The product was purified by recrystallisation from warm water. As precursor material, 3-amino benzoic acid was used, and the amount of the solution comprising sodium sulphite and sodium carbonate was reduced to and mmol, respectively. All other reaction steps were kept the same. In addition, the possible formation of hydroxyphenyl groups due to the presence of aerial humidity was investigated. Changes in the electron density of the aromatic ring at different irradiation times 0— s were monitored by UV—Vis spectroscopy.
The absorption spectra were recorded in the spectral range from to nm in a quartz cuvette. The irradiation was conducted by using a spot curing device high pressure mercury vapour UV light source with a power output of 9. The thermal stability and the decomposition temperature of the azosulphonate salts were investigated via thermogravimetric measurements TGA.
Samples were placed in aluminium oxide crucibles and analysed. A polydispersity index of 1.
The solutions were stored at room temperature in brown glass vials to prevent light-induced decomposition of the PVA-AZO compounds. The gravity settling method was used for preparing thin films GS films. Therefore, 10 g of the solution was poured into polystyrene petri dishes with a diameter of 85 mm, and dried under constant air flow at room temperature. Thin films were also produced by drop casting on CaF 2 disks DC films. FTIR spectra were recorded of the thin DC films prior and after annealing to determine the anticipated formation of ester bonds. In addition, the films were irradiated with a high-pressure mercury vapour UV light 0— s.
Additionally, sample PAII. The changes in the NMR spectra were monitored. To enhance the visualisation of the pattern caused by decomposition of the azo dyes, a combination of phase contrast imaging and polarised light imaging was employed. Optical micrographs were recorded after patterning and after development in deionised water. The two-step synthesis of water-soluble aryl azosulphonates bearing carboxylic moieties on the aromatic ring is shown in Figure 2.
The obtained azosulphonate dyes were dissolved in deuterated water D2O followed by 1 H and 13 C NMR measurements to evaluate the products, as well as their purity. The remaining hydrogen atom is therefore related to the peak at 7. As additional chemical shifts between 7. The remaining five carbon atoms of the aromatic ring can be assigned to the chemical shifts between and ppm and are colour coded in Figure 3 b. The other hydrogen atoms of the aromatic system are unambiguously assigned to a specific chemical shift whereby the change of the dipole moment by only one carboxylic moiety is not as strong as for the AZOII dye.
The position of the carbon atoms of the aromatic ring is colour coded in the range of to ppm see Figure 4 b. From these results, the high purity of the synthesised azosulphonate dyes was evaluated. The dyes were used for the subsequent experiments.
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Photolysis and thermolysis experiments were performed to investigate the thermal stability and the UV sensitivity of both newly synthesised molecules. This is a result of the acidification to pH 2 in the precipitation step, where a residue of carboxylates is left.
This is found for both azo dyes [ 31 , 32 , 33 ]. As a result of UV irradiation, the azo groups of both azo dyes are decomposed, which leads to a nitrogen gas release and splitting off the sulphonic acid moiety.
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This proposed decomposition mechanism is confirmed by the obtained FTIR kinetics and can also be followed by UV—Vis measurements, as the electron density of the aromatic system is altered due to UV-induced decomposition of the chromophore. The change in the UV—Vis absorption spectra at wavelengths of and nm of both azo dyes can be assigned to changes in the electron density of the aromatic system as a result of UV-induced decomposition see Figure 6 a,b. As photoinduced decomposition takes place, a depletion of the absorption at nm is observed Figure 6 a,b. As the 3,5-dicarboxyphenyl azosulphonate sodium salt exhibits high hygroscopy, it takes up aerial water which can be observed as a mass loss of 2.
This first decomposition temperature can be related to fragmentation of the azo groups, which leads to nitrogen gas emission [ 34 ]. Condensation reactions and radical polymerisation enhance the crosslink density in azosulphonate-doped PVA, which could reduce the hydrophilicity and solubility of neat PVA [ 36 ]. The resulting AZO-doped PVA can be used as photoresist, where the illuminated regions are more water resistant than the non-illuminated parts, which are partly dissolved after development in water.
In Figure 8 , the reaction to covalently link the azo dyes to PVA is shown. The amount of the two azosulphonates in PVA was varied, and for these formulations, thin films were produced, which were subjected to further treatment UV, heat. Sample PAII. It is evident that the detection of specific C atoms reaches the detection limit of the measurement system; however, it is possible to determine the structural elements, which are colour coded in Figure 9.
Upon heat treatment, the formation of aromatic ester linkages can be evidenced by the splitting of the carbonyl shift at ppm into two different signals see detail in Figure 9 b [ 31 ].
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One signal is attributed to carboxylic acid, the other to ester units. The evolution of a second carbonyl signal at ppm see Figure 10 b is also attributed to an aromatic ester formation, while the signal at ppm stems from non-converted carboxylic groups [ 37 ]. As these signals are in general very faint, additional FTIR measurements were conducted on thin films to ascertain these findings. The carboxylic groups of the AZO dyes are partially deprotonated for the pristine azosulphonates, as well as for the doped PVA films and the heat-treated samples. Despite these findings, the occurrence of the condensation reaction is uncertain.
The assigned ester and carboxylic bands overlap, and the C—O—C vibration is in the same wavenumber region as the C—O vibrations of carboxylic acid dimers. As a result, the reaction is not clearly evidenced. After the annealing step, investigations of the photolysis kinetics were performed by UV—Vis spectroscopy.
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In the following section, the photolysis behaviour of PAII. As the electron density of the aromatic ring is altered by the cleavage of the azo group, the absorption at nm is diminished and shifted towards lower wavelengths for both azo dyes [ 26 , 30 , 38 ]. This leads to a bathochromic shift of the absorption to nm for PAII. The addition of comparatively small amounts of AZO dyes 2.
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It is also observed that with increasing dye content, the number of surface defects increases after development, which is caused by outgassing of cleavage products nitrogen gas. The obtained structures are a result of decomposition of the chromophore azo groups and the release of nitrogen gas. During the development step, gas bubble formation, most likely nitrogen gas resulting from UV-induced cleavage of the azosulphonate dyes, takes place. The bubbles are formed in the illuminated areas bright lines, see Figure 15 , thus giving further evidence that the patterning step is the origin of this effect.
In a novel approach, the prepared aryl azosulphonate compounds were covalently attached to poly vinyl alcohol.
At first, the coupling between the photoactive carboxylic acids and the polymer by heat-induced esterification was studied. Due to detection limit issues, it is difficult to prove the coupling reaction of the carboxylic moieties and the hydroxyl groups of the polymer by means of FTIR and 13 C NMR spectroscopy. UV—Vis spectroscopy was employed due to its enhanced sensitivity to visualise UV-induced changes in the prepared materials. These novel UV-reactive materials exhibit good contrast behaviour after photolithographic patterning, which can be visualised by polarised light and phase contrast imaging.
This leads to the conclusion that changes of the optical contrast of the thin films are a result of the cleavage of the azosulphonate groups. Due to the thermal stability of both the azosulphonate compounds and PVA, applications for high-temperature coatings with tuneable polarity depending on the chemical environment would be possible, e.
Further adaptions concerning the coupling process of the azosulphonates onto the polymer backbone could lead to full conversion of the carboxylic moieties. Author Contributions Conceptualisation, P. K; supervision, W. Figure 1. All Journals. New Titles. Pick and Choose. Literature Updates.
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Photochemical behavior of PVA as an oxygen-barrier polymer for solar cell encapsulation
Issue 8, Previous Article Next Article. From the journal: RSC Advances. Photochemical behavior of PVA as an oxygen-barrier polymer for solar cell encapsulation. You have access to this article. Please wait while we load your content Something went wrong. Try again? Cited by. Back to tab navigation Download options Please wait Article type: Paper.
DOI: Photochemical behavior of PVA as an oxygen-barrier polymer for solar cell encapsulation J. Gaume, P. Wong-Wah-Chung, A. Rivaton, S.