Rapid Processing of Bottlebrush Coatings through UV-InducedCross-Linking
2020
Bottlebrush polymers can be used
to introduce novel surface properties including hydrophilicity, stimuli-responsiveness,
and reduced friction forces. However, simple, general, and efficient
approaches to cross-linking bottlebrush polymer films and coatings
are limited. Here, we report that bottlebrush polymers with an unsaturated
polynorbornene backbone and thiol-terminated side chains can be cross-linked
on demand by UV irradiation to produce uniform and insoluble bottlebrush
polymer coatings. To quantify the kinetics and efficiency of cross-linking
by UV exposure (254 nm), we measured the normalized residual thickness
(NRT) of bottlebrush and linear polymer films after UV exposure and
solvent washing. For bottlebrush polymers with thiol-terminated polystyrene
(PS) side chains, the NRT exceeded 60% for a UV dose of 1.0 J/cm2, while unfunctionalized linear PS required a dose of 7.9
J/cm2 to achieve similar NRT values. Rapid UV-induced cross-linking
of the bottlebrush PS was attributed to the thiol–ene coupling
of the thiol-terminated side chains with the unsaturated polynorbornene
backbones, as demonstrated through FTIR measurements and control studies
involving bottlebrush polymers with saturated backbones. To establish
the broader applicability of this approach, UV-induced cross-linking
was demonstrated for thin films of bottlebrush polymers with thiol-terminated
poly(methyl acrylate) (BB-PMMA-SH) side chains and those with poly(ethylene
glycol) (BB-PEG) and poly(lactic acid) (BB-PLA) side chains which
do not contain thiol end groups. UV-induced cross-linking of BB-PEG
and BB-PLA films required the use of a multifunctional thiol additive.
Finally, we demonstrated that bottlebrush polymer multilayers can
be fabricated through sequential deposition and UV-induced cross-linking
of different bottlebrush polymer chemistries. The cross-linking process
outlined in this work is simple, general, and efficient and produces
solvent-resistant coatings that preserve the unique properties and
functions of bottlebrush polymers.
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