Dendritic homopolymers and block copolymers: Tuning the morphology and properties

The synthesis and characterization of dendritic homopolymers and block copolymers of ?‐caprolactone and lactide (L ‐lactide and racemic lactide) were performed with multifunctional initiators in combination with living polymerization and the selective placement of branching junctures in a divergent growth strategy. A hexahydroxy‐functional 2,2‐bis(hydroxymethyl) propionic acid derivative was used as an initiator for the stannous‐2‐ethylhexanoate‐catalyzed living ring‐opening polymerization of ?‐caprolactone, L ‐lactide, and racemic L ,D ‐lactide. Branching junctions at the chain ends were introduced with benzylidene‐protected 2,2‐bis(hydroxymethyl) propionic acid. Subsequent generations were then polymerized, after deprotection, from these star‐shaped macroinitiators. Successive chain end capping and initiation produced three generations of polymers with molecular weights in excess of 130,000 g/mol and narrow polydispersities (<1.20). It was possible to prepare diblock and triblock copolymers with phase‐separated morphologies, and with L ‐lactide or D ,L ‐lactide, semicrystalline and amorphous morphologies were demonstrated. The polymers were characterized by ¹H NMR, ¹³C NMR, size exclusion chromatography, and differential scanning calorimetry. The compositions of the block copolymers and the conformational structures of the optically active polymers were also confirmed by optical rotation measurements. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1174–1188, 2004     

Evidence of ultra-fast dissociation in ammonia observed by resonant Auger electron spectroscopy

We present evidence for ultra-fast dissociation of molecular ammonia when photo-excited to the N1s→4a₁ core-hole state. This finding is based on resonant Auger spectroscopical results as well as qualitative arguments concerning the photon energy dependence of the Auger structures. Calculations of the excited state potential based on the Z+1 approximation were performed. Both the calculations and the measurements indicate that the most likely fragmentation pathway for the core excited ammonia molecules leads to NH₂^* and H fragments.     

The rheological properties of AgI and Li2SO4 solid electrolytes,II.

Summary The plastic deformation of the superionic conductors AgI, Li₂SO₄, and Li₂SO₄–K₂SO₄ mixtures rich in Li₂SO₄ has been investigated using stress relaxation. The observed activation volume is inversely proportional to the effective initial stress, but the proportionality constant is smaller than in normal non-superionic substances. This fact can be attributed to the weak steric hindrance for rotation of the lattice units, making the rotatorphases into an additional exception to the general empirical relation found by Kubát and coworkers.The existence of an order-disorder transition in AgI close to 700 K is confirmed by the stress relaxation data and also by measurements of maximum shear stress versus maximum shearing rate. It is found that such data may be conveniently represented by doubly logarithmic plots, in which a change in flow properties can be easily spotted.With 13 figures and 7 tables     

Low Molecular Weight Norbornadiene Derivatives for Molecular Solar‐Thermal Energy Storage

Molecular solar‐thermal energy storage systems are based on molecular switches that reversibly convert solar energy into chemical energy. Herein, we report the synthesis, characterization, and computational evaluation of a series of low molecular weight (193–260 g mol^?1) norbornadiene–quadricyclane systems. The molecules feature cyano acceptor and ethynyl‐substituted aromatic donor groups, leading to a good match with solar irradiation, quantitative photo‐thermal conversion between the norbornadiene and quadricyclane, as well as high energy storage densities (396–629 kJ kg^?1). The spectroscopic properties and energy storage capability have been further evaluated through density functional theory calculations, which indicate that the ethynyl moiety plays a critical role in obtaining the high oscillator strengths seen for these molecules.     

Quasi-Stationary Asymptotics for Perturbed Semi-Markov Processes in Discrete Time

We consider a discrete time semi-Markov process where the characteristics defining the process depend on a small perturbation parameter. It is assumed that the state space consists of one finite communicating class of states and, in addition, one absorbing state. Our main object of interest is the asymptotic behavior of the joint probabilities of the position of the semi-Markov process and the event of non-absorption as time tends to infinity and the perturbation parameter tends to zero. The main result gives exponential expansions of these probabilities together with a recursive algorithm for computing the coefficients in the expansions. An application to perturbed epidemic SIS models is discussed.     

Inventory management of spare parts in an energy company

We address the problem of how to determine control parameters for the inventory of spare parts of an energy company. The prevailing policy is based on an (s, S) system subject to a fill rate constraint. The parameters are decided based mainly on the expert judgment of the planners at different plants. The company is pursuing to conform all planners to the same approach, and to be more cost efficient. Our work focuses on supporting these goals. We test seven demand models using real-world data for about 21?000 items. We find that significant differences in cost and service level may appear from using one or another model. We propose a decision rule to select an appropriate model. Our approach allows us to recommend control parameters for 97.9% of the items. We also explore the impact of pooling inventory for different demand sources and the inaccuracy arising from duplicate item codes.     

Operations Research challenges in forestry: 33 open problems

Annals of Operations Research - Forestry has contributed many problems to the Operations Research (OR) community. At the same time, OR has developed many models and solution methods for use in...