Alphonsus V. Pocius

Radiationless decay in propynal: Dependence upon vibrational state

New time‐resolved measurements of luminescence decay rates and intensities for vibronically selected propynal molecules excited by a pulsed tunable dye laser are reported. The observations allow individual determination of the rates for ’’collision‐free’’ and collision‐induced intersystem crossing and internal conversion from the initially excited 1A″ state. Explicit ’’no parameter’’ calculations of the collision‐free decay rates for the vibrationless level based on the Fermi Golden Rule for time‐independent perturbations give 0.38×104 sec−1 for internal conversion and 0.59×105 sec−1 for intersystem crossing if the broadening mechanism for the initially prepared levels is primarily radiative. The observed decay rate for intersystem crossing from the vibrationless level is 0.52 (+0.5, −0.25) μsec−1, while that for internal conversion is 0.48 (+0.3, −0.48) μsec−1. Results are also presented for collisional quenching of selected vibronic levels of propynal (1A″) by acetonitrile. The enhancement of quenching ...

Intramolecular energy transfer in the 4‐R‐1,2,4‐triazoline‐3,5‐diones: Relation to theory and photochemical reactivity

Various aspects of the intramolecular radiationless decay of the A1A′(1B1) excited electronic state of the 4‐R‐1,2,4‐triazoline‐3,5‐diones (R‐TAD) have been investigated using both laser excited time resolved fluorescence and conventional techniques. The lifetimes of selected vibronic levels of methyl and ethyl TAD under collision free conditions were found to vary approximately linearly from ≃110 to 55 nsec over 2000 cm−1 of excess vibrational energy in the A1A′(1B1) state. Using a somewhat unique technique based upon the fact that vibrational relaxation rather than electronic quenching occurs upon collisions with argon, the quantum yields of fluorescence of selected vibronic levels were determined for methyl TAD and were found to be ≃0.02. Nonradiative rate constants for the selected vibronic levels in A1A′(1B1) of methyl TAD could then be determined and these were found to vary approximately linearly from 8.5×106 to 16.9×106 sec−1 over 2000 cm−1 of excess vibrational energy. Using the experimentally determined variation of the nonradiative rate constant, solution lifetimes, features in the absorption spectrum, and various aspects of the theory of radiationless processes, a choice of the most probable radiationless pathway was made. This choice was intersystem crossing to a lower lying triplet state, either A1A′(1B1) → 3A″(3B1) or A1A′(1B1) → 3A′(3B2) depending upon the R substituent. This choice of nonradiative pathway was found to explain adequately the relative photochemical hydrogen abstraction reactivity of various R‐TADs. It was also found that methyl, ethyl, and n‐butyl TAD all display the same fluorescence lifetime of 92±3 nsec at high pressures of inert gas. This is discussed in terms of the radiationless transition theory, which indicates that some vibrational modes do not accelerate the radiationless decay rate over that of the vibrationless level. This is the first reported experimental verification of the presence of vibrational motions that do not participate in radiationless decay.Various aspects of the intramolecular radiationless decay of the A1A′(1B1) excited electronic state of the 4‐R‐1,2,4‐triazoline‐3,5‐diones (R‐TAD) have been investigated using both laser excited time resolved fluorescence and conventional techniques. The lifetimes of selected vibronic levels of methyl and ethyl TAD under collision free conditions were found to vary approximately linearly from ≃110 to 55 nsec over 2000 cm−1 of excess vibrational energy in the A1A′(1B1) state. Using a somewhat unique technique based upon the fact that vibrational relaxation rather than electronic quenching occurs upon collisions with argon, the quantum yields of fluorescence of selected vibronic levels were determined for methyl TAD and were found to be ≃0.02. Nonradiative rate constants for the selected vibronic levels in A1A′(1B1) of methyl TAD could then be determined and these were found to vary approximately linearly from 8.5×106 to 16.9×106 sec−1 over 2000 cm−1 of excess vibrational energy. Using the experimentally de...

Vibrational relaxation in the A1A′(1B1) electronic state of 4‐methyl‐1,2,4‐triazoline‐3,5‐dione

The fluorescence decay rate from selected vibronic levels of the A1A′(1B1) excited electronic state of 4‐methyl‐1,2,4‐triazoline‐3,5‐dione has been examined as a function of added inert gas pressure. A simple, physically reasonable model is presented from which vibrational relaxation rates may be explicitly determined. The relaxation rate constants have been found to be 6.3×105 Torr−1 sec−1 for collisions with helium, 7×105 Torr−1 sec−1 for argon, 2.5×106 Torr−1 sec−1 for CCl4, 15×105 Torr−1 sec−1 for O2, and 75×105 Torr−1 sec−1 for cyclohexane.

Elastomers Formed In-Situ in Epoxy Resins: Chemistry and Toughening

Abstract The chemistry of the in-situ formation of elastomer particles in epoxy resins is described. Under normal synthetic conditions, if one polymerizes an acrylic monomer in an epoxy resin, the resulting polymer is soluble in the epoxy resin or it grossly phase separates, depending upon the solubility parameter difference between the acrylic polymer and the epoxy resin. This work shows that with the use of a specifically tailored material (which we believe operates as a “polymeric surfactant”) in the reaction mixture, a free-flowing, stable suspension of acrylic elastomer particles in epoxy resin can be obtained. Thus, the reaction product of isocyanatoethylmethacrylate and EPONTM 1009 can be used as the polymeric surfactant in a reaction mixture containing hexyl acrylate, azo-bis(isobutyronitrile) and the diglycidyl ether of bisphenol A. The resulting dispersion is stable to settling even after exposure to 150°C. When properly formulated, the resulting modified epoxy resins are improved in their fract...

THERMALLY ACTIVATED PARAMETERS OF SELF-ADHESION IN ACRYLIC PRESSURE-SENSITIVE ADHESIVE-LIKE NETWORKS

We have conducted an experimental study of acrylic pressure-sensitive adhesive-like networks (PSA-LN) with the goal of establishing deconvolution of interfacial and bulk processes in adhesion. For contact adhesion testing, four types of cylindrically shaped samples were created, each synthetically modified to attain distinct bulk and surface properties. Introduction of small amounts of polar comonomers during synthesis increased the intrinsic adhesion energy, G o , from 65 mJ/m2 for an unmodified acrylic PSA-LN to 129, 158, and 218 mJ/m2 for PSA-LNs modified with 10 wt% of acrylic acid, amino acrylate, and acrylonitrile comonomers, respectively. Following a reversed trend, the critical rate of separation, ν* (below which deadhesion is an intrinsically interfacial process) was more than halved from 496 nm/s for the unmodified PSA-LN to 201, 188, and 212 nm/s for those modified with the same three comonomers (same order, respectively). The adhesion energy was found to be dependent upon the crack propagation rate and sensitive to it at all rates of interfacial separation, including those below the critical crack propagation rate, ν*. In addition, the dependence of the adhesion energy on crack propagation rate was measured at three temperatures. It was found that ν* displayed an Arrhenius dependence on temperature from which an activation energy could be calculated. Those activation energies, as a function of chemical composition, are compared with activation energies derived from shift factors determined from measurements of bulk modulus as a function of temperature. A direct correlation between those activation energies was noted.