No abstract

We report the dielectric and viscoelastic relaxations in undiluted amorphous poly(d,l-lactic acid) (PLA). Three dielectric relaxations designated as Rn, Rs, and β are observed in order of decreasing temperature. The relaxation time for the Rn relaxation increases with increasing molecular weight and is assigned to the normal mode relaxation due to the component of dipole vector aligned in the direction parallel to the chain contour. The R s relaxation is observed about 30 K above the glass transition temperature Tg () 310 K) and is assigned to the local segmental mode due to the transverse component of the monomeric dipoles. The β relaxation is seen in the glassy state and is assigned to the secondary relaxation. From the relaxation strengths for the Rn, Rs, and β relaxations, the effective dipole moments for those relaxation processes are determined and compared with the parallel and transverse components of the dipole moment calculated theoretically with the semiempirical molecular orbital methods. The dielectric relaxation time for the normal mode increases with molecular weight M with the power of 3.5 in the range of molecular weight above the characteristic molecular weight M c () 13 000). The molecular weight between entanglements is calculated to be 7700 from the shear modulus in the rubbery plateau region. It is found that the dielectric normal mode relaxation time agrees approximately with the viscoelastic terminal relaxation time. The relaxation spectra for the viscoelastic relaxation are much broader than those for the dielectric relaxation as observed previously for polyisoprene.

Hot electron processes at metallic heterojunctions are central to optical-to-chemical or electrical energy transduction. Ultrafast nonlinear photoexcitation of graphite (Gr) has been shown to create hot thermalized electrons at temperatures corresponding to the solar photosphere in less than 25 fs. Plasmonic resonances in metallic nanoparticles are also known to efficiently generate hot electrons. Here we deposit Ag nanoclusters (NC) on Gr to study the ultrafast hot electron generation and dynamics in their plasmonic heterojunctions by means of time-resolved two-photon photoemission (2PP) spectroscopy. By tuning the wavelength of p-polarized femtosecond excitation pulses, we find an enhancement of 2PP yields by 2 orders of magnitude, which we attribute to excitation of a surface-normal Mie plasmon mode of Ag/Gr heterojunctions at 3.6 eV. The 2PP spectra include contributions from (i) coherent two-photon absorption of an occupied interface state (IFS) 0.2 eV below the Fermi level, which electronic structure calculations assign to chemisorption-induced charge transfer, and (ii) hot electrons in the π*-band of Gr, which are excited through the coherent screening response of the substrate. Ultrafast pump-probe measurements show that the IFS photoemission occurs via virtual intermediate states, whereas the characteristic lifetimes attribute the hot electrons to population of the π*-band of Gr via the plasmon dephasing. Our study directly probes the mechanisms for enhanced hot electron generation and decay in a model plasmonic heterojunction.