Michael A. McKinney

The Thermal Degradation of Polyacrylonitrile

The volatile products that are evolved during the thermolysis of polyacrylonitrile have been studied by TGA/FTIR techniques. The complementary solid products that are not volatile have also been isolated and characterized, by means of elemental analysis and infra-red spectroscopy. Cyclization of the polymer proceeds before any mass is lost and the driving force for cyclization is the formation of aromatic rings. The extent of cyclization is controlled by the presence of head-to-head linkages within the polymer. Ammonia and hydrogen cyanide are the first gases lost and schemes are proposed to account for their formation. Oligomers are lost from the uncyclized portion of the polymer lying between the cyclized portions and the amount of non-volatile fraction is largely determined by the extent of oligomer loss. A detailed mechanism is presented to account for the observed formation of the volatile products and the structural changes that occur in the residue.

TGA/FTIR studies on the thermal degradation of some polymeric sulfonic and phosphonic acids and their sodium salts

The thermal degradation of poly(vinyl sulfonic acid) and its sodium salt, poly(4-styrenesulfonic acid) and its sodium salt, and poly(vinylphosphonic acid) was studied by a combination of techniques, including TGA/FTIR, to identify the volatile products which were evolved during the degradation as well as analysis of the residues which were obtained in order to propose a mechanism for the degradation. The motivation for the work was to attempt to identify new monomers which could be graft copolymerized onto a polymer in order to improve the thermal stability of that polymer.

Cross-linking of polystyrene by Friedel–Crafts chemistry to improve thermal stability

Abstract Copolymers which contain either alcohol or chloride functionalized polystyrene units have been prepared and they participate in Friedel–Crafts chemistry to give cross-linked polymers by the evolution of either hydrogen chloride or water. Proof of cross-linking comes from the identification of the evolved gas, the insolubility of the product, and the thermal resistance of the newly formed polymer. The onset temperature for the degradation is raised by about 100°C relative to that of polystyrene and the fraction which is not volatile at 800°C ranges from 10% for the alcohol copolymers to 20% for the chloride copolymers.

The isolation and characterization of glutamine-requiring strains of Escherichia coli K12

SummaryMutants of Escherichia coli K12 requiring glutamine as a nitrogen source were isolated, and characterized as lacking glutamine synthetase activity. Temperature sensitive revertants of one of the mutants had a heat labile glutamine synthetase, while temperature insensitive revertants had a glutamine synthetase which was thermostable in vitro, indicating that the mutation was in the structural gene for the enzyme. All of the mutations mapped in the same region of the chromosome suggesting that they might all be in the same gene. The glutamine synthetase gene (gln) was located on the E. coli chromosome by conjugation and P1-mediated transduction at minute 77. The gln gene cotransduced with the genes for oleate degradation (old), and the genes for L-rhamnose utilization (rha). The most probable gene order is old-gln-rha.

Stabilization of polystyrene by Friedel-Crafts chemistry: effect of position of alcohol and the catalyst

Polystyrene has been copolymerized with 4-vinylbenzyl alcohol, 4-(2-hydroxyethyl)styrene, and 4-(3-hydroxypropyl)styrene and it has been shown that thermal cross-linking of these copolymers is dependent upon the alcohol content. When the alcohol content is low, no thermal cross-linking is observed. When various phosphate esters are present as catalysts with these low alcohol content copolymers, cross-linking is observed at temperatures of about 250°C but not at lower temperatures. Cross-linking enhances the thermal stability of the copolymers. Studies of the thermal stability of the copolymers and their blends with the catalysts have been performed using thermogravimetric analysis and thermogravimetric analysis coupled to Fourier transform infrared spectroscopy. There is little difference in the thermal stability of all three copolymers and their blends with the catalysts.

Cyclopropyl halides. Electron transfer in the lithium aluminum hydride reduction of gemdibromo and monobromocyclopropanes.

Abstract The stereochemistry of reduction of mixtures of r-1-bromo-1-deuterio-c- and t-2-phenylcyclopropane and the cyclized products from 1,1-dibromo-2-(3-butenyl cyclopropane upon reduction with lithium aluminum hydride give evidence of a configurationally equilibrated cyclopropyl radical as a reaction intermediate.

Fracture toughness and load relaxation of dentin bonding resin systems

OBJECTIVES The fracture toughness (KIC) and load relaxation of four dentin bonding resins were determined to characterize some of the mechanical properties of these materials after polymerization. METHODS A total of 40 single-edge notch bar specimens were fabricated, 10 each of four commercially available brands, and subjected to three-point bending until fracture, as described in ASTM Standard E399-83 (1991a). The critical stress intensification factor, KIC, was derived for each specimen and compared by analysis of variance and Scheffés multiple comparisons test (p < 0.01). To study the load relaxation characteristics, five rectangular specimens (without notches) of each brand were subjected to three-point loading until a predetermined limiting load value was reached. The test load was allowed to relax for 4 min, after which the specimen was unloaded to the zero load condition, and the load was allowed to build up on its own accord for 3 min. Load relaxation values were measured from the chart, and the mean percent load drop was calculated. The load relaxation data were compared using analysis of variance and Scheffés multiple comparisons test (p < 0.05). RESULTS The fracture toughness (KIC) values of the four adhesive resins studied in this investigation ranged from 0.37-0.94 MPa.m0.5 and were statistically different from each other (p < 0.001). The load relaxation values were found to be greatest within the first 0.5 min, with the total load relaxation of the four bonding agents ranging from 16%-30%. Two of the materials studied showed significantly different short-term load relaxation behavior than the other two resins (p < 0.05). SIGNIFICANCE Bonding agents can be implicated as one of the factors that weaken the interface between the dentin and the composite restorative material. These materials are capable of a rapid short-term response, demonstrating significant load relaxation in the first 0.5 min after loading.

Cross-linking of polystyrene by Friedel-Crafts chemistry: reaction of p-hydroxymethylbenzyl chloride with polystyrene

p-Hydroxymethylbenzyl chloride was found to be an effective cross-linking agent for polystyrene. The reaction was found to occur by Friedel-Crafts alkylation between the benzyl alcohol/chloride functional groups in the additive and phenyl ring in polystyrene. The reaction was studied by TGA-IR to monitor the evolution of hydrogen chloride and water, and the structure of the resultant gel was analyzed by solid state NMR and elemental analysis. The potential application in flame-retardancy was evaluated using Cone calorimetry.

Secondary-site attachment of coliphage lambda near the thr operon

Abstract Phage λ has been integrated at a secondary attachment site near the thr oporon of Escherichia coli . The integration caused a pleiotropic effect since both thrA and thrB enzyme activities were lost. Lysogenic, thr + revertants regained thrA and thrB activities although the enzymes were synthetized constitutively at low levels. Four classes of prophage deletion strains were isolated with deletions extending into trpR and the thr operon. Genetic and biochemical analyses indicated a gene order: trpR .. λ .. thrA .. B .. C . Defective transducing phage carrying thr A, B,C were also isolated.

Nucleophilic cleavage of spiroactivated cyclopropanes.1. Kinetic and thermodynamic parameters for reaction of 6,6-dimethyl-2-phenyl-5,7-dioxaspiro[2,5]octane-4,8-dione (2) with pyridine.

Abstract Reaction of the title spiroactivated cyclopropane(2) with pyridine in acetonitrite yields a zwitterionic addition product. The reaction is reversible which provides conditions under which rates and equilibria constants can be obtained along with the derived activation and thermodynamic parameters of reaction. An extended Bronsted treatment affords a β nuc value for the reaction of 2 with substituted pyridines.