10,134 research outputs found
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
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
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
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
Understanding expertise in surgical gesture by means of Hidden Markov Models
Minimally invasive surgery (MIS) has became very widespread in the last ten years. Due to the difficulties encountered by the surgeons to learn and manage this technique, a huge importance has the improvement of training procedures, the improvement of surgical instrumentation and the robotic automation of surgical gesture. All these purposes require the analysis of surgical performance with the aim to understand it and to define what is expertise in surgical gesture. In this paper for the first time the Hidden Markov Models (HMMs) are used as a tool for the understanding of surgical performance and of the human factors that characterize it. In our experiments we used position data concerning the tools movements during exercises performed on a surgical simulator. Using Hidden Markov theory, we create a model of the expert surgeon performance able to evaluate surgical capability and to distinguish between expert and non-expert surgeons. By analyzing the trained model of the expert surgeon performance we show that it is possible to deduce information about features characterizing the surgical expertise
Using wearable sensor systems for objective assessment of parkinson's disease
This paper presents a novel wearable sensor system based on the integration of miniaturised IMUs for fine hand movement analysis. The system, named SensHand V1, is composed of full 9-axis inertial sensors, placed on the fingers and wrist, which are managed by a cortex-M3 microcontroller. The acquired data are sent to a data logger through the use of Bluetooth communication. In this paper, the system is used for the objective diagnosis of Parkinson's disease, which is commonly assessed by neurologists through visual examination of motor tasks and semi-quantitative rating scales. Here, these motor tasks are also assessed using the SensHand V1, and then compared with the subjective metrics. Results demonstrate that the system is adequate to support neurologists in diagnostic procedures and allows for an objective evaluation of the disease
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