Pressure-sensitive
adhesives (PSAs) are usually made from viscoelastic,
high-molecular-weight copolymers, which are fine-tuned by adjusting
the comonomer ratios, molecular weights, and cross-link densities
to optimize the adhesion properties for the desired end-use. To create
a lightly cross-linked network, an ultraviolet (UV) photoinitiator
can be incorporated. Here, we present the first use of perfluorophenylazide
chemistry to control precisely a polyacrylate network for application
as a PSA. Upon UV irradiation, the highly reactive nitrene from the
azide moiety reacts with nearby molecules through a C–H insertion
reaction, resulting in cross-linking via covalent bonding. This approach
offers three benefits: (1) a means to optimize adhesive properties
without the addition of an external photoinitiator; (2) the ability
to switch off the tack adhesion on demand via a high cross-linking
density; and (3) a platform for additional chemical modification.
A series of poly(n-butyl acrylate-co-2,3,4,5,6-pentafluorobenzyl acrylate) or poly(PFBA-co-BA) copolymers were synthesized and modified post-polymerization
into the photo-reactive poly(n-butyl acrylate-co-4-azido-2,3,5,6-tetrafluorobenzyl acrylate) [azide-modified
poly(PFBA-co-BA)] with various molar contents. When
cast into films, the azide-modified copolymers with a high azide content
achieved a very high shear resistance after UV irradiation, whereas
the tack and peel adhesion decreased strongly with the increase in
azide content, indicating that excessive cross-linking occurred. These
materials are thus photo-switchable. However, in the low range of
azide content, an optimum probe tack adhesion energy was obtained
in films with a 0.3 mol % azide content, where a long stress plateau
(indicating good fibrillation) with a high plateau stress was observed.
An optimum peel adhesion strength was achieved with 0.5 mol % azide.
Thus, the adhesion was finely controlled by the degree of cross-linking
of the PSA, as determined by the azide content of the copolymer chain.
Finally, as a demonstration of the versatility and advantages of the
material platform, we show an azide–aldehyde–amine multicomponent
modification of the azide copolymer to make a dye-functionalized film
that retains its adhesive properties. This first demonstration of
using azide functionality has enormous potential for functional PSA
design.
