Acid-amplifying microcapsules: preparation, characterization, and application to cationic UV curing

Abstract

Microcapsules containing photoacid generators and acid amplifiers have been prepared using a liquid-drying method, for the spatial separation of these molecules from resins. The acid-amplifying microcapsules were applied to a cationic UV curing system of an epoxy resin using a 313 nm light source.

---

Acid-reactive polymers and oligomers have been used for photoreactive materials such as photoresists and UV curing materials by combining with photoacid generators (PAGs). For example, photogenerated acidic species from PAGs catalyze polymer reactions to change the solubility of the polymer in chemically amplified photoresists.¹ To improve the photosensitivity, we have proposed the use of acid-proliferation reactions of acid amplifiers (AAs) that are autocatalytically decomposed into acids by a catalytic amount of acid.² In the reaction system, the number of acid molecules would increase nonlinearly, and the molecules work as catalysts. Various types of AAs, such as acetoacetates,² benzyl sulfonates,³ β-sulfonyloxyketals,⁴ 1,2-diol monosulfonates,⁵ 1,4-diol disulfonates,^6,7 and trioxane derivatives⁸ have been designed to apply the concept of acid proliferation to chemical-amplification systems consisting of PAGs and acid-reactive polymers. However, there is an intrinsic problem in that generated protons are often trapped by resins having some basicity, resulting in inhibition of smooth decomposition of AAs. Our previous study shows that the proliferation reactions are suppressed with proton-accepting solvents such as methanol and dioxane,^2c where the pK_a values of hydroxyl and ether groups are −2.2 (ref. 9a) and 2.92,^9b respectively. Because an AA molecule is surrounded by many weakly basic functional groups of resins such as ether groups and alcohol groups, the influence of the resins on the proliferation reactions is not negligible. To proliferate acidic species from AAs, a double-layered system has been proposed, in which one layer containing PAGs and AAs in polystyrene (PSt), which has no basic functional groups, is separated physically from the other acid-reactive polymer layer.¹⁰ However, the manufacturing process was not simple, and it is difficult to apply such a method for fabrication of cured materials on a large scale. On the other hand, separation of two reaction systems by encapsulation would be another approach. Several stimuli-responsive microcapsules have been introduced to latent materials of self-healing and curing, where the capsules were collapsed by external stimuli such as mechanical stress,^11–13 pH,¹⁴ heat,^15–18 and light.¹⁹

Herein, we have designed a thermally and photoresponsive microcapsule containing PAG and AA as shown in Scheme 1. AA 2 is expected to be autocatalytically decomposed by acidic species from PAG into pentafluorobenzenesulfonic acid (pK_a = −3.3)²⁰ with a tiny amount of acid. Furthermore, this organic acid is stronger than p-toluenesulfonic acid (pK_a = −2.7) that is generated from conventional AAs. The acid-proliferation reactions are effectively caused in PSt microcapsules. Because PSt has no basic moiety, encapsulation of AA 2 in PSt microcapsules would ensure efficient acid proliferation without proton trapping, leading to marked enhancement of curing reactions of epoxy resins.

Scheme 1 Application of acid-amplifying microcapsules to a cationic UV curing system.

Acid-amplifying microcapsules were prepared using a liquid-drying method employing a dichloromethane/water bilayer (see ESI†).²¹ PSt (M_w = 3.5 × 10⁴, M_w/M_n = 1.8) was used as a shell polymer and was dissolved in the dichloromethane layer with PAG 1 and AA 2. On the other hand, polyvinylpyrrolidone and sodium laurate were used as dispersion stabilizer and emulsifier, respectively, in the water layer. It was found that the resulting microcapsules have diameters in the range of 120–560 nm, as observed by SEM (Fig. 1a). The ¹H-NMR spectral measurements showed that the microcapsules consist of PAG 1, AA 2, and PSt (Fig. 1b).

Fig. 1 (a) SEM image of the acid-amplifying microcapsules. (b) ¹H-NMR spectra of PAG 1 (top), AA 2, PSt, and the acid-amplifying microcapsules (bottom, mole ratio: PAG 1/AA 2/Shell (PSt) = 1/13.8/17.6, estimated from the integral ratios). CDCl₃ was used as a solvent where all samples were dissolved.

The acid-amplifying microcapsules were mixed with a polar epoxy resin, EX-512. This resin is cured by acidic species that initiate the ring-opening polymerization of the epoxy groups. In FT-IR spectral measurements, the peak intensity of the epoxy groups observed at 910 cm⁻¹ did not change after irradiation with 313 nm light (Fig. 2). However, the intensity decreased markedly after subsequent heating at 120 °C for 15 min. By increasing the temperature above the glass-transition temperature of PSt (90 °C),²² acidic species would be released out of capsules, resulting in the ring-opening polymerization reactions.

Fig. 2 Changes in the FT-IR spectra of an EX-512 film containing acid-amplifying microcapsules (PAG 1/AA 2/Shell (PSt) = 1/13.8/17.6), before/after 313 nm light irradiation (top, middle, 5000 mJ cm⁻²) and subsequent postbaking at 120 °C for 15 min.

Time courses of the decrease of the epoxy peak in FT-IR measurements are plotted as a function of heating time in Fig. 3. The irradiation energy was varied in the range of 0–5000 mJ cm⁻². It was found that the peak intensity decreased immediately with heating at 120 °C in the exposed cases (Fig. 3a). Furthermore, by using acid-amplifying microcapsules, a clear difference was observed between the unexposed and exposed samples. In the former sample, the normalized peak intensities were relatively higher than those of the latter samples with heating for 5–30 min. This result indirectly indicates the progression of phototriggered acid-proliferation reactions in the PSt microcapsules. The decreasing rate becomes large with irradiation energy, indicating an increase in the total amount of generated acidic species from PAG 1 and AA 2.

Fig. 3 Normalized peak intensities of the epoxy groups (at 910 cm⁻¹) of EX-512 films containing (a) acid-amplifying microcapsules (PAG 1/AA 2/Shell (PSt) = 1/13.8/17.6) or (b) dispersed PAG, AA, and PSt (PAG 1/AA 2/Shell (PSt) = 1/13.8/17.6) from FT-IR measurements, after 313 nm light irradiation of 0 (○), 1000 (●), 2500 (■), and 5000 mJ cm⁻² (◆) plotted as a function of heating time. Postbaking was performed at 120 °C.

On the other hand, in the case that these additives were homogeneously mixed with the EX-512 resin, almost no difference was observed between unexposed and exposed samples under the same postbaking conditions (Fig. 3b). This is due to the retardation of diffusion of generated protons by the many proton-trapping groups such as hydroxyl and ether groups, which inhibit the autocatalytic decomposition of AAs. Furthermore, it is known that the thermal stability of AA 2 in polar media is quite poor.²³ It is considered that instability of the AAs caused an unintended decomposition reaction to occur with the subsequent consumption of the epoxy groups. Comparing these results, it was confirmed that the encapsulation of PAG 1 and AA 2 with a PSt polymer shell indeed contributes to the acid-proliferation reactions in a physically separated place for subsequent efficient acid-catalyzed reactions.

Cured films of EX-512 containing the acid-amplifying microcapsules were then fabricated. Sample films were heated at 120 °C for 10–25 min after UV irradiation because the consumption behaviors of epoxy groups were very different between exposed and unexposed samples with 10–25 min of heating, as shown in Fig. 3a. The hardness of the cured samples was quantified using a pencil-scratch method based on the JIS K5400 standard.²⁴ The hardness range of the pencils used was: 6B (softest), 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H, 7H, 8H, and 9H (hardest). The hardness test results are shown in Fig. 4. The hardness values were below 6B without UV irradiation, while the hardness value was increased in cases with more than of 2500 mJ cm⁻² of irradiation. This is an important result for a photo-controlled cationic UV curing. The maximum hardness of F was obtained with 5000 mJ cm⁻² of irradiation and postbaking at 120 °C for 10 min. A control experiment was performed to confirm the effect of encapsulation. EX-512 films containing the same amount of dispersed PAG 1 and AA 2 gave a value of 3B, even without UV irradiation, after heating at 120 °C for 10 min (data not shown). It is again indicated that encapsulation of PAG 1 and AA 2 with a PSt polymer shell contributes to smooth, adequate autocatalytic decomposition reactions of AA 2 triggered by PAG 1.

Fig. 4 Pencil-hardness of EX-512 films containing 14 wt% of acid-amplifying microcapsules (PAG 1/AA 2/Shell (PSt) = 1/4.4/6.9, with PAG 1 = 1.1 wt% and AA 2 = 5.0 wt% relative to the resin) after 313 nm irradiation and subsequent heating at 120 °C for 10–25 min.

Conclusions

In conclusion, we have prepared acid-amplifying microcapsules with a PSt shell consisting of a PAG and an AA using a liquid-drying method. The AAs were protected from the polar medium by the polymer shell. The acid-amplifying microcapsules were successfully applied to a cationic UV curing system using a 313 nm light source, and were compared with the case using dispersed PAGs and AAs. This is the first example of preparing microcapsules containing AA molecules where the acidic reactivity is double-locked by light and heat. This strategy could be applied to unstable AAs in polar resins with other cationic UV curing systems using resins bearing proton-trapping moieties such as alcohol groups and ether groups.

Acknowledgements

We acknowledge helpful discussions with Dr Yasuyuki Yamada and Dr Takeshi Wakiya of Sekisui Chemical Co., Ltd.

Notes and references

1. H. Ito, Adv. Polym. Sci., 2005, 172, 37.
2. (a) K. Ichimura, K. Arimitsu and K. Kudo, Chem. Lett., 1995, 551; (b) K. Arimitsu, K. Kudo, H. Ohmori and K. Ichimura, J. Photopolym. Sci. Technol., 1995, 8, 43; (c) K. Arimitsu, K. Kudo and K. Ichimura, J. Am. Chem. Soc., 1998, 120, 37.
3. H. Ito and K. Ichimura, Macromol. Chem. Phys., 2000, 201, 132.
4. (a) K. Kudo, K. Arimitsu and K. Ichimura, Mol. Cryst. Liq. Cryst., 1996, 280, 307; (b) K. Kudo, K. Arimitsu, H. Ohmori, H. Ito and K. Ichimura, Chem. Mater., 1999, 11, 2119; (c) K. Arimitsu and K. Ichimura, Macromol. Chem. Phys., 2001, 202, 453.
5. (a) S. Noguchi, K. Arimitsu, K. Ichimura, K. Kudo, T. Ohfuji and M. Sasago, J. Photopolym. Sci. Technol., 1997, 10, 315; (b) A. Teranishi, K. Arimitsu, S. Morino, S. Noguchi and K. Ichimura, J. Photopolym. Sci. Technol., 1998, 11, 159.
6. S. W. Park, K. Arimitsu and K. Ichimura, Macromol. Rapid Commun., 2000, 21, 1050.
7. S. Lee, K. Arimitsu, S. W. Park and K. Ichimura, J. Photopolym. Sci. Technol., 2000, 13, 215.
8. K. Arimitsu and K. Ichimura, Chem. Lett., 1998, 823.
9. (a) V. Gold, K. Laali, K. P. Morris and L. Z. Zdunek, J. Chem. Soc., Perkin Trans. 2, 1985, 865; (b) M. Arnett and C. Y. Wu, J. Am. Chem. Soc., 1960, 82, 4999.
10. K. Arimitsu, K. Kudo, H. Ohmori and K. Ichimura, J. Mater. Chem., 2001, 11, 295.
11. S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown and S. Viswanathan, Nature, 2001, 409, 794.
12. Y. Song, Y. Jo, Y. Lim, S. Cho, H. Yu, B. Ryu, S. Lee and C. Chung, ACS Appl. Mater. Interfaces, 2013, 5, 1378.
13. B. J. Blaiszik, S. L. B. Kramer, M. E. Grady, D. A. McIlroy, J. S. Moore, N. R. Sottos and R. W. Scott, Adv. Mater., 2012, 24, 398.
14. A. P. Esser, N. R. Sottos, S. R. White and J. S. Moore, J. Am. Chem. Soc., 2010, 132, 10266.
15. Z. Zhao, X. Zhou, Q. Tian, X. Wang, W. Li and D. Liu, J. Appl. Polym. Sci., 2014, 131, 41008.
16. H. Jin, C. L. Mangun, A. S. Griffin, J. S. Moore, N. R. Sottos and S. R. White, Adv. Mater., 2014, 26, 282.
17. (a) D. H. Lee, M. Yang, S. H. Kim, M. J. Shin and J. S. Shin, J. Appl. Polym. Sci., 2011, 122, 782; (b) M. J. Shin, J. G. Kim and J. S. Shin, J. Appl. Polym. Sci., 2012, 126, E108.
18. M. Fuensanta, A. Grau, M. D. Romero-Sánchez, C. Guillem and A. M. López-Buendía, Polym. Bull., 2013, 70, 3055.
19. M. F. Bédard, B. G. De Geest, A. G. Skirtach, H. Möhwald and G. B. Sukhorukov, Adv. Colloid Interface Sci., 2010, 158, 2.
20. C. Iwashima, G. Imai, H. Okamura, M. Tsunooka and M. Shirai, RadTech Asia, 2003, 579.
21. R. Mitomi, Y. Taguchi and M. Tanaka, J. Jpn. Soc. Colour Mater., 2006, 79, 487.
22. Differential scanning calorimetry (5 °C min⁻¹, from 30 to 130 °C) was performed using microcapsules prepared with PSt, polyvinylpyrrolidone, and sodium laurate.
23. A. Yoshizawa and K. Arimitsu, Polym. Prepr., 2012, 61, 4212.
24. JIS K5400 defined by Japanese Industrial Standards is a simple method to test the scratch hardness of coatings. In this test, pencils in a range of 6B to 9H hardness grade are used. The pencil is moved scratching over the surface of the coating at a 45° angle with a constant pressure.

---

Footnote

† Electronic supplementary information (ESI) available: Materials, equipment, synthesis, preparation of microcapsules, and experimental procedures. See DOI: 10.1039/c5ra26008f

---