Robert F. Cozzens

Photophysical Processes in Polymers. IV. Excimer Formation in Vinylaromatic Polymers and Copolymers

Excimer formation has been investigated in polystyrene, poly(1‐vinylnaphthalene), poly(2‐vinylnaphthalene), and in copolymers of 1‐vinylnaphthalene with styrene and with methyl methacrylate. Solid films as well as solutions were studied over the temperature range 77–298°K. In the solid polymers, interchain excimer formation is superimposed on the intrachain interaction observed in dilute solutions; intrachain excimer formation is essentially unaffected by the solvent in fluid solutions. Conformations that lead to excimer interaction in a solid solution are fixed by the temperature at which the solid is formed. Activation energies for intramolecular excimer formation for these polymers are somewhat less than those for the corresponding two‐unit model compounds and much less than those for the single‐unit model compounds. While excimer sites act as efficient singlet traps for intramolecularly migrating energy and act to decrease the triplet yield, they do not present a barrier to the transfer of energy to a...

Intramolecular Triplet Energy Transfer in Poly(1‐vinylnaphthalene)

Poly(1‐vinylnaphthalene) in a rigid glass at 77°K exhibits a delayed fluorescence due to triplet–triplet annihilation following intramolecular triplet energy transfer through the naphthalene chromophores. Delayed fluorescence does not appear in the spectrum of 1‐ethylnaphthalene at equivalent concentrations, about 10−3M. Delayed emission from the polymer but not from 1‐ethylnaphthalene is quenched by piperylene. End groups are more important than chain length in the control of triplet migration in the homopolymer; experiments with 1‐vinylnaphthalene‐methyl methacrylate copolymers, however, indicate that some minimum chain length is required for efficient annihilation.

Photophysical Processes in Polymers. III. Properties of Triplet Energy Traps in 1‐Vinylnaphthalene Copolymers

Emission spectra of two‐component atactic copolymers of 1‐vinylnaphthalene with styrene (PVN/S) and with methylmethacrylate have been investigated in a glassy matrix at 77°K. In PVN/S, the rate of intramolecular triplet energy transfer through segments derived from styrene exceeds that of intermolecular triplet transfer from polystyrene to piperylene; the migration occurs through chains having as many as 140 styrene‐derived units. Radiative triplet depletion within the 1‐vinylnaphthalene‐derived segments is manifested by (1) phosphorescence originating from units near the ends of the segments, and (2) delayed fluorescence due to free triplet annihilation within the segment, although some trapping may also take place. In contrast, delayed fluorescence from poly(1‐vinylnaphthalene) appears to result from several annihilation reactions, and serves to demonstrate fundamental differences between the homopolymer and copolymer.

Photophysical Processes in Polymers. V. Triplet Depletion in Solid Vinylnaphthalene Polymers and Copolymers

The results of a study of triplet energy trapping at 77°K are presented for vinylnaphthalene polymers and copolymers with styrene and methyl methacrylate. Delayed emission spectra originating from traps both intrinsic and extrinsic to the polymer chains are identified. In the solid polymers both intramolecular and interchain triplet energy migration occur, with eventual transfer to a trap at collisional distances of about 15 A. Extrinsic traps lead to phosphorescence and delayed fluorescence, while traps that are part of the polymer chain lead to delayed exciplex fluorescence. In the model compound 1‐ethylnaphthalene, only extrinsic trapping phenomena are observed. The influence of concentration on trapping phenomena in both polymers and the model compound is considered from the standpoint of chain or molecular aggregation.

Electron Spin Resonance Study of the Photolysis of Polystyrene and Poly(α‐Methylstyrene)

The photolysis of polystyrene and poly (α‐methylstyrene) was studied by ESR techniques using highly concentrated glassy solutions of the polymers near liquid‐nitrogen temperature. Every attempt was made to insure detection of the earliest free‐radical species. The use of poly (1‐vinylnaphthalene) and polystyrene‐2,3,4,5,6‐d5 verified that the primary radical species is located on the polymer chain and not the ring. The ESR spectra obtained for both polystyrene and poly (α‐methylstyrene) are consistent with the structure HH–C–Ċ–C–HφH. Interpretation of the observed spectra is made on the basis of the stereochemistry of the polymer chain.

High-contrast artificial eyelid for protection of optical sensors

The fabrication, testing and performance of a new device for the protection of optical sensors will be described. The device consists of a transparent substrate, a transparent conducting electrode, insulating polymers, and a reflective top electrode layer. Using standard integrated circuit fabrication techniques, arrays of apertures can be created with sizes ranging from micrometers to millimeters. A stress gradient resulting from different thermal coefficients of expansion between the top polymer layer and the reflective metal electrode, rolls back the composite thin film structure from the aperture area once a release layer is chemically etched away, forming a tightly curled film at one side of the aperture - the open condition. The application of a voltage between the transparent conducting and reflective metal electrodes creates an electrostatic force which unrolls the curled film, closing the artificial eyelid. Fabricated devices have been completed on glass substrates with indium tin oxide electrodes. The curled films have diameters of less than 100micrometers with the arrays having mechanical transparencies of over 80%. Greater transparencies are possible with optimized designs. The electrical and optical results from the testing of the artificial eyelid will be discussed including the optimization of the design and fabrication for applications in systems that extend into the IR spectrum. A primary area of investigation is the choice of the transparent conducting electrode.