Page 38 - Read Online

Page 12 of 28 Park et al. Soft Sci 2024;4:28 https://dx.doi.org/10.20517/ss.2024.22

Following this monomer selection strategy, Lee et al. used EHA as the low T monomer and investigated the
g
impact of various functional monomers on stretchability [126,127] . They incorporated acrylamide (AM), methyl
acrylate (MA), AA, and HEA to create adhesives that fully recovered after 25% elongation, demonstrating
significant flexibility. Stress-strain hysteresis analysis confirmed the recovery properties imparted by the
functional monomers. Cohesion through entanglement and hydrogen bonding with AM or AA resulted in
significant hysteresis loss, while HEA showed minimal loss, indicating its superiority for recovery in
stretchable adhesives. This study suggested HEA as a suitable functional monomer for stretchable adhesives
[Figure 6A]. The same research group developed a fully reversible adhesive with minimal stress-strain
[128]
hysteresis using 2-carboxyethyl acrylate (CEA) as a functional monomer alongside EHA . They
determined that the recovery delay, previously identified due to carboxy groups, was caused by strong
hydrogen bonds formed by these groups after deformation, resulting in residual strain . They found that
[126]
the residual strain was dependent on the amount of CEA and remained consistent even after repeated
experiments. To address this issue, they introduced a pre-strain strategy. The pre-strained adhesive
demonstrated immediate recovery without hysteresis loss after additional deformation [Figure 6B]. This
study is significant as it resolves the recovery delay issue by analyzing the characteristics of the monomer,
particularly when incorporating a carboxy-containing monomer that dramatically enhances adhesion
properties.

Studies have also explored the application of specialized monomers, such as silane acrylates and ethylene
glycol-containing acrylates, in flexible adhesives. Seok et al. demonstrated that using various silane acrylates
increases the polymer’s free volume due to the larger Si−O−Si bond angle compared to the C−O−C bond
[131]
angle, effectively lowering the shear modulus at low temperatures (≤ -20 °C) . They also showed that
ethylene glycol silane acrylate ensures polymer entanglement through physical crosslinking from ethylene
glycol, while the bulky silane group reduces the low-temperature modulus, resulting in a stable modulus
across a wide temperature range . Creep and recovery tests confirmed that the bulky silane group controls
[132]
the elasticity of acrylic adhesives, preventing excessive deformation [Figure 6C].

Additionally, studies are being conducted on hydrogels, demonstrating their potential as adhesives for
stretchable and wearable devices through 2-axis stretch tests [83-86] . These studies utilize substances that act as
movable linkers, such as cyclodextrin or sulfonic acid, or use polymers such as polyvinylpyrrolidone (PVP),
which easily form a hydrogen bond. As adhesives, hydrogels exhibit desirable properties in folding tests or
2-axis stretch tests, stretching and recovering without tearing or buckling issues. For example, Han et al.
designed a novel hydrogel adhesive with a dopamine-containing diacrylate crosslinker . Tri(ethylene
[86]
glycol) diacrylate reacted with dopamine via the aza-Michael reaction to synthesize the crosslinker, which
was then copolymerized with AA to create the hydrogel adhesive. This hydrogel demonstrated a high lab
shear strength of about 70 kPa on skin tissue, attributed to the hydrophilic dopamine unit. It also showed
complete recovery without residual strain up to 150% elongation. Given their biocompatible and stretchable
properties, hydrogels are considered suitable materials for stretchable applications, emphasizing the need to
develop mechanically stable hydrogels similar to conventional acrylic adhesives.

These investigations have highlighted that the structure of monomers can significantly influence the overall
properties of a polymer, such as its T , complex shear modulus, elasticity, and recovery properties. The
g
geometry or composition of monomers and linkers affects not only the structure of the polymer network
but also determines rheological properties such as the temperature stability of the complex shear modulus,
creep, recovery, and adhesion properties.