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A study of vacuum-deposited organic–inorganic hybrid coatings for UV protection of polycarbonate is presented. UV-absorbing compounds, which are commonly used for polycarbonate, were embedded in a silica matrix by thermal co-evaporation under high vacuum. In addition to the optical properties of the coatings, the influence of the silica network on the organic UV absorber and the stability of the intramolecular hydrogen bond (IMHB) are discussed. A model is presented to show the interaction between the organic compound and the silica matrix. It could be shown with UV irradiation experiments that the hydroxyphenyltriazine compound exhibits higher UV stability in the hybrid coating than the hydroxybenzotriazoles.
© 2016 Optical Society of America
1. Introduction
Polymers replace glass in numerous applications. Polycarbonate (PC) in particular is known for its beneficial properties such as high impact strength, stiffness, clarity and temperature stability; however there are disadvantages e.g. low scratch resistance and photo degradation by exposure to sunlight [1]. To improve the mechanical resistance or the optical performance of the PC surface, functional coatings are necessary. The functional coating needs a UV protective component inside to avoid delamination by photo degradation of the polymer surface. The photo degradation of PC is a surface near process and UV absorbers added to the PC bulk material are not able to sufficiently protect the surface [2]. The absorption edge of the coating must be close to 400 nm to block as much of the critical terrestrial UV light as possible without generating a color impression. Wet chemical lacquers containing organic UV absorbers [3], oxide coatings like ZnO and TiO2 [4] or UV blocking dielectric multilayer stacks are state of the art [5]. To fulfil the tough requirements for outdoor glazing parts, common solutions are still not satisfactory with regard to performance and durability. The high scratch resistance of vacuum deposited hard coatings [6] and the UV protection of organic UV absorbing molecules normally used as additives in polymer blends and lacquers is a desirable combination. Therefore, vacuum deposited organic-inorganic hybrid coatings of organic UV absorbers in a silica matrix were investigated. This paper presents the co-evaporation of the organic UV absorber with silica in a high vacuum process. The optical performance and the morphology of the hybrid coating are discussed as well as the interaction of embedded UV absorbers with the silica matrix.
The protic UV absorbers 2,2’-methylenebis(6-(2H-benzotriazol-2-yl)-4-1,1,3,3-tetramethylbutyl)phenol) (Tin360) and 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxy-phenol (Tin1577) are used for the evaporation experiments [7]. The molecular structures of these compounds are shown in Fig. 1.
Fig. 1 Protic UV absorbers 2,2’-methylenebis(6-(2H-benzotriazol-2-yl)-4-1,1,3,3-tetramethylbutyl)phenol) (Tin360) and 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxy-phenol (Tin1577).
The organic compounds are hydrogen transfer-type absorbers. The reversible energy conversion mechanism of Tin1577 and Tin360 is called excited state intramolecular proton transfer (ESIPT). Therefore, the UV absorbers need an intramolecular hydrogen bond (IMHB) between a proton donor (–OH) and a proton acceptor (-N = ). The energy conversion after excitation by absorption is a highly efficient radiation-less decay process from an excited-state proton-transferred form of the molecule back to the ground state. A schematic drawing of the ESIPT cycle is shown in Fig. 2. A more detailed explanation of the proton transfer process is given in [7]. The absorbed photon energy is dissipated as benign heat. In order that the UV protective coatings have a long lifetime, the IMHB has to be intact and unaffected by the matrix material. The stability of these organic UV absorbers during evaporation in a high vacuum deposition process and in the hybrid coating is considered in detail.
2. Experimental
A Balzers BAK 640 high-vacuum chamber equipped with a thermal evaporator (from CreaPhys GmbH) for the organic materials and an electron beam gun evaporator for silica was used. The coatings were deposited without substrate heating, ion or plasma assistance. The organic compounds Tin1577 and Tin360 were purchased from Sigma-Aldrich and used as received. The deposition rate of the organic materials was 0.2 nm/s at a base pressure of 5 × 10−6 mbar. The deposition rate of the organic substance was held constant for all experiments. A silica granulate with a purity of 99,997% (Prot. Feierabend GmbH) was used for evaporation. The silica deposition rate was varied from 0.2 to 1.5 nm/s depending on the desired coating composition. By co-evaporation of both materials at the same time, the hybrid coatings with different organic content were obtained. The deposition rate and film thickness was controlled by means of quartz oscillating monitoring.
The layer density was calculated from X-ray reflectivity (XRR) measurement data as described in [8]. A D5005 diffractometer from Bruker AXS was used on coatings deposited on silicon substrates.
Reflection and transmission spectra were measured with a PerkinElmer Lamda 900 spectrophotometer in the spectral range of 200–850 nm on layers deposited on fused silica substrates allowing the coating characterization without substrate effects. The refractive index n and the extinction coefficient k of the organic single layers were determined from the spectra using LCalc software based on the oscillator model [9].
Infrared spectra of layers deposited on silicon were recorded in transmission from 4000 to 600 cm−1 using a Varian 3100 FTIR spectrometer. Powders were investigated by attenuated total reflectance (ATR) spectroscopy using a Golden GateTM single-reflection diamond ATR accessory from Specac.
The laser induced fluorescence (LIF) measurements of organic-inorganic UV protective coatings have been performed in a similar way to the experimental system described in [10]. Hybrid coatings with varying organic content on fused silica substrates were characterized. The silica substrate shows the mandatory transmittance at the laser operating wavelength. The excitation was carried out at room temperature with the third harmonic of a nanosecond-pulsed Nd:YAG solid state laser (Spectra-Physics Quanta-Ray PRO 290) at 355 nm. The fluorescence response was detected close to the reflected laser beam (“frontface” arrangement). An intensified, gated optical multichannel analyzer (OMA) system with fiber coupling (iStar ICCD, Andor) served for the detection. The spectra were accumulated over 500 pulses with a temporal gate width of ~50 ms. The emitted light was detected within a wavelength range of 350 nm to 700 nm. In the obtained emission spectra, it was not distinguished between fluorescence and phosphorescence radiation. The scattered excitation laser light has been shielded from the detection system using an interference filter with 355 nm suppression.
A Weiss Umwelttechnik GmbH testing device was used for UV irradiation of hybrid layers deposited on PC samples (Makrolon AG2677). The method is described in detail in [11].
3. Results and discussion
3.1 Evaporation stability and morphology of organic-inorganic hybrid coatings
In a preliminary test it was verified that the organic UV absorbers are evaporable in high vacuum without any significant chemical modifications or degradation. In particular, the molecular structure responsible for the energy conversion mechanism after UV absorption have to be unaffected by the evaporation process. Therefore, the organic materials were evaporated in a high vacuum chamber and condensed on a substrate. The raw and the condensed materials were investigated by ATR spectroscopy and the spectra are presented in Fig. 3 [12]. The compound Tin360 consists of two 2-hydroxyphenylbenzotriazoles connected by an alkyl group. The characteristic absorption bands for the hydroxyphenylbenzotriazole at 1004 cm−1 (C-C-C), 1214 cm−1 (C-C-C), 1322 cm−1 (C-N), 1342 cm−1 (C-N), 1418 cm−1 (N-N-N) and 1605 cm−1 (C-C) are also present after the evaporation process. The absorption band at 708 cm−1 in the ATR-spectra is assigned to a C-H-bond which is characteristic for substituents on the benzene ring. This absorption band is reduced for the condensed material compared to the raw material. This is interpreted as a decomposition of the Tin360 in two 2-hydroxyphenylbenzotriazole compounds with an intact IMHB.
Fig. 3 ATR spectra of Tin360 and Tin1577 as pure material before evaporation and as condensed material after vacuum evaporation. For a better comparability the spectra are displaced to avoid an overlap.
The typical absorption bands for the triphenyl-1,3,5-triazine are at 845 cm−1 (C-C triazine ring), 1352 cm−1 (C-N), 1368 cm−1 (C-N), 1417 cm−1 (C-C benzene ring), 1447 cm−1 (C-C benzene ring), 1509 cm−1 (C-N), 1531 cm−1 (C-N) and 1588 cm−1 (C-C benzene ring). This bands are also unaffected by the evaporation process. The spectra for Tin1577 show deviations in the spectral range 850-775 cm−1. The deviation is understood as scission of the alkylic part from the side chain. The alkoxyl group itself is assigned to the bands at 1052 cm−1 and 1256 cm−1.
Both organic compounds are suitable for the evaporation process. The changes in molecular structures of the UV absorbers after evaporation are not important for the ESIPT, and UV functionality is not affected. The vibration band for the hydroxyl group of the IMHB is assumed to be around 3080 cm−1 for Tin360 and 2800 cm−1 for Tin1577 and visible as small peaks in the spectra [7].
The deposited pure organic layers are highly crystalline which results from a planar structure of the molecules [13,14]. The formation of crystals causes high scattering and the organic coatings appear opaque directly after coating deposition (Tin1577) or after storage of several days (Tin360). In Fig. 4 SEM images of the pure organic layers with crystalline morphology are shown.
Fig. 4 SEM images of the pure organic layers from Tin360 (left, top view) and Tin1577 (right, top view and cross section).
For a use as protective coating on PC glazing parts, the hybrid coatings have to be transparent. The coating appears transparent if the coating is amorphous or organic crystals are so small that light of the visible spectral region is not scattered. This is shown for the mixture of Tin1577 in silica by means of SEM images in Fig. 4. From the SEM images in Fig. 5 it could be suggested that the density depends on the organic content of the hybrid coating. This was verified by XRR measurements of the hybrid coatings from which the density in Fig. 6 can be calculated. Depending on the coating composition the density ranges from 2.1 g/cm3 for silica and 1.2 g/cm3 for the pure organic material. On the graph in Fig. 4, the organic content indicated in volume percent showing a linear correlation with the density of the coating. The molar percentage of the organic content is added to show the discrepancy between the numbers of organic species in the mixture compared to the volume they require in comparison to the silica matrix. It could be concluded that the embedded organic species widen the silica network. This is verified by the following spectroscopic investigations.
Fig. 6 Density of the hybrid coating versus the organic content. The organic content was calculated from an EDX element analysis.
3.2 FTIR, UV-VIS and fluorescence spectroscopic investigations of the hybrid layers
Silicon dioxide deposited by electron beam evaporation shows a porous structure as presented in [15]. Additionally, the formation of the silica network is disturbed by the embedding of organic compounds. IR spectroscopy is a useful tool for the structure analysis of organically modified silica [16]. Figure 7 shows FTIR-spectra of Tin1577-silica-layers with varying organic content. SiO2 exhibit three typical vibration modes assigned to different vibrational modes of the Si-O-Si bond at ~460 cm−1, ~800 cm−1 and ~1070 cm−1. The considered mode is at 1070 cm−1 which is caused by an asymmetric stretching motion of the oxygen parallel to the Si-Si axis. The peak at 1070 cm−1 is accompanied by an intense shoulder at the high frequency side. The IR-spectra of the Tin1577-silica hybrid coatings show an increase of this shoulder with increasing organic content. It is known from sol-gel silica films that the intensity of this shoulder increases with porosity. The absorption band at 960-940 cm−1 is assigned to silanol groups and also increases with the organic content of the coating. Silanols are formed by the saturation of free Si-bonds with hydroxyl groups in a less cross-linked silica network.
Fig. 7 FTIR-spectra of Tin1577-silica-layers with varying organic content: Si-O-Si and its shoulder at 1070 cm−1 (left) and the wavenumber region 4000-2500 cm−1 with hydroxyl species (right).
In addition, there are a number of absorption bands of hydroxyl groups at 3800-2900 cm−1 that are also connected to hydrogen bonds [17]. It can be distinguished between water and chemically bonded or physically hydrogen-bonded hydroxyl groups. Hydrogen-bonded OH groups induce a broad absorption band at 3550-3200 cm−1 which is overlapped by a sharp band at 3500-3400 cm−1 from molecular adsorbed water. The sharp 3750 cm−1 peak for free silanol groups (Si-OH) cannot be found in the graph, but there is a band at 3690 cm−1 for hydrogen-bonded silanols. That means the more organic is in the hybrid coating, the more fragmentary the silica network and more free silica bonds are saturated with hydroxyl groups. Increasingly water can penetrate the pores and interact with the silica network and organic UV absorbers. The characteristic absorption band for the IMHB of the Tin1577 at 2800 cm−1 and its interaction cannot be assigned to the broad absorption band of the different hydroxyl species of the hybrid coating with high silica content. The vibration bands at 2800 cm−1 become obvious for hybrid coatings with large organic content (20 mol%). That is an indication for intact IMHBs.
The refractive index and extinction coefficient for Tin1577-silica hybrid coatings with different organic content are presented in Fig. 8. The resulting optical constants of the hybrid coatings are determined from the pure materials and their volume percentage in the mixture. All hybrid coatings are transparent in the visible spectral region and show high absorption for the UV spectral region (λ < 400 nm) dependent on the organic content.
Fig. 8 Refractive indices and extinction coefficients for Tin1577-silica hybrid coatings with different organic content.
The influence of the silica matrix on the absorption behavior of the UV absorber becomes obvious in the UV-VIS spectra of organic layers in comparison with an organic-silica hybrid coating in Fig. 9. The UV-VIS spectra of the pure organic layers show the same absorption peaks as the spectra made from the compound in solution [7,18]. For Tin1577 the main change in the mixture is a broadening of the absorption edge and slightly shifts of the absorption maxima [11]. Tin360 in silica shows major changes in the intensity and position of the absorption maxima. Solvatochromic effects, steric effects, aggregation and chemical reaction are possible reasons for changes in the absorption behavior. There are no significant changes in the curve shape of the spectra so chemical reactions or aggregation [19] are excluded. The rather continuous shift of the spectra assumes steric and solvatochromatic effects.
Fig. 9 Absorption spectra of Tin1577 and Tin1577-silica hybrid coating (left) and Tin 360 and Tin360-silica hybrid coating (right). The organic content in the compared layers is constant.
The bathochromic shift of the absorption edge is known for UV absorbers in a solid solution [20]. The spectral broadening is explained with an increase of vibration and rotation states in the ground state in consequence of a numerous possible conformation of the molecule in the matrix. This steric effect is one of the explanations for the changes in the absorption spectra of the hybrid coating. Solvatochromatic effects, the interaction between the organic compound and the matrix, are the second reason for changes of the absorption properties of the UV absorber. In the case of Tin360 a planar and a non-planar ground state of the molecule are responsible for the formation of different absorption peaks. Both confirmations are shown in Fig. 10. The absence of the absorption maxima at 350 nm and a new absorption peak at 470 nm is systematic for hydroxybenzotriazole compounds in the non-planar conformation. In the planar state the intramolecular hydrogen bond is intact. In the non-planar conformation the ESIPT is interrupted and as a result the compound interacts with the polar silica matrix by intermolecular hydrogen bonds [21, 22].
Fig. 10 Schematic drawing of the (a) planar BZT-configuration with intact IMHB and the (b) non-planar BZT-configuration with open IMHB bond and an intermolecular hydrogen bond to the polar matrix.
Additionally, it was investigated how molecular water in the coating influences the absorption edge shift. A chronologic order of the absorption spectra after deposition shows a shift of the absorption edge to shorter wavelength. The effect is more obvious for Tin1577 than for Tin360, as presented in the graphs in Fig. 11. FTIR spectroscopic measurements show an obvious increase of vibration bands in the wavenumber region 3800-2500 cm−1 assignable to molecular and physical bonded OH-groups of water and silanoles [17]. In lab conditions, atmospheric water penetrates the hybrid coating, free Si-bonds are saturated and molecular water is embedded in the porous matrix. It is assumed that the open hydroxyl group of the Tin360 compound is primarily reacting with unsaturated Si-bonds during layer deposition under vacuum. This interaction is not affected by the penetration of atmospheric water thus the absorption behavior of the UV absorber is uninfluenced. In reference [23] it was presented that the penetrated water have the ability to interrupt hydrogen bonds between the OH of the silica pores and the absorber compound. For Tin1577 it is suggested that the IMHB is interrupted for most of the compounds directly after deposition and are “repaired” by the penetrating water.
Fig. 11 Chronological change of hydroxyl bands in the spectral region of 4000-2500 cm−1 (top) and absorption edge (bottom) for Tin360-silica (left) and Tin1577-silica (right) after deposition.
Fluorescence measurements were carried out with LIF at 355 nm. The idea is that the UV absorber compound needs an alternative way of energy conversion if the ESIPT is interrupted and radiation-less relaxation is not possible. Then, fluorescence or phosphorescence transitions are probable for excited species. Therefore, the dependence of the fluorescence intensity from the organic content was investigated. A measurement of the pure organic layers was not possible due to layer damage by the high laser intensity. In contrast, no laser damage was observed for the organic-inorganic hybrid coatings. In the case of Tin360-silica coatings fluorescence intensity was measurable but did not depend on the organic content as Fig. 12(left) shows. This correlates with the UV-VIS spectroscopic measurements that show an absorption characteristic for non-planar hydroxybenzotriazole compounds with an interrupted IMHB independent from the organic content [12]. For Tin1577 the fluorescence intensity increases with increasing silica content as seen in Fig. 12(right). It is assumed that the probability of interrupting the IMHB rises with increasing silica content. With the increasing silica content, the hybrid coating becomes denser, less water can penetrate and hydrogen bonds with the matrix are formed more often.
Fig. 12 Dependence of the fluorescence intensity from the organic content in Tin360-silica (left) and Tin1577-silica (right) hybrid coatings.
The oscillations in the fluorescence spectra are likely to result from the strong oscillations in the transmission characteristic of the 355 nm blocking filter placed in front of the detection unit.
From the UV-VIS, FTIR and fluorescence spectroscopic investigation presented before the following model of the embedded organic UV absorbing compounds and its interaction with the silica matrix is shown in Fig. 13. The IMHB for Tin360 is generally weaker than for Tin1577 [7] therefore it exhibits a non-planar configuration in the mixture with silica. The ESIPT is interrupted and intermolecular hydrogen bonds between the compound and the silica matrix are formed. The porous structure of the silica leads to a formation of silanol groups at the surface of the pores that build intermolecular hydrogen bonds with the hydroxyl group of the Tin360 compound. Consequently, the UV absorber is no longer photostable. Tin1577 shows an intact IMHB for hybrid coatings with high organic content and with it, high porosity. Molecular water in the pores interacts with the silanol groups and the IMHB of the Tin1577 is unaffected. The higher the silica content is in the hybrid coating, the more likely the interruption of the IMHB by interaction of hydrogen bonds to silanol groups. The ESIPT mechanism is also disturbed and the molecule susceptible for UV degradation. The model only considered the interaction of hydroxyl group from the UV absorber with the matrix. There are many more interactions possible but not considered here.
Fig. 13 Schematic drawing of the silica network with integrated Tin360 species (left) and Tin1577 (right). The Tin360 shows the non-planar configuration with open IMHB in the matrix. The IMHB of the Tin1557 is intact.
3.3 UV stability
UV stability of the UV protective hybrid coatings was tested with artificial UV irradiation. Results with respect to layer adhesion were already published in [12]. A delamination caused by a tape test occurred for the Tin360-silica coating after 600 h irradiation and for Tin1577-silica coatings after 1560 h irradiation. UV-VIS absorption spectra of the hybrid coatings were measured before and after 1200 h of irradiation time. Figure 14 shows for both UV absorbers a decrease of absorption after irradiation. There are only marginal changes of the absorption for Tin1577. In comparison, major changes are observed for Tin360. Here it becomes obvious that the Tin360 photo degrades much faster in a silica matrix than Tin1577. This verifies the consideration according open and intact IMHB of the UV absorber compound that were carried out before.
Fig. 14 Absorption spectra of Tin1577-silica (left) and Tin360-silica (right) before and after 1200 h of artificial UV irradiation.
Additionally, the important FTIR spectra parts of the UV irradiated coatings are shown in Fig. 15. An increasing peak at 1690 cm−1 is detected for Tin360 in silica. This band is assigned to carbonyl groups that are a typical indicator for photooxidation [24]. In the FTIR spectra of Tin1577 the triazine peak at 1531 cm−1 and 1518 cm−1 decrease only in intensity which might be causes by the loss of UV absorber by volatilization out of the coating. Peak changes indicating degradation of the Tin1577 absorber could not be seen.
Fig. 15 Chronological change of the FTIR absorption spectra for Tin1577 (left) and Tin360-silica (right) during the artificial UV irradiation. The scaled up characteristic organic absorption band of the organic UV absorber after baseline correction.
4. Summary
Vacuum coatings are of great interest for weatherproof and mechanical resistant functional coatings on PC glazing parts. UV protective coatings were realized by thermal co-evaporation of organic UV-absorber compounds and an inorganic matrix material. Thermal evaporation and thin film deposition of the organic UV absorber Tin360 and Tin1577 were demonstrated. The compounds are chemically stable in the vacuum evaporation process and the absorption behavior was not influenced by the deposition method.
Hybrid coatings of the organic compounds with a silica matrix were prepared by co-evaporation. With increasing organic content, the number of hydroxyl species in the coating increases, and are also present as molecular water. Because of the embedding of organic compounds in the silica matrix, the SiO2-network is not completely crosslinked, free Si-bonds are saturated with hydroxyl groups and pores are formed filled with molecular water. Changes to the optical properties from the pure organic material in comparison to the hybrid coating are observed. That means a broadening of the absorption edge and shift of the absorption maxima. The origins are steric effects induced by different conformations of the UV absorber molecule in a solid SiO2-matrix and solvatochromatic effects, so the interaction between the organic compound and the matrix. The formation of hydrogen bonds between the silanol groups of the matrix and the hydroxyl group of the protic UV absorber is critical for the energy conversion mechanism ESIPT. Tin360 is more sensitive to the interruption of the IMHB than Tin1577. The IMHB in the Tin360 compound is energetically weaker and the compound tends to be in a non-planar conformation. The open IMHB results in an intermolecular hydrogen bond with the matrix and goes along with a disruption of the energy conversion mechanism ESIPT. This enhances the sensitivity of the species for photodegradation and result in a minimized UV protection ability of the hybrid coating. Tin1577 shows a better photostability because of less interaction with the matrix nevertheless LIF measurements show that the probability for an opened IMHB for Tin1577 increases with increasing silica content. The discussion of the UV absorber-matrix-interaction was focused on the intramolecular and intermolecular hydrogen bonds with respect to their consequence on the photostability. A direct verification of steric and solvatochromatic effects on the absorption properties makes quantenmechanical calculations necessary and would be of great interest for further investigations.
Funding
Bundesministerium für Bildung und Forschung (BMBF) (FKZ 03 X3028 E, Minerva)
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