Elsevier

Polymer

Volume 47, Issue 8, 5 April 2006, Pages 2741-2749
Polymer

Matrix physical structure effect on the electro-optic characteristics of thiol–ene based H-PDLC films

https://doi.org/10.1016/j.polymer.2006.02.049Get rights and content

Abstract

The physical and mechanical properties of several thiol–ene based polymers and their mixtures with the liquid crystal, E7, were characterized to probe their relationship with the liquid crystal film electro-optic performance properties. Kinetic data suggests that high conversion is achieved for each thiol–ene combination. Pre-polymerization phase diagrams indicate that each thiol–ene/E7 mixture phase separates well below room temperature, and thus prior to polymerization at room temperature all are in a single phase. Holographic polymer dispersed liquid crystals (HPDLC) were fabricated for several thiol–ene and E7 mixtures, and electro-optical parameters characterized to probe the relationship between the thiol–ene network properties and the electro-optic performance of the HPDLCs. The photocured matrices exhibited glass transitions and tan δ peak maxima that ranged from temperatures below 0 °C to well above room temperature. There is a clear correlation between the physical nature of the matrix and the electro-optic switching parameters with H-PDLC films fabricated from trithiol-pentaerythritol triallylether, all of which exhibit glass transition temperatures below 0 °C, having the fastest switching times and lowest switching voltages at room temperature. Also, in each case higher liquid crystalline concentration resulted in lower switching voltages.

Introduction

Holographic polymer dispersed liquid crystals (H-PDLCs) are a relatively recent version of the extensively studied polymer dispersed liquid crystals (PDLC) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45]. H-PDLCs, unlike conventional PDLCs, contain alternating planes of polymer matrix and liquid crystal (LC) droplets. Conventional PDLCs are fabricated by the exposure of a photo-reactive monomer/LC mixture sandwiched between two conductive glass plates to a broad-band UV light source. The conversion of monomer to polymer destabilizes the initially homogeneous mixture and results in a phase separated random dispersion of micron-sized liquid crystal droplets surrounded by solid polymer. In the case of H-PDLCs, the reactive monomer/LC mixtures are holographically illuminated using laser light. The phase separated morphology for H-PDLCs consist of alternating planes of sub-micron liquid crystal droplets and solid polymer [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45].

H-PDLCs diffract visible light due to the refractive index modulation that results from the alternating planes of sub-micron liquid crystal droplets and polymer planes. H-PDLCs may be fabricated as transmission or reflection grating and are switchable when subjected to an external electric field. These features of H-PDLCs make them candidates for electro-optic applications such as displays [39], [40], [41], diffractive optics [42], fiber optics [43], remote sensing [44] and many others [45].

Due to the variety of applications, it is crucial to be able to manipulate and improve the electro-optical performance of H-PDLC films such as switching speeds and voltages, diffraction efficiency, and contrast. Bunning et al. [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [41], [42] and Crawford et al. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [40], [44] published several papers dealing with morphological development and the electro-optic properties of H-PDLC films. They evaluated the effect of exposure time and laser beam intensity [15], [17], [22], [32], [37], concentration of LC and polymer matrix [1], [31], [37], and temperature [11], [17], [37] on the H-PDLC electro-optic switching properties. They concluded that electro-optic properties are related to such parameters as the phase separated morphology. Bunning et al. [27] published a recent review article that summarizes the relevant H-PDLC literature.

The majority of the H-PDLC literature is focused on photo-curable acrylate systems [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [21], [22], [24], [26], [29], [31], [32], [33], [34], [35]. Unfortunately, there are distinct disadvantages of acrylate systems; high shrinkage, low conversion, early gel point, oxygen inhibition, and formation of a non-uniform crosslinked matrix due to early micro-gelation. These parameters can have a distinct effect on the operational stability of the grating spacing and the electro-optic properties of the films [5], [21], [22], [28], [30], [36], [38].

Photopolymerizable thiol–ene based resins are alternatives that overcome some of the problems associated with acrylate based H-PDLCs and have received recent attention [28], [30], [48]. Thiol–ene free-radical polymerization processes have been well-known for about fifty years. Progress in this area is well documented by reports from Jacobine et al. [46], Bowman and Cramer et al. [47] and Hoyle et al. [48]. The free-radical chain reaction between a thiol and an ene proceeds by a step-growth process, a free radical addition followed by a chain-transfer reaction. H-PDLCs [18], [19], [20], [28], [30] and conventional PDLCs fabricated from thiol–ene systems exhibit improvements in uniformity of LC droplet size and shape, switching speeds, contrast ratios and high transmission in the on state [28], [30]. All of the PDLC and H-PDLC reports in the literature dealing with thiol–ene based resins have centered on a single system, Norland 65, comprised of a tetrafunctional ene and a trifunctional thiol. Norland 65, when combined with the E7 liquid crystal mixture, gives rise to PDLCs and H-PDLCs with good switching and optical properties, due in great part to the proper balance of the birefringent refractive index properties of the E7 liquid crystal and the thiol–ene matrix. The tetrafunctional ene in Norland 65 has two urethane groups per molecule and thus it is effective in inducing phase separation due to the mismatch in solubility parameters between the urethane groups and the cyanobiphenyl LC components. Moreover, it is well known that urethane groups form hydrogen bonds, thus adding another dimension to network buildup and phase separation between the thiol–ene network and LC components. Urethane groups associate via hydrogen bonding, with the extent of hydrogen bonding depending upon the urethane concentration and the equilibrium constant between the associated and unassociated urethane groups. Introduction of a diluting component, i.e. the liquid crystal, results in a system in which the extent of hydrogen bonded versus free urethane groups changes as a complicated function of the diluting component concentration. Hydrogen bonding is present in both the pre-cured mixture and the final network, although it is assuredly different in each. This can certainly present a profound problem when trying to interpret the basic pre-polymerization phase diagram and its relationship to polymerization kinetics and film formation. Also, hydrogen bonding will also have a distinct effect on the nature of the cured matrix properties and the resulting electro-optic responses. The amount of liquid crystals dissolved in the continuous matrix will affect the hydrogen bonding thus making it more difficult to correlate polymer physical properties and electro-optic response.

As mentioned, all thiol–ene H-PDLCs to date have been fabricated from Norland 65, which we have recently found to have a fairly high glass transition temperature above room temperature. In order to be in a better position to design new matrices for H-PDLCs involving thiol–enes, we wanted to answer a basic question: ‘what role does the physical state of the matrix at the temperature which the electro-optic measurements are made have on the critical switching parameters of voltage and time’. To answer this question, it is necessary to evaluate electro-optic properties at a single temperature for several H-PDLC matrices in different physical states (rubber, glass) at that temperature. It is important to do this in the absence of the influence of hydrogen bonding, which would complicate the interpretation of the results. Accordingly, three thiol–ene systems in which a common trifunctional thiol is photopolymerized with three distinct enes that do not contain urethane groups were evaluated with respect to their physical/mechanical properties in the presence of varying concentrations of liquid crystal. The three cured thiol–ene systems indeed have different physical states at room temperature, and therefore, the electro-optic properties should be reflective of the physical/mechanical configuration of the network, thereby allowing the basic relationship between physical state and switching parameters to be determined. Thermal and dynamic mechanical analysis results of cured networks with a wide variation in the liquid crystal concentration were correlated with the H-PDLC electro-optic performance. A single liquid crystal mixture, E7, was chosen for use in each thiol–ene system. The diffraction properties, which depend upon on the refractive index of the matrix and the liquid crystal mixture among other factors, were therefore not optimized since this would have required selecting a different liquid crystal mixture for each matrix, thus prohibiting a valid comparison of physical state and electro-optic switching properties. By keeping the liquid crystal type constant, a direct comparison between matrix physical state and switching voltage and times was thus obtained.

Section snippets

Materials

Trimethylolpropane tris(3-mercaptopropionate), pentaerythritol triallyl ether, triallyltriazine trione, and triallyloxytriazine were obtained from Aldrich Chemical Company. The nematic liquid crystal mixture, E7, was obtained from EM industries. The initiator 2,2-dimethoxy 2-phenyl acetophenone (DMPA) was obtained from Ciba Specialty Chemicals. All chemicals were used as received without further purification.

Methods

Pre-polymerization monomer/LC mixtures were examined using temperature controlled

Results and discussion

In this report, we present results of a systematic investigation of the material properties of several different thiol–ene based polymer matrices with liquid crystal mixture, E7, present. The question of how structural features of the monomer components affect the properties of the phase separated networks is addressed. Specifically, we carried out a study of basic crosslinked networks formed by reacting a single trifunctional thiol with three trifunctional ene monomers with very different

Conclusions

A systematic investigation of the effect of thiol–ene structure on the physical, mechanical, and electro-optic properties of H-PDLC photocurable systems has been conducted. The results indicate that the chemical structures of the components comprising the thiol–ene/liquid crystal formulation dictate the physical properties of the resultant films and play a role in the electro-optic performance of fabricated H-PDLCs. In each case, as the liquid crystal content increased for a given matrix type,

Acknowledgements

This work was supported by AFOSR (DEPSCoR), NSF (DMR-9512506) and the MRSEC Program of the National Science Foundation under Award Number DMR 0213883.

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