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Considerations on the surface relief grating formation mechanism in case of azo-polymers, using pulse laser irradiation method
Optical Materials 53 (2016) 174–180
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Considerations on the surface relief grating formation mechanism in case of azo-polymers, using pulse laser irradiation method Elena Sava a, Bogdana Simionescu b, Nicolae Hurduc a,⇑, Ion Sava c,⇑ a
«Gheorghe Asachi» Technical University of Iasi, Department of Natural and Synthetic Polymers, Bd. Prof. D. Mangeron 73, 700050 Iasi, Romania «Costin D. Nenitescu» Centre of Organic Chemistry, 202 B Splaiul Independentei, 71141 Bucharest, Romania c «Petru Poni» Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41 A, 700487 Iasi, Romania b
a r t i c l e
i n f o
Article history: Received 5 November 2015 Received in revised form 27 January 2016 Accepted 28 January 2016
Keywords: Azobenzene Polyimides Polysiloxanes Photoisomerization Surface relief gratings
a b s t r a c t Azobenzene-polymers have been the subject of intensive research due to their unique and unexpected properties that allow various applications triggered by light, one of the most investigated being the capability to generate a surface relief gratings (SRG). Despite the effort to understand the SRG formation process, the mechanism remains unclear until now. The paper presents a study concerning the azo-polymer response to the pulse laser irradiation, in the context of the SRG inscription. We propose an inscription mechanism based on the material deformation in the solid state, probably induced by the azo-groups parallelization with the electric field vector. Aromatic polyimides containing azobenzene units were investigated and compared with other azo-polymers having a more flexible main chain, from the photochromic behavior’s perspective and the ability to generate SRG. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The interest for the photosensitive azo-polymers increased continuously during the last period, due to their applications as high performance materials in data storage, solar energy, or biological fields [1–11]. Usually, the trans-cis photoisomerization process is the key of all interesting behaviors corresponding to azomaterials, the most investigated being the capability to generate a surface relief gratings (SRG) as a result of the interaction with the light [12,13]. Few models concerning the SRG formation mechanism have been proposed, none being able to explain the behavior of azo-materials in all operational condition [14–22]. These models can be divided in two main groups: one that supposes the polymer flowing accomplished by the softening/fluidization of the system, and second that presumes only the elastic material deformation, without macromolecules displacement. In our opinion, both possibilities are valuable, the laser irradiation type (in pulses or continuous) being responsible of the inscription mechanism. In the case of pulse laser irradiation, the SRG is generated in few nanoseconds, the polymer flowing being less probable in such a timescale [23,24]. In case of a continuous laser irradiation, the situation is completely different, tens minutes of light exposure being necessary to obtain a relief on the film surface [25–28]. ⇑ Corresponding authors. http://dx.doi.org/10.1016/j.optmat.2016.01.055 0925-3467/Ó 2016 Elsevier B.V. All rights reserved.
A strange characteristic of the SRG formation is that it can take place at temperatures situated much below the azo-polymers Tg values. A special situation appears in the case of polyimides, capable to generate SRG by pulse laser irradiation at room temperature, even if their Tg values are usually above 200 °C [5,23,24,29–32]. The present paper tries to concentrate all the main ideas from our previous studies dedicated to SRG formation using pulse laser irradiation technique, in order to propose a phenomenological mechanism for the film surfaces’ inscription on rigid or flexible azo-materials. Most of the discussions are concentrated on the azo-polyimides that are capable to generate SRG by pulse laser irradiation (365 or 355 nm). The special characteristic of these polyimides is the connection mode of the azo-group in the chain. The azo-groups are attached with an aromatic ring in the mainchain, the second one being situated in the side-chain (Figs. 1 and 2). Because we suppose that during the SRG formation a reordering process in bulky state takes place due to the parallelization of the azo-groups related to the electric field vector, this special architecture of the polymeric chains will play an important role in the re-arrangement processes. The high sensitivity of the relief quality to the operational conditions (laser fluence, number of pulses, light wavelength, film thickness etc.) supports the idea of the re-ordering processes at supramolecular level taking place in the solid state.
E. Sava et al. / Optical Materials 53 (2016) 174–180
2. Experimental The polyimides (PI 1-8) were obtained starting from aromatic dianhydride (such as: benzophenonetetracarboxylic dianhydride or hexafluoroisopropylidene dianhydride or 9,9-Bis[4-(3,4-dicarbox yphenoxy)phenyl]fluorene dianhydride) and different aromatic diamines having azobenzene groups and various substituents on a benzene ring. The copolyimides (PI 9-11) have been synthesized through the polycondensation of (hexafluoroisopropylidene)-diph thalic dianhydride and a mixture of two aromatic diamines, one containing ether groups, such as bis(p-aminophenoxy)-1,4benzene, bis(p-aminophenoxy)-1,3-benzene or bis(p-aminophe noxy)-4,40 -biphenyl, and the other one containing a pendent substituted azobenzene group, namely 2,4-diamino-40 -methylazoben zene. The molar ratio between the two diamines is 3:1, respectively. Details concerning the polymers synthesis and characterization were previously reported [23,24,32]. The starting polysiloxane, containing chlorobenzyl groups in the side chain, was obtained through a two-step reaction starting from dichloro(4-chloromethyl-phenyle thyl)methylsilane (produced by ABCR GmbH & Co). The first step consisted in a hydrolysis reaction, this resulting in a mixture of linear and cyclic oligomers. The second step supposed a cationic equilibration in the presence of trifluoromethanesulfonic acid and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane (Aldrich) as a chain blocker; this resulting in the formation of linear polymers. The polysiloxane containing chlorobenzyl groups in the side-chain were modified afterwards with different azophenols, using a SN2 reaction effectuated in dimethyl sulfoxide. Details concerning polymers’ synthesis and characterization have been previously reported [33]. The modification of poly(chloromethyl styrene) (PCMS) with 4- (phenyl-azophenol) was performed under similar conditions as in case of the polysiloxane [34]. The UV–VIS spectra were registered in solution (chloroform) or in solid state, using a Shimadzu UV-1700 apparatus. The transisomer content was estimated using the strong absorption p - p⁄ band situated in the 350–360 nm region of the spectra. All the samples were maintained for 24 h in dark before the spectrum registration, in order to have all the azo-groups in the trans state. The absorbance maximum at 350 nm was considered to correspond to a polymer having 100% trans isomers (A1). The samples were irradiated a certain period (t) using a non-polarized light source of
175
365 nm (light intensity 10–12 mW/cm2) and the new absorbance value being measured (A2) at 350 nm. The cis-isomer content was calculated using the formula:
Cis-isomerð%Þ ¼ ðA1 A2 Þ=A1 The experimental set-up used for pulsed SRG inscription was a Lloyd interferometric design. As a radiation source for the set-up, we have used a pulsed Nd:YAG laser, working on his third harmonic at 355 nm with a pulse length of 6 ns and a repetition rate of 10 Hz, having 0.6 mrad divergence and 6 mm diameter. In all the experiments a p-p polarization system was used. Details concerning the experimental set-up were previously reported [35]. 3. Results and discussion The polymer chemical structures and samples codes are presented in Figs. 1–3 and the main properties of the polymers are listed in Table 1. UV–VIS spectra, characteristic for each polymer class are presented in Fig. 4. Although the SRG are obtained more quickly using pulse laser irradiation technique, the number of the paper dedicated to this methodology is much lower [36–39], as compared with the continuous irradiation method. This is probably due to the high sensitivity of the method to the operational conditions (laser fluence, number of pulses, pulse duration, film thickness, azo-polymer chemical structure, polymer Tg value, etc.), the relief surface quality being sometimes very poor. Because we supposed that the mechanism responsible for the SRG formation using pulse laser irradiation is based on the material deformation in the solid state, we tried to obtain qualitative information concerning the polymeric chains packing degree, using the photoisomerization studies. It is well known that the trans-cistrans movement of the azo-groups in the solid state imposes the existence of a certain free volume that is proportional with the chains packing degree. As one can see in Fig. 5, the cis-isomer content in the solid state at the equilibrium is significantly lower than in solution, reflecting important steric hindrance generated by the distance between the chains and the individual chain conformation. For the samples PI 1 and PI 2 the cis-isomers content (film) in stationary state is lower than 30% for both samples, while in solution, the cis-isomer
Fig. 1. The chemical structures of the PI 1- PI 8 azo-polyimides.
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Fig. 2. The chemical structures of the PI 9- PI 11 azo-copolyimides.
content is above 50%. In case of the samples PI 3-5, only for PI 5 the cis-isomer content is above 30%, the other two samples presenting a similar behavior as in the case of samples PI 1-2. It must be underlined that not only the cis-isomer content in the stationary state is important for the film characterization, but also the material’s response at short irradiation times (few seconds). One’s reason to obtain this information is the supposition that the re-ordering processes is generated by the parallelization of the azo-groups (located in the trans state) with the direction of the electric field vector corresponding to the laser polarized light (Weigert effect). As consequence, the ratio between trans- and cis-isomers will be essential for a more efficient system reorganization. For all the polyimide samples, after 1 s of irradiation, the cis-isomer content is below 1.5%, which means that after few hundreds of nano-seconds of irradiation (the situation existent during the pulse laser exposure) the cis-isomer content in the film will be negligible. In case of the pulse laser irradiation systems one cannot discuss about trans-cis-trans isomerization cycles, because of the fact that one complete trans-cis-trans cycle needs 50– 100 ls (as a function of the relative position of the azo group related to the electric field vector). A second reason for the interest in the evolution of the cisisomer content at short UV irradiation time, is related to the fact
Fig. 3. The chemical structures of the azo-PCMS and silicon containing azopolymers.
Table 1 The azo-polymers main characteristics.
a b
Sample code
Mw [g/mol]
Tg [°C]
a
PI 1 PI 2 PI 3 PI 4 PI 5 PI 6 PI 7 PI 8 PI 9 PI 10 PI 11 Azo-PCMS CN-Azo-Psi Azo-PSi
17,000 16,000 122,000 105,000 44,500 75,300 68,000 52,500 68,500 63,500 90,300 10,800 18,100 12,500
223 224 191 185 228 227 212 219 202 214 220 87 65 32
345 350 345 345 350 340 345 345 353 353 353 343 361 345
kmax [nm]
Maximum absorption corresponding to the azo-group. The values are valuable at the equilibrium, in the stationary state.
b
b
54.1 57.3 52.5 63.1 69.7 75.3 67.7 70.3 70.2 73.6 76.3 79.6 76.5 90
24.9 29.6 34.4 29.2 39.6 39.7 54.6 40.6 39.8 44.3 39.3 68.6 42.3 64.9
cis-isomer content in solution [%]
cis-isomer content in solid state [%]
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Fig. 6. Photoisomerization kinetic curves at short irradiation times, corresponding to PI 1-5 samples.
Fig. 4. UV–VIS spectra, corresponding to: PI 1, PI 6, PI 9, CN-Azo-Psi, Azo-Psi and Azo-PCMS samples (film thickness 400–500 nm).
Fig. 7. AFM images of the SRG corresponding to PI 5 sample using one laser pulse.
Fig. 5. Plot of the cis-azo groups’ content as a function of the irradiation time, corresponding to the PI 1-2 (A) and PI 3-5 (B) samples.
that these values will give a qualitative image concerning the free volume: a fast growing of the cis-isomer content will reflect enough free volume in the system, while a slow growing will reflect strong sterical hindrances, due to the small distances between the polymeric chains. From this point of view, the less compact structure is corresponding to sample PI 5 which presents around 7% of cis-isomer after 10 s of irradiation (Fig. 6). The most compact film corresponds to sample PI 1, containing only 3.5% cis-isomer after 10 s of irradiation. Taking into consideration that the pressure developed by the azo-groups during photoisomerization is above 1 GPa, one expects very compact structures in case of polyimides [40]. For comparison, the Azo-Psi sample (Table 1),
having a very flexible main-chain, presents 60% cis-isomer content after 10 s of UV irradiation. What is surprising at this polyimides group (regarding the material’s response to the pulse laser irradiation), is the relief quality and the relief amplitude, which are the best for the minimum number of pulses (1–5 pulses) [41]. For example, in the case of PI 5, the best relief quality is obtained for 1 pulse (6 ns) and an incident fluency of 35 mJ/cm2, the amplitude of SRG being 310 nm (Fig. 7). Using 5 laser pulses, the relief amplitude is increasing to 380 nm, but the relief quality is beginning to fail [41]. The relief quality deterioration continues with the increasing of the laser pulse number, reflecting a worst organization of the system. Probably responsible for this behavior is the chemical structure complexity of the polymeric chain and the special connections of the azo-group related to the main-chain. Previous studies of our group reported that, in case of simpler architectures of the polymeric chains [azo-poly(chloromethyl styrene)], the increasing of the laser pulse number (till 200) results in a better relief quality [35]. Moreover, in case of polymers having low Tg values, situated close to the room temperature, the relief surface is not stable in time, that reinforcing the idea of a deformation mechanism in solid state, based on the fast re-ordering chains processes as a result of the azo-groups parallelization [42].
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Fig. 11. 3-D AFM image of a SRG obtained using an Azo-PCMS film irradiated with a fluence of 16.5 mJ/cm2 (200 pulses).
Fig. 8. Plot of the cis-azo groups’ content as a function of the irradiation time, corresponding to the PI 6-8 samples.
Fig. 9. Plot of the cis-azo groups’ content as a function of the irradiation time, corresponding to the PI 9-11 samples.
The next polyimides group (PI 6-8 samples) has a more complex architecture with a bulky substituent in the side-chain. That probable offers more free volume to the photoisomerization process of the azo-groups, the maximum cis-isomer content at the equilibrium being higher as compared to the case of the previous group. As one can see in Fig. 8, in the case of the PI 8 sample, the cis-isomer content is above 40%, but the presence of the chlorine atom connected in para position of the azo-group drastically diminish at only 28% the cis-isomer content in the stationary state. This behavior reflects the importance of the azo-group chemical structure, as well as the sterical aspects.
The next investigated group of polymers have a more complex architecture. More than that, the azo-group content related to the entire polymeric chain is significantly lower as compared to the other polyimides. A first observation related to the photoisomerization process (Fig. 9) is that all samples belonging to this group present a relatively high value of the cis-isomer content in the stationary state (38–44%). This reflects a similar azo-group mobility and similar values of the free volume as compared to the PI 5 - PI 6 samples. All these samples were laser irradiation tested, the polyimides being capable to generate SRG, the material’s response being different as compared to the PI 5 sample. It is surprising that, in spite of the azo-groups low content, these polyimides can react to lower values of the laser fluence, having very different responses depending on the chemical structure (even if the chemical differences are not significant) [32]. One can obtain a good quality of the relief surface even at 10 mJ/cm2 fluence and 10 pulses, only in case of the PI 9 sample, the relief amplitude being only 30–40 nm (Fig. 10 A). For PI 10, only a primary relief geometry having similar amplitudes is generated, as in the case of the PI 9 sample. For PI 11 one may observe a relief formation with a better value of the amplitude (120 nm) as compared to PI 10 sample [32]. Increasing the pulses number to 100, the situation radically changes: this time the PI 9 sample presents a bad relief quality (Fig. 10B), while the PI 10 sample exhibits the best relief quality. This finding reflects the method’s high sensitivity (using pulse laser irradiation) to the operational conditions. For completely different chemical structures [azo-poly(chloromethyl styrene)], similar conclusions were drawn, as well [35]. This argument is sustaining the mechanism of SRG formation based on the chains re-ordering processes due to the parallelization of the azo-groups, because very complex phenomena are expected when the chains are repositioned. By decreasing the azo-polymer Tg value (Azo-PCMS sample - Table 1), better qualities of the relief surface and higher
Fig. 10. AFM images of nanostructured surfaces corresponding to the PI 9 sample, obtained using a fluence of 10 mJ/cm2 and 10 pulses (A) or 100 pulses (B).
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Fig. 12. AFM images of the p-CN-Azo-Psi film: (A) structured surface after UV irradiation (8.4 mJ/cm2, 15 pulses); (B) film surface after the thermal treatment (20 min. at 70 °C) [24].
values of the relief amplitude can be obtained [35]. For example, in Fig. 11 one may observe a SRG obtained for a fluence of 16.5 mJ/ cm2 using 200 pulses. A possible explanation of the better response to the laser irradiation can be the higher main-chains mobility in the solid state as compared to the case of the polyimides. The idea of a better material response through a Tg value decrease is supported by the behavior of an azo-polysiloxane containing CN-Azo groups in the side-chain (Table 1 - CN-Azo-PSi), as well. In Fig. 12A one can observe a good surface relief quality obtained using a laser fluence of only 8.4 mJ/cm2 and 15 pulses [24]. In the case of this sample, a supplementary experiment was performed in order to demonstrate the idea of the film deformation mechanism in the solid state [24]. The structured surface was heated for 20 min at 70 °C, close to the polymer glass transition (Tg = 65 °C), the result of the thermal treatment being the SRG erasing (Fig. 12B). This result suggests an elastic material deformation during laser irradiation by heating the material above Tg, the deformation being completely recuperated, this being in agreement with the elastic deformation mechanism proposed by Saphiannikova [42,43]. Nevertheless, if the azo-polymer Tg value is too low, close to the room temperature, the SRG becomes instable, few hours after the inscription the relief being erased without heating [35]. This is the case of the Azo-Psi sample (Table 1) which has a Tg value of only 32 °C. The disappearance of the SRG at room temperature confirms one more time the idea of an elastic material deformation during laser irradiation, using the pulses technique.
4. Conclusion Starting from a significant number of experiments performed using the pulse laser irradiation method in order to obtain SRG-s, one proposes an inscription mechanism based on the material deformation in solid state, probable induced by the azo-groups parallelization with the electric field vector. This idea is in agreement with the photoisomerization studies’ results obtained using very rigid, rigid and flexible azo-polymeric chains. These photoisomerization studies reflect that in a nano-seconds timescale, the cisisomer content in the film is negligible, making possible the parallelization of a high content of the trans-isomers corresponding to the azo-groups. The parallelization of the azo-groups will induce a very fast re-ordering process of the main-chains, the result being
the relief inscription on the film surface. In the case of polyimides, the connection mode of the azo-groups related to the main-chain is favorable to this mechanism. The surface relief instability, if the polymer is heated above the glass transition, is in agreement with the proposed mechanism. The mechanism concerning the elastic deformation of the material is also supported by the SRG erasing at room temperature if the Tg value of the azo-polymer is lowered enough.
References [1] Y. Ma, H.X. Chen, F. Zhou, H. Li, H. Dong, Y.Y. Li, Z.J. Hu, Q.F. Xu, J.M. Lu, Nanoscale 7 (2015) 7659–7664. [2] D. Hu, Y. Hu, W. Huang, Q. Zhang, Opt. Commun. 285 (2012) 4941–4945. [3] R.J. Moerland, J.E. Koskela, A. Kravchenko, M. Simberg, S. van der Vegte, M. Kaivola, A. Priimagi, R.H. Ras, Mater. Horizons 1 (2014) 74–80. [4] A.M. Kolpak, J.C. Grossman, Nano Lett. 11 (2011) 3156–3162. [5] J. Konieczkowska, M. Wojtowicz, A. Sobolewska, J. Noga, A. Jarczyk-Jedryka, A. Kozanecka-Szmigiel, E. Schab-Balcerzak, Opt. Mater. 48 (2015) 139–149. [6] P. Huh, S.C. Kim, Electron. Mater. Let. 8 (2012) 131–134. [7] K. Kakiage, Y. Aoyama, M. Yamamura, T. Yano, M. Unno, T. Kyomen, M. Hanaya, Silicon 6 (2014) 123–127. [8] H. Baac, J.H. Lee, J.M. Seo, T.H. Park, H. Chung, S.D. Lee, S.J. Kim, Mater. Sci. Eng. C 24 (2004) 209–212. [9] N. Hurduc, A. Macovei, C. Paius, A. Raicu, I. Moleavin, N. Branza-Nichita, M. Hamel, L. Rocha, Mater. Sci. Eng. C 33 (2013) 2440–2445. [10] I. Moleavin, S. Grama, I. Carlescu, D. Scutaru, N. Hurduc, Polym. Bull. 65 (2010) 69–81. [11] D. Pavel, J.M. Ball, S.N. Bhattacharya, R.A. Shanks, N. Hurduc, Comp. Theor. Polym. Sci. 7 (1997) 7–11. [12] P. Rochon, E. Batalla, A. Natansohn, Appl. Phys. Lett. 66 (1995) 136–138. [13] D.Y. Kim, S.K. Tripathy, L. Li, J. Kumar, Appl. Phys. Lett. 66 (1995) 1166–1168. [14] C.J. Barrett, P.L. Rochon, A.L. Natansohn, J. Chem. Phys. 109 (1998) 1505–1516. [15] J. Kumar, L. Li, X.L. Jiang, D.Y. Kim, T.S. Lee, S. Tripathy, Appl. Phys. Lett. 72 (1998) 2096–2098. [16] O. Baldus, S.J. Zilker, Appl. Phys. B: Lasers Opt. 72 (2001) 425–427. [17] A. Ambrosio, P. Maddalena, L. Marrucci, Phys. Rev. Lett. 110 (2013) 146102. 5. [18] C. Fiorini, N. Prudhomme, G. De Veyrac, I. Maurin, P. Raimond, J.M. Nunzi, Synth. Met. 115 (2000) 121–125. [19] T.G. Pedersen, P.M. Johansen, N.C.R. Holme, P.S. Ramanujam, S. Hvilsted, Phys. Rev. Lett. 80 (1998) 89–92. [20] V.P. Toshchevikov, M. Saphiannikova, G. Heinrich, J. Chem. Phys. 137 (2012) 024903. [21] V. Teboul, M. Saiddine, J.M. Nunzi, J.B. Accary, J. Chem. Phys. 134 (2011) 114517. [22] M.L. Juan, J. Plain, R. Bachelot, P. Royer, S.K. Gray, G.P. Wiederrecht, ACS Nano 3 (2009) 1573–1579. [23] I. Sava, A.M. Resmerita, G. Lisa, V. Damian, N. Hurduc, Polymer 49 (2008) 1475–1482. [24] I. Sava, L. Sacarescu, I. Stoica, I. Apostol, V. Damian, N. Hurduc, Polym. Int. 58 (2009) 163–170. [25] M. Kim, J. Lee, C. Chun, D.Y. Kim, S. Higuchi, T. Nakayama, Macromol. Chem. Phys. 208 (2007) 1753–1763.
180
E. Sava et al. / Optical Materials 53 (2016) 174–180
[26] J.E. Koskela, J. Vapaavuori, R.H. Ras, A. Priimagi, ACS Macro Lett. 3 (2014) 1196–1200. [27] A.R. Luca, L. Rocha, A.M. Resmerita, A. Macovei, M. Hamel, A.M. Macsim, N. Nichita, N. Hurduc, Express Polym. Lett. 5 (2011) 959–969. [28] S. Lee, H.S. Kang, J.K. Park, Adv. Mater. 24 (2012) 2069–2103. [29] E. Schab-Balcerzak, J. Konieczkowska, M. Siwy, A. Sobolewska, M. Wojtowicz, M. Wiacek, Opt. Mater. 36 (2014) 892–902. [30] E. Schab-Balcerzak, B. Skorus, M. Siwy, H. Janeczek, A. Sobolewska, J. Konieczkowska, M. Wiacek, Polym. Int. 64 (2015) 76–87. [31] V. Teboul, R. Barille, P. Tajalli, S. Ahmadi-Kandjani, H. Tajalli, S. Zielinska, E. Ortyl, Soft Matter. 11 (2015) 6444–6449. [32] I. Sava, A. Burescu, I. Stoica, V. Musteata, M. Cristea, I. Mihaila, V. Pohoata, I. Topala, RSC Adv. 5 (2015) 10125–10133. [33] K. Kazmierski, N. Hurduc, G. Sauvet, J. Chojnowski, J. Polym. Sci. Part. A: Polym. Chem. 42 (2004) 1682–1692. [34] N. Hurduc, B.C. Donose, A. Macovei, C. Paius, C. Ibanescu, D. Scutaru, M. Hamel, N. Branza-Nichita, L. Rocha, Soft Matter. 10 (2014) 4640–4647.
[35] V. Damian, E. Resmerita, I. Stoica, C. Ibanescu, L. Sacarescu, L. Rocha, N. Hurduc, J. Appl. Polym. Sci. 131 (2014) 41015. 10. [36] Y. Che, O. Sugihara, H. Fujimura, N. Okamoto, C. Egami, Y. Kawata, M. Tsuchimori, O. Watanabe, Opt. Mater. 21 (2002) 79–82. [37] R.D. Schaller, R.J. Saykally, Y.R. Shen, F. Lagugné-Labarthet, Opt. Lett. 28 (2003) 1296–1298. [38] A. Miniewicz, B. Sahraoui, E. Schab-Balcerzak, A.M. Sobolewska, A.C. Mitus, F. Kajzar, Nonlinear Opt., Quantum Opt. 35 (2006) 95–102. [39] S. Yakunin, V. Gaivoronsky, A. Miniewicz, J. Sworakowski, Mol. Cryst. Liq. Cryst. 496 (2008) 310–321. [40] T.A. Singleton, K.S. Ramsay, M.M. Barsan, I.S. Butler, C.J. Barrett, J. Phys. Chem. B 116 (2012) 9860–9865. [41] I. Sava, N. Hurduc, L. Sacarescu, I. Apostol, V. Damian, High Perform. Polym. 25 (2013) 13–24. [42] M. Saphiannikova, V. Toshchevikov, J. Ilnytskyi, Nonlinear Opt., Quantum Opt. 41 (2010) 27–57. [43] M. Saphiannikova, V. Toshchevikov, J. Soc. Inf. Display 23 (2015) 146–153.
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Considerations on the surface relief grating formation mechanism in case of azo-polymers, using pulse laser irradiation method Elena Sava a, Bogdana Simionescu b, Nicolae Hurduc a,⇑, Ion Sava c,⇑ a
«Gheorghe Asachi» Technical University of Iasi, Department of Natural and Synthetic Polymers, Bd. Prof. D. Mangeron 73, 700050 Iasi, Romania «Costin D. Nenitescu» Centre of Organic Chemistry, 202 B Splaiul Independentei, 71141 Bucharest, Romania c «Petru Poni» Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41 A, 700487 Iasi, Romania b
a r t i c l e
i n f o
Article history: Received 5 November 2015 Received in revised form 27 January 2016 Accepted 28 January 2016
Keywords: Azobenzene Polyimides Polysiloxanes Photoisomerization Surface relief gratings
a b s t r a c t Azobenzene-polymers have been the subject of intensive research due to their unique and unexpected properties that allow various applications triggered by light, one of the most investigated being the capability to generate a surface relief gratings (SRG). Despite the effort to understand the SRG formation process, the mechanism remains unclear until now. The paper presents a study concerning the azo-polymer response to the pulse laser irradiation, in the context of the SRG inscription. We propose an inscription mechanism based on the material deformation in the solid state, probably induced by the azo-groups parallelization with the electric field vector. Aromatic polyimides containing azobenzene units were investigated and compared with other azo-polymers having a more flexible main chain, from the photochromic behavior’s perspective and the ability to generate SRG. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The interest for the photosensitive azo-polymers increased continuously during the last period, due to their applications as high performance materials in data storage, solar energy, or biological fields [1–11]. Usually, the trans-cis photoisomerization process is the key of all interesting behaviors corresponding to azomaterials, the most investigated being the capability to generate a surface relief gratings (SRG) as a result of the interaction with the light [12,13]. Few models concerning the SRG formation mechanism have been proposed, none being able to explain the behavior of azo-materials in all operational condition [14–22]. These models can be divided in two main groups: one that supposes the polymer flowing accomplished by the softening/fluidization of the system, and second that presumes only the elastic material deformation, without macromolecules displacement. In our opinion, both possibilities are valuable, the laser irradiation type (in pulses or continuous) being responsible of the inscription mechanism. In the case of pulse laser irradiation, the SRG is generated in few nanoseconds, the polymer flowing being less probable in such a timescale [23,24]. In case of a continuous laser irradiation, the situation is completely different, tens minutes of light exposure being necessary to obtain a relief on the film surface [25–28]. ⇑ Corresponding authors. http://dx.doi.org/10.1016/j.optmat.2016.01.055 0925-3467/Ó 2016 Elsevier B.V. All rights reserved.
A strange characteristic of the SRG formation is that it can take place at temperatures situated much below the azo-polymers Tg values. A special situation appears in the case of polyimides, capable to generate SRG by pulse laser irradiation at room temperature, even if their Tg values are usually above 200 °C [5,23,24,29–32]. The present paper tries to concentrate all the main ideas from our previous studies dedicated to SRG formation using pulse laser irradiation technique, in order to propose a phenomenological mechanism for the film surfaces’ inscription on rigid or flexible azo-materials. Most of the discussions are concentrated on the azo-polyimides that are capable to generate SRG by pulse laser irradiation (365 or 355 nm). The special characteristic of these polyimides is the connection mode of the azo-group in the chain. The azo-groups are attached with an aromatic ring in the mainchain, the second one being situated in the side-chain (Figs. 1 and 2). Because we suppose that during the SRG formation a reordering process in bulky state takes place due to the parallelization of the azo-groups related to the electric field vector, this special architecture of the polymeric chains will play an important role in the re-arrangement processes. The high sensitivity of the relief quality to the operational conditions (laser fluence, number of pulses, light wavelength, film thickness etc.) supports the idea of the re-ordering processes at supramolecular level taking place in the solid state.
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2. Experimental The polyimides (PI 1-8) were obtained starting from aromatic dianhydride (such as: benzophenonetetracarboxylic dianhydride or hexafluoroisopropylidene dianhydride or 9,9-Bis[4-(3,4-dicarbox yphenoxy)phenyl]fluorene dianhydride) and different aromatic diamines having azobenzene groups and various substituents on a benzene ring. The copolyimides (PI 9-11) have been synthesized through the polycondensation of (hexafluoroisopropylidene)-diph thalic dianhydride and a mixture of two aromatic diamines, one containing ether groups, such as bis(p-aminophenoxy)-1,4benzene, bis(p-aminophenoxy)-1,3-benzene or bis(p-aminophe noxy)-4,40 -biphenyl, and the other one containing a pendent substituted azobenzene group, namely 2,4-diamino-40 -methylazoben zene. The molar ratio between the two diamines is 3:1, respectively. Details concerning the polymers synthesis and characterization were previously reported [23,24,32]. The starting polysiloxane, containing chlorobenzyl groups in the side chain, was obtained through a two-step reaction starting from dichloro(4-chloromethyl-phenyle thyl)methylsilane (produced by ABCR GmbH & Co). The first step consisted in a hydrolysis reaction, this resulting in a mixture of linear and cyclic oligomers. The second step supposed a cationic equilibration in the presence of trifluoromethanesulfonic acid and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane (Aldrich) as a chain blocker; this resulting in the formation of linear polymers. The polysiloxane containing chlorobenzyl groups in the side-chain were modified afterwards with different azophenols, using a SN2 reaction effectuated in dimethyl sulfoxide. Details concerning polymers’ synthesis and characterization have been previously reported [33]. The modification of poly(chloromethyl styrene) (PCMS) with 4- (phenyl-azophenol) was performed under similar conditions as in case of the polysiloxane [34]. The UV–VIS spectra were registered in solution (chloroform) or in solid state, using a Shimadzu UV-1700 apparatus. The transisomer content was estimated using the strong absorption p - p⁄ band situated in the 350–360 nm region of the spectra. All the samples were maintained for 24 h in dark before the spectrum registration, in order to have all the azo-groups in the trans state. The absorbance maximum at 350 nm was considered to correspond to a polymer having 100% trans isomers (A1). The samples were irradiated a certain period (t) using a non-polarized light source of
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365 nm (light intensity 10–12 mW/cm2) and the new absorbance value being measured (A2) at 350 nm. The cis-isomer content was calculated using the formula:
Cis-isomerð%Þ ¼ ðA1 A2 Þ=A1 The experimental set-up used for pulsed SRG inscription was a Lloyd interferometric design. As a radiation source for the set-up, we have used a pulsed Nd:YAG laser, working on his third harmonic at 355 nm with a pulse length of 6 ns and a repetition rate of 10 Hz, having 0.6 mrad divergence and 6 mm diameter. In all the experiments a p-p polarization system was used. Details concerning the experimental set-up were previously reported [35]. 3. Results and discussion The polymer chemical structures and samples codes are presented in Figs. 1–3 and the main properties of the polymers are listed in Table 1. UV–VIS spectra, characteristic for each polymer class are presented in Fig. 4. Although the SRG are obtained more quickly using pulse laser irradiation technique, the number of the paper dedicated to this methodology is much lower [36–39], as compared with the continuous irradiation method. This is probably due to the high sensitivity of the method to the operational conditions (laser fluence, number of pulses, pulse duration, film thickness, azo-polymer chemical structure, polymer Tg value, etc.), the relief surface quality being sometimes very poor. Because we supposed that the mechanism responsible for the SRG formation using pulse laser irradiation is based on the material deformation in the solid state, we tried to obtain qualitative information concerning the polymeric chains packing degree, using the photoisomerization studies. It is well known that the trans-cistrans movement of the azo-groups in the solid state imposes the existence of a certain free volume that is proportional with the chains packing degree. As one can see in Fig. 5, the cis-isomer content in the solid state at the equilibrium is significantly lower than in solution, reflecting important steric hindrance generated by the distance between the chains and the individual chain conformation. For the samples PI 1 and PI 2 the cis-isomers content (film) in stationary state is lower than 30% for both samples, while in solution, the cis-isomer
Fig. 1. The chemical structures of the PI 1- PI 8 azo-polyimides.
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Fig. 2. The chemical structures of the PI 9- PI 11 azo-copolyimides.
content is above 50%. In case of the samples PI 3-5, only for PI 5 the cis-isomer content is above 30%, the other two samples presenting a similar behavior as in the case of samples PI 1-2. It must be underlined that not only the cis-isomer content in the stationary state is important for the film characterization, but also the material’s response at short irradiation times (few seconds). One’s reason to obtain this information is the supposition that the re-ordering processes is generated by the parallelization of the azo-groups (located in the trans state) with the direction of the electric field vector corresponding to the laser polarized light (Weigert effect). As consequence, the ratio between trans- and cis-isomers will be essential for a more efficient system reorganization. For all the polyimide samples, after 1 s of irradiation, the cis-isomer content is below 1.5%, which means that after few hundreds of nano-seconds of irradiation (the situation existent during the pulse laser exposure) the cis-isomer content in the film will be negligible. In case of the pulse laser irradiation systems one cannot discuss about trans-cis-trans isomerization cycles, because of the fact that one complete trans-cis-trans cycle needs 50– 100 ls (as a function of the relative position of the azo group related to the electric field vector). A second reason for the interest in the evolution of the cisisomer content at short UV irradiation time, is related to the fact
Fig. 3. The chemical structures of the azo-PCMS and silicon containing azopolymers.
Table 1 The azo-polymers main characteristics.
a b
Sample code
Mw [g/mol]
Tg [°C]
a
PI 1 PI 2 PI 3 PI 4 PI 5 PI 6 PI 7 PI 8 PI 9 PI 10 PI 11 Azo-PCMS CN-Azo-Psi Azo-PSi
17,000 16,000 122,000 105,000 44,500 75,300 68,000 52,500 68,500 63,500 90,300 10,800 18,100 12,500
223 224 191 185 228 227 212 219 202 214 220 87 65 32
345 350 345 345 350 340 345 345 353 353 353 343 361 345
kmax [nm]
Maximum absorption corresponding to the azo-group. The values are valuable at the equilibrium, in the stationary state.
b
b
54.1 57.3 52.5 63.1 69.7 75.3 67.7 70.3 70.2 73.6 76.3 79.6 76.5 90
24.9 29.6 34.4 29.2 39.6 39.7 54.6 40.6 39.8 44.3 39.3 68.6 42.3 64.9
cis-isomer content in solution [%]
cis-isomer content in solid state [%]
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Fig. 6. Photoisomerization kinetic curves at short irradiation times, corresponding to PI 1-5 samples.
Fig. 4. UV–VIS spectra, corresponding to: PI 1, PI 6, PI 9, CN-Azo-Psi, Azo-Psi and Azo-PCMS samples (film thickness 400–500 nm).
Fig. 7. AFM images of the SRG corresponding to PI 5 sample using one laser pulse.
Fig. 5. Plot of the cis-azo groups’ content as a function of the irradiation time, corresponding to the PI 1-2 (A) and PI 3-5 (B) samples.
that these values will give a qualitative image concerning the free volume: a fast growing of the cis-isomer content will reflect enough free volume in the system, while a slow growing will reflect strong sterical hindrances, due to the small distances between the polymeric chains. From this point of view, the less compact structure is corresponding to sample PI 5 which presents around 7% of cis-isomer after 10 s of irradiation (Fig. 6). The most compact film corresponds to sample PI 1, containing only 3.5% cis-isomer after 10 s of irradiation. Taking into consideration that the pressure developed by the azo-groups during photoisomerization is above 1 GPa, one expects very compact structures in case of polyimides [40]. For comparison, the Azo-Psi sample (Table 1),
having a very flexible main-chain, presents 60% cis-isomer content after 10 s of UV irradiation. What is surprising at this polyimides group (regarding the material’s response to the pulse laser irradiation), is the relief quality and the relief amplitude, which are the best for the minimum number of pulses (1–5 pulses) [41]. For example, in the case of PI 5, the best relief quality is obtained for 1 pulse (6 ns) and an incident fluency of 35 mJ/cm2, the amplitude of SRG being 310 nm (Fig. 7). Using 5 laser pulses, the relief amplitude is increasing to 380 nm, but the relief quality is beginning to fail [41]. The relief quality deterioration continues with the increasing of the laser pulse number, reflecting a worst organization of the system. Probably responsible for this behavior is the chemical structure complexity of the polymeric chain and the special connections of the azo-group related to the main-chain. Previous studies of our group reported that, in case of simpler architectures of the polymeric chains [azo-poly(chloromethyl styrene)], the increasing of the laser pulse number (till 200) results in a better relief quality [35]. Moreover, in case of polymers having low Tg values, situated close to the room temperature, the relief surface is not stable in time, that reinforcing the idea of a deformation mechanism in solid state, based on the fast re-ordering chains processes as a result of the azo-groups parallelization [42].
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Fig. 11. 3-D AFM image of a SRG obtained using an Azo-PCMS film irradiated with a fluence of 16.5 mJ/cm2 (200 pulses).
Fig. 8. Plot of the cis-azo groups’ content as a function of the irradiation time, corresponding to the PI 6-8 samples.
Fig. 9. Plot of the cis-azo groups’ content as a function of the irradiation time, corresponding to the PI 9-11 samples.
The next polyimides group (PI 6-8 samples) has a more complex architecture with a bulky substituent in the side-chain. That probable offers more free volume to the photoisomerization process of the azo-groups, the maximum cis-isomer content at the equilibrium being higher as compared to the case of the previous group. As one can see in Fig. 8, in the case of the PI 8 sample, the cis-isomer content is above 40%, but the presence of the chlorine atom connected in para position of the azo-group drastically diminish at only 28% the cis-isomer content in the stationary state. This behavior reflects the importance of the azo-group chemical structure, as well as the sterical aspects.
The next investigated group of polymers have a more complex architecture. More than that, the azo-group content related to the entire polymeric chain is significantly lower as compared to the other polyimides. A first observation related to the photoisomerization process (Fig. 9) is that all samples belonging to this group present a relatively high value of the cis-isomer content in the stationary state (38–44%). This reflects a similar azo-group mobility and similar values of the free volume as compared to the PI 5 - PI 6 samples. All these samples were laser irradiation tested, the polyimides being capable to generate SRG, the material’s response being different as compared to the PI 5 sample. It is surprising that, in spite of the azo-groups low content, these polyimides can react to lower values of the laser fluence, having very different responses depending on the chemical structure (even if the chemical differences are not significant) [32]. One can obtain a good quality of the relief surface even at 10 mJ/cm2 fluence and 10 pulses, only in case of the PI 9 sample, the relief amplitude being only 30–40 nm (Fig. 10 A). For PI 10, only a primary relief geometry having similar amplitudes is generated, as in the case of the PI 9 sample. For PI 11 one may observe a relief formation with a better value of the amplitude (120 nm) as compared to PI 10 sample [32]. Increasing the pulses number to 100, the situation radically changes: this time the PI 9 sample presents a bad relief quality (Fig. 10B), while the PI 10 sample exhibits the best relief quality. This finding reflects the method’s high sensitivity (using pulse laser irradiation) to the operational conditions. For completely different chemical structures [azo-poly(chloromethyl styrene)], similar conclusions were drawn, as well [35]. This argument is sustaining the mechanism of SRG formation based on the chains re-ordering processes due to the parallelization of the azo-groups, because very complex phenomena are expected when the chains are repositioned. By decreasing the azo-polymer Tg value (Azo-PCMS sample - Table 1), better qualities of the relief surface and higher
Fig. 10. AFM images of nanostructured surfaces corresponding to the PI 9 sample, obtained using a fluence of 10 mJ/cm2 and 10 pulses (A) or 100 pulses (B).
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Fig. 12. AFM images of the p-CN-Azo-Psi film: (A) structured surface after UV irradiation (8.4 mJ/cm2, 15 pulses); (B) film surface after the thermal treatment (20 min. at 70 °C) [24].
values of the relief amplitude can be obtained [35]. For example, in Fig. 11 one may observe a SRG obtained for a fluence of 16.5 mJ/ cm2 using 200 pulses. A possible explanation of the better response to the laser irradiation can be the higher main-chains mobility in the solid state as compared to the case of the polyimides. The idea of a better material response through a Tg value decrease is supported by the behavior of an azo-polysiloxane containing CN-Azo groups in the side-chain (Table 1 - CN-Azo-PSi), as well. In Fig. 12A one can observe a good surface relief quality obtained using a laser fluence of only 8.4 mJ/cm2 and 15 pulses [24]. In the case of this sample, a supplementary experiment was performed in order to demonstrate the idea of the film deformation mechanism in the solid state [24]. The structured surface was heated for 20 min at 70 °C, close to the polymer glass transition (Tg = 65 °C), the result of the thermal treatment being the SRG erasing (Fig. 12B). This result suggests an elastic material deformation during laser irradiation by heating the material above Tg, the deformation being completely recuperated, this being in agreement with the elastic deformation mechanism proposed by Saphiannikova [42,43]. Nevertheless, if the azo-polymer Tg value is too low, close to the room temperature, the SRG becomes instable, few hours after the inscription the relief being erased without heating [35]. This is the case of the Azo-Psi sample (Table 1) which has a Tg value of only 32 °C. The disappearance of the SRG at room temperature confirms one more time the idea of an elastic material deformation during laser irradiation, using the pulses technique.
4. Conclusion Starting from a significant number of experiments performed using the pulse laser irradiation method in order to obtain SRG-s, one proposes an inscription mechanism based on the material deformation in solid state, probable induced by the azo-groups parallelization with the electric field vector. This idea is in agreement with the photoisomerization studies’ results obtained using very rigid, rigid and flexible azo-polymeric chains. These photoisomerization studies reflect that in a nano-seconds timescale, the cisisomer content in the film is negligible, making possible the parallelization of a high content of the trans-isomers corresponding to the azo-groups. The parallelization of the azo-groups will induce a very fast re-ordering process of the main-chains, the result being
the relief inscription on the film surface. In the case of polyimides, the connection mode of the azo-groups related to the main-chain is favorable to this mechanism. The surface relief instability, if the polymer is heated above the glass transition, is in agreement with the proposed mechanism. The mechanism concerning the elastic deformation of the material is also supported by the SRG erasing at room temperature if the Tg value of the azo-polymer is lowered enough.
References [1] Y. Ma, H.X. Chen, F. Zhou, H. Li, H. Dong, Y.Y. Li, Z.J. Hu, Q.F. Xu, J.M. Lu, Nanoscale 7 (2015) 7659–7664. [2] D. Hu, Y. Hu, W. Huang, Q. Zhang, Opt. Commun. 285 (2012) 4941–4945. [3] R.J. Moerland, J.E. Koskela, A. Kravchenko, M. Simberg, S. van der Vegte, M. Kaivola, A. Priimagi, R.H. Ras, Mater. Horizons 1 (2014) 74–80. [4] A.M. Kolpak, J.C. Grossman, Nano Lett. 11 (2011) 3156–3162. [5] J. Konieczkowska, M. Wojtowicz, A. Sobolewska, J. Noga, A. Jarczyk-Jedryka, A. Kozanecka-Szmigiel, E. Schab-Balcerzak, Opt. Mater. 48 (2015) 139–149. [6] P. Huh, S.C. Kim, Electron. Mater. Let. 8 (2012) 131–134. [7] K. Kakiage, Y. Aoyama, M. Yamamura, T. Yano, M. Unno, T. Kyomen, M. Hanaya, Silicon 6 (2014) 123–127. [8] H. Baac, J.H. Lee, J.M. Seo, T.H. Park, H. Chung, S.D. Lee, S.J. Kim, Mater. Sci. Eng. C 24 (2004) 209–212. [9] N. Hurduc, A. Macovei, C. Paius, A. Raicu, I. Moleavin, N. Branza-Nichita, M. Hamel, L. Rocha, Mater. Sci. Eng. C 33 (2013) 2440–2445. [10] I. Moleavin, S. Grama, I. Carlescu, D. Scutaru, N. Hurduc, Polym. Bull. 65 (2010) 69–81. [11] D. Pavel, J.M. Ball, S.N. Bhattacharya, R.A. Shanks, N. Hurduc, Comp. Theor. Polym. Sci. 7 (1997) 7–11. [12] P. Rochon, E. Batalla, A. Natansohn, Appl. Phys. Lett. 66 (1995) 136–138. [13] D.Y. Kim, S.K. Tripathy, L. Li, J. Kumar, Appl. Phys. Lett. 66 (1995) 1166–1168. [14] C.J. Barrett, P.L. Rochon, A.L. Natansohn, J. Chem. Phys. 109 (1998) 1505–1516. [15] J. Kumar, L. Li, X.L. Jiang, D.Y. Kim, T.S. Lee, S. Tripathy, Appl. Phys. Lett. 72 (1998) 2096–2098. [16] O. Baldus, S.J. Zilker, Appl. Phys. B: Lasers Opt. 72 (2001) 425–427. [17] A. Ambrosio, P. Maddalena, L. Marrucci, Phys. Rev. Lett. 110 (2013) 146102. 5. [18] C. Fiorini, N. Prudhomme, G. De Veyrac, I. Maurin, P. Raimond, J.M. Nunzi, Synth. Met. 115 (2000) 121–125. [19] T.G. Pedersen, P.M. Johansen, N.C.R. Holme, P.S. Ramanujam, S. Hvilsted, Phys. Rev. Lett. 80 (1998) 89–92. [20] V.P. Toshchevikov, M. Saphiannikova, G. Heinrich, J. Chem. Phys. 137 (2012) 024903. [21] V. Teboul, M. Saiddine, J.M. Nunzi, J.B. Accary, J. Chem. Phys. 134 (2011) 114517. [22] M.L. Juan, J. Plain, R. Bachelot, P. Royer, S.K. Gray, G.P. Wiederrecht, ACS Nano 3 (2009) 1573–1579. [23] I. Sava, A.M. Resmerita, G. Lisa, V. Damian, N. Hurduc, Polymer 49 (2008) 1475–1482. [24] I. Sava, L. Sacarescu, I. Stoica, I. Apostol, V. Damian, N. Hurduc, Polym. Int. 58 (2009) 163–170. [25] M. Kim, J. Lee, C. Chun, D.Y. Kim, S. Higuchi, T. Nakayama, Macromol. Chem. Phys. 208 (2007) 1753–1763.
180
E. Sava et al. / Optical Materials 53 (2016) 174–180
[26] J.E. Koskela, J. Vapaavuori, R.H. Ras, A. Priimagi, ACS Macro Lett. 3 (2014) 1196–1200. [27] A.R. Luca, L. Rocha, A.M. Resmerita, A. Macovei, M. Hamel, A.M. Macsim, N. Nichita, N. Hurduc, Express Polym. Lett. 5 (2011) 959–969. [28] S. Lee, H.S. Kang, J.K. Park, Adv. Mater. 24 (2012) 2069–2103. [29] E. Schab-Balcerzak, J. Konieczkowska, M. Siwy, A. Sobolewska, M. Wojtowicz, M. Wiacek, Opt. Mater. 36 (2014) 892–902. [30] E. Schab-Balcerzak, B. Skorus, M. Siwy, H. Janeczek, A. Sobolewska, J. Konieczkowska, M. Wiacek, Polym. Int. 64 (2015) 76–87. [31] V. Teboul, R. Barille, P. Tajalli, S. Ahmadi-Kandjani, H. Tajalli, S. Zielinska, E. Ortyl, Soft Matter. 11 (2015) 6444–6449. [32] I. Sava, A. Burescu, I. Stoica, V. Musteata, M. Cristea, I. Mihaila, V. Pohoata, I. Topala, RSC Adv. 5 (2015) 10125–10133. [33] K. Kazmierski, N. Hurduc, G. Sauvet, J. Chojnowski, J. Polym. Sci. Part. A: Polym. Chem. 42 (2004) 1682–1692. [34] N. Hurduc, B.C. Donose, A. Macovei, C. Paius, C. Ibanescu, D. Scutaru, M. Hamel, N. Branza-Nichita, L. Rocha, Soft Matter. 10 (2014) 4640–4647.
[35] V. Damian, E. Resmerita, I. Stoica, C. Ibanescu, L. Sacarescu, L. Rocha, N. Hurduc, J. Appl. Polym. Sci. 131 (2014) 41015. 10. [36] Y. Che, O. Sugihara, H. Fujimura, N. Okamoto, C. Egami, Y. Kawata, M. Tsuchimori, O. Watanabe, Opt. Mater. 21 (2002) 79–82. [37] R.D. Schaller, R.J. Saykally, Y.R. Shen, F. Lagugné-Labarthet, Opt. Lett. 28 (2003) 1296–1298. [38] A. Miniewicz, B. Sahraoui, E. Schab-Balcerzak, A.M. Sobolewska, A.C. Mitus, F. Kajzar, Nonlinear Opt., Quantum Opt. 35 (2006) 95–102. [39] S. Yakunin, V. Gaivoronsky, A. Miniewicz, J. Sworakowski, Mol. Cryst. Liq. Cryst. 496 (2008) 310–321. [40] T.A. Singleton, K.S. Ramsay, M.M. Barsan, I.S. Butler, C.J. Barrett, J. Phys. Chem. B 116 (2012) 9860–9865. [41] I. Sava, N. Hurduc, L. Sacarescu, I. Apostol, V. Damian, High Perform. Polym. 25 (2013) 13–24. [42] M. Saphiannikova, V. Toshchevikov, J. Ilnytskyi, Nonlinear Opt., Quantum Opt. 41 (2010) 27–57. [43] M. Saphiannikova, V. Toshchevikov, J. Soc. Inf. Display 23 (2015) 146–153.