G. Fytas

Chain and local dynamics of polyisoprene as probed by experiments and computer simulations

The dynamics of designed short polyisoprene (PI) chains in the melt is investigated on a wide temperature window using dielectric relaxation spectroscopy and pulsed field gradient nuclear magnetic resonance (NMR). At high temperatures, molecular dynamics (MD) simulations performed using two different models (an explicit atom model and a united atom one) capture very well the dynamic properties documented experimentally. Structures pre-equilibrated with end-bridging Monte Carlo are used as initial configurations for MD runs at different temperatures, providing predictions for the temperature dependence of the dynamics of this bulk PI. Local dynamics is unique, independently of the probe (dielectric relaxation, dynamic light scattering, nuclear magnetic resonance, neutron scattering), although mean correlation times are significantly affected, to different extents, by librations. Chain dynamics over the molecular weight and temperature range studied can be described well by the Rouse model, as shown by both...

One-Dimensional Hypersonic Phononic Crystals

We report experimental observation of a normal incidence phononic band gap in one-dimensional periodic (SiO(2)/poly(methyl methacrylate)) multilayer film at gigahertz frequencies using Brillouin spectroscopy. The band gap to midgap ratio of 0.30 occurs for elastic wave propagation along the periodicity direction, whereas for inplane propagation the system displays an effective medium behavior. The phononic properties are well captured by numerical simulations. The porosity in the silica layers presents a structural scaffold for the introduction of secondary active media for potential coupling between phonons and other excitations, such as photons and electrons.

Phonon dispersion and nanomechanical properties of periodic 1D multilayer polymer films.

We report on the first systematic study of phonon propagation in nanostructured composite polymer multilayer films as a function of periodicity and composition using Brillouin light scattering and numerical simulations. The high sensitivity of phonon dispersion to structure and composition allows the probing of the mechanical properties down to the single-layer level. We observe a strikingly different dependence of the longitudinal and shear moduli on confinement effects in the polymer nanolayers. In addition, temperature dependent measurements of sound velocities reveal the presence of distinct glass transition temperatures, indicative of phonon localization in films with large layer thicknesses in agreement with theoretical predictions.

Probing mobility and structural inhomogeneities in grafted hydrogel films by fluorescence correlation spectroscopy

We employed fluorescence correlation spectroscopy to investigate the effect of crosslinking density, annealing in the dry state, temperature, and solvent quality on the one-dimensional swelling, permeability, and mobility of tracer molecules in thermoresponsive hydrogel films. These consist of a carboxylated poly(N-isopropylacrylamide) derivative (PNIPAAm) covalently anchored to glass substrates. Upon increasing the temperature beyond the collapse transition at about 32 °C, the gels shrunk from the swollen to a collapsed state. A molecular dye (Alexa 647) and green fluorescent protein were chosen as tracers as they display only weak interaction with the carboxylated PNIPAAm. At large swelling ratios (low temperatures) the hydrogel layers are spatially homogeneous and both tracers show single Fickian diffusion. Diffusion coefficients scale with the PNIPAAm volume fraction. Upon temperature increase a qualitatively different behavior is observed already in the pretransition region (25–32 °C) concurrently with moderate swelling ratios (<4). This is manifested by an additional, faster Fickian diffusion and structural inhomogeneities, which are also found by optical waveguide mode spectroscopy. Above the collapse transition all diffusants are expelled from the hydrogels at a limiting swelling ratio ∼1.5. Subtle differences in the solvent quality influence the diffusion of tracers in the PNIPAAm hydrogel films. In the transition temperature range structural inhomogeneities at the nanoscale appeared.

Component segmental mobilities in an athermal polymer blend: Quasielastic incoherent neutron scattering versus simulation

The local dynamics of the miscible blend of cis-1,4 polyisoprene (PI) (70% by weight) with 1,2 polybutadiene (PVE) (30% by weight) is studied. Quasielastic incoherent neutron scattering (QENS) experiments have provided the dynamic structure factor for each component in the blend far above the glass transition temperature. Molecular dynamics simulations on the same system have given segmental relaxation functions in good agreement with the experiments. Both methods reveal differences in the mobilities of each component, even at high temperatures, emphasizing intramolecular factors. Remarkably, the segmental relaxation of the PVE component in the PI/PVE blend rich in PI resembles that of PI and not of the pure PVE.

Pressure-induced dynamic homogeneity in an athermal diblock copolymer melt

We report on the effect of pressure on the dual segmental relaxation of an athermal poly(isoprene-b-vinylethylene) (PI-PVE) diblock copolymer melt using dielectric spectroscopy. The quantity (∂ log τ/∂P)T is employed as a probe of the state of dynamic miscibility in the system. We find that pressure, unlike temperature, induces dynamic homogeneity.

Determination of the longitudinal compliance of poly(vinyl acetate) by using the dynamic light scattering technique

Using the Kramers–Kronig relation, we have provided a correlation between the longitudinal compliance and the time correlation function of density fluctuations that can be directly measured in a light scattering experiment. Photon correlation spectroscopic experiments have been carried out to test the theory. Good agreement between the dynamic light scattering data and the mechanical dynamic compressibility data has been obtained.

Self-diffusion of multiarm star polymers in solution far from and near the ordering transition

The self-diffusion of three 128-arm polybutadiene star solutions in toluene was investigated over a broad concentration range from dilute to the ordering region with pulsed field gradient NMR (PFG-NMR). The strong concentration dependence of the self-diffusion coefficient in the fluid state is distinguished clearly from that of linear chains and can be described by a non-Arrhenius VFT-like equation with the concentration playing the role of inverse temperature. In the concentrated regime, the observation of two dynamic phases reflects the coexistence of crystalline and liquid phases over a limited concentration region. The concentration dependences of both the ordinary (fluid-like) diffusion and the completely restricted in-cage motion of these hyper stars are in agreement with the behavior of concentrated colloidal suspensions.

Segmental dynamics of disordered styrene–isoprene tetrablock copolymers

The local segmental dynamics of four styrene-b-isoprene-b-styrene-b-isoprene (SISI) tetrablock copolymers with different styrene composition fs and constant total degree of polymerization N≈120 has been studied in the disordered state in the nano-picosecond time scale, by incoherent quasielastic neutron (QENS), and Brillouin (BS) and depolarized Rayleigh (DRS) light scattering. Far above the glass transition temperature, all three techniques demonstrate the presence of two distinct time scales from which the fast segmental relaxation was quantitatively resolved. This process is associated with the mobility of the polyisoprene (PI) component, and is moderately slower and possesses a broader distribution of relaxation times than in bulk PI. The comparison between the correlation times of DRS and the characteristic times of QENS suggest that segment (hydrogen nucleus) diffusion over a distance of ≈0.8 nm suffices for the loss of local orientation correlations. The faster times of the BS experiment correspond...

Phase Separation and Glass Transition Intervention in a Polymer Blend

We report on a dynamic-light-scattering study of the order parameter fluctuations in a binary mixture of poly(methylphenylsiloxane) and poly(styrene) in the glass transition region. We find that the interdiffusion freezes and the growth of composition fluctuations ceases when the time scales of the segmental motion in the poly(styrene)-rich environment approach the duration of the experiment. In contrast to the single relaxation process found in polymer blends well above Tg, the polymer mixture studied here exhibits an additional slower relaxation mode.