581,452 research outputs found
Semiflexible polymers under external fields confined to two dimensions
The non-equilibrium structural and dynamical properties of semiflexible
polymers confined to two dimensions are investigated by molecular dynamics
simulations. Three different scenarios are considered: The force-extension
relation of tethered polymers, the relaxation of an initially stretched
semiflexible polymer, and semiflexible polymers under shear flow. We find
quantitative agreement with theoretical predictions for the force-extension
relation and the time dependence of the entropically contracting polymer. The
semiflexible polymers under shear flow exhibit significant conformational
changes at large shear rates, where less stiff polymers are extended by the
flow, whereas rather stiff polymers are contracted. In addition, the polymers
are aligned by the flow, thereby the two-dimensional semiflexible polymers
behave similarly to flexible polymers in three dimensions. The tumbling times
display a power-law dependence at high shear rate rates with an exponent
comparable to the one of flexible polymers in three-dimensional systems.Comment: Accepted for publication in J. Chem. Phy
Branched polymers on branched polymers
We study an ensemble of branched polymers which are embedded on other
branched polymers. This is a toy model which allows us to study explicitly the
reaction of a statistical system on an underlying geometrical structure, a
problem of interest in the study of the interaction of matter and quantized
gravity. We find a phase transition at which the embedded polymers begin to
cover the basis polymers. At the phase transition point the susceptibility
exponent takes the value 3/4 and the two-point function develops an
anomalous dimension 1/2.Comment: uuencoded 9 p. ps-file + 2 ps-figure
Ladder polymers for use as high temperature stable resins or coatings
An object of the invention is to synthesize a new class of ladder and partial ladder polymers. In accordance with the invention, the new class of ladder and partial ladder polymers are synthesized by polymerizing a bis-dienophile with a bis-diene. Another object of the invention is to provide a fabricated, electrically conducting, void free composite comprising the new class of the ladder and partial ladder polymers described above. The novelty of the invention relates to a new class of ladder and partial ladder polymers and a process for synthesizing these polymers. These polymers are soluble in common organic solvents and are characterized with a unique dehydration property at temperatures of 300 to 400 C to provide thermo-oxidatively stable pentiptycene units along the polymeric backbone. These polymers are further characterized with high softening points and good thermo-oxidative stability properties. Thus these polymers have potential as processable, matrix resins for high temperature composite applications
Synthesis and characterization of novel low band gap semiconducting polymers for organic photovoltaic and organic field effect transistor applications
PhDThis thesis describes the synthesis, characterization and device properties of a range of conjugated polymers incorporating 3,6-dilakylthieno[3,2-b]thiophene. We report a new and facile synthesis for the preparation of 3,6-dialkylthieno[3,2-b]thiophene, which is readily scaled up to the multi-gram scale. With this synthesis in hand, we initially investigated the properties of poly(thienothiophene-alt-vinylene) polymers incorporating both straight and branched side-chains. Two different polymerization methods were investigated to synthesise the conjugated polymers, namely Stille and Gilch polymerization. The Gilch route was found to lead to high molecular-weight polymers with less cis-defects in the backbone. The polymers were found to be largely amorphous by X-ray diffraction measurements, although there were clear signs of aggregation by optical investigations. Field-effect transistors fabricated with these polymers exhibited charge carrier mobilities up to 0.02 cm2 V-1 s-1 for the straight chain analogue, with the branched polymer displaying lower mobilities. Blends with PC71BM were found to exhibit solar cell device efficiencies up to 2.5 %, with significant differences observed for polymers containing two different side-chains.
In the third chapter we investigated the properties of ethynylene-linked 3,6-dialkylthieno[3,2-b]thiophene polymers. The simple homo-polymers were found to exhibit much worse device performance than the analogous vinylene-containing polymers in transistor devices. Co-polymers with a range of electron accepting monomers were also synthesized. These displayed low optical energy gaps and signs of aggregation in the solid state. Transistors were fabricated and their performance examined.
In the final part of this thesis, co-polymers bearing 3,6-dialkylthieno[3,2-b]thiophene donor and squaraine acceptor units were synthesized. These zwitterionic conjugated polymers displayed band gaps as low as 1.0 eV. The influence of the nature of the side-chains and co-monomer was investigated with regard to their optoelectronic properties
Carboranylmethylene-substituted phosphazenes and polymers thereof
Carboranylmethylene-substituted cyclophosphazenes are described which can be thermally polymerized into carboranylmethylene-substituted phosphazene polymers. The polymers are useful as thermally stable coatings. Also, due to the characteristics of these polymers in acting as a ligand for transition metals, metalocarboranylmethylene phosphazene polymers are described which can act as immobilized catalyst systems, and are electrically conductive and superconductive
Coarse-graining polymers as soft colloids
We show how to coarse grain polymers in a good solvent as single particles,
interacting with density-independent or density-dependent interactions. These
interactions can be between the centres of mass, the mid-points or end-points
of the polymers. We also show how to extend these methods to polymers in poor
solvents and mixtures of polymers. Treating polymers as soft colloids can
greatly speed up the simulation of complex many-polymer systems, including
polymer-colloid mixtures.Comment: to appear in Physica A, special STATPHYS 2001 edition. Content of
invited talk by AA
Synthesis of oligo- and polythiophenes in zeolite hosts
Oligomers and polymers of thiophene derivatives were synthesized in the channels of
zeolite Y and mordenite. Intrazeolite oxidation of monomers such as thiophene , 3-
methylthiophene , and bithiophene by Fe(lll) or Cu(ll) ions results in formation of insoluble
polymers that have spectroscopic properties similar to the corresponding bulk polymers. The
zeolites containing the polymers are nonconducting, but when extracted from the host, the
polymers show d.c. conductivities typical for t he bulk materials. Oligothiophene species with welldefined
electronic transitions could be produced in acidic zeolite Y
Semiflexible polymers in a random environment
We present using simple scaling arguments and one step replica symmetry
breaking a theory for the localization of semiflexible polymers in a quenched
random environment. In contrast to completely flexible polymers, localization
of semiflexible polymers depends not only on the details of the disorder but
also on the ease with which polymers can bend. The interplay of these two
effects can lead to the delocalization of a localized polymer with an increase
in either the disorder density or the stiffness. Our theory provides a general
criterion for the delocalization of polymers with varying degrees of
flexibility and allows us to propose a phase diagram for the highly folded
(localized) states of semiflexible polymers as a function of the disorder
strength and chain rigidity.Comment: 10 pages, 3 figures, Revtex
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