3 research outputs found
Bioinspired Catecholic Flame Retardant Nanocoating for Flexible Polyurethane Foams
An efficient, environmentally friendly,
and water-applied flame
retardant surface nanocoating based on polydopamine (PDA) was developed
for foamed materials such as polyurethane (PU). The PDA nanocoating,
deposited by simple dip-coating in an aqueous dopamine solution, consists
of a planar sublayer and a secondary granular layer structure that
evolve together, eventually turning into a dense, uniform, and conformal
layer on all foam surfaces. In contrast to flexible PU foams that
are known to be highly flammable without flame retardant additives,
micro combustion calorimetry (MCC) and thermogravimetric analysis
(TGA) confirm that the neat PDA is relatively inflammable with a strong
tendency to form carbonaceous, porous char that is highly advantageous
for flame retardancy. By depositing nanocoatings of PDA onto flexible
PU foams, the flammability of the PU foam was significantly reduced
with increasing coating thickness. For the thickest coating (3 days
of PDA deposition), the foam quickly self-extinguished and its original
shape was completely preserved after exposure to a flame in a torch
burn test. In addition to the char forming ability of PDA, it is hypothesized
that its catechol units likely scavenge nearby radicals that typically
evolve additional fuel for the fire as they attack surrounding materials.
This multiple flame retardancy action of PDA (i.e., char formation
+ radical scavenging) enables flame retardant foams with a peak heat
release rate (P-HRR) that is significantly reduced (up to 67%) relative
to control foams, representing much better performance than many conventional
additives reported in the literature at comparable or higher loadings
High-Affinity DNA Base Analogs as Supramolecular, Nanoscale Promoters of Macroscopic Adhesion
Adhesion phenomena are essential
to many biological processes and
to synthetic adhesives and manufactured coatings and composites. Supramolecular
interactions are often implicated in various adhesion mechanisms.
Recently, supramolecular building blocks, such as synthetic DNA base-pair
mimics, have drawn attention in the context of molecular recognition,
self-assembly, and supramolecular polymers. These reversible, hydrogen-bonding
interactions have been studied extensively for their adhesive capabilities
at the nano- and microscale, however, much less is known about their
utility for practical adhesion in macroscopic systems. Herein, we
report the preparation and evaluation of supramolecular coupling agents
based on high-affinity, high-fidelity quadruple hydrogen-bonding units
(e.g., DAN·DeUG, <i>K</i><sub>assoc</sub> = 10<sup>8</sup> M<sup>–1</sup> in chloroform). Macroscopic adhesion
between polystyrene films and glass surfaces modified with 2,7-diamidonaphthyridine
(DAN) and ureido-7-deazaguanine (DeUG) units was evaluated by mechanical
testing. Structure–property relationships indicate that the
designed supramolecular interaction at the nanoscale plays a key role
in the observed macroscopic adhesive response. Experiments probing
reversible adhesion or self-healing properties of bulk samples indicate
that significant recovery of initial strength can be realized after
failure but that the designed noncovalent interaction does not lead
to healing during the process of adhesion loss
Marangoni Instability Driven Surface Relief Grating in an Azobenzene-Containing Polymer Film
The Marangoni effect describes fluid
flow near an interface in
response to a surface tension gradient. Here, we demonstrate that
the Marangoni effect is the underlying mechanism for flow driven feature
formation in an azobenzene-containing polymer film; features formed
in azobenzene-containing polymers are often referred to as surface
relief gratings or SRGs. An amorphous polyÂ(4-(acryloylÂoxyhexylÂoxy)-4′-pentylÂazobenzene)
was synthesized and studied as a model polymer. To isolate the surface
tension driven flow from the surface tension pattern inscription step,
the surface tension gradient was preprogrammed via photoisomerization
of azobenzene in a glassy polymer film without forming topographical
features. Subsequently, the latent image was developed in the absence
of light by annealing above the glass transition temperature where
the polymer is a liquid. The polymer flow direction was controlled
with precision by inducing different surface tension changes in the
exposed regions, in accordance with expectation based on the Marangoni
effect. Finally, the height of the formed features decreased upon
extensive thermal annealing due to capillary leveling with two distinct
rates. A scaling analysis revealed that those rates originated from
dissimilar capillary velocities associated with different azobenzene
isomers