application of the nanofilms. The values of coercive electric field (Ec, Pr = 0) for the FeNCs in this study, approximately 195 MV/m are almost independent of the film thickness (Fig. 4a). Those values are Fig. 3 (a) UV-vis spectra of PVDF(NMP)/pDDA LB nanofilms (50:1), (b) relation between absorbance at 196 nm and number of layers, and (c) PVDF(NMP)/pDDA LB multilayers on a hydrophobic silicon substrate(from 0 to 200 layers). ? 108 ?representative of the extrinsic switching characteristics of the PVDF LB nanofilms. The polarization switching process is dominated by inhomogeneous domain nucleation and growth. The breakdown strength of PVDF homopolymer LB nanofilms exceeds 500 MV/m (Fig. 4a). This value is much higher than that of bulk PVDF (about 300 MV/m). That fact indicates that PVDF LB nanofilms have good electric stability. The fatigue properties of the PVDF LB nanofilms provide useful information for elucidating the operation endurance of PVDF nanofilms thinner than 100 nm. All films from 12 to 81 nm showed long-standing fatigue endurance exceeding 105 cycles (Fig. 4b). In the case of the 81-nm thick PVDF homopolymer film, 106 cycle switching causes only 37 % loss of the Pr value. This is lower than that of a 69-nm thick P(VDF-TrFE) copolymer film. The Pr value reduces 50 % of its initial value under 150 MV/m, 1 kHz after 106 cycles. Therefore, the PVDF homopolymer LB nanofilms in this study occupy a lower fatigue rate than copolymers do.  In Chapter 5, I summarize the thesis. The monolayer behaviors of various PVDF Langmuir films were investigated using Brewster angle microscopy and surface pressure (π)–area (A) isotherms, which show significant dependence on spreading solvents. Thereafter, PVDF homopolymer Langmuir–Blodgett (LB) nanofilms were fabricated successfully by an assistance of amphiphilic pDDA nanosheets onto various substrates. The crystal structure of PVDF in the LB nanofilms is changeable from complete α phase spread from MEK to complete β phase from NMP. In addition, the LB nanofilms show regular layer structures, high film density and high crystallinity with no post-treatment. The ferroelectricity of as-prepared PVDF LB nanofilms was observed with high remanent polarization (Pr) value of 6.6 μC/cm2 and high fatigue endurance exceeding 105 operation cycles. Therefore, the PVDF LB nanofilms are quite promising for non-volatile memory applications. All in all, the thesis proposed a systematic research to finely manipulate the properties of PVDF homopolymer nanofilms from monolayers to devices which have never been reported. It opens up many doors for further researches on ferroelectric polymer nanoelectronics aiming at practical applications. Not limited to ferroelectric polymers, the research results are also instructive and meaningful to crystallization controlling of other semi-crystalline polymers at the nano-scale with tunable film properties.   Fig. 4 (a) Ferroelectric hysteresis loops and (b) Fatigue characteristics of ultrathin PVDF homopolymer LB nanofilms at different thicknesses. ? 109 ?? 110 ?

Studies on Ferroelectric Poly(vinylidene fluoride) Langmuir-Blodgett Nanofilms Assisted by Polymer Nanosheets

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高分子ナノシートを用いた強誘電性ポリフッ化ビニリデンLangmuir-Blodgettナノ薄膜に関する研究

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