Ultrafast dynamics of polybutadiene probed by optically heterodyne-detected optical-Kerr-effect spectroscopy
Introduction
The optically heterodyne-detected optical-Kerr-effect (OHD-OKE) measures the relaxation of optically induced polarizability anisotropy. It has been shown to be an effective probe of the ultrafast molecular dynamics of pure liquids and their binary mixtures [1], [2], [3]. Recently, the technique has been applied to the study of more complex fluids, including confined liquids in microemulsions [4], [5] and nanoporous glasses [6], [7], [8], [9], liquid crystals [10], [11], [12] and natural and synthetic polymers [5], [13], [14], [15], [16]. In the following, we report an investigation of the ultrafast dynamics of polybutadiene (PBD) and compare them with those of 1,3- and 1,4-pentadiene (PD). Measurements are made on both the pure liquids and their solutions in isopentane solvent. The interpretation of the observed PD dynamics permits an assignment of the PBD dynamics.
The OKE and related techniques have previously been employed to study the dynamics of a number of polymers, including polystyrene [5], poly-(2-vinylnaphthalene) [14], poly-(methylphenylsiloxane) [15], polyacrylamide [13] and proteins [16]. These have all been shown to exhibit relaxation involving low frequency intramolecular (often torsional) modes. However, in only a few cases has the evolution of the ultrafast dynamics as a function of molecular size and complexity been investigated, although such studies will aid the assignment of intramolecular modes. In a recent study of polystyrene nanolatexes, Hunt et al. [5] reported the OHD-OKE responses of structural analogues of polystyrene including the monomer styrene, 1,2-diphenylethane and oligomers of polystyrene. It was observed that, as molecular complexity increased, the relaxation pathways contributing to the polarizability anisotropy relaxation underwent a shift to higher frequency, indicating a change from molecular orientational relaxation (in styrene) to relaxation via low frequency intramolecular modes in the polymer. Here this approach is extended to the simpler linear chain PBD.
Section snippets
Experimental
The OHD-OKE spectrometer used here has been described in detail elsewhere, and utilises differential detection and intensity normalisation [4]. The source of ultrafast pulses was a Kerr lens mode-locked titanium sapphire laser (Clark MXR) operating at a wavelength of 800 nm, with a repetition rate of 100 MHz. The pulse duration, measured by second order autocorrelation at the sample position, was 45 fs, with a time-bandwidth product of <0.5.
All chemicals were obtained from Aldrich (99% purity
Results and discussion
(i) Measurements. Fig. 1 displays the OHD-OKE data for each of the three neat liquids. On the picosecond timescale (t > 1.5 ps) the data are well represented by a bi-exponential relaxation function with a finite rise time (Fig. 1a),Such a function (in which the risetime is given by the average frequency of the spectral density, see below) has been found to accurately describe the picosecond dynamics of many liquids and liquid mixtures, as discussed elsewhere [1]
Conclusion
The OHD-OKE responses of poly-(butadiene), 1,3-pentadiene and 1,4-pentadiene have been reported, both as neat liquids and as solutions in isopentane. By comparison with the spectral densities of 1,3- and 1,4-PD it has been possible to assign a portion of the polymer spectral density to intramolecular torsional motion about carbon–carbon single bonds.
Picosecond diffusive orientational relaxation of all three species has been shown to be not well described by the hydrodynamic approximation (1).
Acknowledgement
We are grateful to the EPSRC for financial support.
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