94
Chem. Mater. 1999, 11, 94-100
Photopolymerization of Reactive Mesogenic Schiff Bases and Related Metallomesogens M. Cano, L. Oriol,† M. Pin˜ol,† and J. L. Serrano* Departamento de Quı´mica Orga´ nica, Facultad de Ciencias-ICMA, Universidad de Zaragoza-CSIC, 50009-Zaragoza, Spain, and Departamento de Quı´mica Orga´ nica, Escuela Universitaria Polite´ cnica de Huesca-ICMA, Universidad de Zaragoza-CSIC, 22071-Huesca, Spain Received July 13, 1998. Revised Manuscript Received October 23, 1998
Mesomorphic salicylaldimines and related Cu(II), Pd(II), Zn(II), and VO(IV) complexes modified with acrylate groups have been synthesized. The liquid crystalline properties of the acrylic monomers are reported and, except for the Zn(II) complex, all are mesogenic. Their free-radical polymerization has been undertaken under several conditions, in solution, by using a thermal initiator or a photoinitiator. Use of visible light and a fluorinated diaryltitanocene photoinitiator (Irgacure 784-DC from Ciba-Geigy) have been proved the most adequate conditions to polymerize either the organic or the metal-containing acrylate Schiff bases. The light-induced polymerization has been investigated by optical microscopy, DSC, and in cells, and only Cu(II) inhibits the process. The process has been applied to the preparation of metal-containing elastomers and networks, but the change of mesophase during polymerization and the high transition temperatures of diacrylates have limited the preparation of macroscopically oriented films. Light-activated polymerization has also been attempted for related vinyl-ether Schiff bases but it was unsuccessful.
Introduction Research on metallomesogenic polymers has been an expanding area because of their particular combination of properties. Their liquid crystalline properties together with their ease of processing led to oriented polymers of considerable interest due to their highly anisotropic, either mechanical, optical, or thermal behavior. Besides, the presence of metal atoms, especially transition metals, might introduce additional and unusual properties.1 In this context, we have recently reported the properties of anisotropic fibers of polyazomethines prepared by subjecting the polymeric molecules to an elongational flow.2 The fibers show excellent mechanical properties along the longitudinal axis. In addition, the cross sectional properties of these fibers can be improved by processing the polyazomethines modified with low percentages of Cu2+ ions. However, if the equivalent preparation of thin films with a controlled macroscopic molecular orientation and high metal contents is required, the process becomes comparatively more difficult due to the high viscosity of the polymeric mesophases. In that sense, the photoinitiated polymerization of metal-containing liquid crystals, previously oriented into a mesomorphic monodomain, provides, a priori, a good alternative to overcome this problem. †
Escuela Universitaria Polite´cnica de Huesca-ICMA. (1) Oriol, L.; Serrano, J. L. Adv. Mater. 1995, 7, 348. Oriol, L. In Metallomesogens. Synthesis, Properties and Applications; Serrano, J. L., Ed.; VCH: Weinheim, 1996. Oriol, L.; Pin˜ol, M.; Serrano, J. L. Prog. Polym. Sci. 1997, 22, 873. (2) Cerrada, P.; Oriol, L.; Pin˜ol, M.; Serrano, J. L.; Iribarren, I.; Mun˜oz-Guerra, S. Macromolecules 1996, 29, 2515. Cerrada, P.; Oriol, L.; Pin˜ol, M.; Serrano, J. L. J. Am. Chem. Soc. 1997, 119, 7581.
Functionalized liquid crystalline monomers, such as acrylates, methacrylates, vinyl ethers, or diepoxides, with mesogenic groups having aromatic rings connected, for instance, by ester groups have been subjected to freeradical- or cation-based photoinitiated polymerization. The studies have demonstrated that the initial textures of mechanically oriented monomers can be frozen and, consequently, the orientational order preserved.3,4 The in situ photopolymerization process is also preferred to the analogous thermally activated one due to the compatibility of the polymerization temperatures and the mesogenic temperature intervals.5 However, to date, photopolymerization has been barely applied to reactive metallomesogenic compounds. The technique has been tried with uneven success on discotic Cu(II)-containing phthalocyanines with acrylate or methacrylate groups and on diacetylenic carboxylates, but the process has not been deeply investigated.6,7 Investigation of the in situ photopolymerization of acrylate-modified metal carboxylates has yielded a Zn(II)-containing film with a locally anisotropic structure derived from the hexagonal columnar arrangement of the mesomorphic monomer.8 A few references can be found in the literature related to the polymerization of metal-containing calamitic compounds, and preliminary (3) Hikmet, R. A. M.; Lub, J. Prog. Polym. Sci. 1996, 21, 1165. (4) Broer, D. J. In Radiation Curing in Polymer Science. Vol III. Polymerization Mechanisms; Fouassier, J. P., Rabek, J. F., Eds; Elsevier Science: London 1993; p 383. (5) Kelly, S. M. Liq. Cryst. 1998, 24, 71. (6) van der Pol, J. F.; Neeleman, E.; van Miltenburg, J. C.; Zwikker, J. W.; Nolte, R. J. M.; Drenth, W. Macromolecules 1990, 23, 155. (7) Attard, G. S.; Templer, R. H. J. Mater. Chem. 1993, 3, 207. (8) Marcot, L.; Maldivi, P.; Marchon, J. C.; Guillon, D.; Ibn-Elhaj, M.; Broer, D. J.; Mol, G. N. Chem. Mater. 1997, 9, 2051.
10.1021/cm980491a CCC: $18.00 © 1999 American Chemical Society Published on Web 12/17/1998
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Chem. Mater., Vol. 11, No. 1, 1999 95
Figure 1. Synthetic scheme and nomenclature of the reactive Schiff bases and metal complexes.
investigations for the light-initiated radical polymerization of reactive complexes of the salicylaldiminate type have been reported.9,10 Very recently, the approach has also been applied to metal-containing polymerizable lyotropic liquid crystals for synthesizing polymer-based nanocomposites.11 As a first stage toward the preparation of anisotropic metal-containing polymeric films, our goal was to evaluate the potential of the free-radical photoinitiated polymerization for hydroxy-functionalized Schiff bases and related metal complexes, to find suitable reaction conditions, and to study the process. Firstly, we investigated if the salicyladiminate core itself is prone to prevent the process, either due to the interference of the aromatic hydroxyl group giving phenoxy radicals12 or due to the known photochromism of Schiff bases.13 So far, the polymerization of reactive organic Schiff bases has been reported in solution14 and in the molten state, either in the presence15 or absence16 of a thermally activated initiator. Secondly, the role of the metal in the polymerization was also analyzed, because it was not (9) Caruso, U.; Nacca, A.; Roviello, A.; Sirigu, A. New Polymeric Mater. 1995, 4, 309. (10) Gala´n, J. C.; Oriol, L.; Pin˜ol, M.; Serrano, J. L. In The Wiley Polymer Networks Group Review Series; te Nijenhuis, K., Mijs, W. J., Eds.; John Wiley & Sons Ltd.: Chichester, 1998; Vol. 1. (11) Deng, H.; Ging, D. L.; Smith, R. C. J. Am. Chem. Soc. 1998, 120, 3522. (12) Sastri, S. B.; Stupp, S. I. Macromolecules 1993, 26, 5657. Tanaka, H.; Shibahara, Y.; Sato, T.; Ota, T. Eur. Polym. J. 1993, 29, 1525. (13) Feringa, B. L.; Jager, W. F.; de Lange, B. Tetrahedron 1993, 49, 8267. (14) Haitjema, H. J.; Alberda van Ekenstein, G. O. R.; Yong Tan, Y. Eur. Polym. J. 1992, 28, 1191. Campillos, E.; Marcos, M.; Serrano, J. L.; Alonso, P. J.; Martı´nez, J. I. J. Mater. Chem. 1996, 6, 533. Soto Bustamante, E. A.; Yablonskii, S. V.; Ostrovskii, B. I.; Beresnev, L. A.; Blinov, L. M.; Haasse, W. Liq. Cryst. 1996, 21, 829. (15) Paleos, C. M.; Labes, M. M. Mol. Cryst. Liq. Cryst. 1970, 11, 385. (16) Strzelecki, L.; Liebert, L. Bull. Soc. Chim. Fr. 1973, 597. Strzelecki, L.; Liebert, L. Bull. Soc. Chim. Fr. 1973, 603. Strzelecki, L.; Liebert, L. Bull. Soc. Chim. Fr. 1973, 605. Bouligand, Y.; Cladis, P. E.; Liebert, L.; Strzelecki, L. Mol. Cryst. Liq. Cryst. 1974, 25, 233. Perplies, E.; Ringsdorf, H.; Wendorff, J. H. Polym. Lett. Ed. 1975, 13, 243. Clough, S. B.; Blumstein, A.; Hsu, E. C. Macromolecules 1976, 9, 123.
very obvious. Transition metal complexes have been used as additives to prevent the light degradation of polymers due to their capability of absorbing radiation or quenching excited states of chromophores.17 In this way, these compounds would act as inhibitors of a lightinduced polymerization process. However, there are also examples of photoactive transition metal complexes that function as catalysts of processes such as hydrosilations18 and the possibility of these complexes acting as photosensitizers or coinitiators cannot, a priori, be ruled out. Therefore, to determine the possible influence of the organic core and/or the metal, we synthesized the series of compounds shown in Figure 1. Among the compounds, we synthesized mono- and diacrylate organic Schiff bases in order to investigate the aforementioned influence of the ligand. The acrylate function was selected because of its great reactivity. The metals Cu(II), Pd(II), Zn(II), and VO(IV) were selected due to their different paramagnetic and diamagnetic nature and the different geometry of their salicylaldimine complexes.19 It is known that Cu(II), Pd(II), and VO(IV) give mesogenic Schiff bases complexes but that Zn(II) does not; however, Zn(II) might be of interest in the preparation of blends with low-melting temperatures. In this paper, we report the mesogenic properties as well as the behavior of these acrylates under different free-radical conditions, either in solution, in a melt, or under light radiation, focusing on the latter. In addition, the properties of the polymers obtained by photopolymerization are reported in an attempt to correlate them with the polymerization conditions. As an extension of the work, we describe the initial results on the preparation by photopolymerization of oriented elastomers and networks containing metals. Furthermore, the conclusions have also been applied to the photopolymerization of related vinyl ethers, whose polymerization goes (17) McKellar, J. F.; Allen, N. S. Photochemistry of Man-Made Polymers; Applied Science Publishers Ltd: Essex, 1979. (18) Guo, A.; Fry, B. E.; Neckers, D. C. Chem. Mater. 1998, 10, 531. (19) Sierra, T.; Serrano, J. L. In Metallomesogens. Synthesis, Properties and Applications; Serrano, J. L., Ed.; VCH: Weinheim 1996.
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Cano et al.
through a cationic mechanism that can be promoted by free-radical species.20 Experimental Section Bulk Thermal Polymerization of Acrylates. Polymerizable formulations were prepared by dissolving the appropriate diacrylate with 1% (weight ratio) of a thermal initiator, either 2,2′-azoisobutyronitrile (AIBN) or benzoyl peroxide, in freshly distilled CH2Cl2. The homogeneous solution was evaporated on standing at room-temperature overnight. Polymerization of the samples was performed in a Perkin-Elmer DSC-7 apparatus using unsealed pans under nitrogen atmosphere. The samples were stored and flushed with nitrogen in the sample holder of the DSC for 10 min before the temperature was raised at 10 °C/min. After polymerization, the sample was dissolved in THF and the residual percentage of monomer quantified by GPC. Photoinitiated Polymerization of Acrylates and Vinyl Ethers. Preparation of the Photopolymerizable Formulations. Formulations of diacrylates were prepared by dissolving the corresponding compound in freshly distilled CH2Cl2 together with 1 or 3% (weight ratio) of Irgacure 784 DC (a blend of 30% bis(η5-2,4-cyclopentadien-1-yl)-bis[2,6-difluoro-3-(1H-pyrrol-1yl)phenyl]titanium with 70% Dicalite) and 200 ppm of a thermal inhibitor, 2,6-di-tert-butyl-4-methylphenol. The solution was kept away from light and evaporated by bubbling nitrogen through it at room-temperature overnight. Photopolymerizable blends of vinyl ether derivatives were prepared by dissolving the appropriate compound with 1% of Irgacure 784 DC, as the photosensitizer; 1% bis(p-tolyl)iodonium hexafluorophosphate, as the cationic initiator; and 200 ppm of 2,6-di-tert-butyl-4-methylphenol. The solution was evaporated in the dark by standing at room-temperature overnight and 30 min under vacuum. Photopolymerization. Light-induced polymerization of the blends was carried by irradiation of the molten sample pressed between two glass slides with the appropriate light source, in a thermostatic stage at a controlled temperature. Experiments were performed in a glovebag under nitrogen atmosphere to avoid oxygen inhibition. Photopolymerization of the blends was also performed in a Perkin-Elmer DSC-7 apparatus modified for light irradiation.21 To minimize oxygen inhibition, experiments were carried out in nitrogen atmosphere. The samples were purged for 10 min, heated above the clearing point for 5 min, cooled to the polymerization temperature for 5 min and irradiated during 15 min. When thermally activated polymerization was observed below the clearing point, samples were heated straight to the polymerization temperature. During visible-light-initiated polymerization, the thermal effect of the source induces a jump on the baseline of the DSC scan that overlaps the exotherm recorded due to the heat release during the photopolymerization of the sample (see Figure 3). The effect was minimized by using a water bath between the light source and the sample holder to filter IR radiation. The degree of conversion was calculated from the enthalpy content of the polymerization peak recorded on the DSC, taking 78 kJ/mol as the polymerization enthalpy of an acrylate group.4 The percentage of residual monomer was quantified by GPC. Techniques. Elemental analysis was performed with a Perkin-Elmer 240C microanalyzer. IR spectra were measured on a Perkin-Elmer FTIR 1600, and UV-vis spectra were recorded on a UVICAM UV4. 1H NMR were recorded on a Varian Unit-300 spectrometer operating at 300 MHz. Gel permeation chromatography (GPC) was carried out in a Waters liquid chromatography system equipped with a 600E multisolvent delivery system and a 996 photodiode array detector. Two Ultrastyragel columns (from Waters) of 500 and 104 Å pore size were connected in series using THF as the (20) Bi, Y.; Neckers, D. C. Macromolecules 1994, 27, 3683. (21) Tryson, G. R.; Shultz, A. R. J. Polym. Sci., Polym. Phys. Ed. 1979, 17, 2059.
Figure 2. Thermal polymerization of acrylates: (a) DAcr and 1% AIBN and (b) Zn[LAcr]2 and 1% AIBN. The success of the polymerization was determined by the appearance of an exothermic peak as soon as the sample melts. Calculation of the conversion is hindered by the overlapping of the melting and polymerization enthalpies. mobile phase at 0.8 mL/min flow rate. Calibration was performed with polystyrene standards. Mesogenic behavior and transition temperatures were determined using an Olympus BH-2 polarizing microscope equipped with a Linkam THMS600 hot-stage with a TMS91 central processor and a CS196 cooling system. Differential scanning calorimetry (DSC) was performed with a PerkinElmer DSC-7 in aluminum pans at 10 °C/min scan rate under nitrogen atmosphere. Temperatures were read at the onset or the maximum of the peaks for the low-molecular weight compounds and polymers, respectively. Glass transition temperatures were measured at the midpoint of the heat capacity increase. Visible-light-initiated polymerization was performed using a LAES halogen dichroic lamp, EXN 38°, 50W/12V.
Results and Discussion Synthesis and Mesogenic Properties of the Monomers. The monomers and their intermediates were synthesized according to the route displayed in Figure 1. The starting reactive 4-substituted-2-hydroxybenzaldehyde was obtained by adapting reported procedures.22,23 The ligands (acrylate, LAcr; vinyl ether, LVin) and the organic direactive (acrylate, DAcr; vinyl ether, DVin) compounds were obtained by condensing the aldehyde with the appropriate mono- or diamine.24 (22) Hikmet, R. A. M.; Lub, J.; Tol, A. J. W. Macromolecules 1995, 28, 3313. (23) Hikmet, R. A. M.; Lub, J.; Higgins, J. A. Polymer 1993, 34, 1736. (24) Keller, P.; Liebert, L. Solid State Phys. Suppl. 1978, 14, 19.
Photopolymerization of Mesogenic Schiff Bases
Chem. Mater., Vol. 11, No. 1, 1999 97 Table 1. Optical and Thermal Characterization of the Acrylates compound LAcr
DAcr
Cu[LAcr]2 Pd[LAcr]2 VO[LAcr]2 VO[LAcr]2
mesogenic behavior, T in °C (∆H in kJ mol-1)a C 58.3 (41.51) SC 70.7 (-)b SA 88.4 (1.14) N 94.0 (1.02)‚I I 92.6 (-1.01) N 86.8 (-1.12) SA 68.8 (-)b SC 22.2 (-29.24) C C 99.2 (58.17) SA 156.9 (1.11) N 166.8 (0.60) I I 165.8 (-0.67) N 155.1 (-1.04) SAc gd 26.9 (-31.63) C′ 68.2 (40.25) SA 155.7 (1.21) N 165.5 (0.61) I C 112 (-) SA 118 (-) Ie I 109.0 (-6.89) SA 59.4 (-15.82) C C 132.4 (21.53) SA 170.6 (7.32) I I 169.1 (-7.34) SA 121.0 (-)b SC 117.7 (-4.96) SX 98.2 (-9.26) C C 81.5 (78.51) I Ic gd 26.4 (-17.29) C′ 52.6 (-0.71) C′′ 74.8 (23.14) I C 101.9 (17.47) C′ 138.0 (38.30) I I 129.3 (-6.03) SA 63.6 (-9.29) C′′ C′′ 75.83 (-3.01) C′ 132.4 (35.29) I
a Data from the first heating and first cooling are given in that order. Aditionally, data from the second heating is given when differences from the first one are observed. b Transition temperature determined by optical microscopy. c Crystallization is not observed. d Tg is not detected by DSC. e Transition temperatures determined by optical microscopy. From DSC traces only a peak with a shoulder at 108.3 °C (onset temperature) and ∆H ) 42.06 kJ mol-1 is observable.
Table 2. Optical and Thermal Characterization of the Vinyl Ethers compound
Figure 3. Visible-light-activated polymerization of metalcontaining Schiff bases diacrilates with 3% Irgacure 784 DC: (a) Pd[LAcr]2 and (b) Zn[LAcr]2. From the traces the thermal effect of the light source is observable.
Finally, complexes were obtained by stirring the pertinent metal salt with the ligand.25 The compounds were characterized by elemental analysis and IR, in all the cases, and by 1H NMR, UV, and GPC techniques, when possible. Acrylates of Cu(II), Pd(II), Zn(II), and VO(IV) were readily obtained. During their characterization by 1H NMR and GPC, it was noticed that, in solution, VO[LAcr]2 degrades, losing the metal and yielding the free ligand. Due to this fact, further studies of this complex were abandoned because polymerizable blends need to be prepared in solution. Synthesis of the metal-containing vinyl ethers encountered slightly increasing difficulties; only Cu[LVin]2 and Pd[LVin]2 were obtained, and complexation was unsuccessful with Zn(II) and VO(IV). The mesogenic properties of these compounds have been investigated by DSC and optical microscopy and the results are collected in Tables 1 and 2. From the data some comments are possible: (1) In general, acrylates have lower transition temperatures than the analogous vinyl ethers, the tendency is more apparent if clearing points or metal complexes are compared. This previously observed behavior has been associated with the bulkiness of the acrylate groups.23 (2) Organic compounds, either mono- and difunctionalized, show smectic and nematic phases. However, the nematic (25) Holm, R. H.; Everett, G. W.; Chakravorty, A. Progr. Inorg. Chem. 1966, 7, 83.
LVin DVin Cu[LVin]2 Pd[LVin]2
mesogenic behavior, T in °C (∆H in kJ mol-1)a C 76.2 (38.81) SC 99.8 (0.34) N 113.9 (1.49) I I 112.3 (-1.65) N 98.4 (-0.34) SC 43.5 (-26.03) I C 81.2 (55.44) SA 172.5 (0.91) N 190.0 (1.20) I I 189.2 (-1.10) N 165.7 (-0.42) SA 82.6 (-)b SCc SC 81.3 (-)b SA 164.6 (0.54) N 186.9 (1.49) I C 121.5 (35.65) SA 134.6 (7.82) I I 134.9 (-8.19)d SA 93.9 (-) SC 77.2 (-19.65) C C 133.0 (4.45) C′ 153.4 (15.58) SA 194.8 (7.52) I I 196.8 (-7.46) SA 150.3 (-)b SC 132.9 (-3.57) C′ 115.4 (-6.40) C
a Data from the first heating and first cooling are given in that order. Additionally, data from the second heating is given when differences from the first one are observed. b Transition temperature determined by optical microscopy. c Crystallization is observed on leaving the sample at room temperature. d A peak with a shoulder is observed by DSC.
mesomorphism is lost when the ligand is complexed to either Cu(II), Pd(II), or VO(IV). (3) On the whole, vinyl ethers have more disposition than acrylates to give SC mesophases. In general, during the thermal and optical characterization of acrylates, no evidence was found for decomposition or thermal polymerization either by optical microscopy or by DSC. DAcr presents N and SA mesomorphism and, as remarked on in Table 1, a slightly different behavior was observed from the first heating to further heating scans with a remarkable tendency to supercool. Cu[LAcr]2 shows a SA phase, Pd[LAcr]2 shows in addition a monotropic SC and a more ordered smectic phase, and Zn[LAcr]2 is not mesogenic due to its tetrahedric geometry. Besides, microscopical observations indicate a strong tendency for DAcr and Pd[LAcr]2 to align in a homeotropic fashion. A monotropic SA phase was detected for VO[LAcr]2, however, in this particular case polymerization is observed under
98 Chem. Mater., Vol. 11, No. 1, 1999
manipulation on the microscope, and after 3 h the sample is not fluid. The mesogenic behavior of the vinyl ethers is simpler than the corresponding acrylates. For LVin, the SA phase is lost when compared to the analogous acrylate. DVin shows enantiotropic N and SA phases together with a monotropic SC mesophase. The last phase exhibits a strong tendency toward supercooling, and crystallization is only observed on leaving the sample at room temperature beyond 1 h. As its analogous diacrylate, the compound shows a strong tendency toward homeotropic alignment. For both Cu[LVin]2 and Pd[LVin]2, the enantiotropic SA and the monotropic SC phases were observed. Polymerization of Acrylate Monomers. As has been stated, the polymerization of organic and metalcontaining acrylates was investigated under several conditions such as polymerization in solution and thermal polymerization, but efforts were mainly focused on light-activated polymerization. Polymerization in solution was performed in toluene using AIBN as the initiator. The polymerization was successful for the organic compounds (LAcr and DAcr) but not for the metal-containing ones (Cu[LAcr]2, Pd[LAcr]2 and Zn[LAcr]2). Thermal polymerization in the molten state was investigated on the microscope hot stage, where after standing in the molten state during extended intervals, polymerization can be detected for all the diacrylates except for Cu[LAcr]2. The polymerization of reactive Schiff bases using thermal initiators, AIBN or benzoyl peroxide, was monitored with a DSC apparatus (see Figure 2). From the conversion degrees calculated by GPC, it was concluded that polymerization readily takes place for DAcr, Pd[LAcr]2, and Zn[LAcr]2, but Cu[LAcr]2 gives almost no reaction. Isothermal photoinitiated polymerization was approached but, initially, it was necessary to employ a suitable photoinitiator exhibiting efficient photoactivity in a wavelength range where the target polymerizable compounds do not show strong absorption.26 After several attempts,10 the best system was found to be a visible photoinitiator of the titanocene type (Irgacure 784 DC from Ciba-Geigy) which shows a broad absorbance in the visible region of the spectrum, between 400 and 500 nm.27 Using this photoinitiator, in a first step, the photopolymerization was investigated for the organic monoacrylate, studying the evolution of the reaction and the properties of the final polymer. The conclusions were extrapolated, in a second step, to the photopolymerization of organic and metal-containing diacrylates, which was successful for all of them, except for Cu[LAcr]2. The photopolymerization process was investigated by two procedures: (1) isothermal irradiation at different temperatures in a modified DSC-7 apparatus (see Figure 3) and (2) irradiation of the melt pressed between two glass slides Because a slight variation of the liquid crystalline properties was expected, due to the presence of the initiator, mesophases and transition temperatures were (26) Chang, C.-H.; Mar, A.; Tiefenthaler, A.; Wostratzky, D. in Handbook of Coatings Additives; Calbo, L. J., Ed.; Marcel Dekker: New York, 1992; Vol. 2. (27) Decker, C. Prog. Polym. Sci. 1996, 21, 593.
Cano et al. Table 3. Optical and Thermal Characterization of the Polymerizable Blends Containing the Visible Photoinitiator Irgacure 784 DCa
compound
% initiator
% iodonium salt
LAcr
1
LAcr
3
-
DAcr Pd[LAcr]2 Zn[LAcr]2 LAcr + 5% Pd[LAcr]2 LAcr + 5% Pd[LAcr]2 LAcr + 5% Zn[LAcr]2 LVin DVin Pd[LVin]2
3 3 3 3
-
3
-
3
-
1 1 1
1 1 1
mesogenic behavior, T in °C C 59 Sb 87 N 93 I I 93 N 87 SA 60c SCd C 63 Sb 81 N 92 I I 91 N 82 SA 56c SCd C 100 SAe C 131 SAe C 77 I C 63 Sb 81 N 94 I I 90 N 82 SA 45c SCd C 66 Sb 88 N 94 I I 86 N 80 SA 45c SCd C 63 Sb 81 N 94 I I 90c N 83c SA 56c SCd C 75 SC 100 N 106f C 80 SCg C 133 C′h
a Mesophases and transition temperatures were determined by optical microscopy using a red filter and by DSC using unsealed pans to reproduce polymerization conditions. b The type of smectic mesophase and/or the existence of both SA and SC on heating was difficult to assess due to the use of the red filter. c Transition temperature determined by optical microscopy. d Crystallization on standing at room temperature. e Thermal polymerization observed by DSC above 140 °C. f Thermal polymerization above 111 °C. g Thermal polymerization above 96 °C. h Thermal polymerization above 161 °C.
verified and are collected in Table 3. The calorimetric examination of the curable samples revealed that the initiator induces thermal polymerization above ca. 140 °C. Therefore, the initiator should be used at temperatures low enough that it does not undergo appreciable thermal decomposition. Photopolymerization of the Organic Monoacrylate (LAcr) and Properties of the Side-Chain Polymer. Two curable formulations of LAcr were prepared containing 1 or 3% of initiator, and their photopolymerization was performed at several temperatures: 65, 80, 90, and 100 °C. The temperatures were chosen, according to values collected in Table 3, where the sample was in a different phase (smectic, nematic, or isotropic) in order to determine the possible influence of either the molecular order or the temperature in the light-induced polymerization.4 When LAcr was polymerized in the N and SA phases, in both mesomorphic states, a sudden phase change was observed on the microscope when exposing the sample to a light source, indicating the different mesophase nature of the generated polymer. The sandlike texture of the new mesophase was difficult to identify and it was assigned to an ordered smectic phase. These optical observations also revealed a perceptible slow down of the rate of phase change when the percentage of photoinitiator was reduced from 3 to 1. Assuming a polymerization enthalpy equal to that of conventional acrylates, i.e., 78 kJ/mol,4 conversion degrees were calculated from the integration of the polymerization peak recorded by DSC. Besides the polymerization heat, the peak includes two additional thermal effects that have been ignored. One is the enthalpy of the mesophase transition that takes place during polymerization, which is comparatively small, the other is the thermal effect of the light source. Therefore, conversions were also calculated by GPC
Photopolymerization of Mesogenic Schiff Bases
together with the average molecular weight and the polydispersity index of the polymers. Both series of results are in good agreement. Conversions attained for the 1% formulation at different temperatures are about 92-96%. The resulting polymers have large polydispersity indexes (13-19) and are characterized by a bimodal distribution of molecular weight, having number-average molecular weights (Mn) ranging from 16 000 to 23 000. Slightly lower conversions, 86-96%, are achieved for the 3% formulation showing a small increase associated with an increase of the polymerization temperature. The resulting polymers have markedly lower polydispersity indexes (between 1.7 and 4) and comparable Mn, except for the polymer obtained at 100 °C that was almost unsoluble in THF. The side-chain polymer resulting from the lightactivated radical polymerization was studied by DSC and optical microscopy. Despite the brownish coloration of the 3% blend, in contrast to the yellow coloration of the 1% one, the slight differences in polymerization do not led to noticeable differences in relation to the mesomorphic properties of the generated polymer. A glass transition is detected at 59 °C, followed by endothermic transitions at 152, 163, and 173 °C, with associated enthalpy values of 1.28, 0.75, and 1.19 kJ/ mol, respectively. The SA and N mesophases have been identified above 152 °C; however, it was not possible to assign the type of ordered smectic phase present below this temperature by optical microscopy. Nevertheless, X-ray studies of the same polymer obtained in solution have confirmed the existence of the aforementioned mesophases and revealed the presence of SI and SF phases below 152 °C.28 Furthermore, comparisons should take into consideration possible differences in the physical properties of the polymers. Photopolymerization of the Organic and Metal-Containing Diacrylates. The second step in the present study was to investigate the diacrylates, but in this case, only formulations containing 3% initiator were prepared, to achieve a high initial radical concentration. As was previously mentioned, the usefulness of the titanocene initiator was limited by the thermal polymerization initiated above 140 °C; this fact restrained the study of the photopolymerization at different temperatures and/or fluid phases. With this in mind, the polymerization of the DAcr and Pd[LAcr]2 was only examined at 110 and 135 °C, respectively, on the SA phase. Because the Zn[LAcr]2 is nonmesogenic, it was polymerized at 90 °C in the isotropic phase. With the microscope, it was observed that in all cases the generated cross-linked product had the same texture as the initial monomers. DSC scans recorded on the photopolymerization of metal complexes Pd[LAcr]2 and Zn[LAcr]2 are given in Figure 3. Due to its high melting temperature, in the particular case of the Pd(II) complex, an overlapping of photoinitiated and thermally initiated polymerization at 135 °C cannot be ruled out. Preparation of Metal-Containing Elastomers by Photopolymerization. From the study of the pure compounds, the photopolymerization of blends composed (28) Pue´rtolas, J. A.; Diaz, R.; Barbera´, J.; Oriol, L.; Pin˜ol, M.; Serrano, J. L. Acta Polym. Accepted for publication.
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of monoacrylates and low percentages of diacrylates was approached. The polymerization of these systems yielded elastomeric materials that have a number of interesting properties.29 At the same time, the preparation of blends limits shortcomings of the pure compounds, such as the high-temperature transitions of DAcr and Pd[LAcr]2 and the nonmesogenic behavior of the Zn[LAcr]2. Therefore, mixtures of LAcr containing 5% (weight ratio) of the corresponding diacrylate were prepared with 3% initiator. Their mesogenic properties are summarized in Table 3. In all the samples, as was seen for LAcr, a mesophase transition was observed with the microscope as the light-induced polymerization took place. The sandlike texture of the final elastomeric material resembles that of the ordered smectic observed for LAcr above 152 °C. The texture is recovered after heating above the clearing temperature and subsequent cooling of the material. Oriented Samples. The key to obtain anisotropic films by in situ photopolymerization is that a macroalignment of the molecules needs to be accomplished in the monomeric state prior to polymerization, either by mechanical shearing or by bringing the molecules into contact with specially treated substances. Accordingly, photopolymerization was conducted in cells with unidirectionally polyamide-rubbed coatings in order to obtain films with a planar alignment. From the fluid phases, the nematic mesophase is the most suitable because the lower viscosity makes its processing easier. For LAcr, cells of 2 µm thickness were filled by capillary flow at the nematic temperature and, macroscopically, it was observed that the initial transparent film turned opaque with ongoing polymerization. The mesophase change induced during polymerization is probably responsible for the loss of orientational order and scattering due to domain formation. This situation was reproduced when oriented cells containing LAcr and 5% of any diacrylate (DAcr, Pd[LAcr]2, or Zn[LAcr]2) were exposed to light irradiation. For DAcr or Pd[LAcr]2, the high viscosity of the smectic phases below 140 °C and their thermal polymerization in the nematic phase hinder obtaining macroscopically planar aligned films, even by mechanical shearing. The strong tendency of diacrylates to develop homeotropic textures for the SA phase was observed by optical microscopy. For this reason, DAcr was oriented in the smectic phase by mechanical shearing between two glass substrates treated with polyimide for homeotropic alignment. A film with perfectly oriented areas was obtained upon irradiating at 135 °C during 10 min. A morphological study of the film by scanning electron microscopy (SEM) (see Figure 4) reveals a structure of uniaxially oriented sheets perpendicular to the surface, providing evidence of the freezing of the molecular organization prevalent at the start of the polymerization. Light-Initiated Polymerization of Vinyl Ethers. With the results of the light-induced polymerization of acrylates as the starting point, curable formulations of the analogous vinyl ethers with 1% of a cationic initiator and 1% of the titanocene photoactive compound, which would act as a visible photosensitizer, were investigated. (29) Davis, F. J. J. Mater. Chem. 1993, 3, 551.
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Figure 4. SEM photograph of the fracture surface of a homeotropic film of DACr. The sample was prepared by tearing up the film along a longitudinal direction in liquid nitrogen.
The selected cationic initiator was a iodonium salt, bis(ptolyl)iodonium hexafluorophosphate. The study of the Cu[LVin]2 was not undertaken due to the quenching activity shown by the analogous diacrylate. Photopolymerization of LVin at 85 and 90 °C took place at low rates, yielding very low degrees of conversion: about 52% of the initial monomer remain unreacted and the photoprocess yields only oligomers (Mn ) 1796). For the diacrylates DVin and Pd[LVin]2, photopolymerization in fluid phases was not possible due to thermal decomposition of the initiator. Conclusions The reaction of the acrylate organic Schiff bases (LAcr and DAcr) under different conditions proved that there is no interference from the salicylaldimine core, either due to the presence of free-radicals or photoinduced structural changes in the molecule. On the other hand, metal-containing diacrylates do not react in solution, but they can be polymerized in bulk, a circumstance likely related to the higher concentration of radicals when the process is carried out in absence of solvent. Palladium(II) and zinc(II) diacrylates (Pd[LAcr]2 and Zn[LAcr]2) were thermally polymerized, even if the process proceeds at low rates. The polymerization is
Cano et al.
accelerated by light using an appropriate initiator, a fluorinated titanocene derivative. However, the Cu(II) complex (Cu[LAcr]2) does not react under any of the tested conditions. The inhibition of the photoinitiated polymerization by Cu(II) has been previously observed.6,8 The mechanism that accounts for the quenching has not been investigated, but as the absorption spectrum of the Cu[LAcr]2 shows no remarkable differences when compared for instance to Pd[LAcr]2, we more likely relate the lack of reactivity to the paramagnetic nature of Cu(II). The photopolymerization has been successfully applied to the preparation of metal-containing elastomers and networks, but the formation of oriented specimens has encountered several obstacles. In the preparation of oriented elastomers, the different mesophase nature of the initial monomeric blend versus the final polymeric material results in a loss of orientation. In relation to oriented networks, the lack of a nematic phase at low temperatures and the high viscosity of the smectic phase hindered the achievement of macroscopic planar alignment. However, the tendency of diacrylates to give homeotropic smectic arrangements has allowed the locking of this molecular order by in situ photopolymerization. Finally, the photopolymerization of analogous vinyl ethers has been confirmed to be less effective, mainly related to an overlapping of the high transition temperatures of these compounds and the thermal activation of the initiator at these temperatures. Acknowledgment. This work was supported by the CICYT Spanish Project MAT96-1073-C02. We are grateful to Prof. D. J. Broer from Philips Research Laboratories in Eindhoven, The Netherlands, for all the helpful discussions as well as to Prof. R. Sastre from the Instituto de Ciencia y Tecnologı´a de Polı´meros, CSIC, in Madrid, Spain. We kindly acknowledge Ciba-Geigy for supplying a sample of the photoinitiator Irgacure 784-DC. Supporting Information Available: Synthesis procedures and characterization data for the reactive compounds (10 pages). Ordering information is given on any current masthead page. CM980491A
