Letter | Published:

# Structural colour using organized microfibrillation in glassy polymer films

## Abstract

The formation of microscopic cavities and microfibrils at stress hotspots in polymers is typically undesirable and is a contributor to material failure. This type of stress crazing is accelerated by solvents that are typically weak enough not to dissolve the polymer substantially, but which permeate and plasticize the polymer to facilitate the cavity and microfibril formation process1,2,3. Here we show that microfibril and cavity formation in polymer films can be controlled and harnessed using standing-wave optics to design a periodic stress field within the film4. We can then develop the periodic stress field with a weak solvent to create alternating layers of cavity and microfibril-filled polymers, in a process that we call organized stress microfibrillation. These multi-layered porous structures show structural colour across the full visible spectrum, and the colour can be tuned by varying the temperature and solvent conditions under which the films are developed. By further use of standard lithographic and masking tools, the organized stress microfibrillation process becomes an inkless, large-scale colour printing process generating images at resolutions of up to 14,000 dots per inch on a number of flexible and transparent formats5,6.

## Access optionsAccess options

from\$8.99

All prices are NET prices.

## Code availability

The code used for optical analysis in this study is available from the corresponding authors upon reasonable request.

## Data availability

The data that supports the findings of this study are available from the corresponding authors upon reasonable request.

## References

1. 1.

Kambour, R. P. Review of crazing and fracture in thermoplastics. J. Polym. Sci. D 7, 1–154 (1973).

2. 2.

Robeson, L. M. Environmental stress cracking: a review. Polym. Eng. Sci. 53, 453–467 (2013).

3. 3.

Ward, A. L., Lu, X., Huang, Y. & Brown, N. The mechanism of slow crack growth in polyethylene by an environmental stress cracking agent. Polymer 32, 2172–2178 (1991).

4. 4.

Connes, P. Silver salts and standing waves: the history of interference colour photography. J. Opt. 18, 147–166 (1987).

5. 5.

Kumar, K. et al. Printing colour at the optical diffraction limit. Nat. Nanotechnol. 7, 557–561 (2012).

6. 6.

Carroll, K. M. et al. Fabricating nanoscale chemical gradients with thermochemical nanolithography. Langmuir 29, 8675–8682 (2013).

7. 7.

Sakoda, K. Optical Properties of Photonic Crystals Vol. 80 (Springer, 2004).

8. 8.

Born, M. & Wolf, E. Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light (Elsevier, 2013).

9. 9.

Chung, J. Y., Chastek, T. Q., Fasolka, M. J., Ro, H. W. & Stafford, C. M. Quantifying residual stress in nanoscale thin polymer films via surface wrinkling. ACS Nano 3, 844–852 (2009).

10. 10.

Kang, C. et al. Full color stop bands in hybrid organic/inorganic block copolymer photonic gels by swelling−freezing. J. Am. Chem. Soc. 131, 7538–7539 (2009).

11. 11.

Li, T., Zhou, C. & Jiang, M. UV absorption spectra of polystyrene. Polym. Bull. 25, 211–216 (1991).

12. 12.

Bray, J. C. & Hopfenberg, H. B. The effect of polymer molecular weight on the solvent crazing of polystyrene. J. Polym. Sci. B 7, 679–684 (1969).

13. 13.

Donald, A. M. & Kramer, E. J. Effect of molecular entanglements on craze microstructure in glassy polymers. J. Polym. Sci. Polym. Phys. Ed. 20, 899–909 (1982).

14. 14.

Kolle, M. et al. Mimicking the colourful wing scale structure of the Papilio blumei butterfly. Nat. Nanotechnol. 5, 511–515 (2010).

15. 15.

Chung, W.-J. et al. Biomimetic self-templating supramolecular structures. Nature 478, 364–368 (2011).

16. 16.

Hansen, C. M. 50 years with solubility parameters—past and future. Prog. Org. Coat. 51, 77–84 (2004).

17. 17.

Hansen, C. M. & Just, L. Prediction of environmental stress cracking in plastics with Hansen solubility parameters. Ind. Eng. Chem. Res. 40, 21–25 (2001).

18. 18.

Nadal, E. et al. Plasmon-enhanced diffraction in nanoparticle gratings fabricated by in situ photo-reduction of gold chloride doped polymer thin films by laser interference patterning. J. Mater. Chem. C 5, 3553–3560 (2017).

19. 19.

Matsubayashi, A., Fukunaga, K., Tsuji, T., Ataka, K. & Ohsaki, H. Multilayered ordering of the metal nanoparticles in polymer thin films under photoirradiation. Langmuir 27, 733–740 (2011).

20. 20.

Smirnov, J. R. C. et al. Adaptable ultraviolet reflecting polymeric multilayer coatings of high refractive index contrast. Adv. Opt. Mater. 3, 1633–1639 (2015).

21. 21.

Park, S.-G. & Yang, S.-M. Multicolor patterning using holographic woodpile photonic crystals at visible wavelengths. Nanoscale 5, 4110 (2013).

22. 22.

Yuan, L. & Herman, P. R. Laser scanning holographic lithography for flexible 3D fabrication of multi-scale integrated nano-structures and optical biosensors. Sci. Rep. 6, 22294 (2016).

23. 23.

Suzuki, N., Tomita, Y. & Kojima, T. Holographic recording in TiO2 nanoparticle-dispersed methacrylate photopolymer films. Appl. Phys. Lett. 81, 4121–4123 (2002).

24. 24.

Campbell, M., Sharp, D. N., Harrison, M. T., Denning, R. G. & Turberfield, A. J. Fabrication of photonic crystals for the visible spectrum by holographic lithography. Nature 404, 53–56 (2000).

25. 25.

Nam, K. H., Park, I. H. & Ko, S. H. Patterning by controlled cracking. Nature 485, 221–224 (2012).

26. 26.

Choi, Y. W. et al. Ultra-sensitive pressure sensor based on guided straight mechanical cracks. Sci. Rep. 7, 40116 (2017).

27. 27.

Dubois, V., Niklaus, F. & Stemme, G. Design and fabrication of crack-junctions. Microsyst. Nanoeng. 3, 17042 (2017).

28. 28.

Yang, C. et al. All-solution-processed, scalable, self-cracking Ag network transparent conductor. Phys. Status Solidi 215, 1700504 (2018).

29. 29.

Kim, M., Ha, D. & Kim, T. Cracking-assisted photolithography for mixed-scale patterning and nanofluidic applications. Nat. Commun. 6, 6247 (2015).

30. 30.

Thurn-Albrecht, T. et al. Nanoscopic templates from oriented block copolymer films. Adv. Mater. 12, 787–791 (2000).

31. 31.

Hori, K., Matsuno, H. & Tanaka, K. Sorption kinetics of methanol in thin poly(methyl methacrylate) films studied by optical reflectivity. Soft Matter 7, 10319 (2011).

32. 32.

Nikolov, I. D. & Ivanov, C. D. Optical plastic refractive measurements in the visible and the near-infrared regions. Appl. Opt. 39, 2067–2070 (2000).

33. 33.

El-Kashef, H. The necessary requirements imposed on polar dielectric laser dye solvents. Phys. B 279, 295–301 (2000).

34. 34.

Aspnes, D. E. & Studna, A. A. Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV. Phys. Rev. B 27, 985–1009 (1983).

35. 35.

Taflove, A. & Hagness, S. C. Computational Electrodynamics: The Finite-Difference Time-Domain Method (Artech, 2005).

36. 36.

Oskooi, A. F. et al. Meep: A flexible free-software package for electromagnetic simulations by the FDTD method. Comput. Phys. Commun. 181, 687–702 (2010).

37. 37.

Li, Z.-Y. & Lin, L.-L. Photonic band structures solved by a plane-wave-based transfer-matrix method. Phys. Rev. E 67, 046607 (2003).

38. 38.

Hansen, C. M. On predicting environmental stress cracking in polymers. Polym. Degrad. Stabil. 77, 43–53 (2002).

39. 39.

Akcay, C., Parrein, P. & Rolland, J. P. Estimation of longitudinal resolution in optical coherence imaging. Appl. Opt. 41, 5256–5262 (2002).

40. 40.

Wu, D. Y., Meure, S. & Solomon, D. Self-healing polymeric materials: a review of recent developments. Prog. Polym. Sci. 33, 479–522 (2008).

## Acknowledgements

We thank D. Hirayama for assistance with sample preparation and the support of K. Kuroda, head of the JST-Presto Sakigake programme. We thank the Analysis Centre at iCeMS, KUIAS, Kyoto University for access to their SEM and confocal microscope. We thank the Nanohub at Kyoto University for access to their clean room facilities and micro-LED. This work was supported by JST-PRESTO (JPMJPR1417) and the central facilities are supported by the World Premier International Research Initiative (WPI), MEXT, Japan.

### Reviewer information

Nature thanks Seung Hwan Ko, Shin-Hyun Kim and Marta Rink for their contribution to the peer review of this work.

## Author information

M.M.I. and E.S. initiated, managed and planned the overall project. M.M.I. established the printing protocols. A.H.G. developed the microscopy elements of this work. D.Q., D. Yamamoto, D. Yamaguchi and H.J. all performed the experiments. All authors contributed to data analysis and manuscript refinement and preparation.

Correspondence to Masateru M. Ito or Easan Sivaniah.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Standing-wave modelling and light coherence.

a, Reflecting light within the thin film interferes to form standing waves. As the thickness of the film increases, the minimum intensity remains close to zero while the maximum intensity of the standing wave oscillates within the film, as shown by finite difference time domain simulations of polystyrene on silicon. b, Profiles of standing waves in films of different thickness. The colour shows the light intensity relative to the maximum light intensity that can be achieved in this system (the colour scale shows relative light intensity, unitless). At any thickness, for example those indicated by A and B in a, standing-wave interference occurs, although the anti-node intensity can vary by a factor of up to 0.6 in this polystyrene system. Fluctuations in thickness of the thin films therefore do not prevent crosslinking from occurring as long as the applied dose is high enough. The table below outlines the coherence length of the LED light sources used. Approximating Gaussian light sources, the coherence length is $${l}_{{\rm{c}}}=\sqrt{\frac{2ln\left(2\right)}{{\rm{\pi }}}}\frac{{\lambda }_{0}^{2}}{n{\rm{\Delta }}{\lambda }_{0}}$$ where λ0 is the free space wavelength of the light source, Δλ0 is the spectral width (full-width at half-maximum, FWHM), and n is the refractive index of the medium39. The polymer films in this study had thicknesses of less than 1 μm so that coherent interference could be achieved with light sources that have coherence lengths on the micrometre scale.

### Extended Data Fig. 2 Colour change during layer formation in acetic acid.

Crosslinked polystyrene film (28 kDa) is submerged in acetic acid and porous layers are formed within one minute. Images show snapshots of the stepwise colour change that occurs during the formation of the layers. The time after submersion is indicated in each image. Images were captured using a digital single-lens reflex camera with a macro-lens (EOS Kiss X5, Canon and 60-mm EFS, Canon); see Supplementary Video 1. We found that the large-scale colour change can begin from any location on the film including the centre (as shown), edges and corners, depending on the local film condition. The final structure and in-plane layer formation dynamics as discussed within this paper are independent of these large-scale colour change effects.

### Extended Data Fig. 3 Spectroscopic analysis of the development of films.

a, Reflectance spectrum of the developed polystyrene (28 kDa) films and a model photonic multilayer film spectrum produced using the transfer matrix method. bd, Real-time spectroscopy of the immersion process in acetic acid at various conditions. b, Polystyrene film crosslinked and developed in an acetic acid bath (20 °C). The film initially undergoes a series of sharp changes in spectra, indicating changes in film thickness, and then after formation of a Bragg peak (green arrows), the spectrum persists. c, Polystyrene film crosslinked and developed in an acetic acid bath (30 °C). Unlike in b, the final Bragg peak decays. d, Polystyrene film crosslinked, and then annealed at 160 °C for 2 days and developed in an acetic acid bath (30 °C). The heat map shows a smooth change in spectra, indicating a gradual increase towards a final swollen film without microfibril formation. e, After crosslinking, films were post-annealed at a range of temperatures (60–160 °C) to remove stresses in the films (thermal de-stressing). Clear stepwise changes in the spectrum are seen in films that were thermally de-stressed below the glass transition temperature (approximately 110 °C). After thermal de-stressing at higher temperatures the stepwise expansion effect is reduced and the films simply expand smoothly. f, The film thickness expansion over time is derived from real-time spectra where 1.0 corresponds to the initial film thickness. After microstructure expansion finishes the films relax to equilibrium thickness. Without any stress or external force, crazes in glassy polymers undergo collapse under certain conditions40, as seen by the decay of the expansion ratio.

### Extended Data Fig. 4 Hansen parameter plots.

Specific solvent conditions associated with Hansen solubility theory were identified for each of the polymers used in this study: polystyrene (PS), PMMA, polycarbonate (PC) and PSF. RED, relative energy difference. The blue dots and red crosses denote successful and unsuccessful application of organized stress microfibrillation respectively. The regions within the dotted lines are magnified in the insets. The numeric labelling of each solvent is detailed in the corresponding table where the weight ratio of components in a mixture is indicated in parentheses, for example, ‘16(1:5)’ refers to a solution of toluene and methanol with a weight ratio of 1:5. THF, tetrahydrofuran.

### Extended Data Fig. 5 The recording of reflected and transmitted colour.

a, Illustration for experimental setup to record reflected and transmitted colour of polystyrene film (192 kDa) with BDABP printed on a transparent support. b, An example showing reflected and transmitted colour of the Kyoto University logo printed on a PET substrate. A camera (EOS Kiss X5, Canon) with macro-lens (60-mm EFS, Canon) was used to take the photos. The film reflects blue light and transmits the complementary yellow colour, which can be seen in the shadow of the film. The light source is a 190-W ultrahigh-performance mercury lamp (BenQ, China).

### Extended Data Fig. 6 Micro-LED crosslinking of films.

a, b, The image to be printed is turned into to a two-tone image with Floyd–Steinberg dithering using GIMP software (https://www.gimp.org/) (b). c, The image is then converted to a CAD file using Tanner L-Edit IC Layout software (Mentor Graphics, https://www.mentor.com/tannereda/l-edit). The micro-LED system can control the on/off state of an array (1,024 × 768) of 1 μm × 1 μm light sources, which produce a single frame. The dashed grids in c indicate how such composite images are divided into individual frames. d, An SEM image shows the structure and boundary of a pixel formed in 35-kDa PS/PQ through micro-LED illumination. Scale bar, 1 μm. e, The image resolution is calculated from the printed stripes (light blue) in 35-kDa PS/PQ, which are 1.8 μm wide, corresponding to 14,000 dots per inch. The average linewidths were calculated by first converting the image to a binary image, where the light blue and dark blue were mapped to white and black respectively. The average width of the patterned lines is then calculated as the total area of the white pixels divided by the height of the image and total number of lines. Scale bar, 10 μm.

## Supplementary information

### Supplementary Video 1 | Colour change of films during development.

Video of polystyrene (28 kDa) film crosslinked by 254 nm light developing in acetic acid. The colour of the film changes in a step-wise pattern as new microfibril separated layers are formed. Video was acquired using a DSLR camera with a macro lens (EOS Kiss X5, Canon. EFS 60 mm, Canon).

## Rights and permissions

Reprints and Permissions

• #### DOI

https://doi.org/10.1038/s41586-019-1299-8