Scale-free static and dynamical correlations in melts of monodisperse and Flory-distributed homopolymers: A review of recent bond-fluctuation model studies

It has been assumed until very recently that all long-range correlations are screened in three-dimensional melts of linear homopolymers on distances beyond the correlation length $\xi$ characterizing the decay of the density fluctuations. Summarizing simulation results obtained by means of a variant of the bond-fluctuation model with finite monomer excluded volume interactions and topology violating local and global Monte Carlo moves, we show that due to an interplay of the chain connectivity and the incompressibility constraint, both static and dynamical correlations arise on distances $r \gg \xi$. These correlations are scale-free and, surprisingly, do not depend explicitly on the compressibility of the solution. Both monodisperse and (essentially) Flory-distributed equilibrium polymers are considered.Comment: 60 pages, 49 figure

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 f(s) and constant total degree of polymerization Napproximate to120 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 approximate to0.8 nm suffices for the loss of local orientation correlations. The faster times of the BS experiment correspond to shorter displacements, approximate to0.3 nm. These results demonstrate that the segmental dynamics of the PI are much faster than would be inferred from the monomeric friction factor of PI previously extracted by diffusion and viscosity measurements in the same tetrablock matrices. This, in turn, indicates a substantial local spatial heterogeneity in the segmental dynamics. The slow process is due to the PS segments, which do not relax, appreciably on the time scales accessible here. (C) 2002 American Institute of Physics

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 experimental data and a normal mode analysis on simulation trajectories. Deviations from the Rouse model emerge for the high modes at short times; still, this model offers a rather accurate picture. (C) 2003 American Institute of Physics

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 f(s) and constant total degree of polymerization Napproximate to120 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 approximate to0.8 nm suffices for the loss of local orientation correlations. The faster times of the BS experiment correspond to shorter displacements, approximate to0.3 nm. These results demonstrate that the segmental dynamics of the PI are much faster than would be inferred from the monomeric friction factor of PI previously extracted by diffusion and viscosity measurements in the same tetrablock matrices. This, in turn, indicates a substantial local spatial heterogeneity in the segmental dynamics. The slow process is due to the PS segments, which do not relax, appreciably on the time scales accessible here. (C) 2002 American Institute of Physics

Technological properties and non-enzymatic browning of Lupinus albus protein-enriched spaghetti

Spaghetti was prepared by replacing semolina with different amounts of lupin protein, in order to increase the protein content. A detailed investigation of the rheological properties of the dough and the cooking quality of pasta was performed in comparison to standard semolina spaghetti. Moreover, the effect of the addition of lupin protein on non-enzymatic browning was evaluated by measuring epsilon-furoylmethyllysine (furosine) and 5-hydroxymethyl-2-furancarboxaldehyde (HMF), which are considered useful indices of semolina quality and pasta processing conditions. Dried spaghetti fortified with 5% of lupin protein isolate has a colour and rheological features comparable with the semolina sample and also the behaviour during cooking results to be satisfactory. As far as the thermal damage is concerned, the furosine values of fortified spaghetti differ only marginally from standard pasta and the percentage lysine loss is quite small (ranging from 12.1% to 15.7%)