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Alignment of polymer dispersed nanometric films of ferroelectric liquid crystalline molecules
Journal of Molecular Liquids 294 (2019) 111664
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Alignment of polymer dispersed nanometric films of ferroelectric liquid crystalline molecules Ramneek Kaur a,⁎, Gurpreet Kaur Bhullar a, K.K. Raina b,c a b c
P.G. Department of Physics, Mata Gujri College, Fatehgarh Sahib, India Materials Research Laboratory, School of Physics and Materials Science, Thapar Institute of engineering and technology, Patiala, India Department of Physics, DIT University, Dehradun, India
a r t i c l e
i n f o
Article history: Received 19 April 2019 Received in revised form 3 August 2019 Accepted 30 August 2019 Available online 31 August 2019 Keywords: Ferroelectric liquid crystal Polymer Atomic force microscopy Langmuir Blodgett technique
a b s t r a c t Composite films of polyvinyl alcohol dispersed in ferroelectric liquid crystal (FLC) matrix are fabricated by using Langmuir–Blodgett technique. Pressure–area isotherms show that the composite system formed well aligned monolayer as compared to undoped FLC. Due to molecular interactions, monolayers and multilayers possess good spreading properties and we are able to transfer them onto solid substrates from air-liquid interface. FTIR spectroscopy was used to examine presence of all transferred functional groups on substrate. X-Ray spectrum indicates that even due to presence of PVA in FLC matrix, smectic layer ordering of FLC molecules remains unaltered. During non-contact mode of atomic force microscopy, dense and well aligned homogenous pattern is observed in scanned images. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Liquid crystalline is a state of matter which has properties intermediate to isotropic liquids and solid crystals. They exhibit different phases due to different positional and orientational ordering of their constituting molecules [1–5]. Ferroelectric liquid crystals (FLCs), also referred as chiral smectic C* LC, are an important class of smart electro optic materials. They have lamellar modulated helical structure and polarization is associated with helix. In the recent past, significant research works have been carried out on several important FLC materials to understand their material physics for display devices. They create stable compressible monolayers at the air–water interface which are easily transferrable onto solid substrates [6–9]. To enhance properties ferroelectric liquid crystals are dispersed with nanoparticles and polymers. Homogenous dispersion of nanoparticles in FLC matrix with low concentration doping has been reported [10–12]. In current research paper polyvinyl alcohol (PVA) is dispersed at low concentration in FLC matrix. PVA is a synthetic polymer with high flexibility and ease to film formation [13,14]. Stable films of dispersed
⁎ Corresponding author. E-mail address: [email protected] (R. Kaur).
https://doi.org/10.1016/j.molliq.2019.111664 0167-7322/© 2019 Elsevier B.V. All rights reserved.
system can be formed by using Langmuir Blodgett (LB) technique. LB provides excellent and simple method to deposit ultrathin films on solid substrates with controlled thickness. Stability of LB film strongly depend upon hydrophilic and hydrophobic balance of depositing molecules. Researchers have reported LB films of PVA by using Maleic acid (MA) acid as crosslinker of PVA [15], Naphthalene labelled polyvinylalcohol (NAPVA) films mixed with stearic acid [16], Poly(vinyl alcohol)s films bearing Azobenzene Sides [17], LB films of acetalized poly(vinyl alcohol)s [18]. Films of PVA molecules are also deposited by using Layer by layer assembly method [19,20]. In past decade thin films of FLC- polymer composite films have been formed at micrometer thickness. The methods used for polymer dispersion in FLC such as Polymer induced phase separation (PIPS), Solvent induced phase separation (SIPS) and temperature induced phase separation (TIPS) are effected by reduced solubility and proportion of LC, variation in parameters due to temperature and cooling rate, difficult to control evaporation rate of solvent respectively [21]. Meanwhile, we report films at nanometric scale by using LB method. It provides well controlled architecture at room temperature [22]. Dispersed monolayer is formed with constant compression of barriers. Due to reduced thickness these films are of great interest as they can have different physical properties such as high contrast ratio, a low threshold voltage, and a low switching
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time [23]. They may have many potential applications in technology due to enhanced properties.
2. 2. Materials and Experiment The structural detail of ZLI-3654 (FLC) dispersed with Polyvinyl alcohol is shown in Fig. 1 (a, b). FLC molecules are consisting of biphenyl rings and long alkyl chains attached at ends for providing contribution toward hydrophobicity. While oxygen being electronegative element, nitrogen present at meta carbon position of benzene ring, halide chlorine attached to chiral carbon and cyanide group provides hydrophilic character to FLC molecule. Thus FLC is amphiphilic in nature with average length ~2.7836 nm and exhibit SmC* phase at room temperature. PVA(C2H4O)n where Degree of polymerization (n) ~ 1700‐1800 has 44.0 g/mol molecular weight for each repeating unit. PVA has glass transition (Tg) temperature 85.78 ͦ C and melting point is about 190 °C [24]. In molecular formula it has–OH group [25], which provide hydrophilic character whereas hydrophobic character is provided by repeated chain of alkyl groups [26,27]. Composites of FLC-PVA were made by weight mixing 1 and 5 wt% of polymer - FLC in chloroform solution via ultrasonication for 30 min. Langmuir Blodgett experimental setup (KSV-NIMA) is consist of four major parts: Mini trough (160 ml without dipping well), Wilhelmy plate (38 mm × 19.62 mm × 10 mm), Derlin Barriers and clipper stand to hold substrate. Mini trough is made of Teflon; it is cleaned with ethanol [Merck (AR)] and then rinsed with deionized water. Wilhelmy plate is used to measure surface pressure within a precision of 0.1 mN m‐1 [28,29]. Barriers are used to compress the molecules for aligning them at water subphase at constant rate
(10 mm min ‐1). Substrate is held vertically, Y-type deposition mode is used to deposit films in which monolayers are transferred on both emersion and immersion of the substrate through subphase [30–32]. To obtain isotherm of undoped FLC, 1 mg ml ‐1 solution of FLC material was prepared in chloroform by sonicating the mixture for about half an hour. 100 μl of this solution was spread on subphase, constant compression of molecules result in increased surface pressure. 1 and 5 wt% of PVA – FLC composite materials were prepared in chloroform (1 mg ml ‐1 ) after 30 min of sonication. The proper dispersion of polymer in FLC matrix was ensured by clear solution obtained after sonication. 100 μl mixtures of both composites were spread on water sub phase separately. Isotherms were obtained by constant compression. Monolayers were transferred to individual Silicon substrates and multilayers on glass substrate. During Y type deposition transfer ratio was nearly 1.0. Transfer Ratio is used measure the extend of deposition on the substrate, it is defined as the ratio of the coated area of the substrate and decrease of area occupied by the monolayer on water subphase at constant pressure [33].
3. Results and discussion 3.1. Surface pressure–AREA (П–A) isotherms Fig. 2 shows surface pressure area isotherm for 1 and 5 wt% composite system of PVA in FLC matrix. Initially when barriers are far apart, there are no molecular interactions among molecules of PVA and FLC, it is shown schematically in Fig. 3(I), in this phase of FLC molecules are not close to each other, just like gas molecules. Due
Fig. 1. (a) shows molecular structure and 3-dimensional view of atoms consisting ferroelectric liquid crystal (FLC, ZLI-3654) and (b) shows molecular structure and 3D view of monomer unit forming Polyvinyl alcohol (PVA). (Atoms are represented as Red: Oxygen, Green: Chlorine, White: Hydrogen, Royal Blue: Nitrogen and Sky Blue: Carbon).
R. Kaur et al. / Journal of Molecular Liquids 294 (2019) 111664
Fig. 2. Surface pressure–area isotherm profiles of 1 and 5 wt% of PVA dispersed in FLC matrix. Inset figure shows П-A isotherm profile of undoped FLC, having three distinct phases (Gaseous, Liquid and Condensed) on basis of their molecular arrangement with constant compression.
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to constant compression of barriers, molecules come closer to each other as shown in Fig. 3(II). Alignment of molecules is attained with further compression of barriers as shown in Fig. 3(III) [34,35], this phase is termed as condensed phase. In condensed phase oxygen of PVA being hydrophilic is attracted toward water while alkyl chains being hydrophobic are repelled toward air. FLC molecules get aligned inside chain network of PVA such that it's hydrophilic part gets affinity from water and oxygen of PVA, and it's hydrophobic groups are aligned upwards. Surface pressure start increasing with decrease in mean molecular area due to compression of monolayer with barriers. In case of 5 wt% composite systems, surface pressure was about 2 mNm‐1 at 25nm 2 / molecule. With further closing of barriers molecules attained condensed phase at 18 mN m‐1. In case of 1 wt% composite system surface pressure follows the similar increasing trend as 5 wt% composite system but surface pressure reaches up to 13 mN m ‐1. This is due to lesser concentration of PVA in FLC matrix. It is observed that surface pressure in both composite systems is higher as compared to undoped FLC (inset Fig. 2) [36]. FLC shows three distinct (Gaseous, liquid expanded, condensed) phases during compression of monolayer [6,37]. Initially, when the compression process was not started, molecules of FLC were far apart just like gas molecules and this phase is termed as gaseous phase. With constant compression of barriers molecules came closer and resulted in formation of liquid expanded phase. In condensed
Fig. 3. Schematic view of interaction process (I, II and III) during alignment of PVA/FLC molecules with constant compression of barriers.
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phase, due to compression, FLC molecules aligned on water surface resulting in formation of stable monolayer. Thus dispersion of PVA results in different interactions among molecules, which has increased the maximum surface pressure attained by molecules in condensed phase. FLC composite film has attained higher surface pressure as compared to undoped FLC, thus FLC composite systems form much stable monolayer at air-water interface. Monolayers and multilayers were deposited on silicon and glass substrate respectively. 3.2. Fourier Transform Infrared (FTIR) spectroscopy of LB films FTIR (BX-II, Perkin Elmer) was used to confirm the complete deposition of molecules of 1 and 5 wt% composite systems (Fig. 4).The distinct broad absorption band at 3708–3427 cm ‐1 is due to the stretching vibration of O\\H groups of PVA [15]. Symmetrical stretching absorptions due to CH 2 –CH 2 bands appear at 2959.25 cm ‐1 and 2924.66 cm ‐1 , and absorptions at 2872.90 cm‐1and 2854.40 cm‐1are due to asymmetrical stretching of CH2–CH2 bands of alkyl chains of FLC matrix and carbohydrate part of PVA. Absorption at 1620.38 cm ‐1 corresponded to C\\C aromatic structure of FLC and 1527.23 cm ‐1 corresponds to N\\H bond of FLC. Absorption at 1433.14 cm‐1 is due to C\\H bending vibrations of composite system. Absorption at 1261.89 cm‐1 and 1277.89 cm‐ 1 corresponded to ϕ – O\\C stretch (1270–1230 cm‐1), where ϕ denotes an aromatic substitution. The absorptions at 1137.28 cm ‐1 and 1168.74 cm ‐1 of the FLC spectra corresponded to C\\O stretching(C\\O\\C; 1200–1050 cm‐1), with alkyl substitution. Absorption at 825.300 cm ‐1 is due to C\\C stretching vibration. Absorption at 716.15 cm ‐1 confirms the presence of distinctive halogen (C\\Cl; 800–700 cm‐1) of ferroelectric liquid crystal molecule which has participated in formation of monolayer at air water
Fig. 4. FTIR spectrum of LB films of 1 and 5 wt% PVA in FLC matrix deposited @ 13 and 16 mNm‐1 respectively.
interface [38–40]. Each specific chemical bond of 1 and 5 wt% composite system has a unique absorption wave number as shown in spectra. 3.3. X-ray diffraction LB composite films were deposited on glass substrate, these films were exposed to Cu–Kα radiation (Philips XPERT–PRO MPD). Fig. 5 shows X-ray diffraction spectra of 1 wt% PVA dispersed FLC composite film at angle range 2θ = (2.5 ͦ - 27 ͦ). The Xray diffraction profile showed an intense peak at 2θ = 3.2° [40,41], corresponding to the smectic layer ordering of liquid crystal molecules with a layer spacing of ∼2.752 nm [42]. It points toward retaining of ferroelectric phase with low-concentration doping of polymer in FLC matrix. 1 wt% PVA dispersed FLC, LB film was again exposed to Cu-Kα radiation at different angle range 2θ = 10°–30°. A broad peak at 2θ = 20 ͦ is due to diffraction from aligned hydrophobic and hydrophilic groups of PVA [4] as shown in inset Fig. 5. Fig. 6 shows similar XRD profiles for 5 wt% PVA dispersed FLC composite film at angle range 2θ = (2.5°–30°). It is observed that, with increased concentration of PVA in FLC matrix, intensity decreases for peak at 2θ = 3.2° and increases for broad peak of PVA at 2θ = 20° (inset Fig. 6). 3.4. AFM imaging of LB films Atomic Force Microscopy (Solver-NEXT NT-MDT) was used to analyze the surface structure of deposited films with superior resolution at 1 Hz raster scanning rate. SiN tip with curvature radius 2 nm is attached to a flexible cantilever, having resonance frequency 87–230 kHz and force constant 5.1 nN m‐1. This assembly forms a probe to interact with the surface of deposited film. The deflections in cantilever during scanning result in formation of image. Initially, the undeposited substrate is scanned to confirm surface smoothness. So that we can obtain exact topography of deposited films while scanning the film in non- contact mode. This mode is preferred because films of soft materials can get damaged during contact or intermittent mode
Fig. 5. XRD spectra of LB film of 1 wt % PVA dispersed in FLC with 2θ = (2.5° - 27° ) on glass substrate . Inset figure shows XRD profile with 2θ = (10° - 30° ).
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Fig. 6. XRD spectra of LB film of 5 wt% PVA dispersed in FLC with 2θ = (2.5°–30°) on glass substrate . Inset figure shows XRD profile with 2θ = (10°–30°).
[43–46].Topography of properly cleaned silicon substrate is examined under non-contact mode. AFM image in scanning range 1 × 1 μm 2 and average height profile are shown in Fig. 7. It is observed that roughness is b1 nm, it ensures that substrate is suitable for depositing Langmuir monolayers. AFM image of 1 wt% polymer dispersed FLC films deposited at 13 mN m –1 on silicon substrate in the scanning range of 2 × 2 μm2 is shown in Fig. 8. Fig. 8(a) shows two dimensional topography, brighter regions are at height about 4‐8 nm indicating presence of polymer in FLC (2-3 nm) matrix. The completely dark zones are due to topography of substrate, indicating height b 1 nm. There is no deposition in dark zones. It can be observed that maximum surface of substrate is deposited with composite material. Major covering of substrate surface with depositing materials is a supporting result to transfer ratio value obtained during deposition process at constant surface pressure [47].
Fig. 8(b) shows 3D view of film, sharp peaks of greater height (5‐14 nm) are due to polymer and smaller peaks (2‐3 nm) indicate presence of FLC molecules. Fig. 8(c) indicate average height profile up to 6 nm and Fig. 8(d) shows particle count is higher in range of 2–6 nm which indicates the uttermost deposition on the substrate. Enlarged and rotated view of some portion of Fig. 8(b) is shown in Fig. 9. Alignment of FLC molecules can be clearly observed along black lines drawn on image. FLC are aligned as low heighted mountains (2‐3 nm) along the drawn black lines, while PVA molecules are shown as high mountains (5‐14 nm) [6]. AFM image of 5 wt% polymer dispersed FLC composite film deposited at 16 mN m ‐1 on silicon substrate in the scanning range of 5 × 5 μm 2 is shown in Fig. 10.2D view of topography is shown in Fig. 10(a), it indicates contrast of image has been changed with increasing concentration of PVA molecules.
Fig. 7. AFM 2D-image and average height profile of silicon substrate.
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Fig. 8. AFM non-contact mode imaging of PVA-FLC composite film in the scanning range 2 × 2 μm 2 , deposited on silicon wafer. a, b, c and d show two-dimensional image, three-dimensional view, topographical height variation and counts as per size distribution of 1 wt% PVA dispersed FLC composite films deposited at 13 mN m ‐1 .
Majority of the substrate possess bright regions as compared to Fig. 10(a) because of increased concentration of PVA in deposited film. Fig. 10(b) shows 3D view of LB film, it has more peaks of polymer dispersed in FLC matrix as compared to Fig. 8 (b). Fig. 10(c) indicates average height profile up to 10 nm.
Size wise particle count in Fig. 10(d) is also increased to 2‐ 15 nm. Thus the composite materials formed uniform and dense films on the substrate. 4. Conclusions Isotherms of PVA-FLC composite system indicate that PVA and FLC have experienced interactions among hydrophilic and hydrophobic molecules; they are well aligned at water surface such that stable and transferrable layers are formed. The intense diffraction peak observed at 2θ = 3.2 ͦ inferred that dispersion of PVA molecules in FLC matrix has not altered smectic structure of ferroelectric liquid crystalline molecules. Homogenous deposition of composite systems is examined from atomic force microscopic images. Peaks of height (2‐3 nm) shows aligned FLC matrix and PVA molecules are embedded in it having peaks of height (514 nm). These kinds of FLC-polymer composite films can find innumerable applications for fabricating opto-electrical devices at nano scale. Acknowledgement
Fig. 9. Enlarged and rotated view of some portion of Fig. 8(b) showing alignment of polymer dispersed FLC composite system.
Authors are thankful to reviewers for their valuable suggestions to improve quality of work. Authors (R. Kaur and G.K. Bhullar) also wish to thank Ms. L. K. Brar for training on Langmuir–Blodgett and Atomic Force Microscopy experimental set ups.
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Fig. 10. AFM images of 5 wt% PVA dispersed FLC composite films deposited at 16 mN m‐1. a, b, c and d indicates similar to Fig. 8.
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Alignment of polymer dispersed nanometric films of ferroelectric liquid crystalline molecules Ramneek Kaur a,⁎, Gurpreet Kaur Bhullar a, K.K. Raina b,c a b c
P.G. Department of Physics, Mata Gujri College, Fatehgarh Sahib, India Materials Research Laboratory, School of Physics and Materials Science, Thapar Institute of engineering and technology, Patiala, India Department of Physics, DIT University, Dehradun, India
a r t i c l e
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Article history: Received 19 April 2019 Received in revised form 3 August 2019 Accepted 30 August 2019 Available online 31 August 2019 Keywords: Ferroelectric liquid crystal Polymer Atomic force microscopy Langmuir Blodgett technique
a b s t r a c t Composite films of polyvinyl alcohol dispersed in ferroelectric liquid crystal (FLC) matrix are fabricated by using Langmuir–Blodgett technique. Pressure–area isotherms show that the composite system formed well aligned monolayer as compared to undoped FLC. Due to molecular interactions, monolayers and multilayers possess good spreading properties and we are able to transfer them onto solid substrates from air-liquid interface. FTIR spectroscopy was used to examine presence of all transferred functional groups on substrate. X-Ray spectrum indicates that even due to presence of PVA in FLC matrix, smectic layer ordering of FLC molecules remains unaltered. During non-contact mode of atomic force microscopy, dense and well aligned homogenous pattern is observed in scanned images. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Liquid crystalline is a state of matter which has properties intermediate to isotropic liquids and solid crystals. They exhibit different phases due to different positional and orientational ordering of their constituting molecules [1–5]. Ferroelectric liquid crystals (FLCs), also referred as chiral smectic C* LC, are an important class of smart electro optic materials. They have lamellar modulated helical structure and polarization is associated with helix. In the recent past, significant research works have been carried out on several important FLC materials to understand their material physics for display devices. They create stable compressible monolayers at the air–water interface which are easily transferrable onto solid substrates [6–9]. To enhance properties ferroelectric liquid crystals are dispersed with nanoparticles and polymers. Homogenous dispersion of nanoparticles in FLC matrix with low concentration doping has been reported [10–12]. In current research paper polyvinyl alcohol (PVA) is dispersed at low concentration in FLC matrix. PVA is a synthetic polymer with high flexibility and ease to film formation [13,14]. Stable films of dispersed
⁎ Corresponding author. E-mail address: [email protected] (R. Kaur).
https://doi.org/10.1016/j.molliq.2019.111664 0167-7322/© 2019 Elsevier B.V. All rights reserved.
system can be formed by using Langmuir Blodgett (LB) technique. LB provides excellent and simple method to deposit ultrathin films on solid substrates with controlled thickness. Stability of LB film strongly depend upon hydrophilic and hydrophobic balance of depositing molecules. Researchers have reported LB films of PVA by using Maleic acid (MA) acid as crosslinker of PVA [15], Naphthalene labelled polyvinylalcohol (NAPVA) films mixed with stearic acid [16], Poly(vinyl alcohol)s films bearing Azobenzene Sides [17], LB films of acetalized poly(vinyl alcohol)s [18]. Films of PVA molecules are also deposited by using Layer by layer assembly method [19,20]. In past decade thin films of FLC- polymer composite films have been formed at micrometer thickness. The methods used for polymer dispersion in FLC such as Polymer induced phase separation (PIPS), Solvent induced phase separation (SIPS) and temperature induced phase separation (TIPS) are effected by reduced solubility and proportion of LC, variation in parameters due to temperature and cooling rate, difficult to control evaporation rate of solvent respectively [21]. Meanwhile, we report films at nanometric scale by using LB method. It provides well controlled architecture at room temperature [22]. Dispersed monolayer is formed with constant compression of barriers. Due to reduced thickness these films are of great interest as they can have different physical properties such as high contrast ratio, a low threshold voltage, and a low switching
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time [23]. They may have many potential applications in technology due to enhanced properties.
2. 2. Materials and Experiment The structural detail of ZLI-3654 (FLC) dispersed with Polyvinyl alcohol is shown in Fig. 1 (a, b). FLC molecules are consisting of biphenyl rings and long alkyl chains attached at ends for providing contribution toward hydrophobicity. While oxygen being electronegative element, nitrogen present at meta carbon position of benzene ring, halide chlorine attached to chiral carbon and cyanide group provides hydrophilic character to FLC molecule. Thus FLC is amphiphilic in nature with average length ~2.7836 nm and exhibit SmC* phase at room temperature. PVA(C2H4O)n where Degree of polymerization (n) ~ 1700‐1800 has 44.0 g/mol molecular weight for each repeating unit. PVA has glass transition (Tg) temperature 85.78 ͦ C and melting point is about 190 °C [24]. In molecular formula it has–OH group [25], which provide hydrophilic character whereas hydrophobic character is provided by repeated chain of alkyl groups [26,27]. Composites of FLC-PVA were made by weight mixing 1 and 5 wt% of polymer - FLC in chloroform solution via ultrasonication for 30 min. Langmuir Blodgett experimental setup (KSV-NIMA) is consist of four major parts: Mini trough (160 ml without dipping well), Wilhelmy plate (38 mm × 19.62 mm × 10 mm), Derlin Barriers and clipper stand to hold substrate. Mini trough is made of Teflon; it is cleaned with ethanol [Merck (AR)] and then rinsed with deionized water. Wilhelmy plate is used to measure surface pressure within a precision of 0.1 mN m‐1 [28,29]. Barriers are used to compress the molecules for aligning them at water subphase at constant rate
(10 mm min ‐1). Substrate is held vertically, Y-type deposition mode is used to deposit films in which monolayers are transferred on both emersion and immersion of the substrate through subphase [30–32]. To obtain isotherm of undoped FLC, 1 mg ml ‐1 solution of FLC material was prepared in chloroform by sonicating the mixture for about half an hour. 100 μl of this solution was spread on subphase, constant compression of molecules result in increased surface pressure. 1 and 5 wt% of PVA – FLC composite materials were prepared in chloroform (1 mg ml ‐1 ) after 30 min of sonication. The proper dispersion of polymer in FLC matrix was ensured by clear solution obtained after sonication. 100 μl mixtures of both composites were spread on water sub phase separately. Isotherms were obtained by constant compression. Monolayers were transferred to individual Silicon substrates and multilayers on glass substrate. During Y type deposition transfer ratio was nearly 1.0. Transfer Ratio is used measure the extend of deposition on the substrate, it is defined as the ratio of the coated area of the substrate and decrease of area occupied by the monolayer on water subphase at constant pressure [33].
3. Results and discussion 3.1. Surface pressure–AREA (П–A) isotherms Fig. 2 shows surface pressure area isotherm for 1 and 5 wt% composite system of PVA in FLC matrix. Initially when barriers are far apart, there are no molecular interactions among molecules of PVA and FLC, it is shown schematically in Fig. 3(I), in this phase of FLC molecules are not close to each other, just like gas molecules. Due
Fig. 1. (a) shows molecular structure and 3-dimensional view of atoms consisting ferroelectric liquid crystal (FLC, ZLI-3654) and (b) shows molecular structure and 3D view of monomer unit forming Polyvinyl alcohol (PVA). (Atoms are represented as Red: Oxygen, Green: Chlorine, White: Hydrogen, Royal Blue: Nitrogen and Sky Blue: Carbon).
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Fig. 2. Surface pressure–area isotherm profiles of 1 and 5 wt% of PVA dispersed in FLC matrix. Inset figure shows П-A isotherm profile of undoped FLC, having three distinct phases (Gaseous, Liquid and Condensed) on basis of their molecular arrangement with constant compression.
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to constant compression of barriers, molecules come closer to each other as shown in Fig. 3(II). Alignment of molecules is attained with further compression of barriers as shown in Fig. 3(III) [34,35], this phase is termed as condensed phase. In condensed phase oxygen of PVA being hydrophilic is attracted toward water while alkyl chains being hydrophobic are repelled toward air. FLC molecules get aligned inside chain network of PVA such that it's hydrophilic part gets affinity from water and oxygen of PVA, and it's hydrophobic groups are aligned upwards. Surface pressure start increasing with decrease in mean molecular area due to compression of monolayer with barriers. In case of 5 wt% composite systems, surface pressure was about 2 mNm‐1 at 25nm 2 / molecule. With further closing of barriers molecules attained condensed phase at 18 mN m‐1. In case of 1 wt% composite system surface pressure follows the similar increasing trend as 5 wt% composite system but surface pressure reaches up to 13 mN m ‐1. This is due to lesser concentration of PVA in FLC matrix. It is observed that surface pressure in both composite systems is higher as compared to undoped FLC (inset Fig. 2) [36]. FLC shows three distinct (Gaseous, liquid expanded, condensed) phases during compression of monolayer [6,37]. Initially, when the compression process was not started, molecules of FLC were far apart just like gas molecules and this phase is termed as gaseous phase. With constant compression of barriers molecules came closer and resulted in formation of liquid expanded phase. In condensed
Fig. 3. Schematic view of interaction process (I, II and III) during alignment of PVA/FLC molecules with constant compression of barriers.
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phase, due to compression, FLC molecules aligned on water surface resulting in formation of stable monolayer. Thus dispersion of PVA results in different interactions among molecules, which has increased the maximum surface pressure attained by molecules in condensed phase. FLC composite film has attained higher surface pressure as compared to undoped FLC, thus FLC composite systems form much stable monolayer at air-water interface. Monolayers and multilayers were deposited on silicon and glass substrate respectively. 3.2. Fourier Transform Infrared (FTIR) spectroscopy of LB films FTIR (BX-II, Perkin Elmer) was used to confirm the complete deposition of molecules of 1 and 5 wt% composite systems (Fig. 4).The distinct broad absorption band at 3708–3427 cm ‐1 is due to the stretching vibration of O\\H groups of PVA [15]. Symmetrical stretching absorptions due to CH 2 –CH 2 bands appear at 2959.25 cm ‐1 and 2924.66 cm ‐1 , and absorptions at 2872.90 cm‐1and 2854.40 cm‐1are due to asymmetrical stretching of CH2–CH2 bands of alkyl chains of FLC matrix and carbohydrate part of PVA. Absorption at 1620.38 cm ‐1 corresponded to C\\C aromatic structure of FLC and 1527.23 cm ‐1 corresponds to N\\H bond of FLC. Absorption at 1433.14 cm‐1 is due to C\\H bending vibrations of composite system. Absorption at 1261.89 cm‐1 and 1277.89 cm‐ 1 corresponded to ϕ – O\\C stretch (1270–1230 cm‐1), where ϕ denotes an aromatic substitution. The absorptions at 1137.28 cm ‐1 and 1168.74 cm ‐1 of the FLC spectra corresponded to C\\O stretching(C\\O\\C; 1200–1050 cm‐1), with alkyl substitution. Absorption at 825.300 cm ‐1 is due to C\\C stretching vibration. Absorption at 716.15 cm ‐1 confirms the presence of distinctive halogen (C\\Cl; 800–700 cm‐1) of ferroelectric liquid crystal molecule which has participated in formation of monolayer at air water
Fig. 4. FTIR spectrum of LB films of 1 and 5 wt% PVA in FLC matrix deposited @ 13 and 16 mNm‐1 respectively.
interface [38–40]. Each specific chemical bond of 1 and 5 wt% composite system has a unique absorption wave number as shown in spectra. 3.3. X-ray diffraction LB composite films were deposited on glass substrate, these films were exposed to Cu–Kα radiation (Philips XPERT–PRO MPD). Fig. 5 shows X-ray diffraction spectra of 1 wt% PVA dispersed FLC composite film at angle range 2θ = (2.5 ͦ - 27 ͦ). The Xray diffraction profile showed an intense peak at 2θ = 3.2° [40,41], corresponding to the smectic layer ordering of liquid crystal molecules with a layer spacing of ∼2.752 nm [42]. It points toward retaining of ferroelectric phase with low-concentration doping of polymer in FLC matrix. 1 wt% PVA dispersed FLC, LB film was again exposed to Cu-Kα radiation at different angle range 2θ = 10°–30°. A broad peak at 2θ = 20 ͦ is due to diffraction from aligned hydrophobic and hydrophilic groups of PVA [4] as shown in inset Fig. 5. Fig. 6 shows similar XRD profiles for 5 wt% PVA dispersed FLC composite film at angle range 2θ = (2.5°–30°). It is observed that, with increased concentration of PVA in FLC matrix, intensity decreases for peak at 2θ = 3.2° and increases for broad peak of PVA at 2θ = 20° (inset Fig. 6). 3.4. AFM imaging of LB films Atomic Force Microscopy (Solver-NEXT NT-MDT) was used to analyze the surface structure of deposited films with superior resolution at 1 Hz raster scanning rate. SiN tip with curvature radius 2 nm is attached to a flexible cantilever, having resonance frequency 87–230 kHz and force constant 5.1 nN m‐1. This assembly forms a probe to interact with the surface of deposited film. The deflections in cantilever during scanning result in formation of image. Initially, the undeposited substrate is scanned to confirm surface smoothness. So that we can obtain exact topography of deposited films while scanning the film in non- contact mode. This mode is preferred because films of soft materials can get damaged during contact or intermittent mode
Fig. 5. XRD spectra of LB film of 1 wt % PVA dispersed in FLC with 2θ = (2.5° - 27° ) on glass substrate . Inset figure shows XRD profile with 2θ = (10° - 30° ).
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Fig. 6. XRD spectra of LB film of 5 wt% PVA dispersed in FLC with 2θ = (2.5°–30°) on glass substrate . Inset figure shows XRD profile with 2θ = (10°–30°).
[43–46].Topography of properly cleaned silicon substrate is examined under non-contact mode. AFM image in scanning range 1 × 1 μm 2 and average height profile are shown in Fig. 7. It is observed that roughness is b1 nm, it ensures that substrate is suitable for depositing Langmuir monolayers. AFM image of 1 wt% polymer dispersed FLC films deposited at 13 mN m –1 on silicon substrate in the scanning range of 2 × 2 μm2 is shown in Fig. 8. Fig. 8(a) shows two dimensional topography, brighter regions are at height about 4‐8 nm indicating presence of polymer in FLC (2-3 nm) matrix. The completely dark zones are due to topography of substrate, indicating height b 1 nm. There is no deposition in dark zones. It can be observed that maximum surface of substrate is deposited with composite material. Major covering of substrate surface with depositing materials is a supporting result to transfer ratio value obtained during deposition process at constant surface pressure [47].
Fig. 8(b) shows 3D view of film, sharp peaks of greater height (5‐14 nm) are due to polymer and smaller peaks (2‐3 nm) indicate presence of FLC molecules. Fig. 8(c) indicate average height profile up to 6 nm and Fig. 8(d) shows particle count is higher in range of 2–6 nm which indicates the uttermost deposition on the substrate. Enlarged and rotated view of some portion of Fig. 8(b) is shown in Fig. 9. Alignment of FLC molecules can be clearly observed along black lines drawn on image. FLC are aligned as low heighted mountains (2‐3 nm) along the drawn black lines, while PVA molecules are shown as high mountains (5‐14 nm) [6]. AFM image of 5 wt% polymer dispersed FLC composite film deposited at 16 mN m ‐1 on silicon substrate in the scanning range of 5 × 5 μm 2 is shown in Fig. 10.2D view of topography is shown in Fig. 10(a), it indicates contrast of image has been changed with increasing concentration of PVA molecules.
Fig. 7. AFM 2D-image and average height profile of silicon substrate.
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Fig. 8. AFM non-contact mode imaging of PVA-FLC composite film in the scanning range 2 × 2 μm 2 , deposited on silicon wafer. a, b, c and d show two-dimensional image, three-dimensional view, topographical height variation and counts as per size distribution of 1 wt% PVA dispersed FLC composite films deposited at 13 mN m ‐1 .
Majority of the substrate possess bright regions as compared to Fig. 10(a) because of increased concentration of PVA in deposited film. Fig. 10(b) shows 3D view of LB film, it has more peaks of polymer dispersed in FLC matrix as compared to Fig. 8 (b). Fig. 10(c) indicates average height profile up to 10 nm.
Size wise particle count in Fig. 10(d) is also increased to 2‐ 15 nm. Thus the composite materials formed uniform and dense films on the substrate. 4. Conclusions Isotherms of PVA-FLC composite system indicate that PVA and FLC have experienced interactions among hydrophilic and hydrophobic molecules; they are well aligned at water surface such that stable and transferrable layers are formed. The intense diffraction peak observed at 2θ = 3.2 ͦ inferred that dispersion of PVA molecules in FLC matrix has not altered smectic structure of ferroelectric liquid crystalline molecules. Homogenous deposition of composite systems is examined from atomic force microscopic images. Peaks of height (2‐3 nm) shows aligned FLC matrix and PVA molecules are embedded in it having peaks of height (514 nm). These kinds of FLC-polymer composite films can find innumerable applications for fabricating opto-electrical devices at nano scale. Acknowledgement
Fig. 9. Enlarged and rotated view of some portion of Fig. 8(b) showing alignment of polymer dispersed FLC composite system.
Authors are thankful to reviewers for their valuable suggestions to improve quality of work. Authors (R. Kaur and G.K. Bhullar) also wish to thank Ms. L. K. Brar for training on Langmuir–Blodgett and Atomic Force Microscopy experimental set ups.
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Fig. 10. AFM images of 5 wt% PVA dispersed FLC composite films deposited at 16 mN m‐1. a, b, c and d indicates similar to Fig. 8.
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