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Initiatorless Photopolymerization of Liquid Crystal Monomers Kyung Min Lee,†,‡ Taylor H. Ware,†,‡,§ Vincent P. Tondiglia,†,‡ Matthew K. McBride,∥ Xinpeng Zhang,∥ Christopher N. Bowman,∥ and Timothy J. White*,† †
Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433-7750, United States ‡ Azimuth Corporation, 4027 Colonel Glenn Hwy, Beavercreek, Ohio 45431, United States § Department of Bioengineering, The University of Texas at Dallas, Richardson, Texas 75080, United States ∥ Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, United States S Supporting Information *
ABSTRACT: Liquid crystal monomers are widely employed in industry to prepare optical compensating films as well as extend or enhance the properties of certain display modes. Because of the thermotropic nature of liquid crystalline materials, polymerization of liquid crystalline monomers (sometimes referred to as reactive mesogens) is often initiated by radical photoinitiation (photopolymerization) of (meth)acrylate functional groups. Here, we report on the initiatorless photopolymerization of commercially available liquid crystalline monomers upon exposure to 365 nm UV light. Initiatorless polymerization is employed to prepare thin films as well as polymer stabilizing networks in mixtures with low-molar-mass liquid crystals. EPR and FTIR confirm radical generation upon exposure to 365 nm light and conversion of the acrylate functional groups. A potential mechanism is proposed, informed by control experiments that indicate that the monomers undergo a type II Norrish mechanism. The initiatorless polymerization of the liquid crystalline monomers yield liquid crystalline polymer networks with mechanical properties that can be equal to those prepared with conventional radical photoinitiators. We demonstrate that initiatorless polymerization of display modes significantly increases the voltage holding ratio, which could result in a reduction in drive voltages in flat-panel televisions and hand-held devices, extending battery life and reducing power consumption. KEYWORDS: liquid crystal polymers, liquid crystals, photopolymerization, photoinitiator, displays
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INTRODUCTION
light, and charge transfer (CT) polymerization including donor−acceptor molecules.30−37 Generally speaking, photoinitiation occurs via either Norrish type I or type II mechanisms. The Norrish type I mechanism is the photochemical homolysis of aldehydes and ketones which produces two free radical intermediates. The photochemical process starts with the absorption of a photon by the molecule and the accompanying change in energy to the excited singlet state. Thereafter, the molecule moves to an excited triplet state through intersystem crossing. Upon cleavage of the α-carbon bond, two radical fragments are obtained.38 The Norrish type II reaction occurs through intermolecular abstraction of a hydrogen from proton donating molecules by the excited carbonyl compound to produce a 1,4-biradical as a primary photoproduct.39 Liquid crystal (LC) materials are ubiquitous in the modern era due to their wide employment in displays. Displays commonly employ low-molar-mass liquid crystalline materials
Because of their excellent mechanical and optical properties, acrylate polymers are widely used in numerous applications, such as liquid crystal displays,1−7 optical fibers and compact discs,8,9 waveguides,10 lenses,11−13 and actuators.14,15 The radical polymerization of acrylate monomers can be initiated by heat or light (photopolymerization).16 Photopolymerization has many advantages relating to the ease of spatial or temporal control of light, energy savings, and processing. Lightresponsive molecules or molecular systems capable of generating radicals in the presence of light are commonly known as photoinitiators. Often the photoinitiator is comparatively expensive relative to the monomer precursors, and their byproducts can produce undesirable yellowing. Not surprisingly, initiatorless photopolymerization of acrylate and other monomers has been pursued. Several investigations have detailed the initiatorless photopolymerization of acrylate monomers including radiation using an electron beam (EB)17−20 or a short wavelength excimer lamp (220−225 nm),21−25 bicomponent systems (such as brominated acrylate/ acrylate monomers,26 vinyl acrylate/acrylate monomers,27 and thiol/ene monomers28,29) upon exposure of 313−365 nm UV © XXXX American Chemical Society
Received: July 26, 2016 Accepted: September 16, 2016
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DOI: 10.1021/acsami.6b09144 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. Absorption spectra of (a) 6 × 10−5 M RM82 in THF and pure RM82 in the nematic phase at 100 °C (1 μm thickness) as well as (b) pure RM82 in the nematic state 100 °C () and isotropic state 120 °C (- - -). Confirmation of the phase behavior is evident in the inset polarized optical micrographs at 100 and 120 °C. (c) Photographs of polymeric films prepared from the direct exposure of pure liquid crystal monomers RM82, RM257, M04301, and SLO4151. Samples were polymerized at 100 °C for 1 h with exposure to 50 mW/cm2 of 365 nm light. (d) Thermomechanical properties of a film composed of a liquid crystalline polymer network prepared from the initiatorless polymerization of RM82.
structures, J-type52,53 and H-type, of liquid crystals have been reported. These aggregates have different stacking offset angles due to a transverse slippage between mesogens. The slipped stacking arrangement (degree of slippage) is optimized by a source of steric hindrance, which distorts the π−π stacking.51,54 J-type aggregates show tilted stacks52,53 and red-shifted absorption (bathochromic) whereas H-aggregates show untilted or less tilted stacks and blue-shifted absorption. In this article, we report on the spontaneous initiatorless photopolymerization of single component liquid crystal monomers. Through control experiments, we confirm that the reaction is radical in nature with indications that the reaction proceeds via a Norrish type II mechanism. The initiatorless photopolymerization could be beneficial in many current applications of liquid crystals, including reducing the energy consumption of stabilized display modes.
that are thermotropic and typically in the nematic phase. In a series of papers in the late 1980s, researchers from Philips documented monomers that also formed liquid crystalline phases (liquid crystal monomers (LCMs), also known as reactive mesogens).2,40−44 The polymerization of liquid crystal monomers generated films with interesting thermal and optical properties and are also pervasively employed in displays to improve viewing angle or enhance energy efficiency.3,4,45 Since these materials and mixtures of them typically exhibit liquid crystalline phases at elevated temperatures, processing is greatly simplified by photopolymerization of these monomers. The display industry has developed alignment techniques and drive schemes to generate a variety of display modes, some of which require stabilization of low concentrations of polymers prepared from liquid crystalline monomers. These materials have also been subject to recent interest in preparing shapechanging topographies and structures.14 In order to realize initiatorless photopolymerization, a material system must absorb the incident photons and subsequently form radical species. The absorption of liquid crystals, owing to their aromatic nature, has a comparably redshifted absorption to more conventional (meth)acrylate monomers. Furthermore, it is well-known that self-assembly of dye molecules or liquid crystals even when dispersed in solvents (5−30 wt %) can show unexpected shifts in absorption.46−51 Driven by the fundamental thermodynamics of self-assembly, liquid crystals favor organization driven primarily by π−π and head−tail interaction between the mesogens. The π−π interaction occurs in between molecules with molecular p-orbitals, and two self-assembled stacked
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EXPERIMENTAL SECTION
Photopolymerization of Liquid Crystalline Monomers. The polymerizations examined here employed the commercially available monomer RM82 (1,4-bis[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2methylbenzene) (materials procured from Merck, Synthon, and Alpha Micron were examined). Liquid crystalline polymer networks (LCNs) prepared by initiatorless polymerization of RM82 were compared to those prepared by conventional photoinitiation by mixing 2 wt % of the photoinitiator Irgacure 369 (I-369) (Ciba). The chemical structure of RM82 is the inset in Figure 1. Samples were prepared by melting the mixtures at 100−130 °C (nematic to isotropic range) and drawing through capillary action between two glass slides spaced by glass rod spacers. Reactions were initiated by exposure to 365 nm light. Other liquid crystalline monomers were also examined B
DOI: 10.1021/acsami.6b09144 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. (a) Real-time FTIR examination of the polymerization reaction of RM82 cured with UV light irradiation (i) at 110 °C in the nematic state (initiatorless), (ii) at 120 °C in the isotropic state (initiatorless), and (iii) at 120 °C in the isotropic state with 1 wt % photoinitiator (I-369). (b) EPR signal of 62 wt % RM82 mixed in THF without I-369: (i) before and (ii) after UV exposure (50 mW/cm2).
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including RM257 (1,4-bis[4-(3-acryloyloxypropyloxy)benzoyloxy]-2methylbenzene) (Synthon, Merck), M04301 (1,4-bis[4-(11acryloyloxyundecyloxy)benzoyloxy]-2-methylbenzene) (Alpha Micron Inc. and Synthon), and SLO4151 (4-[[(3S)-3-methyl-6-[(1-oxo-2propenyl)oxy]hexyl]oxy]-1,4-phenylene ester) (Alpha Micron Inc.). After polymerization, the films were removed from the glass substrates. Polymer stabilized nematic liquid crystal samples were prepared in 15 μm thick planar alignment cells with ITO coatings in the same reaction conditions. Alignment cells were prepared in our laboratories with previously described methods.55 Characterization Methods. The thermomechanical properties of the LCN films were examined in specimens with 6 mm × 1 mm × 0.02 mm (length, width, and thickness, respectively). Storage modulus (E′), loss modulus (E″), and loss tangent (tan δ) were determined by dynamic mechanical analysis (DMA) (Q800, TA Instruments) operating at a strain of 0.5% and frequency of 1 Hz with a heating rate of 2 °C/min over the temperature range of 20−200 °C. Glass transition temperatures (Tg) are reported from the peak value of the tan δ curve. Absorption spectra for all samples were collected with a Cary 5000 UV−vis spectrometer. The voltage holding ratio (VHR) of polymer stabilized liquid crystal mixtures with or without a photoinitiator (Irgacure 369, Merck) was measured by using a HP Elsicon VHR-100. A peak voltage of 9 V, 64 ms pulse at a 60 Hz period was used, and the VHR value of an empty cell with 4 μm thickness with planar geometry was higher than 99%. Real-time FTIR (RTIR) measurements were performed on a Nicolet 6700 with a custom-built horizontal heating stage. Acrylate conversion was measured as the decrease in the peak area from 830 to 800 cm−1. Series scans were taken 1 scan per second. The sample was heated to the melt and pressed in between sodium chloride plates. Light exposure was done with a mercury bulb (EXFO Acticure 4000) with a 365 nm filter. Electron paramagnetic resonance (EPR) tests of radical generation upon UV irradiation were performed on Bruker Elexsys E 500 EPR spectrometer equipped with Bruker superhigh sensitivity resonator (SHQE cavity). Typical EPR parameters for radical generation were a 4 G modulation amplitude with a modulation frequency of 100 kHz, a receiver gain of 60, and an attenuation of 25 dB. 1 H and 13C NMR for RM82 monomer was measured by Bruker Advance 400 MHz spectrometer. Chemical shifts were referenced to the tetramethylsilane (TMS). The NMR spectra for the monomer examined here are included in the Supporting Information (Figure S1). Elemental analysis for RM257 and RM82 monomers was conducted by MIDWEST MICROLAB, Inc. Carbon, hydrogen, oxygen, and nitrogen analyses were performed via combustion at 990 °C with the elemental analyzer. Oxygen content was measured by pyrolysis at 1200 °C and determined gravimetrically. The calculated and measured results are well matched and summarized in Table S1, confirming the purity of the monomers.
RESULTS AND DISCUSSION The absorption spectra of the liquid crystal monomer (RM82, inset chemical structure) in solution with THF and in the nematic phase are shown in Figure 1. Both samples exhibit an absorption peak centered around 265 nm. The RM82 melt at 100 °C (nematic phase) exhibits a comparably broader absorption peak and can absorb appreciable light at long wavelengths. This red-shift can be indicative of electronic interactions relating to aggregation and organization expected of liquid crystalline materials. Such broad aggregates absorb light of various wavelengths, showing a broad absorption peak and a new secondary peak.50 This absorption is attributable to the aromatic structure of these monomers. For comparison, the absorption spectrum of the common aliphatic diacrylate monomer (1,6-hexanediol diacrylate, HDDA) is included in Figure S2. As illustrated in Figure 1b, the absorption spectrum of RM82 is largely unchanged when heated into the isotropic state. This indicates that the phase behavior of these materials is not the dominant factor in generating the comparatively broad absorption but simply the molecular-level interaction of the monomers. To our knowledge, no prior examination has documented the initiatorless polymerization of these materials although initiatorless polymerization of these materials may have been observed but not described.56,57 Here, by exposing a liquid crystal alignment cell filled with simply RM82 at 100 °C with exposure of ∼50 mW/cm2 365 nm wavelength UV light for 1 h a robust film (Figure1c) can be harvested with a glass transition temperature (Tg) of 117 °C and 1.5 GPa storage modulus (E′) at 30 °C in Figure 1d. For comparison, an LCN prepared of RM82 initiated with 2 wt % Irgacure 369 at the same light conditions exhibits a slightly higher Tg (125 °C) and E′ (1.6 GPa) (Figure S3). It should be noted that simply subjecting the initiatorless mixtures to the process conditions (e.g., elevated temperature) does not generate a solid film. Other common diacrylate monomers, such as RM257, M04301, and even chiral versions such as SLO4151 received from various manufacturers (Merck, Synthon Chemicals and AlphaMicron, Inc.), are also capable of initiatorless polymerization, illustrated in Figure 1c. RM82 secured from all three manufacturers photopolymerized by irradiation with UV light in the absence of any photoinitiator. The reaction kinetics and overall conversion of the acrylate group during UV exposure were examined with real-time FT-IR (RTIR). Three reactions were examined: the initiatorless polymerization of RM82 at 110 °C (nematic phase) and both C
DOI: 10.1021/acsami.6b09144 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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can be excited by absorbing the 365 nm UV light, and the excited aggregates can abstract the hydrogen in the proton donating molecules (acrylate group). The radicals from the proton donating molecules are generated by hydrogen abstraction process, which is type II initiation. The generated radicals can attack the acrylate groups in LC monomers and start photopolymerization without photoinitiators. The type II hydrogen abstraction initiation is briefly explained in Scheme 1.
initiatorless and photoinitiated polymerization of RM82 at 120 °C (isotropic phase) (Figure 2a). All samples were polymerized in bulk without solvent with 50 mW/cm2 light intensity. The change in area of the peak at 810 cm−1, which corresponds to the acrylic carbon−carbon double bond, as monitored to determine the monomer conversion. At 110 °C conversion reaches ∼50%, whereas 75% conversion at 120 °C is observed. Acrylate conversion increases at a higher temperature due to increased chain mobility. The LC monomer with photoinitiator (1 wt % I-369) shows higher acrylate conversion (∼90%) than LCM without photoinitiator. The acrylate conversion without photoinitiator reaches ∼75% conversion in the equivalent polymerization conditions here, which is about 90% of the conversion of LCM with photoinitiator. Because of higher conversion of the LCM with photoinitiator, Tg and E′ are higher than those of the LCM without photoinitiator, as can be seen in Figure S3. Electron paramagnetic resonance (EPR) is used to directly study the radical concentration and environment during photopolymerization of various acrylate monomers.58 Figure 2b shows EPR of 62 wt % RM82 in deoxygenated THF solvent without a photoinitiator. Before UV exposure, no radical signals are observed. Evident in Figure 2b, a clear radical signal is apparent after the material is exposed to UV light. In dilute solutions in which RM82 exhibits minimal absorption (Figure 1a), no radical signal is apparent before or after UV irradiation, indicating the reaction does not proceed (Figure S4). To further investigate the initiation, an amine molecule (ethyl 4-(dimethylamino)benzoate, chemical structure inset into Figure 3) was added as a proton donating molecule. As can
Scheme 1. A Proposed Hydrogen Abstraction Type II Initiation Mechanism
We believe that the initiatorless polymerization of liquid crystal monomers could be beneficial to applications of these materials in displays. Increasingly, display modes, for example the vertical alignment mode, employ polymer stabilization by including these monomers and reacting them via photoinitiated polymerization. Ionic impurities cause several deleterious side effects in LC displays such as image flickering, image sticking, reduced voltage holding ratio, lowered switching speed, increased threshold voltage, and grayscale shift.59−63 The photoinitiator is one of the major sources of ions.55,64,65 The impact of ions on the performance of LC displays is often evaluated in a metric referred to as the voltage holding ratio (VHR). VHR is determined by charging LC cells with low holding voltage that is accompanied by a short voltage pulse. The voltage across the cell is then monitored after this perturbation. Because of the sensitivity and mobility of ions to the voltage spike, the applied voltage will decrease. If there are no mobile ions, as would be expected by two electrodes separated by only air, the applied voltage observed after the voltage spike will be constant over time and approximately equal to 1. The VHR is measured by eq 1:66
Figure 3. Real-time FTIR examination of the double-bond conversion of (i) the RM82 melt (in the isotropic state) and (ii) a mixture of 9:1 ratio of RM82:amine (in the isotropic state). Neither mixture included any photoinitiator. Both samples were irradiated with 50 mW/cm2 UV light at 120 °C.
be seen in Figure 3, adding the proton donating amine molecule does increase the conversion rate and final conversion, indicating a type II hydrogen abstraction initiation. If the initiatorless reaction was proceeding by type I (e.g., unimolecular bond cleavage to generate radical species), the kinetics would not depend or be expedited by an additive such as the amine included here. Liquid crystal mesogens in high concentration mixtures or the pure LC melt can be highly aggregated through π−π interaction, and the conjugated LC aggregates can absorb longer wavelength light, inducing a broad absorption peak and a new peak at higher wavelengths. The conjugated LC aggregates
VHR = VRMS/Vpeak
(1)
where VRMS and Vpeak are root-mean-square voltage and peak voltage. A peak voltage of 9 V, 64 ms pulse at a 60 Hz period was used for the VHR measurement. The VHR value of an empty 4 μm thickness cell with planar geometry was higher D
DOI: 10.1021/acsami.6b09144 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. Voltage holding ratio (VHR) of polymer-stabilized LC samples prepared by (a) initiatorless polymerization and (b) standard photopolymerization initiated with 1 wt % I-369. Polymerization was initiated in both samples by exposure to 50 mW/cm2 UV for 3 min.
Figure 5. Electro-optic response of negative dielectric anisotropy cholesteric mixture prepared with 5 wt % SLO4151 but without additional photoinitiator: (a) before and (b) after 50 mW/cm2 UV exposure for 3 min.
than 0.99. The cells were filled with LC mixtures with or without photoinitiator (I-369), exposed to the UV light for 3 min to initiate polymerization. As can be seen in Figure 4, the VHR of polymer stabilized LC prepared by initiatorless polymerization of RM82 is 0.972, whereas the VHR of a polymer stabilized LC prepared by standard, photoinitiated polymerization with 1 wt % I-369 is 0.866. Recently, we have reported control of the reflection notch position and bandwidth in polymer stabilized cholesteric liquid crystals, formulated from negative dielectric anisotropy liquid crystals.55,67,68 Here, in samples prepared without photoinitiator we observe similar electro-optic (EO) response of PSCLC prepared without photoinitiator (Figure 5). Before UV exposure, CLC mixture shows no response to applied field (spectra in Figure 5a are almost perfectly overlapped). However, after UV exposure and initiatorless polymerization, we see that upon application of as much as 60 V dc field we see symmetric broadening of the reflection notch. As we have detailed in prior reports, the electro-optic response in this systems is induced by the movement of polymer network in the CLC medium, which induces pitch variation across the cell thickness. The negative dielectric LC host aligned in the planar geometry does not realign to the dc field.
which the material can organize into liquid crystal phases or in isotropic mixtures with concentration to enable sufficient interaction. We hypothesize that the UV irradiation of the monomers forms a yet to be determined excited state that subsequently abstracts hydrogen from adjacent molecules to generate radical species capable of initiating polymerization. Control experiments with amine additives further improve the reactivity which strongly indicates that the reaction proceeds via a Norrish type II initiation.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09144. Additional EPR, spectroscopy, and mechanical data references in the text (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (T.J.W.). Notes
The authors declare no competing financial interest.
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CONCLUSION A number of diacrylate liquid crystal monomers are polymerized spontaneously without a photoinitiator by 365 nm UV exposure. The initiatorless polymerization is enabled by the red-shifted absorption of the materials at concentrations in
ACKNOWLEDGMENTS The authors are grateful for financial support from the Air Force Office of Scientific Research and the Materials and Manufacturing Directorate of the Air Force Research Laboratory. The authors also thank Professor L.-C. Chien E
DOI: 10.1021/acsami.6b09144 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.6b09144 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
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DOI: 10.1021/acsami.6b09144 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
