Polymer Network Liquid Crystal Research Articles

Design of polarization-dependent, flexural–torsional deformation in photo responsive liquid crystalline polymer networks

Light responsive materials that exhibit wirelessly actuated, multidimensional deformation are excellent candidates for programmable matter applications such as morphing structures or soft robotics. A central challenge to designing adaptive structures from these materials is the ability accurately predict three dimensional deformations. Previous modeling efforts have focused almost exclusively on pure bending. Herein we examine key material parameters affecting light driven flexural-torsional response in azobenzene functionalized liquid crystal polymer networks. We show that a great deal of control can be obtained by specifying material alignment and actuating the material with polarized light. Insight gained from the theoretical framework here lays the foundation for more extensive modeling efforts to combine polarization controlled flexural-torsional deformations with complex geometry, boundary conditions, and loading conditions.

Liquid‐Crystal Polymers: Topography from Topology: Photoinduced Surface Features Generated in Liquid Crystal Polymer Networks (Adv. Mater. 41/2013)

Light can induce rich and diverse topographical transformations of initially flat films composed of photoresponsive, azobenzene-functionalized liquid-crystalline polymer networks. The N-fold symmetry of the topography of the photoinduced response of defect-containing azo-LCN films is examined by T. J. White and co-workers on page 5880. It is found to be inherently coupled to the strength of the defect, confirming that the director profile emanating from the topological defect governs the behavior. The topography of a film subsumed with a +10 defect resting on water after exposure to light is shown here.

Recent advances in polymer network liquid crystal spatial light modulators

ABSTRACTPolymer network liquid crystal (PNLC) spatial light modulators are attractive for display and photonic applications because they can achieve submillisecond response time while keeping a large phase change. However, their on‐state scattering caused by the grain boundary of LC multidomains hinders their applications. In this article, we review recent progress on the development of scattering‐free PNLCs extending from short‐wavelength infrared to visible region by reducing the domain sizes to ∼200 nm through low temperature curing process. To reduce operation voltage, both transmissive and reflective modes, LC material properties (birefringence and dielectric anisotropy), polymer composition and concentration, and pretilt angle effect are analyzed. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 183–192

Characterization of azobenzene polymer networks using in situ solid state NMR and temperature dependent photostriction

Azobenzene liquid crystal polymer networks (azo-LCNs) undergo a complex light-driven molecular conformation change of the azobenzene chromophore which imparts a macroscopic shape change within a glassy polymer network. To better understand molecular conformational changes which underlie macroscopic polymer deformation, we have collected solid-state nuclear magnetic resonance (NMR) data on 19F fluorinated side-chain azo-LCNs using an in situ visible light (450–458 nm) LED light source. We illustrate measurable changes in 19F NMR lineshapes under light irradiation, indicating that conformational changes can be probed by NMR. The measured effects of light on NMR spectra are also found to be reversible upon removal of the light source. We further show that sample heating does not affect azobenzene isomerization through analysis of temperature-dependent magic-angle-spinning NMR lineshapes. These results illustrate a narrowing of the lineshapes, but no change in the NMR peak positions, indicating that heating from 30 and 60 ° C affects molecular dynamics but does not change the azobenzene conformation. In addition to NMR data, benchtop photomechanical uni-axial measurements are taken over a temperature range from 23 to 60 ° C. Samples with the fluorinated side-chain azo-LCNs are compared to samples composed only of non-fluorinated main-chain azo-LCN composition. Similar stress relaxation was observed in both compositions under high pre-stretch. The amount of stress relaxation was found to depend on the pre-stretch, the ambient temperature, and the polarization of light.

Topography from Topology: Photoinduced Surface Features Generated in Liquid Crystal Polymer Networks

Films subsumed with topological defects are transformed into complex, topographical surface features with light irradiation of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs). Using a specially designed optical setup and photoalignment materials, azo-LCN films containing either singular or multiple defects with strengths ranging from |½| to as much as |10| are examined. The local order of an azo-LCN material for a given defect strength dictates a complex, mechanical response observed as topographical surface features.

P.1: Distinguished Student Poster Paper: Submillisecond‐response Polymer Network Liquid Crystal Cylindrical Microlens Array for 3D Displays

AbstractWe demonstrate a submillisecond‐response tunable‐focus cylindrical microlens array based on polymer network liquid crystal. The focal length can be tuned from 1.9 cm to 5 cm by increasing the voltage from 0 to 80V. This microlens array has potential applications in autostereoscopic 3D displays.

Reconfigurable fabrication of scattering-free polymer network liquid crystal prism/grating/lens

We report a simple reconfigurable printing method for fabricating submillisecond-response and scattering-free polymer network liquid crystal photonic devices, such as prism, grating, and lens. To suppress light scattering in the visible region, we reduce the domain sizes by controlling polymer concentration, selecting a high viscosity liquid crystal (LC) host, and performing UV curing at a low temperature. To demonstrate the configurability, we printed a LC micro-prism array with ∼300-μm feature size and a circular lens with 1.3-mm radius without any pre-patterned templates. This reconfigurable printing technique enables fast design iterations and should have widespread applications for fabricating display and photonic devices.

A microlens array based on polymer network liquid crystal

Using UV light to expose a homogeneous cell containing liquid crystal (LC)/monomer mixture through a patterned photomask, we prepared a polymer network liquid crystal (PNLC) microlens array. In each microlens, the formed polymer network presents a central-symmetrical inhomogeneous morphology and LC exhibits a gradient refractive index distribution. By applying an external voltage to the cell, the gradient of the LC refractive index is changed. As a result, the focal length of the microlens can be tuned. Our PNLC microlens array has the advantages of low operating voltage, easy fabrication, and good stability. This kind of microlens array has potential applications in image processing, optical communications, and switchable 2D/3D displays.

Electric polarization frozen by a polymer network in nematic liquid crystals

We constructed nematic liquid crystal–polymer network composites by photon-polymerizing monomers in nematic phase under externally applied electric fields. The monomers formed a polymer network which can stabilize not only the director of the liquid crystal but also the electric polarization of the liquid crystal. The composites exhibited different dielectric behaviors depending on whether the monomers were polymerized under DC or AC voltages. When the monomers were polymerized under AC voltages, the formed composite responded to applied electric fields as a dielectric medium. When the monomers were polymerized under DC voltages, the formed composite exhibited a net polarization.

Torsional mechanical responses in azobenzene functionalized liquid crystalline polymer networks

Soft materials capable of both planar and flexural–torsional responses could enable the development of soft robotic elements that emulate the dexterity and functionality of a multitude of creatures in the animal kingdom. Here, we examine the response of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs) specifically focusing on realizing large magnitude flexural–torsional responses observed as out-of-plane twisting or coiling. Towards this end, azo-LCNs were prepared in either the twisted nematic (TN) or hybrid orientations. The characterization of the flexural–torsional photomechanical responses is complimented with examination of thermomechanical properties. The diverse range of tailorable photomechanical responses is shown to be strongly dependent on the alignment of the nematic director to the film geometry and the actinic light intensity.

Gradient polymer network liquid crystal with a large refractive index change

A simple approach for preparing gradient polymer network liquid crystal (PNLC) with a large refractive index change is demonstrated. To control the effective refractive index at a given cell position, we applied a voltage to a homogeneous cell containing LC/diacrylate monomer mixture to generate the desired tilt angle and then stabilize the LC orientation with UV-induced polymer network. By varying the applied voltage along with the cells' movement, a PNLC with a gradient refractive index distribution is obtained. In comparison with conventional approaches using patterned photomask or electrode, our method offers following advantages: large refractive index change, freedom to design specific index profile, and large panel capability. Potential applications include tunable-focus lenses, prism gratings, phase modulators, and other adaptive photonic devices.

Engineering of Complex Order and the Macroscopic Deformation of Liquid Crystal Polymer Networks

Rise or fall: Complex-structured freestanding polymer films with molecular order in three dimensions were prepared through photoalignment of polymerizable liquid crystals. The resulting films deform into cone and saddle shapes upon heating.

Engineering of Complex Order and the Macroscopic Deformation of Liquid Crystal Polymer Networks

Engineering of Complex Order and the Macroscopic Deformation of Liquid Crystal Polymer Networks

Photomechanical Response of Pre‐strained Azobenzene‐Functionalized Polyimide Materials

AbstractRecently, we have reported that azobenzene‐functionalized polyimide materials exhibit a substantial increase in photogenerated stress in comparison to azobenzene‐functionalized acrylate‐based liquid crystal polymer networks. Here, we show that pre‐straining the azo‐CP2‐20 material further increases the magnitude of the photomechanical response visualized as bending in the cantilever geometry as well as in direct measurements of photogenerated stress. Additionally, we also report on the ability to optically fix both optically and mechanically generated shapes in azo‐CP2‐20. The optically fixable shape memory of azo‐CP2‐20 is enabled by the introduction of excess free volume during thermal processing as the material was pre‐strained.magnified image

Submillisecond-response and scattering-free infrared liquid crystal phase modulators

We demonstrate a submillisecond-response and scattering-free infrared phase modulator using a polymer network liquid crystal (PNLC). The required voltage for achieving 2π phase change at λ = 1.06 µm is 70V (or 5.8 V/μm) and the measured response time is ~200 µs at 25°C and 30 µs at 70°C. Opposite to our conventional understanding, a high viscosity LC helps to achieve small domain size during polymerization process, which in turn reduces the response time and light scattering. We use Rayleigh-Gans-Debye scattering model to analyze the voltage-on state transmission spectra. When the domain size is comparable to the wavelength, the model fits with experimental results well. But when the domain size is smaller than the wavelength, the simple Rayleigh model works well.

Photochemical Mechanism and Photothermal Considerations in the Mechanical Response of Monodomain, Azobenzene-Functionalized Liquid Crystal Polymer Networks

The potential for wireless transduction of input light energy into mechanical outputs has led to a reinvigorated pursuit of photomechanical effects in polymeric materials and composites. We report here on factors influencing the photochemical mechanism (and thus the mechanical output) in monodomain azobenzene-functionalized liquid crystal polymer networks. Through systematic examination of a representative material with both mechanics and spectroscopic characterization the prevalence of the trans–cis and trans–cis–trans mechanisms is elucidated. Furthermore, the role of light intensity in generating heat (photothermal effects) is also reported.

A polarization-independent liquid crystal phase modulation using polymer-network liquid crystals in a 90° twisted cell

A polarization-independent liquid crystal phase modulation using polymer-network liquid crystals in a 90° twisted cell (T-PNLC) is demonstrated. T-PNLC consists of three layers. Liquid crystal (LC) directors in the two layers near glass substrates are orthogonal to each other and those two layers modulate two eigen-polarizations of an incident light. As a result, two eigen-polarizations of an incident light experience the same phase shift. In the middle layer, LC directors are perpendicular to the glass substrate and contribute no phase shift. The phase shift of T-PNLC is electrically tunable and polarization-independent. T-PNLC does not require any bias voltage for operation. The phase shift is 0.28 π rad for the voltage of 30 Vrms. By measuring and analyzing the optical phase shift of T-PNLC at the oblique incidence of transverse magnetic wave, the pretilt angle of LC directors and the effective thickness of three layers are obtained and discussed. The potential applications are spatial light modulators, laser beam steering, and micro-lens arrays.

Photomechanical bending mechanics of polydomain azobenzene liquid crystal polymer network films

Glassy, polydomain azobenzene liquid crystal polymer networks (azo-LCNs) have been synthesized, characterized, and modeled to understand composition dependence on large amplitude, bidirectional bending, and twisting deformation upon irradiation with linearly polarized blue-green (440–514 nm) light. These materials exhibit interesting properties for adaptive structure applications in which the shape of the photoresponsive material can be rapidly reconfigured with light. The basis for the photomechanical output observed in these materials is absorption of actinic light by azobenzene, which upon photoisomerization dictates an internal stress within the local polymer network. The photoinduced evolution of the underlying liquid crystal microstructure is manifested as macroscopic deformation of the glassy polymer film. Accordingly, this work examines the polarization-controlled bidirectional bending of highly concentrated azo-LCN materials and correlates the macroscopic output (observed as bending) to measured blocked stresses upon irradiation with blue-green light of varying polarization. The resulting photomechanical output is highly dependent on the concentration of crosslinked azobenzene mesogens employed in the formulation. Experiments that quantify photomechanical bending and photogenerated stress are compared to a large deformation photomechanical shell model to quantify the effect of polarized light interactions with the material during static and dynamic polarized light induced deformation. The model comparisons illustrate differences in internal photostrain and deformation rates as a function of composition and external mechanical constraints.

Synthesis and phase behavior of chiral liquid crystalline polymeric networks derived from menthol

The synthesis of new chiral monomer 4-(menthyloxyacetoxy- benzoyloxy)biphenyl-4′-(2-(undec-10-e noyloxy)ethoxy)benzoate (ML), crosslinking agent 4-(undec-10-enoyloxy)biphenyl-4′-(2-(undec-10-enoyloxy)ethoxy)benzoate (CA), and liquid crystal polymer networks (E1−E5) containing menthyl group is presented. Their chemical structures and phase behavior were characterized with Fourier transform infrared (FT-IR), proton nuclear magnetic resonance (1H-NMR), elemental analyses, polarizing optical microscopy, differential scanning calorimetry, thermogravimetric analysis (TGA), and X-ray diffraction. The selective reflection of light for ML was investigated with ultraviolet/visible/near infrared (UV/Visible/NIR). By inserting a flexible spacer between the mesogenic core and the terminal menthyl groups, MLcould form mesophase and show a chiral smectic C phase, cholesteric phase and cubic blue phase. CA displayed a smectic A phase and nematic phase. The polymer networks containing less than 12 mol% of the crosslinking units showed reversible cholesteric phase transition, wide mesophase temperature range, and excellent thermal stability. With increasing the content of crosslinking unit, the corresponding Tg increased, the Ti decreased, and the mesophase temperature range narrowed for E1−E5. TGA showed that the Td(5%) was greater than 330°C for E1−E5.

Discrimination of Alcohol Molecules Using Hydrogen-Bridged Cholesteric Polymer Networks

This paper presents a novel hydrogen-bridged cholesteric liquid crystal (CLC) polymer networks with a porosity. This kind of material is adopted to distinguish methanol from ethanol. Because of the diversity of molecular affinities in ethanol and methanol with the hydrogen-bridged cholesteric polymer networks, the reflected colors of the cholesteric polymer networks are obviously different upon the uptake of the alcohol solutions with different ratios of methanol/ethanol. Furthermore, the composition of the monomers was altered to obtain the different sensitivities of the different alcohol concentrations and ratios of methanol/ethanol. The reduction of cross-link density and increasing amount of alkyloxybenzoic acid in polymer networks can enhance the distinguishing ability between methanol and ethanol. The function of change in transmission-valley wavelength of CLC versus the alcohol concentration and the ratio of methanol/ethanol was also obtained from the experimental results.