Energy transport in aromatic polymer blends and copolymers.

Abstract

The morphology of solid-state linear aromatic polymer blends and copolymers is studied non-destructively via electronic excitation transport by monitoring the steady-state emission spectra as well as the long-lived decays of phosphorescence and delayed fluorescence. For comparison, we employ differential scanning calorimetry and find it to be partially destructive. In blends composed of Poly(methylmethacrylate) (PMMA) with other polymers including Poly(1-vinylnaphthalene) (P1VN), Polystyrene (PS), and Poly(styrenemethylmethacrylate) (PS/MMA), the relative intensity of excimer to monomer fluorescence is experimentally shown to be highly dependent on the concentration of chromophores, the configuration and the conformation of the macromolecular matrix, as well as its thermal history. The triplet-triplet annihilation in P1VN/PMMA blends at 77K is shown to follow pseudo-unimolecular (heterofusion) kinetics, which is based on the fusion of mobile triplets with trapped triplets. The formation of a quasi-one-dimensional tenuously connected network of guest P1VN chains embedded in host PMMA chains (at about 10% by weight) is probed via the heterogeneity exponent, h. The value of h, characterizing the low dimensional diffusion-limited fractal-like kinetics, is determined by monitoring the decays of delayed fluorescence and phosphorescence. The long-lived decays are also well described by the modified stretched exponential model, with a critical exponent, $\beta$. The theoretical relationship between $\beta$ and h has been successfully proved from the decay measurements. Unlike classical kinetics in homogeneous media, fractal-like kinetics in low dimensional heterogeneous media can be used to easily distinguish between reaction-limited and diffusion-limited reactions. The triplet transport in P1VN/PMMA blends is also shown to be dependent on the excitation intensity and duration. The distribution of triplets created by transient excitation pulses is uniformly random but that generated by steady-state excitation is not. For prolonged excitation at 77K, photophysical degradation is observed; it can be reversed by thermal annealing even at temperatures below their glass transition. The phosphorescence decay rates of PS in various structural forms are shown to increase with temperature and molecular weight, due to thermal detrapping and more efficient chain coiling, respectively. Appended are comparative measurements on polycrystalline naphthalene.