910 research outputs found
Polymerizable ultraviolet stabilizers for outdoor use
Polymeric materials that are stable enough to use outdoors without changes in excess of 20 years are investigated. Ultraviolet stabilizers or plastic materials were synthesized, polymerizable ultraviolet stabilizers, particularly of the 2(2-hydroxyphenyl)2H-benzotriazole family were prepared their polymerization, copolymerization and grafting onto other polymers were demonstrated, and ultraviolet stabilizing systems were devised. These materials were evaluated from the photophysical point of view
Stabilized unsaturated polyesters
An unsaturated polyester, such as propylene glycolmaleic acid phthalic acid prepolymer dissolved in styrene is interpolymerized with an ultraviolet absorber and/or an antioxidant. The unsaturated chain may be filled with H or lower alkyl such as methyl and tertiary alkyl such as tertiary butyl. A polymer stable to exposure to the outdoors without degradation by ultraviolet radiation, thermal and/or photooxidation is formed
Magnetic interactions of substitutional Mn pairs in GaAs
We employ a kinetic-exchange tight-binding model to calculate the magnetic
interaction and anisotropy energies of a pair of substitutional Mn atoms in
GaAs as a function of their separation distance and direction. We find that the
most energetically stable configuration is usually one in which the spins are
ferromagnetically aligned along the vector connecting the Mn atoms. The
ferromagnetic configuration is characterized by a splitting of the topmost
unoccupied acceptor levels, which is visible in scanning tunneling microscope
studies when the pair is close to the surface and is strongly dependent on pair
orientation. The largest acceptor splittings occur when the Mn pair is oriented
along the symmetry direction, and the smallest when they are oriented
along . We show explicitly that the acceptor splitting is not simply
related to the effective exchange interaction between the Mn local moments. The
exchange interaction constant is instead more directly related to the width of
the distribution of all impurity levels -- occupied and unoccupied. When the Mn
pair is at the (110) GaAs surface, both acceptor splitting and effective
exchange interaction are very small except for the smallest possible Mn
separation.Comment: 25 figure
Magnetic properties of substitutional Mn in (110) GaAs surface and subsurface layers
Motivated by recent STM experiments, we present a theoretical study of the
electronic and magnetic properties of the Mn-induced acceptor level obtained by
substituting a single Ga atom in the (110) surface layer of GaAs or in one of
the atoms layers below the surface. We employ a kinetic-exchange tight-binding
model in which the relaxation of the (110) surface is taken into account. The
acceptor wave function is strongly anisotropic in space and its detailed
features depend on the depth of the sublayer in which the Mn atom is located.
The local-density-of-states (LDOS) on the (110) surface associated with the
acceptor level is more sensitive to the direction of the Mn magnetic moment
when the Mn atom is located further below the surface. We show that the total
magnetic anisotropy energy of the system is due almost entirely to the
dependence of the acceptor level energy on Mn spin orientation, and that this
quantity is strongly dependent on the depth of the Mn atom.Comment: 14 pages, 13 figure
Grafting of 2 (2-hydroxy-5-vinylphenyl) 2H-benzotriazole onto polymers with aliphatic groups. Synthesis and polymerization of 2 (2-hydroxy-5-isopropenylphenyl) 2H-benzotriazole and a new synthesis of 2 (2-hydroxy-5-vinylphenyl) 2H-benzotriazole
Successful grafting of 2(2-hydroxy-5-vinylphenyl)2H-benzotriazole onto saturated aliphatic C-H groups of polymers has been accomplished. When the grafting reaction was carried out in chlorobenzene at 150 C = 160 C with di-tertiarybutylperoxide as the grafting initiator, grafts as high as 20 percent - 30 percent at a grafting efficiency of 50 percent and 80 percent have readily been obtained. The grafting reaction was carried out in tubes sealed under high vacuum since trace amounts of oxygen cause complete inhibition of the grafting reaction by the phenolic monomer. On a variety of different polymers including atactic polypropylene, ethylene/vinyl acetate copolymer, poly(methyl methacrylate), poly(butyl acrylate), and polycarbonate were used
Reduktion von Aminosäuren zu Aminoalkoholen mittels Lithiumaluminiumhydrid
Durch Umsetzung mit Lithiumaluminiumhydrid in Tetrahydrofuran wurde eine Anzahl freier Aminosäuren zu den entsprechenden Aminoalkoholen reduziert. Der Anwendungsbereich der Methode wird an Hand von Beispielen erläutert.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41695/1/706_2004_Article_BF00904133.pd
Realistic Tight Binding Model for the Electronic Structure of II-VI Semiconductors
We analyze the electronic structure of group II-VI semiconductors obtained
within LMTO approach in order to arrive at a realistic and minimal tight
binding model, parameterized to provide an accurate description of both valence
and conduction bands. It is shown that a nearest-neighbor model is
fairly sufficient to describe to a large extent the electronic structure of
these systems over a wide energy range, obviating the use of any fictitious
orbital. The obtained hopping parameters obey the universal scaling law
proposed by Harrison, ensuring transferability to other systems. Furthermore,
we show that certain subtle features in the bonding of these compounds require
the inclusion of anion-anion interactions in addition to the nearest-neighbor
cation-anion interactions.Comment: 9 pages, 9 figure
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