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Page 6 of 28 Park et al. Soft Sci 2024;4:28 https://dx.doi.org/10.20517/ss.2024.22

Table 1. The required properties of flexible adhesives for various forms and applications
Flat Foldable Rollable Stretchable
Thickness (μm) 100~200 ≤ 50 50~100 50~100
Peel strength (N/cm) > 7 > 1 > 5 > 1
Recommended adhesive behavior Hyperelastic Viscoelastic + hyperelastic Viscoelastic + hyperelastic Hyperelastic
Material group Acrylate, silicone Acrylate Acrylate, silicone Acrylate, silicone, rubber,
hydrogel
Glass transition temperature (T , g - < -30 °C < -40 °C -
°C)
Storage modulus (G’, Pa, @RT) ~10 6 ~10 4 ~10 5 ~10 4
Creep (%, @RT) - 100%~200% > 150% > 250%
Recovery (%, @RT) - > 80% > 85% > 90%
Required environmental stability -20 °C -20 °C -40 °C -
60 °C / 90% RH 60 °C / 90% RH 85 °C / 85% RH
85 °C 85 °C 105 °C
a c
Fatigue stability (@RT, - > 1,000,000 times / 720 h > 100,000 times / 1,000 h > 1,000 times / N.D.
dynamic/static)
a b c
Fatigue stability (@Environ. , - > 50,000 times / 300 h > 10,000 times / 500 h N.D.
dynamic/static)
2 2 2 c
UV stability 2.4 W/m 2.4 W/m 55 W/m N.D.
300 h 300 h 500 h
Applications TV, IT, mobile IT, mobile TV, auto Medical sensor
Textile display
Additional requirements - UV-blocking - Biocompatibility
Water-resistance
Detachable
a b
Fatigue stability is synonymous with dynamic durability or mechanical cycle stability; Environmental conditions are the same as those listed in
c
the row immediately above; Not defined. The required properties are not yet defined and will need to be established in the near future. T :
g
Transition temperature; RT: room temperature; RH: relative humidity; N.D.: not defined; UV: ultraviolet; IT: information technology.

Creep and recovery (Stress relaxation)
Adhesives for flexible devices must deform effectively in response to dynamic changes and return to their
original state once external stress is removed . Otherwise, repeated deformations could lead to permanent
[92]
damage, such as buckling and screen distortion from wrinkles. As mentioned in Section “Low modulus
across a wide temperature range”, although adhesives with an extremely low modulus offer superior stress
relaxation compared to conventional adhesives, their enhanced flowability can adversely affect their
recovery characteristics. Enhancing the recovery properties necessitates adjusting the polymer network
through chemical/physical crosslinking and increasing polymer entanglement. The adhesive’s creep and
recovery characteristics, as well as its stress relaxation properties, are analyzed using dynamic mechanical
analysis (DMA) and a rheometer [Figure 3B].

To control the viscoelastic properties of the adhesive effectively, it is essential to manage both the
combination of monomers and the properties of the polymer network that forms the adhesive. This
polymer network is created through crosslinking and polymer entanglement. Polymer entanglement is
closely related to viscoelasticity, while crosslinked networks primarily affect elasticity. The polymer network
determines the rheological properties of the adhesive, which vary with temperature, frequency, and shear
force. Rheological testing methods, such as frequency sweep, temperature sweep, and creep and recovery,
enable detailed analysis of the adhesive polymer’s network [93-99] . Based on these analytical results, the
adhesive polymer structure can be precisely adjusted to optimize its rheological properties, thereby
enhancing the mechanical stability of flexible devices.