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Thermal analyses of dye Disperse Red 1 grafted onto silica nanoparticles.

Abstract Dye-grafted silica nanoparticles (GSi[O.sub.2]) were synthesized via a dual-step process involving, first, attachment of the organic dye Disperse Red 1 (DRf) to the coupling agent, 3-isocyanatopropyltriethoxysilane, by means of urethane bonds, and then grafting of the silylated-DRl onto silica nanoparticles (~6 nm) prepared by hydrolysis and condensation of tetraethoxysilane in a sol-gel process. Dye-adsorbed silica nanoparticles (Dsi[O.sub.2]) were also prepared for comparison, for which DR1 was bound only physically to silica instead of covalent bonds. The thermal behaviors of the formed Gsi[O.sub.2] and DSi[O.sub.2] were examined by means of differential scanning calorimetry and thermal gravimetric analysis. The results showed that both the particle size and silica content have significant effects on the thermal behaviors of the dye-adsorbed and dye-grafted silica. Moreover, crystalline DR1 lost significantly its crystallinity after being adsorbed on silica, and became virtually amorphous after being grafted onto silica. The formed particles were UV-cured with a multifunctional acrylic monomer to yield color coatings on glass substrates. UV-visible spectra indicated that brightness and color saturation of the coaling comprising GSi[O.sub.2] could be maintained better than that comprising DSi[O.sub.2] after heat treatment at 280[degrees]C.

Keywords Dye, Silica, Thermal behavior. Thermal stability, Crystallinity, Degradation

Introduction

Dye-doped silica is a new class of organic-inorganic composite materials, (1-16) which find potential in a wide variety of industrial applications, such as colored coatings, (1) fluorescence, visible light and near-infrared emitters, (2) 5 biomedical and chemical sensors, (5-7) molecular imprints, (8) nonlinear optical devices, (9-11) etc., depending on the properties of the dye. Dye-doped silica can easily be prepared through physical means such as solution blending, in which dye molecules are made to adsorb on silica particles via secondary interactive forces, such as hydrogen bonds. The durability of such composite material sometimes has been unsatisfactory due to poor dye-particle interface bonding. In contrast, the chemical approach adopted a synthetic procedure, whereby dye molecules were covalently grafted onto silica surfaces with the bridging of a coupling agent. Many such related studies have been reported.For example, Seckin et al. grafted rhodamine B on mesoporous silica through the inter-linkage of [gamma]-glycidyloxypropyltrimethoxysilane, and found that the main absorption bands of the dye remain unchanged. (13) Cui et al. used 3-isocynatopropyltriethoxysilane to modify the azo chromophore, CI Dispersed Red 1 (DR1) first, and then it was grafted on silica by co-condensation with the precursors, tetraethoxysilane (TEOS) and anilinomethyltriethoxysilane (AMTES), via a sol-gel process. The aniline group of AMTES was found to improve the thermal stability of optical nonlinearity in the formed film. (11) Hou and Schmidt prepared silica-based photochromic coatings that incorporated silylated spirooxazine dyestuff. (16) A de-coloration study was performed and the results indicated that photochromic response of the dye was not affected by silylation.

In the current work, referring to Cui et al.'s method, (11) we synthesized silylated-DR1 by reaction of -N[H.sub.2] groups in the dye with -NCO groups in 3-isocyanatopropyltriethoxysilane (ICPTES) in the first step, while a modification was made subsequently--the silylated-DR1 was chemically bonded to colloidal silica (~ 6 nm prepared separately) via a sol--gel process. The synthesis pathway can be found in Scheme 1. On the other hand, DR1-doped silica through physical adsorption was also prepared by direct mixing of pristine DR1 with a silica sol (6 or 400 nm) for comparison. Differential scanning calorimetric (DSC) and thermogravimelric (TG) analyses of both types of DR1-silica powder were performed. Quite interestingly, dramatic decreases of crystallinity with respect to unmodified DR1 were found for the adsorbed and grafted DR1. As far as our understanding, such unusual behaviors have never been reported in the literature. Furthermore, colored hard coatings were prepared and post-treated at 280[degrees]C for 1 h to see their color stability at an elevated temperature. Promising results, based on light transmittance and CIE color coordinates, were found for the coating containing chemically grafted dye. The detailed preparation procedures and tested results are presented and discussed in the sections that follow.

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Experimental

Materials

The dye, 2-[N-ethyl-4-[(4-nitrophenyl)diazenyl]anilino]ethanol (CI Disperse Red 1, DR1), was purchased from Aldrich. TEOS (>98%) was purchased from Fluka. ICPTES (>95%) was purchased from Aldrich. The former was used to synthesize Si[O.sub.2], whereas the latter was used as a coupling agent. Methyl ethyl ketone (MEK, 99.5%) purchased from Fluka was used as the solvent for all reactions. Dibutyltin dilaurate (T-12) was purchased from Aldrich and was used as the catalyst for the synthesis of the ICPTES-grafted dye (silylated-DRl). Aqueous hydrochloric acid (37 wt%) was purchased from J.T. Baker. The coupling agent, 3-(trimethoxysilyl)propyl methacrylate (MSMA, 98%), and the /rexa-functional crosslinking agent, dipentaerythrilol hexaacrylate (DPHA, regent grade), were purchased from Aldrich. The photoinitiator, 2-hydroxy-2-methyl-l-phenyl-l-propanone, (Darocure 1173), was purchased from Ciba-Geigy. All materials were used as received.

Modification of the dye

The solvent, MEK, was de-aerated with dehumidified nitrogen gas for 0.5 h and heated to 30[degrees]C. Then, a suitable amount of DR1 was added in the solvent under constant agitation. After complete dissolution being attained, ICPTES and then T-12 were added to invoke the dye-modification reaction (formation of urethane linkage), cf. Scheme la, which was allowed to proceed for 3 h. The molar ratio of DR1/ICPTES was set to 1, T-12 was added at 0.1% of the total weight of DR1 plus ICPTES, and MEK consisted 97 wt% of the total reaction mixture. In due courses of the reaction, samples were taken for FTIR analyses, and at the end, the product was vacuum-distilled and then oven-dried (80[degrees]C, 2 h) to yield the silane-modified dye (silylated-DR1, termed MR hereinafter) powder.

Preparation of dye-grafted silica nanoparticles

Silica particles with an average particle size of either ~6 or 400 nm were prepared by the sol--gel method, which involved acid- or base-catalyzed hydrolysis and condensation of TEOS in 2-propanol aqueous solutions. The detailed preparation procedures can be found in our previous publications. (17-21) The dye-grafted silica (termed Gsi[O.sub.2]) was prepared by adding MR and additional H[Cl.sub.(aq)] (pH 1.2) to the ~6 nm silica sol. The condensation reaction between Si-OH or between Si-OH and Si-(O[C.sub.2][H.sub.5]), cf. Scheme lb, was continued for 3 h. Alternatively, unmodified DR1 was also added to the above silica sol as a comparison to see the effect of secondary bonding (e.g., hydrogen bonds between DR1 and the surface -OH of Si[O.sub.2] particles) on the thermal property of the dye. The formed dye-adsorbed silica particles are abbreviated as DSi[O.sub.2]. Powdery products were obtained by the same drying procedure as mentioned previously. The compositions of various DSi[O.sub.2] and Gsi[O.sub.2] are listed in Table 1, for which the symbols DR and GR denote dye-adsorbed and dye-grafted silica, respectively, and the number following the symbol denotes the dye content.

Preparation of colored coatings

To enhance the compatibility between dye-grafted silica nanoparticles and the binder DPHA, ~6 nm silica nanoparticles were modified first with MSMA (cf. references (17, 19)) to form MSMA-grafted silica (termed Msi[O.sub.2]), and then reacted with MR (or just mixed with pure DR1 for comparison) using the same procedure as that described in the section, preparation of dye-grafted silica nanoparticles. To prepare a UV-curable coating formulation, appropriate amounts of DPHA and photoiniliator, Darocure 1173, were added into the as-prepared sol under mild agitation with a solid content adjusted to 15 wt%. Then, the homogeneous sol was spin-coated on a glass substrate, prebaked at 80[degrees]C for 30 s, followed by UV irradiation to yield a cured film. In subsequent discussions, the colored coatings were termed CDR and CGR for those containing dye-adsorbed and dye-grafted silica, respectively, and the weight ratios for the two samples were set at DPHA/DR1/Msi[O.sub.2] = 80/10/10 and DPHA/ DR1/ICPTES/Msi[O.sub.2] = 70/10/10/10, respectively. The coatings were heated at 280[degrees]C for 1 h to examine their color stability against thermal attack.

Characterization

The following methods were employed to characterize the modified and unmodified dye:

(1) Infrared absorption spectra of MR and Gsi[O.sub.2] were obtained using a Fourier transform infrared spectrophotometer (Nicolet Spectrometer 550, USA). For all scans, the spectra were collected over the wavenumber range of 400-4000 [cm.sup.-1] with a resolution of 4 [cm.sup.-1].

(2) The melting temperature ([T.sub.m]) and heat of fusion of DSi[O.sub.2] and Gsi[O.sub.2] were obtained by differential scanning calorimeter (DSC, model 2010, TA Instrument Ltd.). The calorimeter was calibrated with indium standard before running the tests. For a typical experiment, 10 mg of a dried sample was sealed in an aluminum pan and placed in the heating chamber together with an empty reference pan. The temperature was raised from 25 to 250[degrees]C at a constant rate of 10[degrees]C/min under nitrogen flow.

(3) A thermal gravimetric analyzer (TGA, Hi-Rcs TGA 2950, TA Instruments Ltd.) was used to examine the thermal degradation behavior of the pristine and modified dye. Samples (8-12 mg in the platinum pan) were heated from room temperature to 700[degrees]C with a heating rate of 10[degrees]C/min under nitrogen flow.

(4) Light transmittance of the prepared coatings was measured by a UV/Vis spectrometer (UV500, Unicam) over the wavelength range of 400-700 nm with respect to a glass substrate.

(5) The color temperature and CIE chromaticity coordinates of the coatings were measured using a MFS-630 optical characteristics measuring system (Hong Ming Technology, Taiwan).

Results and discussion

Chemical analyses

Figure la shows the FTIR spectra of ICPTES, DR1, and their equal molar mixture. The absorptions due to -NCO and C=O groups of ICPTES are located at 2272 and 1725 [cm.sup.-1], respectively, which are absent from the spectrum of DR1. The bands at 1104 and 1081 [cm.sup.-1] for ICPTES correspond to the vibration of -SiOC groups. The -OH of DR1 appears as a broad band around 3500 [cm.sup.-1]. At different times along silylation of DR1, c.f. Scheme 1, samples were withdrawn and IR analyzed, for which four typical spectra are shown in Fig. 1b. The appearance of -NH absorption at 1518 and 3332 [cm.sup.-1] signifies the formation of urethane linkage via reaction between -NCO of ICPTES and -OH of DR1. The signal of -C=O shifts from 1725 to 1692 [cm.sup.-1], which further evidences conversion of -N=C=0 during urethanation. On the other hand, the -NCO signal at 2271 [cm.sup.-1] declines progressively and vanishes virtually after 3 h, indicating complete reaction being approached at this time. The bands at 1104 and 1079 [cm.sup.-1] for -SiOC are maintained throughout the reaction; hence it can be used in subsequent sol--gel synthesis of Gsi[O.sub.2], c.f. Scheme 1c. FTIR analysis of a typical sol--gel process can be found in the literature. (17-20)

Thermal analyses of the dye-adsorbed and dye-grafted silica

Figure 2 demonstrates the DSC thermograms of DR1 and DSi[O.sub.2] with different dye contents. For the pristine DR1, a major crystal melting peak at 168[degrees]C is indicated (a small hump around 144[degrees]C is also observed, which may arise from impurities) with a large heat of fusion, [DELTA]H = 87.4 J/g. After being attached onto silica particles (with the size 400 nm), the melting endotherm of the dye decreased dramatically. For instance, [DELTA]H became 9.9 J/g for the sample DRL20, which contained 20% DR1. As silica was essentially amorphous, this value corresponded to a melting heat of 49.5 J/g of DR1 in the sample, or a significant decrease of ~43% in crystallinity. Obviously, secondary bonds between silica and DR1 have effectively rendered a large portion of the dye molecules unable to orient into an ordered crystalline lattice. This trend was even more obvious for samples with lower dye contents. As shown in Fig. 2d, heat of fusion for DRL10 was 0.79 J/g (7.9 J/ g-dye); in other words, only ~9% of the dye in the sample has eventually crystallized.

When the size of silica particles was reduced to 6 nm, the total secondary interactions exerted on the dye molecules rose, due to increased contact between these two species. As a result, crystallinity of the dye declined further. As shown in Fig. 3, the heat of fusion for DR20 became as small as 0.42 J/g (2.1 J/g-dye), amounting to a crystallinity only ~l/24 that of DRL20 having the same dye content. Inhibition of crystallization by silica has also been demonstrated previously for the formation of phosphotungstic acid (PWA)/silica nanocomposites, (20) wherein the crystallinity and hence the photochromic response of PWA was tuned by interacting with silica particles of different sizes. For the case of dye-grafted silica, DR1 molecules were each covalently pinched by silica particles (with the size ~6 nm); that is, their motions were considerably restricted, and hence it was impossible for them to crystallize. For example, as shown in Fig. 3c, the DSC pattern for the sample GR20 was practically linear over the range 50-250[degrees]C, without evidence of any identifiable crystal melting behavior.

The thermal decomposition diagrams of pure DR1, MR, and various prepared dye-adsorbed and dye-grafted silica are shown in Fig. 4. DR1 underwent a typical one-stage thermal decomposition, which initiated at ~200[degrees]C, and proceeded very rapidly over the range of 250-350[degrees]C. The 5% weight loss point ([T.sub.d]) was found at 260[degrees]C, and a char yield of 16.75 wt% was obtained at the end (700[degrees]C). The decomposition behavior of the modified dye MR was a little more complicated, as it involved, in addition to the decomposition of DR1 moiety significantly over the 260-400[degrees]C range, the contribution from condensation between silanes to form Si-O-Si, commencing relatively early at ~50[degrees]C and accounting for ~4.7 wt% weight loss up to 200[degrees]C. This activity might have continued, yet with decreased contribution at higher temperatures. Furthermore, a minor amount of weight loss due to decomposition of urethane bonds could be expected. On the whole, the above three possibilities resulted in a final char yield of 40.9%.

The TGA patterns of dye-adsorbed silica (DRL20 and DR20) and dye-grafted silica GR20 are shown in Fig. 4b. Just as with pristine DR1, all three samples decomposed most sharply between 250 and 350[degrees]C, and then the rate slowed down until the end, however, with very high char yields of 76.8-82.6 wt%. The condensation between surface -OH of silica particles accounted for most of the early weight losses (e.g., prior to 250[degrees]C) in the thermal decomposition process. It is interesting to find that the char yield increased following the order: GR20 > DR20 > DRL20; that is, higher amounts of residues being obtained for samples with better dye-silica contacts. It is conceivable that the interfacial moieties attached strongly to silica may form nondegradable bonds (e.g., Si-O, Si-C) or converted to nonvolatile species (e.g., carbon black) attaching strongly to the silica surface at elevated temperatures.

Thermal stability of the coatings

Light transmittance of the coating, CDR containing dye-adsorbed silica, before and after being heated at 280[degrees]C for 1 h was measured, and the spectra are shown in Fig. 5a. Obviously, heat treatment has caused a significant change of transmittance; e.g., the intensity at 650 nm dropped from 67% to 61%, yet it increased from 0.14% to 5.4% at 500 nm. The major absorption occurred over the green--blue range, below which transmittance became null. A small blue-shift of ~25 nm was found after the heating procedure; hence a change of hue toward orange-yellow could be expected. The CIE color coordinates of the coatings were measured, cf. Figure 6 and Table 2. It appears that both x- and y-values decreased substantially after the stringent thermal treatment; therefore, a loss of color saturation is evidently observed in Fig. 5b. Unlike CDR, the coating CGR (containing dye-grafted silica) demonstrated superior color stability against thermal impact. As indicated in Table 2 and Fig. 6, the color coordinates (hence, saturation and hue) and light transmittance changed much less than CDR. Such marked improvement is associated with the fact that liner dispersion (i.e., better dye-silica contact) of the dye has been achieved by chemical modification.

Conclusion

The dye DR1 was attached to silica nanoparticles either physically via secondary interactions (dye-adsorbed silica, Dsi[O.sub.2]) or covalently through the coupling of ICPTES (dye-grafted silica, Gsi[O.sub.2]). These particles possessed lower crystallinity and higher char yield than pristine DR1, as was evident from DSC and TGA analyses. In the extreme case (Gsi[O.sub.2] with 20% dye content), the dye became totally amorphous without detectable crystal melting endotherm. For Dsi[O.sub.2] particles, the changes in crystallinity and char yield were less obvious, and they depended upon the size of Si[O.sub.2] particles in the sample. Furthermore, coatings were prepared by UV-curing of the modified particles together with the crosslinking agent DPHA. Thermal tests of the coatings at 280[degrees]C indicated that Gsi[O.sub.2] was able to maintain the color saturation and hue better than Dsi[O.sub.2].

DOI 10.1007/s11998-015-9670-7

C.-C. Chang. F.-H. Huang. Z.-M. Lin, L.-P. Cheng ([mail])

Department of Chemical and Materials Engineering, Tamkang University, New Taipei City 25137. Taiwan e-mail: lpcheng@mail.tku.edu.tw

C.-C. Chang, L.-P. Cheng

Energy and Opto-Electronic Materials Research Center. Tamkang University, New Taipei City 25137, Taiwan

Acknowledgments The authors thank the Ministry of Science and Technology of Taiwan for financial support (Grant No. NSC 99-2221-E-032-002-MY3).

References

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Table 1: Compositions of dye-adsorbed (DR) and dye-grafted (GR) silica

Sample name            Composition (wt%)

              DR1   MR   Silica (6 nm)   Silica (400 nm)

DRL10         10    --        --               90
DRL15         15    --        --               85
DRL20         20    --        --               80
DR15          15    --        85
DR20          20    --        80
GR20          --    20        80               --

Table 2: Optical properties of the coatings CDR and
CGR before and after heat treatment

Sample name    CIE chromaticity   Light transmittance
                  coordinates            (%)

                 X         y      500 nm    650 nm

CDR            0.449     0.393     0.14       67
CDR-280        0.382     0.371      5.4       61
CGR            0.501     0.371       0        64
CGR-280        0.476     0.366       0        59


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Author:Chang, Chao-Ching; Huang, Feng-Hsi; Lin, Zi-Min; Cheng, Liao-Ping
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Date:Jul 1, 2015
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