Reports: DNI552743-DNI5: Gas-Phase UV Grafting on Surface-Initiated ROMP Coatings for Improved Stability
Bradley J. Berron, PhD, University of Kentucky
This effort is
focused on the investigation of the chemistry, structuring and stability of
coatings formed from surface-initiated ring-opening metathesis polymerization
(SI-ROMP) coatings. We focused this investigation on two distinct areas: 1) the
study of the stability of the underlying self-assembled monolayer, and 2) the
study of solvent-induced degradation of the SI-ROMP coatings. In each we
explored the role of the proposed thiol-grafting in the stability of these
coatings.
Improving the stability
of the self-assembled monolayer adsorbate. In our study of the nature of
monolayer-gold bonding, we observed poor solvent and electrochemical stability
of the base monolayer used to grow the ROMP coatings. Specifically, the
solvents used in SI-ROMP would lower the crystallinity of the alkyl region of
the monolayer, and reducing the component of monolayer stability associated
with intra-chain stabilization. We proceeded with a hypothesis that a y-shaped
adsorbate with two gold-interfacing thiol bonds would provide a more stable
monolayer and an environment-interfacing group with the conformational freedom
required for further functionalization and polymerization. Adsorbates are
synthesized through the thiol-yne addition of two thiol-containing head groups
to an alkyne-containing tail group. The resulting monolayers have two distinct
phases: a highly crystalline head phase adjacent to the gold substrate, and a
reduced density tail phase, which is in contact with the environment (Figure 1).
The ellipsometric thickness of 27 Å is consistent with the proposed structure,
where a densely packed decanedithiol monolayer is capped with an 11 carbon
long, second layer at 50% lateral chain density. The Fourier transform infrared
peaks associated with asymmetric and symmetric methylene stretching are shifted
toward higher wavenumbers compared to those of well-packed self-assembled
monolayers (SAMs), which shows a lower average crystallinity of the thiol-yne
monolayers compared to a typical monolayer. Contact angle measurements indicate
an intermediate surface energy for the thiol-yne monolayer surface, owing to
the contribution of exposed methylene functionality at the surface in addition
to the acid terminal group. The conformational freedom at the surface was
demonstrated through remodeling the thiol-yne surface under an applied
potential. Changes in the receding contact angle in response to an external
potential support the capacity for reorientation of the surface presenting
groups. Despite the low packing at the solution interface, thiol-yne monolayers
are resistant to water and ion transport, supporting the presence of a densely
structured layer at the gold surface. Further, the electrochemical stability of
the thiol-yne adsorbates exceeded that of well-packed SAMs, requiring a more
reductive potential to desorb the thiol-yne monolayers from the gold surface.
The thiol-yne monolayer approach is not limited to carboxylate functionality
and is readily adapted for low-density monolayers of varied functionality. This
work is published as an article in Langmuir.
Figure 1. The design of
thiol-yne adsorbates for low interfacial density and high monolayer stability.
(a) Solution phase synthesis of a bifunctional adsorbate through
click-chemistry and subsequent adsorption. (b) Description of stabilizing
forces in thiol-yne SAMs.
We are continuing in
this promising area of surface chemistry with the simplified synthesis of these
highly stable thiol-yne adsorbates through a surface grafting approach. A manuscript
which describes the surface modification approach and characterizes these
coatings is in preparation for publication as an article in Langmuir.
Stability of SI-ROMP. Here, we investigate
the grafting of thiols to surface-initiated polynorbornene (pNB) as a novel
approach towards highly-stable, ROMP-type conformal coatings (Figure 2). We
have developed the grafting reaction in the solution phase, where alkyl or
fluorocarbon thiols are grafted to the pNB backbone at olefinic sites.
Conversion of olefin groups is >95% by FTIR, and the resulting FTIR spectra
have appropriate additional peaks for linear alkyl or fluorocarbon groups. The
removal of the olefin functionality from the polymer backbone decreased the
rate of coating damage from solvents by over 50%. As expected, the coatings are
still damaged by solvents, as the removal of olefin is incomplete, and 100%
conversion would be required for complete protection by thiol-ene grafting.
These findings perfectly motivate our originally-proposed Objective 3, where
crosslinking of the coatings by a dithiol species will both remove the majority
of the olefin in the backbone, and also provide covalent reinforcement around
potential cleavage site. Work on this task is ongoing.
Figure 2. Surface-initiated
ROMP polymers are unstable in oxygenated solvent. Gas-phase grafting of thiols
to the unsaturated backbone improves polymer stability by consuming alkene
functionality.