Correlation between the method of preparation and the properties of polymers and liquid crystals materials
-
BENMOUNA Réda*, BENYOUCEF Boumediène
-
Research Unit ²Materials and Renewable Energies²
-
University Aboubekr Belkaïd, Faculty of Sciences, Department of Physics
-
Tlemcen BP119, Algeria.
-
* To whom correspondence should be addressed: redabenmouna@yahoo.com
ABSTRACT
In this paper we discuss the correlation between the conditions of preparation and the performance of Polymer Dispersed Liquid Crystals. These composite materials exhibit a swiss cheese morphology with randomly dispersed liquid crystal inclusions in a solid polymer matrix. The criteria for assessing the performance of such materials in practical applications are based on the quality of their electro-optic responses. These in turn are highly sensitive to the conditions of preparation. This work presents results dedicated to the characterization of such sensitivity. The systems considered here are composed of thiol-ene formulations with the eutectic mixture of low molecular weight liquid crystals E7. All systems are elaborated using in-situ photopolymerization under radiation curing according to the mechanism of Polymerization Induced Phase Separation.
1. INTRODUCTION
In this work, we will be interested on Polymer and liquid crystal (LCs) composites known as Polymer Dispersed Liquid Crystals, in short PDLCs. These systems consist of nearly spherical droplets with diameter in the mm or nm scale randomly dispersed in a solid polymer matrix. They are interesting for their applications in display devices such as TV, computer screens, in smart windows with controllable light transmission and telecommunications systems1-3.
PDLCs are also interesting because of the fundamental problem they bring as multicomponent systems. It is certainly interesting from the fundamental point of view to study the properties of composite materials made of widely different species such as high molecular weight flexible polymers and small rigid LC molecules. From the synergy of the properties of different components, one can produce new materials combining the advantages of each one of them. Sometimes, new materials are obtained with completely different properties depending on the conditions of preparation.
Most applications of PDLCs are based upon their electro-optical functionality. Under normal mode conditions, they can be switched from an opaque state to a transparent state by applying an electric field. If no electric field is applied (off state), the droplets directors are randomly oriented inducing orientational fluctuations and a strong scattering of light. If an electric field is applied (on state), they are activated and the droplets directors align progressively as the strength of the field increases. Orientation fluctuations of the directors are suppressed and the film becomes transparent. The scattered intensity is high when the size of droplets is comparable to the wavelength of radiation which means in the mm scale since we are dealing with light in the visible range.
Systems with droplets in the nanosize scale are characterized by a low scattering level. However, there is a change in the phase of incident light which in turn produces a change in the birefringence4-7. PDLCs with nanosized dimensions include Holographic PDLCs (HPDLC) with gratings8-10 and Cholesteric gels11. These systems are used for spectral filtering of fiber optics and signals in telecommunication devices. By applying the Bragg diffraction condition of multiple reflections, one could tune the size of droplets or apply an electric field to reflect the light in a certain wavelength range. For PDLCs with gratings, if matching of the refractive index of polymer and LC is satisfied, the incident beam passes through. When an electric field is applied, there is a change in the LC index of refraction inducing a reflection of part of the beam so that only part of it is transmitted. The same goal is achieved with PDLCs with randomly distributed nanosized droplets instead of gratings. All PDLCs may be elaborated according to different techniques. The type of procedure used and the conditions of preparation are essential in determining the final properties of the materials and their performance in practical applications1-3. There are essentially three methods of preparation based on phase separation phenomena. Here, we are concerned with films elaborated according to in-situ polymerisation / crosslinking under radiation curing. This procedure uses the Polymerisation Induced Phase Separation (PIPS) method whereby the kinetics of phase separation is tuned with the kinetics of polymerization. Clearly, the conditions of film preparation have a strong influence on the interplay between these competitive phenomena and control the subsequent properties of the materials and their performance in practical applications. The accordance of the curing light source with the absorption spectrum of the photoinitiator has a strong impact on the morphology, thermal behavior and electro-optical parameters of the PDLC films. The present work is an effort to assess some of these effects considering a variety of properties but with a particular emphasis on the effects of curing radiation and photoinitiator.
14
2. EXPERIMENTAL CONSIDERATIONS
2.1. Materials
In this work, we consider a laboratory made system based on thiol-ene. Its formulation contains two molecular species: a multifunctional acrylate and a thiol. The exact composition has been given in references6-7. The low molecular weight LC E7 consists of an eutectic mixture of 4 paraphenylenes with the following composition: 51wt.-% 4-cyano-4’-pentyl-biphenyl (5CB), 25wt.-% of 4-cyano-4’-heptylbiphenyl (7CB), 16wt.-% of 4-cyano-4’-octyloxybiphenyl (8OCB), and 8wt.-% of 4-cyano-4”-pentyl-p-terphenyl (5CB). This compound is convenient since it presents a wide range of temperature with good working conditions. It has a nematic to isotropic transition at TNI=61°C, a glass transition at Tg=-61°C, a large positive birefringence and a large positive dielectric anisotropy.
The main challenge is always to find the optimal combination source/photoinitiator and the right parameters (source intensity, concentration of photoinitiator etc) to achieve a product with desired characteristics and high performance in practical applications.
Polymerization Induced Phase Separation or PIPS is the method used for samples preparation combined with the radiation curing. The basic principle of this method is to irradiate a homogeneous single-phase mixture (monomer or oligomer + LC + photoinitiator) with certain intensity for a given period of time. In the dynamic regime, the sample is placed on a conveyor moving with a selected velocity chosen to achieve the prescribed dose.12-15 It is important to keep in mind that the thiol-ene formulation used here presents the advantage of not being sensitive to oxygen.
2.2. Techniques of measurement
Differential Scanning Calorimetry (DSC)
An apparatus of type Mettler DSC 30 was used. Typically 10mg of a homogeneous mixture was deposited on an aluminium pan before photocuring. The samples were then covered to start DSC measurements. The thermal cycle applied includes a cooling ramp of 5°C/min allowing detection of the major thermal events. A wide range of temperature (- 100°C to +100°C) was scanned to explore the thermophysical behaviour of uncured and photocured samples. At each composition, duplicates were considered to validate the results.
Scanning Electron Microscopy (SEM)
A high resolution SEM apparatus was employed to study the sample morphology using Cambridge Instruments 250 in the transmission mode with 10kV accelerating voltage. Films of the cured samples were peeled off from the glass substrate with a razor blade and the LC extracted by immersion into methanol during one hour. Dried samples were coated with a thin metallic film prior to the SEM measurements.
Fourier Transform Infra Red spectroscopy (FTIR)
Initial solutions were mixed with an agitator to reach a homogeneous state. Drops of the solutions were put on NaCl plates and covered with PET sheets of 100µm thickness. Both sides of PET sheets were scraped with fine sandpapers to prevent interference bands. FTIR Spectra were recorded at room temperature in the transmission mode employing a Perkin Elmer 2000 apparatus before and after exposure to the radiation. The elapsed time between the end of curing and the FTIR experiments was kept the same. Measurements were repeated three times to validate the results. Scan parameters were in the range between 700 and 4000cm-1 with 16 scans of resolution 4cm-1 at intervals of 1cm-1.
15
3. RESULTS AND DISCUSSION
3.1. Thermophysical
Differential Scanning Calorimetry (DSC) is among the standard tools to characterize the thermophysical properties. It yields the transition temperatures and the enthalpy exchanges at these transitions. This information is required to obtain the limit of solubility of LC in the polymer and the amount of LC dissolved in the matrix.
Figure 1 shows the results for the thiol-ene/E7 formulation at different compositions. It exhibits peaks corresponding to the characteristic transitions of this compound, namely the glass transition and the Nematic-Isotropic (NI) transition. For the latter transition the position of the peak indicates the temperature of NI transition while the area yields the amount of heat exchange DHNI. Collecting the results obtained at different composition enables us to deduce crucial information on the miscibility limit and the amount of LC segregated in the droplets. The results are displayed in Figure 2a for DHNI versus LC concentration and Figure 2b for a, the amount of LC segregated in droplets. The latter information is extracted from both DHNI and DCp.
16
Figure 1: DSC thermograms of thiol-ene/E7 systems at different compositions. From top to bottom 90wt%E7, 35wt%E7 and pure thiol-ene.
(a) (b)
Figure 2: (a) DHNI versus LC concentration. Intersection with x-axis yields the limit of miscibility of LC. (b) Amount of LC segregated in the droplets. Symbols represent the experimental values; solid and dashed lines are theoretical results16.
The amount of LC segregated in
the droplets has been determined according to Smith’s formula16.
The limit of solubility of the LC in the polymer matrix determined from the
extrapolation of the 
and 
curves to the x-axis. These results illustrate well the
correlation between thermophysical properties and the procedure used for
sample preparation.
3.2. Morphology
The morphology of PDLC systems can be explored by different techniques such as Optical Microscopy, Scanning Electron Microscopy (SEM), FTIR and AFM17. Here, we have used the SEM to explore the nanodroplets size distribution. The latter is expressed by histograms that are analyzed with a Gaussian fit to obtain the mean diameter and the width of the distribution (or variance). Figure 3(a) and (b) gives examples of micrographs obtained on the same sample viewed at different scales (i.e. bares represent 150nm for the picture on the left and 600nm for the other). The histogram accompanying the micrographs yields the droplet diameter distribution throughout the films. We see that the droplet size remains within the range of few tens of nm. The mean diameter and the width of the distribution both increase linearly with the LC concentration.
(a) (b) (c)
Figure 3: SEM images of thiol-ene/E7 at 35wt-%E7. Scale bars correspond to: 150nm for the picture on the left hand side and 600nm for that on the right hand side. These samples were cured using Laser Argon. Part (c) gives the corresponding histogram for the droplets size distribution.
17
Figure 3c gives a particular example of histograms established on the basis of repeated calculations of droplets diameter assuming a nearly spherical shape and scrutinizing various regions of the films. The statistics were improved by enhancing the sampling procedure and increasing the number of sampled regions. The essential feature of these results is to assess that the droplet mean diameter is in the nanometer scale and tend to increase with the amount of diluent.
3.3. Monomer conversion: FTIR Spectroscopy Study
FTIR Spectroscopy was chosen as the appropriate tool to evaluate the rate of monomer conversion by monitoring the peaks of the carbone carbone double bond C=C and the S-H bond in the thiol-ene component. Our investigation focused on the effects of the nature and the concentration of photoinitiator, LC concentration, curing radiation and temperature of polymerization. The results were useful in assessing the efficiency of the polymerization process and its kinetics. The spectrum in figure 4 gives the identification of the S-H and C=C peaks which we are dealing within the present study.
Figure 4: Identification of the S-H and C=C peaks in the FTIR Spectrum used to study the kinetics of monomer conversion. The samples were cured using Halogen source and the photoinitiator Eosine at a concentration of 0.5wt% with respect to the monomer content.
Figure 5 describes the way in which these peaks disappear when the dose of curing radiation increases. Both peaks were quite sensitive to the dose of curing but not to the same extent. Other peaks did not show such a sensitivity and therefore could not be used to evaluate the rate of conversion.
The conversion ratio was
calculated as 
where AD is the absorption coefficient at dose D
and A0 the corresponding coefficient prior to radiation exposure
which was calculated using Beer-Lambert law6,7.
18
The spectra of uncured samples were taken between NaCl plates while cured films were sandwiched between an NaCl plate and a PET sheet knowing that PET was not sensitive to the radiation used.
(a) (b)
Figure 5: FTIR spectra showing the absorbance versus wavenumber (cm-1) for the formulation thiol-ene at different doses of the curing radiation. (a) S-H bond at 2570 cm-1. (b) C=C bond at 1647 cm-1.
