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The scaled particle theory and the polarizable continuum model are studied to describe the thermodynamics of solvation of the fullerness C(sub 60) and C(sub 70) in aromatic solvents, hexane, and water. The model was able to reproduce the trends in solution behavior of C(sub 60) along a series of aromatic solvents.

Polymer chains, confined to cavities or polymer layers with dimensions less than the chain radius of gyration, appear in many phenomena, such as gel chromatography, rubber elasticity, viscolelasticity of high molar mass polymer melts, the translocation of polymers through nanopores and nanotubes, polymer adsorption, etc. Thus, the description of how the constraints alter polymer thermodynamic properties is a recurrent theoretical problem. A realistic treatment requires the incorporation of impenetrable interacting (attractive or repulsive) boundaries, a process that introduces significant mathematical complications. The standard approach involves developing the generalized diffusion equation description of the interaction of flexible polymers with impenetrable confining surfaces into a discrete eigenfunction expansion, where the solutions are normally truncated at the first mode (the “ground state dominance” approximation). This approximation is mathematically well justified under conditions of strong confinement, i.e., a confinement length scale much smaller than the chain radius of gyration, but becomes unreliable when the polymers are confined to dimensions comparable to their typically nanoscale size. We extend a general approach to describe polymers under conditions of weak to moderate confinement and apply this semianalytic method specifically to determine the thermodynamics and static structure factor for a flexible polymer confined between impenetrable interacting parallel plate boundaries. The method is first illustrated by analyzing chain partitioning between a pore and a large external reservoir, a model system with application to chromatography. Improved agreement is found for the partition coefficients of a polymer chain in the pore geometry. An expression is derived for the structure factor S(k) in a slit geometry to assist in more accurately estimating chain dimensions from scattering measurements for thin polymer films. [ABSTRACT FROM AUTHOR]

The entropy theory of glass formation, which has previously been developed to describe general classes of polymeric glass-forming liquids, is extended here to model the thermodynamic and dynamic properties of poly(α-olefins). By combining this thermodynamic theory with the Adam–Gibbs model (which relates the configurational entropy to the rate of structural relaxation), we provide systematic computations for all four characteristic temperatures (TA, Tc, Tg, T0), governing the position and breadth of the glass transition, and the fragility parameters (D,m) describing the strength of the temperature dependence of the structural relaxation time, where TA is the temperature below which the relaxation is non-Arrhenius, Tc is the crossover or empirical mode-coupling temperature, Tg is the glass transition temperature, and T0 is the temperature at which the extrapolated relaxation time diverges. These temperatures and fragility parameters are evaluated as a function of molar mass, pressure, and the length n of the α-olefin side chains. The nearest neighbor interaction energy and local chain rigidities are found to strongly influence the four characteristic temperatures and the low temperature fragility. We also observe an “internal plasticization” of the poly(α-olefins) wherein the fragility decreases as the number n of “flexible” side group units increases. Our computations provide solid support for a pressure counterpart of the Vogel–Fulcher–Tammann relation. The entropy theory of glass formation predicts systematic changes in fragility with chain stiffness, cohesive energy, polymerization index, and side chain length, and qualitative trends in these parameters are discussed. [ABSTRACT FROM AUTHOR]

We examine the rheological and dielectric properties of solutions of equilibrium self-assembling particles and molecules that form polydisperse chains whose average length depends on temperature and concentration (free association model). Relaxation of the self-assembling clusters proceeds by motions associated either with cluster rotations, with diffusive internal chain dynamics, or with interchain entanglement interactions. A hierarchy of models is used to emphasize different physical effects: Unentangled rodlike clusters, unentangled flexible polymers, and entangled chains. All models yield a multistep relaxation for low polymer scission rates (“persistent polymers”). The short time relaxation is nearly exponential and is dominated by the monomeric species and solvent, and the long time relaxation is approximately a stretched exponential, exp[-(t/τ)β], a behavior that arises from an averaging over the equilibrium chain length distribution and the internal relaxation modes of the assembled structures. Relaxation functions indicate a bifurcation of the relaxation function into fast and slow contributions upon passing through the polymerization transition. The apparent activation energy for the long time relaxation becomes temperature dependent, while the fast monomeric relaxation process remains Arrhenius. The effective exponent β(T), describing the long time relaxation process, varies monotonically from near unity above the polymerization temperature to a low temperature limit, β≃13, when the self-assembly process is complete. The variation in the relaxation function with temperature is represented as a function of molecular parameters, such as the average chain length, friction coefficient, solvent viscosity, and the reaction rates for particle association and dissociation. [ABSTRACT FROM AUTHOR]

Cluster formation and disintegration greatly complicate the description of relaxation processes in complex fluids. We systematically contrast the viscoelastic and dielectric properties for models of equilibrium polymers whose thermodynamic properties have previously been established. In particular, the monomer-mediated model allows chain growth to proceed only by monomer addition, while the scission-recombination model enables all particles to associate democratically, so that chain scission and fusion occur at the interior segments as well as at chain ends. The minimal models neglect hydrodynamic and entanglement interactions and are designed to explore systematically the competition between chemical reaction and internal chain relaxation and how this coupling modifies the dynamics from that of a polydisperse solution of Rouse chains with fixed lengths (i.e., “frozen” chains). As expected, the stress relaxation is nearly single exponential when the assembly-disassembly reaction is fast on the time scale of structural chain rearrangements, while multiexponential or nearly stretched exponential relaxation is obtained when this reaction rate is slow compared to the broad relaxation spectrum of almost unperturbed, nearly “dead” chains of intrinsically polydisperse equilibrium polymer solutions. More generally, a complicated intermediate behavior emerges from the interplay between the chemical kinetic events and internal chain motions. [ABSTRACT FROM AUTHOR]

General discrete one-dimensional stochastic models to describe the transport of single molecules along coupled parallel lattices with period N are developed. Theoretical analysis that allows to calculate explicitly the steady-state dynamic properties of single molecules, such as mean velocity V and dispersion D, is presented for N=1 and N=2 models. For the systems with N>2 exact analytic expressions for the large-time dynamic properties are obtained in the limit of strong coupling between the lattices that leads to dynamic equilibrium between two parallel kinetic pathways. It is shown that for all systems dispersion is maximal when the coupling between channels is weak. [ABSTRACT FROM AUTHOR]

The polymerization dynamics of double-stranded polymers, such as actin filaments, is investigated theoretically using simple chemical kinetic models that explicitly take into account some microscopic details of the polymer structure and the lateral interactions between the protofilaments. By considering all possible molecular configurations, the exact analytical expressions for the growth velocity and dispersion for two-stranded polymers are obtained in the case of the growing at only one end, and for the growth from both polymer ends. Exact theoretical calculations are compared with the predictions of approximate multilayer models that consider only a finite number of the most relevant polymer configurations. Our theoretical approach is applied to analyze the experimental data on the growth and fluctuations dynamics of individual single actin filaments. [ABSTRACT FROM AUTHOR]

A combination of electrochemical state, electronic, fluorescence and ESR spectra and density functional theory (DFT) calculations are used for the 22 known [C.sub.60][(C[F.sub.3]).sub.n] derivatives. The results for the [C.sub.60][(C[F.sub.3]).sub.n] compounds and the additional [C.sub.60][(X).sub.n] compounds have provided insights into the frontier orbitals of [C.sub.60][(X).sub.n] derivatives.

Self-healing polymeric materialsare systems that after damagecan revert to their original state with full or partial recovery ofmechanical strength. Using scaling theory we study a simple modelof autonomic self-healing of unentangled polymer networks. In thismodel one of the two end monomers of each polymer chain is fixed inspace mimicking dangling chains attachment to a polymer network, whilethe sticky monomer at the other end of each chain can form pairwisereversible bond with the sticky end of another chain. We study thereaction kinetics of reversible bonds in this simple model and analyzethe different stages in the self-repair process. The formation ofbridges and the recovery of the material strength across the fracturedinterface during the healing period occur appreciably faster aftershorter waiting time, during which the fractured surfaces are keptapart. We observe the slowest formation of bridges for self-adhesionafter bringing into contact two bare surfaces with equilibrium (verylow) density of open stickers in comparison with self-healing. Theprimary role of anomalous diffusion in material self-repair for shortwaiting times is established, while at long waiting times the recoveryof bonds across fractured interface is due to hopping diffusion ofstickers between different bonded partners. Acceleration in bridgeformation for self-healing compared to self-adhesion is due to excessnonequilibrium concentration of open stickers. Full recovery of reversiblebonds across fractured interface (formation of bridges) occurs afterappreciably longer time than the equilibration time of the concentrationof reversible bonds in the bulk. [ABSTRACT FROM AUTHOR]

The frontier orbitals of 22 isolated and characterized C60(CF3)n derivatives, including seven reported here for the first time, have been investigated by electronic spectroscopy (n = 2 [1], 4 [1], 6 [2], 8 [5], 10 [6], 12 [3]; the number of isomers for each composition is shown in square brackets) fluorescence spectroscopy (n = 10 [4]), cyclic voltammetry under air-free conditions (all compounds with n = 12), ESR spectroscopy of C60(CF3)n- radical anions at 25 00 (n = 4 [1] and 10 [1]), and quantum chemical calculations at the DFT level of theory (all compounds including n = 16 [3] and 18 [2]). DFT calculations are also reported for severaI hypothetical C60(CF3)n derivatives. The X-ray structure of one of the compounds, 1,6,11,16,18,26,36,41,44,57-C60(CF3)10, is reported here for the first time. Most of the compounds with n ≤ 12 exhibit two or three quasi-reversible reductions at scan rates from 20 my s-1 up to 5.0 V s-1 respectively. The 18 experimental 0/- E1/2 values (vs C60-1) are a linear function of the DFT-predicted LUMO energies (average E1/2 deviation from the least-squares line is 0.02 V). This linear relationship was used to predict the 0/- E1/2 values for the n = 16 and 18 derivatives, and none of the predicted values is more positive than the 0/- E1/2 value for one of the isomers of C60(CF3)n. In general, reduction potentials for the 0/- couple are shifted anodically relative to the C600/- couple. However, the 0/- E1/2 values for a given composition are strongly dependent on the addition pattern of the CF3 groups. In addition, LUMO energies for isomers of C60(X)n (n = 2, 4, 6, 8, 10, and 12) that are structurally related to many of the CF3 derivatives were calculated and compared for X = CH3, H, Ph, NH2, CH2F, CHF2, F, NO2, and ON. The experimental and computational results for the C60(F3)n compounds and the computational results for more than 50 additional C60(X)n compounds provide new insights about the frontier orbitals of C60(X)n derivatives. For a given substituent, X, the addition pattern is as important, if not more important in many cases, than the number of substituents, n, in determining E1/2 values. Those addition patterns with double bonds in pentagons having two C(sp²) nearest neighbors result in the strongest electron acceptors. [ABSTRACT FROM AUTHOR]