A recent article in Nature Communications introduced a dually cross-linked mechanically interlocked network (MIN) of [2]rotaxane, which can dissipate stress within wrinkles without disrupting the network. It also uses quadruple H-bonding to highlight the importance of mechanical bonds in wrinkle regulation.
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Background
Artificial wrinkles with responsive erasure/regeneration behaviors are promising for smart applications. These are developed using bilayer polymer systems with functional structures, often employing dynamic bonds to create intricate patterns and enable adjustable behaviors.
However, current wrinkle modulation methods, which rely on network rearrangement, face bottlenecks in in situ wrinkle regeneration. While efficient in erasing wrinkles, these methods have inherent limitations in wrinkle recovery.
In contrast, in situ regulation of wrinkle erasure and regeneration can be achieved through mechanical bonds with reversible intramolecular motions, which do not depend on dynamic bond rearrangement.
This study explores the fabrication and in situ regulation of stimuli-responsive wrinkles using a dually cross-linked MIN comprising [2]rotaxane and quadruple H-bonding.
Methods
Three MIN specimens were prepared on polydimethylsiloxane (PDMS) substrates by varying the ratios of benzo-24-crown-8-based [2]rotaxane, nitrobenzyl-caged ureidopyrimidinone (NB-UPy), and butyl methacrylate (BMA) (molar ratios of 0.75: 0.25: 100, 0.5: 0.5: 100, and 0.25: 0.75: 100) using free radical polymerization. Two control samples with singly cross-linked networks (molar ratio of [2]rotaxane, NB-UPy, and BMA was 1: 0: 100 and 0: 1: 100) were prepared for comparison.
To prepare the wrinkle pattern on the MINs/PDMS bilayer, a mixture of tetrahydrofuran solution comprising monomers [2]rotaxane and NB-UPy (molar ratio of 1:1) was spin-coated onto PDMS elastomer and heat-treated.
Wrinkles formed after cooling due to the mismatch in thermal expansion ratios and moduli between the stiff top layer and the soft PDMS substrate. The samples were then exposed to HCl or Et3N gas to erase the wrinkled patterns.
The mechanical properties of the MINs, including Young’s modulus, toughness, energy dissipation, and damping capacity, were determined through standard stress-strain experiments. Additionally, the specimens were characterized using nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy.
Thermal stability was also analyzed through a thermogravimetric analyzer (TGA), and transition temperatures were determined using differential scanning calorimetry (DSC). The wrinkled surfaces were observed using a laser scanning confocal microscope (LSCM).
Results and Discussion
Coating the MIN with nitrobenzyl-caged UPy on a PDMS substrate created a bilayer system. Photoirradiation-induced deprotection of UPy formed quadruple H-bonding cross-linking points, enhancing the modulus of the top layer and promoting wrinkle formation, which could be patterned using photomasks.
The dually cross-linked MIN, combining [2]rotaxane and UPy quadruple H-bonding, demonstrated energy dissipation through both mechanical bonds and quadruple H-bonding in tensile and rheological measurements.
The [2]rotaxane cross-link dissipated energy through the sliding motion of mechanical bonds, while the quadruple H-bonding depended on the dissociation of UPy dimers and the disruption of the network structure.
Acid stimulation with HCl triggered the dissociation of quadruple H-bonding, eliminating MIN wrinkles within 120 seconds due to rapid network disruption. These wrinkles could be regenerated by thermal treatment, but with an altered microstructure, making in situ wrinkle recovery unfeasible due to the necessary network rearrangement.
In contrast, the Et3N treatment eliminated wrinkles without continuous stimulation, occurring after the sample was removed from the Et3N atmosphere. This was attributed to the disrupted host-guest interaction of [2]rotaxane by Et3N, leading to energy dissipation through the sliding motion of [2]rotaxane and gradual wrinkle elimination while preserving the texture.
The wrinkles regenerated upon thermal treatment are nearly identical to the original, demonstrating in situ recovery of the surface microstructure.
Conclusion
The researchers successfully achieved in situ regulation of stimuli-responsive wrinkles using a dually cross-linked MIN. The [2]rotaxane cross-link dissipated stress within the wrinkles through sliding motion without disrupting the network, while the second cross-link of quadruple H-bonding highlighted the advantages of mechanical bonds in wrinkle regulation.
The quadruple H-bonding induced dynamic responsiveness to specific stimuli, altering the network structures, but required contact-based stimulation to cease the response. In contrast, energy dissipation through [2]rotaxane movement was slower, leading to a gradual wrinkle erasure.
Combining these dynamic interactions enabled the integration of different behaviors of wrinkle regulation, including contact/non-contact responses, acid/alkali responses, fast/slow responses, and irreversible/reversible regeneration in a single system. This demonstrates fascinating and non-interfering dynamic properties, promoting the application of dynamic wrinkles in smart and high-precision materials.
Journal Reference
Yang, M., et al. (2024). Stimuli-responsive mechanically interlocked polymer wrinkles. Nature Communications. DOI: 10.1038/s41467-024-49750-8
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