Effect of the polyethylene confinement and topology on its crystallisation within semicrystalline ABC triblock copolymers

The isothermal crystallisation behaviour of the polyethylene block within polystyrene-b-polyethylene-b-poly(ε-caprolactone), SEC, triblock copolymers was studied by differential scanning calorimetry. The morphology was observed by transmission electron microscopy. Melting scans after isothermal crystallisation performed at different times were employed to determine the crystallisation kinetics one step at a time (“isothermal step crystallisation”). Double melting endotherms were observed after isothermal crystallisation and they were interpreted as a result of the melting of two lamellar populations. These arise from the intrinsic short chain branching distribution within the hydrogenated polybutadiene chains that conform the PE blocks and from their location within the copolymer microdomains. The Hoffman-Weeks procedure failed to yield reasonable values for the equilibrium melting point of the PE blocks as a result of the distribution of linear sequences present in these blocks. The results indicate that as the degree of PE confinement increases the Avrami index decreases to values that are even lower than 1, a result that can be explained by the nature of the homogeneous nucleation process that is in between sporadic and instantaneous.     

Interconversion of the thiazine and thiazolidine system of β-lactam antibiotics,electrochemical cleavage of kamiya's disulphide promoted by bromide ion

A new transformation of Kamiya's disulphide ($\text{1}$) into the bromopenam derivative ($\text{4}$) has been carried out by electrolyzing the disulphide ($\text{1}$) in protic solvents in the presence of Me₄NBr. The transformation occurs through an intermediate episulphonium-bromide ion pair in which the bromide appears to tightly linked to the sulphur.     

Q-switched Ti:Er:LiNbO3 waveguide laser

Q-switched operation of a Ti:Er:LiNbO3 waveguide laser has been achieved with up to 2.4 W pulse peak power (0.18J) at 2kHz repetition frequency with an intracavity Mach–Zehnder-type amplitude modulator. A theoretical model has been developed to describe the laser operation. Good agreement between numerical and experimental results has been achieved. Significant potential for improvement is predicted.     

A survey of product form queueing networks with blocking and their equivalences

Queueing network models have been extensively used to represent and analyze resource sharing systems, such as production, communication and information systems. Queueing networks with blocking are used to represent systems with finite capacity resources and with resource constraints. Different blocking mechanisms have been defined and analyzed in the literature to represent distinct behaviors of real systems with limited resources. Exact product form solutions of queueing networks with blocking have been derived, under special constraints, for different blocking mechanisms. In this paper we present a survey of product form solutions of queueing networks with blocking and equivalence properties among different blocking network models. By using such equivalences we can extend product form solutions to queueing network models with different blocking mechanisms. The equivalence properties include relationships between open and closed product form queueing networks with different blocking mechanisms.This work has been partially supported by CNR Project Research Funds Progetto Finalizzato Sistemi Informatici e Calcolo Parallelo and by MURST Project Research Funds Performability hw/sw di sistemi distribuiti e paralleli.     

Thermal characterization of polycarbonate/polycaprolactone blends

In this work, we prepared blends of bisphenol A polycarbonate (PC) and poly(ϵ‐caprolactone) (PCL) in a wide composition range by melt mixing and solution mixing. Two different molecular weights of PCL were used (nominally, 10.000 g/mol, PCL10, and 80.000 g/mol, PCL80). The thermal behavior of both systems was studied via differential scanning calorimetry under dynamic and isothermal conditions. The blends were miscible in the entire composition range in the liquid and amorphous states, as indicated by the single glass‐transition temperature (T_g) exhibited by both the PC/PCL10 and PC/PCL80 blends. The compositional variation of the T_g was accurately described by the Fox equation for the PC/PCL80 blends, whereas slight deviations from this equation were exhibited by the PC/PCL10 blends. For blend compositions containing 40% or more PCL, either one or both blend components crystallized. Crystallization occurred during cooling from the melt or during subsequent heating in the form of cold crystallization. Although PCL crystallization was reduced and its crystallization rate decreased with the addition of PC, PCL was a very effective macromolecular plasticizer for PC, to the extent that crystallization during the scan was detected for some blend compositions. Isothermal crystallization experiments allowed the determination of equilibrium melting points (T) by the Hoffman–Weeks extrapolation method. A T depression was found for both PCL and PC components as the content of the other blend component was increased. The Avrami equation was closely obeyed by both blend components during the isothermal overall crystallization kinetics up to crystalline conversion degrees of 60–70% and with values of Avrami indices ranging from 3 to 4, depending on the crystallization temperature employed. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 771–785, 2001     

Conjugative and inductive effects in the ring opening of 1-aryl substituted oxiranes. Some reactions of 1-(p-methoxyphenyl)- and 1-(m-methoxyphenyl)-1,2-epoxycyclohexanes under acidic conditions

The reactions of 1-(p-methoxyphenyl)- and 1-(m-methoxyphenyl)-1,2-epoxycyclohexanes with trichloroacetic acid in benzene and with dilute sulphuric acid have been studied and compared with those of the parent compound, 1-phenyl-1,2-epoxycyclohexane. Both the steric course of the ring opening and the amounts of rearrangement products are strictly related to the effect of the substituent in the phenyl moiety, confirming that the tendency of aryloxiranes to undergo syn addition is linked with the amount of charge developed at the intermediate benzylic cationic centre.     

Reaction of Hg2+ Insertion into Cysteine Pairs Within Bovine Insulin Crystals Followed via Raman Spectroscopy

Chemical modifications of protein crystals may be achieved via soaking of reactants from their precipitating solution, through the solvent channel, into the protein matrix. We describe a Raman microscopy approach to follow mercury insertion into cysteine pairs within protein single crystals, via soaking in an aqueous Hg²⁺ solution. The method has been developed using bovine insulin as the model system. Applying an efficient mercuration protocol, consisting of a first step of disulphide bridge TCEP-induced reduction within the crystal, followed by overnight reaction with a HgCl₂ solution, we obtained Hg-derivative crystals. Raman spectra collected on these derivative crystals, kept in the mother liquor, reveal a characteristic Raman band at 335 cm^?1, which has been assigned to a –S–Hg–S– bridge. The analysis provides Raman-based markers of mercury binding to cysteines, and thus of mercury intoxication.     

Separable solutions for Markov processes in random environments

In this paper we address the problem of efficiently deriving the steady-state distribution for a continuous time Markov chain (CTMC) S evolving in a random environment E. The process underlying E is also a CTMC. S is called Markov modulated process. Markov modulated processes have been widely studied in literature since they are applicable when an environment influences the behaviour of a system. For instance, this is the case of a wireless link, whose quality may depend on the state of some random factors such as the intensity of the noise in the environment. In this paper we study the class of Markov modulated processes which exhibits separable, product-form stationary distribution. We show that several models that have been proposed in literature can be studied applying the Extended Reversed Compound Agent Theorem (ERCAT), and also new product-forms are derived. We also address the problem of the necessity of ERCAT for product-forms and show a meaningful example of product-form not derivable via ERCAT.     

Ellipsoidal core-shell cylindrical microphases in PS-b-PB-b-PCL triblock copolymers with a crystallizable matrix

Polystyrene-block-polybutadiene-block-poly(ϵ-caprolactone) SBC triblock copolymers with a PCL matrix exhibit microphase separation into three different phases within the spherulitic superstructures. Mixing of the PS-block can occur upon melting of the PCL-block if the molecular weight is low enough. Even though the crystallization takes place well below the PS-glass transition, a deformation of the amorphous microphases into ellipsoidal core-shell cylindrical microdomains is observed by TEM. These copolymers have mechanical properties which are dominated by the PCL (poly(ϵ-caprolactone)) matrix with an influence of the amorphous blocks.