Photo-optical properties of photopolymerizable cholesteric compositions

Colloid Polym Sci (2007) 285:681–686 DOI 10.1007/s00396-006-1593-5 SHORT COMMUNICATION Photo-optical properties of photopolymerizable cholesteric compositions Natalia Dadivanyan & Alexey Bobrovsky & Valery Shibaev Received: 27 July 2006 / Accepted: 4 October 2006 / Published online: 19 January 2007 # Springer-Verlag 2007 Abstract Compositions consisting of nematogenic diacrylate monomer and chiral dopants capable of forming the cholesteric mesophase and of photopolymerizing were obtained and studied. For the first time, the change of optical properties and cholesteric helix pitch during photocrosslinking of diacrylate under UV irradiation (365 nm) was investigated. The kinetics of photopolymerization was studied and several stages of this process were observed. Cholesteric photopolymerizable blends containing chiral photosensitive dopant capable of E–Z isomerizing under UV irradiation (313 nm) were studied. The decrease of the helix twisting power of the dopant during photoisomerization was observed resulting in untwisting of the helix and shift of the selective light reflection peak into the longwavelength region of the spectrum. The possibility of the photoregulation of optical properties of such blends with following fixation of structure and of these properties by means of photopolymerization under UV irradiation (365 nm) was demonstrated. Keywords Cholesteric photopolymerizing mixtures . Chiral-photochromic dopant . E–Z isomerization . Selective light reflection peak photoregulation Introduction Among liquid crystalline (LC) systems, cholesteric LC polymers, and blends are of great interest because of their unique optical properties. The first of these properties is the N. Dadivanyan : A. Bobrovsky (*) : V. Shibaev Department of Chemistry, Moscow State University, Lenin Hills, Moscow 119992, Russia e-mail: bbrvsky@yahoo.com selective circularly polarized light reflection. This property gives one the opportunity to use such materials in polarizing filters and reflectors of light in different regions of the spectrum in optics and optoelectronics. There are several ways to prepare polymer cholesteric LC systems [1–5]. One of them is the synthesis of comblike copolymers containing nematogenic groups that form the mesophase and chiral groups responsible for cholesteric helix formation. Another possibility is related to obtaining blends consisting of the nematogenic polymerizable monomer and chiral dopant, which is responsible for the formation of the cholesteric helix. From such blends, not only linear polymers, but also polymer networks can be obtained. The advantage of such crosslinked systems is the fixation of the helix pitch (P). This method allows getting films with constant wavelength maximum (1λ max) of the selective light reflection. One of the methods of network preparation is the photopolymerization of a blend containing a bifunctional nematogenic monomer which is a crosslinker and, at the same time, a chiral dopant. Such experiments are described by Broer et al. [6, 7], Lub et al. [8, 9], and others [10–13]. It is of practical interest to get from one blend cholesteric films samples with different values of 1λ max which are related to P : lmax ¼ nP, where n is the average refractive index. There are several methods of cholesteric helix pitch changing. The first method is based on the dependence of the helix pitch on temperature. Variation of the temperature allows one to obtain films with selective light reflection in different regions of the spectra. On the other hand, the P value can be changed by introducing photosensitive chiral groups into the cholesteric matrix. These groups can change their geometry under irradiation, which results in the change of P. This approach was used, for example, in [14–18]. In [14], the conformational transition in chiral 682 Colloid Polym Sci (2007) 285:681–686 azobenzene derivative fragments was described. In these substances, E–Z isomerization of N=N-bond under UV irradiation takes place. In [15–17], the analogous isomerization of C=C-bond is observed. In [18], the isomerization of a chiral dopant based on isosorbide derivatives was described. In all the above mentioned cases, E–Z isomerization leads to a decrease of the chiral fragments anisometry, helix untwisting, and shift of the selective light reflection to the long-wavelength region of the spectrum. In this work, the combination of the two methods is presented: on one hand, photocrosslinking of blends containing bifunctional nematogenic monomer and chiral photochromic dopant; on the other hand, change of the helix pitch by photoisomerization of chiral photochromic dopant (Fig. 1). The changes of the optical properties during irradiation were studied in detail. As objects for this study, two types of blends consisting of photopolymerizable bifunctional nematogenic monomer and low-molar-mass chiral dopant were chosen. In both cases, diacrylate monomer DA was used as nematogenic monomer. The main difference between Cinsorb and Hexsorb is the presence of the double bonds in Cinsorb. This means that under UV irradiation of the system containing this substance, two processes may take place: photopolymerization of diacrylate monomer and photoisomerization of chiral dopant. In addition, the photoinitiator Irgacure 651 was added into both blends (2.0 wt.%): CH 3 O OCH 3 Irgacure 651 O In order to prevent unwanted uncontrolled thermopolymerization the inhibitor, 4-methyl-2,6-ditert-butylphenol was introduced (2.0 wt.%): OH O O O O O DA O O O O O For the first blend, we used a chiral derivative of isosorbide Hexsorb (8.5 wt.%): For the second blend, a derivative of isosorbide and cinnamic acid Cinsorb was chosen (8.5 wt.%). This substance contains double bonds, and under UV irradiation, E–Z isomerization is possible: OCH3 UV O The absorbance maximum of the photoinitiator is 335 nm (Fig. 2). Therefore, the irradiation with 365 nm leads to formation of the network in both types of blends (Fig. 1). Chiral photochromic dopant Cinsorb absorbs in the short-wave region of the spectrum (313 nm, Fig. 2). UVirradiation (313 nm) of the blend containing Cinsorb induces E–Z isomerization of the dopant, which results in helix untwisting which can be fixed afterwards by 365 nm irradiation (Fig. 1). Other components of mixtures (DA, Hexsorb and inhibitor) absorb light at shorter wavelengths, below 300 nm and they are not involved directly in photochemical processes described above. Cinsorb O H H O O O O O C H 3O O H H O UV O O O OCH 3 C H3O OCH 3 O O H H O O O O OCH 3 Fig. 1 The scheme of the photoinduced processes in films of cholesteric photopolymerizable blends, P helix pitch, P2 >P1 Colloid Polym Sci (2007) 285:681–686 683 313 nm 1.4 1.2 Absorbance measurements was 0.1 °C. The spectra were obtained after annealing of the samples. Cinsorb Irgacure651 Photopolymerization 1.0 0.8 365 nm 0.6 0.4 0.2 0.0 300 350 400 λ / nm Fig. 2 UV-spectra of photoinitiator Irgacure 651 and chiral-photochromic dopant Cinsorb in dichloroethane solution. The concentration of substances in solution is almost equal to mixture composition The samples were photopolymerized at different temperatures in the LC state. The study of the photo-optical properties of the blends was performed using a specially constructed optical set-up including mercury lamp DRSh250. Light with λ=313 and 365 nm was separated using interference filters. To avoid heating of the sample because of the IR irradiation of the lamp, a water filter was used. To obtain the planar-parallel beam of light, a quartz lens was used. The light intensity was determined by intensity analyzer (gauge) Laser Mate-Q (Coherent). Results and discussion Phase behavior Experimental part Preparation of blends and films Blends were prepared by dissolution of the components in chloroform and then were dried on air. Residual solvent was removed by heating in vacuum at a temperature a little bit lower than the isotropization temperature (110–115 °C). Samples of planar-oriented films were prepared between two glasses; the thickness was fixed using glass spacers (11 μm). For samples’ preparation, a thermostatic cell “Mettler FP-86” was used. The samples were made at temperatures 10–15 °C below the clearing temperature (Tcl). Planar texture of the samples was formed as a result of mechanical shear. Afterwards, the samples were annealed for 15–20 min at the same temperature. Methods Polarization microscopy The phase transition temperatures were determined using polarization microscope “LOMO R-112” equipped with controlled hot-stage “Mettler FP-86.” Observations were made using crossed polarizers. The selective light reflection studies Selective light reflection was studied by measuring light absorbance using spectrometer J&M TIDAS. For studying temperature dependence of the selective light reflection, a specially constructed cell was used compatible with the thermostatic cell “Mettler FP-80.” The accuracy of such According to the polarization microscopy study, both blends (DA/Cinsorb and DA/Hexsorb) are crystalline at room temperature and, after heating, first become LC and then transform into isotropic state (Table 1). It is clearly seen from optical microscopy that the phase transition from crystalline to LC state is accompanied by an appearance of “oily strikes,” which are typical for the cholesteric phase [19]. Besides, the selective light reflection above the melting point is observed. This also confirms cholesteric structure formation. Temperature dependences of the selective light reflection maximum It is well known that cholesterics reflect the circularly polarized light [20]. The directions of the polarization of the light and of the cholesteric helix are the same. This allows one to study the optical properties of cholesterics by measuring transmittance (Tr) of the thin films using a spectrometer. Thus, the transmittance minimum corresponds to the reflection maximum. In Fig. 3, the dependence of the logarithm of the transmittance of the right-handed circularly polarized light for the planarly oriented film of one of the blends as a function of the wavelength is Table 1 Phase transition and optical properties of cholesteric blends Blend Phase transition temperatures, °C 1λ max at T=0.8 Tcl, nm DA/Cinsorb DA/Hexsorb Cr 104–105 N* 129.3 I Cr 105–107N* 128.8 I 469 545 684 Colloid Polym Sci (2007) 285:681–686 800 1.0 -lg Tr DA/Hexsorb DA/Cinsorb 700 λmax, nm Fig. 3 The temperature dependence of the wavelength maximum for blends DA/Cinsorb and DA/Hexsorb and the dependence of lgTr as a function of the wavelength for DA/Hexsorb (t=112 °C) 0.5 0.0 400 500 600 600 λ , nm 500 400 60 80 100 120 140 0 T, C Photopolymerization of cholesteric blends During irradiation of the DA/Hexsorb and DA/Cinsorb samples with 1λ= 365 nm, fast crosslinking is observed. During this process, the selective light reflection peak position is fixed. After irradiation, the films become insoluble in chloroform, while before that the blends were completely soluble in this solvent. Polarization microscopy shows that the texture of the samples doesn’t change during irradiation. During irradiation, the position of the selective light reflection peak changes. The kinetic curves obtained during photopolymerization are presented in Fig. 4. They can be divided into four portions. The first one (I) corresponds to the induction time, which is typical for the radical polymerization. During this time, the value of 1λ max for the blend DA/Cinsorb doesn’t change; however, in the case of the blend DA/Hexsorb, 1λ max slightly changes during the first few seconds. This means that the helix pitch is almost constant and the structure is rather stable. During the second time period (II), the value of 1λ max increases; in other words, the helix pitch increases. This probably can be explained by the decreasing in the order parameter because of the oligomer products formation, resulting in appearance of the helix defects and its untwisting. The third region (III) of the curve shows λmax decreasing 1 2 3 510 110 500 100 560 II λmax, nm 540 520 IV DA/Hexsorb I 490 90 480 80 470 II 500 λmax/ nm III III IV I DA/Cinsorb 460 Width / nm presented. This spectrum corresponds to almost complete reflection of the right-handed circularly polarized light. Using the selective light reflection spectra obtained at different temperatures, the temperature dependences of the wavelength maximum of the selective light reflection were derived for both blends (Fig. 3). The shift of the wavelength maximum into the longwavelength region of the spectrum with the temperature decrease is observed. Probably, it is explained by an appearance of the smectic order elements during cooling of the sample. This effect was observed earlier [19] and was explained by increasing of the twisting constant of liquid crystal during the formation of layer structure elements. 0 2 4 6 8 10 70 t / min 480 0 20 40 100 120 140 t, s Fig. 4 The time dependence of the wavelength maximum for blends DA/Cinsorb and DA/Hexsorb; 1λ irr =365 nm, Tirr =115 °C Fig. 5 The dependence of the wavelength maximum on the irradiation time for DA/Cinsorb blend with different photoinitiator concentration (1–2%, 2–0.5%) and the dependence of the selective light reflection peak width on the irradiation time (3–0.5% of photoinitiator); 1λ irr =365 nm, Tirr =115 °C Colloid Polym Sci (2007) 285:681–686 685 defects in the formed network as a result of the distortion of the helical structure of the sample during shrinkage. In Fig. 6, temperature dependences of the wavelength maximum of the selective light reflection for the crosslinked samples are presented. It is clearly seen that after irradiation there is almost no temperature dependence of 1λ max, whereas before irradiation this dependence was obvious (Fig. 3). This means that during irradiation, a polymer network with fixed helix pitch was obtained. Thus, crosslinking leads to stabilization of optical properties up to decomposition temperature of the blend samples. 600 DA/Hexsorb 500 DA/Cinsorb 450 400 20 40 60 80 100 120 140 160 180 0 Helix pitch photoregulation due to photoisomerization of chiral photochromic dopant T, C Fig. 6 The temperature dependence of the 1λ max for blends DA/ Cinsorb and DA/Hexsorb after irradiation; 1λ irr =365 nm, Tirr =115 °C During irradiation of the DA/Cinsorb sample with 1λ=313 nm E–Z isomerization of C=C double bond in dopant Cinsorb is observed, which results in the shift of the selective light reflection peak to the long-wavelength region of the spectrum after 2–3 min of irradiation (Fig. 7). During further irradiation (5 min), the selective light reflection peak degeneration takes place. This is the result of cholesteric helix deterioration, which is clearly seen in the polarizing microscope. Thus, it is impossible to fix the selective light reflection during irradiation with this wavelength. To solve this problem, we divided the experiment into two parts (Fig. 1). Firstly, the samples were irradiated with 1λ=313 nm for 2 min. This time is enough for Cinsorb E–Z isomerization. Afterwards, the sample was irradiated with 1λ=365 nm for 10 s. This results in network formation with planar texture and selective light reflection in the “green” region of the spectrum. For this system, the temperature dependence of the wavelength maximum of the selective light reflection was obtained. 1λ max doesn’t change with temperature, as it was shown previously, but before irradiation there was a strong dependence. The influence of the irradiation light intensity was studied. The samples of DA/Cinsorb were irradiated with 1λ=313 nm using the different intensity of the UV-light. It is pointing out the pitch decrease because of the network shrinkage [13]. Finally, in the fourth portion (IV) of the curve, the values of 1λ max become constant again. This means that the helix pitch is fixed in the solid state and the helical supramolecular structure is preserved. Thus, these data confirm the stable network formation. It should be mentioned that the value of 1λ max after irradiation differs from the initial one. This difference is small for DA/ Cinsorb (less then 10 nm), while for DA/Hexsorb, Δλ= 30 nm. The time of helix shrinkage in region III for blend DA/Hexsorb is four times greater than for DA/Cinsorb. The initiator concentration effect on kinetics of photocrosslinking is shown in Fig. 5. The four-fold decrease of the initiator concentration results in increasing of the full reaction time from 40 s to 10 min. The increasing of the initiator concentration doesn’t influence Δ1λ . One of the most important characteristics of cholesterics is the selective light reflection peak width. We studied the change of this parameter during irradiation (Fig. 5). It was shown that the peak width increases, especially in portion III of the kinetic curve. This peak widening can probably be explained by the growth of the amount of the structural 1.2 time 650 1.0 *2 600 0.8 - lg Tr Fig. 7 The dependence of lg Tr from the wavelength and the dependence of the wavelength maximum as a function of the irradiation time for DA/Cinsorb blend at different light intensities of the irradiation (1–I= 0.22 mW/cm2, 2−I=0.44 mW/ cm2, asterisks represent the deterioration of the selective light reflection peak); λirr =313 nm, Tirr =115 °C 0s 30 s 1 min 1.5 min 2 min 0.6 0.4 0.2 0.0 400 500 600 λ, nm 700 800 λmax, nm λmax, nm 550 550 *1 500 450 0 5 10 t, min 15 20 686 seen (Fig. 7) that the two-fold increasing of the intensity leads to the decreasing of the complete peak deterioration time by approximately three times. The increase in the intensity results in the variation of the λmax value, which can be fixed later by the photopolymerization process. Conclusions For the first time, the optical properties and cholesteric helix pitch changes during photopolymerization of the blends consisting of bifunctional nematogenic diacrylate monomer and chiral dopant were studied. The photocrosslinking kinetics was investigated in details and several stages of this process were discovered and described. Optical properties of the blend containing chiral photochromic dopant capable of E–Z photoisomerization under UV irradiation with 1λ=313 nm were studied. The decreasing of the dopant helical twisting power during photoisomerization is observed. This leads to the helix untwisting and shift of the selective light reflection peak to the longwavelength region of the spectrum. The possibility of photoregulation of optical properties of such blends with further fixation of structure by means of photocrosslinking during irradiation with 1λ=365 nm was demonstrated. Acknowledgments This research was supported by the Russian Foundation of Fundamental Research (05-03-33193) and Federal Scientific Technical Program (contract no. 02.434.11.2025). A.B. gratefully acknowledges the Russian Science Support Foundation, the research fellowships provided by INTAS (reference no. 03-55-956). View publication stats Colloid Polym Sci (2007) 285:681–686 References 1. Shibaev V, Freidzon Ya (1989) Cholesteric polymers with side chain mesogenic groups: structure, optical properties and intramolecular mobility. In: McArdle CB (ed) Side chain liquid crystal polymers. Blackie, Glasgow, p 260 2. Shibaev V, Bobrovsky A, Boiko N (2003) Prog Polym Sci 28:729 3. Bobrovsky A, Boiko N, Shibaev V (1998) Polym Sci Ser A 40:232 4. Bobrovsky A, Boiko N, Shibaev V, Springer J (2001) Liq Cryst 28:919 5. Yarmolenko S, Shepeleva L, Kutyulya L, Vashchenko V, Drushlyak T, Ponomarev O (1995) Zh Obshchey Khimii 65:145 6. Broer DJ, Boven J, Mol GN (1989) Makromol Chem 190:2255 7. 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