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Underwater adhesives have drawn enormous interests for biomedical and engineering applications. In this work,
a strong underwater adhesive is developed by addition reaction between bis(3-aminopropyl) terminated polydimethylsiloxane
(PDMS) and hexamethylene diisocyanate (HDI). The PDMS-based adhesive was totally cured in
aqueous solutions and exhibited strong adhesion with various substrates. The highest underwater adhesion
strength of the adhesive on polyethylene terephthalate (PET) substrate reached 2.8 MPa, which is comparable
with those commercial glues that cured in air. Such strong underwater adhesion is mainly caused by the combination
of chemical crosslinks, hydrogen bonds and other physical interactions. Further study reveals that
increasing the curing temperature can decrease the mechanical strength of the adhesive, which leads to a
decrease in adhesion strength. Moreover, increasing the salt concentration in aqueous solution would screen the
electrostatic charges at the interface and impair the underwater adhesion. Besides, the tensile adhesion tests
suggested that the underwater adhesion strength was enhanced by 3 times as the curing time extended from 3 h
to 6 h. The increased adhesion is mainly caused by the enhanced mechanical strength of the polymeric adhesive
as well as the increased interactions at the adhesive-substrate interface. In addition, the strong adhesion force of
the adhesive is also demonstrated from the microscale by employing atomic-force-microscope force measurement.
This work provides both practical and fundamental insights into developing underwater adhesives with
strong adhesion.