While
polymer brushes in contact with liquids have been researched
intensively, the characteristics of brushes in equilibrium with vapors
have been largely unexplored, despite their relevance for many applications,
including sensors and smart adhesives. Here, we use molecular dynamics
simulations to show that solvent and polymer density distributions
for brushes exposed to vapors are qualitatively different from those
of brushes exposed to liquids. Polymer density profiles for vapor-solvated
brushes decay more sharply than for liquid-solvated brushes. Moreover,
adsorption layers of enhanced solvent density are formed at the brush–vapor
interface. Interestingly and despite all of these effects, we find
that solvent sorption in the brush is described rather well with a
simple mean-field Flory–Huggins model that incorporates an
entropic penalty for stretching of the brush polymers, provided that
parameters such as the polymer–solvent interaction parameter,
grafting density, and relative vapor pressure are varied individually.
Stable and precise control of humidity is imperative for a wide variety of experiments. However, commercially available humidistats (devices that maintain a constant humidity) are often prohibitively expensive. Here, we present a simple yet effective humidistat for laboratory-scale applications that can be easily and affordably (<€250) constructed based on an Arduino Uno as microcontroller, a set of proportional miniature solenoid valves, a gas washing bottle, and a humidity sensor. The microcontroller implements a PID controller that regulates the ratio of a dry and humid airflow. The design and implementation of the device, including a custom driver circuit for the solenoids, are described in detail, and the firmware is freely available online. Finally, we demonstrate its proper operation and performance through step response and long-term stability tests, which shows settling times of approx. 30 s and an attainable relative humidity range of 10–95%
Polymer brushes,
coatings of polymers covalently end-grafted
to
a surface, have been proposed as a more stable alternative to traditional
physisorbed coatings. However, when such coatings are applied in settings
such as vapor sensing and gas separation technologies, their responsiveness
to solvent vapors becomes an important consideration. It can be anticipated
that the end-anchoring in polymer brushes reduces the translational
entropy of the polymers and instead introduces an entropic penalty
against stretching when vapor is absorbed. Therefore, swelling can
be expected to be diminished in brushes compared to nongrafted films.
Here, we study the effect of the anchoring-constraint on vapor sorption
in polymer coatings using coarse-grained molecular dynamics simulations
as well as humidity-controlled ellipsometry on chemically identical
polymer brushes and nongrafted films. We find a qualitative agreement
between simulations and experiments, with both indicating that brushes
certainly swell less than physisorbed films, although this effect
is minor for common grafting densities. Our results imply that polymer
brushes indeed hold great potential for the intended applications.
We present a straightforward procedure for preparing thick (up to 300 nm) poly(3-sulfopropyl methacrylate) brushes using SI-ARGET-ATRP by conducting the reaction in a fluid film between the substrate and a coverslip. This method is advantageous in a number of ways: it does not require deoxygenation of the reaction solution, and the monomer conversion is much higher than usual since only a minimal amount of solution (microliters) is used, resulting in a tremendous reduction (~50x) of wasted reagents. Moreover, this method is particularly suited for grafting brushes to large substrates.
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