353 research outputs found
Macromolecules at interfaces : a flexible theory for hard system
A statistical theory for flexible macromolecules at interfaces has been developed. The theory is based on a lattice model in which the equilibrium set of molecular conformations in a concentration profile is evaluated, using a selfconsistent procedure. In this way, the Flory-Huggins theory for polymer solutions is extended to inhomogeneous solutions of macromolecules without any additional assumption. Apart from the Flory-Huggins polymer-solvent interaction parameter χ, a similar parameter χ s is used to describe the interaction of polymer segments with a solid interface. The average number of molecules in each particular conformation can be computed, so that a very detailed picture of the interfacial structure is obtained. Thus also the train, loop, and tail size distributions of adsorbed polymer can be calculated. In principle, there are no adjustable parameters in the theory. Moreover, there are no restrictions on the system parameters such as polymer concentration, chain length, number of species in a mixture or solvent quality, although in some cases numerical problems may occur. Results are given for adsorption of homopolymers, polydisperse polymer, polyelectrolytes, and star-branched polymer, for the structure of lipid bilayers and of the amorphous phase of semicrystalline polymer, and for the interaction between surfaces due to the presence of adsorbing or nonadsorbing polymer. Available experimental data on adsorption isotherms, bound fraction, layer thickness, surface fractionation, steric stabilization, and polymer bridging agree very well with the theoretical predictions
Effect of Adsorbing and Nonadsorbing Polymer on the Interaction Between Colloidal Particles
In this paper it is described how a recent theoretical model can be applied to a system of two colloidal particles in the presence of adsorbing and nonadsorbing polymer. It turns out that in the case of adsorption the most suitable boundary condition is restricted equilibrium, in which a constant amount of polymer is in local equilibrium inside the gap between two particles. At a low polymer dose the formation of bridges gives rise to bridging flocculation, at higher amounts of polymer steric stabilization occurs due to the mutual repulsion of two extended polymer layers. If the polymer does not adsorb on the particles, full equilibrium applies in which the chemical potentials of solvent and polymer in the gap are the same as in the equilibrium bulk solution, The depletion of polymer near the surface may lead to depletion flocculation in not too concentrated polymer solutions. In very concentrated systems the thickness of the depletion zone is relatively small, and the attraction between the particles becomes too weak to overcome the particle entropy, Then the system is restabilized
Irreversible Adsorption from Dilute Polymer Solutions
We study irreversible polymer adsorption from dilute solutions theoretically.
Universal features of the resultant non-equilibrium layers are predicted. Two
cases are considered, distinguished by the value of the local monomer-surface
sticking rate Q: chemisorption (very small Q) and physisorption (large Q).
Early stages of layer formation entail single chain adsorption. While single
chain physisorption times tau_ads are typically microsecs, for chemisorbing
chains of N units we find experimentally accessible times tau_ads = Q^{-1}
N^{3/5}, ranging from secs to hrs. We establish 3 chemisorption universality
classes, determined by a critical contact exponent: zipping, accelerated
zipping and homogeneous collapse. For dilute solutions, the mechanism is
accelerated zipping: zipping propagates outwards from the first attachment,
accelerated by occasional formation of large loops which nucleate further
zipping. This leads to a transient distribution omega(s) \sim s^{-7/5} of loop
lengths s up to a size s_max \approx (Q t)^{5/3} after time t. By tau_ads the
entire chain is adsorbed. The outcome of the single chain adsorption episode is
a monolayer of fully collapsed chains. Having only a few vacant sites to adsorb
onto, late arriving chains form a diffuse outer layer. In a simple picture we
find for both chemisorption and physisorption a final loop distribution
Omega(s) \sim s^{-11/5} and density profile c(z) \sim z^{-4/3} whose forms are
the same as for equilibrium layers. In contrast to equilibrium layers, however,
the statistical properties of a given chain depend on its adsorption time; the
outer layer contains many classes of chain, each characterized by different
fraction of adsorbed monomers f. Consistent with strong physisorption
experiments, we find the f values follow a distribution P(f) \sim f^{-4/5}.Comment: 18 pages, submitted to Eur. Phys. J. E, expanded discussion sectio
Entropy-induced smectic phases in rod-coil copolymers
We present a self-consistent field theory (SCFT) of semiflexible (wormlike)
diblock copolymers, each consisting of a rigid and a flexible part. The
segments of the polymers are otherwise identical, in particular with regard to
their interactions, which are taken to be of an Onsager excluded-volume type.
The theory is developed in a general three-dimensional form, as well as in a
simpler one-dimensional version. Using the latter, we demonstrate that the
theory predicts the formation of a partial-bilayer smectic-A phase in this
system, as shown by profiles of the local density and orientational
distribution functions. The phase diagram of the system, which includes the
isotropic and nematic phases, is obtained in terms of the mean density and
rigid-rod fraction of each molecule. The nematic-smectic transition is found to
be second order. Since the smectic phase is induced solely by the difference in
the rigidities, the onset of smectic ordering is shown to be an entropic effect
and therefore does not have to rely on additional Flory-Huggins-type repulsive
interactions between unlike chain segments. These findings are compared with
other recent SCFT studies of similar copolymer models and with computer
simulations of several molecular models.Comment: 13 pages, 8 figure
Hierarchical simulations of hybrid polymer-solid materials
Complex polymer-solid materials have gained a lot of attention during the last 2-3 decades due to the fundamental physical problems and the broad spectrum of technological applications in which they are involved. Therefore, significant progress concerning the simulations of such hybrid soft-hard nanostructured systems has been made in the last few years. Simulation techniques vary from quantum to microscopic (atomistic) up to mesoscopic (coarse-grained) level. Here we give a short overview of simulation approaches on model polymer-solid interfacial systems for all different levels of description. In addition, we also present a brief outlook concerning the open questions in this field, from the point of view of both physical problems and computational methodologies
Understanding adhesion: a means for preventing fouling
Adhesion of particulate materials is an important step in the formation
of fouling. Because the size of such materials is generally less than 1μm,
the phenomenon can be described in terms of colloid chemistry. Accordingly,
the net force of interaction between foulants and the surface has
been described in terms of DLVO theory (van der Waals attraction and
electrostatic double-layer repulsion). However, those forces are sometimes
not sufficient to describe the formation of fouling. Recent works have made
it possible to calculate the effect of hydrophobic interactions and steric
forces, which can also be taken into account. In aqueous media, the various
types of interactions can be strongly affected by the pH, the ionic strength,
the type of ions, and the presence of polymeric molecules. The objective of
this work is to give a general overview of the basic physicochemical factors
playing a role in fouling and to outline some practical aspects related to the
theoretical reasoning to help prevent or at least mitigate fouling
Probing the interaction of nanoparticles with mucin for drug delivery applications using dynamic light scattering
Drug delivery via the eye, nose, gastrointestinal tract and lung is of great interest as they represent patient-compliant and facile methods to administer drugs. However, for a drug to reach the systemic circulation it must penetrate the “mucus barrier”. An understanding of the characteristics of the mucus barrier is therefore important in the design of mucus penetrating drug delivery vehicles e.g. nanoparticles. Here, a range of nanoparticles – silica, aluminium coated silica, poly (lactic-co-glycolic acid) (PLGA) and PEGylated PLGA – each with known but different physicochemical characteristics were examined in the presence of mucin to identify those characteristics that engender nanoparticle/mucin interactions and thus, to define “design rules” for mucus penetrating (nano)particles (MPP), at least in terms of the surface characteristics of charge and hydrophilicity. Dynamic light scattering (DLS) and rheology have been used to assess the interaction between such nanoparticles and mucin. It was found that negatively charged and hydrophilic nanoparticles do not exhibit an interaction with mucin whereas positively charged and hydrophobic nanoparticles show a strong interaction. Surface grafted poly (ethylene glycol) (PEG) chains significantly reduced this interaction. This study clearly demonstrates that the established colloid science techniques of DLS and rheology are very powerful screening tools to probe nanoparticle/mucin interactions
Self-Consistent Field study of Polyelectrolyte Brushes
We formulate a self-consistent field theory for polyelectrolyte brushes in
the presence of counterions. We numerically solve the self-consistent field
equations and study the monomer density profile, the distribution of
counterions, and the total charge distribution. We study the scaling relations
for the brush height and compare them to the prediction of other theories. We
find a weak dependence of the brush height on the grafting density.We fit the
counterion distribution outside the brush by the Gouy-Chapman solution for a
virtual charged wall. We calculate the amount of counterions outside the brush
and find that it saturates as the charge of the polyelectrolytes increases
- …