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Entropic Effects in Solvent-Free Bidisperse Polymer Brushes Investigated Using Density Functional Theories
journal contribution
posted on 2019-12-12, 15:07 authored by Ching-Heng Tai, Guan-Ting Pan, Hsiu-Yu YuSolvent-free polymer-functionalized nanoparticles form
a special
type of colloid composed of inorganic cores self-suspended by their
grafted coronas. In the absence of intervening solvent molecules,
the fluidity of the system is provided by these tethered polymers
as they fill the space. Here, we study the structure and interaction
of neighboring polymer-grafted surfaces in the solvent-free condition
using mean-field density functional theories. For opposing flat surfaces,
the brush configuration and the associated energy landscape are semianalytically
investigated given the incompressibility of the tethered entropic
chains. The effect of brush polydispersity (including variations in
both chain length and surface grafting density) is considered by two
bidisperse models corresponding to different physical scenarios: one
for opposing brushes uniformly mixed with two species at a fixed grafting
density, and the other for opposing brushes with distinct chain lengths
and grafting densities. The space-filling capabilities of the neighboring
coronas differ not only by their ratio of radii of gyration for the
composing polymers but also by their ratio of grafting densities.
We show that the system energy depicts a steric repulsion as the brushes
are compressed, which is typical for hairy particles in a solvent.
However, as the interwall separation increases, the cooperative stretching
of the chains leads to an entropic attraction between them, a unique
characteristic of solventless systems. The corresponding brush profiles
change from a bell-like shape to a more step-function-like feature
as the interwall spacing increases significantly. The interwall separation
associated with the overall free energy minimum therefore characterizes
the favorable interparticle spacing for solvent-free polymer-functionalized
particles. The limiting accessible parameter space of polymer sizes
and grafting densities subjected to the space-filling constraint is
comprehensively explored for representative interparticle spacing
characterizing the compressed, relaxed, and stretched regimes for
a given polymer species, respectively. Such information would be useful
for guiding the design of experimental solvent-free polymer-functionalized
nanoparticles.
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chain lengthschain lengthbrush configurationbidisperse modelsSuch informationinterwall separationbell-like shapeDensity Functional Theories Solvent-free polymer-functionalized nanoparticles formspace-filling constraintinterparticle spacingsolvent-free polymer-functionalized particlescores self-suspendedrepresentative interparticle spacingstep-function-like featurepolymer-grafted surfacessolvent-free conditionSolvent-Free Bidisperse Polymer Brushes Investigatedpolymer speciesbrush polydispersitysolvent-free polymer-functionalized nanoparticlesEntropic Effectsentropic attractionsystem energyinterwall separation increasespolymer sizesmean-field densityentropic chainsbrush profiles changespace-filling capabilitiesparameter spacesolventless systemssteric repulsionenergy landscapeinterwall spacing increases
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