Excimer and Exciton Fusion of Blends and Molecularly Doped Polymers--A New Morphological Tool.
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EXCIMER AND EXCITON FUSION OF BLENDS AND MOLECULARLY DOPED POLYMERS--A NEW MORPHOLOGICAL TOOL. ZHONG-YOU SHI, CHING-SHAN LI and RAOUL KOPELMAN, Department of Chemistry, The University of Michigan, Ann Arbor, MI 48109-1055. ABSTRACT Exciton-exciton and exciton-excimer triplet fusion kinetics is monitored in medium molecular weight P1VN/PMMA solvent cast films with concentrations from 0.005 to 100% (weight), at temperatures of 77 to 300 K, via time resolved fluorescence and phosphorescence (10 ns to 10 sec). The heterogeneity exponent (h) is 0.5 for isolated P1VN chains, zero (classical) for pure P1VN and "fractal-like" throughout certain concentration regimes. However, h is not monotonic with blend concentration but rather oscillates between zero and 0.5. Correlation is made with morphology changes (phase separation, filamentation). •s expected, the triplet exciton kinetics is dominated by short-range hops (about 5 A) and thus monitors the primary topology of the chains. At concentrations below 0.01%, the excitons are constrained to a truly one-dimensional topology. At higher concentrations there is a fractal-like topology. Similar studies were conducted on naphthalene-doped PMMA (1-20% weight). The lower concentration samples are neither segregated nor random solution phases. INTRODUCTION Can luminescence techniques establish themselves as important tools for polymer morphology? Industrial materials are becoming more complex, structurally, as they are being optimized for specific performance criteria. This complexity goes beyond the chemistry of copolymers and into the specific physical arrangement of composite materials containing monomers, homopolymers, and copolymers. This leads to heterogeneous mixtures containing microphase and interphase domains. There is a general belief that the microscopic structure of the material and its dynamic response are responsible for its desirable properties [11. However, we know little about the morphology and dynamics at the 50 to 500 A scale (typical polymer dimensions). Characterizing the structure and dynamics on this scale might lead to: (1) a better understanding of materials performance, (2) improved analytical methods for quality control, and 3) guidance for future synthesis of improved materials. While spectroscopic methods have been around for a long time, the recent advances in laser technology and in the theory of photophysical processes have led to new approaches. Fluorescence and phosphorescence approaches, based largely on energy transfer (ET) and quenching, have been of much recent interest[1-7]. Here we focus on a relatively new approach of monitoring longer-range energy transport and energy fusion kinetics. This approach has been applied successfully to simple organic crystals, mixed crystals, crystallites embedded in porous membranes and glasses, and vapor-deposited organic films [8,9]. Most powerful has been the utilization of triplet excitation transport and fusion, via delayed fluorescence and phosphorescence. We apply this approach to the study of polymer b
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