There is increased technological interest to use blends of various dissimilar elastomers in applications for which service, material availability, or cost of a single elastomer do not provide the necessary processing, vulcanizate, or economic properties. Properties of polyblends are sensitive to variations in the amounts of the individual polymers used. Therefore, there is a need for developing a variety of analytical tools that will enable the compounder to monitor the consistency of blend compositions. In this study, the feasibility of using pyrolysis-GC/MS and thermogravimetric analysis (TGA) to estimate the blend composition of SBR/NBR blends was investigated. Pyrolysis-GC/MS degradation products that are characteristic of each polymer were identified. The GC/MS peak areas were used to determine the blend composition. The blend compositions were estimated by TGA from the linear correlation between the polymer composition and the temperature required to pyrolyze a sample to a specific “% weight loss.” The results obtained by pyrolysis-GC/MS and TGA were compared to calculated blend ratios of SBR/NBR in order to estimate the accuracy of the test methods presented in this study.

ABSTRACT:There is increased technological interest in using blends of various dissimilar elastomers in applications for which service, material availability, or cost of a single elastomer do not provide the necessary processing, vulcanizate, or economic properties. The properties of these polyblends are sensitive to small variations in the amounts of the individual polymers used. Accurately estimating the elastomer composition of blends is of vital importance to the elastomer industry. This study illustrates the feasibility of using mid-infrared (MIR) and near-infrared (NIR) spectroscopy to estimate the amount of styrene-butadiene and acrylonitrile-butadiene copolymers in blends composed of varying ratios of the two elastomers. Sometimes it is difficult to obtain a film of an elastomer amenable to IR analysis; to address this problem, several techniques were developed in this study [MIR transmission of a film, attenuated total internal reflection (ATR)-FTIR of a chunk, and NIR using a fiber-optic probe]. A plot of the absorbance ratio (absorbance of the characteristic peak for styrene-butadiene rubber or acrylonitrile-butadiene rubber/absorbance of the CAC stretching vibration of polybutadiene) versus the amount of each elastomer in the blend was used to predict the blend composition. In addition, the blends were also characterized by ATR-FTIR using a plot of the characteristic peak absorbance versus the polymeric content for a series of standards. A partial least-squares algorithm was used to develop a calibration curve for the NIR region. Finally, the accuracy of the test methods developed in this work is compared to results obtained by pyrolysis-GC/MS and thermogravimetric analysis.

Photolytic initiation of free radical reactions is important to many areas of technology; time-resolved monitoring of submicromolar concentrations of radicals produced during the course of these reactions is needed to provide information about the rate of initiation and its competition with radical recombination. In this work, time-resolved laser-induced fluorescence is evaluated for monitoring of diphenylketyl radicals produced by photoreduction of the triplet state of benzophenone. Fluorescence from the doublet-doublet transition of the radical is excited with a continuous wave laser and provides a sensitive method to detect these intermediates at nanomolar concentrations and to study their kinetics in solution on time scales from a few microseconds to hundreds of milliseconds. The ketyl radical fluorescence measurements of radical initiation reactions allowed the H atom abstraction rate constant by triplet benzophenone from both 2-propanol and benzhydrol to be determined, where k(H) = (2.1 ± 0.1) × 10(6) M(-)(1) s(-)(1) for 2-propanol and k(H) = (4.4 ± 0.1) × 10(6) M(-)(1) s(-)(1) for benzhydrol. The diphenylketyl radical recombination rate constant was also determined by time-resolved fluorescence monitoring of the decay of the radical population and found to be k(r) = (1.9 ± 0.2) × 10(8) M(-)(1) s(-)(1). Formation kinetics could be measured on a microsecond time scale from radical populations as low as 45 nM; decay kinetics could be followed on a millisecond time scale from 20 nM radical concentrations.

Electronic excitation of a probe chromophore can lead to a change in dipole moment that influences its activity or solubility in solution and changes its relative affinity for partitioning between two phases. Photoexcitation of a probe molecule can, therefore, perturb a sorption equilibrium, and the relaxation kinetics of the probe to the new equilibrium conditions can be monitored in a time-resolved luminescence experiment. The adsorption/desorption kinetics of rose bengal, distributed between a C-18 derivatized porous-silica surface and a liquid mobile-phase solution, were investigated. These kinetics were determined by observing their effect on the phosphorescence decay of the triplet state of rose bengal and its quenching by ferricyanide. The methanol/water solvent compositions were varied to alter the fraction of adsorbed rose bengal. The adsorption rate constant for the triplet state was determined from the dependence of the phosphorescence relaxation rate on dye concentration in solution. The results indicate that the adsorption kinetics are diffusion controlled and that the relaxation is influenced by efficient triplet-energy transfer between excited- and ground-state rose bengal at the C-18 silica/solution interface.

The triplet-state decay kinetics of erythrosin-ITC, chemically bound to aminated porous silica, provides information about the local environment of the liquid/solid interface and its influence on interfacial reaction kinetics. A dispersion in phosphorescence decay rates of the immobilized erythrosin was observed. From the pH response of the decay kinetics, two distinct populations appear to arise from domains of protonated amine and silanol sites on the silica surface. The decay rates of the excited-triplet populations are sufficiently slow to probe quenching encounters with solution-phase azulene, which can diffuse over relatively long distances during the lifetime of the excited-state. By variation of the solvent conditions to influence adsorption of azulene, it was found that adsorbed azulene does not participate in interfacial quenching. The quenching process, therefore, involves contact between solution-phase azulene and the immobilized probe. The rates of this process for the short-lived (solution-associated) erythrosin population were slower than free-solution rates by a factor of 4.3, which is expected from surface-immobilization of the probe. The results indicate that transport of azulene to this population is efficient and that the pore network through which the azulene diffuses is well connected over a distance scale of ∼0.5 μm. The quenching kinetics for the longer-lived (surface-associated) erythrosin were somewhat slower, possibly due to steric hindrance or local exclusion of azulene, or less efficient molecular transport over longer distances within the porous silica.