Kenkichi Murakami

Solvent effects on the coupling processes of polystyryl radicals

The molecular weight distributions of polystyrene prepared in various solvents at 60 °C showed broadening with increasing solvent viscosity, whereas the effects of solvent power were not conspicuous. Simulations of polymerization reactions with several models showed that the coupling probability of polystyryl radicals is not independent of chain length at 60 °C and that the chain length dependence becomes apparent with increasing solvent viscosity. These results were discussed in terms of the dynamical behavior of polymer radicals in solution and it was tentatively concluded that the chain length dependence of coupling probability of polystyryl radicals originates mainly from the hydrodynamic interactions between polymer segments.

Chain length dependence of termination rate constant: computer experiment of effects on the kinetics of radical polymerization of styrene

Abstract The kinetics of the radical polymerization of styrene, initiated by AIBN, have been studied by computer experiments. The models adopted assume a termination rate constant for polystyryl radicals with degrees of polymerization m and n , given by: k t,mn =k α m -α n -α Deviations from simple kinetic rules have been found for α ≉ 0.0 . For example, the rate of polymerization is roughly proportional to the 0.45 power of initiator concentration for α = 0.1. Conventional methods for the kinetic study of radical polymerizations are shown to come to erroneous conclusions if the effects of the chain length dependence of termination rate constant are not taken into account.

Studies of network structure of rubber vulcanizates by a cryoscopic method: 4

Abstract The freezing-point depression, ΔT, of swollen dicumene peroxide cured natural rubber in benzene has been measured. The parameter aH, introduced by Kuhn, which varies inversely as ΔT, should be regarded as an indication of the depression. The anomalous depression is considered to occur due to a non-frozen benzene area formed on the surrounding network chains. From comparison of the scaling law and the experimental results, it was clear that aH can reflect not only the quantity corresponding to network chain density determined by conventional methods but also the difference in three-dimensional network structure between degraded and undegraded rubber vulcanizates.

Chemorheology of irradiation-cured natural rubbers: 1. Stress relaxation mechanisms for various curing systems in natural rubber at high temperature

Abstract Stress relaxation mechanisms were investigated for various curing systems in natural rubber, especially the ‘complex’ crosslinking irradiation cure systems containing sulphur or tetramethylthiuram disulphide (TMTD) in both air and nitrogen mainly at 100°C. With the systems, the chemical stress relaxation in air showed that in the main oxidative scission was the major cause of stress decay. However, the number of moles of main chain scission, qm(t) seemed to be dependent on the chemical structure of the crosslink. In the TMTD curing system and irradiation-TMTD curing systems, qm(t) is independent of the ratio, ρ, of the effective chain density Nc(0) based on the carbon-carbon crosslinkages to Nm(0) based on mono- and di-sulphide linkages. Stress decay in air of both accelerated-sulphur cures and irradiation-sulphur cures was shown to be due to both the interchange reaction of polysulphide linkages and random scission of the main chain. On the other hand, in nitrogen there are two simultaneous reactions at the crosslink:interchange and thermal scission.

Interaction and structure of PVA-Cu(II) complex: 1. Binding of a hydrophobic dye toward PVA-Cu(II) complex

Abstract The interaction between PVA-Cu(II) complex and a hydrophobic dye, p-aminoazobenzene, was investigated by means of absorption spectrometry. The binding behaviour of the dye could be analysed on the basis of the method proposed by Klotz et al.1. A structure model of the complex which was termed ‘cluster model’ was found to be consistent with the following experimental results: the binding affinity of the dye appeared only when the complex is formed, and the degree of binding is principally determined by the concentration of cupric ion.

Complex formation and ionic association in methanol solutions of poly[ethylene oxide] and alkaline earth salts

Abstract Complex formation of poly(ethylene oxide) (PEO) with divalent barium and strontium salts was investigated in methanol. In these systems the complexation was accompanied by a considerable degree of ionic association. An analytical model for the polymer-ion complexation based on a one-dimensional lattice model was proposed. According to this model, the electrostatic effects between the bound ions were separated from the total free energy change of the binding. Three binding constants, i.e., the ionic association constant K A, the cation binding constant, K c, and the anion binding constant, K a, could be estimated. K A for barium and strontium salts was comparable, and the effect of counteranions on K A was not large. K c for barium salts was almost independent of the kind of counteranion and larger than that for corresponding strontium salts, indicating stronger polymer-ion interaction for barium salts. The anion binding constant, K a, was strongly dependent upon the kind of anion, and the order w...

Chemorheology of irradiation-cured natural rubbers: 2. Stress relaxation mechanisms of sulphur-containing cured systems in air

Abstract Stress relaxation mechanisms in air were investigated for irradiation-cured natural rubbers (NR) containing the various levels of sulphur, in relation to the crosslinked structure. All the stress relation curves of these vulcanizates except the sulphur-free sample were the sum of two exponential terms. The relaxation rate in the long time region increased with increasing sulphur contents, in spite of having the same initial chain density. This behaviour could be explained by introducing the assumption, that this system has two independent networks, i.e. one ( N p (O)) based on the polysulphide cross-linkages and another ( N c , m (O)) based on the mono- and di-sulphide linkages and the carbon—carbon bonds. N p (O) increased with increasing sulphur contents and N c , m (O) decreased with increasing sulphur contents. The q m , c ( t ) (the number of moles of main chain scission) were in agreement with each other, independent of the sulphur contents. However, this value was larger than that of the sulphur-free sample. This was similar to irradiation-cured tetramethylthiuram disulphide NR, in which the crosslinking sites consisted of mono- and di-sulphides and carbon—carbon bonds.

Behaviour of the spin-spin relaxation time for natural rubber vulcanizates under large deformation

Abstract The spin-spin relaxation time, T2, for DCP-cured natural rubber with various crosslink densities, ve, has been measured under various deformation. T2 is separated into two components: one is the long T2 component, T2L, for the mobility of amorphous network chains, the other is the short one, T2S, for that of the strain-induced crystalline chains. T2L decreased exponentially with increasing extension ratio,α, and the decreasing rate was more remarkable with increasing ve. The α and ve dependence of T2L has been quantitatively explained by the equation experimentally derived by Nishi et al.T2L under various extension increased and became almost constant with increasing temperature, while the corrected fraction of T2S, T2S (%), gradually decreased. The apparent melting point, Tm, at which the corrected T2S (%) was zero under various deformation was determined. The α dependence of Tm, has been discussed by using Florys equation.

Influence of oxygen content in the atmosphere on chemical stress relaxation of natural rubber vulcanizates: 1. Relaxation mechanisms of sulphur-cured systems

Abstract Stress relaxation of sulphur-cured natural rubbers of different crosslinked structures was investigated in an atmosphere containing variable amounts of oxygen. At high temperature, even very low contents of oxygen strongly affected the degradation of these vulcanizates. The stress relaxation of each vulcanizate was analysed in relation to the sol fraction and the swelling of sample degraded under the same condition. The scission mechanism of the vulcanizates depended on the quantity of oxygen in the atmosphere. Under nitrogen, the scission occured mainly near the crosslink site in all the samples. However, in the case of their being small amounts of oxygen present it took place near the crosslink site in the initial stages of degradation and then along the main chain at the later stages. In the atmosphere of comparative high oxygen content, the degradation seemed to result from the oxidative scission along the main chain. The degradation mechanisms of sulphur-cured natural rubbers were also discussed.

Chemorheological Study of Vulcanized Rubbers under Mechanical Stress

Abstract Below the glass transition temperature, the cleavage of molecular chains of extended vulcanized rubbers will occur. The crosslink density of vulcanized rubbers, which were placed at room temperature for about 4 h in a nitrogen atmosphere, increased about 17 per cent after extension of 200 per cent at −76° C. This may be due to the recombination of cleft chains, and this can be shown by the Maxwellian type of curves resulting from a plot of crosslink density as a function of forced strain. The decreasing proportion of crosslink density in the presence of radical acceptors in the rubbers and the increasing crosslink density in the absence of radical acceptors, because of recombination of cleft chains, was ascribed to the recombination of unstable chains produced by the added mechanical stimulus.