2D organic conjugated polymer nanofilm has shown promising potential applications in organic solar cells and flexible electronics due to its tunable electronic and mechanical properties. However, its multifunctionality is largely hindered by weak mechanical performances. Here, a new strategy of harnessing buckling‐driven delamination is proposed for achieving highly stretchable, free‐standing organic nanosheets with largely improved multifunctionality in mechanical, electrical, and wetting properties. A model system of organic conjugated polymeric (P3BT/C60) nanosheets on prestrained elastomers is fabricated through both spin‐coating and transfer‐printing methods. It is found that the free‐standing nanosheet exhibits both superior mechanical and electrical properties with two times higher in fracture strength, and one order of magnitude higher in electrical conductivity than the spin‐coated nanofilm. Compared to wrinkled spin‐coated nanofilms with orthogonal cracks, the crack‐free, buckle‐delaminated free‐standing nanosheet shows not only stable electrical properties with high stretchability but also a large enhancement in both wetting anisotropy and parallel contact angle due to its higher‐aspect‐ratio features. Lastly, measuring the nanofilm's fracture strength and interfacial toughness from the metrology of cracking and buckle‐delaminated micropatterns is demonstrated. It is shown that such metrology‐based approaches can be applied to various nanofilm–substrate systems for thin film and interfacial mechanical properties measurement.
Small degree of anisotropic wetting on self-similar hierarchical wrinkled surfaces
We studied the wetting behavior of multiscale self-similar hierarchical wrinkled surfaces. The hierarchical surface was fabricated on poly(dimethylsiloxane) (PDMS) substrates by manipulating the sequential strain release and combined plasma/ultraviolet ozone (UVO) treatment. The generated structured surface shows an independently controlled dual-scale roughness with level-1 small-wavelength wrinkles (wavelength of 700–1500 nm and amplitude of 50–500 nm) resting on level-2 large-wavelength wrinkles (wavelength of 15–35 μm and amplitude of 3.5–5 μm), as well as accompanying orthogonal cracks. By tuning the aspect ratio of hierarchical wrinkles, the degree of wetting anisotropy in hierarchical wrinkled surfaces, defined as the contact angle difference between the parallel and perpendicular directions to the wrinkle grooves, is found to change between 3° and 9°. Through both experimental characterization (confocal fluorescence imaging) and theoretical analyses, we showed that the wetting state in the hierarchical wrinkled surface is in the Wenzel wetting state. We found that the measured apparent contact angle is larger than the theoretically predicted Wenzel contact angle, which is found to be attributed to the three-phase contact line pinning effect of both wrinkles and cracks that generates energetic barriers during the contact line motion. This is evidenced by the observed sudden drop of over 20° in the static contact angles along both perpendicular and parallel directions after slight vibration perturbation. Finally, we concluded that the observed small degree of wetting anisotropy in the hierarchical wrinkled surfaces mainly arises from the competition between orthogonal wrinkles and cracks in the contact line pinning.
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- Award ID(s):
- 1745921
- NSF-PAR ID:
- 10073039
- Date Published:
- Journal Name:
- Soft Matter
- Volume:
- 14
- Issue:
- 9
- ISSN:
- 1744-683X
- Page Range / eLocation ID:
- 1517 to 1529
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C7SM02208E
