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Page 14 of 28 Park et al. Soft Sci 2024;4:28 https://dx.doi.org/10.20517/ss.2024.22

The use of diacrylate crosslinkers to induce chemical crosslinking is common practice. Adjusting the
structure and quantity of the crosslinker allows control over the crosslinking density. Generally, shorter,
structurally rigid crosslinkers, and linkers with stronger secondary interactions result in higher polymer
crosslinking density, thereby enhancing cohesion properties. To increase the cohesion of flexible adhesives,
researchers have introduced chemical and physical crosslinking methods or explored novel linker designs.
These studies are crucial for guiding the design of polymer networks using crosslinkers.

Yi et al. also investigated the rheological properties resulting from the formation of crosslinking networks
[87]
using cyclodextrin-based acrylate (CDA) and 1,6-Hexanediol diacrylate (HDDA) . They studied three
network structures: the covalent crosslinking network (CCN) formed solely with HDDA, the movable
crosslinking network (MCN) with CDA alone, and the confined sliding network (CSN) with both HDDA
and CDA. The investigation revealed significant differences in rheological properties, particularly in strain
recovery. Polymers with CCN and CSN, which possess chemical crosslinking, demonstrated over 80% strain
recovery due to their elasticity. In contrast, the MCN polymer, featuring only physical crosslinking,
exhibited poor strain recovery of 50% due to plastic deformation. During a 3mmR folding test, the CCN
polymer sample cracked and delaminated, while the CSN polymer sample showed no defects [Figure 7A].
This suggested that the strictly crosslinked structure in the CCN polymer, resulting from full chemical
crosslinking, induces a stiff network that leads to crack initiation and subsequent delamination.

Park et al. overcame the limitations of traditional crosslinkers, such as their rigid structure and consequent
[2]
reduction in adhesion, by designing a novel urethane acrylate linker . They incorporated two key concepts
into the core structure. First, they used m-Xylylene diisocyanate (XDI) and 1,3-bis(isocyanatomethyl)-
cyclohexane (H XDI) to enhance light stability and strength; Second, they employed poly(ethylene glycol)
6
(PEG)-acrylate to improve chain mobility and cohesion, resulting in the creation of PEG-contained
urethane diacrylate. The XDI-PEG linker (XPD) and H XDI-PEG linker (HPD) were compared to the
6
commercially used linker HDDA. The findings showed that, while maintaining similar adhesion properties,
the recovery speed at 20% strain was significantly faster [Figure 7B]. Notably, HPD exhibited a strain
recovery speed of over ten times faster. This rapid recovery is attributed to the cyclohexane in HPD acting
as a soft segment with PEG, providing flexibility and limiting urethane hydrogen bond formation due to
steric hindrance. Conversely, XPD showed a slower recovery speed compared to HPD, as the benzene acts
as a hard segment in the linker and allows urethane hydrogen bond formation. This study suggests that in
designing crosslinkers, ensuring sufficient mobility through the use of soft moieties and suppressing
hydrogen bond formation can significantly improve the strain recovery rate of adhesives.

Zhang et al. studied how network structure affects the behavior of stretchable adhesives . They found that
[140]
adhesives with hyperelastic properties recover well without wrinkles after deformation, unlike those with
viscoelastic properties. Their adhesive system involves coating an elastic adhesive surface with a viscoelastic
adhesive, creating interlinks between the elastic and viscoelastic polymers, forming a heterogeneous
network. This combination of interlinks and conventional crosslinks preserves the overall elasticity of the
adhesive while maintaining interface adhesion and shear flow due to the viscoelastic polymer [Figure 7C]. A
similar strategy involving the formation of a heterogeneous network was also employed by Jeong et al., who
developed an adhesive using PDMS as the hyperelastic layer . This adhesive maintains the extremely low
[42]
modulus even at -50 °C, ensuring foldability without delaminations or cracks across a broad temperature
range from -50 to 100 °C. The significance of their research lies in maintaining the heterogeneous network
while ensuring low modulus across the harsh temperatures. These studies highlight the importance of
elasticity for flexible adhesives by introducing new lamellar structured adhesives.