Thermal stability of “living” polymer–lithium systems

The thermal stabilities of polybutadienyllithium, polyisoprenyllithium, and polystyryllithium solutions have been determined in hydrocarbon solvents. Kinetic analysis indicated that a complex mechanism was involved in the thermolysis of polybutadienyllithium. The thermal stability was observed to increase with increasing lithium concentration, suggesting the presence of competitive reactions in addition to the expected elimination of lithium hydride. The thermal stability of the three systems studied was consistent with their reported degrees of association: dimeric polystyryllithium was less stable than tetrameric polyisoprenyllithium or hexameric polybutadienyllithium.     

Theory of “Cohesive” vs “Adhesive” Separation in an Adhering System

It is shown that there is limited validitity to the doctrine that true interfacial separation, in an adhering system, is highly improbable. An analysis employing the Griffith-Irwin crack theory yields these results: The important parameters are, difference in elastic moduli, ΔE; differences in g, the energy dissipation per unit crack extension; thickness, Δ₁ or δ₂, of the region where dissipation occurs; and the presence or absence of strong interfacial bonds. If the forces across the interface are appreciably weaker than the cohesive forces in either phase, there is a strong minimum in g at the interface. For flaws of equal size, an interfacial flaw will be the site of initiation of failure. If strong interfacial bonds are present, then if Δg and ΔE have the same sign, failure is most probable, deep within one phase. If Δg and ΔE have opposite signs, failure may be initiated, and may propagate, at a distance δ from the interface, in the phase with lower g. This may be mistaken for weak-boundary layer failure.     

A note on the “ Danckwerts ” boundary conditions for continuous flow reactors

A mathematical justification is given for the use of the “ Danckwerts ” boundary conditions for continuous flow reactors. It is shown that the apparent indeterminacy, which D resolves intuitively, is caused by the use of a discontinuous coefficient of diffusion. By treating this as the limit of a continuous function and imposing continuity of the reactant concentration as the physically relevant boundary condition, the Danckwerts solution is obtained in the limit.     

“HETEROGENIZING” HOMOGENEOUS CATALYSTS

For many years, it has been customary to classify catalysts as “homogeneous” or “heterogeneous.” The former commonly operate through the formation of “intermediate compounds,” and the latter, by adsorption of the reactants on the catalyst surface. The line between the two is a fine one, for the distinction between adsorption and compound formation is not at all clear, and seems to be becoming less and less clear as we learn more about adsorption. In recent years, several writers [l-7] have stressed the point that there is a good deal of overlap between homogeneous and heterogeneous catalysis. Experimental evidence supporting this point of view is accumulating, and while we are not prepared to say that there is no distinction, we can say with certainty that many homogeneous catalysts can be converted into heterogeneous ones, retaining the advantages of great activity and selectivity inherent in homogeneity and, at the same time, assuming the ready recovery which is the great advantage of heterogenity. In practice, of course, the matter is not quite that simple, for other factors must be considered. On the whole, however, many advantages have been found in the use of heterogenized homogeneous catalysts.     

Toward “Rubbery” nanoparticles

The synthesis of electrically Conducting Natural Rubber (CNR) nanoparticles from natural rubber (cis 1, 4 polyisoprene) by a simple chemical doping technique is reported for the first time. Much before the establishment of conjugation as a precondition for polymers to be conducting a typical nonconjugated polymer like cis 1,4 polyisoprene was shown to develop intrinsic conductivity on doping. However, the possibility of developing conducting nanoparticles of natural rubber by doping has never been explored. Doping of natural rubber solution with Antimony pentchloride is found to lead to the formation of nanosized rubber particles with improved thermal stability and lower degradation characteristics than that of pristine rubber. Transmission electron microscopy and Dynamic Light Scattering experiments revealed a highly uniform dispersion of the particles with sizes in the range of 4 nm. The doped nanoparticles are found to retain “rubbery” properties of natural rubber and therefore these can be rightly termed as Rubber Nano particles. The development of nanoparticles of rubber assumes great significance in that it would lead to hitherto unknown applications for natural rubber in micro applications‐like sensors, and optoelectronics devices to macro applications such as compatible reinforcing fillers for elastomers and plastics to replace conventional fillers like carbon particles. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Properties and structure of PVP–lignin “blend films”

Blend films of poly (4‐vinylpyridine) and lignin were prepared by the casting method. Their structure and properties were studied by Fourier transform infrared (FTIR), wide‐angle X‐ray diffraction (WXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TG), and differential scanning calorimetry (DSC). The IR spectra of the blend films indicated that hydrogen‐bonding interaction occurred between poly (4‐vinylpyridine) and lignin. The glass transition temperature of these blends increased with the increase of lignin content, which indicated that these blends were able to form a miscible phase due to the formation of intermolecular hydrogen bonding between the hydroxyl of lignin and the pyridine ring of poly (4‐vinylpyridine). The thermostability of these blends decreased with the increase of lignin content. Initially, an appreciable increase in the measured tensile strength was achieved with a lignin content of 15%, at which the maximum value of 33.03 MPa tensile strength was reached. At a 10% lignin incorporation level, the blend film exhibited a maximum value of 9.03% strain. When the threshold in lignin content for blend films exceeded that limit of 10% lignin, the strain behavior of these blend films deteriorated. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1405–1411, 2005     

On the so-called “semi-ionic” C–F bond character in fluorine–GIC

The short-range structures of fluorine–graphite intercalation compounds with stage-1 structures, C_xF (x = 2.47, 2.84, 3.61), were analyzed by means of neutron diffraction. It has been shown that the so-called “semi-ionic” C–F bond character in C_xF is essentially covalent with the bond length of 0.140 nm, and the original planar graphene sheets are buckled at the sp³-hybridized carbon atoms bound to fluorine atoms. Conjugated C–C double bonds with the bond length of 0.142 nm are preserved between the carbon atoms unbound to fluorine atoms in C_xF, while other C–C bonds are single bonds with the bond lengths of 0.153 nm. The C–F bond order in C_xF is slightly lower than those in poly(carbon monofluoride) ((CF)_n) and poly(dicarbon monofluoride) ((C₂F)_n), which is explained by the hyperconjugation involving the C–C bonds on the carbon sheets and C–F bond.