An ultra-high resolution SEM study of the morphology of electrically switchable volume gratings formed from polymer-dispersed liquid crystals

Low-voltage, ultra-high resolution SEM (UHR SEM) is becoming a valuable complementary technique to TEM, X-ray diffraction, and the scanning probe microscopies for determining polymer morphology and polymer structure-property relationships. Imaging organic materials at low voltage without significant loss in resolution allows for the visualization of structurally interesting features on the order of 50 - 1000 Å with reduced charging and improved topographic contrast. The easily interpretable nature of the data obtained from this technique and the ease of sample preparation offer advantages over more commonly used polymer morphology characterization techniques.Electrically switchable polymer dispersed liquid crystal (PDLC) volume gratings are of considerable interest for applications in diffractive optics. The system presently under investigation is a PDLC diffraction grating formed by a single-step laser induced polymerization of a penta-acrylate monomer, blended with a photoinitiator, crosslinker, and E7 liquid crystal (LC). Upon polymerization, the liquid crystal phase-separates into liquid crystal-rich planes with a periodicity of 0.56 μm.

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High-Performance Polymer Dispersed Liquid Crystal Enabled by Uniquely Designed Acrylate Monomer

Polymers ◽

10.3390/polym12081625 ◽

2020 ◽

Vol 12 (8) ◽

pp. 1625 ◽

Cited By ~ 1

Author(s):

Rijeesh Kizhakidathazhath ◽

Hiroya Nishikawa ◽

Yasushi Okumura ◽

Hiroki Higuchi ◽

Hirotsugu Kikuchi

Keyword(s):

Optical Properties ◽

Liquid Crystal ◽

Low Voltage ◽

Polymer Networks ◽

Mesh Size ◽

Laser Scanning Microscopy ◽

Present System ◽

Driving Voltage ◽

Acrylate Monomer ◽

Polymer Dispersed

The widespread electro–optical applications of polymer dispersed liquid crystals (PDLCs) are hampered by their high-driving voltage. Attempts to fabricate PDLC devices with low driving voltage sacrifice other desirable features of PDLCs. There is thus a clear need to develop a method to reduce the driving voltage without diminishing other revolutionary features of PDLCs. Herein, we report a low-voltage driven PDLC system achieved through an elegantly simple and uniquely designed acrylate monomer (A3DA) featuring a benzene moiety with a dodecyl terminal chain. The PDLC films were fabricated by the photopolymerization of mono- and di-functional acrylate monomers (19.2 wt%) mixed in a nematic liquid crystal E7 (80 wt%). The PDLC film with A3DA exhibited an abrupt decline of driving voltage by 75% (0.55 V/μm) with a high contrast ratio (16.82) while maintaining other electro–optical properties almost the same as the reference cell. The response time was adjusted to satisfactory by tuning the monomer concentration while maintaining the voltage significantly low (3 ms for a voltage of 0.98 V/μm). Confocal laser scanning microscopy confirmed the polyhedral foam texture morphology with an average mesh size of approximately 2.6 μm, which is less in comparison with the mesh size of reference PDLC (3.4 μm), yet the A3DA-PDLC showed low switching voltage. Thus, the promoted electro–optical properties are believed to be originated from the unique polymer networks formed by A3DA and its weak anchoring behavior on LCs. The present system with such a huge reduction in driving voltage and enhanced electro–optical performance opens up an excellent way for abundant perspective applications of PDLCs.

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Holographic Polymer Dispersed Liquid Crystals: Effect of Partial Matrix Fluorination on Electro-Optical and Morphological Properties

MRS Proceedings ◽

10.1557/proc-709-cc6.7.1 ◽

2001 ◽

Vol 709 ◽

Author(s):

Michael D. Schulte ◽

Stephen J. Clarson ◽

Lalgudi V. Natarajan ◽

C. Allan Guymon ◽

Timothy J. Bunning

Keyword(s):

Liquid Crystal ◽

Low Voltage ◽

Morphological Properties ◽

Acrylate Monomer ◽

Polymer Dispersed Liquid Crystal ◽

Morphological Studies ◽

Polymer Dispersed Liquid Crystals ◽

The Matrix ◽

Polymer Dispersed ◽

Morphological Differences

ABSTRACTHolographic polymer dispersed liquid crystal (H-PDLC) films with partially fluorinated matrices were investigated. Electro-optical and morphological studies revealed that fluorinated composites were substantially different from non-fluorinated analogues. The addition of a fluorinated monofunctional acrylate monomer to a pentaacrylate-derived polymer matrix resulted in improved diffraction efficiency. These findings suggest that the partial fluorination of the host polymer decreases the compatibility between the matrix and liquid crystal phase. Morphological differences between fluorinated films and non-fluorinated control specimens were verified using low-voltage, high-resolution scanning electron microscopy (LVHRSEM).

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Electrically switchable reflection gratings in polymer dispersed liquid crystals

MRS Proceedings ◽

10.1557/proc-559-109 ◽

1999 ◽

Vol 559 ◽

Cited By ~ 7

Author(s):

L. V. Natarajan ◽

R. L. Sutherland ◽

V. P. Tondiglia ◽

S. Siwecki ◽

R. Pogue ◽

...

Keyword(s):

Liquid Crystal ◽

Liquid Crystals ◽

Electric Field ◽

Low Voltage ◽

Blue Shift ◽

Maximum Efficiency ◽

Acrylate Monomer ◽

Polymer Dispersed ◽

Reflection Gratings ◽

Grating Formation

ABSTRACTElectrically switchable volume reflection holograms were written by inhomogeneous illumination of a prepolymer syrup containing a nematic liquid crystal and a multifunctional acrylate monomer. Switchable holograms are diffractive optics structures and the diffraction efficiency can be controlled by the application of an electric field. Reflection gratings with grating spacing varying between 0.16-0.27 µm were made during the phase separation of liquid crystals from the fast curing prepolymer syrup. The reflection efficiency of the holograms were electrically modulated with the applied field of ∼10-15V/µm. Real time study of the grating formation revealed that the maximum efficiency is reached in ∼15 seconds. The shrinkage of the host polymer during grating formation resulted in the blue shift of the reflection notch. The response time of the grating in an electric field is ∼50 µs. Low voltage scanning electron microscope studies showed the presence of discrete nematic droplet domains of sizes 30-60 nm in liquid crystal rich region.

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ELECTRO-OPTICAL SWITCHING CHARACTERISTICS OF VOLUME HOLOGRAMS IN POLYMER DISPERSED LIQUID CRYSTALS

Journal of Nonlinear Optical Physics & Materials ◽

10.1142/s021886359600009x ◽

1996 ◽

Vol 05 (01) ◽

pp. 89-98 ◽

Cited By ~ 98

Author(s):

L.V. NATARAJAN ◽

R.L. SUTHERLAND ◽

V.P. TONDIGLIA ◽

T.J. BUNNING ◽

W.W. ADAMS

Keyword(s):

Liquid Crystal ◽

Electric Fields ◽

Optical Switching ◽

Low Voltage ◽

Response Times ◽

Single Step ◽

Material System ◽

Volume Holograms ◽

Polymer Dispersed ◽

Switching Characteristics

Electrically switchable volume holograms lead to the possiblity of real-time electro-optical control of diffractive optic components. We report here on the development of a novel photopolymer-liquid crystal composite material system for writing in a fast single step, high diffraction efficiency volume holograms, capable of switching in applied electric fields of low voltage. Switching of a first-order Bragg diffracted beam into the zero-order with an applied field of ~10 V/µm was observed. With the addition of a surfactant to our pre-polymer syrup, we observed lowering of the switching fields to ~5 V/µm. We report response times for switching and relaxation in the order of microseconds. Low voltage, high resolution scanning electron microscopy studies show that the Bragg gratings formed consist of periodic polymer dispersed liquid crystal planes. The addition of surfactant leads to formation of very uniform small (20–40 nm) nematic droplets. A simple model based on the shape of the liquid crystal droplets was applied to explain the switching fields and response times.

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Low-voltage high-resolution SEM: A valuable resource for the polymer morphologist

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100150174 ◽

1993 ◽

Vol 51 ◽

pp. 868-869

Author(s):

D. L. Vezie ◽

W. W. Adams ◽

E. L. Thomas

Keyword(s):

Electron Microscopy ◽

High Resolution ◽

Secondary Electron ◽

Low Voltage ◽

Significant Loss ◽

Noise Component ◽

Polymer Morphology ◽

X Ray ◽

X Ray Scattering ◽

Ray Scattering

The elucidation of polymer morphology has historically been accomplished using optical microscopy (OM), conventional transmission electron microscopy (TEM), X-ray scattering and more recently for crystalline polymers, high resolution TEM (HRTEM). Significant work using scanning electron microscopy (SEM) to study synthetic polymers began in the late 1960's, but for twenty years after these first studies, the smallest features imaged by SEM in polymer samples have typically been 1-10 μm. The evolution of the low voltage high resolution SEM (LVHRSEM) now provides polymer scientists with the opportunity to image structures topographically on the order of 50-100 Å, a level of microstructure important in basic polymer physics research. The potential for improved understanding of polymer morphology is significant, especially when HRSEM data is combined with information from other complementary techniques such as TEM, X-ray scattering, and the scanning probe microscopies (SPM, AFM, STM).The advantages of LVHRSEM over conventional thermionic source SEM as a polymer characterization technique include: i) low keV operation reduces or eliminates charging in uncoated insulating polymers, ii) the high brightness, low energy spread, small spot size field emission gun allows for operation at low keV without significant loss of resolution (practically, resolution on the order of 50 Å at 1.0 keV in a polymer sample is attainable), iii) imaging of surface topography is improved at low keV as the beam/sample interaction volume is smaller and closer to the surface, resulting in more secondary electrons being generated within the secondary electron escape depth, and iv) the immersion lens system improves the signal to noise ratio by decreasing the noise component in the secondary electron signal.

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Holographic Polymer-Dispersed Liquid Crystals: Materials, Formation, and Applications

Advances in OptoElectronics ◽

10.1155/2008/684349 ◽

2008 ◽

Vol 2008 ◽

pp. 1-52 ◽

Cited By ~ 56

Author(s):

Y. J. Liu ◽

X. W. Sun

Keyword(s):

Liquid Crystal ◽

Liquid Crystals ◽

Refractive Index Profile ◽

Single Step ◽

Fast Switching ◽

Polymer Dispersed Liquid Crystal ◽

Current State ◽

Index Matching ◽

Polymer Dispersed ◽

Materials Used

By combining polymer-dispersed liquid crystal (PDLC) and holography, holographic PDLC (H-PDLC) has emerged as a new composite material for switchable or tunable optical devices. Generally, H-PDLC structures are created in a liquid crystal cell filled with polymer-dispersed liquid crystal materials by recording the interference pattern generated by two or more coherent laser beams which is a fast and single-step fabrication. With a relatively ideal phase separation between liquid crystals and polymers, periodic refractive index profile is formed in the cell and thus light can be diffracted. Under a suitable electric field, the light diffraction behavior disappears due to the index matching between liquid crystals and polymers. H-PDLCs show a fast switching time due to the small size of the liquid crystal droplets. So far, H-PDLCs have been applied in many promising applications in photonics, such as flat panel displays, switchable gratings, switchable lasers, switchable microlenses, and switchable photonic crystals. In this paper, we review the current state-of-the-art of H-PDLCs including the materials used to date, the grating formation dynamics and simulations, the optimization of electro-optical properties, the photonic applications, and the issues existed in H-PDLCs.

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