10,959 research outputs found
A finite excluded volume bond-fluctuation model: Static properties of dense polymer melts revisited
The classical bond-fluctuation model (BFM) is an efficient lattice Monte
Carlo algorithm for coarse-grained polymer chains where each monomer occupies
exclusively a certain number of lattice sites. In this paper we propose a
generalization of the BFM where we relax this constraint and allow the overlap
of monomers subject to a finite energy penalty \overlap. This is done to vary
systematically the dimensionless compressibility of the solution in order
to investigate the influence of density fluctuations in dense polymer melts on
various s tatic properties at constant overall monomer density. The
compressibility is obtained directly from the low-wavevector limit of the
static structure fa ctor. We consider, e.g., the intrachain bond-bond
correlation function, , of two bonds separated by monomers along the
chain. It is shown that the excluded volume interactions are never fully
screened for very long chains. If distances smaller than the thermal blob size
are probed () the chains are swollen acc ording to the classical
Fixman expansion where, e.g., . More importantly, the
polymers behave on larger distances () like swollen chains of
incompressible blobs with P(s) \si m g^0s^{-3/2}.Comment: 46 pages, 12 figure
Non-extensivity of the chemical potential of polymer melts
Following Flory's ideality hypothesis the chemical potential of a test chain
of length immersed into a dense solution of chemically identical polymers
of length distribution P(N) is extensive in . We argue that an additional
contribution arises ( being the
monomer density) for all if which can be traced back to the
overall incompressibility of the solution leading to a long-range repulsion
between monomers. Focusing on Flory distributed melts we obtain for , hence,
if is similar to the typical
length of the bath . Similar results are obtained for monodisperse
solutions. Our perturbation calculations are checked numerically by analyzing
the annealed length distribution P(N) of linear equilibrium polymers generated
by Monte Carlo simulation of the bond-fluctuation model. As predicted we find,
e.g., the non-exponentiality parameter to decay
as for all moments of the distribution.Comment: 14 pages, 6 figures, submitted to EPJ
On the Dynamics and Disentanglement in Thin and Two-Dimensional Polymer Films
We present results from molecular dynamics simulations of strictly
two-dimensional (2D) polymer melts and thin polymer films in a slit geometry of
thickness of the order of the radius of gyration. We find that the dynamics of
the 2D melt is qualitatively different from that of the films. The 2D monomer
mean-square displacement shows a power law at intermediate times
instead of the law expected from Rouse theory for nonentangled
chains. In films of finite thickness, chain entanglements may occur. The impact
of confinement on the entanglement length has been analyzed by a
primitive path analysis. The analysis reveals that increases
strongly with decreasing film thickness.Comment: 6 pages, 3 figures, proceedings 3rd International Workshop on
Dynamics in Confinement (CONFIT 2006
Single chain structure in thin polymer films: Corrections to Flory's and Silberberg's hypotheses
Conformational properties of polymer melts confined between two hard
structureless walls are investigated by Monte Carlo simulation of the
bond-fluctuation model. Parallel and perpendicular components of chain
extension, bond-bond correlation function and structure factor are computed and
compared with recent theoretical approaches attempting to go beyond Flory's and
Silberberg's hypotheses. We demonstrate that for ultrathin films where the
thickness, , is smaller than the excluded volume screening length (blob
size), , the chain size parallel to the walls diverges logarithmically,
with . The corresponding bond-bond
correlation function decreases like a power law, with
being the curvilinear distance between bonds and . % Upon increasing
the film thickness, , we find -- in contrast to Flory's hypothesis -- the
bulk exponent and, more importantly, an {\em decreasing}
that gives direct evidence for an {\em enhanced} self-interaction of chain
segments reflected at the walls. Systematic deviations from the Kratky plateau
as a function of are found for the single chain form factor parallel to the
walls in agreement with the {\em non-monotonous} behaviour predicted by theory.
This structure in the Kratky plateau might give rise to an erroneous estimation
of the chain extension from scattering experiments. For large the
deviations are linear with the wave vector, , but are very weak. In
contrast, for ultrathin films, , very strong corrections are found
(albeit logarithmic in ) suggesting a possible experimental verification of
our results.Comment: 16 pages, 7 figures. Dedicated to L. Sch\"afer on the occasion of his
60th birthda
Are polymer melts "ideal"?
It is commonly accepted that in concentrated solutions or melts
high-molecular weight polymers display random-walk conformational properties
without long-range correlations between subsequent bonds. This absence of
memory means, for instance, that the bond-bond correlation function, , of
two bonds separated by monomers along the chain should exponentially decay
with . Presenting numerical results and theoretical arguments for both
monodisperse chains and self-assembled (essentially Flory size-distributed)
equilibrium polymers we demonstrate that some long-range correlations remain
due to self-interactions of the chains caused by the chain connectivity and the
incompressibility of the melt. Suggesting a profound analogy with the
well-known long-range velocity correlations in liquids we find, for instance,
to decay algebraically as . Our study suggests a precise
method for obtaining the statistical segment length \bstar in a computer
experiment.Comment: 4 pages, 3 figure
The Halogen Bond in the Design of Functional Supramolecular Materials: Recent Advances
Halogen bonding is an emerging noncovalent interaction for constructing supramolecular assemblies. Though similar to the more familiar hydrogen bonding, four primary differences between these two interactions make halogen bonding a unique tool for molecular recognition and the design of functional materials. First, halogen bonds tend to be much more directional than (single) hydrogen bonds. Second, the interaction strength scales with the polarizability of the bond-donor atom, a feature that researchers can tune through single-atom mutation. In addition, halogen bonds are hydrophobic whereas hydrogen bonds are hydrophilic. Lastly, the size of the bond-donor atom (halogen) is significantly larger than hydrogen. As a result, halogen bonding provides supramolecular chemists with design tools that cannot be easily met with other types of noncovalent interactions and opens up unprecedented possibilities in the design of smart functional materials.
This Account highlights the recent advances in the design of halogen-bond-based functional materials. Each of the unique features of halogen bonding, directionality, tunable interaction strength, hydrophobicity, and large donor atom size, makes a difference. Taking advantage of the hydrophobicity, researchers have designed small-size ion transporters. The large halogen atom size provided a platform for constructing all-organic light-emitting crystals that efficiently generate triplet electrons and have a high phosphorescence quantum yield. The tunable interaction strengths provide tools for understanding light-induced macroscopic motions in photoresponsive azobenzene-containing polymers, and the directionality renders halogen bonding useful in the design on functional supramolecular liquid crystals and gel-phase materials. Although halogen bond based functional materials design is still in its infancy, we foresee a bright future for this field. We expect that materials designed based on halogen bonding could lead to applications in biomimetics, optics/photonics, functional surfaces, and photoswitchable supramolecules
Surface segregation of conformationally asymmetric polymer blends
We have generalized the Edwards' method of collective description of dense
polymer systems in terms of effective potentials to polymer blends in the
presence of a surface. With this method we have studied conformationally
asymmetric athermic polymer blends in the presence of a hard wall to the first
order in effective potentials. For polymers with the same gyration radius
but different statistical segment lengths and the excess
concentration of stiffer polymers at the surface is derived as % \delta \rho
_{A}(z=0)\sim (l_{B}^{-2}-l_{A}^{-2}){\ln (}R_{g}^{2}/l_{c}^{2}{)%}, where
is a local length below of which the incompressibility of the polymer
blend is violated. For polymer blends differing only in degrees of
polymerization the shorter polymer enriches the wall.Comment: 11 pages, 7 figures, revtex
Supramolecular hierarchy among halogen and hydrogen bond donors in light-induced surface patterning
Halogen bonding, a noncovalent interaction possessing several unique features compared to the more familiar hydrogen bonding, is emerging as a powerful tool in functional materials design. Herein, we unambiguously show that one of these characteristic features, namely high directionality, renders halogen bonding the interaction of choice when developing azobenzene-containing supramolecular polymers for light-induced surface patterning. The study is conducted by using an extensive library of azobenzene molecules that differ only in terms of the bond-donor unit. We introduce a new tetrafluorophenol-containing azobenzene photoswitch capable of forming strong hydrogen bonds, and show that an iodoethynyl-containing azobenzene comes out on top of the supramolecular hierarchy to provide unprecedented photoinduced surface patterning efficiency. Specifically, the iodoethynyl motif seems highly promising in future development of polymeric optical and photoactive materials driven by halogen bonding
Hierarchical Self-Assembly of Halogen-Bonded Block Copolymer Complexes into Upright Cylindrical Domains
Self-assembly of block copolymers into well-defined, ordered arrangements of chemically distinct domains is a reliable strategy for preparing tailored nanostructures. Microphase separation results from the system, minimizing repulsive interactions between dissimilar blocks and maximizing attractive interactions between similar blocks. Supramolecular methods have also achieved this separation by introducing small-molecule additives binding specifically to one block by noncovalent interactions. Here, we use halogen bonding as a supramolecular tool that directs the hierarchical self-assembly of low-molecular-weight perfluorinated molecules and diblock copolymers. Microphase separation results in a lamellar-within-cylindrical arrangement and promotes upright cylindrical alignment in films upon rapid casting and without further annealing. Such cylindrical domains with internal lamellar self-assemblies can be cleaved by solvent treatment of bulk films, resulting in separated and segmented cylindrical micelles stabilized by halogen-bond-based supramolecular crosslinks. These features, alongside the reversible nature of halogen bonding, provide a robust modular approach for nanofabricatio
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